Revision 12

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ISBN: 978-1-921007-85-9

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Acknowledgements

IDC Technologies expresses its sincere thanks to all those engineers and technicians on our training workshops who freely made available their expertise in preparing this manual.

Contents


1 Introduction to HVAC 1


1.1 General 1

1.2 Principles of thermodynamics 1

1.3 Temperature and its measurement 5

1.4 Pressure and temperature relationship 6

1.5 Laws of thermodynamics 6

1.6 Fundamentals of heat transfer 7

1.7 Fundamentals of fluid flow 9


2 Psychrometry 15


2.1 Introduction to psychrometry 15

2.2 The properties of air 15

2.3 Understanding the psychrometric charts 25

2.4 Psychrometric processes 32

2.5 Air conditioning systems-Summer and winter 43


3 Requirements of Comfort air conditioning 53


3.1 General 53

3.2 Air purification methods 56

3.3 Thermodynamics of the human body 57

3.4 Role of clothing 58

3.5 Comfort and comfort chart 60

3.6 Design considerations 64

3.7 Requirements of temperature and humidity-high heat load industries 64

3.8 Recommended inside design conditions 65

3.9 Outside summer design conditions for some foreign cities 66

3.10 Types of Ventilation Systems 68

3.11 Effect of vertical temperature gradient & corrective measures 73

3.12 Factors considered in air distribution systems indoor 75

3.13 Indoor Air Quality 77

3.14 Design of ventilation systems 85


4 Heating & Cooling Load Calculation Procedure 101


4.1 General 101

4.2 Definitions 102

4.3 Design considerations 102

4.4 Internal Sensible and Latent Heat Load components 103

4.5 Comfort air conditioning 104

4.6 Heat gain classification 105

4.7 Miscellaneous heat sources 114

4.8 Fresh air load 115

4.9 Design of air-conditioning system 116

4.10 By-pass factor (bf) consideration 121


5 HVAC Systems 125


5.1 Heating systems 125

5.2 Warm air heating systems 126

5.3 Sizing heating systems 131

5.4 Hot water heating system 131

5.5 Steam heating systems 134

5.6 Electric heating systems 135

5.7 District heating system 135

5.8 Warm air curtains 136

5.9 Air-conditioning systems: General 137

5.10 Heat pumps 158

5.11 Air handling units 162

5.12 Functional variations in the design 166

5.13 Fan coil unit 181

5.14 Capacity calculation of an air handling unit 183


6 Variable air volume (VAV) systems 187


6.1 General 187

6.2 System concept 187

6.3 Different VAV systems 189


7 Duct design, air flow and its distribution 195


7.1 Air flow and pressure losses 195

7.2 Dynamic losses in ducts 199

7.3 Duct design 202

7.4 Duct arrangement systems 204

7.5 Air distribution system inside space 207

7.6 Ventilation systems 210

7.7 Effect of vertical temperature gradient and corrective measures 212


8 Insulation of Air-conditioning systems 217


8.1 Introduction 217

8.2 Desired properties of an ideal insulating material 217

8.3 Factors affecting thermal conductivity 218

8.4 Types of insulation materials 219

8.5 Heat transfer through insulation 222

8.6 Economical thickness of insulation 223

8.7 Insulated systems 224

8.8 Importance of relative humidity for the selection of insulation 227


9 Air-conditioning equipment 229


9.1 Air filters 229

9.2 Humidifiers 238

9.3 Dehumidifiers 243

9.4 Fans and blowers 248

9.5 Grills and registers 254


10 Refrigeration 257


10.1 General 257

10.2 Methods of refrigeration 257

10.3 Air refrigeration system 263

10.4 Vapor compression refrigeration system 265

10.5 Absorption refrigeration system 266

10.6 Refrigerants 271

10.7 Refrigerant nomenclature 276

10.8 Important refrigerants 276

10.9 Refrigeration equipment 279


11 Controls and Instrumentation 299


11.1 Objectives 299

11.2 Introduction 299

11.3 Definitions 300

11.4 Elements of control 304

11.5 Types of control system 312

11.6 Methods of control 315

11.7 Selection of a control system 320

11.8 Typical control systems 321

11.9 Control specifications 325

11.10 Conclusion 327


12 Installation, Commissioning, Operation, Testing & Maintenance 329


12.1 Objectives 329

12.2 Installation 329

12.3 Charging the refrigeration unit 330

12.4 Adding oil to the compressor 335

12.5 Commissioning 336

12.6 Other service operations 341

12.7 Operational activities 343

12.8 Do’s and don’ts 345

12.9 Maintenance 346

12.10 Economics 352


13 Fault finding and troubleshooting 355


13.1 Objectives 355

13.2 Introduction 355

13.3 Faults 355

13.4 Troubleshooting 357


14 Green House effect and future refrigerants 367


14.1 Objectives 367

14.2 General 367

14.3 The greenhouse effect 368

14.4 History of CFCs 371

14.5 Ozone depletion by CFCs and the greenhouse effect 372

14.6 Global warming potential (GWP) and Ozone depleting potential(ODP) 373

14.7 Montreal protocol (1987) 374

14.8 Kyoto protocol 375

14.9 Future refrigerants to replace CFCs 376


Appendices 381


Appendix A: Psychrometry 381

Appendix B: Properties of refrigerants 395

Appendix C: Conversions and tables 403

Appendix D: Psychrometric charts plotting 419

Appendix E: Testing, Adjusting and Balancing in HVAC systems 425

Appendix F: Practical Exercises 443

Appendix G: Practical Exercises - Answers 459


1


Introduction

Objectives

After reading this chapter the student should be able to:

  • Refresh his knowledge on the engineering basics
  • Understand the laws of thermodynamics

1.1 General

Air conditioning for human comfort was considered a luxury a few decades ago, but now it has become a necessity in life. The air conditioning industry is rapidly developing throughout the world. More than 10 million window installations are being installed each year and residential central cooling installations are enjoying similar popularity.

Apart from reasons for comfort alone, air conditioning is commonly used nowadays in various industries such as food, automobiles, hotels, textiles and many more. On Earth, not only pollution from smoke is on the rise but pollution from dust is also playing havoc with our lives. Air conditioning plays a vital role in keeping out smoke and dust which could harm our health. Similarly, air conditioning has an important role to play in the preservation of food.

At present, there is hardly any sector of the economy that is not dependent on this industry. In fact in most areas of industry, HVAC systems are considered to be a basic necessity.

It is thus important to become part of this industry and this course is targeted at providing you with the basic knowledge and technology to play a role in designing, installing and commissioning HVAC systems.

The following gives an overview of the basic principles of thermodynamics, which play a key role in understanding HVAC systems.

1.2 Principles of Thermodynamics

1.2.1 Force, Newtons

In simple language, force is defined as a push or a pull. It is anything that has a tendency to set a body into motion, to bring a body to rest or change the direction of any motion.

1.2.2 Pressure, Pascals

Pressure is the force exerted per unit area. It may be described as the measure of intensity of a force exerted on any given point on the contact surface. Whenever a force is evenly distributed over a given area the pressure at any point on the surface is the same. It can be calculated by dividing the total force exerted by the area (on which the force is exerted).

Atmospheric pressure

The Earth is surrounded by an envelope of air called the atmosphere, which extends upward from the surface of the earth. Air has mass and due to gravity exerts a force called weight. The force per unit area is called pressure. This pressure exerted on the Earth’s surface is known as atmospheric pressure.

Gauge pressure

Most pressure measuring instruments measure the difference between the pressure of a fluid and the atmospheric pressure. This is referred to as gauge pressure.

Absolute pressure

Absolute pressure is the sum of gauge pressure and atmospheric pressure.

Vacuum

If the pressure is lower than the atmospheric pressure, its gauge pressure is negative and the term vacuum is applied to the magnitude of the gauge pressure when the absolute pressure is zero (i.e. there is no air present whatsoever).

The relationships among absolute pressure, gauge pressure, atmospheric pressure and vacuum are shown graphically in the Figure 1.1.

Figure 1.1
Relationship between absolute, gauge and vacuum pressures

In the above figure

Pa is the atmospheric pressure

Pgauge is the gauge pressure

Pab is the absolute pressure

Pvacuum is the vacuum pressure

1.2.3 Density

It is defined as the mass of a substance divided by its volume or the mass per unit volume.

ρ = mass/volume

Specific Volume is defined as the reciprocal of density or volume per unit mass.

v = V/m

Specific Weight (Ws) is defined as the weight of a substance divided by its volume or the weight per unit volume.

Ws = m/V

1.2.4 Work

If a system undergoes a displacement under the action of a force, work is said to be done; the amount of work being equal to the product of force and the component of displacement parallel to the force. If a system as a whole exerts a force on its surrounding and a displacement takes place, the work that is done either by or on the system is said to be external work.

1.2.5 Energy

A body is said to possess energy when it is capable of doing work. In more general terms, energy is the capacity of a body for producing an effect. Energy is classified as

  1. Stored Energy; examples are (a) Chemical energy in fuel and (b) Energy stored in dams
  2. Energy in Transition: examples are (a) Heat and (b) Work

The following are the various forms of energy.

Potential energy (P.E)

It is the energy stored in the system due to its position in the gravitational force field. If a heavy object such as a building stone is lifted from the ground to the roof, the energy required to lift the stone is stored in it as potential energy. This stored potential energy remains unchanged as long as the stone remains in its position.

P×E = mgH

Where

H = height of the object above the datum

Kinetic Energy (K.E), Joules= Newton meter

If a body weighing one kg is moving with a velocity of v m/sec with respect to the observer, then the kinetic energy stored in the body is given by:

This energy will remain stored in the body as long as it continues in motion at a constant velocity. When the velocity is zero, the kinetic energy is also zero.

Internal Energy

Molecules possess mass. They possess motion of transactional and rotational nature in liquid and gaseous states. Owing to the mass and motion these molecules have a large amount of kinetic energy stored in them. Any change in the temperature results in the change in the molecular kinetic energy since molecular velocity is a function of temperature.

Also the molecules are attracted towards each other by forces, which are very large in their solid state and tend to vanish once they are in a perfect gas state. In the melting of a solid or vaporization of a liquid it is necessary to overcome these forces. The energy required to bring about this change is stored in molecules as potential energy.

The internal energy is defined as the total energy of the body - chemical, nuclear, heat, gravitational, or any other type of energy. This energy is stored within the body which is denoted by the symbol ‘µ’. It is obvious from the above definition that it is impossible to measure the absolute value of the internal energy. However, we can measure the changes occurring in the internal energy. Since thermodynamics deals with the change in the internal energy of the system, it is important to know what causes the internal energy to change. The change in the internal energy can be caused by either due to absorption or release of heat in the system or the work done by or on the system., or if any matter enters or leaves the system.

1.2.6 Heat

Heat is one of the many forms of energy. This is evident from the fact that heat can be converted into other forms of energy and that other forms of energy can be converted into heat. Heat as molecular energy is universally accepted and heat as internal energy of the matter is thermodynamics.

Since all other forms of energy may be converted into heat, it is considered to be energy in its lowest form. The availability of heat energy to do work depends on temperature differential.

1.2.7 Heat capacity

It may be defined as the energy that must be added or removed from one kilogram of a substance to change its temperature by one degree Centigrade. In refrigeration technology heat capacity is used to determine how much heat should be removed to refrigerate various products.

1.2.7.1 Sensible heat (QS)

Heat which results in an increase or decrease in the temperature without it changing its phase is called sensible heat. A change in sensible heat is given by the equation when there is a change in temperature

QS = m× CS (T2 – T1)

Note: CS is the heat capacity at constant pressure

m = mass of the substance in kg

(T2 – T1) = Temperature difference in °C

1.2.7.2 Latent heat (QL)

Latent heat is the heat at which a substance changes its phase without any increase or decrease in the temperature. It is the amount of heat required to change the state of a substance.

QL = m×Cw(w2 – w1)

Note: Cw is the heat capacity of moisture

m = mass of the substance in kg

(w2 – w1) = change in moisture content in g/kg

1.2.7.3 Total heat (QT)

Total heat is the sum of sensible heat and latent heat. Heat measurements are taken above a specified datum. These measurements with water are at zero degrees C, since below this temperature water is solid. Refrigerant heat measurements are at –400C. For example: The sensible heat, latent heat and total heat for steam are shown in Figure 1.2 below.

QT = QS + QL

Figure 1.2
Total Heat Chart Of –400C Ice To Steam at 100 0C

a-b is sensible heat, b-c is latent heat of fusion, c-d is Sensible heat, d-e is latent heat of vaporization, e-f is super heat.

1.3 Temperature and its measurement

Temperature is a property of matter. It is the measure of intensity of heat contained in matter and its relative value. A substance is said to be hot or cold when its temperature is compared with some other reference temperature. A high temperature indicates a high level of heat intensity or thermal pressure and a body is said to be hot.

Like other forms of energy heat can be measured because it has quantity and intensity. Heat is not visible but manifests itself in its effects on various substances either by changing its state or by creating relative degrees of sensation when in contact with the human body.

Since temperature is a measure of heat content, the temperature can be measured by measuring the effects of heat on different properties of matter as follows;

  • Addition of heat increases the volume of the substance or pressure at constant volume. This property is used for measuring the temperature with the help of a mercury thermometer.
  • With the increase in temperature, the resistivity of metals increases which is utilized in resistance thermometers
  • If two junctions made of two dissimilar metals are maintained at different temperatures, a current flows in the circuit. This property is used in measuring with a thermocouple.
  • When the temperature of a substance increases, the color also changes. This property is used for measuring the temperature in radiation pyrometers.

1.4 Pressure and temperature relationship

Water boils at 1000C when the pressure on it is atmospheric at sea level. If the pressure is increased above the atmospheric pressure, i.e. in a deep mine shaft the boiling point increases and when the pressure is reduced below atmospheric, i.e. on top of a mountain, it reduces.

Boiling water does not necessarily have to be hot because if there is vacuum, water boils at a very low temperature. The same is true when it comes to other liquids, such as various refrigerants. These refrigerants have the same properties as water except their boiling point ranges are lower.

This pressure temperature relationship is used in most air conditioning and refrigeration systems.

1.5 Laws of Thermodynamics

1.5.1 First law of Thermodynamics and Energy Conservation

It is a fundamental principle that matter can neither be created nor destroyed though it may be made to take different forms. Similarly, energy cannot be created or destroyed. It can be converted from one form to another. The first law of thermodynamics states that the total energy in a system always remains constant.

This law is mainly based on observation and can be best studied with the help of observations.

In the following examples, we can see that heat, work, electricity and chemical energy are various forms of energy and they are mutually convertible.

  • An electric Iron converts electricity into heat.
  • An electric fan converts electricity into work.
  • Water flowing through a turbine converts its potential energy into work.
  • Churning of water converts work into heat.

The first law of thermodynamics can be represented by the equation:

E1 + Qa – Qt = E2

Where:

E1 is the energy possessed by the system initially

E2 is the energy possessed by the system after the work is done

Qa is the energy added to the system

Qt is the energy taken away from the system.

1.5.2 Second law of Thermodynamics

The second law of thermodynamics can be stated in a number of ways as:

  • Heat flows from a body at higher temperature to a body at lower temperature irrespective of the mass and material of the body participating in the heat transfer. This heat flow is possible without the addition of external work.
  • Work has the tendency to convert into heat but the heat cannot be converted into work.
  • Every engine or a refrigerator ejects heat to the surroundings.

With a brief discussion on the various thermodynamic principles, let us now study the fundamentals of Heating, Ventilation and Air conditioning in the next chapters.

1.6 Fundamentals of Heat Transfer

1.6.1 Modes of Transferring Heat

Heat is always transferred when a temperature difference exists between two bodies. There are three basic modes of heat transfer:

  • Conduction involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred.
  • Convection involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid.
  • Radiation, or radiant heat transfer, involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body.

1.6.2 Heat Flux

The rate at which heat is transferred is represented by the symbol. Common units for heat Q transfer rate is Watts. Sometimes it is important to determine the heat transfer rate per unit area, or heat flux, which has the symbol. Units for heat flux are W/m2. The heat flux can be Qhf determined by dividing the heat transfer rate by the area through which the heat is being transferred.

1.6.3 Thermal Conductivity

The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity (k) measured in W/m.K. It is a measure of a substance’s ability to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For vapors, it depends upon pressure.

1 W/(m.K) = 1 W/(m.oC) = 0.85984 kcal/(hr.m.oC)

Table: 1.1
Thermal conductivity values for various materials at 300 K
Material Thermal conductivity W/m.K
Copper 399
Gold 317
Aluminum 237
Iron 80.2
Carbon steel 43
Stainless Steel (18/8) 15.1
Glass 0.81
Plastics 0.2 – 0.3
Wood (shredded/cemented) 0.087
Cork 0.039
Water 0.6
Ethylene glycol 0.26
Hydrogen 0.18
Benzene 0.159
Air 0.026

1.6.4 Log Mean Temperature Difference (LMTD)

In heat exchanger applications, the inlet and outlet temperatures are commonly specified based on the fluid in the tubes. The temperature change that takes place across the heat exchanger from the entrance to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature difference (LMTD or ΔTlm).

1.6.5 Convective Heat Transfer Coefficient

The convective heat transfer coefficient (hc), defines, in part, the heat transfer due to convection. The convective heat transfer coefficient is sometimes referred to as a film coefficient and represents the thermal resistance of a relatively stagnant layer of fluid between a heat transfer surface and the fluid medium. Common units used to measure the convective heat transfer coefficient are (W/m2K).

Figure 1.2
Typical order-of magnitude values of convective heat transfer coefficients
Type of fluid and flow Convective heat transfer coefficient
hc, (W/m2 K)
Air, free convection 6 – 30
Water, free convection 20 – 100
Air or superheated steam, forced convection 30 – 300
Oil, forced convection 60 – 1800
Water, forced convection 300 – 18000
Synthetic refrigerants, boiling 500 - 3000
Water, boiling 3000 – 60000
Synthetic refrigerants, condensing 1500 - 5000
Steam, condensing 6000 – 120000

1.6.7 Overall Heat Transfer Coefficient

In the case of combined heat transfer, it is common practice to relate the total rate of heat transfer Q the overall cross-sectional area for heat transfer (Ao), and the overall temperature difference (ΔTo) using the overall heat transfer coefficient (Uo). The overall heat transfer coefficient combines the heat transfer coefficient of the two heat exchanger fluids and the thermal conductivity of the heat exchanger tubes. Uo is specific to the heat exchanger and the fluids that are used in the heat exchanger.

Q = Uo Ao ΔTo

Where: Q = The rate of heat transfer (W)

Uo = the overall heat transfer coefficient (W/m2 oK)

Ao = the overall cross-sectional area for heat transfer (m2)

ΔTo = the overall temperature difference (oK)

1.6.8 Bulk Temperature

The fluid temperature (Tb), referred to as the bulk temperature, varies according to the details of the situation. For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid that is "far" from the surface, for instance, the center of the flow channel. For boiling or condensation, Tb is equal to the saturation temperature.

1.7 Fundamentals of Fluid Flow

Fluid flow is an important part of most industrial processes; especially those involving the transfer of heat. Frequently, when it is desired to remove heat from the point at which it is generated, some type of fluid is involved in the heat transfer process. Examples of this are the cooling water circulated through cooling coils in HVAC, the air flow past the heating and cooling coils, from fans and blowers, duct work, terminal units, packaged air conditioning units etc., Unlike solids, the particles of fluids move through piping and components at different velocities and are often subjected to different accelerations.

The basic principles of fluid flow include three concepts or principles:

  1. The first is the principle of momentum (Equations of fluid forces)
  2. The second is the conservation of energy (First Law of Thermodynamics).
  3. The third is the conservation of mass (Continuity equation)

1.7.1 Properties of Fluids

A fluid is any substance which flows because its particles are not rigidly attached to one another. This includes liquids, gases and even some materials which are normally considered solids, such as glass. Fluids are materials which have no repeating crystalline structure.

There are several properties, including temperature, pressure, mass, specific volume, density, and Buoyancy.

  • Temperature was defined as the relative measure of how hot or cold a material is. It can be used to predict the direction that heat will be transferred.
  • Pressure was defined as the force per unit area. Common units for pressure are Pascal.
  • Mass was defined as the quantity of matter contained in a body and is to be distinguished from weight, which is measured by the pull of gravity on a body.
  • The specific volume of a substance is the volume per unit mass of the are substance. Typical units are m3/kg .
  • Density, on the other hand, is the mass of a substance per unit volume. Typical units are kg/m3. Density and specific volume are the inverse of one another. Both density and specific volume is dependant on the temperature and somewhat on the pressure of the fluid. As the temperature of the fluid increases, the density decreases and the specific volume increases. Since liquids are considered incompressible, an increase in pressure will result in no change in density or specific volume of the liquid. In actuality, liquids can be slightly compressed at high pressures, resulting in a slight increase in density and a slight decrease in specific volume of the liquid.
  • Buoyancy is defined as the tendency of a body to float or rise when submerged in a fluid. When a body is placed in a fluid, it is buoyed up by a force equal to the weight of the water that it displaces.
  • Compressibility is the measure of the change in volume a substance undergoes when a pressure is exerted on the substance. Liquids are generally considered to be incompressible. For instance, a pressure of 1110 kg/ cm 2 will cause a given volume of water to decrease by only 5% from its volume at atmospheric pressure. Gases on the other hand, are very compressible. The volume of a gas can be readily changed by exerting an external pressure on the gas.

1.7.2 Pascal’s Law

Pascal's law, or the Principle of transmission of fluid-pressure, states that "pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same."

where

ΔP is the hydrostatic pressure (given in pascals in the SI system), or the difference in pressure at two points within a fluid column, due to the weight of the fluid;

ρ is the fluid density (in kilograms per cubic meter in the SI system);

g is acceleration due to gravity (normally using the sea level acceleration due to Earth’s gravity in metres per second squared);

Δh is the height of fluid above the point of measurement, or the difference in elevation between the two points within the fluid column (in metres in SI).

1.7.3 Control Volume

In thermodynamics, a control volume was defined as a fixed region in space where one studies the masses and energies crossing the boundaries of the region. This concept of a control volume is also very useful in analyzing fluid flow problems. The boundary of a control volume for fluid flow is usually taken as the physical boundary of the part through which the flow is occurring.

The control volume concept is used in fluid dynamics applications, utilizing the continuity, momentum, and energy principles

1.7.4 Volumetric Flow Rate

The volumetric flow rate V of a system is a measure of the volume of fluid passing a point in the system per unit time. The volumetric flow rate can be calculated as the product of the cross sectional area (A) for flow and the average flow velocity (v).

˙V = A x v

The area is measured in square meter and velocity in meters per second, results in volumetric flow rate measured in cubic meter per second. Other common units for volumetric flow is liters per minute.

1.7.5 Mass Flow Rate

The mass flow rate (m ) of a system is a measure of the mass of fluid passing a point in the system per unit time. The mass flow rate is related to the volumetric flow rate.

Mass flowrate = Density x Volumetric flowrate

m = ρ x V

The volumetric flow rate is in m 3 /s and the density is kg/m 3 results in mass flow rate measured in kilograms per second

1.7.8 Conservation of Mass

In thermodynamics, we know that the energy can neither be created nor destroyed, only changed from one form to another form. The same is true for mass. Conservation of mass is a principle of engineering that states that all mass flow rates into a control volume are equal to all mass flow rates out of the control volume plus the rate of change of mass within the control volume.

1.7.9 Steady-State Flow

Steady-state flow refers to the condition where the fluid properties at any single point in the system do not change over time. These fluid properties include temperature, pressure, and velocity. One of the most significant properties that is constant in a steady-state flow system is the system mass flow rate. This means that there is no accumulation of mass within any component in the system.

1.7.10 Continuity Equation

The continuity equation is simply a mathematical expression of the principle of conservation of mass. For a control volume that has a single inlet and a single outlet, the principle of conservation of mass states that, for steady-state flow, the mass flow rate into the volume must equal the mass flow rate out. The continuity equation for this situation is expressed by the following equation:

mIN = mOUT

ρ x A x v (inlet) = ρ x A x v (Outlet)

1.7.11 Head Loss

Head loss is a measure of the reduction in the total head (sum of elevation head, velocity head and pressure head) of the fluid as it moves through a fluid system. Head loss is unavoidable in real fluids. It is present because of: the friction between the fluid and the walls of the pipe; the friction between adjacent fluid particles as they move relative to one another; and the turbulence caused whenever the flow is redirected or affected in any way by such components as piping entrances and exits, pumps, valves, flow reducers, and fittings.

1.7.12 Frictional Loss

Frictional loss is that part of the total head loss that occurs as the fluid flows through straight pipes. The head loss for fluid flow is directly proportional to the length of pipe, the square of the fluid velocity, and a term accounting for fluid friction called the friction factor. The head loss is inversely proportional to the diameter of the pipe.

1.7.13 Frictional Factor

The friction factor has been determined to depend on the Reynolds number for the flow and the degree of roughness of the pipe’s inner surface. The quantity used to measure the roughness of the pipe is called the relative roughness, which equals the average height of surface irregularities “”divided by the pipe diameter “D”

The value of the friction factor is usually obtained from the Moody Chart.

1.7.14 Darcy’s Equation

The frictional head loss can be calculated using a mathematical relationship that is known as Darcy’s equation for head loss. The equation takes two distinct forms. The first form of Darcy’s equation determines the losses in the system associated with the length of the pipe.

1.7.15 Minor Losses

The losses that occur in pipelines due to bends, elbows, joints, valves, etc. are sometimes called minor losses. This is a misnomer because in many cases these losses are more important than the losses due to pipe friction, considered in the preceding section. For all minor losses in turbulent flow, the head loss varies as the square of the velocity. Thus a convenient method of expressing the minor losses in flow is by means of a loss coefficient (k). Values of the loss coefficient (k) for typical situations and fittings is found in standard handbooks. The form of Darcy’s equation used to calculate minor losses of individual fluid system components is expressed by Equation:

1.7.16 Equivalent Piping Length

Minor losses may be expressed in terms of the equivalent length (Leq) of pipe that would have the same head loss for the same discharge flow rate. This relationship can be found by setting the two forms of Darcy’s equation equal to each other.


2


Psychrometry

Objectives

At the conclusion of this chapter, students should be able to:

  • Understand psychrometry and read a psychrometric chart.
  • Construct a psychrometric chart
  • Describe various psychrometric processes
  • Understand various air-conditioning systems

2.1 Introduction to psychrometry

Psychrometry is a science that involves the property of moist air (a mixture of dry air and water vapor) and the process in which the temperature and/or the water vapor content of the mixture are changed.

As per ASHRAE definition, the psychrometry as that branch of physics concerned with the measurement or determination of atmospheric conditions, particularly the moisture in the air.

The Psychrometric chart is a convenient tool for determining the moist air psychrometric properties and visualizing the changes of moist air properties in various sequences of psychrometric processes. These charts are also drawn on the basis of specified barometric pressure or elevation with respect to the sea level.

The Psychrometric tables exhibit more accurate changes occurring in air and moisture mixtures in the air conditioning processes, but the psychrometric charts are more convenient to use in all practical purposes.

2.2 The properties of air

The atmospheric air is a mixture of dry air and water vapor (moisture). The air in natural state; always contain certain amount (3.5%) of water vapor. The dry air and water vapor, do not react chemically with one another. Although they are present as mixture, each acts independent of the other.

2.2.1 Dalton’s law

Dalton’s law states that two gases can occupy the same space (Volume) at the same time, but each acts independently of the other and each exerts its own pressure.

Total pressure = Partial pressure of dry air + partial pressure of water vapor.

In common usage, total pressure is referred to as “Barometric Pressure” or “Atmospheric Pressure”.

2.2.2 Air density and specific volume

Air has its own weight.

The density of standard air is 1.2 kg/m3 and specific volume 0.83 m3/kg

For example, a fan in an air conditioning system is 300 m3/min,

Then the weight of the air handled will be 300 x 1.2 = 360 kg/min.

2.2.3 Dry air

The dry air in the atmosphere is mixture of oxygen (21%) and nitrogen (78%). The balance (1%) consists of other gases, such as argon, carbon dioxide, hydrogen etc. Both oxygen and nitrogen are in highly superheated state and therefore, the dry air is also in super heated state. Due to this state, the air conditioning processes make only slight changes in the density/ volume of dry air.

When dry air is heated or cooled, only the sensible heat is added or deleted, without any effect on the latent heat.

The specific heat of dry air = 0.133 kcal/kg

2.2.4 Moist air

It is a mixture of dry air and water vapor. The content of water vapor depends upon the temperature of air and its quantity may change from zero to maximum, i.e the saturation capacity of air.

The mass of water vapor associated with the dry air is not constant. But how the water vapor is added to the dry air? The following points will illustrate how this is being carried out:

  1. The water vapor constantly evaporating from the lake, sea and oceans into the earth’s atmosphere and returns as precipitation to the earth.
  2. Water vapor is added to the air from our homes, buildings by infiltration, perspiration, respiration, cooking, cloth washing, plants and trees from residential areas and forest.
  3. Water vapor is added to the air from the building materials and furnishings
  4. Water vapor is added to the air by humidification or evaporative cooling processes.

The table below shows the composition of the water vapor for calculating the molecular mass.

Table 2.1
Composition of water vapor
Substance Atoms Atomic mass Molecular mass
Hydrogen (H2) 2 1.00794 2.01588
Oxygen (O) 1 15.9994 15.99940
Total     18.01528

The pressure exerted by the water vapor in a mixture of air, will depend upon the amount of vapor present or the percentage of saturation. It is a known fact that the saturation pressure will be achieved only if the water and vapor formed are inside a container. Therefore, it is obvious that the pressure of water vapor present in atmosphere need not be the saturation pressure at the corresponding temperature.

The density of water vapor is very low and it is 0.0253 kg/m3. So the smaller units of grams (g) or grains (gr) are used to express its density.

(1 Lb = 7000 grains) (1 grain=0.06g).

The following table shows the saturated water vapor and density at different temperatures.

Table 2.2
Saturated vapor pressure

2.2.5 Dry Bulb Temperature, t db

Dry-bulb temperature is the temperature of the air measured by an ordinary thermometer or a temperature sensor like thermocouple, thermister, RTD, bi-metal and mercury bulb

It is the true temperature of moist air at rest, and not subjected to evaporation, condensation or radiation.

Since air is a mixture of dry air and water vapor, the dry-bulb temperature is the temperature of not only the dry air component but also the temperature of the water –vapor component.

The usage of dry-bulb temperature measurement;

(a) In calculating the sensible energy knowing the beginning and ending points, the mass flow of air, and the specific heat capacity of the moist air.

q = m Cp (t2 – t1)

Where: q = Sensible heat

m = Mass of dry-air)

Cp = Specific heat of water vapor

t2 = Entry temperature

t1 = Exit temperature

(b) In psychrometric charts as bottom X-axis coordinate, to calculate other properties of moist air

(c) To calculate Enthalpy of mixed air (dry air + Water vapor) at a particular temperature measured.

2.2.6 Wet Bulb Temperature, twb

The temperature measured by the thermometer with its bulb covered with a wet cloth and exposed to a current of moving air at 3 to 4 m/s is known as wet bulb temperature (WBT).

As the air passes over the wet wick of the thermometer the water of the wick tends to evaporate. The cooling effect of evaporation lowers the temperature measured by the wet bulb thermometer corresponding to the rate of evaporation. When the temperature measured by the WBT reaches a steady state, then the heat absorbed by the bulb for evaporation of water vapor is equal to the heat given by air (by convection) to the thermometer. This means that the total heat of air leaving the thermometer remains constant.

The heat necessary to cause evaporation in the manner stated above is present in air in the form of sensible heat. During the process of evaporation, sensible heat is converted into latent heat of vaporization maintaining the total heat of air constant. This conversion to latent heat is accomplished without change in total heat.

The evaporation rate from the wet wick depends on the condition of the air passing over it. If the surrounding air is dry then the evaporation rate will be more rapid and the drop in temperature (difference between temp. measured by WBT and DBT) will be appreciable. When the surrounding air is moist, then the evaporation rate will be slower; so will be the drop in temperature. This shows that the wet bulb temperature is a measure of degree of saturation or the relative humidity. Air with high relative humidity will have lesser drop in temperature compared to air with low relative humidity. Air with 100% relative humidity will have no drop in temperature.

The equipment used for measuring dry bulb temperature and wet bulb temperature simultaneously is called a psychrometer. There are different types of psychrometers, as listed below.

(a) Laboratory Psychrometer

This is a simple instrument, which houses both the dry bulb thermometer and the wet bulb thermometer. This is generally used in college laboratories.(Figure 2.1)

(b) Sling Psychrometer

This psychrometer consists of two mercury thermometers mounted on a frame, which has a handle. The handle of the frame helps in the rotating of the psychrometer to produce the necessary air motion. One bulb of the two thermometers is covered with a wet wick to measure the WBT. The rotating motion of the sling provides necessary air velocity over the thermometers. This air movement passing the wick helps to bring the air at temperature (WBT) in immediate contact with the wet wick.(Figure 2.2)

(c) Aspirating Psychrometer

This is similar to the other psychrometers with the exception of the blower, which provides a rapid motion of air over the thermometers. These types are used to measure the temperatures after a particular period of time mostly to measure the atmospheric conditions of cities throughout the day and year. The motor is connected to the time switch as per the interval required for the measurement of temperature.(Figure 2.3)

Figure 2.1
Laboratory Psychrometer
Figure 2.2
Sling Psychrometer
Figure 2.3
Aspirating Psychrometer

2.2.7 Relative Humidity

Relative Humidity can be defined in two ways:

“The ratio of the actual amount of moisture content in one unit volume of dry air at a certain temperature to the amount of moisture needed to saturate it at that temperature”

“The ratio of the actual pressure of water vapor of a certain unsaturated moist air at a given temperature to the vapor pressure when saturated at the same temperature”.

Relative humidity signifies the absorption capacity of air. More moisture will be absorbed by air if the initial relative humidity is less. It is derived by the equation:

Where Pv is vapor pressure;

Pvs is saturated vapor pressure.

Referring to the table on saturated vapor pressure shown;

At 21.1°C (70°F), the air is holding 0.072 g/cc of moisture and that is saturated

At 26.7 °C (80°F), the air is holding 0.098 g/cc of moisture and that is saturated

For the above two conditions, the relative humidity is 100%

Here, if we need to increase the temperature only from 21.1°C to 26.7°F without increasing the moisture content, then the relative humidity will be:

Table 2.3
Relationship between temperature, density and RH
Temperature °C (°F) Actual Density g/cc Saturated Density g/cc Relative Humidity %
21.1 (70) 0.072 0.072 100%
26.7 (80) 0.072 0.098 73.4%
32.2 (90) 0.072 0.133 54.1%
37.8 (100) 0.072 0.177 40.7%

The above chart indicate that if we increase the temperature of the air without increasing the moisture content, the relative humidity comes down and actually the air being dried. Drying is really means the removal or reduction of water vapor content

As the temperature increases, the amount of water vapor needed for saturation also increases. When we push more water vapor into the same volume of air, the pressure exerted by the water vapor increases. This is seen from the table also. But increasing the temperature without adding water vapor, the pressure increase is appreciably low.

2.2.8 Dew point

The temperature at which the water vapor contained in an air sample just starts to condense is called its “Dew Point”. Another defining statement is that the dew point is the temperature at which the moisture contained in the air at a particular temperature becomes saturated.

When the R.H value reaches 100% with respect to the temperature and the moisture content, for example, at 21.1°C, the moisture content is 0.072 g/cc, the relative humidity is 100% and any further cooling of air below 21.1°C, some of the moisture will condense into water.
Any object at a temperature below the dew point of the surrounding air, it will condense some moisture out of the air. the sweating observed on the outside of a glass of ice water is due to the condensation of moisture from air on to the cold surface of the glass.

When we need to remove moisture from the air and condense it to liquid water, only the latent heat of the amount of water vapor to be condensed has also to be removed.. Since the latent heat of water being high,-an average of 555 kacl/kg of moisture-the load on the cooling system increases.

2.2.9 Humidity ratio

Humidity ratio is the ratio of the mass of water vapor to the mass of dry air, in a sample or volume of moist air.

Where; W = Humidity ratio in kgwv/kgda

mwv = mass of water vapor in the space or sample of moist air

mda = mass of dry air in the space or sample of moist air.

The measurement of humidity ratio can be done by utilizing “Gravimetric Hygrometer”

The following equations are derived from the humidity ratio, water vapor pressure and relative humidity.

The Ideal gas equation: pV = mRT

Humidity ratio: W = mwv/mda

Rearranging the ideal gas equation: m = pV/RT

As “m” mass represents the mass of air, which is the sum of mwv & mda, we can individually write the equation as:

Mass of water vapor mwv = pwv Vwv / Rwv Twv

mda = pda Vda / Rda Tda

As per Dalton’s Law, the water vapor and dry air occupy the same volume and are at the same temperature. Therefore eliminating the volume and temperatures terms:

mwv = pwv / Rwv

mda = pda / Rda

Here again, Rwv and Rda represent the Specific Gas Constant for water vapor and dry air.

Since, RH equation, 100 x pwv = RH x pwvs,

Where: pwv = Partial pressure of the water-vgapor component of moist air mixture

pda = Partial pressure of the dry air component of the moist air mixture

pbar = Total pressure, i.e., atmospheric or barometric pressure

pwvs = Partial pressure of saturated water vapor at dry-bulb temperature

R.H = Relative humidity expressed in percentage

Humidity ratio is sometimes incorrectly called “Specific Humidity” or “Absolute Humidity”

To avoid this confusion, both specific humidity and absolute humidity is defined as follows:

“Specific humidity is the weight of water vapor in unit mass of dry air (g/kg)”

“Absolute humidity is the weight of moisture per unit volume of dry air (g/cc)”

2.2.10 Sensible heat flow

Sensible heat is dry heat causing change in temperature but not in the moisture content. The sensible heat flow can be expressed as

Qs = cp ρ q Δt / 3600

Where: Qs = sensible heat flow (kW)

cp = specific heat of air (kJ/kg K) = 1.0 kJ/kg.k

ρ = air density at standard conditions = 1.202kg/m3

q = air flow (m3/hr)

Δt = temperature (oC)

2.2.11 Latent heat flow

Latent heat is the heat, when supplied to or removed from air, results in a change of moisture content - the temperature of the air is not changed

The latent heat flow can be expressed as:

Ql = hwe ρ q Δx / 3600

Where Ql = latent heat flow (kW)

hwe = 2465.56 - latent heat of vaporization of water (kJ/kg)

ρ = 1.202 - air density at standard conditions (kg/m3)

q = air flow (m3/hr)

Δx = humidity ratio difference (kg water/kg dry air)

2.2.12 Specific Volume

The Specific volume in psychrometrics is the volume per unit mass of the dry air component and expressed as m3 / kgda

The specific volume is used in process calculations in converting between moist air volumetric flow (m3/s) and the mass flow (kgda/s) of the dry air component.

The equation for specific volume , applying Ideal Gas Equation is as follows:

2.2.13 Enthalpy or Heat content of air

“The enthalpy of moist air is the sum of the enthalpy of the dry air and the enthalpy of the water vapour. In atmospheric air, water vapor content varies from 0 to 3% by mass. The enthalpy of moist air includes the:

  1. Enthalpy of the dry air – The sensible heat
  2. Enthalpy of water vapor – The latent heat

For moist air, the enthalpy of dry air is given a zero value at 0°C, and for water vapour the enthalpy of saturated water is taken as zero at 0°C.

The enthalpy of moist air is given by (h)

h = ha + W hw

Where:

h = specific enthalpy of moist air (kJ/kg)

ha = specific enthalpy of dry air (kJ/kg)

W = humidity ratio ( kgwv / kgda)

hw = specific enthalpy of water vapor (kJ/kg)

Specific Enthalpy of Dry Air - Sensible Heat (ha)

Assuming constant pressure conditions the specific enthalpy of dry air can be expressed as:

ha = cpa t

Where:

cpa = specific heat capacity of air at constant pressure (kJ/kg°C)

(For air temperature between -100°C and 100°C the specific heat capacity can be set to

cpa = 1.006 (kJ/kg°C)

t = air temperature (°C)

Specific Enthalpy of Water Vapor - Latent Heat (hw)

Assuming constant pressure conditions the specific enthalpy of water vapor can be expressed as:

hw = cpw t + hwe

Where

cpw = specific heat of water vapor at constant pressure (kJ/kg°C)

t = water vapor temperature (°C)

hwe = evaporation heat of water at °C (kJ/kg)

For water vapor the specific heat capacity can be set to cpw = 1.84 kJ/kg°C

The heat of evaporation (water at °C) can be set to hwe = 2501 kJ/kg

Therefore, the enthalpy of moist air is summed up as:

h = cpa t + W [cpw t + hwe]

Where

cpa= specific heat of dry air at constant pressure, kJ/kg°C, 1.006 kJ/kg°C

cpw= specific heat of water vapor, kJ/kg°C, 1.84 kJ/Kg°C

t = Dry-bulb temperature of air-vapor mixture, °C

W = Humidity ratio, kg of water vapor/kg of dry air

hwe = enthalpy of water vapor at temperature t, kJ/kg

The unit of h is kJ/kg of dry air. Substituting the approximate values of cpa and cpw ,we obtain:

h = 1.006 t + W (1.84 t + 2501)

2.3 Understanding the psychrometric charts

2.3.1 Dry Bulb Temperature – Tdb

The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature.

Figure 2.4
Dry-bulb Temperature

The Dry Bulb Temperature refers basically to the ambient air temperature. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air.

Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. The temperature is usually given in degrees Celsius (oC) or degrees Fahrenheit (oF). The SI unit is Kelvin (K). Zero Kelvin equals to - 273oC.

The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of the psychrometric chart. Constant dry bulb temperatures appear as vertical lines in the psychrometric chart.

2.3.2 Wet Bulb Temperature - Twb

The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.

Figure 2.5
Wet-Bulb Temperature

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor.

The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity (the air is at the saturation line).

Combining the dry bulb and wet bulb temperature in a psychrometric diagram or Mollier chart, gives the state of the humid air. Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the Psychrometric Chart.

2.3.3 Dew Point Temperature - Tdp

The Dew Point is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated. Above this temperature the moisture will stay in the air.

Figure 2.6
Dew Point Temperature

If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low.

If moisture condensates on a cold bottle from the refrigerator, the dew-point temperature of the air is above the temperature in the refrigerator.

The Dew Point temperature can be measured by filling a metal can with water and ice cubes. Stir by a thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.

The Dew Point is given by the saturation line in the psychrometric chart.

2.3.4 Humidity Ratio or Moisture content

Specific Humidity is the water vapor content of air, given in grams of water vapor per kg of dry air (i.e., kg of moisture/kg of dry air). It is also known as moisture content or humidity ratio. Air at a given temperature can support only a certain amount of moisture and no more. This is referred to as the saturation humidity.

Humidity ratio is represented on the chart by lines that run horizontally and the values are on the right hand side (Y-axis) of the chart increasing from bottom to top.

Figure 2.7
Humidity Ratio or Moisture content

2.3.5 Specific Air Volume

Specific Volume is the volume that a certain weight of air occupies at a specific set of conditions. The specific volume of air is basically the reciprocal of air density.

As the temperature of the air increases, its density will decrease as its molecules vibrate more and take up more space (as per Boyle’s law). Thus the specific volume will increase with increasing temperature.

Since warm air is less dense than cool air which causes warmed air to rise. This phenomenon is known as thermal buoyancy. By similar reasoning, warmer air has greater specific volume and is hence lighter than cool air.

The specific volume of air is also affected by humidity levels and overall atmospheric pressure. The more the moisture vapor present in the air, the greater shall be the specific volume. With increased atmospheric pressure, the greater the density of the air - so the lower its specific volume. The unit of measure used for specific volume is cubic meter / kg of dry air.

Specific volume is represented on Psychrometric Chart by lines that slant from the lower right hand corner towards the upper left hand corner at a steeper angle than the lines of wet bulb temperature and enthalpy.

Figure 2.8
Specific Air Volume

2.3.6 Sensible Heat Ratio (SHF)

Figure 2.9
Sensible Heat Ratio

The sensible heat ratio helps to determine the percentage of sensible heat and latent heat contribution to the total cooling load. ASHRAE psychrometric chart uses a protractor to plot the slope of the line representing the sensible heat ratio.

Figure 2.10
Sensible Heat RatioProtractor

2.3.7 Relative Humidity (RH)

Relative humidity (RH) is a measure of the amount of water air can hold at a certain temperature. Air temperature (dry-bulb) is important because warmer air can hold more moisture than cold air.

Lines of constant relative humidity are represented by the curved lines running from the bottom left and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the chart.

Figure 2.11
Relative humidity

2.3.8 Enthalpy

Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat. Sensible heat is the heat (energy) in the air due to the temperature of the air and the latent heat is the heat (energy) in the air due to the moisture of the air.

The sum of the latent energy and the sensible energy is called the air enthalpy. Enthalpy is expressed in Btu per pound of dry air (kilojoules per kilogram (kJ/kg).

Enthalpy is useful in air heating and cooling applications. Air with same amount of energy may either be dry hot air (high sensible heat) or cool moist air (high latent heat).The enthalpy scale is located above the saturation, upper boundary of the chart. Lines of constant enthalpy run diagonally downward from left to right across the chart; follow almost exactly the line of constant wet bulb temperature.

The enthalpy of moist air, in kJ/kg, is therefore:

h = (1.007*t - 0.026) + g*(2501 + 1.84*t)

Where g is the water content in kg/kg of dry air

Figure 2.12
Enthalpy

2.3.9 Combination of properties

The chart below is the complete chart combining most of the lines and other parameters so far discussed:

Figure 2.13
Combination of properties
  1. Represents Sensible Heating
  2. Sensible Heating and Humidification
  3. Chemical Dehydration
  4. Sensible Cooling
  5. Cooling & dehumidification
  6. Evaporative cooling
  7. Latent heat addition-Humidification
  8. Latent heat removal-Dehumidification

2.4 Psychrometric processes

The psychrometric process happens when the air at an initial state transforms and changes to final state. The transformation of air undergoes four basic processes

  1. Sensible heating only (Heat addition into the air takes place without altering the moisture content)
  2. Sensible cooing only (Heat removal from the air takes place without altering the moisture content)
  3. Humidification only (Latent energy addition-Latent heating only-No change in dry-bulb temperature)
  4. Dehumidification only (Latent energy removal-Latent cooling only-No change in dry bulb temperature)
Figure 2.14
Four Basic Processes

In general the above four process involves the phase changes in water content, which are represented by the following figure:

Figure 2.15
The Phase Change of water

There are other processes involving both heat and water vapor transformation too, and they are classified as:

Single processes

  1. Cooling and Dehumidification process (involving coils in air washer with chilled spray of water)
  2. Evaporative cooling process (Involving adiabatic process called Sensible cooling and humidification-Constant wet-bulb temperature)
  3. Water spray process
  4. Chemical Dehumidification process (Involving chemical or sorbent materials in adiabatic dehumidification-Constant wet-bulb temperature)
  5. Mixing of two air stream (Involving adiabatic process with no heat transfer)
  6. Room effect (Changes to supply air due to sensible and latent heat gains in the room)
  7. Fan Heat (Including fan, motor and drive(Similar to a sensible heating process-No change in water vapor)
  8. Enthalpy Wheel (Mixing process)

Two or more processes in sequence

  1. Face & Bypass of mixing air-2 process in sequence
  2. Return Air Face & Bypass
  3. Reheat with cool and Dehumidification
  4. Sensible Precooling followed by Evaporative cooling
  5. Sensible Heating followed by Humidification
  6. Typical air conditioning cycle

Now let us consider the first four basic processes

2.4.1 Sensible Heating only

Figure 2.16
Sensible Heating only

This is the process where the temperature of the air stream is increased without any change in moisture content or specific humidity. The transfer heat into the air stream is done by one of the following devices:

  1. Steam coil
  2. Hot water coil
  3. Heat pipe
  4. Air-to-air heat exchanger
  5. Sensible only rotary heat wheel.
  6. Electrical heating coil
  7. Furnace

2.4.2 Latent Heating Process

A latent heating process occurs when water is evaporated without changing the dry bulb temperature. This is shown as vertical line in psychrometric chart.

Figure 2.17
Latent Heating only

2.4.3 Sensible Cooling only process (Cooling without change in water vapor content)

The transfer of heat from air using one of the following devices:

  1. Chilled water
  2. Refrigerant cooling coil
  3. Indirect evaporative cooler
  4. Heat pipe
  5. Air-to-air heat exchanger
  6. Sensible only rotary heat wheel
  7. Air washers

On a psychrometric chart, the sensible cooling process proceeds horizontally to the left along a line of constant humidity ratio towards the saturation line. In this process, there is no change in dew-point temperature, water vapor pressure, or humidity ratio.

The heating and cooling explained above are represented on psychrometric charts as shown in the following figures.

Figure 2.18
Heating
Figure 2.19
Cooling

2.4.4 Heating and Humidification

In this process, the air first passes through a heating coil and then through the humidifier where steam at a mass flow rate of required value and specific enthalpy hx is sprayed into the air stream.

The heating and humidification of the air can be considered as two separate processes in sequence.

Figure 2.20
Heating and Humidification Psychrometric chart

Referring to the psychrometric chart above, from point 1 to 2, the air passes through the heating coil and the sensible heat transfer takes place without altering the moisture content. From point 2 to 3, the moisture is added and the humidification of air occurs.

During the sensible heating process of the moist air, the energy added is calculated by the following equation:

Q = ma (h2 – h1)

Where: Q = Rate of energy added, KJ/hr

ma = mass flow rate of dry air through the process

h2 = Specific enthalpy of moist air downstream of heating coil

h1 = Specific enthalpy of moist air upstream of heating coil

During the humidification process, the energy equation is;

ma (h3 – h2) = mw hw

Where: h3 = The specific enthalpy of the moist air downstream of the humidifier

h2 = Specific enthalpy of moist air upstream of the humidifier

hw = Specific enthalpy of the steam

mw = Mass flow rate of the steam

The rate of moisture addition to the air, mw, is determined by a water vapor mass balance

mw = ma (w3 – w2)

Where: w2 = Humidity ratio of the moist air upstream of the humidifier

w3 = Humidity ratio of the moist air downstream of the humidifier

Combining the equations,

ma (h3 – h2) = mw hw and mw = ma (w3 – w2)

ma (h3 – h2) = ma (w3 – w2) x hw

Where the left hand side of the equation represents the slope of the humidification process on a psychrometric chart. Thus the direction of the process can be determined from the enthalpy of the steam added to the air stream and the enthalpy-Moisture protractor on a psychrometric chart.

The specific humidity of air can also be increased by the injection of a predetermined quantity of steam into the air. It is important here that the steam is dry and saturated and there is no condensation at all.

It is not possible to spray steam below 100°C (at atmospheric pressure) as it is necessary to spray steam though the nozzles, which require higher pressure than atmospheric. Hence the lowest possible enthalpy carried with steam is the total heat of steam at 100°C when the steam is fully dry and saturated.

The amount of steam sprayed per kg of air is given by (W2 – W1).

2.4.5 Cooling and Dehumidification

The removal of water vapor from air is termed dehumidification. It is only possible when the air is cooled below its dew point temperature. For effective dehumidification, it is necessary to maintain the cooling coil surface below the dew point temperature of air.

Figure 2.21
Cooling and Dehumidification

Let us take an example:

Air is to be cooled from 35°C DB and 24°C WB to 20°C DB and 17.6°C WB. Take a psychrometric chart and mark these values. If we join these two points and draw a parallel line from the reference point to intersect the sensible heat factor line, we will notice that it intersects at 0.74 indicating that there is 26% latent heat removal and 74% sensible heat removal.

The process of cooling and dehumidification is represented in the chart as follows:

The cooling and dehumidifying process is shown in the psychrometric chart below. It begins at point 1 and ends at point 2.

The refrigeration capacity required to accomplish this QR, is obtained from the energy balance.

Figure 2.22
Cooling and Dehumidification Psychrometric chart

Where hw is the enthalpy of saturated liquid at temperature t2.

The second term in the square bracket is the enthalpy associated with the liquid condensate as it runs out of the cooling coil. This term is small compared to (h1-h2) which is the enthalpy difference to cool the air and condense the water.

The approximation is often made where the process is divided into sensible (S) and latent (L), components.

The rate at which the moisture removed from the air is:

mw = ma (W1 – W2)

2.4.6 Cooling with adiabatic humidification of air

In this process, air is passed over a spray chamber. A spray chamber is a chamber with nozzles, which spray water. The temperature of the spraying water is more than the WBT of entering air and below the temperature of air. When air passes over this chamber, part of the water evaporates and is carried away by the air, increasing the specific humidity of air as shown in the figure below.

Figure 2.23
Cooling with Adiabatic saturation

Air provides the heat required for the evaporation of water. During this time, the temperature of air decreases keeping the total enthalpy constant.

Generally, complete humidification of the air in not possible thus the effectiveness of the spray chamber can be defined as:

E = T1 – T3/(T1 – T2)

Where: (T1 – T3) is the actual drop in the DBT

(T1 – T2) is the ideal drop in DBT

The humidifying efficiency is given by; efficiency = 100 × E.

2.4.7 Adiabatic chemical dehumidification

When the high humid air is passed over a solid absorbent bed or a liquid absorbent spray, part of the water vapor will be absorbed reducing the water content in the air. The latent heat released is absorbed by air increasing its DBT and the total enthalpy remains constant. Thus the chemical dehumidification process follows the path along the constant enthalpy line.

The effectiveness of the dehumidifier is defined as:

E = (T3 – T1)/(T2 – T1)

2.4.8 Evaporative Cooling Systems

The evaporative cooling can be classified as:

  1. Direct evaporative system
  2. Indirect evaporative system
  3. Multi-stage evaporative systems

2.4.8.1 Direct evaporative system

The figure below shows the schematic of an elementary direct, evaporative cooling system and the process on a psychrometric chart

Figure 2.24A
Direct Evaporative Cooling system

As shown in the figure, the direct evaporative cooling, the conditioned air comes in direct contact with the wetted surface, and gets cooled and humidified.

In this process:

  1. The hot and dry outdoor air is first filtered and then is brought in contact with the wetted surface or spray of water droplets in the air washer.
  2. The air gets cooled and dehumidified due to simultaneous transfer of sensible and latent heats between air and water (process o-s).
  3. The cooled and humidified air is supplied to the conditioned space, where it extracts the sensible and latent heat from the conditioned space (process s-i).
  4. Finally the air is exhausted at state i.

In an ideal case when the air washer is perfectly insulated and an infinite amount of contact area is available between air and the wetted surface, then the cooling and humidification process follows the constant wet bulb temperature line and the temperature at the exit of the air washer is equal to the wet bulb temperature of the entering air (to,WBT), i.e., the process becomes an adiabatic saturation process. However, in an actual system the temperature at the exit of the air washer will be higher than the inlet wet bulb temperature due to heat leaks from the surroundings and also due to finite contact area.

One can define the saturation efficiency or effectiveness of the evaporative cooling system ɛ as:

Where,

ɛ = Saturation efficiency

to = Outside air entering temperature

tS = Supply air to conditioned space after evaporative cooling

to,WBT = Saturated wet-bulb temperature

The amount of supply air required can be obtained by writing energy balance equation for the conditioned space, i.e.

Where,

mS = The amount of supply air

Qt = The Total heat transfer rate (QS+Ql)

hi = Enthalpy of return or exhaust air

hS = Enthalpy of supply air

Advantages

  1. Compared to the conventional refrigeration based air conditioning systems, the amount of airflow rate required for a given amount of cooling is much larger in case of evaporative cooling systems.
  2. The evaporative coolers are very useful essentially in dry climates

Disadvantages

  1. The evaporative coolers cannot provide comfort as the cooling and humidification line lies above the conditioned space condition ‘i’.
  2. For a given outdoor dry bulb temperature, as the moisture content of outdoor air increases, the required amount of supply air flow rate increases rapidly
  3. The conventional refrigeration based air conditioning systems can be used in any type of climate.

2.4.8.2 Indirect evaporative cooling system:

The figure below shows the schematic of a basic, indirect evaporative cooling system and the process on a psychrometric chart.

As shown in the figure, in an indirect evaporative cooling process, two streams of air - primary and secondary are used.

Stream-1-The primary air stream becomes cooled and humidified by coming in direct contact with the wetted surface (o-o’),
Stream-2-The secondary stream which is used as supply air to the conditioned space, decreases its temperature by exchanging only sensible heat with the cooled and humidified air stream (o-s).

The moisture content of the supply air remains constant in an indirect evaporative cooling system, while its temperature drops. Obviously, everything else remaining constant, the temperature drop obtained in a direct evaporative cooling system is larger compared to that obtained in an indirect system, in addition the direct evaporative cooling system is also simpler and hence, relatively inexpensive. However, since the moisture content of supply air remains constant in an indirect evaporation process, this may provide greater degree of comfort in regions with higher humidity ratio. The commercially available indirect evaporative coolers have saturation efficiency as high as 80%.

Figure 2.24B
Indirect Evaporative Cooling system

2.4.8.3 Multi-stage evaporative cooling systems:

Figure below shows a typical two-stage evaporative cooling system and the process on a psychrometric chart. As shown in the figure, in the first stage the primary air cooled and humidified (o -o’) due to direct contact with a wet surface cools the secondary air sensibly (o -1) in a heat exchanger. In the second stage, the secondary air stream is further cooled by a direct evaporation process (1-2). Thus in an ideal case, the final exit temperature of the supply air (t2) is several degrees lower than the wet bulb temperature of the inlet air to the system (to).

Figure 2.25
Two-stage Evaporative Cooling system

To improve efficiency of the evaporative cooling systems first sensibly cool the outdoor air before sending it to the evaporative cooler by exchanging heat with the exhaust air from the conditioned space. This is possible since the temperature of the outdoor air will be much higher than the exhaust air. It is also possible to mix outdoor and return air in some proportion so that the temperature at the inlet to the evaporative cooler can be reduced, thereby improving the performance. For example, one can use multistage evaporative cooling systems and obtain supply air temperatures lower than the wet bulb temperature of the outdoor air. Thus multistage systems can be used even in locations where the humidity levels are high.

2.5 Air-conditioning systems- Summer and Winter

There are two basic systems in air-conditioning:

  • Summer air-conditioning systems
  • Winter air-conditioning systems

Lets us now briefly study the various methods used for the above air-conditioning systems.

2.5.1 Summer air-conditioning systems

Summer air-conditioning system for hot and dry outdoor conditions

As the name suggests these systems are used for hot and dry atmospheric conditions like temperature of 38–42°C and relative humidity of about 20–25%.

In this process our purpose would be to reduce the air temperature and increase its relative humidity where the required comfort conditions are 24°C and 60% RH. The general arrangement of the equipment and the psychrometric process are represented in the figures below.

Figure 2.26
Summer air conditioning system for hot and dry outdoor conditions
Figure 2.27
Representation of psychrometric process

Atmospheric air is passed through the dampers and gets filtered before passing over the cooling coil. When the air is passed over the cooling coil, its temperature is reduced by sensible cooling as represented by point 2 on the psychrometric process chart.

The air coming out from the cooling coil at point 2 is passed into an adiabatic humidifier where the water vapor increases the humidity of air and the conditioned air leaves the humidifier at point 3.

The efficiency of the humidifier is given by the equation:

Efficiency = [(T2 – T3) /(T2 – T5 )] × 100

If the quantity of atmospheric air supplied is V L/sec, then the capacities of the cooling coil and the humidifier are given by:

Total capacity of cooling coil = (V / Hf) × [( h3 – h1)/1000] KW of refrigeration

Where: V is the volume of handled air in L/sec

Hf is the density of moist air Kg/m3.

Capacity of humidifier = (V / Vs) × [(w3 – w2) / 1000] kg/sec

Summer air-conditioning system for hot and humid outdoor conditions

As the name suggests these systems are used for hot and humid atmospheric conditions like; temperature of 32–38°C and relative humidity of about 70–75 %.

Figure 2.28
Summer air conditioning system for hot and wet weather

In this process our purpose would be to reduce the air temperature and its relative humidity where the required comfort conditions are the same: 24°C and 60 % RH. The general arrangement of the equipment and the psychrometric process are represented in the following figures.

Figure 2.29
Representation of psychrometric process

Here the air is filtered and then passed over the cooling coil for dehumidification. As air is passed over the cooling coil whose temperature is below the dew point temperature of incoming air, the temperature and humidity of air is reduced and it comes out at point 3.

The capacity of the cooling coil is given by the equation;

Total capacity of cooling coil = Hf×(V) × [( h1 – h3 ) / 1000] KW of refrigeration

The air then enters the heating coil condition 3 and leaves at condition 5

Capacity of heating coil = Hf × (V) × (h5 – h3)/1000 KW

Summer air-conditioning system with single cooling coil and mixing

This type of system is used to reduce the load on the cooling coil as part of the air going out of the room, which at a lower temperature than the outdoor condition, is mixed with fresh air. The arrangement of the system is shown in figure and the corresponding processes are represented on the psychometric chart

Figure 2.30
Arrangement for the components for the given air-conditioning system
Figure 2.31
Psychrometric process for the given system

Condition (4) is the mixing of air at conditions (2) and (3). Condition (5) is the condition of air leaving the cooling coil and 5–1 represents the heating of air passing through the blower due to friction. The process 1–2 represents the condition of air passing through the air-conditioned room taking the load in the room.

The details of the cooling system (refrigeration) used in single coil direct expansion system are shown in following Figure.

Figure 2.32
Direct expansion refrigeration system for cooling and dehumidifying of hot and moist air

This system is known as a direct expansion system as the refrigerant is directly used for cooling the air in the evaporator. But in large systems, used for comfort air-conditioning and having several cooling coils, a centrifugal refrigeration plant processes chilled water and chilled water is further circulated to the various cooling coils.

This system is known as an indirect cooling system. A centrifugal compressor using would be used for producing chilling water, as it has to handle a large quantity of refrigerant.

Summer air-conditioning with single coil and bypass mixing

This system is used to control DBT in the air-conditioned room as per the load in the room. The arrangement of the system is shown in Figure.

Figure 2.33
Arrangement for the components for the given air-conditioning system

Condition 4 is the mixing of air at conditions 2 and 3. Condition 5 is the condition of air coming out of the cooling air. Condition 6 is the mixing of air at conditions 5 and 2. Process 4–5 represents the cooling and dehumidifying of air passing through the cooling coil. Process 6-1 is heat generated by fan and motor. Process 1–2 represents the condition of air passing through the room as it takes the load in the room. The re-heating of air passing through the blower due to friction is neglected for plotting on the psychrometric chart.

The previous system used has some limitations, as the temperature in the air conditioned room cannot be controlled according to the load in the room. The control of DBT is more important than humidity control as long as humidity is not excessive.

The present system is used during partial load operation. The face dampers on the cooling coil and bypass dampers are controlled by a motor, which positions them so as to maintain a constant DBT. As the sensible heat gain of the air-conditioned space decreases, more re-circulated air is bypassed. However with direct expansion cooling coil, the air, which passes across the coil, may be more thoroughly dehumidified than when the full air quantity is handled. Thus satisfactory space humidity conditions may be maintained during some partial load conditions without the need for re-heating.

Summer air-conditioning with single cooling coil and absorbent dehumidifier:

The cooling coil discussed in the above methods for cooling the air, also produces some dehumidification in conjunction with the cooling process. The dehumidification of air by a refrigerant cooling coil has limitations. If the coil surface temperature is less than 0°C, frost forms on the coil and the heat transfer rate reduces. A defrosting system is required and reheating of the air is needed before passing into the air-conditioned space. This refrigeration system becomes more complicated and more expansive to own and operate as the required air dew point temperature is reduced.

The absorbent system shown in the following figure can reduce the required surface temperature of the cooling coil and completely avoids the possibility of frosting the coil as the required coil temperature is always above 0°C. Therefore this method produces extremely low air dew-point temperatures, more reliably and more economically than the refrigeration method. The psychrometric processes for the above-described system are shown in the following Figure.

Figure 2.34
Arrangement for the components for the given air-conditioning system
Figure 2.35
Psychrometric Processes For Given System

Condition 4 is the mixing of airs at conditions 2 and 3. Process 4–5 represents the adiabatic dehumidification of air passing through the absorbent dehumidifier. Process 5–6 is the sensible cooling of air passing through the cooling coil whose surface temperature is considerably above the temperature required for frosting. Process 6–1 is heat generated by fan and fan motor.
Process 1–2 is the condition of air passing through the air-conditioned room, taking the existing load.

Summer air-conditioning with evaporating cooling:

Comfort air-conditioning systems capable of maintaining optimum thermal conditions may be expensive to own and operate. Partially effective systems, which involve much less costs, may be attractive where finances preclude the installation of a completely effective system. In hot dry regions evaporating cooling systems may be capable of providing considerable relief in enclosed spaces.

The evaporative cooling system commonly used is shown in Figure below and the corresponding processes.

Figure 2.36
Arrangement for the components for the given air-conditioning system

Process 3–1 represents evaporative cooling and process 1–2 represents room load taken by the air passing through the room. State 2 is an acceptable space condition although not necessarily an optimum one. State 3 of the outdoor air is at a much higher temperature but lower RH than state 2. As the air-washer is the only processing device in the system, the cost of the system is considerably lower than the system used for optimum comfortable conditions.

Generally, a much higher flow rate of air is used with an evaporative cooling system (2 to 3 times of conventional) than with conventional systems. A high rate of air movement past a person allows the same degree of comfort but with higher effective temperatures as compared to the situations where air-movement is low.

2.5.2 Winter air-conditioning systems

The required comfort air conditions are the same as in summer. The typical arrangement of the required equipment and its representation on the psychrometric chart are shown in the following Figure below.

Figure 2.37
Winter air conditioning system

Air is passed through the resistance heater known as the preheating coil, and then through the humidifier. It is then passed through the second preheating coil.

Winter air-conditioning with double reheat coils and air washer

During severe winter conditions it is always necessary to increase the DBT and RH of the air. The arrangement of the components in this system and its representation on the psychrometric chart are shown in the following figures.

Figure 2.38
Arrangement for the components for the given air-conditioning system

Condition 4 is the mixing of air at conditions 2 and 3. Process 4–5 is the sensible heating in the preheat coil. Process 5–6 is the adiabatic cooling of the air passing through the air washers and process 6–1 is the sensible heating in the reheat coil. Process 1–2 is the cooling and dehumidifying of the air passing through the conditioned room. This compensates for the heat and the vapor loss of the air in the conditioned room. In large systems it is a common practice to use re-circulating air fans as well as supply air fans. However this condition does not affect the process represented in the above psychrometric chart.

Winter air-conditioning using 100% outdoor air with pre-heating (by waste heat of the exhaust)

In designing any air-conditioning system every effort has to be made to utilize internal heat emission wherever economically feasible. Such a system is shown in the following figure where the waste heat from the exhaust is used for preheating fresh air.

Figure 2.39
Arrangement for the components for the given air-conditioning system

The air washers serve as humidifying devices to offset the moisture losses in the air conditioned space and in addition to this, it cleans the air. The reheat coil regulates the heat supply thus controlling the DBT of the air-conditioned space as required.

Process 4–5 is the preheating of fresh air by using the waste heat in the exhaust air. Here process 2–3 shows the cooling of exhaust air. Process 5–6 is the humidification of air by using steam and process 6–1 is the sensible heating in the reheat coil.

Process 1–2 is the cooling and dehumidification of air to compensate for the heat and vapor loss in the conditioned space. In winter air-conditioning systems where heating is required, the use of outdoor air should be kept to a minimum.


3


Requirements of comfort air-conditioning

Objectives

After reading this chapter the student should be able to:

  • Explain the requirements of comfort air conditioning
  • Understand the importance of indoor air quality
  • Read a comfort chart and use it to design an air conditioning system
  • Know the various air purification methods
  • Understand the ventilation standards set by ASHRAE
  • Investigate indoor air quality

3.1 General

The feeling of comfort people experience in an air-conditioned place depends upon the following five main factors:

  • Supply of oxygen and removal of carbon-dioxide
  • Removal of body heat dissipated by the occupants
  • Removal of body moisture dissipated by the occupants
  • To provide sufficient air movement and air distribution in occupied space
  • To maintain the purity of air by removing odor and dust

Real comfort cannot be achieved unless the above-mentioned factors are properly controlled. Simultaneous control of all these factors is essential in order to produce a satisfactory environment for human comfort.

Let’s discuss each factor mentioned above in detail.

3.1.1 Oxygen supply

Like other machines, the human body requires adequate supply of oxygen to sustain combustion (food digestion), which in turn converts chemical energy into work, and dissipates carbon dioxide as exhaust gas. Each person requires nearly 0.65 m3 of oxygen per hour under normal conditions and produces 0.2 m3 of carbon dioxide. The rise in concentration of CO2 is an index of oxygen consumption.

The percentage of CO2 in the atmosphere is nearly 0.03% by volume and it is necessary to maintain this percentage for proper functioning of the respiratory system. When the percentage of CO2 in air exceeds 2%, the partial pressure of oxygen reduces to a value such that breathing becomes more difficult. Extreme discomfort exists when the percentage of CO2 reaches 6% and a person can become unconscious at levels of 10% (of CO2). The quantity of air-supply to an air-conditioned space should be maintained in such a way that the percentage of CO2 should not increase beyond the minimum.

3.1.2 Heat removal

The human body can be considered as an engine that converts thermal energy into mechanical work with a thermal efficiency of 20%, the remaining amount of heat being dissipated into the atmosphere. The atmosphere surrounding the person must be capable of absorbing the heat dissipated. When a person is not doing any external work, he still does internal work such as pumping of the blood through the body and muscular work required for respiration.

If a space of 6 m3 is provided to each person and if there is no transfer of heat and air from an outside source, then the space temperature will rise 0.136°C for each kJ of heat added to this space. Thus there will be a rise in temperature of 43°C per hour as the human body dissipates 320 kJ of heat per hour.

Q = mlΔt = lv

The objective of the ventilation system is to provide sufficient circulation of air to avoid excessive rise in temperature in air-conditioned space and to establish an atmosphere in which occupants can live and work satisfactorily.

3.1.3 Moisture removal

The moisture loss from the body is nearly 50 grams per hour when a person is at rest. The body’s ability to dispose of heat by evaporation (to the atmosphere) decreases as the air humidity increases. High humidity of air reduces the apparent freshness in an enclosed space and increases the difficulty of disposing of body heat. The ventilation system must be capable of maintaining the relative humidity below 70%.

3.1.4 Air motion

Increased air velocity increases the heat transfer from the body by reducing the thickness of the film of air adjacent to the body. The effect of increased velocity is to increase the body heat loss and reduce the feeling of discomfort when the ambient air is at a temperature lower than the body surface temperature.

The sensible heat transfer will be in the opposite direction if the air temperature exceeds that of the body temperature; so the effect of increased velocity is to increase the already existing discomfort.

Secondly, increased velocity reduces the thickness of the layer of the saturated vapor near the body surface and helps evaporation. The heat loss by evaporation is usually greater than the heating effect by convection when dew-point temperature is below 30°C. Hence increased velocity is always advantageous.

Air velocity in the air-conditioned space should not be more than 0.04 to 0.12 m/s at 20°C and 0.05 to 0.17 m/s at 22°C.

The following table gives the comfortable ranges of air velocity and humidity with respect to room air temperature.

Table 3.1
Air Velocity and humidity with respect to room air temperature
Room air temp. °C Velocity m/sec R.H.%
Minimum Maximum Minimum Maximum
20 0.04 0.12 35 65
21 0.04 0.14 35 65
22 0.05 0.17 35 65
23 0.07 0.21 35 65
24 0.09 0.24 35 65
25 0.12 0.32 35 65
26 0.16 0.40 35 65

Air motion alone has no meaning without proper air distribution. Air distribution is described as the uniform supply of air in an air-conditioned system. Air motion with distribution creates a local cooling sensation known as a draft.

3.1.5 Purity of air

Odor, dust, toxic gases and bacteria are the contents in air, which define the purity of air. The evaporation on the body surface adds odor to the air. Smoke is objectionable due to its bad effects on the nose, eyes and heart. The removal of toxic gases is very essential to avoid the irritation they cause. The control of bacteria is most important to prevent its bad effects on human health and this is done by sterilization of air.

The factors of the environment also affect the indoor air conditions. The prime factors are:

  • Human population and density
  • Working area
  • Degree of ventilation
  • Hygienic conditions of human occupants.
  • Proximity of the people to microbial sources.

ASHRAE precisely defines the parameters to be considered in an indoor environment.

They are:

  • Indoor air temperature (IAT) and mean radiant temperature (MRT)
  • Indoor relative humidity (IRH)
  • Outdoor ventilation provided
  • Air cleanliness
  • Air path and movement
  • Sound level
  • Pressure difference between space and surroundings

Formation of microbes like bacteria, algae and viruses is a very common phenomenon and considered as an irritant in a conditioned indoor environment. Normally the microbial population is measured in terms of colony forming units (CFU) per cubic meter (CFU/m3).

The following table shows the particles and their relative sizes.

Table 3.2
Particles and their relative sizes
Particles Sizes Source
Dust <100 μm Natural and mechanical processes
Smoke 0.1–0.3 μm Incomplete combustion
Fumes <1 μm Condensation of vapor
Fog 2–60 μm Condensation of vapor
Mist 60–200 μm Atomizing and spraying
Microbes 0.003–0.06 μm Virus
Bacteria 0.4–5.0 μm Environmental
Fungal Spores 10–100 μm Environmental

3.2 Air purification methods

Filters are widely used in various industries and applications to prevent entry of unwanted debris in the working area. The filters can be classified based on the type of filtration and its porosity. Porosity is defined as the number of pores per unit area in a filter. Various types of filters used are:

3.2.1 High efficient particulate air filters (HEPA)

These filters are efficient for removal of air particulates. Pre-filters that are used in clean rooms are rated in the range of 30-95 % efficiency. The final filters to be used should be HEPA filters having an efficiency of about 99.97 to 99.99 % on 0.3 micron particles. These filters are made of glass fibers of sub micrometer diameter that are formed into pleated paper mats.

3.2.2 Ultra low penetration air filters (ULPA)

ULPA filters have an efficiency of 99.99 % on 0.3 micron particles of air. An ULPA filter contains a filter media similar to that of a HEPA filter. Both its filter media and sealing (depending on the construction of the frames) are more efficient than HEPA filters.

3.2.3 Activated carbon filters

These filters are most widely used to remove objectionable odors and irritating vapors of gaseous airborne particles typically ranging from 0.003 to 0.006 microns in size. Low efficiency filters are used as pre-filters for protection. Pleated filters of 50 micron depth are added to most packaged plants.

3.2.4 Plasma air purifier

These kinds of filters are also used in split, window and package air conditioners. With the use of this technology, the smallest germs, bacteria, smoke, unpleasant odors and microbes are removed easily.

3.3 Thermodynamics of the human body

The heat produced in the body is dissipated to the atmosphere through four different modes of heat transfer viz. convection, radiation, evaporation, and respiration. The heat lost from the body by each mode depends on the atmospheric conditions.

The human body can be comfortable only when the heat is dissipated from the body at the rate at which it is produced; else there is heat accumulation in the body leading to discomfort.

The phenomenon of heat lost from the body can be represented by the equation;

*

Hm – W = He ± Hc ± Hr ± Hs ± respiration……….

Where:

Hm is the metabolic heat produced by the body

W is the useful rate of working

(Hm – W) is the heat to be dissipated from the body

He is the heat lost by evaporation

Hc is the heat lost or gained by convection

Hr is the heat lost or gained by radiation

Hs is the heat stored in the body

The convective heat lost from the body is given by the equation:

Hc = hc × A (Tb – Ta)………..

Where:

hc is the heat transfer coefficient of the body = 13.5V0.6 W/m2 °C

V is the wind speed of about 2.5 m/s

A is the surface area of the body i.e. 1.5 to 2.5 m2 for a normal human being

Tb is the mean body surface temperature, 31–33°C

Ta is the temperature of the surroundings, 23°C

When the atmospheric temperature is more than the body temperature, heat is gained by the body. Higher velocities of air impart more discomfort to the body when the surrounding temperature is more than the body temperature; e.g. when the DBT is 40°C with a wind speed of about 2.5 m/s, the convective heat gained by an unclothed body is about 1800 kJ/hr, which is equal to the metabolic heat produced during heavy work.

Hc = (13.5×2.50.6)×1.8(42–32) = 421W, which is the metabolic rate of heavy work.

3.3.1 Radiation Heat Loss

The Stefan–Boltzmann law, also known as Stefan's law, states that the total energy radiated per unit surface area of a black body per unit time.

Hf, is directly proportional to the fourth power of the black body's thermodynamic temperature T (also called absolute temperature):

Where,

Hf is the radiation heat loss, W/m2

σ is the Stefan-Boltzmann constant, 5.6704 × 10-8 kg s-3 K-4

T is the thermodynamic temperature,oK

A more general case is of a grey body, the one that doesn't absorb or emit the full amount of radiative flux. Instead, it radiates a portion of it, characterized by its emissivity, ɛ

To find the total absolute power of energy radiated for an object we have to take into account the surface area, A(in m2):

Thermodynamic temperature = (Mean body surface temperature – Surrounding temp.)

Where:

Tb is the mean body surface temperature, 31–33°C

Ta is the temperature of the surroundings, 23°C

The heat lost due to evaporation is given by the equation:

He = Kd × A (Pvs – Pv) × Hfg × Kc……….

Where:

Kd is the diffusion coefficient in kg of water evaporated per m2 per hr per N pressure difference, kg/Pa.s.m2

Pvs is the saturation vapor pressure corresponding to the skin temperature

Pv is the vapor pressure of surrounding air

Hfg is the latent heat of vaporization, which is taken as 2450 kJ/kg

Kc is the factor that affects the type of clothing

He becomes zero when Pvs = Pv i.e. when the air is saturated.

3.4 Role of clothing

Clothing interferes with the air movement across the skin and as a result decreases the convective heat transfer and the potential evaporation. This decrease may vary form 20 to 50% depending on the texture of clothing.

The heat transfers He, Hr and Hc are mostly dependent upon the following environmental conditions:

  • Dry bulb temperature and mean radiant temperature
  • Absolute humidity
  • Air velocity

The main purpose of comfort air-conditioning systems is to control the abovementioned factors of conditioned air in such a way that the sum of heat losses ( Hc + Hr + He ) must balance Hm (metabolic heat produced) thereby making Hs (stored heat) zero.

The metabolic heat produced (Hm) depends upon the activity of the human being. The experimental values of ‘Hm’ for different rates of working and their sensible and latent components are listed in the table below:

Real heat gain from occupants in air-conditioned space at 24°C

Table 3.3
Metabloic Heat produced in different working conditions
Degrees of activity Typical Application H’m=Hm-W & H8=0, Total heat dissipated per person in kJ/hr (watts) Sensible heat in kJ/hr (watts) Latent heat in kJ/hr (watts) O2-Consumed in liters per hour
Seated at rest Theatre 360 (100) 241 (67) 119 (33) 0.24
Seated with light work Offices and hotels 431 (150) 252 (70) 179 (50) 0.30
Moderately active Offices and hotels 468 (130) 252 (70) 216 (60) 0.4
Standing with light work Departmental stores 468 (130) 252 (70) 216 (60) 0.4-0.5
Walking slow Banks 540 (150) 273 (76) 267 (74) 0.5-0.6
Light Bench work Factory (small production parts) 791 (220) 306 (85) 485 (135) 0.7–1.1
Moderate Dancing Dancing hall 900 (250) 342 (95) 558 (155) 2.5–3.1
Moderately heavy work Factory (Heavy production parts or foundry) 971 (270) 360 (100) 611 (170) 3.1-3.4
Heavy work Blowing factory 1079 (300) 396 (110) 383 (190) 3.4–5.2
Maximum Activity possible 1547 (430) 554 (154) 993 (276)  

The following graphs show the proportional heat release rates from a human body by different modes of heat transfer.

Figure 3.1
Human body heat release Vs Heat dissipation
Figure 3.2
Human body heat release Vs Surrounding air temperature

3.5 Comfort and comfort chart

In theory human comfort exists when the rate of heat production becomes equal to the rate of heat loss. This equilibrium condition is maintained when proper conditions of temperature, humidity, air velocity and purity are maintained in the air-conditioned space. The feeling of comfort experienced by an individual depends upon various factors such as eating habits, type of clothing, duration of stay, age, sex, rate of activity and so on.

Since the feeling of comfort is controlled by the number of variables, there is no proper method to measure it. A proper control of dry bulb temperature and the relative humidity can ensure reasonable closeness to a feeling of comfort.

The American Society of Heating and Refrigeration Engineers (ASHRAE) have conducted exhaustive tests on various people subjected to wide variations of combinations of temperature, relative humidity and air motion. A scientific method to measure comfort feeling of human beings introduces the concept of ‘Effective temperature’.

3.5.1 Effective temperature

Effective temperature is a measure of feeling warm or cold of the human body in response to the air temperature, moisture content and the air motion.

The feeling of warmth by the body depends on the heat stored in the body (Hs), which is always positive during summer. The most comfortable feeling exists when the Hs is zero.

The effect of metabolic heat produced by a body depends on the dry bulb temperature (DBT) and the relative humidity of air at a selected velocity. If the air at different DBT and RH conditions carries the same amount of heat as the heat carried by air at temperature ‘T’ and 100 % RH , then the temperature ‘T’ is defined as effective temperature. This can be better clarified in the following tables:

Table 3.4
Dry bulb temperature and Relative Humidity values at 21°C
DBT°C RH % of people feeling comfort ET
21 100%    
23 88% 90% 21
25 76%    
27 64%    
29 52%    

In the table above, if the different combinations of DBT and RH that carry the same quantity of heat from the body, then the temperature ‘21°C’ at saturated conditions is the effective temperature.

Similarly in Table II below, for the given combinations of DBT and RH the effective temperature is 25°C.

Table 3.5
Dry bulb temperature and Relative Humidity values at 25C
DBT°C RH % of people feeling comfort ET
25 100%    
27 90% 80% 25
29 80%    
31 70%    
32 60%    

As the effective temperature increases or decreases beyond a particular value then the percentage of people feeling comfort decreases. The effective temperature at which the majority of the people feel comfortable is the most comfortable effective temperature, which is shown in the following comfort chart.

Figure 3.3
Comfort chart (not to scale) – air motion used 5m/min to 8m/min

The experiments by the American Society of Heating and Ventilation Engineers (ASHVE) have resulted in a comfort chart showing percentage of people feeling comfortable at different effective temperatures.

The comfort chart has DBT on its X-axis and WBT on its Y-axis. The 100 % RH line is drawn as DBT and WBT and is the same on any point on this line. Taking the points from Table I and Table II above, the effective temperature lines are drawn.

The effective temperature lines b1–b1’ and b2–b2’ show that the percentage of people feeling comfort is zero in summer. The maximum (100 %) of people feeling comfort in summer is shown by line BB. The condition of air in an air-conditioned space in summer should ideally lie on line BB. Similarly the effective temperature lines a1–a1’ and a2–a2’ show that the percentage of people feeling comfort is zero in winter. The maximum (100 %) of people feeling comfort in winter is shown by line AA. The condition of air in an air-conditioned space during winter should ideally lie on line AA.

When the relative humidity (RH) of air is low, the evaporation from the body surface increases, causing under-cooling and dry skin. On the other hand when the RH is high, evaporation from the body surface ceases and causes discomfort due to stickiness. The most desirable relative humidity range lies between 30% and 70%.

Considering the relative humidity limitations, the desired comfort conditions must lie between B’B” for summer conditions and between A’A” for winter conditions. The area A’A”B’B” shown on the comfort chart is used for year round air-conditioning. A good compromise is 22.5°C @ 30–70% RH.

3.5.2 Economic consideration for selecting the comfort point

On the comfort chart, all points lying on the line B’B” give the same comfort. Selecting the point for the most economical working of the air-conditioning system, can be explained by plotting the line B’B” on the psychometric chart as shown below.

Figure 3.4
Comfort point selection in psychrometric chart

If the point B’ is chosen, the load on the cooling system is given by the equation:

Load = (V /Vs ) × [ ( h1 – h2 )/3.5] Tons of Refrigeration …………………. Eq.1

Where V is the volume of outside air circulated per sec. And point “O” represents the atmospheric Conditions on the psychrometric chart.

If point B” is chosen, the load on the cooling system is given by the equation:

Load = (V /Vs) x[(h1 – h3 )/3.5] Tons of Refrigeration …………………..…. Eq.2

The load given by the Eq. 2 is quite higher than the load given by the Eq.1. Thus point B’ is the most economical point for the required maximum comfort for the number of maximum people.

What is Vs?

3.5.3 Factors governing the optimum effective temperature

The comfort chart shows roughly the percentage of people comfortable at various effective temperatures. In actual practice, conditions may vary depending on person-to-person, nation-to-nation, different food habits, climatic conditions and altitude. These factors are responsible for changing the optimum effective temperature.

Climatic and seasonal difference: People living in colder climatic conditions are comfortable at lower temperatures than people living in warmer regions. The comfort chart shows that the optimum effective temperature in winter is 19°C and 22°C in summer.

Clothing: Clothing affects the effective temperature because the loss by convection and radiation depends on the body surface temperature. Light clothing requires less effective temperature compared with heavy clothing.

Age and sex: The metabolic rate of a woman is less than that of a man; so a woman requires greater effective temperature as compared to a man.

Air velocity: Higher air velocities require less difference between outdoor effective temperature and inside effective temperature. The air velocity in the conditioned space should be low enough to avoid objectionable noise and draft. It should be high enough to carry (out) heat from human bodies.

3.6 Design considerations

The following figure shows the comfort zone and health data needed for the purpose of design. The designer must estimate the extreme environmental conditioning requirements to avoid hazards. The selected design conditions should not constitute a danger to the health of the occupants

Effective temperatures of 27.7°C in summer or 22°C in winter are found to be comfortable with relative humidity in the range of 35 to 65%. Figure 3.5 below, shows the areas of definite hazard.

Figure 3.5
Areas of Definite Hazards

3.7 Requirements of temperature and humidity - high heat load industries

The physiological effect of high humidity on the health of industrial workers is known as thermal stress. With sustained temperature beyond 37.7°C and 80–90°C R.H. a worker may experience heat stroke, vomiting, giddiness and nausea. Abnormal thermal stress reduces the appetite, increases the heart rate and consequently, workers lose weight within a few months. The problem of thermal stress in industries in tropical countries is far greater than that prevailing in the West.

The safe upper limit of tolerable temperature thus established is 25.5°C (85°F) effective temperature at 30 m/min air velocity (0.5m/s).

The prevailing conditions in the textile industries are generally uncomfortable since the process requires high humidity. There is heat build-up in the body and the sweat on the body cannot evaporate due to high existing humidity and the metabolic heat accumulated in the body is unconsumed. In addition to this there is a risk of oxygen depletion in the existing atmosphere as steam is added in the process (to increase the RH).

Wherever the source of heat is located near the worker, it can be reduced by providing a radiation shield between the worker and the hot machine surfaces. Sufficiently cool air may be supplied to the machine and the hot air may be exhausted separately to reduce the effect of heat load on the human body.

On the outside, the overall heat load can be kept to a minimum by shading of roofs, walls and windows by the use of false roofs, louvers, shade trees, creepers on walls, Venetian blinds, reflecting paint on roof, spray cooling of roof and walls and installation of insulated ceilings.

28°C WBT is considered to be a limit. This was based on research work by COMRO in SA gold mines.

3.8 Recommended inside design conditions

The DBT and RH conditions recommended for different residential buildings and industries purposes are shown in the following Table.

Desirable inside design conditions for different buildings (summer conditions)

Table 3.6
Desirable inside design conditions for different buildings (summer conditions)
Type of Building DBT (˚C) R.H.
1. Residences Living room
Bed rooms
Hall and corridors
20–30
15–20
15–20
55 to 65%
55 to 65%
55 to 65%
2. Offices   21–24 40 to 60%
3. Hospitals Wards
Operating theaters
24
20
60%
65%
4. Restaurants (*)   23–26 55 to 60%
5.Churches and Museums   20–23
15–20
60 to 65%
55 to 60%
6. Warehouses   12–16 50 to 60%
7. Cinema Houses (*)   23 60 to 65%
8. Factories Heavy work
Light work
Sedentary work
15
18
22
50%
55%
60%
9. Schools   19–23 60%
10. Hothouses   25–28 50 to 60%

* Ref: ASHRAE 1999 Applications Handbook Chapter 3.2

Table 3.7
Desirable conditions for Industrial purposes
No. Type of Industry DBT (˚C) R.H.
1 Brewing 5–10 30–50
2 Confectionery 15–21 30–65
3 Drug 24–26 20–40
4 Electrical 15–40 5–7
5 Food 5–21 40–85
6 Fur 2–5 50–65
7 Leather 32–50 95
8 Paint 15–21 25–50
9 Photographic 21–24 50–70
10 Printing 20–30 45–80
11 Rubber 25–35 25–50
12 Textile 24–26 50–85
13 Tobacco 21–32 55–85

3.9 Outside summer design conditions for some foreign cities

The DBT and WBT for different cities are listed in a Table for average outside summer conditions.

Table 3.8
Outside summer design conditions
CITY DBT°C WBT°C RH
Adelaide 37.0 21.4  
Auckland 27.0 21.0  
Bangkok 35.5 33.3  
Berlin 25.5 18.9 55
Bern 24.5 17.8 54
Brisbane 30.8 24.9  
Brussels 24.5 9.5 64
Bonn 24.5 18.9 60
Cairo 35.5 23.5 37
Chicago 35 17.2 40
Copenhagen 23.3 17.9 60
Colombo 30 27 30
Havana 33.3 27.2 67
Hong Kong 31 27 74
Kabul 34 18 20
Kansas 37.8 24.5 33
Karachi 33.9 26.7 59
Katmandu 30 24.6 65
Lahore 39 27.2 41
London 23.5 18.3 60
Los-Angeles 32.2 21.1 38
Manchester 32.2 22.8 45
Mells 34.6 20.4  
Moscow 25.5 18.3 51
New-Haven 35 23.6 46
New-York 29.5 22.6 64
Ottawa 24.5 19.5 63
Oxford 22.8 17.8 64
Paris 25.5 19.5 58
Perth 36.6 22.4  
Rangoon 35 28.3 61
Rome 28.9 22.2 55
San-Francisco 29.5 18.8 34
Saigon 35 29.5 66
Singapore 33.3 26.7 61
Sydney 31.1 22.7  
Tel-Aviv 32.3 22.2 40
Tokyo 30.0 26.1 74
Washington 35 25.5 53
Table 3.9
DBT vs Occupancy
Outside DBT (˚C) Occupancy over 40 minutes Occupancy below 40 minutes
  DBT (°C) WBT (°C) RH% ET DBT (°C) WBT (°C) RH% ET
26.5 23.8
25
26
18.3
17.2
16.2
60
47
35
21.7
21.7
21.7
24.4
25.6
26.6
18.9
17.8
16.6
61
47
36
22.2
22.2
22.2
29.5 24.4
25.6
26.5
18.9
17.8
16.6
61
47
36
22.2
22.2
22.2
25
26
27.2
19.4
18.5
17.2
61
48
36
22.8
22.8
22.8
32.5 23
24
27.2
19.4
18.3
17.2
61
48
36
22.8
22.8
22.8
25.6
26.4
28.8
20.6
19.4
18.3
64
52
40
23.3
23.3
23.3
35.5 25.6
26.6
27.8
26.6
19.4
18.3
64
52
40
23.3
23.3
23.3
26
27.2
28.8
21.1
20
18.9
65
52
41
23.9
23.3
23.9
37.5 26
27.2
28.3
21.1
20
18.9
65
52
41
23.9
23.9
23.9
26
27.2
28.3
21.7
20.6
19.4
63
50
38
24.4
24.4
24.4
40.5 26.6
27.8
28.9
21.7
20.6
19.4
65
52
42
24.2
24.2
24.2
27.2
28.3
29.5
22.2
21.4
20
65
54
41
24.7
24.7
24.7
43.5 26.6
27.8
28.9
21.7
20.6
19.4
65
52
42
24.5
24.5
24.5
27.2
28.3
29.5
22.2
21.7
20
65
54
41
25
25
25
46.5 26.6
27.8
28.9
21.7
20.6
19.4
65
52
42
24.8
24.8
24.8
27.2
28.3
29.5
22.2
21.7
20
65
54
41
25.3
25.3
25.3
49.5 26.6
27.8
28.9
21.7
20.6
19.4
65
52
42
25.2
25.2
25.2
27.2
28.3
29.5
22.1
21.7
20
65
54
41
25.6
25.6
25.6

Note: In a shopping center where people come from high ambient outdoor conditions – a higher temperature is needed to satisfy comfort requirements.

Table 3.10
Recommended Commercial Conditions
Types of use DBT (°C) WBT (°C) RH% Effective Temperature
Deluxe Application 25.6 18.9 50 22.4
Normal Application 26.6 20 51 23.4
15 to 40 minutes Occupancy 27.8 21.1 49 24.1

3.10 Types of Ventilation Systems

In general term, ventilation is the flow of air into and out of a space and its purpose is:

  1. To supply clean outdoor air into a space, replacing the contaminated and stale air from the space. This may be achieved either by natural ventilation or mechanical ventilation. This is called “ The Intentional ventilation”
  2. To take care of the outside infiltrating air into the space by gaps and cracks in the building envelope, for example heavy wind speed, entry space for conveyors, frequent opening of doors for human trafficking. Air infiltration not only adds to the quantity of air entering the building but may also distort the intended air flow pattern to the detriment of overall indoor air quality and comfort.
  3. To take care of the exfiltration air loss from an enclosed space, for example, differential pressure across the adjacent space or room.

An improper ventilation and unacceptable indoor air quality may lead to:

  1. Offsetting the performance of heating or cooling inside a space.
  2. Causing health problem for personnel, such as Headaches, Eye irritations, fatigue, breathing problem. asthma attack, dizziness, depression, skin irritation etc.
  3. Air leakage from the seams and joints of ventilation, heating and air conditioning circulation ducts
  4. Possibility of pollutants may be drawn into the building through the openings

It should be noted that the ventilation is not:

  1. An air conditioning process, since the ventilation displaces a contaminated air by a clean air. It does not , alter the temperature and humidity conditions inside the space
  2. The recirculated air may be filtered for removing the dust, but it does replenish the oxygen and does not remove the metabolic pollutants (carbon dioxide and odour). We may need fresh air.

Therefore, if a space requires both ventilation and air conditioning together, the air handling must be designed in such a way to achieve this two objectives.

3.10.1 Types of Ventilation systems

The ventilation systems can be classified into:

  • Natural Ventilation
  • Mechanical Ventilation
  • Heat Recovery Ventilation

Natural Ventilation

The flow of air caused due to pressure difference between the ambient air and the air in the conditioned space is known as natural ventilation system. The airflow caused by the wind speed prevailing at the atmosphere surrounding this roof mounted ventilators or generally called as “Wind Turbines”. Wind blow is always uncertain, both in its direction and in speed. There is no control of dust and odor carried by incoming air. Consequently natural ventilation has no significance in air conditioning systems

Natural ventilation is the process of supplying and removing air through an indoor space by natural means. There are two types of natural ventilation occurring in buildings:

  • wind driven ventilation
  • stack ventilation.

The pressures generated by buoyancy, also known as 'the stack effect', are quite low (typical values: 0.3 Pa to 3 Pa) while wind pressures are usually far greater (~1 Pa to 35 Pa). The majority of buildings employing natural ventilation rely primarily on wind driven ventilation, but stack ventilation has several benefits. The most efficient design for a natural ventilation building should implement both types of ventilation.

Wind driven ventilation

Wind driven ventilation or roof mounted ventilation design in buildings provides ventilation to occupants using the least amount of resources. . By utilizing the design of the building, Wind driven ventilation takes advantage of the natural passage of air without the need for high energy consuming equipment. Wind catchers are able to aid wind driven ventilation by directing air in and out of buildings.

Wind driven ventilation has several significant benefits:

  • Greater magnitude and effectiveness
  • Readily available (natural occurring force)
  • Relatively economic implementation
  • User friendly (when provisions for control are provided to occupants)

Some of the important limitations of wind driven ventilation:

  • Unpredictableness and difficulties in harnessing due to speed and direction variations
  • The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area
  • May create strong draughts, discomfort.
Figure 3.6
Natural Ventilation
Figure 3.7
Hybrid ventilator driven by Electronic commutating motor

How does wind driven unit work?

A well-designed turbine ventilator takes advantage of the wind to create a positive flow through the throat of the ventilator. The wind influences the performance of the ventilator in two ways.

  1. As the Wind approaches and strikes the ventilator, it jumps, creating an area of low pressure on the leeward side of the turbine. This low pressure zone is fed by drawing air from the turbine, causing a continuous extraction of air from the building.
  2. As the turbine rotates, the centripetal forces associated with the rotation fling air outwards form the tips of the vanes. Replacement air is drawn into the throat of the ventilator from the building causing continuous ventilation.

The turbine will even rotate and exhaust in the absence of wind using the thermal currents developed within the building.

Stack Ventilation

One way to ventilate a building that is hotter or colder on the inside than outside is to use what is known as "stack effect". Because of the temperature difference, the air inside the building is either more or less dense than the air outside. If there is an opening high in the building and another low in the building, a natural flow will be caused. If the air in the building is warmer than the outside, this warmer air will float out the top opening, being replaced with cooler air from outside. If the air inside is cooler than that outside, the cooler air will drain out the low opening, being replaced with warmer air from outside.

One common use for stack effect would be nighttime flushing of a building's interior, to cool it for the next day.

Figure 3.8
Stack Ventilation

Mechanical ventilation

The controlled ventilation (flow quantity and direction) caused with the use of a fan is called mechanical ventilation. This is essential when the temperature, humidity and air motion are to be controlled in an air-conditioned space. Generally, three different systems of mechanical ventilation are used in practice.

Extraction System

This system is most widely used for ventilation in an air conditioning plant. The air from the air-conditioned space is withdrawn by using a fan. This is accomplished either by using propeller type exhaust fans or by means of a duct, depending on the requirements.

The exhaust produces a low-pressure zone around it, which causes a flow of air towards the fan. The desirable and undesirable location of inlet points is shown in the following figures.

Figure 3.9
Desirable Position of Inlet

Exhaust ventilation- Removes the contaminant at it’s source, like a bath fan or kitchen exhaust hood. While this qualifies as mechanical ventilation it is hardly optimal. Exhaust only leads to unbalanced pressures, drafts and potentially unsafe backdrafting.

Figure 3.10
Undesirable Position of Inlet

As shown in figure 3.10, the short-circuit caused will not give required ventilation in remote parts of the air-conditioned space. The effectiveness of an extraction system largely depends on the location of inlets with respect to the exhaust outlets. The distance between the inlet and the exhaust should not be excessive enough to contaminate the air while it traverses the occupied space.

Supply systems

In this system, air is supplied under pressure to the conditioned space by means of a fan.

The advantages of this system over the extraction system are:

  • Since conditioned air is introduced under positive pressure, better control over quantity, velocity and its distribution is possible.
  • The re-circulation of interior air for heating systems can be achieved more conveniently.
  • Since a positive pressure is maintained in the conditioned space, it eliminates all possibilities of creating a negative draft.

Supply Ventilation- Provides a means of introducing fresh air into the home to replace exhausted air and help with dilution of home contaminants. Traditionally a duct is ran from the cold air return of your HVAC system to the outside metered by a barometric damper or an electronic damper that opens and closes based on preset conditions. By bringing the air through the return we can condition and filter the air before it is introduced into the living area. Coupled with quality bath and kitchen exhaust fans this setup is effective and should be your minimum standard.

Combined Supply and Extraction system

To ensure uniform fresh air, a combined system is always preferred. In this system, fresh

air is introduced at required points and its uniform distribution in the occupied space is achieved with the help of extraction and supply fans.

In a combined system, the capacity of the supply fan should be 20 % more than the exhaust fan to maintain the pressure in the conditioned space, above atmosphere. This also prevents negative draft and infiltration of dust and other air-borne contaminants.

With rooms of moderate widths, it is preferred that the inlet air and the exhaust are located on the walls opposite to each other. If the distance between the walls is more, then it is important to provide an extracting duct to avoid the possibility of short-circuiting.

This is illustrated by the following figure.

Figure 3.11
Combined Supply and Extraction system

Heat Recovery Ventilation

HRV/ERV-Stands for heat recovery ventilator and energy recovery ventilator. An HRV/ERV is a balanced exhaust system that runs the exhaust and intake air through a heat exchanger, pulling the heat out of our conditioned air before we send it outside. An ERV is the same setup that also pulls out humidity making it a great fit for hot humid areas. HRV’s can be installed stand alone or tied into the duct system. Based on conditions set by your installer the unit will exhaust and replace the same volume of air with reduced heating/cooling load due to the heat exchanger. Another fantastic idea that I saw involves a remote mounted bath fan that pulls air from several bathrooms that ties into the ERV. Instead of sending that warm steamy air outside it is ran through the ERV.

3.11 Effect of vertical temperature gradient and corrective measures

Uniform temperature in an air-conditioned room is the prime requirement of comfort. However a temperature gradient can exists irrespective of all measures taken during the design. Its magnitude depends on the volume of air-conditioned space and the method of air distribution used. In cases where several temperature gradients exist, corrective measures should be taken to eliminate the gradient for comfort.

The temperature gradient depicts the ascent of hotter and lighter air and the descent of colder and heavier air within an enclosure. Temperatures will be highest near the ceiling and lowest near the floor.

Temperature gradient is the critical factor in the cost of heating an enclosure because it is the colder zone, from 1 meter to 1.5 meters off the floor, where the requirements for human comfort are determined. Yet, it is the ceiling and the upper region of the outside facing walls, where the heat loss is at its maximum. Insulation will retard the amount of heat loss but will not eliminate the cost of maintaining higher temperature in the upper regions of the enclosure.

3.11.1 Corrective actions

Once the problem areas are identified, the next step is to take corrective measures. Ceiling height, freestanding obstructions and heat emitting equipment are the factors, which contribute to the temperature gradients. Poor air circulation is mainly responsible for temperature gradient. Hot air cannot be prevented from rising but proper circulation can prevent it from staying in one place. The air circulation should ensure proper mixing of hot air and colder air, which exist at different elevations within the room.

The ideal air patterns are those which push hot air downward in a cone shape with a spread of approximately 100 to 300 degrees, as shown in the figure below.

Figure 3.12
Ideal Air Pattern Is Depicted Which Aspirates Hot Upper Air

This activates the phenomenon of aspiration to draw the stagnant hot air away from the ceiling and mix with the downward flow of new air.

Vents used to discharge air into the enclosed space are adjustable so that the shape and direction of initial flow of air can be influenced to suit the installation. Airflow can also be directed to a shallower angle towards the walls as shown in the following figure. This will aspirate the upper air as the new air current is deflected downwards along the vertical surface.

Figure 3.13
Ideal Air Pattern causing discomfort to workers working below diffusers can be adjusted to ‘bounce’ air off adjacent walls down to occupied level

Along with these adjustments in the air circulation patterns, repositioning the return air grills may be beneficial. The floor level return grills are not advisable if the same space is cooled during summer. In such cases, grills can be designed to heat the room at either ceiling or the floor. The ceiling return can be fitted with ductwork extending to the floor and terminating with another grill. During the heating season, the upper grill will be closed and the bottom one will be open. During the cooling season the upper grill will be open and the bottom one will remain closed.

A downward air flow in air distribution systems is a more common type of air conditioning system as it offers the following advantages; where cooling is always required:

  • The air speed required to overcome the convection of up-currents is only about 0.3 m/sec; therefore the power consumed is only 33 % of the horizontal flow patterns.
  • There is complete freedom of movement and access for occupants everywhere in the air-conditioned space.
  • Supply air temperatures can be low, therefore duct sizing and capital costs are reduced.
Table. 3.11
Recommended Velocities in air conditioning equipment
Individual Duct System Trunk Duct System
Equipment Velocity In m/sec. Equipment Velocity In m/sec.
Horizontal Duct 3.5 to 6 Main Duct 4.5 to 6
Vetrical Duct 2 to 4 Branch Duct 3 to 5
Wall Registers 1 to 2 Risers 1.3 to 2.5
Floor Registers 0.6 to 0.95 Registers and Grilles 1 to 2

3.12 Factors considered in air distribution systems indoor

The requirement of a good air distribution system is to provide a proper combination of DBT, RH and air motion in the air-conditioned space. This comfort condition is generally required at 1.8 meters above the floor level. The maximum variation in temperature should not be more than one 0C. The desirable terminal air velocity lies between 0.125 to 0.15 m/sec.

3.12.1 Draft

It is defined as the feeling of warmth or coolness due to air motion at the required DBT and RH. It is measured above and below the controlled room conditions of 24.5 °C DBT and a terminal air velocity of 0.15 m/sec. Therefore to avoid the feeling of draft, proper air distribution in the room is absolutely essential. The desirable velocities of air taking into consideration the permissible sound level are listed below:

Table.3.12
Draft Velocities
Application Broad-casting Studios Residences and Private Offices Cinema Theatres General Offices Store main floor Store Upper floor
Velocity in m/sec. 1.5 to 2.5 2.5 to 3.75 5 5 to 6.25 6 6 to 7

To maintain the required DBT and RH with proper air circulation in the conditioned space, fresh inlet air and room air should mix properly with minimum pressure loss.

The principles of air distribution involve the following factors:

3.12.2 Throw

It is the distance traveled by the air stream in the horizontal plane after leaving the air outlet and reaching a velocity of 0.125 m/sec. at a height of 1.8 meters above the floor level.

3.12.3 Drop

It is the vertical distance the air moves after it leaves the outlet and reaches the end of throw.

3.12.4 Entrainment or Induction Ratio

The air leaving the outlet is known as primary air and the air entrained by the primary air from the room is known as secondary air. The sum of primary and secondary air is known as total air. The entrainment or induction ratio is:

Induction ratio = Total air / primary air

Higher value of induction ratio is desirable for proper mixing and uniform DBT and RH of air in the room. The throw depends upon the supply air velocity, temperature difference between the primary air and the room air and the induction ratio.

3.12.5 Spread

It is defined as the angle of divergence of an air stream after it leaves the outlet. The spread depends on the type of outlet used. Air outlets can have three types of vanes:

  • Straight,
  • Converging
  • Diverging.

Outlets with straight vanes produce a spread angle of 14 to 24 degrees in the horizontal as well as vertical plane.

Outlets with converging vanes produce the same angle as straight vanes but the throw is 15 to 20 % higher. Outlets with diverging vanes give a fanning effect.

3.12.6 Types of supply air ducts

The outlets are classified as sidewall outlets, ceiling and floor outlets according to their locations. The basic types that are commonly used are described below.

3.12.7 Grill outlets

These outlets have adjustable bar grills, which are the most common types with horizontal and vertical vanes. These are similar to the grills used for evaporative type home coolers and are rarely used for comfort air conditioning as they create draft.

3.12.8 Slot diffuser

It is an elongated outlet with an aspect ratio of 25:1 and maximum height of 75 mm. These types of outlets are used on the sidewalls at a higher elevation and along the periphery of the floor.

3.12.9 Ceiling outlets

These types of outlets are mounted in the ceiling. Multi passage, round, square or rectangular are the most common types. They are also provided with adjustable louvers to vary the amount of air.

3.13 Indoor Air Quality

So far we have emphasized the quality of air supplied to an air-conditioning system but the quantity of air supplied is equally important. The quantity of air required by each person for breathing is nearly 6 m3/hour. The percentage of CO2 in the atmosphere is 0.03% and it should not increase above 0.6% as breathing becomes difficult with an increase in percentage of CO2. The quantity of air required to fulfill this requirement is 1.2 m3 free air per minute per person.

Generally, engineers provide 0.3 m3 of outside air per person per minute, which maintains the level of CO2 below the minimum required and provides sufficient O2 for easy breathing. In present day ventilation standards, the oxygen content in the air is not the determining factor for the amount of fresh air supplied. The amount of outside air required is governed almost entirely by the sources of contamination within the space.

The chief sources include body odors from materials, cooking odors and tobacco smoke. Body odors are not particularly harmful except that they cause a feeling of stuffiness and discomfort: if the odors are sufficiently strong, they will cause loss of appetite; lack of desire for activity and headache. The effect of the odor depends upon the space provided per person in an air-conditioned space, the amount of fresh air circulated and the cleanliness of the people.

No data is available concerning the quantity of air required for different types of odors and their concentrations. The ventilation requirement of such a place is best left to individual judgment.

There are no absolute standards of ventilation but certain local codes must be satisfied as per the rules of the local Government. The local codes depend upon the density of people (residence or Cinema Hall) the type of odor sources (human odor or industrial), and the percentage of re-circulated air.

3.13.1 Indoor air quality (IAQ)

Concern about indoor air quality (IAQ) and the study of air quality issues is a fairly recent phenomenon. In the mid 70s or so, IAQ studies primarily involved comparing inside air to outside air. The level of outdoor pollution was a chief concern and the goal was to ensure that indoor air was of better quality than the outdoor air subjected to pollutants. As studies increased in sophistication, other measurable factors came into play

A reduction in natural ventilation, or ‘fresh’ air, in the interest of saving energy became a concern and, finally, people realized that pollutants could actually originate within a building.

The World Health Organization (WHO) estimated that more than 30 percent of all commercial buildings have significant IAQ problem The quality of indoor air affects productivity, personal comfort, building maintenance costs and even health and safety, either positively or negatively depending on how air quality is managed.

Ensuring satisfactory air quality requires a good understanding of the building itself. The design, physical layout, mechanical systems, equipment and space usage are all essential elements that can affect air quality. The air distribution system requires particular attention.

3.13.2 Effects of poor quality air

When we talk about factors affecting air quality, these are over and above the proven hazardous materials where exposure limits have been set and personal protective equipment prescribed. The factors considered here are those that exist in ‘normal’ air in offices, schools, libraries, churches, hospitals and other interior spaces where we spend time without expecting to face any risks.

Actually there is no universal reaction to a measured amount of a particular material. Different people have different tolerance levels. It is difficult to assign standards or guidelines to set acceptable levels of all the airborne pollutants. Many indoor air contaminants are actually new, bred from the ever-changing technology that so many of us are exposed to daily. From alternative energy sources to photocopiers, we are generating new pollutants at an ever-increasing pace.

Symptoms caused by air quality problems vary greatly according to an individual’s sensitivity and may include chills, sweating, eye irritation, allergies, coughing, sneezing, nausea, fatigue, skin irritation, breathing difficulty and others. Indoor air quality is a growing concern and gaining attention. It is necessary to take a proactive approach and address any issues that could potentially have adverse affects on indoor air quality.

3.13.3 Pollutants affecting the air quality

1. Biological
The common biological pollutants that affect the indoor air quality are bacteria, fungi, viruses, molds, pollen, animal hair, dander and excrement.

2. Chemical
There are certain air borne chemicals, which also affect the indoor air quality, such as cleaners, solvents, fuels, adhesives, various combustion by-products and emissions from furnishings and floor and wall coverings.

3. Particles and Aerosols
These are minute solids or liquids that are light enough to be suspended in air. These can be categorized as dust, smoke, mist, fume and condensates.

Controlling sources of pollutants

In a typical building, pollutants can be categorized as:

  • Those that enter the building from the outside
  • Those generated within the building itself

Pollutant sources must be located and controlled to ensure good indoor air quality. Keep in mind that both sources and pathways are essential components that must be well understood for effective problem remediation. Pathways are created as pollutants travel by air movement or from relative positive to relative negative pressure areas through even the smallest of openings.

Once the source is identified, several methods can be adopted for managing the pollutant source, which include:

  • Removing the source
  • Repairing the source so it no longer contributes pollutants
  • Isolating the source with a physical barrier
  • Isolating the source using air pressure differential
  • Minimizing the time people are exposed
  • Diluting pollutants and removing them from the building with increased ventilation
  • Increasing filtration to clean the air and remove pollutants

Investigating indoor air quality

In order to obtain the best comfortable conditions in an air-conditioned space it is important that the existing air quality is known.

A typical IAQ investigation requires several steps:

Planning

  • Obtain the background information about the building and its systems.
  • Talk to affected people to understand the complaints and symptoms.
  • Check for patterns as to where and when they occur.
  • Set objectives.
  • Determine the strategy to be employed.

Gathering data

  • Carry out necessary measurements throughout the building, which include temperature, humidity, CO2, CO, particles, VOCs, chemicals and bioaerosols.

Analyzing the data

  • Compare the measurement readings with acceptable measurements.
  • Eliminate certain areas or suspected problems that direct you to areas requiring additional focus (there can be multiple problems).

Report findings

  • Report all the results indicating a need for corrective action

Offering assistance

  • Prepare an IAQ management plan that includes setting policies and conducting routine measurements to ensure good air quality is maintained.

In order to make a thorough investigation the affected occupants should be asked questions such as:

  • What symptoms are you experiencing?
  • When did the symptoms begin?
  • Are the symptoms present all the time or just during certain periods of time?
  • Where do the symptoms occur?
  • Do symptoms subside when you leave the affected the area?
  • Have there been changes to the area such as new furniture, carpet, paint, remodeling or construction projects, etc?
  • Is there a smoking or parking area nearby?
  • Have you recently moved?
  • Have you had a significant change in your activities?
  • Does anyone else near the affected area have symptoms similar to yours?

3.13.4 Measurements used to determine air quality

Several parameters must be measured and analyzed to determine the quality of indoor air and whether or not corrective action is appropriate. Once a problem is discovered and corrected, an area should be routinely monitored to prevent reoccurrence. This also helps to detect emerging problems early so that they can be remedied before they become difficult and expensive to manage.

3.13.5 The criteria for determining indoor air quality can be categorized as comfort and health

Comfort and productivity

Comfort is a way of measuring occupant satisfaction, which, in turn, can directly affect concentration and productivity and impact the cost of doing business. It is a phenomenon that is both physical and psychological, and it varies greatly from person to person. It can depend upon factors like type of clothing worn, level and type of activity and physical surroundings, including people, furnishings and adjacent spaces. Attaining optimum comfort is not practical. As a general rule of thumb, the best one can hope to achieve is satisfying about 80% of the occupants.

The common measurable characteristics of comfort include temperature, humidity, air velocity, ventilation, vibration and noise.

The factors that are difficult to quantify but have an impact on individual comfort arelight glare, odors, physical space layout, proximity to other areas, and ergonomics.

Let us discuss the following measurements often used to determine comfort level:

  • Temperature
  • Humidity
  • Velocity
  • Volume
  • Ventilation

Note: When making a measurement, allow sufficient time for the instrument to capture a ‘stable’ reading..

Temperature

Temperature is one of the basic IAQ measurements that have a direct impact on perceived comfort and, in turn, concentration and productivity. According to ASHRAE Standard 55, the recommended temperature ranges perceived as ‘comfortable’ are 22.8°C to 26.1°C in summer and 20.0°C to 23.6°C in winter.

How to measure?

Measurements should be taken periodically at many areas of the building to be sure that air is distributed evenly and temperatures are consistent.

Humidity

Too little humidity in a space may create static build-up and people will sense that their skin feels dry. Too much humidity will make people feel sticky. According to ASHRAE Standard 55, indoor humidity levels should be maintained between 30% and 65% for optimum comfort.

ASHRAE Standard 55 links temperature and humidity together to provide a measure of thermal comfort. The objective should be to set the appropriate temperature and humidity levels so as to maximize occupant comfort while controlling energy consumption.

How to measure?

Humidity can be measured in several ways. Typically, references such as relative humidity, wet bulb, dry bulb, humidity ratio and absolute humidity are used. Whichever method is chosen, measurements should be taken periodically and spread throughout the building to ensure that air is distributed evenly and humidity levels are consistent.

Velocity

One of the first checks, often overlooked, in a comfort study is making sure that sufficient air is moving in a conditioned space. Air movement can affect human comfort level. Too much air is perceived as ‘drafty’ or ‘chilly’ and too little may create a sensation of stuffiness. Thus measuring velocity and maintaining it becomes vital.

How to measure?

As a practical measurement a quick spot check at the supply diffuser will show if sufficient air is entering a given space. This will assure there are no unexpected blockages in the air system, such as a closed damper. Velocity is also a good indicator that air is being appropriately distributed or balanced throughout the building and reaching all the spaces intended. Measurements should also be taken in the actual occupied ‘zones’ to assess how air velocity affects individuals.

Air velocity is rarely uniform across any section of an air duct. The shape of a duct, its turns and branches and friction all affect the movement of air. In general, air tends to move slower towards the edges or corners and faster in the center of a duct. The average air velocity can be determined using a straight average for both round and rectangular ducts using the log-Tchebycheff method – a method that accounts for velocity losses due to friction. Velocity measurements should be taken at a minimum of 25 points for rectangular ducts and, for round ducts, symmetrically disposed diameters with at least 6 points on each should be used. For the greatest accuracy, take these measurements at least 7.5 diameters downstream or 3 diameters upstream from any disturbance such as an elbow, venturi or take-off.

Volume

ASHRAE Standard 62 lists recommended outdoor air requirements expressed in terms of cubic feet per minute (cfm) per person depending on the type of space and activity. Air volume or flow into an area affects the air change rates or exchange of air between outside and inside. This results from leakage and natural or mechanical ventilation systems. The exchange of air can have a large impact on indoor air quality as it may increase the amount of outdoor pollutants being introduced or, conversely, dilute and help remove contaminants generated inside.

How to measure?

In order to determine the volumetric flow rate, the average measured air velocity as mentioned above is multiplied by the cross-sectional area of a duct.

For example:

If a duct is 600 mm × 600 mm (cross-sectional area = 0.371 m2) and the average measured air velocity is 0.762 m/s , the resulting flow rate is 0.371 × 0.762 = 0.282 m3/s or 600 cfm.

An air capture hood can also be used to determine airflow. Capture hoods provide quick, direct measurements of airflow from diffusers, vents or grilles. They are capable of collecting and storing real-time flow measurements and they are also valuable when balancing the system for proper flow in all areas.

Ventilation

The introduction of fresh air helps dilute unwanted pollutants and gets them out of the building faster. ASHRAE Standard 62 presents recommendations pertaining to ventilation, or the amount of fresh air introduced into a given area. It recommends a minimum volume per person over time, depending on the type of space and activity being performed, expressed in cubic feet per minute per person.

How to measure?

A good indicator of proper ventilation is the level of CO2 present in a space. Carbon dioxide is a normal by-product of respiration, combustion and other processes.

Elevated levels of CO2 may indicate that additional ventilation is required. ASHRAE Standard 62 recommends an indoor level not to exceed 650 ppm above outdoor ambient air, which is typically about 300 to 400 ppm. Under normal conditions, even elevated CO2 levels are rarely a health hazard since levels up to 10,000 parts per million (ppm) can be tolerated without ill effects by healthy people. Measurements should be taken between different areas, in air distribution zones, at varying heights and between indoor and outdoor areas to ensure that the building is properly ventilated.

The following table lists a few examples of the amount of outside air needed per person for good ventilation as per ASHRAE standard 62 in different environments.

Table.3.13
Outside air quantity required per person
Application Outside air m3/person
Dining room 34
Hotel/Motel room 25.5
Kitchen 25.5
Office Space 34
Conference room 34
Public Rest room 85
Retail stores 25.5
Gymnasium 34
Auditoriums 25.5
Libraries 25.5
Patient rooms 37.5
Residential Living areas 25.5

Health and safety

We have seen the various factors of indoor air quality that can affect the comfort conditions of the occupants. While comfort is important in maintaining productivity and concentration, many unwanted airborne contaminants can actually pose a threat to human health. Unhealthy IAQ conditions occur whenever vapors, gases or airborne particulates are present in concentrations that adversely affect one or more occupants of a space.

Potentially toxic, infectious, allergenic, irritating or otherwise harmful substances are almost always around us. Usually they exist in such small concentrations but if they rise beyond a certain level can be harmful to the occupants. One cannot fully ignore these while investigating the indoor air quality.

The pollutants that affect the health and are concerned with the safety of the occupants are :

  • Carbon Monoxide
  • Airborne particles
  • Ultra fine particles
  • Bioaerosols
  • Chemical in aerosol form

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless, poisonous gas that is a by-product of incomplete combustion. When inhaled, it readily mixes with hemoglobin in the blood, inhibiting the blood’s ability to carry and exchange oxygen. Carbon monoxide does not readily leave the body once it enters and treatment in some cases may even require a blood transfusion. Excessive exposure to carbon monoxide can starve the body of oxygen and lead to death. The exposure limits for CO are a maximum of 35 ppm for one hour, not more than one time per year, or 9 ppm over any eight-hour period.

How to measure?

Measurements of carbon monoxide should be taken periodically and spread throughout many areas in a building to be sure that air is being distributed evenly and no dangerous levels of CO are detected. Pay particular attention to areas in which any form of combustion takes place. Typical examples of outdoor CO sources in a building include vehicular emissions from traffic or parking areas and building exhaust stacks. Indoor sources include furnaces, boilers, stoves and smoking areas.

Airborne particles affect respiration weakening the body’s natural defenses. Particles with an aerodynamic diameter of 10 microns (PM 10) are inhalable while particles with diameters less than 4 microns are respirable and readily enter the lungs.

ASHRAE Standard 62 recommends a maximum exposure limit for PM10 particles of 0.15 mg/m3 for a 24-hour average and 0.05 mg/m3 for an annual average exposure.

It is important to minimize airborne particles in a conditioned space by good housekeeping practices, upgrading filters, maintaining a positive pressure relative to the outdoors and having proper exhaust design.

In spite of these practices airborne particles may enter spaces, which necessitate measurement. Two basic methods typically are used:

  • Air sampling over time
  • Measurements employing real-time instruments.

With air sampling over time, materials are most often collected on a filter medium and subsequently analyzed in an environmental laboratory located away from the sampling location. With real-time instruments, measurements are made and results obtained on-site. Three types of instruments photometers, optical particle counters (OPC) and condensation particle counters (CPC) are normally used for real-time measurements.

Ultrafine particles

Ultrafine particles (UFPs) are defined as particles less than 0.1 micron diameter. These are produced by combustion and some chemical reactions. They are so small that they can pass easily through the body’s natural defense mechanisms to the deepest areas of the lungs. Certain people are extremely sensitive to ultrafine particles, sometimes regardless of chemical composition. No specific guidelines or standards have been developed for ultrafine particles. Much research currently is being done, however, and some initial results have linked UFPs to potentially adverse health conditions.

How to measure?

The only practical instrument for detecting ultrafine particles is a condensation particle counter (CPC). It is a device that ‘grows’ the small particles to a size large enough to be counted using conventional particle counting techniques.

A basic understanding of the ventilation system and how fresh air is introduced, filtered and distributed throughout the building is necessary for an effective investigation. If levels of ultrafine particles significantly higher than expected are found anywhere in the building, take steps to locate and identify the source. Using the particle counter, ultrafines can be traced quickly and easily directly to their source. Once a source is located remedial action to control, repair or remove is often straightforward.

Bioaerosols

Bioaerosols are defined as airborne particles, large molecules or volatile compounds that are living or released from living organisms. A few examples are plants, including fungi, yeasts, molds, mildews and pollen, as well as bacteria, endotoxins, viruses, antigens, and animal parts. Bioaerosols range in size from less than 0.1 micron to 100 microns in diameter. Some of these bioaerosols contain dangerous toxins that in extreme cases can cause a range of adverse health effects, including death.

How to measure?

Most biological growth requires some kind of food and water. Condensation, plumbing leaks or roof leaks can foster unwanted growth and must be checked and corrected. Atthis time, bioaerosols such as molds, fungi and bacteria must be collected, cultured and analyzed in an environmental microbiology laboratory setting to determine exactly what they are and how large a presence they have. Sampling often consists of collecting material through an air sample on different sized filter media.

We have seen how various pollutants affect the indoor air quality. The following points need to be kept in mind while investigating any indoor air quality situation:

  • Be aware of the entire picture.
  • Many parameters that may be contributing to an overall problem must be considered and checked.
  • Keep in mind that it is not uncommon to find multi-layered problems, and finding and solving one issue may not get to the root cause.
  • Think of an investigation as peeling an onion; as each layer is removed, another is exposed.
  • Be sure to understand the exact time and place that problems are suspected, since many IAQ problems come and go in a moment.
  • Use common sense along with the proper tools and keep investigating and correcting problems until complaints stop.

As a guideline the following table lists the various parameters that should be considered while investigating indoor air quality along with their limits.

Table.3.14
Indoor Air Quality Standard
Parameter Limit / Range Reference
Temperature Winter-22.7° to 26.1°C
Summer-21.1° to 23.6°C
ASHRAE Standard 55
Relative humidity 30% to 65% ASHRAE Standard 55
Air movement (Velocity) 0.25 m/s WHO
Ventilation 2.55 to 102 m3/h ASHRAE Standard 62
CO2 Levels < 1000 ppm ASHRAE Standard 62
Filtration 25 to 30% minimum efficiency ASHRAE Standard 52.2
Inhalable particles 0.15 µgm/m3 over 24 hr. ASHRAE Standard 62
Carbon Monoxide 9 ppm over 8 hours or 35 ppm in one hour per year maximum EPA-National ambient air quality standards

3.14 Design of Ventilation Systems

A design procedure of ventilation systems, with air flow rates, heat and cooling loads, air shifts according occupants, air supply principles.

A ventilation system may be designed more or less according the following procedure:

  • Calculate heat or cooling load, including sensible and latent heat
  • Calculate necessary air shifts according the number of occupants and their activity or any other special process in the rooms
  • Calculate air supply temperature
  • Calculate circulated mass of air
  • Calculate temperature loss in ducts
  • Calculate the outputs of components - heaters, coolers, washers, humidifiers
  • Calculate boiler or heater size
  • Design and calculate the duct system

3.14.1 Calculate Heat and Cooling Loads

Calculate heat and cooling loads by:

  • Calculating indoor heat or cooling loads
  • Calculating surrounding heat or cooling loads

A quick selection guide of ventilation systems in comfort environments

The cooling capacities of air-condition systems are often determinant for selecting the types and principles. The cooling limits for the systems are often set by comfort requirements. Typical maximum cooling capacities of some common types of displacement and mixing systems in different kind of buildings are roughly set in the following table.

Table.3.15
Ventilation principles and cooling loads

The heat load in typical rooms are influenced by the use of lights, heat emission from machines like computers, the number of people and the sun radiation through windows and other building elements. The heat load is typically very dynamic and for larger building and systems some kind of computerized tools should always be used in the design.

Heat loads in some typical rooms can be found in the following table.

Table.3.16
Heat Loads
Typical rooms and heat loads Heat Load (W/m2)
Normal offices without automatic 0 - 30
Normal offices with automatic 30 - 50
Conference rooms 20 - 75
Data rooms > 60
Guest rooms, normal standard 0 - 25
Guest rooms, high standard 25 - 50
Patients rooms 0 - 20
Treatment rooms 20 - 60
Intensive rooms > 50
Conference rooms 20 - 75
Theatre, cinema 40 - 60
Restaurants 30 - 70
Class rooms 20 - 50
Food 20 - 40
Normal 30 - 60

3.14.2 Calculate Air Shifts according the Occupants or any Processes

CO2 acceptance and comfort levels

Indoor air quality includes:

  • temperature
  • odor
  • high or low levels of gases

Since CO2 is exhaled by people at predictable levels the content of Carbon Dioxide - CO2 - in air may be a significant indication of air quality. A measure of CO2 indicates the amount of fresh air supply:

  • 25.5 m3/h ventilation rate per occupant corresponds to 1000 ppm CO2
  • 34 m3/h ventilation rate per occupant corresponds to 800 ppm CO2

Normal CO2 Levels

The effects of increased CO2 levels on adults at good health can be summarized:

  • normal outdoor level: 350 - 450 ppm
  • acceptable levels: < 600 ppm
  • complaints of stiffness and odors: 600 - 1000 ppm
  • ASHRAE and OSHA standards: 1000 ppm
  • general drowsiness: 1000 - 2500 ppm
  • adverse health effects expected: 2500 - 5000 ppm
  • maximum allowed concentration within a 8 hour working period: 5000 ppm

The levels above are quite normal and maximum levels may occasionally happen from time to time.

Extreme and Dangerous CO2 Levels

  • slightly intoxicating, breathing and pulse rate increase, nausea: 30,000 ppm
  • above plus headaches and sight impairment: 50,000 ppm
  • unconscious, further exposure death: 100.000 ppm

Carbon Dioxide Standard Levels

The recommendation in ASHRAE standard 62-1989 are:

  • classrooms and conference rooms 25.5 m3/h per occupant
  • office space and restaurants 34 m3/h per occupant
  • hospitals 42.5 m3/h per occupant

3.14.3 Carbon Dioxide Emission from People

Table.3.17
The carbon dioxide emission from persons and their activity
Activity Respiration per Person (m3/h) Carbon Dioxide Emission per Person (m3/h)
Sleep 0.3 0.013
Resting or low activity work 0.5 0.02
Normal work 2 - 3 0.08 - 0.13
Hard work 7 - 8 0.33 - 0.38

The carbon dioxide concentration in a room with persons can after a time - t - be expressed as

c = (q / n V) [1 - (1 / en t)] + (c0 - ci) (1 / en t) + ci

Where: c = carbon dioxide concentration in the room (m3/m3)

q = carbon dioxide supplied to the room (m3/h)

V = volume of the room (m3)

e = the constant 2.718.....

n = number of air shifts per hour (1/h)

t = time (hour, h)

ci = carbon dioxide concentration in the inlet ventilation air (m3/m3)

c0 = carbon dioxide concentration in the room at start, t = 0 (m3/m3)

Example - Carbon Dioxide Emission in a Cinema

In a cinema with 100 persons at low to normal activity the emission of carbon dioxide will be in the range of 0.02 to 0.08 m3/h per person.

The total emission will be in the range of 2 to 8 m3/h.

3.14.4 Pollution Concentration in Rooms

Concentration of a pollution in a limited space as a room depends on the amount of polluted material spread in the room, the supply of fresh air, the outlets position and construction, the principles used for supply and outlet from the space, the fluid flow in the space .

The concentration of a pollution in a limited space as a room, container, tank etc. depends on:

  • amount of polluted material spread in the room or space
  • amount of fresh supply air or fluid
  • the outlet positions and constructions
  • the principles used for supply and outlet from the space
  • fluid flow in the space

The concentration of a pollution can be calculated like:

c = q / n V (1 - e-nt) + (c1 - c2) e-nt + c2

Where: c = concentration in the space at perfect mix (m3/m3) (kg/kg)

q = amount of pollution added to the space (m3/h) (kg/h)

n = volume changes per hour (h-1)

V = volume or mass of the space (m3) (kg)

e = the Number = 2.718281828459045235..

t = time (h)

c1 = concentration in the space at start (m3/m3) (kg/kg)

c2 = concentration in the supply fluid (m3/m3) (kg/kg)

If the start concentration (t = 0) in the space and the concentration in the supply fluid is zero, equation can be reduced to

c = q / n V (1 - e-nt)

Note: After some time the concentration in the room will stabilize. The table below shows the value of c when the amount of pollution q = 1, and the volume of the space V = 1.

Table.3.18
Pollution Concentrations

3.14.5 Persons and Metabolic Heat Gain

Metabolic heat gain from occupants in air conditioned spaces at different degrees of activities - in Watts

The table below indicates the sensible and latent heat from people. The values can be used to indicate the heat loads handled by air conditioning systems.

Note that the values are based on older ISO and ASHRAE standards. Later ISO and ASHRAE standards should be checked for updated values.

Table.3.19
Persons and Metabolic Heat Gain

3.14.6 Area per Person

Common area per person in buildings - values can in general be used calculating indoor climate loads

The table below can be used as a guide to what is often regarded as required area (square meter or square feet) per person inside some typical buildings and rooms. The values can also be used to calculate human sensible and latent heat load to indoor climate.

Table.3.20
Area requirement per Person

3.14.7 Clothing, Activity and Human Metabolism

Human metabolism at low and high activity with different levels of clothing.

Table.3.21
Clothing, Activity and Metabolism

3.14.8 Air Change Rate

How to calculate air change rates - air change rate equations in imperial and SI units

Table.3.22
Air Change Rates
Building / Room Air Change Rates - n - (1/hr)
All spaces in general min 4
Attic spaces for cooling 12 - 15
Auditoriums 8 - 15
Banks 4 - 10
Barber Shops 6 - 10
Bars 20 - 30
Beauty Shops 6 - 10
Boiler rooms 15 - 20
Bowling Alleys 10 - 15
Cafeterias 12 - 15
Churches 8 - 15
Clubhouses 20 - 30
Cocktail Lounges 20 - 30
Computer Rooms 15 - 20
Court Houses 4 - 10
Dental Centers 8 - 12
Department Stores 6 - 10
Dining Halls 12 -15
Dining rooms hotels 5
Dress Shops 6 - 10
Drug Shops 6 - 10
Engine rooms 4 - 6
Factory buildings, ordinary 2 - 4
Factory buildings, fumes and moisture 10 - 15
Fire Stations 4 - 10
Foundries 15 - 20
Galvanizing plants 20 - 30
Garages repair 20 - 30
Garages storage 4 - 6
Homes, night cooling 10 - 18
Jewelry shops 6 - 10
Kitchens 15 - 60
Laundries 10 - 15
Libraries, public 4
Lunch Rooms 12 -15
Luncheonettes 12 -15
Nightclubs 20 - 30
Malls 6 - 10
Medical Centers 8 - 12
Medical Clinics 8 - 12
Medical Offices 8 - 12
Mills, paper 15 - 20
Mills, textile general buildings 4
Mills, textile dye houses 15 - 20
Municipal Buildings 4 - 10
Museums 12 -15
Offices, public 3
Offices, private 4
Police Stations 4 - 10
Post Offices 4 - 10
Precision Manufacturing 10 - 50
Pump rooms 5
Restaurants 8 - 12
Retail 6 - 10
School Classrooms 4 - 12
Shoe Shops 6 - 10
Shopping Centers 6 - 10
Shops, machine 5
Shops, paint 15 - 20
Shops, woodworking 5
Substation, electric 5 - 10
Supermarkets 4 - 10
Town Halls 4 - 10
Taverns 20 - 30
Theaters 8 - 15
Turbine rooms, electric 5 - 10
Warehouses 2
Waiting rooms, public 4

Note that in many cases local regulations and codes will govern the ventilation requirements.

The fresh air supply to a room can be calculated as:

q = n V

Where: q = fresh air supply (m3/h)

n = air change rate (1/n)

V = volume of room (m3)

Example - Fresh Air Supply to a Public Library

The fresh air supply to a public library with volume 1000 m3 can be calculated as

Q = (4 1/h) (1000 m3)

= 4000 m3/h

Air Change Rate - SI Units

Air change rate expressed in SI-units:

n = 3600 q / V

Where: r = air change rate per hour

q = fresh air flow through the room (m3/s)

V = volume of the room (m3)

Example - Air Change Rate SI Units

With an air flow of 3 m3/s in a 20000 m3 room the air flow rate can be calculated as

n = 3600 (3 m3/s) / (20000 m3)

= 0.54

3. Calculate Air Supply Temperature

Calculate air supply temperature. Common guidelines:

  • For heating, 38 - 50oC may be suitable
  • For cooling where the inlets are near occupied zones - 6 - 8oC below room temperature
  • For cooling where high velocity diffusing jets are used - 17oC below room temperature

4. Calculate Air Quantity

Air Heating

If air is used for heating, the needed air flow rate may be expressed as:

qh = Hh / ρ cp (ts - tr)

Where: qh = volume of air for heating (m3/s)

Hh =heat load (W)

cp = specific heat capacity of air (J/kg K)

ts = supply temperature (oC)

tr = room temperature (oC)

ρ = density of air (kg/m3)

Air Cooling

If air is used for cooling, the needed air flow rate may be expressed as

qc = Hc / ρ cp (to - tr)

Where: qc = volume of airfor cooling (m3/s)

Hc =cooling load (W)

to= outlet temperature (oC) where to = tr if the air in the room is mixed

Example - heating load:

If the heat load is Hh = 400 W, supply temperature ts = 30 oC and the room temperature tr = 22 oC, the air flow rate can be calculated as:

qh = (400 W) / (1.2 kg/m3) (1005 J/kg K) ((30 oC) - (22 oC))

= 0.041 m3/s

= 149 m3/h

Moisture

If it is necessary to humidify the indoor air, the amount of supply air needed may be calculated as:

qmh = Qh / ρ (x2 - x1)

Where: qm = volume of air for humidifying (m3/s)

Qh = moisture to be supplied (kg/s)

ρ = density of air (kg/m3)

x2 = humidity of room air (kg/kg)

x1 = humidity of supply air (kg/kg)

Dehumidifying

If it is necessary to dehumidify the indoor air, the amount of supply air needed may be calculated as:

qmd = Qd / ρ (x1 - x2)

Where: qmd = volume of air for dehumidifying (m3/s)

Qd = moisture to be dehumified (kg/s)

Example - humidifying

If added moisture Qh = 0.003 kg/s, room humidity x1 = 0.001 kg/kg and supply air humidity x2 = 0.008 kg/kg, the amount of air can expressed as:

qmh = (0.003 kg/s) / (1.2 kg/m3) ((0.008 kg/kg)- (0.001 kg/kg))

= 0.36 m3/s

Alternatively the air quantity is determined by the requirements of occupants or processes.

5. Temperature loss in ducts

The heat loss from a duct can be expressed as:

H = A k ( (t1 + t2) / (2 - tr) )

Where: H = heat loss (W)

A = area of duct walls(m2)

t1 = initial temperature in duct (oC)

t2 = final temperature in duct (oC)

k = heat loss coefficient of duct walls (W/m2 K) (5.68 W/m2 K for sheet metal ducts, 2.3 W/m2 K for insulated ducts)

tr = surrounding room temperature (oC)

The heat loss in the air flow can be expressed as:

H = q cp (t1 - t2)

Where: q = mass of air flowing (kg/s)

cp = specific heat capacity of air (kJ/kg K)

Combining H = A k ( (t1 + t2) / (2 - tr) ) & H = q cp (t1 - t2)

H = A k ((t1 + t2) / 2 - tr)) = q cp (t1 - t2)

For large temperature drops should logarithmic mean temperatures be used.

6. Selecting Heaters, Washers, Humidifiers and Coolers

Units as heaters, filters etc. must on basis of of air quantity and capacity be selected from manufactures catalogues.

7. Boiler

The boiler rating can be expressed as:

B = H (1 + x)

Where: B = boiler rating (kW)

H = total heat load of all heater units in system (kW)

x = margin for heating up the system, it is common to use values 0.1 to 0.2

Boiler with correct rating must be selected from manufacturer catalogues.

8. Sizing Ducts

Air speed in a duct can be expressed as:

v = Q / A

Where: v = air velocity (m/s)

Q = air volume (m3/s)

A = cross section of duct (m2)

Overall pressure loss in ducts can be expressed as:

dpt = dpf + dps + dpc

Where: dpt = total pressure loss in system (Pa, N/m2)

dpf = major pressure loss in ducts due to friction (Pa, N/m2)

dps = minor pressure loss in fittings, bends etc. (Pa, N/m2)

dpc = minor pressure loss in components as filters, heaters etc. (Pa, N/m2)

Major pressure loss in ducts due to friction can be expressed as

dpf = R l

Where: R = duct friction resistance per unit length (Pa, N/m2 per m duct)

l = length of duct (m)

Duct friction resistance per unit length can be expressed as

R = λ / dh (ρ v2 / 2)

Where: R = pressure loss (Pa, N/m2)

λ = friction coefficient

dh = hydraulic diameter (m)

Recommended Relative Humidity in Production and Process Environments

Recommended Relative Humidity - RH - in production and process environments like libraries, breweries, storages and more.

To avoid damage of products, or to achieve proper process conditions, it is often important to keep the environment and the indoor climate within certain temperature and humidity limits.

Low relative humidity may dry up the product, or high relative humidity may increase the water activity growing mould in the production process lines.

The table below can be used as a guidance to recommended Relative Humidity - RH - in some common production and process environments.

Table.3.23
Recommended Relative Humidity
Production and Process Environment Recommended Relative Humidity - RH - (%)
Sugar Storage 20 - 35%
Breweries 35 - 45%
Coffee Powder 30 - 40%
Milk Powder Storage 20 - 35%
Seed Storage 35 - 45%
Unpacked Medicine 20 - 35%
Transformer Winding 15 - 30%
Semiconductors 30 - 50%
Books and Paper Archive 40 - 55%
Paper Storage 35 - 45%
Preventing Rust and Corrosion below 55%,
< 40% for no rust generation
Library 50 - 55%
Spray Paint 30 - 50%
Laboratory electronics 45 - 60%
Plastic Pallets 5 - 30%
Computer Peripherals 50 - 60%
Rust Resistance Below 40%
Medical Syrups 30 - 40%
Capsule Storage 30 - 45%
Powder Storage 30 - 45%
Wood Drying 25 - 35%
Explosives 35 - 50%
Note! low RH may generate static electricity and spark ignitions
Normal Storage 50 - 55%
Musical Instrument 45 - 55%
Leather Product 40 - 55%
Cable Wrapping 15 - 25%
Chemical Laboratory 30 - 45%

3.14.9 STP - Standard Temperature and Pressure & NTP - Normal Temperature and Pressure

The definition of STP - Standard Temperature and Pressure and NTP – Normal Temperature and Pressure

Since temperature and air pressure may vary form place to place it is necessary with standard reference conditions for testing and documentation of chemical and physical processes.

Note: There is a variety of alternative definitions for the standard reference conditions of temperature and pressure. STP, NTP and other definitions should therefore be used with care. It is always important to know the reference temperature and reference pressure for the actual definition used.

STP - Standard Temperature and Pressure

STP is commonly used to define standard conditions for temperature and pressure which is important for the measurements and documentation of chemical and physical processes:

  • STP - Standard Temperature and Pressure - is defined by IUPAC (International Union of Pure and Applied Chemistry) as air at 0oC (273.15 K, 32 oF) and 105 pascals (1 Pa = 10-6 N/mm2 = 10-5 bar = 0.1020 kp/m2 = 1.02x10-4 m H2O = 9.869x10-6 atm = 1.45x10-4 psi (lbf/in2))
  • STP - commonly used in the Imperial and USA system of units - as air at 60 oF (520 oR) and 14.696 psia (15.6oC, 1 atm)
  • Note that the earlier IUAPC definition of STP to 273.15 K and 1 atm (1.01325 105 Pa) is discontinued.

NTP - Normal Temperature and Pressure

NTP is commonly used as a standard condition for testing and documentation of fan capacities:

  • NTP - Normal Temperature and Pressure - is defined as air at 20oC (293.15 K, 68oF) and 1 atm ( 101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 29.92 in Hg, 760 torr). Density 1.204 kg/m3 (0.075 pounds per cubic foot)

SATP - Standard Ambient Temperature and Pressure

SATP - Standard Ambient Temperature and Pressure is also used in chemistry as a reference:

  • SATP - Standard Ambient Temperature and Pressure is a reference with temperature of 25 degC (298.15 K) and pressure of 101 kPa.

ISA - International Standard Atmosphere

ISA - International Standard Atmosphere is used as a reference to aircraft performance:

  • ISA - International Standard Atmosphere is defined to 101.325 kPa, 15 degC and 0% humidity.

ICAO Standard Atmosphere

Standard model of the atmosphere adopted by the International Civil Aviation Organization (ICAO):

  • Atmospheric pressure: 760 mmHg = 14.7 lbs-force/sq inch
  • Temperature: 15oC = 288.15 K = 59oF

4


Heating and Cooling Load Calculation Procedure

Objectives

After reading this chapter, a student should be able to:

  • Understand various factors involved in heating and cooling load calculation process
  • Understand the necessity of this calculation for further selection of air conditioning equipments
  • Carry out calculations for designing an air conditioning system on his own

4.1 General

For air conditioning a space or a room, there are basically two parameters has to be taken into consideration:

(a) Temperature inside the space

The temperature inside must be lower than the surrounding temperature. Naturally, the heat flows from a higher temperature to lower temperature and in order to maintain a lower temperature inside the space or room, heat transfer takes place from outside to inside, through various means.

(b) Humidity or Moisture level inside the space

Moisture content must be maintained at a lower level than the atmospheric level.

In hot summer, the surface temperature is higher than that of the body, so there is no possibility of flow of heat from the skin to the surroundings, thus the persons feel hot. At this situation, the water from the body evaporates at the skin surface dissipating the heat due to metabolism. This helps to maintain normal body temperature. But if the surrounding air is not only hot but highly humid as well, very little evaporation of water can take place from the skin surface, and so the person feels hot and uncomfortable.

The heat emanating from the solar radiation, affect the outside walls and roofs and end up in transferring heat to the inside space.

Surrounding air infiltrates into the space conditioned, whenever the doors are open as well as small cracks and gaps in the doors and windows and in the wall of the building.

A small portion of the fresh air is taken inside for the ventilation purposes for the comfort of the personnel working.

As we know, while laying air conditioning ducts and pipelines for air, water and steam etc, certain amount of heat is also picked up from the non-conditioned space adds heating load inside the room

As the human always sweat, machines and equipments release heat, lights inside the building emit heat to indoor, all of them adds up heat load inside.

Considering the above facts, for the purpose of calculation, we look at few terms and definition relevant to this process

4.2 Definitions

4.2.1 Air Conditioning

A process which heats, cools, cleans and circulates air and controls its moisture content. This process is done simultaneously and all year round.

4.3 Design Considerations

4.3.1 Design Conditions

Outdoor Air and Indoor Air Temperature (Dry bulb and Wet Bulb), Humidity, Moisture Content.

4.3.2 Orientation of building

Location of the space to be air conditioned with respect to:

  • Compass points - sun and wind effects
  • Nearby permanent structures - shading effects
  • Reflective surfaces - water, sand, parking lots, etc.

4.3.3 Dimensions of space

Length, width, and height-Ceiling height, Floor to floor height, floor to ceiling, clearance between suspended ceiling and beams.

4.3.4 Construction materials

Materials and thickness of walls, roof, ceiling, floors and partitions and their relative position in the structure.

4.3.5 Surrounding conditions

Exterior color of walls and roof, shaded by adjacent building or sunlit. Attic spaces - vented or un-vented, gravity or forced ventilation. Surrounding spaces conditioned or unconditioned-temperature of non-conditioned adjacent spaces, such as furnace, boiler room, kitchen etc. Floor on ground basement etc.

4.3.6 Doors and Windows

Doors-Location, type, size and frequency of use.
Windows-Size and location, wood or metal sash, single or double hung. Type of glass - single or multi-pane. Type of shading device. Dimensions of reveals and overhangs.

4.3.7 People

Number, duration of occupancy, nature of activity, any special concentration. At times, it is required to estimate the number of people on the basis of square meter per person or on average traffic.

4.3.8 Lighting

Wattage at peak. Type - incandescent, fluorescent, recessed, exposed. If the light are recessed, the type of air flow over the lights, exhaust, return or supply, should be anticipated. At times, it is required to estimate the wattage on a basis of watts per square meter, due to lack of information.

4.3.9 Motors

Location, name plate and brake horsepower and usage. The latter is of great significance and should be carefully evaluated. It is always advisable to measure power input where possible. This is especially important in estimates for industrial installations where the motor machine load is normally a major portion of the cooling load.

4.3.10 Appliances, business machines, electronic equipment

Locations, rated wattage, steam or gas consumption, hooded or un-hooded, exhaust air quantity installed or required, and usage. Avoid pyramiding as not all machines will be used at the same time.

Electronics equipment often requires individual air conditioning – the manufacturer’s recommendation for temperature and humidity variation must be followed.

4.3.11 Ventilation

Cubic meter/minute per person, cubic meter / minute, scheduled ventilation. Excessive smoking or odours, code requirement, exhaust fans-type, size, speed, cubic meter/minute delivery.

4.3.12 Thermal storage

Operating schedule (12, 16 or 24 hours per day, specifically during peak outdoor conditions, permissible temperature swing in space during design day on floor, nature of surface materials enclosing the space.

4.3.13 Continuous or intermittent operation

Whether system be required to operate every business day during cooling season, or only occasionally, such as ballrooms and churches. If intermittent eg. churches, ballrooms, determine duration of time available for pre-cooling or pull-down.

4.4 Internal Sensible and Latent Heat Load Components

A. Load components can be divided into two (2) types:

  1. SENSIBLE LOAD results when heat entering the conditioned space that causes dry bulb temperature (DB) to increase.
  2. LATENT LOAD results when moisture entering the space causes the humidity to increase.

B. A load component may be all sensible, all latent, or a combination of the two.

C. The effective room total heat (Room Sensible Heat + Latent Heat) which determines the quantity and temperature-Humidity condition of the supply air from Air Handling units.

D. Grand Total heat (ERTH + the load due to fresh air intake, chilled water system, pump’s hp etc.), which determines the capacity of the refrigeration plant.

There are few terms and ratios which we often come across in heat load calculations:

(d) Apparatus Dew Point (ADP)

The supply air coming out of the cooling coil depends upon the surface temperature of the cooling coil and must be controlled to get the desired conditions.

This effective surface temperature is called “Apparatus Dew Point” (ADP).

(e) Bypass Factor

It is the ratio of the quantity of the bypass air, which escapes through the gaps between the fins and tubes of the cooling coil and to that of the total air passing through the coil is called “Bypass Factor”.

4.5 Comfort Air Conditioning

4.5.1 Outside & Inside design conditions

The temperature, relative humidity and air movement within the conditioned space, have to be maintained in such a way that the dissipation of the heat due to body metabolism is steady, to maintain the normal body temperature with comfort.

Experiments show that the various combinations of temperature, relative humidity and air movement can create the same amount of feeling of comfort. With the fixed air movement, the comfort conditions can be obtained with different combinations of temperature and relative humidity.

For example,

  1. If the temperature is raised, a reduction in relative humidity can give the same comfort feeling (Winter Heating)
  2. If the relative humidity is raised, a reduction in temperature can give the same comfort feeling.(Summer Cooling)

Effective Temperatures for Comfort air-conditioning

Unless otherwise specified the inside condition shall be(Summer Cooling):

24° ± 1°C, 50 to 60% RH, 4.5 to 7.5m/min

The inside condition shall be (Winter Heating):

20° ± 1°C, 35 to 40 % RH

4.6 Heat Gain Classification

4.6.1 Room Sensible Heat Gain-RSH (External + Internal)

The external loads considered are:

Solar Heat Gain through Glass

Sun rays entering windows

When solar radiation falls on glass and other partially transparent material some of the incident energy is reflected, some is absorbed by the material, due to the latent heat addition of those materials and the rest is transmitted to the inside of the building.

The windows may have plain, colored and tinted glass and the value heat radiation calculated based on the heat load per unit area of the window glass.

The heat gain = A x R x MF

Where:

A = Area of glass, in m2

R = Solar gain in kcal/h/m2 (Refer Table1)

MF = Multiplying factors for the type of glass, shading etc.(Refer Table:2)

Table 1: Peak Solar heat gain thought ordinary glass "R" Value
Table 2

Solar Heating – Transmission gain through the walls and roofs

Sun rays striking walls and roofs

Due to absorption of heat by solar radiation, the temperature of the wall and the roof rises above the ambient temperature.

The heat gain = A x U x EqTD

Where:

A = Area of the wall or roof in m2

U = Transmission coefficient in kcal/h/m2/°C (Table-3&4)

EqTD = Corrected equivalent temperature difference in °C

Table 3:
Table 4: Roofs-Heating transmission Coefficient,"U"

Transmission gain through glass, partitions and floors

The air temperature outside the conditioned space

The heat gained by the partition walls / glass or plywood partitions inside the conditioned space.

The heat transmission also takes place between two adjacent rooms of different temperature.

The heat gain = A x U x TD

Where:

A = Area of the glass / partitions in m2

U = Transmission coefficient for glass/ partitions in kcal/h/m2/°C(Table-5)

TD = Temperature difference between the surroundings and the conditioned space °C

Table 5: "U" Values for Floor, ceiling, Partition and Door

Infiltration

The wind blowing against a side of the building. Surrounding air infiltrates into the space conditioned, whenever the doors are open as well as small cracks and gaps in the doors and windows and in the wall of the building.

The heat gain due to infiltration= Qi x 60 x TD x ρ x C

Where:

Qi = Infiltration air quantity in m3/min

TD = Temperature difference between the surroundings and the conditioned space °C

ρ = Density of air (1.2 Kg/m3)

C = Specific heat of air (0.24)

Table 6: Coil Bypass Factors

By-passed Fresh Air

Outdoor air usually required for ventilation purposes

The heat gain = QV x BF x 60 x TD x ρ x C

Where:

QV = Bypassed air quantity in m3/min

BF = Coil bypass factor (Table-6)

TD = Temperature difference between the surroundings and the Conditioned space

ρ= Density of air (1.2 Kg/m3)

C = Specific heat of air (0.24)

Internal Heat Gain Loads

The Internal loads considered are:

People Occupancy

The heat generated within the human body by the metabolism. The metabolic rate varies with the type of activity of the individual person. The heat gain considered for both Sensible Heat + Latent Heat.

Sensible heat gain from persons

Sensible heat gain= N x SP

Where:

N = Number of persons occupying in the conditioned space

SP = Sensible heat per person, in Kcal/hr (See the standard table provided for SP and LP)

Latent heat gain from persons

Latent heat gain = N x wP x h

Where:

N = Number of persons occupying in the conditioned space

wP = Moisture released per person per hour, in Kg/person

h = latent heat of condensation of moisture, in kcal/kg (See the standard table provided for SP and LP)

Table 7: Persons and Metabolic Heat Gain

Lights

The heat load contributed by the lights to the sensible heat only. Some of the standard lighting and the load contributed by them is given below:

  1. Fluorescent Light -- Total watts x 1.25 x 0.86 kcal/hr.
  2. Incandescent light -- Total watts x 0.86 kcal/hr.

We can also use the standard tabulated values for different lightings, (see table)

Fan H.P in draw-thru systems

To calculate the heat gain from fans:

The heat gain from Fans =N x BHP x f

Where:

N = Number of fans in the system

BHP = Brake horse power of the fans, in KW

f = A constant , 641 to get kcal/h

Appliances and others

Some of the appliances give off both Sensible heat and latent heat, like cooking, drying, etc

The heat gain from appliances and others =W x fw

Where:

W = Watts of each appliances

fw = 0.86 to get in kcal/hr.

Table 8: Appliances Sensible and Latent Heat load

Safety factor-5% of room sensible heat gain

An additional of 5% on Room Sensible Heat (RSH) is taken as a safety factor, in order to account for heat gain from supply duct, leakages etc.

4.6.2 Effective Room Sensible Heat Gain (ERSH)

The Effective room sensible heat gain is the sum of the following, which are calculated earlier:

(a) Sensible Heat Gain

Solar heat gain through glass

Solar Heating – Transmission gain through the walls and roofs

Transmission gain through glass, partitions and floors

Infiltration

Bypassed fresh air

Internal Heat Gain Loads (People Occupancy, Lights, Fan H.P in draw-thru systems, Appliances and others)

(b) Safety factor

5% of (a) sensible heat gain

Therefore,

Effective Room Sensible Heat gain (ERSH) = (a) Sensible Heat Gain + (b) Safety Factor

4.6.3 Room Latent Heat Gain (RLH)

Room Latent Heat (RLH) consists of the following:

Infiltration

Infiltration latent heat gain = Qi x 60 x (wo – wi) x ρ x h

Where:

Qi = Infiltration air quantity in m3/min

(wo – wi) = Difference between outside air &inside air moisture conditions, in g/kg

ρ= Density of air (1.2 Kg/m3)

h = Latent heat of condensation of moisture, in Kcal/Kg

Outside air (By-passed air)

The latent heat removal for condensation of excess moisture from the bypassed fresh air forms part of the room latent heat gain.

The latent heat gain = QV x 60 x BF x (wo – wi) x ρ x h

Where:

QV = Fresh air quantity in m3/min.

BF = Bypass factor of the cooling coil

(wo – wi) = Difference between outside air &inside air moisture conditions, in g/kg

ρ= Density of air (1.2 kg/m3)

h = Latent heat of condensation of moisture, in Kcal/Kg

People (Occupancy)

The moisture released by the occupants due to various activities, has to be condensed. Refer table for values

The latent heat gain = N x w x h

Where:

N = Number of persons occupied in the space or room

w = Moisture released per person in g/person

h = latent heat of condensation of moisture in Kcal/ gram

Table 9
Apparatus Dew Points

Appliances

In comfort application, the steam appliances like water kettle, steam cookers etc. In commercial applications, the steam used from the boilers in steam heating process has to be taken into account.

The latent heat gain = WS x hS

Where:

WS = Moisture generated per hour from steam appliances in grams

hS = latent heat of steam, in Kcal/Kg

Therefore, the sum of room latent heat gain

(c) Infiltration+ Outside air (By-passed air)+ People (Occupancy)+ Appliances

Safety factor-5% of room latent heat gain

An additional of 5% on Room Latent Heat (RLH) is taken as a safety factor, in order to account for Effective Room Latent Heat Gain

(c) x 5/100 = (d) Safety factor

4.6.4 Effective Room Latent Heat Gain (ERLH)

Sum of the following:

Room Latent Heat Gain + Safety Factor

4.6.5 Effective Room Total Heat (ERTH)

The effective room total heat is the sum of the following:

Effective room sensible heat gain (ERSH) and Effective room latent heat gain (ERLH)

4.6.6 Outside Air Heat gain (Ventilation Air)

Consider a 25% of total air quantity is used for ventilation purpose.

Sensible Heat Gain = QV x 60 x C x (1-BF) x ρ x TD

Where:

QV = 25% of total air quantity in m3/hr

BF = Coil bypass factor

TD = Temperature difference between the surroundings and the conditioned space °C

ρ = Density of air (1.2 Kg/m3)

C = Specific heat of air (0.24)

Latent Heat Gain = QV x (1-BF) x 60 x (wo – wi) x ρ x h

Where:

QV = 25% of total air quantity in m3/hr

BF = Coil bypass factor

(wo – wi) = Difference between outside air & inside air moisture conditions, in g/kg

ρ = Density of air (1.2 Kg/m3)

h = Latent heat of condensation of moisture in Kcal/ Kg

4.6.7 Return Duct Heat Gain (Fan Horse power heat gain)

= 641 x BHP of fan in Kcal/hr.

4.6.8 Grand Total Heat

The Grand Total Heat is the sum of the following:

Effective Room Total Heat (ERTH)

Outside Air Heat gain (Ventilation Air)

Return Duct Heat Gain (Fan Horse power heat gain)

The air quantity that has to be handled by the air handling equipment is calculated based on the following.

4.6.9 Air quantity handled by the Air-Handling Unit (AHU)

The air quantity into the space to be conditioned should offset the room sensible and latent heat load. It should also handle the total sensible and latent heat loads, i.e, including the outdoor (fresh) air loads, etc.

Air quantity (dehumidified supply air) based on Heat load

Where:

Qda = Dehumidified air quantity in m3/min.

ERSH = Effective room sensible heat in Kcal/hr

trm = Room design temperature in °C

tadp = Apparatus dew point temperature in °C

BF = Bypass factor of the coil

ρ = Density of air (1.2 kg/m3)

C = Specific heat of air (0.24)

4.7 Miscellaneous heat sources

4.7.1 Duct heat gain

The heat gain through the ducts is calculated by using the equation;

Q = Ad × U { Ta – [ ( Ti + To )/ 2 ] }

Where:

Ad is the area of the duct.

U is the overall heat transfer coefficient.

Ta is the ambient air temperature.

Ti is the air temperature entering the duct.

To is the air temperature leaving the duct.

This calculated heat gain of the duct running in an air-conditioned space is to be added to the sensible heat load of the space.

The heat gain in the return air duct need not be added to the sensible heat load of the space, but needs to be added to the total system load.

4.7.2 Fan load

The power consumed by the fan is converted into heat which is transferred to the air. If the fan is located before the air conditioner then the energy is added to the total load and if the fan is located after the air conditioner then the heat is added to the room’s sensible heat load.

4.8 Fresh air load

Fresh air is supplied to the air-conditioned room to reduce the concentration of odour and smoke and simultaneously to maintain the minimum required level of oxygen.

The quantity of heat removed by the cooling coil is calculated from the psychrometric chart and the equations are given as:

Sensible heat removed from fresh air per kilogram of dry air;

= ( hc – hb )

Latent heat removed from fresh air per kilogram of dry air:

= ( ha – hc )

Where:

ha is the heat content in the outside air

hb is the heat content in air leaving the conditioner and entering the room.

hc is the heat content in the air leaving the room.

Figure 4.4
Fresh air load
Figure 4.5
Fresh air load (Saturated)

4.9 Design of air-conditioning system

The following figure shows the heat flow system for a room and an air-conditioner.

Figure 4.6A
Heat flow system schematic

The air at the outlet of the conditioner enters the room at condition ‘a’ and leaves the room at condition ‘b’ after removing the heat Qs and Ql from the room.

The air condition at ‘a’ must satisfy the following equation:

QS = m× CS (Tb – Ta)

Note: CS is the heat capacity at constant pressure

m = mass of the substance in kg

(Tb – Ta) = Temperature difference in °C

QL = m×Cw(wb – wa)

Note: Cw is the heat capacity of moisture

m = mass of the substance in kg

(wb – wa) = change in moisture content in g/kg

When the quantity of air required is known, the condition of air at ‘a’ can be calculated from the psychrometric chart as shown in the following figure.

Figure 4.6B
Condition of air which enters room at ‘a’

The condition of air entering the room can be calculated from the following equation;

Qt = ( Qs + Ql ) = m ( hb – ha )

The value of hb is known according to the required comfort condition in the room. If (Qs+ Ql) is known then ha can be calculated from the above equation.

The Sensible Heat Ratio ( SHR ) is defined as:

SHR = Qs / ( Qs + Ql )

The high value of SHR indicates low latent heat load and low value of SHR indicates high latent heat load. The common values of SHR for comfort air-conditioning purposes range between 0.6 to 0.8

The capacity of the conditioner is given by the equation;

Capacity = { ( hb – ha ) } / 3.5 (kW cooling)

4.9.1 Cooling load and air quantities

The air circulated in the room should be adequate to take the load from the air-conditioned room. The condition of the air entering the room (DBT) should be able to take the load and follow the path along the room SHR line. The lower the supply temperature, the lesser is the quantity of air required for circulation. The minimum temperature of supply air required is determined by the arrangement of the air-conditioned system. Air-conditioned installations for summer season are commonly designed to supply air at 10oC below the comfort temperature exit conditions.

The different types of air conditioning arrangements are discussed below:

All fresh air used:

Figure 4.7A
All fresh air used schematic
Figure 4.7B
All fresh air used-Psychrometric chart

The above arrangement shows that there is no re-circulation of conditioned air and all fresh outdoor air is used. This arrangement is highly uneconomical unless the outdoor conditions are very close to the required inside conditions.

C is the condition of outside air.

B is the condition of room outlet air.

Draw the room SHR line through point ‘b’ which cuts the saturation line of the psychrometric chart at point ‘e’ which is the room dew-point temperature. Point ‘d’ is referred to as the apparatus dew-point. Joining line ‘cd’ cuts the line ‘eb’ at point ‘a’, which is the condition of air entering the room.

4.9.2 Re-circulated air type arrangement

Figure 4.8A
Part recirculated air used
Figure 4.8B
Part recirculated air used-Psychrometric chart

When fresh air at condition ‘c’ is mixed with the room outlet air at condition ‘b’ added the resulting condition of the air becomes ‘f’. The rest of the process of finding the condition of air entering the room remains the same as above.

Re-circulated air with re-heat coil

If the latent heat load of the room is high enough, it is necessary to cool the air below the required air temperature at the inlet of the room to remove the moisture. Under this condition, re-heating of the air leaving the conditioner is necessary.

Figure 4.8C
Recirculated air with re-heat coil

Re-circulated air used for heating the air coming out of the conditioner:

Figure 4.9A
Re-circulated air with/without bypass
Figure 4.9B
Re-circulated air with/without bypass-Psychrometric chart

Line ‘da’ on the psychrometric chart represents the conditioner line and ‘eb’ represents the room SHR line, which cuts line ‘da’ at point ‘c’. Point ‘c’ represents the air leaving the conditioner coil. The quantity of air at condition ‘b’ is mixed with the quantity of air at condition ‘c’ in a proportion such that the resultant air condition is ‘f’ as represented in the chart. In this arrangement the quantity of re-circulated air required is considerably high (80 to 85 %) compared with other arrangements.

The amount of air passing through the room is given by the equation:

Ma = Qt / ( hb – hf )……….where

Qt is the total room load in kJ/sec.

4.10 By-pass factor (BF) consideration

The by-pass factor is a function of the physical and operating characteristics of the cooling or heating coil. The parameters that affect by-pass factor (BF) are listed below:

  • Smaller heat transfer surface area of the coil results in an increase in the by-pass factor.
  • Increase in the air velocity increases the by-pass factor as the air remains in contact with the coil for a lesser time.

The following table gives suggested by-pass factors for various applications:

Table 10
Bypass factors
Type of application By-pass factor Example
A small total load 0.3 to 0.5 Residence
Comfort application with relatively small total load. 0.2 to 0.3 Residence, factory, Retail shop
Typical comfort application 0.1 to 0.2 Dept. store, banks
Applications with high internal sensible load. 0.05 to 0.1 Restaurants
All outdoor Air 0 to 0.1 Hospital operating theatre, Intensive Care Units

4.10.1 Position of the fan for considering its load

All of the energy added to the fan is to be accounted for in the load. If the motor is in-track of the air-stream then the motor losses need to be added as well. See ASHRAE Pocket Guide Page 95 for more details.

The following figures show two different positions of the fan.

Figure 4.10
Positioning of fan before cooler

As shown in the above figure, when the fan is located before the cooler, the heat of the fan motor is given to air and the heating load will be undesirably taken by the cooler.

On the other hand if the blower is located after the cooler as shown below, the blower discharge capacity will be reduced as it has to handle the air at lower temperature. Also the heat of the fan motor given to the air will reduce the heat load of the heater.

Figure 4.11
Positioning the blower after the cooler

4.10.2 Cooling coils

The cooling coils used in air conditioning systems are mainly of two types:

Direct expansion coils

In direct expansion coils, the refrigerant from the expansion valve is directly expanded in the coil used for cooling. The arrangement of heat transfer from the air to the refrigerant may be parallel, counter or cross-flow. In most of the applications the coils used are provided with fins for increasing the effective heat transfer.

Secondary refrigerant coils

Secondary refrigerant coils carrying chilled water are used for large air conditioning systems. In this system, the water or brine is cooled in the evaporator of the refrigeration circuit in a centralized place and then distributed for further cooling of air used for air conditioning purposes.

4.10.3 Dehumidifying air-washers

The common type of air washer used for cooling and dehumidification is shown in the following figure.

Figure 4.12
Dehumidifying air washer schematic

The evaporator of the refrigeration circuit chills the water circulated through it. The chilled water is then sprayed in the path of the airflow. The temperature of water required is below the dew point temperature of the air for positive dehumidification. These kinds of air washers are commonly used in industries like textile and printing.

If humidity control is important it is done by changing the spray water temperature.

The condition of air passing through the air washer is illustrated by the following figure.

Figure 4.13
Condition of air passing through air washer

‘a’ = Condition of air entering the air washer.

‘b’ = Condition of air leaving the air washer.

‘c’ = Condition of air water entering the air washer.

‘d’ = Condition of water leaving the spray chamber.

‘Ma’ = Quantity of air in kg/min

‘Mw’ = Quantity of water in kg/min.

The energy balance between air and water is expressed as:

Ma ( ha – hb ) = Cpw×Mw(Td – Tc )

This type of air washer performs the reverse duty of a cooling tower. Air washers using hot water give the same effect as cooling towers. The effectiveness of air washers in terms of enthalpy is given by the equation:

E = ( ha – hb ) / ( ha – hd )

Point ‘d’ shown in the above chart is the outlet condition of air if the air washer is 100% effective.

The effectiveness of air washers can be increased either by increasing the contact area of air and water or be decreasing the velocity of air.

The disadvantages of air washers compared to cooling coils:

  • The size of the air washer per ton of refrigeration is 2 to 3 times more than cooling coils.
  • It is necessary to use cooled brine instead of water in air washers when the dew-point temperature of air is below zero deg C. The possibility of corrosion of air washer parts is greater with brine.

5


HVAC systems

Objectives

After reading this chapter, the student should be able to

  • Understand the various HVAC systems
  • Explain the various heating systems
  • Explain the various cooling systems in an HVAC system
  • Describe the arrangement of all components in an HVAC system

5.1 Heating systems

In many parts of the world, during winter, it becomes necessary to heat the houses by supplying heat from external sources to maintain the comfort conditions for the occupants. To maintain the required comfort conditions heat supplied to the conditioned room should be equal to the heat lost to the atmosphere.

The heating systems require certain essential elements such as:

  • A fluid for conveying heat.
  • A combustion chamber where fuel is burned and heat is given to the circulation fluid.
  • Piping network for conveying the heated fluid to the conditioned spaces.
  • Radiators or convectors for dissipating the heat in the conditioned spaces.

The classifications of heating systems are as follows:

5.2 Warm air heating systems

These systems are further classified into natural circulation systems and forced circulation systems.

5.2.1 Natural circulation system

The figure below shows the arrangement of a common type of natural circulation heating system for small buildings:

Figure 5.1
Warm air heating system with natural circulation

The air is heated in the furnace, becomes lighter and rises through the ducts to the rooms. The air supplied to the room is also partly mixed with fresh air.

The hot air is usually supplied to the rooms at floor level instead of ceiling level as it provides better air circulation. The difference in the density of warm air in the rising ducts and the cold air in the rooms causes a continuous flow of air through the system.

Advantages of warm air heating system by natural circulation are:

  • The capital cost is relatively lower than water or steam heating systems.
  • It is simple to operate.
  • Operating and maintenance costs are lower.
  • Humidification of this system is easier.
  • The room air temperature is adjustable to weather conditions and the re- circulation of air helps in obtaining uniform air temperature.
  • Noiseless system as far as air circulation is concerned.

Disadvantages of warm air heating system by natural circulation are:

  • It cannot be used for big buildings or industries because it works on the pressure difference between the fuel gases and cold air, which is limited.
  • Equal air distribution through the room is not possible.
  • Proper control of humidity is not possible.
  • The use of these systems is limited to ground floor buildings only.

5.2.2 Forced circulation system

This system comprises a fan for circulating air through it. The fan provides necessary power required for circulating the necessary amount of air through the building.

The forced warm heating system is further classified according to the arrangements of the ducting, viz.

Perimeter loop systems

These perimeter loop systems are used for small residences. In this system, return air is taken back from the ceiling and is forced downward over the heating surface. A typical arrangement is shown in the following figure.

Figure 5.2
Bottom warm air system

The heated air from the furnace plenum chamber flows through the radial feeder ducts, which are connected to the single loop perimeter ducts as shown in the figure below. It is further discharged into the space to be heated. The air passing through the feeder ducts heats the floor sufficiently and the perimeter loop heats the edges.

Figure 5.3
Perimeter loop-system

Perimeter radial system

A typical arrangement of this type is shown in the following figure:

Figure 5.4
Perimeter radial system of warm-air heating

The discharge points of heated air are located at the end of each radial feeder. The main disadvantage of this arrangement is the areas in the room corners are likely to cool for human comfort during severe winter conditions.

Blend air system

Figure 5.5
Blend-air heating system

In this system, part of the total air supplied to the rooms comes from the furnace. The hot air coming from the furnace is passed through blending units, which induce a flow of re-circulated air. This re-circulated air mixes with hot air and provides the required temperature of the supplied quantity. The air for each room enters at the top of the blending units.

Advantages of forced circulation system over natural circulation system

  • These systems can be used for multi-storey buildings.
  • Heating systems occupy less space.
  • More effective air-filters can be used for cleaning of air.
  • The duct size required is less, since the required velocities can be achieved by the fan.
  • Less heat transfer surface areas are required.

It is recommended that the temperature of hot air not exceed 70°C for natural circulation systems and 65°C for forced circulation systems.

5.2.3 Humidification of air

During winter, air in the atmosphere contains less moisture. Humidification is always necessary in such conditions.

Most of the air heating systems have evaporators required for humidification. A humidifier used for this purpose should have a larger water surface area exposed to hot air and it should receive sufficient heat through conduction and radiation from the heating system.

The two types of humidifiers generally used are:

  • Pan type
    This is shown in Figure 5.1 and its performance depends upon the surface area exposed to the air.
  • Jet type
    This type of humidifier vaporizes water by breaking it into minute particles. A fan is used for such humidifiers equipped with a humidity control system.

5.3 Sizing heating systems

The surface area provided by heating for heating the air under natural or forced circulation is given by the following equation:

Output from the furnace = 175 × Ah KJ/hr

Where:

Ah is the total heating surface area.

5.4 Hot water heating system

Hot water heating systems are very popular for central heating. Flow of water through the plant is achieved because of the difference in density between hot and cold water. Alternatively, hot water is circulated through the system with a pump.

The following figure shows a typical hot water heating system used for residences.

Figure 5.6
Hot water heating system for residences

In this system, the expansion tank is located at the highest point of the system. In case the water in the system boils, vapor/steam can easily escape to the atmosphere from the expansion tank.

There are mainly two types of piping arrangements used for hot water heating systems. A typical schematic of each of the arrangements is shown in the following figures.

5.4.1 One pipe system

In this system, the hot water supply pipe from the boiler supplies hot water to all the radiators. The same pipe carries back the condensed steam to the boiler. It is very important to know that this pipe should always be sloping towards the boiler.

Figure 5.7
One-pipe system

5.4.2 Two pipe system

In this system, there are two separate pipes for supplying the hot water and returning the condensed steam. The return pipe should always be placed at a lower level.

Figure 5.8
Two-pipe system

All the arrangements shown above are types of gravity flow circulation. The flow through the system is achieved by natural circulation due to the density difference between hot and cold water.

These systems can be converted into forced circulation systems by simply adding a pump in the circuit. In this case, the pump takes care of the circulation of water through the system. In a forced circulation system, the pump needs to be installed in the return line of the boiler to avoid cavitation problems. Also care needs to be taken that the expansion tank is a closed one.

5.4.3 Calculation of heat required for natural water circulation

The pressure difference causing the flow of hot water through the system is given by the equation:

Where

ρP is the net pressure difference causing the flow.

H is the height between the boiler datum and radiator datum.

ρc is the density of return water

ρh is the density of supply water

The head developed should be greater than the head lost due to friction to maintain circulation.

Forced circulation is normally used in large installations where more than one building is supplied hot water from one point. For such systems the capacity of the pump required for circulation is determined by the following equation:

V = Q / {Cpw (Th – Tc) ρ × 60}

Where:

V is the volume of water circulated in liters/min.

Q is the heat delivered in kJ/hr

ρ is the density of water in kg/lit.

Th is the temperature of supply water.

Tc is the temperature of return water.

Cpw is the specific heat of water.

5.4.4 Expansion tank

Like all liquids, water expands with the rise in its temperature. In water heating systems, an expansion tank is necessary to take care of the expanded volume of water. The capacity of the expansion tank depends on the water hold-up of the heating system and the maximum required temperature of water.

In an open tank system, the water temperature can go up to a maximum of 90°C. Water expands by 3.6 %, for a temperature rise from 20°C to 90°C.

In a closed tank system, when the volume of water increases with the increase in temperature, the expanded volume of water enters the expansion tank compressing the air in it. This pressurizes the system increasing the boiling point of water. Due to the increased boiling point of water, such systems can be used where the required water temperatures are above 90°C. Higher operating water temperature permits a higher temperature differential across the radiators. The radiators and the piping can be sized smaller accordingly, giving an economical advantage over natural circulation systems.

5.4.5 Advantages of water heating systems

  • The density of water is higher compared to steam and air; hence, the pipe diameter required is less than that of steam and air heating systems.
  • Leakage problems are not severe compared to steam heating.
  • Handling of water is easier.

5.4.6 Disadvantages of water heating systems

  • The frictional losses in the pipes are more compared to other systems.
  • Heat carrying capacity is less. Heat conveyed by one kilogram of water is 1/15th of the heat carried by one kilogram of steam.
  • Power required to pump the water for the same heat carrying capacity is more than the steam heating systems.

5.5 Steam heating systems

The steam heating system is preferable to other systems because it carries more heat per kilogram of steam compared to that of water heating systems.

The quantity of heat given by the condensation of steam at atmospheric pressure is equivalent to the latent heat of vaporization, which is 2160 kJ/kg of steam. The condensed steam is further sub cooled to obtain an additional heat gain to the system amounting to about 80 to 120 kJ/kg of steam. It is always advisable to supply steam at a pressure of 1.5 to 2 kg/cm2, as pressure has little effect on the heat delivered per kilogram of steam. However, low-pressure steam requires larger pipe sizes and more heat transfer surface area of radiators and convectors.

High-pressure steam dissipates more quantity of heat per square meter of the radiator and requires smaller diameter pipes. However, other costs such as the boiler and pipes, being required to withstand higher pressures are quite high.

The piping and radiators in steam heating systems should be arranged with a view to perform the following three functions successfully:

  • To carry steam to the radiators and convectors
  • To remove air from the system.
  • To carry the condensed steam back to the boiler.

Following are the type of steam heating systems generally used:

  • Two pipe gravity return system.
  • Vapor system.
  • Vacuum system with vacuum pump.

5.5.1 Two pipe gravity return system

In the two pipe system the main headers pitch away from the boilers. Steam passes through the main supply headers to the radiators and returns back to the boiler through the main return header. The steam trap is located at the point where the main supplier header is connected to the main return header to prevent passing of steam from one part to another while allowing condensate to return.

5.5.2 Vapor system

The layout of the vapor system is similar to that of the two-pipe gravity system as stated above. The steam pressure in this system is just above atmospheric pressure, which is required only to maintain a flow. The sub-atmospheric pressure is obtained by condensation of steam in the radiators. This condensate mostly returns to the boiler by gravity. The radiators used with this system are partly filled with steam unless the heat demand is at the maximum. The advantage of this system is that heating is achieved with less thermal potential, which prevents overheating.

Vacuum system with vacuum pump

In this system, the pump provided on the return line of the boiler maintains the pressure in the return line below atmospheric. This helps in preventing air from accumulating in the radiators and ensures uniform heating. Changing the amount of vacuum can vary the rate of flow of steam through the radiator. A thermostatic trap is provided at the outlet of each radiator, which returns the condensate to the main return header and then to the boiler.

A slope given in the return header helps the condensate to flow by gravity to the pump. The function of the vacuum pump is to withdraw the air passed from the piping and radiators. It also draws condensate and discharges back to the boiler.

5.5.3 Advantages of steam heating systems

  • Heat carrying capacity of steam is higher compared to that of water.
  • Precise control of temperature is possible.
  • Pumping power required for steam heating system is less compared to the power required for pumping water carrying the same amount of heat.
  • The required sizes of the radiators and convectors are smaller than water heating systems.

5.5.4 Disadvantages of steam heating systems

  • Since density of steam is less than water, the pipe sizes required are larger than water heating systems.
  • Since the pipes are subjected to higher temperatures, corrosion is one of the major problems.
  • The temperature of steam cannot be varied widely to suit the highly varied room temperature requirements. This limitation results in serious temperature variation in the room.

5.6 Electric heating systems

Electric heating systems are quite common today because they maintain cleanliness and complete control of required temperature.

Special resistance wires are manufactured for electric panel heating and they (wires) are embedded in the ceiling or the walls. The output capacity of an electric heating panel is about 178 w/m2.

5.6.1 Advantages of electric heating systems over conventional ones

  • Electrical energy is the most efficient form of energy to be converted into heat source.
  • Transmission and distribution of heat source produced by electrical energy is also effortless.
  • Virtually, no manpower is required for maintenance and operation of electric heating systems.
  • Easier and efficient control of temperature is possible. It becomes economical to provide individual control for each space to prevent overheating and unnecessary wastage of energy.

5.7 District heating system

District heating systems use a central heating plant for distributing heat to various buildings within a defined area.

Central heating plants are efficient in fuel consumption, require less manpower and are economical to install and operate. The district heating systems were first introduced for distributing heat in the form of steam produced at central steam generating stations. Now-a-days, high-pressure hot water systems are more common, due to the developments in distribution systems.

5.7.1 Methods of distribution

Efficient distribution with minimum heat loss is equally important to the economical production of heat. The system must ensure minimum transmission losses from the generation station to the building by means of proper insulation.

There are mainly two types of distribution system viz. the trunk system and feeder system. The trunk system, which is more common, uses a central trunk, which is of a larger diameter near the plant and decreases in diameter towards its remote end.

The feeder system makes use of the feeder pipes that extend from the plant to the strategic points in the distribution network.

The steam pressure in the distribution system varies based on its application. The low-pressure steam is exhausted from the condenser at a pressure of 0.4 to 0.8 bar and can be used for space heating.

Underground pipes

There are four major points to be considered in the design of underground steam piping.

  • Heat insulation.
  • Protection of pipe from mechanical damage and corrosion.
  • Drainage of condensation.
  • Provision for expansion joints.

The pipes are enclosed within a conduit which permits free expansion (of pipes) and strong enough to withstand the earth pressure.

5.8 Warm air curtains

It is accepted by the management of supermarkets and departmental stores, which sell or provide service to the public, that sales increase considerably if the main doors of the premises are kept open during business hours. In cold weather, in order to prevent the ingress of cold air and to keep the entrance comfortable, warm air curtains are economical to use.

In the arrangement of a simple warm air curtain, air is blown downwards across the length of the entrance from an outlet duct and grill fitted above the entrance. The air processing plant comprising of filter, heater and fan may be located inside the plant room or adjacent to the curtained entrance in the form of a packaged assembly.

The velocity of air within the walking zone must be restricted to an air speed at which discomfort at head level is avoided (less than 1 m/s).

Tests on existing installations have come up with certain standard design data for limiting conditions.

Table 5.1
Air curtain specifications
Parameter Unit Value
Discharge of air quantity M3/m2 of grille 0.56 to 1.4
Discharge air temperature °C 28 to 65
Discharge air speed m/sec 5 to 15
Maximum outdoor temperature °C 5 to 8
Maximum outdoor wind speed m/sec 3 to 5
Height of curtained entrance Meters 2.2 to 3.5

5.9 Air-conditioning systems: General

An air-conditioning system is defined as an assembly of various equipment used to produce a specified condition of air within a required space or building.

An ideal air-conditioning system should maintain correct temperature, humidity, air-purity, air movement and noise level. These are mainly classified as:

  • Central air-conditioning system.
  • Unitary air-conditioning system.
  • District air-conditioning system.
  • Self-contained air-conditioning units.

Another method of classification is by the type of fluid used for heating or cooling. They are:

  • All water system.
  • All air system.
  • Combined system.
  • Heat pump system.

5.9.1 Central air-conditioning system

In central air-conditioning systems, all the components of the system are grouped together in one central room and conditioned air is distributed to the required places through extensive ducts. These systems are used for conditioning loads above 79 KW of refrigeration and 42 m3/sec of conditioned air. Below this load, unitary systems are more economical.

A central plant requires the following components:

  • Cooling and dehumidifying coils
  • Heating coils
  • Fan with motor
  • Sprays for cooling, dehumidifying or washing
  • Air cleaning equipment
  • Controls

The physical size of the coil and filter banks is the limiting factor in the design of central air-conditioning systems. The coil face area of 0.2 m2 for a four row coil is required to handle 0.5 m3/sec of air at the face velocity of 2.5 m/s. Due to larger surface area requirements and compressor sizes, a practical limit of 350 kW capacity or 24m3/s flow of air is adopted for a single unit.

The blowers are manufactured with capacities up to 100 m3/s, but the associated equipment for handling such a huge quantity of air is difficult to fabricate and install.

The central air-conditioning plant handling 20 m3/sec as a limit is generally used for auditoriums and large multi-storey buildings where the ducting can be easily laid down. In office buildings, schools and smaller buildings, air-conditioning units of 5 to 17 m3/sec are more commonly used.

This system may use the following methods to supply conditioned air:

  • Conditioned air from the central plant is supplied to the required space with controlled air discharge.
  • Chilled water from the chiller installed centrally is supplied to air supply units installed in the required space.
  • Individual evaporator in each room with thermostatic flow control can be used.

Advantages

  • The capital and running costs are less per unit of refrigeration.
  • Noise and vibration troubles are less to the people utilizing the conditioned air as the air-conditioning plant is quite far away.
  • Better accessibility for maintenance.

5.9.2 Unitary air-conditioning system

In unitary air-conditioning systems, all the components are factory assembled. These assembled units are usually installed where the refrigeration load is 17 kW or above.

Advantages

  • The installation cost is less.
  • Zoning or extensive ductwork is eliminated.
  • Since these units can be installed locally, they can be switched ON and OFF individually, based on the requirements.
  • Unitary systems can meet the requirement of each room independently, which a central system cannot.

5.9.3 District air-conditioning system

District cooling systems are not as popular as district heating systems. The world’s first district cooling plant, which runs parallel to the district heating system, was installed at New York’s Kennedy Airport in the year 1957.

District cooling plants are best suited for a concentrated load. The larger the air-conditioning plant, the greater is the advantage.

Advantages

  • The capital cost of the equipment is less, since larger units cost less per ton of refrigeration.
  • Individual buildings save in capital cost by eliminating the space requirement in the basements for air-conditioning equipment.
  • Design, construction and control can be optimized with larger machines resulting in efficient working and less operating costs.
  • Since the consumption of fuel and power is high, these can be made available at lower cost.
  • Automatic air-conditioning plant needs less manpower.
  • Maintenance and labor costs are less compared to small plants.
  • Air pollution being an important consideration in heating and cooling systems, efficient smoke control equipment can be installed.

Disadvantages

  • The protection of underground pipes against corrosion and electrolytic action is necessary.
  • The installation cost of the pipes is higher since they are run underground.
  • The operating costs are affected by the increase in pipe length as heat losses increase.
  • The chilled water pumping costs are high.

A district air-conditioning system is generally decided upon centralization of load and controls taking into consideration diversity and future growth requirements.

The minimum capacity of a refrigeration system for district conditioning purposes should be 5 MW of refrigeration. Otherwise, the investment cost for the piping distribution would be so high that it would offset the saving due to lower operating cost. Machines using water as refrigerant and steam as the power source are ideal for such systems.

5.9.4 Self contained air-conditioning units

Self-contained units are available widely and are generally of three types:

Room cooler

These units are small in capacity ranging from 0.25 to 1.5 tons of refrigeration. These are window mounted and air cooled units. The room heat is rejected to outdoor air and the condensers are located outside.

Store coolers

These are also self-contained small units with capacities ranging from 3 to 20 tons of refrigeration. These coolers are water-cooled. A short run of ductwork with built-in grillers is also used to distribute conditioned air to the space.

Residential air-conditioning units

These units are used with various methods, each involving a different arrangement like:

  • A self-contained unit incorporating water-cooled or air-cooled refrigeration system and gas or oil fired heating system.
  • In some self-contained units, the cooling coil is provided in the plenum and the refrigerant to the cooling coil is supplied from a remote air-cooled condensing unit.

Now let us study the various systems classified according to the fluid in the system.

5.9.5 All water system

In an all water system, the air is conditioned i.e. either heated or cooled by using hot water or cold brine before entering the air-conditioned room. This is illustrated in the following figure:

Figure 5.9
A typical all water system

In this system, individual air supply units are installed in each room. These air supply units consist of a heating or cooling coil and a fan to blow air into the conditioned room. The airflow generated by the fan is heated or cooled by flowing hot or cold water through the heating or cooling coil provided in the air supply unit.

Many times, sun exposed zones may need cooling while other zones may need heating. Zoning and proper piping distribution is required to reduce such operational difficulties.

Advantages:

  • The greatest advantage of this system is its flexibility, adapting it for many building module requirements.
  • The fan coil system is one of the lowest capital cost central plant systems.
  • On smaller systems it does not require ventilation air ducts and is easy to install in existing structures.
  • It can incorporate electrical heating.

Disadvantages:

  • Seasonal changeover is required in most climates.

Hydronics heating and cooling systems

Hydronics is the use of water as the heat-transfer medium in heating and cooling systems. Some of the oldest and most common examples are steam and hot-water radiators. In large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning.

The water within the system is neither the source of the heat nor its destination; only its “conveyor belt.” Heat is absorbed by the water at a heat source, conveyed by the water through the distribution piping, and finally released into a heated space by a heat emitter.

Figure 5.10
Typical Hydronic system

In a Hydronics system:

  1. Water cooling is provided by chilled water from chillers and cooling towers.
  2. Water heating is provided by Boilers.

Classification of Hydronic systems

Hydronic systems used by HVAC are classified as follows:

  1. Gravity flow systems or Forced flow systems
  2. Low, medium and high temperature applications
  3. Low, medium and high pressure applications
  4. Pumping arrangements-Single, parallel and series pumps
  5. Piping arrangements-Single, two, three, four and series piping

Gravity flow systems

It is classified as UP-FEED and OVERHEAD systems

When you heat water in a boiler, it will rise up into the pipes because it's lighter that the relatively cold water in the system piping. That colder water, in turn, falls back down into the boiler by gravity.

In an Up-Feed gravity system, hot water from the boiler raises to the top radiator by the principle of “Hot water raises, Cold water sinks”. This is also due to the density difference between the hot and cold water. It is to be noted that no circulating pumps are used in this system. This system uses an Expansion tank, because the hot water expands by 5% approximately. The speed at which the hot water moves up depends on:

Figure 5.11
Typical Gravity Flow Hydronic system
  1. Height of the building and the number of floors, a three storey building is practical to to install up-feed gravity flow system
  2. The pipe sizing-The larger the piping, faster the hot water flow
  3. The temperature of hot water shall be around 80 ° C
  4. The temperature difference between hot and cold water should be around 6 to 10 deg.C
  5. The pipes used should be very smooth inside, to avoid friction

Overhead Flow Systems

In Overhead systems, water goes first to the attic or to a main suspended from the top floor ceiling and then feeds down to the radiators. Because this raiser pipe is very large, it offers less frictional resistance to the water and the hot water moves more quickly from the boiler to the radiators than it would in the upfeed system.

Figure 5.12
Typical Overhead Flow Hydronic System

The plus point of this overhead gravity feed system is that the cooler water pulls the hot water through the radiators as it falls down the return risers pipe. This force counteracts the effects of friction and makes the radiators heat faster.

Like the up-feed gravity system, this overhead gravity feed system also uses an expansion tank in the attic to take care of the extra water due to expansion.

Forced Flow systems

As the name implies, the hot water is forced into the radiators by additional components like Circulators, Feed valves, Pressure reducing valves and a Compression tank.

Figure 5.13
Typical Expansion Tanks in Hydronic System

The forced flow system is a closed hydronic system with cold water and then heat it to a high limit,, the water will expand about 5%, which has to be accommodated somewhere. The compression tank, as per closed system, has air trapped inside the tank about 1/3 the volume and the air get pressurized due to expanding water volume.

How does a Hydronic system transfer heat?

To understand why hydronic heating is so effective it is important to understand how heat transfer takes place. The transfer of heat occurs in three ways:

  1. Conduction – This is the movement of heat through objects that physically touch, where heat moves from the warmer object to the colder. Standing barefoot on a beach with hot sand or a cold kitchen tile floor are good examples of this process. Density affects an objects ability to be conductive, which is why liquids are much better conductors than gases.
  2. Convection – This occurs when fluids or gases transfer heat while they are being circulated from one area to another. Traditional heating systems that use forced air are perfect examples of this type of heat movement.
  3. Radiation – Thermal radiation is heat that travels in invisible waves through empty space. It is not something that can be blown away by the wind or moved. It is simply absorbed by the person or object that is in the path of the beam of energy, and is a far more effective means of transferring heat.

Heating Exchangers and Their Locations

  • Underfloor Radiant Loops – Radiant flooring has become very popular for new home construction, as it is much easier to add into the concrete or floor joist system during the original building of the home. It can provide consistent comfortable heating evenly throughout the home.
  • Baseboard Heaters/Radiators – Hydronic baseboard units and radiators can be much easier to install for home remodels because they require less tubing below the floor. They can be inconspicuously located along the walls of a room, but you should make sure they are not blocked by furniture, so that the heat can radiate out into the room.
  • Walls and Ceilings – Similar to installing a radiant floor as above, wall and ceiling radiant panels are available to be placed behind walls, and can heat a broad area. These panels are most often run with electric radiant heat, as opposed to hydronic, due to water damage possibilities. They can be great additions to residential hydronic heat, especially in spaces where large areas of floor are covered by cabinetry, such as kitchens.

How to Install Hydronic Heating?

There are many different ways to install a residential hydronic heating system in your home. Whether you are building a new custom home from the ground up or undergoing an extensive remodel, hydronic systems can be installed in almost any situation. The following is a look at a few of the most common installation methods.

Hydronic Radiant Flooring

Radiant floors are either installed with what is called a “wet installation” method or “dry installation” method. All methods require insulation to be placed below to ensure the heat is directed into the home and not away. The following is an explanation of each method and when they are used.

  • Wet – Using a wet installation involves placing the radiant tubing into a bed of concrete. This can be very effective because the concrete acts to protect the tubing, while also providing a thermal mass to absorb the heat and radiate the warmth evenly throughout the room. The two different types of wet application are for:
  • Slab on grade foundations: During this process the radiant tubing is secured to the rebar or reinforcing structure within the slab before the wet concrete is poured into the foundation.
  • Thin Slab: These are created on top of subfloor systems where the tubing is attached to the subfloor and a thin layer of self leveling concrete is poured over the top. This can raise the floor height anywhere from ½ inch to 1.5 inches, and requires the structural engineering of the home to be designed to support the extra weight the concrete creates. The bonus of this installation is that it allows second floor rooms, as well as areas above basements and crawlspaces to have radiant floors installed.
  • Dry – Dry systems of radiant flooring installation, often called plate systems, utilize prebuilt panels that have tracks for the radiant tubing within their design. This makes it easy for installers to loop the tubes as needed before covering with flooring material. Without the thermal mass provided by concrete, a dry installation method can require more careful placement of insulation and the addition of heat reflectors to help separate the heating zones and direct the warmth in the right direction.

Hydronic Radiator and Baseboard Installation

Radiators and baseboard hydronic heaters are much simpler to install because there is far less piping to utilized overall. From the plumbing manifold, the different zones of pipe are placed within the walls or through the floor joists to connect with their units. Careful placement of the radiator or baseboard unit is important because you will not want it blocked with furniture in the future. In addition, the plumbing lines for the heated and returning water should be run through condition spaces and insulated. This will ensure that there is little heat loss as the water is moved to the heating units, and as much heat is recovered when it is returned to the boiler.

What are the Benefits of a Hydronic Heating System?

While radiant heating has become more and more popular over the last several years, there are still many people who are not aware of the many hydronic heating system benefits when compared to a traditional heating plan. Radiant heat is clean and comfortable, while extremely energy efficient and flexible in design.

Comfort

The single most important element that a heating system must give you and your family is comfort. Hydronic heating takes comfortable to a new level, as can be seen from the many benefits below:

  • Multi Zones – Radiant heating allows for personalized temperature control through the use of multiple zones throughout the home. This way parents and children can custom set their bedroom temperatures to their personal taste, while the kitchen and family room are kept comfortable for everyone. In addition, you don’t have to worry about keeping the doors closed in your rooms to trap heat. This is because radiant heat has no bursts of air that push warmth out of the areas you want it and into the spaces you don’t.
  • Warm Tiles and Floors – There is nothing as discouraging to the thought of getting out of bed in the morning then that frigid walk across cold bathroom tile on your way to the shower. Radiant flooring solves this problem by infusing the floors with heat, so that those previously chilly tiles become your source of warmth. People aren’t the only ones affected, since pets love nothing better than to stretch out on a warm floor.
  • Balanced Humidity Levels – It is far easier to maintain a balanced humidity level in the home with radiant hydronic heat because it will not dry out your home. Cold winter days already do enough to dry out skin without help from a forced air heating system, which sucks additional moisture out of the house in the process of heating the air.
  • Quieter – Radiant flooring and hydronic heating equipment work in silence while providing warmth for the whole house. There are no sounds of the heating unit kicking on and off during the night. Instead, steady heat radiates into the room with nothing to notice but the comfortable temperatures.

Health

Dust and allergens can be a concern for any family. They create sneezes and make it hard to breathe at times, especially for those with strong allergies or asthma. Unfortunately, traditional heating equipment pushes air around the home, which stirs up the allergens and dust, and carries it throughout the house. This is why filters are so necessary for force air systems, and why the infrequent changing of filters can have an impact on your family’s health.

Hydronic heat avoids this issue entirely, as it does not require the movement of any air to warm the home. This provides a healthier environment.

Efficiency

Energy efficiency is talked about more and more these days. Aside from the benefit that reduced energy consumption can have on the planet, the immediate impact of a hydronic system is the money that you will save on your energy bill each month.

Hydronic heat is more efficient than traditional air heating for many reasons. For one, water is a better heat conductor than air. This means the warmth is more easily transferred throughout the home, so less energy is needed in heating the medium. In addition, forced air units create pressure which pushes heat out through the gaps in insulation. Any home with poor insulation or drafts will suffer from heat loss, but radiant heat does not raise the air pressure in the home, so that warmer air is actively pushed out through the nooks and crannies. You will need to speak with local installers to get a specific understanding of the savings potential in your climate when using hydronic radiant heating, but it has been shown to produce comfortable living conditions at a 20-40% lower cost than traditional heat systems.

Versatile Installation

The versatility of installing hydronic heating allows homeowners to design a system around their needs. Whether your local fuel source is propane, natural gas, electricity, or oil you will be able to find a hydronic boiler that works for you. Hydronic piping can be placed in the walls and easily routed around the home, unlike traditional systems that require extensive ductwork and return air systems with large chases, which eat into your usable square footage in the home.

Hydronic system piping

The hydronic system piping is sub-divided into the following categories:

  1. Open and Closed type
  2. One pipe series loop system (Fig.5.14)
  3. One pipe parallel loop system (Fig.5.15)
  4. Two pipe system-Direct return piping (Fig.5.16)
  5. Two pipe system-Reverse Return Piping (Fig.5.17)
  6. Three pipe system-Direct return piping (fig.5.18)
  7. Three pipe system-Reverse return piping
  8. Four pipe system- Direct return piping (Fig.5.19)
  9. Four pipe system- Reverse return piping
  10. Primary loop piping (Fig.5.20)
  11. Secondary loop piping (Fig.5.21)

A hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning.

Chillers and cooling towers are used separately or together as means to provide water cooling, while boilers heat water.

Many larger cities have a district heating system that provides, through underground piping, publicly available steam and chilled water.

Open and Closed Type

An open system has a break in the piping and the water is “open” to the atmosphere. The water-cooled condenser and cooling tower loop of the air conditioning system is an open piping system.

The type of pipes used are black steel (commonly used).

A closed system has no break in the piping and the water is “closed” to the atmosphere. The loop from the water cooler to the chilled water coil is a closed piping system and from chiller to AHU or FCU.

A typical air conditioning chiller gives examples of both the open and closed piping system.;

The type of pipes used are galvanized ion (GI) (commonly used).

One Pipe System

  • The One Pipe Series Loop uses less pipe than any other hydronic piping arrangement therefore it is less expensive to install the piping but you need bigger radiators or longer baseboards at the end of the loop because the last part of the loop will have less heat. This is shown in figure 5.14
Figure 5.14
One pipe Series Loop in Hydronic System
  • The radiators or baseboards at the beginning of the loop use most of the heat thus the reason for the larger radiators and baseboards at the end of the loop.
  • There is also a larger temperature drop in this type of loop between the supply and the return versus other types of hydronic piping arrangements. The near boiler piping may need to be modified to prevent large ΔT between supply and return.
  • Water flow is accomplished by using TEES with flow restrictors.
  • On one pipe parallel loop, balancing of equal flow through parallel piping is accomplished by installing balancing valves or reducing / increasing pipe size in the supply and return lines. This is shown in figure 5.15.
Figure 5.15
One pipe Parallel Loop in Hydronic System

Two Pipe System

  1. Two pipe system-Direct return piping (Figure5.16)
  2. Two pipe system-Reverse Return Piping (Figure 5.17)

Two pipe system-Direct return piping

  • The Two Pipe Direct Return Loop utilizes more pipe than the one pipe series loop but all radiators and baseboards receive the same temperature of water therefore it is more even heat than in all the radiators and/or baseboards than the one pipe series loop.
  • In a two pipe direct return system, the radiators are connected to the supply/return piping like a rungs of a ladder.
  • Another advantage of two pipe direct return loop over the one pipe series loop is that it can be zoned.
  • Zoning gives you more control over where and when you want heat and this can save you money on the cost of heating.
  • As with many hydronic loop systems the two pipe direct return needs balancing valves. The near boiler piping may need to be modified to prevent large delta T between supply and return.
Figure 5.16
Two pipe system-Direct return piping

Advantages:

  • Low cost of return piping in most (but not all) applications, and the supply and return piping are separated.

Disadvantages:

  • This system can be difficult to balance due to the supply line being a different length than the return.
  • The further the heat transfer device is from the boiler the more pronounced the pressure difference.
  • Because of this it is always recommended to: minimize the distribution piping pressure drops; use a pump with a flat head characteristic, include balancing and flow measuring devices at each terminal or branch circuit; and use control valves with a high head loss at the terminals..

Two pipe system-Reverse Return Piping

In this system, the radiators are still connected between the supply return like rungs of a ladder. However, reverse return has its supply/return connected at opposite ends-pushing/pulling with equal force through all the connected radiators that have identical or similar head losses.

Figure 5.17
Two pipe system-Reverse Return Piping
  • The Two Pipe Reverse Return Hydronic Loop uses more pipe than the two pipe direct return hydronic loop but the flow is more balanced and even than the two pipe direct return hydronic loop.
  • All baseboards and radiators receive the same temperature of water so is the same as the two pipe direct return but an advantage over the one pipe series hydronic loop.
  • The two pipe reverse return hydronic loop can also be zoned offering you savings on your heating bill by taking advantage of hydronic loop zoning and large pressure drop.

Advantages:

  • This system is often considered "self balancing", however, valves should always be included.

Disadvantages:

  • The installer or repair person cannot trust that every system is self balancing without properly testing it.
  • Very large scale systems can be built using the two-pipe principle. For example, rather than heating individual radiators, the steam may be used in the reheat coils of large air handlers to heat an entire floor of a building

Three pipe system

A three-pipe system has two supply mains and one return main. One supply circulates chilled water from the chiller(s), and the other supply circulates heated water from the boiler(s). The return main carries water from each coil back to either the chiller or boiler. (Refer Figure 5.18)

Figure 5.18
Three pipe system

It is further sub-divided into:

  1. Three pipe system-Direct return piping
  2. Three pipe system-Reverse return piping

A three-way valve at the inlet of each coil delivers either cold or hot water to the coil. It is to be noted that the supply water streams are not mixed. When both cold and hot water are available, any coil can either heat or cool without regard to the operation of any other coil.

Typically during the year, sometimes, the HVAC system is simultaneously heating and cooling, with the return pipe carrying a mixture of both hot and cold water. The result is that both the chiller and the boiler receive warm water and must use more energy in order to supply their proper water temperature.

Four-Pipe System

  • Systems for induction and radiant panel or fan-coil systems derive the name four-pipe systems because of the four pipes to each terminal unit.
  • The four pipes consist of a cold water supply, a cold water return, a warm water supply, and a warm water return.
  • The four-pipe system satisfies variations in cooling and heating to the induction units using temperature primary air, secondary chilled water, and secondary hot water.
  • Terminal units are provided with two independent secondary water coils one served by hot water and the other by cold water.
Figure 5.19
Four-Pipe system Piping

Description of Direct or Reverse –Return piping

A direct-return piping system is routed to bring the water back to the pump by the shortest possible path. The heating or cooling coils are piped so that the first coil supplied is the first returned and the last coil supplied is the last returned.

Balancing valves are required for flow adjustments since water will follow the path of least resistance, and the coils closest to the pump will tend to receive too much water while the coils farthest from the pump will be starved.

A reverse-return piping system is designed so the length of the circuit to each coil and back to the pump is essentially equal in pressure drop. The coils are piped so that the first coil supplied is the last returned, and the last coil supplied is the first returned. Reverse-return systems generally need more piping than direct-return systems. Reverse-return systems are sometimes considered self-balancing because the intent of the design is to have equal pressure drops throughout the loop. However, because of varying circumstances in design or installation, reverse-return systems are usually not self-balancing, and balancing valves are still required for proper flow adjustments

Primary Looping

  • Primary-secondary variable-flow systems: Water flows through the chiller or boiler primary loop at a constant rate, and water flows through the secondary loop, which serves air handlers or fan coils, at a variable rate.
Figure 5.20
Primary Loop Piping
  • The decoupled section (shown as common piping in the diagram below) isolates the two systems hydraulically. Primary-secondary variable-flow systems are more energy efficient than constant-flow systems, because they allow the secondary variable-speed pump to use only as much energy as necessary to meet the system demand.
  • These are chilled-water or heating-water systems with a single variable-flow loop.
  • A two-way bypass valve is typically used to maintain a minimum specified flow rate through the chiller or boiler.
  • Primary-loop variable-flow systems are more efficient than primary-secondary variable-flow systems.

Secondary looping

  • Secondary loop systems have all of the refrigeration associated with liquid chilling located within the central machine room of the supermarket.
Figure 5.21
Flow Diagram for a Secondary Loop Refrigeration System
  • The chiller system resembles the multiplex refrigeration system since multiple parallel compressors are employed in both systems.
  • The compressors are utilized based upon suction pressure which controls the supply temperature of the chilled fluid.
  • The use of multiple compressors allows the refrigeration capacity to conform to changing operating conditions, resulting in better fluid temperature control and lower compressor energy use.

5.9.6 All air system

In an all air system, the air-conditioning plant is located remotely in a central place and conditioned air is sent through ducts and distributed into the air-conditioned space.

It is applied in buildings requiring individual control of conditions and having multiple zones such as office buildings, schools, laboratories, hotels, and ships.

In this system, the required conditions of air-conditioned space are kept within the desired limits by various methods to cope with the load changes.

Volume control

This type of control system varies the volume of air required based on the condition of required air. This works efficiently when the variation in the required load is within 20 %; otherwise, with a lesser volume of air, there may be a resultant air movement and distribution problem. This type of system is known as all air variable volume constant temperature system.

The components of this system for varying load are shown in the figure below:

Figure 5.22
Variable air quantity control

Significant advantages of the variable volume systems are low initial cost and low operating costs. The capital cost of the system is far lower in comparison to other systems because it requires only a single run of ductwork and simple control at the air terminal. Since the volume of air is reduced with the reduction in load, the refrigeration and fan power load can be optimally utilized.

Reheat control

The plant is designed to take peak loads and any reduction in the heat load is taken care of by introducing a reheat coil at the entry of air into the air-conditioned space. This coil can be provided in the control duct of a multi-zone system or in the warm air duct of a dual duct system.

The purpose of the reheat system is to allow control of areas of unequal loading or to provide heating or cooling of perimeter areas with different magnitudes of exposure. A typical arrangement of this kind of control is illustrated in the following figure:

Figure 5.23
Reheat control

A control thermostat activates the reheat unit when the temperature falls below the set point.

Dual duct system

A typical arrangement of this type of control system is illustrated in the following figure:

Figure 5.24
Dual duct system

In this system, the central air-conditioning plant supplies warm air through one duct and cold air through another duct. A thermostat that mixes the warm and cool air in the required proportion controls the temperature in an individual space.

With simultaneous availability of cold and warm air at each terminal at all times, this system provides greater flexibility in satisfying multiple loads and provides prompt temperature response as required.

A common characteristic of multi-room or multi-storey buildings is their highly variable sensible heat load. A properly designed dual duct system can efficiently take care of such varying load.

For the best performance, some form of constant volume regulation should be incorporated in the system to maintain a constant flow of air.

5.9.7 Air water system

Elementary conceptual illustration of air-water system is shown in the following figure.

Figure 5.25
Air-water air conditioning system

The conditioned air (cold or hot as per the requirements) is brought up to the conditioned space through ducts and the remaining cooling or heating is done by the coil located in the conditioned space.

The airside of an air water system is comprised of central air conditioning equipment, a duct distribution system and a room terminal. Air is supplied at constant volume and is referred to as primary air to distinguish it from room air, which is re-circulated over the room coil. The waterside (of the system) consists of a pump and piping to convey water to the coil in the room. The heating or cooling coil may be an integral part of the air terminal or a completely separate unit within the conditioned space. For cooling coils water circulated through the coil is cooled either by direct refrigeration or by chilled brine. To distinguish it from the primary chilling water circuit, the waterside is referred to as the secondary water loop. Individual room control is obtained by varying the capacity of the coil. This is done by either reducing the flow of water in the coil or airflow over it.

5.9.8 Air water induction system

A typical arrangement of this type of air water system is illustrated below:

Figure 5.26
Air water induction system

In this system, a central station is provided for conditioning the primary air alone. The primary conditioned air is then discharged to induction units in each of the spaces to be conditioned. The nozzles in the induction unit, through which the primary air passes, induce a fixed ratio of secondary air (room air) to flow over the water coil, which mixes with the primary air. The mixture of primary air and secondary air is then discharged to the room through supply grills. The room air is either heated or cooled by the water coil. To maintain the desired room conditions, the flow of water through the coil is automatically controlled.

5.10 Heat Pumps (ref: https://www.energygroove.net/heatpumps.php)

The principle of operation of the heat pumps is based on the First Law of Thermodynamics and Energy Conservation:

“Matter can neither be created nor destroyed though it may be made to take different forms. Similarly, energy cannot be created or destroyed. It can take any form.”

This means that heat can be created from hot or cold sources, when external energy is applied to the original system. Ultimately heat can be extracted from anything that is above absolute zero.

In this capacity heat pumps can operate as both heating and cooling devices by either transferring a temperature from one area to another, expanding/contracting a substance, or both.

There are three basic types of heat pumps. These are:

  • Air source heat pumps
  • Absorption heat pumps
  • Ground source heat pumps

5.10.1 Air Source Heat Pumps

Figure 5.27
Air source heat pumps

A cold refrigerant circulates inside the refrigerant coils where it becomes heated by outside air being blown by a fan, and a compressor that further increases the temperature through compressing the refrigerant. The heated refrigerant is then moved inside the space where it passes through another set of heating coils where a fan extracts the heat from the coils by blowing air on it. The heated air can then be distributed about the building through air ducts. Lastly the refrigerant is passed through an expansion valve that cools it down to begin the cycle all over again.

The cooling process is virtually the reverse of the heating process, whereby a reversing valve near the compressor changes the direction of the refrigerant flow.

Figure 5.28
Summer and Winter process

Air source heat pumps are driven by electricity, and systems exist that are powered by solar panels, making them both clean and energy efficient.

5.10.2 Absorption Heat Pumps

Absorption heat pumps work similar to air source heat pumps but instead of using electricity to compress a refrigerant, they use heated water generated from solar boilers, geothermal resources or natural gas in combination with an absorption pump and a pressure pump. The absorption pump absorbs ammonia or lithium bromide into water. This mix is then pressurized by the pressure pump. The ammonia or lithium bromide is then boiled out of the water by the heat from the heated water creating heat that can be used inside. However, unlike air source heat pumps, absorption heat pumps are not reversible.

Figure 5.29
Absorption Heat Pumps

5.10.3 Ground Source Heat Pumps

Ground source heat pumps use the constant, well insulated temperature that exists just below the ground or in a body of water, e.g. a pond, to transfer heating or cooling to a building. This is accomplished by transferring heat or cold from below the ground via underground piping that contains a refrigerant. There are several variations to this, including:

  1. Direct exchange
  2. Closed loop
    1. Vertical
    2. Horizontal
    3. Pond
  3. Open loop
    1. Standing column well
Figure 5.30
Ground source heat pumps

Direct Exchange

The direct exchange system is the simplest, most efficient and also least expensive. It involves a heat pump that circulates a refrigerant through underground copper pipes, where heat is transferred from the ground to the refrigerant through the copper piping. Although this system is limited by the thermal conductivity of the ground, its lack of additional mechanisms, e.g., water pump and heat exchanger, make its overall energy efficiency very high.

Closed Loop

The closed loop system involves two sets of piping, one that contains water and anti-freeze and passes below the ground to absorb heat and transfer it through a heat exchanger to the second pipe that contains refrigerant and is in contact with the heat pump that distributes the heat throughout the building. This system also requires a water pump to move the water and anti-freeze below the ground. The name “closed loop” comes from the fact that the liquids in both piping systems remain contained, without being refreshed, i.e., they are continuously reused.

Figure 5.31
Ground source heat pumps-Closed loop
Figure 5.32
Ground source heat pumps-Open loop

Closed loop variations include distributing the underground pipes either vertically, horizontally or under water. Determining which method is best depends on factors such as cost, availability of land, underground geology and proximity to water. Horizontal is cheaper than vertical, but requires more land, and wet environments are best for transferring heat.

Figure 5.33
Ground source heat pumps-Vertical loop

Open Loop

An open loop heat pump operates like a closed loop system in that it uses two piping loops with a heat exchanger. The difference is with the underground loop that, instead of reusing the same liquid, accesses water from an underground source or pond. In this case water is continuously renewed throughout the loop. An open loop system is only practical where there is easy access to water. Issues with this type of system include pipe contamination from minerals in the water, and also the possibility that such a system may drain or contaminate natural aquifers or wells.

5.11 Air Handling Units

5.11.1 Types of Air Handlers

  1. Ducted Air handler
  2. Ductless Air Handlers
  3. Packaged Unit (water cooled or air cooled)

Ducted Air Handlers

Air handlers usually connect to ductwork that distributes the conditioned air through the building and returns it to the AHU.

Figure 5.34
Ducted Air Handlers

Ductless Air Handler

Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork.

Small air handlers, for local use, are called terminal units, and may only include an air filter, coil, and blower; these simple terminal units are called:

  • Blower coils –VAV Units
  • Fan coil units.

A larger air handler that conditions 100% outside air, and no re-circulated air, is known as a makeup air unit (MAU).

An air handler designed for outdoor use, typically on roofs, is known as a packaged unit (PU) or rooftop unit (RTU).

Figure 5.35
Roof Top Ductless Air Handlers

5.11.2 Air handler components

The main components of an air handler are:

  1. Blower/fan
  2. Heating and/or cooling elements
  3. Filters
  4. Humidifier
  5. Mixing chamber
  6. Heat recovery device
  7. Controls
  8. Vibration isolators

Blower/fan

Air handlers typically employ a large squirrel cage blower driven by an AC induction electric motor to move the air. The blower may operate at a single speed, offer a variety of set speeds, or be driven by a Variable Frequency Drive to allow a wide range of air flow rates. Flow rate may also be controlled by inlet vanes or outlet dampers on the fan. Some residential air handlers (central 'furnaces' or 'air conditioners') use a brushless DC electric motor that has variable speed capabilities.

Multiple blowers may be present in large commercial air handling units, typically placed at the end of the AHU and the beginning of the supply ductwork (therefore also called "supply fans"). They are often augmented by fans in the return air duct ("return fans") pushing the air into the AHU.

Heating and/or cooling elements

Air handlers may need to provide heating, cooling, or both to change the supply air temperature depending on the location and the application. Smaller air handlers may contain a fuel-burning heater or a refrigeration evaporator, placed directly in the air stream. Electric resistance and heat pumps can be used as well. Evaporative cooling is possible in dry climates.

Large commercial air handling units contain coils that circulate hot water or steam for heating, and chilled water for cooling. Coils are typically manufactured from copper for the tubes, with copper or aluminum fins to aid heat transfer. Cooling coils will also employ eliminator plates to remove and drain condensate. The hot water or steam is provided by a central boiler, and the chilled water is provided by a central chiller. Downstream temperature sensors are typically used to monitor and control 'off coil' temperatures, in conjunction with an appropriate motorized control valve prior to the coil.

Filters

Air filtration is almost always present in order to provide clean dust-free air to the building occupants. It may be via simple low-MERV pleated media, HEPA, electrostatic, or a combination of techniques. Gas-phase and ultraviolet air treatments may be employed as well.

It is typically placed first in the AHU in order to keep all its components clean. Depending upon the grade of filtration required, typically filters will be arranged in two (or more) banks with a coarse-grade panel filter provided in front of a fine-grade bag filter, or other 'final' filtration medium. The panel filter is cheaper to replace and maintain, and thus protects the more expensive bag filters.

The life of a filter may be assessed by monitoring the pressure drop through the filter medium at design air volume flow rate. This may be done by means of a visual display, using a pressure gauge, or by a pressure switch linked to an alarm point on the building control system. Failure to replace a filter may eventually lead to its collapse, as the forces exerted upon it by the fan overcome its inherent strength, resulting in collapse and thus contamination of the air handler and downstream ductwork.

Humidifier

Humidification is often necessary in colder climates where continuous heating will make the air drier, resulting in uncomfortable air quality and increased static electricity. Various types of humidification may be used:

  • Evaporative: dry air blown over a reservoir will evaporate some of the water. The rate of evaporation can be increased by spraying the water onto baffles in the air stream.
  • Vaporizer: steam or vapor from a boiler is blown directly into the air stream.
  • Spray mist: water is diffused either by a nozzle or other mechanical means into fine droplets and carried by the air.
  • Ultrasonic: A tray of fresh water in the air stream is excited by an ultrasonic device forming a fog or water mist.
  • Wetted medium: A fine fibrous medium in the air stream is kept moist with fresh water from a header pipe with a series of small outlets. As the air passes through the medium it entrains the water in fine droplets. This type of humidifier can quickly clog if the primary air filtration is not maintained in good order.

Mixing chamber

In order to maintain indoor air quality, air handlers commonly have provisions to allow the introduction of outside air into, and the exhausting of air from the building. In temperate climates, mixing the right amount of cooler outside air with warmer return air can be used to approach the desired supply air temperature. A mixing chamber is therefore used which has dampers controlling the ratio between the return, outside, and exhaust air.

Heat recovery device

A heat recovery device heat exchanger of many types, may be fitted to the air handler between supply and extract airstreams for energy savings and increasing capacity. These types more commonly include for:

Cross Plate Heat exchanger: A sandwich of plastic or metal plates with interlaced air paths. Heat is transferred between airstreams from one side of the plate to the other. The plates are typically spaced at 4 to 6mm apart. Can also be used to recover cooling. Heat recovery. efficiency up to 70%.

Thermal Wheel: A slowly rotating matrix of finely corrugated metal, operating in both opposing airstreams, heat is absorbed as air passes through the matrix in the exhaust air stream, during one half rotations, and released during the second half rotation into the supply air stream in a continuous process. Can also be used to recover and cool. Heat recovery efficiency is up to 85%. Wheels are also available with a hydroscopic coating to provide latent heat transfer and also the drying or humidification of airstreams.

Run around coil: Two number air to liquid heat exchanger coils, in opposing airstreams, piped together with a circulating pump and using water or a brine as the heat transfer medium. This device, although not very efficient, allows heat recovery between remote and sometimes multiple supply and exhaust airstreams. Heat recovery efficiency is up to 50%.

Heat Pipe: Operating in both opposing air paths, using a confined refrigerant as a heat transfer medium. The 'pipe' is multiple sealed pipes mounted in a coil configuration with fins to increase heat transfer. Heat is absorbed on one side of the pipe, by evaporation of the refrigerant, and released at the other side, by condensation of the refrigerant. Condensed refrigerant flows by gravity to the first side of the pipe to repeat the process. Heat recovery efficiency is up to 65%.

Controls

Controls are necessary to regulate every aspect of an air handler, such as: flow rate of air, supply air temperature, mixed air temperature, humidity, air quality. They may be as simple as an off/on thermostat or as complex as a building automation system (BAS) using BACnet or LonWorks, for example.

Common control components include temperature sensors, humidity sensors, sail switches, actuators, motors, and controllers.

Vibration isolators

The blowers in an air handler can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the occupants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler and often also between the fan compartment and the rest of the AHU. The rubberized canvas-like material of these sections allows the air handler to vibrate without transmitting much vibration to the attached ducts.

The fan compartment can be further isolated by placing it on a spring suspension, which will mitigate the transfer of vibration through the floor.

5.12 Functional variations in the design

Air handling units may be adapted to all types of air conditioning systems, including human comfort applications (Ventilation) as well as heating and cooling of the space. Their main function is to supply air into the space or room to control the humidity and temperature levels.

Air Handling units are designed with various air distribution systems, such as:

  1. Constant Volume Single-zone system
  2. Constant Volume Single-zone with economizer
  3. Constant Volume Multi-zone system
  4. Constant Volume Dual-duct system
  5. Constant Volume Reheat system
  6. Constant Volume Single-zone system AHU with Refrigerant
  7. Constant volume, variable temperature system with bypass control and economy cycle
  8. Variable volume constant temperature system with economy cycle
  9. Constant volume induction system with economy cycle
  10. Heat Recovery system
  11. Variable Air Volume-Single duct system
  12. Variable Air Volume-Dual Duct system
  13. Packaged Units-Water and Air cooled

5.12.1 General functional description

An air handling unit, is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning system. An air handling unit consists of a large metal box containing Dampers, Filters, Heating coil, Cooling coil, Humidifier, Blower, Sound attenuators, and Dampers. Ducted Air handling units normally have supply air ducting and Return or Re-circulating air ducting provisions built into it. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork also. A typical schematic of single zone air handling unit shown in figure below:

Figure 5.36
General functional arrangement schematic

Small air handling units, located near the space to be heated or cooled are called terminal units, and may only include an air filter, coil, and blower; these simple terminal units are called blower coils or fan coil units. These units may function as Constant Air Volume system as well as variable Air Volume systems. Finally the conditioned air enters the space through units called, Registers or Diffusers into the space.

Both Air Handling Units (AHU) and Make-up Air Handling units may be coupled together as one unit or they can be supplied individually. These may slo be termed as ALL-AIR SYSTEMS.

5.12.2 Typical Control Schematic of an AHU

The air handling unit affects to the temperature and humidity inside the building. In this case the control is based on several principles. The supply air temperature should be kept constant to allow the adjustment of temperature in each room with separate thermostats or dampers. The supply air humidity (ME10) must not exceed a certain level. Also the circulation of air should always be sufficient. The temperature control can be roughly divided in to two scenarios. When the outside temperature is lower than the indoor temperature, the sequence of operation is as follows: first the cooling valve (TV52) is closed after which the speed of the heat recovery wheel (SC50) is increased. If the set point temperature is not achieved with the (HRW), then additional heating is acquired by opening the preheating valve (TV45) and after that the post-heating valve (TV46). When the outside temperature is higher than the indoor temperature, the (HRW) should be always on. If the cooling power of the heat recovery wheel is insufficient additional cooling is acquired by opening the cooling valve (TV52). In case the (HRU) starts to frost, its rotation speed is decreased according to a pressure difference measurement (PDIE50).

The circulation of air is kept at an adequate level with the exhaust (PF1.1) and supply air (TF1.1) fans. Their run speeds are adjusted according to pressure measurements (PE30) and PE10. When the supply air humidity exceeds a predefined level, it will be dried by adjusting the cooling coil valve (TV52). When this happens, the supply air temperature is kept at the desired, constant level with the HRU and the pre- and post heaters. Whenever an alarm that is severe enough is triggered, the system will be shut down. All three variable frequency drives (SC10, SC30, SC50) are set to zero speed and the exhaust and intake air dampers are closed.

A separate frost protection device will prevent the preheating coil from freezing by overriding the control signal of the heating valve (TV45) if the temperature (TE45) of the heating coil is too low. Pressure difference measurements (PDIE01, PDIE02 and PDIE30) will cause an alarm in case a filter gets too dirty and the pressure difference gets too big.

Figure 5.37
Typical control schematic

5.12.3 Constant Volume Single-zone system AHU

This air handling system can be installed either within the space or remote location, with or without duct work. The proper functioning of this system is achieved, if this unit is connected to only one zone. The figure below shows the schematics of a single-zone system constant volume all-air system.

In this system, the room temperature is set by the thermostat T1 and the measurement is fed to the supply air discharge thermostat. If there is a variation in room temperature, the discharge thermostat modulates the heating and cooling coil valves by opening or closing the appropriate valve. Since the zone (room) thermostat and the heating valve are both direct acting and normally open, any increase in temperature of the room, the heating valve will close progressively to restrict the hot water flow into the heating coil.

Since the cooling water valve, which is normally closed, will remain closed as long the heating requirement is adequate. When cooling is required, the cooling valve will respond with the signal from thermostat.

When the air delivered by the supply fan is constant, the rate of outside air intake is determined by the setting of the dampers. The outside air damper, have a motor to drive them from the closed to open position. The re-circulated dampers are manually adjustable. The re-circulated air dampers work in tandem with the outside air dampers as well as with the exhaust dampers.

Figure 5.38
Constant Volume Single Zone AHU Schematic

5.12.4 Constant Volume Single Zone with Economizer AHU

In many climates there are substantial periods of time when cooling is required and the return air from the space is warmer and moister than the outside air. During these periods, you can reduce the cooling load on the cooling coil by bringing in more outside air than that required for ventilation. This can be accomplished by expanding the design of the basic air-conditioning system to include an economizer.

The economizer consists of three (or four) additional components as shown in figure below:

Figure 5.39
Constant Volume Single Zone with Economizer AHU Schematic

(1) Expanded air intake and damper, sized for 100% system flow.

(2) Relief air outlet with automatic damper, to exhaust excess air to outside.

(3) Return air damper, to adjust the flow of return air into the mixing chamber.

(4) (Optional) Return fan in the return air duct. The return fan is often added on economizer systems, particularly on larger systems. If there is no return fan, the main supply fan must provide enough positive pressure in the space to force the return air out through any ducting and the relief dampers. This can cause unacceptable pressures in the space, making doors slam and difficult to open. When the return air fan is added it will overcome the resistance of the return duct and relief damper, so the space pressure stays near neutral to outside.

The economizer is a very valuable energy saver for climates with long periods of cool weather. For climates with warm moist weather most of the year, the additional cost is not recovered in savings. Also, for spaces where the relative humidity must be maintained above -45%, operation in very cold weather is uneconomic. This is because cold outside air is very dry, and considerable supplementary humidification energy is required to humidify the additional outside air.

5.12.5 Constant Volume Multi-zone System AHU

The multi-zone air handling units provide a single supply duct for each zone and achieve zone control by mixing hot and cold air at the central unit in response to room or zone thermostats. If there are many zones, the control becomes easy and flexible. However, there are limitations too in total number of zones controlled by a central unit. The fan system must be a blow through with the heating and cooling coils in parallel downstream of the fan.

The air handling unit has both a heating coil and a cooling coil in parallel. Supply air can be diverted to pass either through the heating coil, cooling coil, or a combination of the two. This allows the supply air temperature to be varied to meet the requirements of the zone.

The figure below shows a multi-zone blow through system.

Figure 5.40
Constant Volume Multi- Zone AHU Schematic

The application of multi-zone, blow through system is suitable in location where there are high sensible heat loads and limited ventilation outside air requirement. Blow-through units add the fan heat before the cooling coil. The leaving air temperature from the cooling coil then becomes the supply air temperature. This provides the maximum temperature rise between the cooling air and the space design temperature. (The least amount of supply air will be required.) Since the air is often fully saturated and moisture may be an issue, blow-through should not be used with final filters downstream of the coils.

The multi-zone air handling unit has many sets of dampers which allow the total supply air volume to be divided as required among the zones it serves. With more advanced controls, it is possible to avoid simultaneous heating and cooling by using only one deck at a time and maintaining good zone temperature control.

The air handling unit can incorporate economizers to take advantage of free cooling during mild weather. It may also have a preheat coil to provide the proper supply air temperature in cold weather.

Three-Deck Multi-Zone Units

A modification of the two-deck system is the three-deck system, with the third deck being bypass air (return air with ventilation air introduced). The decks are arranged in such away that when the zone is calling for heating, bypass air and hot deck air are mixed to meet the zone requirements.

In cooling, air from the cooling deck and bypass air mix to meet the cooling load. The result: no simultaneous heating and cooling.

Typical Applications

Two-deck multi-zone systems are not commonly used in new construction because of their poor energy performance. They are more common in retrofit applications, since many schools were built with multi-zone systems in the past. Upgrading to a three-deck multi-zone system resolves many issues, while utilizing existing ductwork and controls.

Common applications include Schools.

5.12.6 Constant Volume Dual duct system

The dual duct or Double duct system, the supply fan supplies warm air through one duct and cold air through other duct. The temperature inside a room or space can be controlled individually by mixing warm and cold air at definite proportion.

Dual Duct HVAC is best used when numerous spaces with highly variable sensible loads are combined with the need for high flow rate ventilation. VAV adds some energy savings, but by sacrificing desired flow rates and ventilation.

By setting the thermostat in the room or space individually, year round temperature control is possible with this system.

A constant volume regulation provided will maintain constant flow of air. All the outdoor air supply can be used if the outlet temperature is low enough to handle the cooling load.

The system incorporated also control automatically reset the cold air supply to the highest temperature and hot air supply to the lowest temperature.

Figure 5.41
Constant Volume Dual Duct AHU Schematic

5.12.7 Constant Volume Reheat system

The reheat system is a modification of the single zone constant volume system. The conditioned air is supplied at a fixed temperature from the central unit and designed to offset the maximum cooling load in all zones of the space. The reheat heater is located at each zone and controlled by the thermostats on each individual zones. It heats the air when the temperature in the zone falls below the set value. A fixed amount of air is delivered to each zone at a supply air temperature. This air is adequate to meet the zone’s peak load requirement. If the actual load is less than the peak, then the reheat coil provides supplemental load equal to the difference between the peak and the actual load.

This reheat system is used in hospitals, laboratories, office buildings or other spaces, where wide fluctuations of load are expected.

The main disadvantage of this reheat system is the energy inefficiency. For example, whenever the cooling load is less than the peak load, over 90% of time, the cooling effect and reheat are working against each other to neutralize the situations.

This system controls the temperature and humidity of an air-conditioned space in a much better way than previous systems did. The relative humidity of the conditioned space is maintained constant under all variable load conditions.

The refrigeration capacity of the coil is controlled from the dew point thermostat at the outlet of the dehumidifier. The thermostat is located after the blower if the blower power and duct heat gain is comparatively large. The thermostat can also control the return and outdoor dampers to use the cooling effect of outdoor air during marginal weather conditions.

There are also some variations possible in the system design such as the terminal reheat units may heat the primary air directly or may heat secondary air or induced air or room air directly.

Figure 5.42
Constant Volume Reheat AHU Schematic

5.12.8 Constant Volume Single-zone system AHU with Refrigerant

In this system, the quantity of refrigerant in the direct expansion system or chilled water through the coil is controlled with the use of a thermostat located in the return air duct. Direct expansion type with on-off control is also used when the load is small or medium. For larger loads, chilled water coils with modulated controls are used.

The thermostatic control also regulates the quantities of maximum outdoor air and return air to provide cooling by outside air during marginal weather conditions. The heating coil takes care of winter heating and the preheating coil takes care of the situation when the temperature of mixture or return and outdoor air is below the required supply temperature.

This system is mostly preferred for stable load characteristics and minimum ventilation requirements.

Figure 5.43
Constant volume variable temperature system with the help of refrigeration control

5.12.9 Constant volume, variable temperature system with bypass control and economy cycle

The arrangement of various components in this system is shown in the following figure:

Figure 5.44
Constant volume, variable temperature system with bypass control

In this arrangement a part of the return air is by-passed around the coil. The face dampers control refrigeration capacity required in the coil. The temperature of air leaving the coil reduces with the increase in the bypass factor. This helps in lowering the humidity. The refrigeration plant stops with the close of the AFS. The bypass and face dampers can regulate the outdoor air for cooling in marginal weather when the plant is stopped.

The sizing of return, bypass and outdoor ducts is such that the outdoor air is not short-circuited through the bypass.

5.12.10 Variable volume constant temperature system with economy cycle

This system is used where the load can be met by changing the volume flow between 100% and 75% at full and minimum load conditions. If the solar heat gain load is considerable when compared with other heat loads, then this system is not favored, as there will be drastic variation in the sensible heat load factor.

The arrangement of its components is shown in the following figure.

Figure 5.45
Variable volume constant temperature system

The coils are designed to take the variations in the load without the use of re-heaters in individual zones. The variable load is taken care of by controlling the air volume at the inlet ducts to each system or at the outlet ducts in the conditioned space. The thermostat (T) regulates the capacity of the dehumidifier in summer, and return and outdoor air in marginal weather conditions. The air conditioning is maintained by a thermostat, which controls the air volume to the room or zone.

At partial loads the sensible heat ratio (S.H.R) decreases and requires low dew point temperature (DPT) of supply air. Lower dew point temperature is obtained by controlling the flow of chilled water through the dehumidifier. This system is preferred for a multi-zone application.

5.12.11 Constant volume induction system with economy cycle

The arrangement of components is shown in the following figure.

Outdoor air or a mixture of outdoor air and return air is supplied to the air-conditioned space, flowing through the filter and coil. Chilled water is circulated through the coil while hot water or steam is supplied to each of the induction units located in individual air-conditioned space.

A constant volume of primary air is supplied to each air-conditioned space that takes care of cooling through the induction unit. This primary air entering the rooms through the induction units induces room air that is heated by allowing flow over the heating coil of the induction unit. This takes care of cooling during summer or winter heating, as required. The temperature control in the room is achieved by an automatic control of hot water or steam flowing through the induction coil.

Figure 5.46
Constant volume induction system

This system is particularly suitable for multi room buildings with high latent heat gains. The details of the induction units used in air-conditioned spaces are shown in the figure above.

Figure 5.47
Details of an induction unit

Centrally conditioned primary air is supplied to the unit plenum at high pressure. The high-pressure air flows through the induction nozzles and induces secondary air from the room. This secondary air flows over the induction coil when it enters the induction unit.

Here it is either heated or cooled depending on the season or room requirements or both. The primary and secondary air are then mixed and discharged into the room.

Features of this system are:

  • The temperature in each room is controlled without the problems involved in zoning.
  • The primary air supply is centralized.
  • The control system is much simplified since it is done by a separate thermostat for each room.
  • Quiet operation due to remote location of the fans and related parts.
  • Low and centralized maintenance.
  • Centralized dehumidification avoids condensation problems in the air conditioned rooms.

5.12.12 Heat recovery system

In large commercial applications, a considerable amount of heat is generated internally. This condition usually occurs within the central spaces, which do not have exterior walls. Considerable saving in energy can be realized if the heat energy from the interior spaces and exhaust air is recovered and used in heating the exterior parts of the structure.

Heat energy from the exhaust fans can be recovered by using an air-to-air heat exchanger. Different types of heat exchangers used for heat recovery are shown in the following schematic.

Figure 5.48
Rotating heat exchanger (Air-to-Air Type)
Figure 5.49
Cross flow air-to-air heat exchanger
Figure 5.50
Run-around Coil

5.12.13 Variable-air-volume (VAV) single-duct systems

Variable air volume (VAV) is a technique for controlling the capacity of a heating, ventilating, and/or air-conditioning (HVAC) system. The simplest VAV system incorporates one supply duct that, when in cooling mode, distributes approximately 13 °C supply air. Because the supply air temperature, in this simplest of VAV systems, is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served.

There are two primary advantages to VAV systems. The fan capacity control, especially with modern electronic variable speed drives, reduces the energy consumed by fans which can be a substantial part of the total cooling energy requirements of a building.

Dehumidification is greater with VAV systems than it is with constant volume system, which modulates the discharge air temperature to attain part load cooling capacity.

The air blower's flow rate is variable. For a single VAV air handler that serves multiple thermal zones, the flow rate to each zone must be varied as well.

A VAV terminal unit, often called a VAV box, is the zone-level flow control device. It is basically a quality, calibrated air damper with an automatic actuator. The VAV terminal unit is connected to either a local or a central control system. Historically, pneumatic control was commonplace, but electronic direct digital control systems are popular especially for mid-to-large size applications. Hybrid control, for example having pneumatic actuators with digital data collection, is popular as well.

Control of the system's fan capacity is critical in VAV systems. Without proper and rapid flow rate control, the system's ductwork, or its sealing, can easily be damaged by over-pressurization.

In VAV systems, chilled air is distributed to spaces from an air handling unit, and the temperature of individual spaces is controlled by throttling the quantity of air into each space. The throttling is accomplished by terminal units that are controlled by the space thermostats.

VAV systems were originally introduced as a more efficient alternative to constant-volume reheat systems. The VAV concept offers two major efficiency improvements: (1) it reduces or eliminates reheat and (2) it minimizes fan power. These are used to provide space heating, or to reheat the chilled air to allow a minimum air flow to be maintained in the spaces, or for both purposes.

Also, you can save both reheat energy and fan energy by using accurate fan modulation to match cooling or heating load changes. VAV systems also allow us to use energy saving thermostatic controls, including dead-band thermostats and temperature setback thermostats.

Figure 5.51
Variable Air Volume with Reheat

5.12.14 Variable-air-volume dual-duct systems

VAV dual-duct systems have the potential of being efficient and comfortable, but they often have significant opportunities for improvement.

In dual-duct systems, the air handling unit has two coils, a continuously operating cooling coil and a continuously operating heating coil. The cooling coil feeds chilled air into a cold air duct. The heating coil feeds hot air into a hot air duct. The two ducts run in parallel throughout the building. At each space, air is tapped from the two ducts by a terminal unit. The terminal unit has a hot air damper and a cold air damper. When the space thermostat calls for heating, the hot air damper opens. When the thermostat calls for cooling, the cold air damper opens.

Efficiency suffers if a terminal unit mixes chilled air with heated under any conditions. The system may be designed to do this deliberately under low conditioning loads to maintain a minimum air flow into the spaces.

“Triple-duct” systems avoid air mixing. They are similar in appearance to dual-duct systems. The main difference is a third duct that carries unconditioned air (a mixture of return air and outside air) for mixing with either the heated air or the chilled air. If properly installed, triple-duct systems have no mixing losses, except for leakage that occurs inside the terminal units.

The dual-duct VAV system design is especially favorable for exploiting the energy saving principle of the outside air economizer cycle.

Figure 5.52
Variable Air Volume Dual-Duct system

5.12.15 Packaged Air Conditioners with Water Cooled Condenser

In these packaged air conditions the condenser is cooled by the water. The condenser is of shell and tube type, with refrigerant flowing along the tube side and the cooling water flowing along the shell side. The water has to be supplied continuously in these systems to maintain functioning of the air conditioning system.

The shell and tube type of condenser is compact in shape and it is enclosed in a single casing along with the compressor, expansion valve, and the air handling unit including the cooling coil or the evaporator. This whole packaged air conditioning unit externally looks like a box with the control panel located externally.

Figure 5.53
Packaged Unit

In the packaged units with the water cooled condenser, the compressor is located at the bottom along with the condenser (refer the figure). Above these components the evaporator or the cooling coil is located. The air handling unit comprising of the centrifugal blower and the air filter is located above the cooling coil. The centrifugal blower has the capacity to handle large volume of air required for cooling a number of rooms. From the top of the package air conditioners the duct comes out that extends to the various rooms that are to be cooled.

All the components of this package AC are assembled at the factory site. The gas charging is also done at the factory thus one does not have to perform the complicated operations of the laying the piping, evacuation, gas charging, and leak testing at the site. The unit can be transported very easily to the site and is installed easily on the plane surface. Since all the components are assembled at the factory, the high quality of the packaged unit is ensured.

5.13 Fan coil unit

Figure 5.54
Fan Coil Unit

5.13.1 Functional description

Fan coil units are used for attaining a desired temperature in a small area e.g. a small room. FCU is a simple device consisting of a heating or cooling coil and fan. It is part of an HVAC system found in residential, commercial, and industrial buildings. Typically a fan coil unit is not connected to ductwork, and is used to control the temperature in the space where it is installed, or serve multiple spaces. It is controlled either by a manual on/off switch or by thermostat.

Unit configurations are:

  1. horizontal (ceiling mounted)
  2. vertical (floor mounted).
  3. Wall mounted

….and typically include an appropriate enclosure to protect and conceal the fan coil unit itself, with return air grille and supply air diffuser set into that enclosure to distribute the air.

Due to their simplicity, fan coil units are more economic to install than ducted or central heating systems with air handling units. However, they can be noisy because the fan is within the same space.

FCUs are supplied with pre-cooled air coming from fresh air handling units. To further cool the supplied air fan coil units usually use chilled water coming either from chillers or a heat exchanger. The important mechanical parts involved in the operation of FCUs are the following:

  • Control valve (modulating type)
  • Fan (usually three speeds)
  • Filters for the purification of the supplied air
  • Strainers for the filtration of chilled water
  • Cooling coil/Heating coil

The desired temperature in the serving area is achieved by the simultaneous control of the control valve and the fan. The control valve regulates the flow of cold water inside the cooling coil. The fan blows the supplied air through the cooling coil further decreasing its temperature before it comes to the serving area.

When further cooling is not required the control valve is closed preventing the cold water from flowing through the cooling coil. The fan is often designed in such a way that it operates in more than one speed.(3-speeds)

The temperature of the serving area is controlled according to a set point and a temperature measurement. The controller regulates both the fan speed and the control valve according to the deviation of the current room temperature and the set point.

The fan can be operated either with manually or automatically. In manual mode the fan speed can be set to operate always in one of the three speeds. In automatic mode the fan speed is determined by the valve position.

5.13.2 Design and operation

The coil receives hot or cold water from a central plant, and removes or adds heat from the air through heat transfer. Traditionally fan coil units can contain their own internal thermostat, or can be wired to operate with a remote thermostat.

Fan coil units circulate hot or cold water through a coil in order to condition a space. The unit gets its hot or cold water from a central plant, or mechanical room containing equipment for removing heat from the central building's closed-loop. The equipment used can consist of machines used to remove heat such as a chiller or a cooling tower and equipment for adding heat to the building's water such as a boiler or a commercial water heater.

Fan coil units are divided into two types:

(a) Two (2) pipe fan coil units

Two pipe fan coil units have one (1) supply and one (1) return pipe. The supply pipe supplies either cold or hot water to the unit depending on the time of year.

(b) Four (4) pipe fan coil units

Four (4) pipe fan coil units have two (2) supply pipes and two (2) return pipes. This allows either hot or cold water to enter the unit at any given time. Since it is often necessary to heat and cool different areas of a building at the same time, due to differences in internal heat loss or heat gains, the four (4) pipe fan coil unit is most commonly used.

Fan coil units may be connected to piping networks using various topology designs, such as "direct return", "reverse return", or "series decoupled".

Depending upon the selected operating conditions, it is very likely that the cooling coil will be designed to dehumidify the entering air stream, and as a by product of this process, it will at times produce a condensate which will need to be carried to drain. The fan coil unit will contain a purpose designed drip tray with drain connection for this purpose.

Speed control of the fan motors within a fan coil unit is effectively used to control the heating and cooling output desired from the unit. This is normally achieved by manually adjusting the taps on an AC transformer supplying the power to the fan motor. Fan motors are typically AC type motors.

5.13.3 Application of Fan Coil Unit

Fan coil units are typically used in spaces where economic installations are preferred such as unoccupied storage rooms, corridors, loading docks.

In high-rise buildings, fan coils may be stacked, located one above the other from floor to floor and all interconnected by the same piping loop.

Fan coil units are an excellent delivery mechanism for hydronic chiller boiler systems in large residential and light commercial applications. In these applications the fan coil units are mounted in bathroom ceilings and can be used to provide unlimited comfort zones - with the ability to turn off unused areas of the structure to save energy.

5.14 Capacity calculation of an air handling unit

Cooling Coil in the Main Air Handling Unit (AHU)

AHU cooling coil can use:

  1. Chilled water
  2. Direct expansion system.
  3. Chilled water temperature varies between 5-7o C.
  4. Refrigerant temperatures at direct expansion systems can be as low as 2o C.
Figure 5.55
Cooling Coil in AHU Units

The cooling capacity of an air handling unit is calculated based on the total heat (Sensible heat + Latent heat) removed from the air passing through the cooling coil in the air handling unit. This will be the difference between the inlet enthalpy of the air entering the cooling coil and the outlet enthalpy immediately after the air passes the cooling coil. Obviously, the air entering the cooling coil is a mixture of return air from the space to be cooled and the fresh air entering to air handling unit to mix with the return air. The air mixing takes place before the filter in the mixing plenum. The measurement of air condition ( dry bulb temperature and the wet bulb temperature) to calculate the inlet enthalpy, shall be taken near the inlet side of the cooling coil. The measurement of air outlet condition after passing through the cooling coil is taken between the cooling coil and the fan inlet.

From the reading of dry bulb and wet bulb temperatures, plot them on a psychrometric chart to find out the inlet and outlet enthalpies. The resultant enthalpy is the cooling capacity in terms of kcal/kg.

Cooling coil calculation

In calculating the required cooling effect, it is important to consider that part of the effect is used to separate the water (latent heat) and the remaining effect to lower the air temperature (sensible heat). It is therefore, necessary to incorporate enthalpy differences in the calculations to absorb the latent heat part.

Here, for calculation purpose, we may use the thermodynamic properties of ai as standard air, which is 21.1°C with a pressure of 760 mmHg.

THUMB RULE: 10 to 11.3 m3/min is taken as the cooling coil capacity required per ton of cooling capacity.

Heating Coil Calculation

Heating Coil in the Main Air Handling Unit (AHU)

Once the cold season is hot water or steam is supplied to the heating coils in the AHU. This coil heats the leaving the AHU, sending warm air to all of the rooms. The coil consists of a copper header supplying steam to a copper tube which passes through a continuous aluminum fin for added heat transfer surface area.

When a certain volume of air passes a heating coil, air temperature is increased from t1 to t2. This process takes place at constant air humidity.

Air velocity

The air velocity across the heating coil is calculated as follows:

L = A x v

v = Velocity, m/s

A = Area of heating coil face, m2

Air Heating

If air is used for heating, the needed air flow rate may be expressed as:

qh = H h / ρ cp (ts - tr)

Where:

qh = volume of air for heating (m3/s)

Hh =heat load (W)

cp = specific heat capacity of air, 1005 J/kg.K

ts = supply temperature (oC)

tr = room temperature (oC)

ρ = density of air (kg/m3)

Example - heating load:

If the heat load is Hh = 400 W, supply temperature ts = 30 oC and the room temperature tr = 22 oC, the air flow rate can be calculated as:

qh = (400 W) / (1.2 kg/m3) (1005 J/kg K) (30 oC - 22 oC)

= 0.041 m3/s = 149 m3/h

Air Cooling

If air is used for cooling, the needed air flow rate may be expressed as:

qc = Hc / ρ cp (to - tr)

Where:

qc = volume of airfor cooling (m3/s)

Hc =cooling load (W)

to = outlet temperature (oC) where to = tr if the air in the room is mixed

Moisture

If it is necessary to humidify the indoor air, the amount of supply air needed may be calculated as:

qmh = Qh / ρ (x2 - x1)

Where :

qm = volume of air for humidifying (m3/s)

Qh = moisture to be supplied (kg/s)

ρ = density of air (kg/m3)

x2 = humidity of room air (kg/kg)

x1 = humidity of supply air (kg/kg)

Dehumidifying

If it is necessary to dehumidify the indoor air, the amount of supply air needed may be calculated as:

qmd = Qd / ρ (x1 - x2)

Where:

qmd = volume of air for dehumidifying (m3/s)

Qd = moisture to be dehumified (kg/s)

Example - humidifying

If added moisture Qh = 0.003 kg/s, room humidity x1 = 0.001 kg/kg and supply air humidity x2 = 0.008 kg/kg, the amount of air can expressed as:

qmh = (0.003 kg/s) / (1.2 kg/m3) ((0.008 kg/kg)- (0.001 kg/kg))

= 0.36 m3/s

Alternatively the air quantity is determined by the requirements of occupants or processes.

Temperature loss in ducts

The heat loss from a duct can be expressed as:

H = A k ( (t1 + t2) / (2 - tr) )

Where:

H = heat loss (W)

A = area of duct walls(m2)

t1 = initial temperature in duct (oC)

t2 = final temperature in duct (oC)

k = heat loss coefficient of duct walls (W/m2 K) (5.68 W/m2 K for sheet metal ducts, 2.3 W/m2 K for insulated ducts)

tr = surrounding room temperature (oC)

The heat loss in the air flow can be expressed as:

H = q cp (t1 - t2)

Where:

q = mass of air flowing (kg/s)

cp = specific heat capacity of air (kJ/kg K)

Heat loss from duct and airflow can be combined to get the formula:

H = A k ((t1 + t2) / 2 - tr)) = q cp (t1 - t2)

For large temperature drops should logarithmic mean temperatures be used.

The following equation quantifies the heat-transfer process:

Q = U × A × LMTD

Where:

Q = amount of heat transferred, (W)

U = heat-transfer coefficient, (W/m2 • °K)

A = effective surface area for heat transfer,(m2)

LMTD = log-mean temperature difference across the coil surface,(°C)

Increasing any one of these variables (heat-transfer coefficient, surface area, or log-mean temperature difference) results in more heat transfer and ultimately improves the life-cycle value of the cooling coil.

In the context of a chilled-water cooling coil, LMTD describes the difference between the temperatures of the air passing across the coil fins and the water flowing through the coil tubes:

LMTD = (TD2TD1) / ln (TD2 / TD1)

Where:

TD1 = leaving-air and entering-water temperature difference at the coil,°C

TD2 = entering-air and leaving-water temperature difference at the coil,°C

One way to increase LMTD is to supply the coil with colder water.


6


Variable air volume (VAV) systems

Objectives

After studying this chapter, a student should be able to:

  • Understand the concept of Variable Air Volume (VAV) systems
  • Describe the Characteristics of VAV systems
  • Explain the various VAV systems
  • Explain the arrangements of terminals in a VAV system

6.1 General

The air-conditioning industry today is advanced in energy saving technologies and these must be applied to optimize air-conditioning installations. The variable air volume system is considered a high energy saving system. It is applicable to most buildings where an all-air system can be successfully applied. It is ideally suited to interior zones of single, multi-storey and multi-room buildings that require cooling whenever they are occupied.

6.2 System concept

High velocity variable air volume (VAV) systems control space conditions by controlling air quantity instead of temperature. Constant volume systems such as terminal reheat, multi-zone and double duct systems maintain a constant flow of air and regulate the temperature by using reheat coils or mixing hot and cold air streams.

The VAV system is much more difficult to design and control airflows due to the widely fluctuating airflows and static pressures encountered.

VAV is an all-air system. This system needs only one supply air duct as shown in the figure below. The supply air temperature is constant and the required comfort conditions as per the load are achieved by varying the quantity of air supplied to each room of the building.

Figure 6.1
Variable volume system

The Concept of modulating the cooling capacity by varying the air is simple and more energy efficient. VAV is the only system that caters entirely to the non simultaneous cooling load. For example; an air-conditioned building with rooms in all four exposures has cooling loads that follow the sun around the building from East to West. In such conditions, a constant volume system would deliver a constant volume of air at varying temperatures. The air handling system would always deliver the same amount of air. With a VAV system, the supply air quantity is varied. The major advantage deriving from this reduced air volume would be a reduction in the energy consumed by the fans. The cooling capacity would also be reduced because of absence of terminal reheat.

The air used in a VAV system is first cooled and dehumidified in a central station and then introduced into the conditioned area via terminals having the capacity to vary the quantity of supply air. Each terminal of a VAV system regulates the quantity of supply air to maintain the same temperature.

The successful use of VAV systems depends upon the use of suitable air distribution terminals that are capable of maintaining adequate air distribution over a wide range of discharge air quantities. This demands the use of specifically designed units that can maintain high induction rates, ensure rapid air mixing and good circulation over the entire range of modulating air quantities.

6.2.1 Characteristics of a VAV system

In order to realize the maximum potential benefits of the all-air variable volume system, the design engineer should be familiar with and provide for the maximum utilization of the following major characteristics:

  • Automatic room temperature control should be provided for individual rooms.
  • Zoning of extensive zoned ductwork is not required since the individually controlled variable volume terminals act as separate zones.
  • Because of high induction rates and uniform air distribution, lower supply air temperature can be used without causing uncomfortable cold spots even at reduced volume. This keeps duct sizes and number of terminals to a minimum.
  • Equipment sizes, capital costs and operating costs are reduced.
  • System operating cost and performance is not affected by the excess capacity of the system. The terminals will automatically adjust to the actual load, without penalizing operating cost when operated at reduced loads.
  • Centralized apparatus location minimizes operating and maintenance costs.
  • Centrally located variable volume apparatus permits economical use of high efficiency filtration equipment to provide cleaner air.
  • System permits the use of sprayed coil dehumidifiers to provide better quality air supply and winter humidification.
  • Remotely located mechanical equipment reduces the noise levels in the conditioned space.
  • System changeover problems are eliminated because the heating system is controlled independently (of the cooling system).

6.3 Different VAV systems

Several variations in the basic VAV system are possible according to the load profile in an air-conditioned area.

6.3.1 VAV without heating

This is considered the simplest form of the system. The load profile shown in the figure below shows that the system will be able to meet all the needs of an interior zone, because the cooling load is all year round. Air-conditioning load consists of heat from occupants, lights and miscellaneous equipment. This is a consistent year around load irrespective of the ambient temperature. The interior-cooling load varies only in magnitude depending upon occupancy and use. Therefore, the variation in supplied quantity of cooled and dehumidified air will satisfy the changes in the load without the need to heat partial loads.

Figure 6.2
Typical load profile - interior zone of office building

6.3.2 VAV with heating

The effect of perimeter transmission gains or losses and of solar gain gives a load pattern of exterior areas, which differs from interior areas. The following figure shows how the ambient to room temperature difference affects the transmission gain or loss in terms of sensible heat.

Figure 6.3
Typical load profile – perimeter zone of office building

Any air-conditioning system should have the capacity to meet the changes in loads, which at any given ambient condition can vary between heating and cooling requirements depending upon the variables. They can adequately cope with these variables but do not provide facilities of simultaneously heating and cooling. It is necessary to provide an alternate source of heating at the perimeter so that the room temperature is maintained uniform throughout.

The following figure illustrates the heating requirement at the perimeter in conjunction with the variable air volume system.

Figure 6.4
Typical variable air volume load profile for perimeter zone with perimeter heating

6.3.3 Central plant for a VAV system

To maximize the economy of the plant operation, the all-air VAV system comprises of a central air handling unit with economizer dampers, an exhaust fan, and a medium pressure supply fan having a constant pressure device and a filtration and cooling system. A medium pressure air distribution ductwork system is also necessary to connect the air terminals.

The following figure shows a typical central station system suitable for a variable air volume application in a separate plant.

Figure 6.5
Components of central station apparatus for variable air volume systems

The return and supply air fans are controlled by variable inlet guide vanes, which are operated by a static pressure controller. Return air, exhaust and outside air dampers are controlled to provide maximum free cooling whenever the ambient temperature is lower than the return air. When the outdoor air is below the desired dew point setting a constant DPT is maintained by mixing return and outdoor air without the use of refrigeration. Refrigeration comes into the picture to control the constant DPT when the free cooling does not satisfy the demand.

6.3.4 Terminal diffuser for a VAV system

The growth of VAV systems remained slow due to the lack of proper terminals at the outlet of the supply system. Conventional ceiling diffusers do not give satisfactory air distribution when used with a VAV system.

The supply system used with VAV systems must satisfy the following requirements:

  • It should have a volume-regulating device, which maintains a constant supply of air regardless of the varying duct pressure.
  • It should have a high induction ratio.
  • It must provide free air distribution with supply air quantity even at 20% of full-load air volume.
  • It should possess a thermostat for individual room control.

Carrier International Corporation designs the terminal fulfilling all the above requirements.

This unit is a part of the duct system. The insulated plenum is used for insulation and sound attenuation. The air passes through a sound proof plenum and automatic dampers. The dampers maintain the air quantity as set on the regulator and control the room temperature in response to the thermostat. The air is then discharged through the diffuser’s slot and mixes immediately with the induced air. Air distribution takes place very quickly and the rate of induction is very high. A large temperature difference between the room and supply air reduces the air temperature and the air quantity, which reduces the energy consumption of the supply fan. Irrespective of the load, it is required to maintain the air throw constant, which requires considerable energy for inducing the secondary air.

Since the secondary source of energy is not available, the required energy at reduced volume is taken from the primary air jet. If the supplied air quantity is reduced, the air mass and its velocity also change linearly and the kinetic energy varies as the cube of air quantity. If the mass flow and the velocity are reduced simultaneously the available kinetic energy drops and becomes insufficient to produce the correct throw.

6.3.5 Arrangement of terminals in a VAV System

The following points have to be kept in mind for arranging the terminals in a VAV system.

  • The terminals must integrate with the light fittings within the ceiling.
  • They should be capable of providing adequate air distribution within the zone throughout the spectrum of air volumes, when the zone is occupied.
  • They should operate within the sound level of the space.
  • The terminal should have the flexibility of control such that a single or group of terminals can be controlled as the requirement is varied.

The location terminals largely depend on the type of control used at the terminal. Various approaches to a common perimeter area are illustrated in the following figures.

Figure 6.6a
Typical ceiling layout ‘Moduline’ system with wet perimeter heating
Figure 6.6b
Typical ceiling layout - Dual conduit system

The above figures show systems of the self-powered type where only one thermostat is located in the terminal unit itself.

Figure 6.6c
Typical ceiling layout ‘master satellite’ system with wet perimeter heating
Figure 6.6d
Terminal reheat VAV system

The above figures show arrangements, which have either electric or pneumatic controls, which are powered, from a remote source and are controlled from a room thermostat.

The primary function of a VAV is to provide the correct amount of air to satisfy the exact load in a given area. So it is essential that the designer considers whether the terminal system chosen has the capacity to change the thermostat controls and control positions to meet the changing needs of the building.


7


Duct design, air flow and its distribution

Objectives

After reading this chapter the student should be able to

  • Understand the basics of air flow through the ducts
  • Calculate the sizes required for ducting for conditioned air
  • Describe and select the various air distribution systems for air conditioning

7.1 Air flow and pressure losses

The successful operation of any air-conditioning system is dependent upon the efficient circulation of air in the air-conditioned space. A careful estimation of pressure losses is necessary for the selection of a proper duct size. The cost of ducting in an air-conditioning system is 10 to 30 % of the total capital cost. The power required by the fan contributes substantially to the operating costs. It is therefore important to correctly design an air-duct system such that the capital cost is kept to the minimum and operating cost is low.

The total energy required by the fan to pump air must be equal or more than the pressure losses encountered in the ducting. The fan delivers energy to air in the form of static pressure and dynamic pressure. The sum of both the pressures given by the fan is used to overcome pressure losses caused in the airflow path.

The ducting system typically produces three types of losses.

7.1.1 Shock loss

The pressure loss, which takes place due to an abrupt change in the air direction or in a duct cross-section, is known as shock loss.

7.1.2 Friction loss

Friction loss is directly proportional to the length of the duct and the velocity head. It is inversely proportional to the duct diameter. To minimize the friction losses, the duct diameter should be the same as the fan inlet and outlet diameters.

The frictional resistance of a duct of any cross-section is given by the equation, as follows:

Where:

ΔP is the pressure loss due to friction in Pa.

f is the friction factor.

L is the length of the duct in meters.

D is the diameter of the duct.

V is the velocity in m/sec

e is the air density Kg/m3

Figure 7.1
Moody’s Diagram

The friction factor (f) in the equation above can be found on the Moody diagram or by using the following relationship:

Reynolds Number is given by the equation:

Re = (ρ× D × V) / μ

Where:

ρ is the density of air in kg/m3

D is the diameter of the duct.

V is the velocity of air in m/sec.

μ is the dynamic viscosity of air in kg/msec.

After calculating the Reynolds number from the above equation, the friction factor for smooth ducts can be computed as follows.

The Reynolds Number can be used to determine if flow is laminar, transient or turbulent. The flow is:

  • laminar when Re < 2300
  • transient when 2300 < Re < 4000
  • turbulent when Re > 4000

f = 0.3164 / (Re)0.25 …….when Re >2300 and < 200000

For rough duct surfaces, friction factor (f) is given by the equation:

f = 1 / [1.74 – 2 log e ( 2ɛ /D ) ] …. Where

ɛ is the roughness in m

The following table gives the recommended roughness for pipes of different materials:

Table 7.1
Absolute roughness of the ducts/pipes

7.1.3 System effect loss

These losses are those that affect the flow pattern of air entering or leaving the fan. Since these flow distortions reduce the fan pressure, they are typically much more serious than shock and friction losses. Unfortunately, system effect losses are often overlooked, because design manuals rarely treat these losses as extensively as they do shock and friction losses.

A few examples of system effect losses are:

  • The right angle elbow placed close to the fan inlet can cause flow distortion. The elbow turns the air causing it to swirl within the inlet.
  • The damper placed immediately at the fan outlet can produce a resistance of 1.75 times than that produced by a similar object placed away from the fan.

The actual total pressure requirements of a duct system are determined in two ways. For residential and light commercial applications, the heating and cooling loads determine the heating, cooling and air moving equipment. Therefore, the fan characteristics can be known before the duct design is done. Also, the pressure losses in all other elements except the supply and return ducts are known. The total pressure available for the duct is then the difference between the total head available from the fan and the sum of pressure losses of all other elements in the system excluding the ducts.

The following figure shows a typical total pressure profile for a residential or light commercial application.

Figure 7.2
Total pressure profile for a typical residential or light commercial system

In the above case, the fan is capable of developing 15 mm of water column (mmWC) at rated capacity. The return grill, filter, coils and diffusers have a combined pressure drop of 10.45 mmWC. Therefore, the total available pressure for which the duct must be designed is 4.05 mmWC.

7.2 Dynamic losses in ducts

When there is a change in the direction or velocity of flow through the duct, pressure drop is inevitable. The additional loss in excess of the straight duct friction loss is the dynamic loss. The dynamic losses are due to:

  • Change in the direction of flow through bends and elbows.
  • Sudden change in the cross-section area due to enlargement, contraction, damper controls, suction and discharge openings.

The dynamic pressure drop is expressed as a product of dynamic velocity pressure and the dynamic pressure drop coefficient (Kd).

The losses in elbows, bends, fittings and valves are expressed in terms of equivalent length (Le) of the duct.

Let us now study the computation method of each of the dynamic losses.

Pressure losses in elbows

The different types of bends and tees used in ducting are shown below:

Figure 7.3
Different types of bends and tees used with ducts

The values of (Le /D) for different types of elbows are given in the following table.

Table 7.2
The values of (Le /D) for different types of elbows

Loss due to sudden enlargement of the cross-section area

As the air flows from the smaller area into a larger one, the velocity decreases and the difference in velocity is converted into static pressure.

Figure 7.4
Sudden Enlargement of duct cross-section

The pressure drop is given by the equation:

ΔP = [ (A1 / A2) – 1 ]2 [ 0.5 × ρ × V2 2 ]

Where:

A2 is the enlarged cross-section area.in m2

A1 is the cross-section area of the duct.in m2

V2 is the velocity head at the enlarged cross-section area.in m/sec.

ρ is the density of air in kg/m3

Losses due to sudden contraction

An abrupt contraction is geometrically the reverse of an abrupt enlargement. Here also the streamlines cannot follow the abrupt change of geometry and hence gradually converge from an upstream section of the larger tube. However, immediately downstream of the junction of area contraction, the cross-sectional area of the stream tube becomes the minimum and less than that of the smaller pipe. This section of the stream tube is known as vena contracta, after which the stream widens again to fill the pipe.

Figure 7.5
Flow through a sudden contraction

The velocity of flow in the converging part of the stream tube from Sec. 1-1 to Sec. c-c (vena contracta) increases due to continuity and the pressure decreases in the direction of flow accordingly in compliance with the Bernoulli’s theorem.

Where Ac represents the cross-sectional area of the vena contracta, and Cc is the coefficient of contraction defined by:

Equation is usually expressed as:

Where:

Although the area A1 is not explicitly involved, the value of Cc depends on the ratio A2/A1. For coaxial circular pipes and at fairly high Reynolds numbers.

Table below gives representative values of the coefficient K.

Table 7.3
Area Ratio Vs Coefficient K

Pressure loss in the fittings

When air is diverted from the main duct to the branch duct, the velocity reduces in the main duct. If there is no loss, the change in the velocity is converted into static pressure. But the actual static pressure regain is reduced by a regain factor (Rg). The value of Rg for a well-designed round duct is 0.9. In case of rectangular ducts, the value of Rg should be as low as 0.5.

7.3 Duct design

The ducts should be designed to carry the required quantity of air from the fan to the air-conditioned space with minimum losses and optimum sizing.

The velocities in the duct should be high enough so that the size of the duct can be kept to the minimum and it should be low enough to reduce the noise level and pressure losses.

The velocities recommended for various purposes are tabulated below

Table 7.4
Recommended air velocities in ducts (m/sec.)

There are three basic methods commonly used for duct design:

7.3.1 Equal friction loss (pressure drop) method

The size of the duct is designed to give equal friction loss per meter length of the duct.

The advantage of this system is:

  • If the duct layout is symmetrical giving the same length in each run, then dampers are not required to balance the system, as this method gives equal pressure drops in all branches.

The disadvantages are:

  • If the duct runs are of different lengths, the shortest run will have minimum pressure drop and air will flow with high pressure.
  • This method does not balance the pressure at the outlet if the lengths of the duct runs are different and dampers are needed for balancing the pressures.

7.3.2 The static regain method

For a perfect balancing of the air duct layout, the pressure at all outlets must be the same. Equalizing the pressure drops in all the branches can do this.

For this, the friction loss in each run should be equal to the pressure gain by reduction in velocity. The pressure gain due to reduction in velocity is given by the equation:

= 0.5 [ ( v12 – v22 ) / 2 × g ]

Where 0.5 indicates the regain efficiency.

The advantages of this system are:

  • It is possible to design long runs as well as short runs for complete regain.
  • It is sufficient to design the main duct for complete regain and use the same pressure at the outlet of the branches.

This method allows for balancing but reducing the velocity increases the duct size and it should not exceed the economic limit.

7.3.3 Velocity reduction method (equal velocity method)

The ducts are designed in such a way that the velocity decreases as flow proceeds. The pressure drops are calculated for these velocities for respective branches and main the duct. The fan is designed to overcome the pressure losses along any single run including the losses of main duct, branch duct, elbows, valves etc. Dampers in the respective ducts adjust the pressure at the outlet.

The advantages of this system are:

  • This method is easiest among all methods in sizing the duct diameters.
  • The velocities can be adjusted to avoid noise.
  • This is adopted only for simple systems.

The major disadvantage is that it requires considerable judgment in selecting the velocities to make the system optimum in economy and power.
The procedure for designing the duct system using each of the methods described above is illustrated in the sample calculations of this chapter.

7.4 Duct arrangement systems

The ducts carry conditioned air from the air handling units to the air supply openings in the room and carry the return air from the room back to the air-conditioner for re-circulation.

Different duct layout patterns are described below:

7.4.1 Perimeter system

The perimeter system may be of loop type or radial type as shown in the following figures:

Figure 7.6a
Primary loop system
Figure 7.6b
Radial perimeter system

The air conditioner is usually placed in the basement and located near the geometric center of all the outlets. The supply outlets are placed closed to the ceiling level. The ducts run through the basement and connect the air conditioner to the outlet grills. The return grills are generally located on the bottom side of the inside wall. This is a common arrangement generally used for a residential system.

7.4.2 Extended plenum system

The arrangement of ducts in this system is shown in the following figure:

Figure 7.7
Extended volume system

The advantage of this system is that the grills can be located wherever required according to the structural needs. The air-conditioner can be located in the attic, in the basement or any other convenient location. Such layouts are used for residential or commercial applications.

The system, which is commonly used in commercial applications, is shown in the figure below.

Figure 7.8
Arrangement of duct for commercial purpose

7.5 Air distribution system inside space

The main objective of the air distribution system in an air-conditioning system is to create a proper combination of temperature, humidity and air circulation in the occupied zone of the conditioned space. To obtain the required comfort conditions within this zone, standard limits have been established as acceptable effective temperatures. This term comprises of air temperature, air motion, relative humidity and their effects on the human body.

Conditioned air must be delivered to the air-conditioned space and distributed to the desired point at the desired temperature and velocity. The temperature variation should not be more than 2°C. The desirable air circulation around the human body is 0.125 m/sec.

The following points are preferred in an air distribution system:

  • Flow direction of air should be towards the faces of the people.
  • Downward flow is preferred over an upward flow.

There are several methods of air distribution systems that can be efficiently used for different purposes. Let us discuss the common distribution systems.

The distribution system shown below is commonly used for many installations and more suitable for summer air conditioning systems. The supply and exhaust grills are located in the same wall. This system is economical and simple in construction.

Figure 7.9
Excellent for cooling and heating

The velocity at the outlet of the supply grill must be sufficient to carry it out. It should not be high enough to cause impingement on the opposite wall that may cause objectionable downward and reverse currents.

The pan type arrangement of the supply air as shown in figure 7.10a gives uniform discharge around its periphery.

Figure 7.10a
Supply air –Pan type arrangement

Similarly, the distribution system shown in figure 7.10b below combines the supply and return openings in a single unit. This method is used for cooling as well as heating applications. If used for cooling, the effectiveness is assisted by natural upward convection currents. This system is more useful when the ducts cannot run in partitions or columns.

Figure 7.10b
Excellent for cooling and heating

When only ventilation is desired the inlet and outlet grills can be located as shown in the following figures.

Figure 7.11a
Preferable for heating
Figure 7.11b
Locating supply and return grills

The incoming air should have a low velocity to avoid uncomfortable drafts. This distribution system is used for winter air conditioning in single-storey buildings.

Perforated ceiling panels are nowadays commonly used to distribute conditioned air. As shown in the figure below, it is possible to deliver a large quantity of conditioned air through such panels without the uncomfortable draft affecting the people as the diffusing effect of the air stream creates mixing within the conditioned space. The only disadvantage of this system is its high cost of construction.

Figure 7.12
Preferable for cooling

7.6 Ventilation systems

7.6.1 Natural ventilation

The flow of air caused due to pressure difference between the ambient air and the air in the conditioned space is known as natural ventilation system. The flow of air caused due to this is termed wind. Wind blow is always uncertain, both in its direction and in speed. There is no control of dust and odor carried by incoming air. Consequently natural ventilation has no significance in air conditioning systems.

7.6.2 Mechanical ventilation

The controlled ventilation (flow quantity and direction) caused with the use of a fan is called mechanical ventilation. This is essential when the temperature, humidity and air motion are to be controlled in an air-conditioned space. Generally, three different systems of mechanical ventilation are used in practice.

Extraction System

This system is most widely used for ventilation in an air conditioning plant. The air from the air-conditioned space is withdrawn by using a fan. This is accomplished either by using propeller type exhaust fans or by means of a duct, depending on the requirements. The exhaust produces a low-pressure zone around it, which causes a flow of air towards the fan.

The desirable and undesirable location of inlet points is shown in the following figures.

Figure 7.13a
Desirable Position of Inlet
Figure 7.13b
Un Desirable Position of Inlet

As shown in figure 7.13b, the short-circuit caused will not give required ventilation in remote parts of the air-conditioned space. The effectiveness of an extraction system largely depends on the location of inlets with respect to the exhaust outlets. The distance between the inlet and the exhaust should not be excessive enough to contaminate the air while it traverses the occupied space.

Supply systems

In this system, air is supplied under pressure to the conditioned space by means of a fan. The advantages of this system over the extraction system are:

  • Since conditioned air is introduced under positive pressure, better control over quantity, velocity and its distribution is possible.
  • The re-circulation of interior air for heating systems can be achieved more conveniently.
  • Since a positive pressure is maintained in the conditioned space, it eliminates all possibilities of creating a negative draft.

Combined Supply and Extraction system

To ensure uniform fresh air, a combined system is always preferred. In this system, fresh air is introduced at required points and its uniform distribution in the occupied space is achieved with the help of extraction and supply fans.

In a combined system, the capacity of the supply fan should be 20 % more than the exhaust fan to maintain the pressure in the conditioned space, above atmosphere. This also prevents negative draft and infiltration of dust and other air-borne contaminants.

With rooms of moderate widths, it is preferred that the inlet air and the exhaust are located on the walls opposite to each other. If the distance between the walls is more, then it is important to provide an extracting duct to avoid the possibility of short-circuiting. This is illustrated by the following figure.

Figure 7.14
Locating extracting duct

7.7 Effect of vertical temperature gradient and corrective measures

Uniform temperature in an air-conditioned room is the prime requirement of comfort. However a temperature gradient can exists irrespective of all measures taken during the design. Its magnitude depends on the volume of air-conditioned space and the method of air distribution used. In cases where several temperature gradients exist, corrective measures should be taken to eliminate the gradient for comfort.

The temperature gradient depicts the ascent of hotter and lighter air and the descent of colder and heavier air within an enclosure. Temperatures will be highest near the ceiling and lowest near the floor.

Temperature gradient is the critical factor in the cost of heating an enclosure because it is the colder zone, from 1 meter to 1.5 meters off the floor, where the requirements for human comfort are determined. Yet, it is the ceiling and the upper region of the outside facing walls, where the heat loss is at its maximum. Insulation will retard the amount of heat loss but will not eliminate the cost of maintaining higher temperature in the upper regions of the enclosure.

7.7.1 Corrective actions

Once the problem areas are identified, the next step is to take corrective measures. Ceiling height, free standing obstructions and heat emitting equipment are the factors, which contribute to the temperature gradients. Poor air circulation is mainly responsible for temperature gradient. Hot air cannot be prevented from rising but proper circulation can prevent it from staying in one place. The air circulation should ensure proper mixing of hot air and colder air, which exist at different elevations within the room.

The ideal air patterns are those which push hot air downward in a cone shape with a spread of approximately 100 to 300 degrees, as shown in the figure below.

Figure 7.15
Ideal Air Pattern Is Depicted Which Aspirates Hot Upper Air

This activates the phenomenon of aspiration to draw the stagnant hot air away from the ceiling and mix with the downward flow of new air.

Vents used to discharge air into the enclosed space are adjustable so that the shape and direction of initial flow of air can be influenced to suit the installation. Air flow can also be directed to a shallower angle towards the walls as shown in the following figure. This will aspirate the upper air as the new air current is deflected downwards along the vertical surface.

Figure 7.16
Ideal Air Pattern causing discomfort to workers working below diffusers can be adjusted to ‘bounce’ air off adjacent walls down to occupied level

Along with these adjustments in the air circulation patterns, repositioning the return air grills may be beneficial. The floor level return grills are not advisable if the same space is cooled during summer. In such cases, grills can be designed to heat the room at either ceiling or the floor. The ceiling return can be fitted with ductwork extending to the floor and terminating with another grill. During the heating season, the upper grill will be closed and the bottom one will be open. During the cooling season the upper grill will be open and the bottom one will remain closed.

A downward air flow in air distribution systems is a more common type of air-conditioning system as it offers the following advantages; where cooling is always required:

  • The air speed required to overcome the convection of up-currents is only about 0.3 m/sec; therefore the power consumed is only 33 % of the horizontal flow patterns.
  • There is complete freedom of movement and access for occupants everywhere in the air-conditioned space.
  • Supply air temperatures can be low, therefore duct sizing and capital costs are reduced.
Table 7.5
Recommended air velocities in air-conditioning equipments (m/sec.)

7.7.2 Factors considered in air distribution systems

The requirement of a good air distribution system is to provide a proper combination of DBT, RH and air motion in the air-conditioned space. This comfort condition is generally required at 1.8 meters above the floor level. The maximum variation in temperature should not be more than one 0C. The desirable terminal air velocity lies between 0.125 to 0.15 m/sec.

Draft

It is defined as the feeling of warmth or coolness due to air motion at the required DBT and RH. It is measured above and below the controlled room conditions of 24.5 °C DBT and a terminal air velocity of 0.15 m/sec. Therefore to avoid the feeling of draft, proper air distribution in the room is absolutely essential. The desirable velocities of air taking into consideration the permissible sound level are listed below:

Table 7.6
Desirable Velocities of air in places (m/sec.)

To maintain the required DBT and RH with proper air circulation in the conditioned space, fresh inlet air and room air should mix properly with minimum pressure loss.

The principles of air distribution involve the following factors:

Throw

It is the distance traveled by the air stream in the horizontal plane after leaving the air outlet and reaching a velocity of 0.125 m/sec. at a height of 1.8 meters above the floor level.

Drop

It is the vertical distance the air moves after it leaves the outlet and reaches the end of throw.

Entrainment or Induction Ratio

The air leaving the outlet is known as primary air and the air entrained by the primary air from the room is known as secondary air. The sum of primary and secondary air is known as total air. The entrainment or induction ratio is: Induction ratio = Total air / primary air

Higher value of induction ratio is desirable for proper mixing and uniform DBT and RH of air in the room. The throw depends upon the supply air velocity, temperature difference between the primary air and the room air and the induction ratio.

Spread

It is defined as the angle of divergence of an air stream after it leaves the outlet. The spread depends on the type of outlet used. Air outlets can have three types of vanes: straight, converging and diverging.

Outlets with straight vanes produce a spread angle of 14 to 24 degrees in the horizontal as well as vertical plane.

Outlets with converging vanes produce the same angle as straight vanes but the throw is 15 to 20 % higher. Outlets with diverging vanes give a fanning effect.

Types of supply air ducts

The outlets are classified as sidewall outlets, ceiling and floor outlets according to their locations. The basic types that are commonly used are described below.

Grill outlets

These outlets have adjustable bar grills, which are the most common types with horizontal and vertical vanes. These are similar to the grills used for evaporative type home coolers and are rarely used for comfort air conditioning as they create draft.

Slot diffuser

It is an elongated outlet with an aspect ratio of 25:1 and maximum height of 75 mm. These types of outlets are used on the sidewalls at a higher elevation and along the periphery of the floor.

Ceiling outlets

These types of outlets are mounted in the ceiling. Multi passage, round, square or rectangular are the most common types. They are also provided with adjustable louvers to vary the amount of air.


8


Insulation of Air-Conditioning Systems

Objectives

After reading this chapter the student will be able to

  • Understand the requirements of insulation in HVAC systems
  • Know the various insulation material available and their properties
  • Calculate the required insulation thickness

8.1 Introduction

The rapidly increasing cost of fuels in recent years has made it very important to ensure that all air-conditioning systems are well insulated. This has become imperative from the energy conservation point of view.

Materials having extremely low thermal conductivity are called insulating materials. The heat flow rates (from buildings to outside in winter air-conditioning and from outside to buildings in summer air-conditioning) are reduced substantially with the use of correct insulation.

There are numerous kinds of insulating material. The selection of an insulating material for a particular purpose depends on the number of required properties of the insulating material. The selection is also done based on economic and structural considerations.

8.2 Desired properties of an ideal insulating material

The required properties of an ideal insulating material are:

  • Low heat conductivity
    The ability of material to conduct heat through it is known as its thermal conductivity. It must be as low as possible to reduce the required thickness of insulation.
  • Permanence
    Insulating material may disintegrate as a result of internal chemical activity over a period of time or due to its exposure to the surroundings. It should have high resistance to such disintegration.
  • Strength
    The insulating material used must withstand the pressures exerted on it. Some materials are more suitable to various types of constructions. Structural strength is generally obtained by the use of wood or framework.
  • Light weight
    This is necessary to avoid the use of heavy structural members. This point becomes very important for air-conditioning systems used in automobile, railway, marine and aircraft applications.
  • Water repellent
    Moisture absorbed by the insulating material increases its conductivity and reduces strength. Material that resists absorption either as free water or water vapor should be used.
  • Sanitary
    Material that provides a medium for vermin infestation should be excluded.
  • Odorless
    It should not produce any type of objectionable odor when wet or dry. Insulation materials derived from vegetable sources are always subject to decomposition over a period of time. Such materials may develop odors and should not be used as insulating materials.

Insulating material generally has four basic uses, all of which govern the selection of the kind of materials to be selected.

  • To retard heat flow.
  • To prevent surface condensation.
  • To add structural rigidity.
  • To control noise and vibration.

8.2.1 Advantages of providing insulation

Any thermal insulation prevents loss of heat, in or out of the system and provides the following advantages:

  • Reducing the overall operating cost by reducing the fuel consumption.
  • Reduces the heating or cooling requirement by cutting down the heat losses or heat gain.
  • Better process control can be achieved by reducing the temperature drop in heating systems and temperature gain in cooling systems.
  • It provides fire protection to the plant and piping (provided the material is non-flammable).
  • It helps to reduce the noise level in the system and absorbs vibrations.

8.3 Factors affecting thermal conductivity

Low thermal conductivity is the prime requirement of an insulating material used in either refrigeration or air-conditioning. Therefore, it is necessary to know the parameters that affect this important property of insulating material.

The major parameters that affect the thermal conductivity are density, pressure, moisture, temperature and porosity. The qualitative effects of these parameters are shown in the following figure:

Figure 8.1
Factors affecting thermal conductivity

The molecules in high-density material or under high pressure are nearer to each other, which help to conduct heat in a much better way. Therefore, the increase in the density and pressure increases the thermal conductivity. Moisture also increases the conductivity, as water is a good conductor of heat, compared to air. Water conducts heat at least 25 times more than air. Porosity is a measure of how porous the material is. Highly porous materials have lots of air pockets and air is a poor conductor of heat.

8.4 Types of insulation materials

These are different types of insulating materials, the selection of which depends on the use for which it is intended.

There are five basic types of insulation, which can be used alone or in combination:

  • Flake insulation is composed of small particles that finely divide the air space. Vermiculite or mica is the most common material.
  • Fibrous insulation is composed of small diameter fibers that finely divide the air space. They may be organic or inorganic and may or may not be bonded. Inorganic fibers such as glass wool or rock wool are the most common materials.
  • Granular insulation is composed of small nodules that contain voids. It is not considered a true cellular material because gas can be transferred between individual spaces. Typical materials are magnesia and calcium silicate.
  • Cellular insulation is composed of small individual cells that finely divide the air space. It is produced from glass, rubber or plastic.
  • Reflective insulation is composed of parallel thin sheets of foil having high thermal reflectance spaced to restrict radiant heat transfer across the space. Aluminum and stainless steel foils are used for this purpose.

8.4.1 Basic properties of common insulation materials

Table 8.1
Basic properties of common insulation materials

Wool type insulators

Rock wool, mineral wool, glass wool and slag wool are the names given to insulating material made by melting the principle element from which they are formed, and air blown into fibrous form, after which they are usually ground.

Foils

Aluminum foil as insulation is used in refrigerated rail cars, refrigerated compartments on board ships, household cabinets and other applications where lightweight is needed and a big Δt where there could be high radiation losses.

The required thickness is obtained by means of successive layers of aluminum foil each about 0.001 mm thick.

It has the following advantages:

  • Moisture and vermin proof.
  • Free from odors.
  • High resistance to fire.
  • Good strength and light in weight.

8.4.2 Special insulating material

Many high quality insulating materials have been developed over the last decade with extensive research work. Let’s discuss some of them.

Silica gel

When silica gel is dried by heating at normal pressure, it shrinks to one-fifth of its original volume. When the water in the aqua gel is replaced by alcohol and the product is heated to the critical temperature of alcohol with a pressure in excess of critical pressure, shrinkage is eliminated and the product left is a bulky substance. The density of this substance is about 90 kg/m3. It has very low thermal conductivity.

Foam glass

It is the trade name of porous glass block insulation with sealed pores. It is suitable for exterior walls and floors or low temperature rooms due to its structural strength.

Fiberglass

It is found that fibrous insulation materials are the most suitable and efficient ones among the wide range of insulating materials available.

It has the following advantages:

  • Fiberglass serves a dual purpose of providing thermal as well as sound insulation when applied to ceilings and walls.
  • It is light in weight and low in cost.
  • It reduces the operating cost of an air-conditioning plant due to its low thermal conductivity and less wear and tear.
  • It is non-hygroscopic.
  • It is non flammable. It has class-I spread of flame when tested in accordance with BS : 476.

Plastic foams

Foamed plastics are finding rapidly growing applications as thermal insulation for modern air-conditioning systems. The demand is for foamed polystyrene and polyurethane.

Key considerations in selecting plastic foam as insulating material are thermal conductivity, compressibility strength, water vapor transmission, dimensional stability and method of installation.

A few of the plastic foam insulating materials are discussed below.

  • Polystyrene
    Principally this material is available in the form of sheet or pipe covering and is used in cold storage rooms, tanks and vessels. Another well established application of extruded polystyrene foam is as a core for sandwich panels, encased in aluminum, steel or plywood.
  • Urethane
    Rigid urethane foam is a cellular plastic form of material. It is formed when two liquids, isocyanates and polyols are mixed in the presence of certain additives and catalytic agents. The mixture foam after 10 to 30 seconds expands to 30 times its original volume. It fills the cavity to be insulated and hardens into an airtight mass with many tiny cells; each filled with gas produced by the foaming process.

It is available in a variety of forms like slabs, panels, moldings, pipes, ducts and liquid that can be poured on the job to form an insulating layer. Urethane is the only insulating material, which can be sprayed as a liquid to reach virtually inaccessible areas.

Urethane foams can be used for temperature ranges of –200°C to 120°C. In addition to the above properties, they are chemically inert and under normal usage do not deteriorate when exposed to the atmosphere.

Various properties of plastic foam insulating material are shown in the following table.

Table 8.2
Various properties of plastic foam insulating material
Insulating Material Urethane Fiber glass Polys tyrene Flexible Vinyl Rigid Vinyl Foam Glass Vermiculite
Tensile strength kgf/cm2 3.8 7.4 2 - 4.2 - -
Shear strength kgf/cm2 2.0 - - - 2.2 - -
Compressive strength
kgf/cm2
2.5 0.20 1.2 - 2.2 7.2 -
Service Temp. range in °C –220 to +120 –20 to +280 –20 to +110 –27 to +105 –20 to +85 - –250 to +1050
Flammability (self-ignition temp. is 500°C) Self ext. Non combustible Self ext. Self ext. Self ext. Non combustible Non combustible
Thickness of equal Insulation Value 1 2 2.5 2.5 1.5 3.2 4.3
Types available Foam board pipe Batt.
pipe
Board Sheet pipe Board Board Looses fill
Conductivity (K) kcal/m-hr-°C (w/mk) 0.165
(0.023)
0.330
(0.036)
0.420
(0.035)
0.420 0.255 0.525 0.720
(0.068)
Vapor Permeability per-m-cm 3.25 - 5.0 0.30 <0.25 0.25 -
Water absorption % by volume 3 - Zero to 3 3.5 1 to 2 0.2 500
  • Baytherm Rigid Polyurethane foam
    Baytherm is one of the engineering plastics from Bayer’s extensive range of polyurethanes. It is rigid foam of low specific gravity and has a closed cell structure. Baytherm is a two component engineering plastic produced by a reaction between Desmodur and Baytherm. The closed cells of gelled foam are filled with vaporized blowing agent. Hence, the thermal conductivity is as low as 0.019 W/m-K. It is the most efficient of all known insulators and is very easy to apply.

Its wide use in refrigeration is due to its twin function as insulator and strengthener. Its excellent heat insulation, bonding action and ability to penetrate into the cavity has made the manufacture of refrigeration equipment most economical.

8.5 Heat transfer through insulation

The heat transfer through the structure takes place by conduction, convection and radiation. The insulating material reduces the heat transfer by replacing the short path of low resistance by a long path high resistance.

The heat flow by conduction is reduced due to the long path followed through the insulating material and due to low conductivities of air gap and the material itself.

If there is a no continuous path of air inside the insulating material then the heat transferred by convection is neglected. Convection heat transfers generally take place in materials like glass wool, which provides a continuous path of airflow.

The different modes of heat transfer in insulating materials are shown in the following figure.

Figure 8.2
Different modes of heat transfer in insulating materials

8.6 Economical thickness of insulation

As the insulation thickness increases, the cost of insulation increases. The net saving in the heat losses is the function of cost of insulation. The graphs for insulation cost and saving in cost due to heat reduction with respect to the insulation thickness are shown below. Combining the effect of the two, total cost graph can be obtained as shown in the same figure. A method for calculating the most economical thickness of insulation has been proposed by Stone as:

X = [ A × K / B ] ½ – [Rt × K]

Where:

X is the most economical thickness in cm

K is the conductivity of insulation W/mo C

Rt is the sum of all heat resistances

A = Y ( T1 – T2 ) N / 200,000

Where:

Y is the hours of operation year.

(T1–T2) is the overall temperature difference.

N is the cost of removing 200,000 kJ of heat.

B = (I / S) + (Interest rate / 2 ) × (S + I)/S

Where:

I is the investment per sq.m area of per cm thickness.

S is the expected life in years.

Figure 8.3
Insulation Cost

8.7 Insulated systems

8.7.1 Insulation applied to hot surfaces

A typical arrangement of insulation is shown in the following figure.

Figure 8.4
Insulation applied to hot surface

When the insulation is applied to a hot surface, moisture from the insulation vaporizes and the vapor pressure near the hot surface increases, which tries to escape into the low-pressure ambient air. Thus, the heated surface tries to dry out the insulation in contact with it.

As shown in the figure, a weather barrier is always provided on the insulation to prevent the insulation from absorbing moisture from the air. The barrier material provided for hot surfaces should not be a good vapor barrier; otherwise, the vapor pushed out form the hot surface condenses on the inner surface of the barrier.

8.7.2. Insulation applied to cold surfaces

When the insulation is applied to cold surfaces, the water vapor from ambient air tries to pass through the insulation towards the cold surface, as the vapor pressure of ambient air is more than that of cold surfaces. This vapor condenses when subjected to freezing temperature. Heat losses are thus increased, as the conductivity of water is 23 times that of air. Moisture has another serious effect on the insulation because the water increases when it freezes and thus ruptures the insulation.

To prevent this, it is necessary to provide a perfect weather barrier. It is also necessary to remove the initial moisture from the insulating material.

8.7.3 Insulation applied to heated buildings

The construction of the building that is provided with air-conditioning is different from ordinary buildings. The walls of an air-conditioned building are composed of three layers of building material as shown below.

Figure 8.5
Insulation applied to heated buildings

For example, if the room is maintained at a temperature of 27oC and RH of 50% with the ambient outside temperature of 5oC and RH of 80%, the vapor pressure of the room and the outside air would be 1766 Pa and 700 Pa respectively. This vapor pressure difference of 1066 Pa will force the vapor to move towards the atmosphere. At the above room conditions, the dew-point temperature (DPT) of air is 15.6oC. The flow of moisture through the walls should be prevented otherwise the escaping vapors would condense when subjected to a temperature less than its DPT. Hence a vapor barrier becomes of utmost importance to prevent the rotting of building material.

8.7.4. Insulation applied to cooled buildings

In this case, the vapor barrier must be provided outside the insulating material to prevent moisture from entering and condensing on the insulation. The arrangement of this system is shown below.

Figure 8.6
Insulation applied to cooled buildings

As a rule, the vapor barrier should be provided on the hotter side of the insulation when the insulation separates two spaces of air at different temperatures.

Two vapor barriers are provided, one inside and one outside the insulating material, when the building is provided with heating in winter and cooling in summer. This arrangement is shown in the figure below.

Figure 8.7
Vapor pressure in ambient air is low in winter and high in summer

8.8 Importance of relative humidity for the selection of insulation

The relative humidity of an air-conditioned building plays a very important role in the selection of insulation. The RH values are limited to 70% for summer cooling and 35% for winter heating as per the comfort requirements.

The condition of air is characterized by its DBT and RH. The water vapor in air is condensed if it is cooled below its Dew Point. Higher humidity in air is more prone to condense as its dew point is higher.

The condensation on the pipe surface carrying cold fluid will start earlier if it is surrounded by air at 80% RH compared with air at 50% RH.

To avoid condensation, pipe surface temperature should be greater than tb if surrounded by 80% RH air or greater than ta if surrounded by 50% RH air. For increasing the surface temperature of the pipe carrying cold refrigerant, insulation should be provided.

The insulation thickness for higher relative humidity air should be more than the thickness required for lesser relative humidity air when the refrigerant temperature is the same in both cases.

Designing an insulation thickness to avoid condensation is a difficult task as it depends on many factors as listed below:

  • Temperature of the cold fluid carried by the pipe.
  • Dew point of the surrounding temperature.
  • Heat transfer coefficient of the outer surface of insulation.
  • The surface characteristics of the insulating material.

Thickness of insulation (T) = ƒ (Ti – DPT) × h × q

Where:

Ti is the temperature of the cold fluid flowing in the pipe.

DPT is the dew-point temperature of the surrounding air.

‘h is the heat transfer coefficient of the outer surface

q is the heat transfer per unit length of pipe

The thickness of insulation should be such that:

Ts (outer surface temperature) > DPT (dew-point temperature of air)

Heat transfer coefficient (h) is important in calculating the economical thickness.

‘h’ comprises of convective heat transfer coefficient (hc) and radiation heat transfer coefficient (hr). The value of ‘hc’ depends on the insulation thickness while the value of ‘hr’ depends on the emissivity of the insulated surface. As the emissivity of the surface decreases, the thickness increases.

The table below shows insulation thickness required for a pipe of 40 mm diameter at various pipe temperatures and relative humidity.

Table 8.3
Insulation thickness for various temperatures and R.H

9


Air conditioning equipment

Objectives

After reading this chapter the student will be able to:

  • Understand the various components used in HVAC systems
  • Describe in detail the components of HVAC systems including
  • Air Filters
  • Humidifiers
  • Dehumidifiers
  • Fans
  • Blowers

9.1 Air filters

The air taken from the atmosphere carries dust, bacteria and odour in the air-conditioned systems, which are harmful to human health. To safeguard the health of the occupants from the harmful sources, it is necessary to remove all the harmful ingredients from the air before being taken into the air-conditioning system.

The removal of all kinds of impurities from the air provides the following advantages:

  • It improves the quality of the product.
  • Protects machines and equipment from corrosion.
  • Ensures better health of occupants and increases efficient work in the air-conditioned space.
  • Eliminates the dust nuisance.

The impurities in air are mainly divided into six forms as described below.

  • Dust is small earthly particles and does not involve a chemical reaction. Dust of coal, cement, corn and small earthly particles fall into this category.
  • Fumes are solid particles generated by chemical reaction such as oxidation.
  • Smoke is the term used for visible aerosols. It is the effect of combustion of organic material like wood, coal, oil or tobacco.
  • Fog is the microscopic water particles resting on small solid particles suspended in air.
  • Pollens from trees, flowers and vegetables are carried with outdoor air into air-conditioned spaces.
  • Bacteria are minute living organisms; some of them are harmless while some are dangerous to human health.

Odours do not come in the category of dust, as they cannot be visualized

Methods of air cleaning

Air cleaning is done by various methods according to the nature of the dust. Most of the dust is removed by air-filters. Dust like bacteria and pollen are killed by air-sterilization and odours are suppressed by the application of ozone.

Air-filtration

Many types of dust are removed only by proper filtration of air. The selection of a particular air filter depends on the nature, diameter and concentration of the dust in air. The diameters of different types of dust and their concentration in different areas are given in the table below.

Table 9.1
The diameters of different types of dust and their concentration in different areas

Air sterilization

The sterilization of air is done to remove the bacteria and germs from the air. Ultra-violet rays eliminate bacteria from supply air when mounted on supply ducts. Bactericidal mists are also used to kill the bacteria. This mist consists of tiny droplets of either propylene glycol or tri-ethylene glycol and is dispersed in air with concentration of one gram to 50 to 200 million cubic centimeters of air. The application of air sterilization is not economical and is done only at places where it is absolutely essential.

Odour suppression

Odour suppression or removal from air is necessary for comfort air-conditioning. The odour creates dullness and reduces the working efficiency. Activated carbon placed in a perforated container is used to remove odour from the air. Whenever the activated carbon becomes ineffective, it can be made effective by heating it up to 550oC.

The use of ozone for the treatment of odours has been proven successful from many years of experience. The unwanted odours are suppressed by the masking of the ozone.

The air filters used in air-conditioning systems are broadly classified into five groups:

  • Dry Filter
  • Viscous filter
  • Wet filter
  • Electric filter
  • Centrifugal dust collector

9.1.1 Dry Filters

Dry filters are subdivided into two forms, as cleanable filters and throw away filters. Dry filters are usually made of cloth, coarse paper, wool or cellulose felt. The dust in the air is trapped or screened while passing through these filters. The velocity of air allowed through the filter ranges between 0.03 to 0.25 m/sec. The filtering material is provided in the form of bags called bag filters to provide the maximum filtration area in less occupied space. The cleaning of these filters is done by providing a jerk, shaking or rapping action to the filters while in operation. Depending on type & thickness, dry filters are capable of trapping up to 99% dust of about 0.5-micron size from the air. A typical bag filter is illustrated in the figure below.

Figure 9.1
Dry filter

Throw away filters are made of glass wool, plastic fibers or vegetable fibers. The pads of one of these materials are set into cardboard frames to form a filter media.

The dry type air filters remove dust particles of diameter 0.3 to 10 microns very satisfactorily. Since it has a limited duct holding capacity, it cannot be used where the dust concentration in the air is higher than 2.5 grams per 1000 m3 of air. However, these types of filters are not capable of removing smoke from air.

9.1.2 Viscous filters

These filters are made in the form of pads and bats using glass wool, steel wool, plastic fibers or copper mesh. These pads are impregnated with viscosine, which is an oil substance. These pads are generally throw away type. However, a few of them can be washed in gasoline and reused again. The viscosine used, should have the following properties.

  • It must have a constant viscosity over a wide temperature range.
  • It must have a germicidal action to prevent growth of bacteria.
  • It should not evaporate more than 1% of its weight during the lifetime of the filter.

9.1.3 Self cleaning viscous filter

These filters take the form of a continuous roll of material coated with oil and are driven by a motor across the air stream as shown in the figure below.

The filtering screen is continuously running over the rolls at the top and bottom, while the bottom roll is in the sump filled with viscous liquid. This viscous liquid washes the screen as it passes through the sump.

Figure 9.2
Self cleaning viscous fluid filter

The principle advantage of the self-cleaning filters is the maintenance cost as compared to the other filters. These filters are generally used for airflow of 80 m3/s to 600 m3/s.

9.1.4 Electronic air filters

The application of electronic air filters for removing the dust is very recent. Dr.Cottel in 1906 developed this filter, which was commercially launched only in the year 1937. Its principle is illustrated in the following figure.

Figure 9.3
Electric filter

Air is passed between two oppositely charged conductors and it ionizes as the voltage applied between the conductors is around 8000 to 15000 volts DC. As the air is passed through the ionized chamber, both negative and positive ions are formed, the latter being large in quantity. The air passing through the ionized chamber is further passed over the collecting unit. This unit consists of metal plates placed 15 to 20 mm apart. Alternate plates are positively charged and earthed and attract negatively and positively charged ions respectively. As the alternate plates are grounded, high intensity electrostatic field exists between the plates. When charged dust particles are passed between the plates, the electrostatic field exerts a force on the charged particles and drives them towards the grounded plates. To remove the accumulated dust, the collector plates are cleaned periodically by washing them with hot water.

The advantages of this air cleaner are:

  • It assures low initial cost, lower operation and maintenance cost.
  • It requires lesser installation space.
  • This filter is very effective for smaller particles like smoke or mist.
  • The power requirement is considerably less as a unit handling 800 m3 of air requires only 200 watts of power.

Few drawbacks of this filter are:

  • Due the closely located charged plates and high potentials used, it is advisable to protect the entire apparatus by placing a fire mesh in the air stream before the ionizing chamber.
  • A pre-filter of another type is necessary to reduce load on this filter.

9.1.5 Centrifugal dust collector

The centrifugal dust collectors are mostly used in industrial air-conditioning.

In this filter, a high velocity air stream is directed into a conical chamber as shown in the figure below. This produces a whirling air current in the chamber and throws the heavier dust particles to the sides. This dust falls out of the air stream and gets collected at the bottom. Clean air is taken out form the top of the dust collector.

Figure 9.4
Centrifugal dust collector

The main advantage of this filter is its ruggedness, which does not require maintenance. However, it is effective only for removing larger dust particles. It cannot remove any dust particle, which remains in suspension with air.

9.1.6 Use of activated carbon filter

Activated carbon filters are used in re-circulation air systems to clean air for re-use and where gaseous and vaporous contaminants need to be removed from ventilated air.

It removes organic contaminants effectively such as odours of occupants, chemicals and solvents. The particles of dust visible to the naked eye measure about 175 microns and these filters can remove particles in one-micron size and above. Carbon works on the principle of adsorption. Sub micron material is accumulated and held throughout the carbon media on the surface of the carbon molecules rather than within them as in adsorption. Different carbon materials capture different size particles.

Unlike other filters, the increase in pressure drop is not there within the activated carbon filter when it accumulates contaminants. When the activated carbon filters are exhausted, it is re-activated to its original state by heating it at a high temperature in a controlled process, which makes carbon porous.

These kinds of filters are used in buildings or closed places where some or all the air is re-circulated.

9.1.7 Selection of air filters

The factors to be considered while evaluating the suitability of an air filter for a particular application are:

  • The degree of air cleanliness required
  • Amount of air handled.
  • Type and amount of particulate matter in the air to be filtered
  • The method of disposal of the collected dust.

These factors determine the capital cost, operating cost and the maintenance requirements of the filter installation.

Let us briefly compare the relative significance of the above factors on different filters discussed above.

Table 9.2
Comparison of different air filters
  Viscous Filters Wet filters Electronic filters Dry Filters
Capital cost Low High High Low
Maintenance Cost Low Low high Low
Filtration efficiency Low high high high
Application Suitable for unitary air-conditioning system For installation handling large airflows and normally unattended installations For installation where the dust load is low. Any kind of air conditioning systems
Filtration capacity Cannot filter fine dust. Can filter fine dust. Not suitable for collection of coarse dust. Can filter fine dust

The above comparison should be considered as a general guide only. In selection of air filters, each installation should be considered on its merits taking into account the three important filter operating characteristics:

  • Filtration efficiency
  • Resistance to air flows.
  • Dust holding capacity.

9.1.8 Performance of air filters

The selection of a particular type of air filter depends on the nature and size of dust. Coarse particles are more easily removed from air than fine particles.

The efficiency of the air filter is given by the equation:

Efficiency = [ (m1 – m2 ) / m1 ] × 100

Where:

m1 is the mass of dust particles per unit volume of air before entering the air filter.

m2 is the mass of dust particles per unit volume of air leaving the air filter.

The efficiency for different filters are given in the following table:

Table 9.3
Efficiencies of Air filters
Type of filter Efficiency
Dry Fabric ( Cotton-wool pads ) 25 to 35 %
Dry fabric ( fine glass wool ) 50 to 75 %
Viscous filters 5 to 15 %
Electronic filters 70 to 90 %

The following table gives the selection of filter for different types of dust.

Table 9.4
Selection of filter for different types of dust.

Odour removal

Controlling odour in an air-conditioned space is a health as well as comfort necessity. The principle sources of odours are body odours resulting from perspiration, breathing and organic decomposition.

Sources of odours are so many and diverse that it is difficult to suggest specific remedies for the emissions they create.

Various materials that are responsible for odours carried in air are given in the following table:

Table 9.5
Odour sources and their concentrations
Odour Causing element Source Concentration in kg per mg/m3 of air
Butyric Acid Body odour 1.5
Valeric acid Body odour 0.01
Pyridine Burning tobacco 0.035
Iodoform -- 0.0255
Cresol -- 0.85
Aerolin Frying fat 1
Essential oils Perfumes 1

Most odours are either eliminated or reduced in concentration by the following methods.

  • Modification of the process
  • Masking and neutralization
  • Absorption method
  • Combustion or incineration
  • Condensation
  • Scrubbing
  • Fresh air circulation

Let us briefly discuss all of the above methods.

Modification of process

Many times, small changes in the process minimize odour generation and are more effective and economical than control devices. Modification could involve lowering the process temperature or other parameters.

Masking and neutralization

Masking does not alter the composition of the original odour whereas neutralization does alter the composition. Perfumes, colognes and deodourants are the masking elements, which release a pleasant odour to overcome the unpleasant smell. Use of ozone for odour control is also a masking method.

Odour neutralization is the method used to eliminate the intensity of the original odour. Selection of neutralization agents depends mainly on the experience of people. A series of pre-catalogued agents is available for treating a variety of specific odours.

Absorption

Activated carbon is used mostly in locations where noxious gases are prevalent. It is an absorber of odours in the same way as silica gel is an absorber of moisture.

9.2 Humidifiers

The humidification of air is one of the important phases of air conditioning in specific applications. The success of many of these specialized air-conditioning systems – particularly where close environmental control is required – depends on the proper functioning of humidifiers. Humidification is achieved by using one of the following methods:

  • Injection of steam
  • Atomizing of water
  • Evaporation of water
  • By air washing

Common consideration for humidification

The selection of water quality for humidification is very important as the water introduced to increase the RH affects the equipments. Hard water contains calcium and magnesium salts which settle on the surface of equipments. Hence, it is essential that the water used is softened.

All humidification systems except steam humidifiers consist of an arrangement for exposing the water surface in the form of small droplets. In order to convert water into vapor, nearly 2400 kJ of heat is required for every liter of water. The heat is taken from the air, which is to be humidified.

9.2.1 Humidification by Steam Injection

In this system, steam is injected into the air just above the atmospheric pressure to carry out humidification. The steam condenses to a very fine mist as it is dispersed and evaporates instantly to the gaseous state raising the RH. The temperature rise during this process is negligible. The following figure shows a typical steam humidifier.

Figure 9.5
Steam injection type humidifiers

In this system, steam from the available source flows through a strainer and pressure reducer into the outer jacket of the distribution manifold. Steam heats the manifold and prevents condensation. The strainer prevents dirt from entering the parts while the pressure reducer regulates the steam at required pressure for the humidifier. A control valve actuated by a humidistat, in the steam line controls the steam injection in the humidifier. When the valve opens steam flows through the separator to remove condensate and then enters the distribution manifold. The condensate from the trap is either drained or returned back to the system.

Steam humidifiers are generally located inside the ducts of air handling system. Steam releases its latent heat of vaporization as it condenses, slightly increasing the duct temperature. This is a disadvantage if the humidified air is being cooled.

The advantages of the steam humidifier are :

  • The initial cost of a steam humidifier is quite low when steam is already available.
  • The steam humidifier can be easily installed without any necessary reinforcement due to its compactness and lightweight.
  • The noise during operation is negligible.
  • It does not carry any harmful impurities with air.

Some drawbacks are:

  • It carries odours, which are unpleasant to occupants, hence seldom used in comfort air conditioning.
  • When the humidification is required for cooling applications, steam can waste energy because the air gets heated up due to steam.
  • The steam humidifiers sometimes provide more heat than needed to the enclosure due to the high temperature of the steam.

9.2.2 Humidification by atomizing the water in air

Atomization type humidifier

Water from the supply tank is drawn with the help of compressed air by aspiration and is blown in the duct in the form of a fine mist. An effective humidification can be achieved by this method.

A typical arrangement of this humidifier is shown in the following figure:

Figure 9.6
Atomization type humidifier

In this humidifier, the venturi principle is used. The compressed air is passed through a narrow section of pipe at a high velocity. This creates a suction effect, which lifts the water from the reservoir and mixes with air. The mixture of air and mist is then passed into the air-conditioned space through the main duct.

This type of humidifier does not add heat to the room. The heat required for evaporation of water mist is taken from the air thus reducing its DBT.

Atomizing humidifier capacities range from 5 to 50 kg/hr. For higher requirements, multiple units are used. This humidifier is noisy owing to the high velocity of air. It is generally preferred for industrial buildings where noise is not objectionable.

Impact Type Humidifier

This type of humidifier is more practical. It makes use of a fine jet of water, which is sprayed on a hard target with a high velocity. The impact of water on the hard surface breaks up the stream of water into fine spray. Air is forced into the chamber, which picks up the mist, which evaporates with the heat available in the air. There are eliminators placed in the path of the air stream to remove the water droplets carried with the air.

The arrangement of this system is shown in the following figure:

Figure 9.7
Impact type humidifier

10 to 20 % of water supplied by the impact humidifier is carried with air in vapor form. The evaporation rate depends on the velocity of water jet, temperature and RH of entering air. To effect higher evaporation rate it is necessary to have a higher velocity of water jet, higher temperature of water, lower RH of the entering air and better mixing of air and water spray.

The impact humidifier is quieter in operation. The percentage of evaporation can be improved by using a hot coil surface as the target of the water jet.

9.2.3 Humidification by evaporation of water in air

These type of humidifiers discharge water vapor into the air to be humidified. The process is endothermic and the heat required is provided by the equipment itself.

Pan and coil type humidifier

In this system of humidification, when the humidistat controls calls for humidity in the pan coil evaporative unit, the heating element in the water pan is energized. The heating device may be either steam, electric or hot water. The heating of water in the pan causes evaporation. The air flowing over the pan in the unit carries water vapor coming from the surface of water in the pan. The rate of evaporation of water depends on the area of water surface exposed to air and the rate of air flow.

Humidifier using steam heating is illustrated below:

Figure 9.8
Forced evaporation humidifier

A large shallow pan contains water and is heated by a steam coil immersed in the pan. The temperature of water is controlled by the quantity of steam supplied. The required level of water in the pan is maintained with a float switch.

This humidifier is simple in operation and its maintenance cost is low as there is no moving part. Precise control of RH is not possible with this humidifier.

Heated water type humidifier

A typical arrangement is shown in the following figure.

Figure 9.9
Heated water type humidifier

This system consists of an evaporative media, over which air is forced before flowing into the ducting. Warm water is passed through this evaporative media. Air moving over this evaporates some of the water and carries the water vapor into the duct.

Heated air type humidifier

As shown in the figure below, a rotating evaporative media moves through a reservoir of cold water. The fan and the heating coils are also energized at the same time. Incoming air flows over the heater to raise its temperature. The heated air then flows over the wetted evaporative media, picks up the water vapor generated due to its temperature, and flows into the ducting.

All types of evaporative humidifiers can be installed in the ducts of the air handling system. Most units have an automatic bleed off drain or blow down arrangement to flush the scales formed.

These units are available in capacities ranging from 5 to 20 kg/hr.

9.2.4 Humidification by air washing

The equipment under this category is capable of cooling, cleaning and humidifying air without using cooling coils.

Spray type air washer

This is the most effective and commonly used type of humidifier. The bank of water spray nozzles is shown in the figure below. A small pump supplies water to the spraying nozzles under high pressure. The un-evaporated water is collected and re-circulated for humidification purpose. The eliminator pads provided after the spray nozzles trap the water droplets, which escape with the spray.

Figure 9.10
Air-washer humidifier

The advantages of these humidifiers are :

  • By regulating the quantity of water and water temperature passing through the nozzles, the desired temperature and RH of the air can be easily achieved.
  • As air is exposed to water, the soluble and objectionable gases are dissolved in water leaving the air free from gases.
  • It is easy to operate and its maintenance cost is low.

The humidifying efficiency of this humidifier depends on the number of banks used and the direction of water spray.

9.3 Dehumidifiers

Dehumidification of air is the process of removal of moisture from air. This is necessary for storage, manufacture and packing and variety of products in addition to comfort air conditioning.

There are three common methods to accomplish dehumidification:

  • By reducing the temperature of air below its DPT. This is accomplished by passing air over the cooling coil whose surface temperature is maintained below the DPT of air.
  • By absorption of moisture from air. This is accomplished by passing air through an absorption media.
  • Adsorption of moisture from air. This is accomplished by passing air through a chemical.

Let us briefly study, how dehumidification is achieved with each of the above methods.

By reducing air temperature below its DPT

This can be accomplished by two methods.

  • The air may be passed over a cold surface whose surface temperature is below the DPT of air.
  • Air may be passed through a spray of water whose temperature is below the DPT of air.

Most cooling coils used for dehumidification are of fin or tube type. Water can be used as the cooling medium if the DPT of air is above zero oC , but if DPT is below zero deg.C, cooling is accomplished by using brine or by direct expansion of the refrigerant.

9.3.1 Spray type dehumidifier

This type of dehumidifier is similar to the air washer type humidifier, except the temperature of water used for spraying is below zero oC. A typical arrangement of the spraying and cooling system is shown in the following figure.

Figure 9.11
Spray-Type-Dehumidifier

A refrigeration machine is used to cool the water supplied for spraying. This type of dehumidifier is extensively used in large installations where the duct system carries conditioned air from a central unit to various rooms.

9.3.2 Dehumidification by adsorption method

An absorber type of material takes up water vapor from air and holds it without any chemical reaction. The action of absorber is similar to the action of a blotter on ink.

Silica gel is a good absorber for water vapor in air. It is capable of holding and adsorbing 40% of its own weight. The moisture thus adsorbed can be removed by the reactivation process, which is nothing but heating the bed containing the adsorbed silica gel.

When silica gel is used for dehumidification, two beds are provided. One is used while the other is dried out.

The arrangement of dehumidifying the air using one bed and reactivation of the other bed is shown in the figure below.

Figure 9.12a
Dehumidification by adsorption method
Figure 9.12b
Dehumidification by adsorption method-Reactivation

9.3.3 Liquid Absorption Dehumidifier

The liquid absorbent dehumidifier is a very recent development. Niagara Blower Company developed an absorbent liquid known as “hygrol” for the first time.

Nowadays the liquid absorbents commonly used are lithium bromide and lithium chloride solutions. The stronger the concentration and lower the temperature of the solution, the more vapor will be absorbed from the air. Lithium chloride also removes certain bacteria from air.

9.3.4 Lithium bromide absorption system

The arrangement of this system is shown in the following figure.

Figure 9.13
Liquid Absorption Type De Humidifier using Lithium Bromide Solution as Absorbent

A fan, usually on the dry airside, draws moist air into the unit while the absorbent is sprayed over a finned coil or a surface containing the cooling medium. As the air gives up moisture, the coolant simultaneously cools it. The temperature of air during dehumidification remains constant and the condition of air theoretically follows the path 1-2 as shown in the graph below.

Figure 9.14
Dehumidification of air-psychrometric chart

As represented in the above graph, the water vapor removed by the absorbent is (w1 – w2) and the heat taken from the cooling coil is ( H1-H2) per kilogram of air. Controlling the flow of refrigerant or brine through the cooling coil can control the temperature of air. Such a system is designed by Midland Ross Corporation, USA.

An eliminator in the discharge section of both, the dehumidifier and the regenerator, removes the entrained droplets of the solution. A liquid dehumidifier is automatically controlled by a system that monitors the solution level. An increase in the level indicates that the system is dehumidifying the air.

This type of unit can be used for continuous dehumidification. The advantage of liquid absorption systems for domestic and commercial applications is that air at a very low dew-point can be produced and cold air gives no icing problems.

Dehumidifiers from different reputed Companies

  • POOL Dehumidifier: Poolpak Co., Pennsylvania offers high quality and high efficiency dehumidifiers for commercial and residential applications.
  • Heavy duty Dehumidifier: The Alachua , Florida, offers dehumidifiers capable of operating in environments from 16oC to 35oC inlet air with RH of 40 to 60%.
  • Somerset Technologies: This Company at Brunswick, New Jersey has developed new energy efficient dehumidifier, which uses the principle of chemical absorption of water vapor from air.

9.4 Fans and blowers

These are important parts of the air-conditioning systems. The term fan and blower are quite often used synonymously. When the pressure of air to be handled is large, then it is known as a blower and when the pressure of air-handled is less, it is termed a fan.

The blowers or fans are divided mainly into two types according to the airflow pattern.

  • Axial flow fan and propeller
  • Centrifugal fans
  • Mixed flow fans

9.4.1 Axial fan

An axial flow fan causes the air to flow parallel to the axis of the fan. These are propeller type fans enclosed in short cylindrical housing. Vane axial fans incorporate specially designed vanes, which may be either upstream or downstream of the fan wheel.

Advantages

  • Available in low to extremely high volume range.
  • Higher pressures attainable with staging (using fans in series to boost pressure) more simply than with centrifugal fans because of straight-line air flow.
  • It requires minimum weight and space per cfm.

Limitations

  • For the same capacity, the sound levels are higher than the centrifugal fans.
  • Motor and bearings cannot be completely protected.
  • Poor accessibility of bearings for maintenance
  • Cannot be used for higher temperature application.
  • Noisy and unstable in operation when the resistance increases.

9.4.2 Centrifugal Fans

These are also known as radial fans. The air enters the fan axially and is discharged radially from the fan. Centrifugal fans are very often used in air conditioning system compared with axial fans as they develop more pressure than axial fans. These fans are categorized based on the curvature of the blades relative to the impeller radius.

The three types of blades are forward curved blades, radial curved blades and backward curved blades.

Forward curve bladed centrifugal fan

These are characterized by a large number of wide shallow blades, having a very large inlet area relative to the wheel diameter. These fans operate at low speeds compared to other centrifugal fans for equal volume and pressure.

Advantages

  • Excellent among centrifugal fan group for any volume at low to moderate pressure and relatively clean air.
  • Most home and commercial heating and ventilation systems use this type because of its low speed and quiet operation.
  • More efficient and generally lower sound level than axial fans.

Limitations

  • Not as efficient as backward inclined blade fans.
  • Space required is high as compared to axial fans.
  • Should not be used where high dust loaded air is handled.

Backward curve bladed centrifugal fan

This type of fan is capable of the highest efficiency and lowest sound levels.

Advantages

  • Can be used for moderate to high volume requirements.
  • Static pressure range can go up to 750 mmWC.
  • Has the highest efficiency.
  • Sound level is the lowest as compared to other fans.

Limitations

  • Space required and weight is high compared to axial fans
  • Should not be used for higher duct loading

Radial blade centrifugal fan

This fan is characterized by simple, rugged construction. They are usually referred to as industrial exhaust fans.

Advantages

  • From maintenance point of view, the best fan to consider for severe duty.
  • Simple construction often makes field
  • Repair feasible while it might be difficult in other types of fans.
  • Can be used for almost any industrial application from clean air to extremely heavy dust loadings

Limitations

  • Lowest efficiency in centrifugal fan group.
  • Highest sound levels.
Figure 9.15
Types of Impellers and blade angles

From the above figures, it is clear that the absolute velocities of radial and backward curved blades are less than the absolute velocity of the forward curved blades for the same speed and impeller diameter. In order to obtain the same capacity under similar conditions, the radial and backward blade fans must be operated at higher speed than the forward curve bladed fan.

9.4.3 Fan power and efficiency

Total Head developed by a fan is given by the equation:

Ht = Hp + HV

Where:

Ht is the total head developed in mm of Water Column (mmWC)

Hp is the static pressure in mm of Water Column (mmWC)

Hv is the velocity head in mm of Water Column (mmWC)

Total brake-power (B.P) is given by the equation:

Where:

ΔP = The pressure difference, Pa

Q = is the volume of air at standard atmospheric conditions

η = efficiency

9.4.4 Fan performance curves

A performance curve is a graph, which shows a fan’s volume flow rate plotted against either horsepower required or against the resistance pressure of the ducts, dampers, registers etc. The typical fan performance curve of each of the three types of centrifugal fans is shown in the following graphs.

Figure 9.16
Fan performance curve

As shown in the graphs, there is a clear relationship between the fan flow rate and the static pressure available at the fan outlet. The lower the static pressure, the higher is the fan flow rate.

  • The backward curve has a non-overloading characteristic
  • To produce the same static pressure, the backward curve blade fans should be operated at a higher speed than the forward curve blade fans.
  • The backward curved blade fans do not have a point of inflection in their static pressure curve.
  • The maximum operating efficiency on a backward curved fan is achieved when it is operated at its maximum power consumption.

Adoption of fan to systems

The total pressure of the system at the outlet of the fan increases with the increase in quantity of air through the system. The resistance curve is drawn against the flow rate as shown in the figure below.

Figure 9.17
System curve for fans

In the above graph, let’s assume the fan operates at a speed ‘N1’ delivering air through a system having resistance ‘R1”. The operating point on the graph in this case is ‘a’.

To reduce the flow rate of the fan from ‘Qa’ to ‘Qb’ , two methods can be used.

  • Increasing the system resistance ( by partially closing the dampers ), which establishes the new resistance curve ‘R2’ intersecting the fan characteristics at point ‘b’. The corresponding power consumed and the pressure developed, can be plotted on the curve.
  • Reducing the fan speed from ‘N1’ to ‘N2’, changes the fan characteristics. In this case the fan operating curve will lie on the system resistance curve and the operating point will be ‘c’.

The power required at the point ‘c’ is much less than the power required at point ‘b’.
This shows that speed reduction is a more economical method than changing the system resistance for reducing the fan flow rate. It is necessary to always keep the system resistance minimum and make adjustments in the flow rate by reducing the fan speed.

9.4.5 Fan Laws

Fan laws have been derived from the functional relationship of volume, pressure, power, speed and size of fans operating on the same point on its characteristic curve.

Practically, these laws do not apply exactly because of design considerations and manufacturing tolerances, but they are useful in estimating approximate outputs of similar fans of different diameters and speeds

  1. Air flow rate varies as (fan diameter)3 and as rpm
  2. Static Pressure developed varies as (fan diameter)2 and as (rpm)2
  3. Brake Horse Power absorbed by the fan varies as (fan diameter)5 and as (rpm)3

Selection of fans

The selection of a fan for a particular air-conditioning system requires the following data.

  • Volume of air Q, required to be circulated.
  • Total system ΔP that includes the ΔP offered by air filters, washers, coils, duct etc.

9.4.6 Fan noise

Noise in a fan is simply defined as unwanted sound. Measurement of the sound level in the vicinity of the fan already installed is a relatively simple matter. There are many sound level meters in the market adequate for this measurement. A fan or a fan system may generate enough noise to be extremely annoying even though it may be below the acceptable limits. If the problem can be foreseen and eliminated by selecting the best type of fan and installing it properly, it is much less costly. However, this is not always the case. The following guidelines would help us in resolving the noise problem in a fan.

Inspect the fan for possible mechanical sources of noise. Investigate the following possibilities

  • Worn or dry bearings or coupling
  • Loose set screws on the impeller
  • Broken or looses bolts or fasteners
  • Bent fan shaft
  • Impeller imbalance
  • Weak or unstable foundation or fan mounting
  • Looses dampers or inlet vanes
  • Fan speed too high
  • Fan rotating in the wrong direction
  • Foreign material in the fan
  • Fan impeller rubbing on the housing
  • Vibration transmitted to the fan from some other source
  • Fan blade surfaces coated with dust

Install a good vibration isolation base for the fan and motor assembly and provide a rigid foundation.

Install flexible connection (canvass ducts) between the fan inlet or outlet and the connecting ductwork. Do not use the fan housing to support the ductwork.

Install sound attenuators (silencers) on the fan inlet.

If only a moderate noise reduction is needed, insulating the fan casing will serve the purpose.

9.5 Grills and Registers

A proper air supply to the air-conditioned space is done by the use of grill or registers. A grill or a register can be located in the floor, on the high side of a wall or in the ceiling. The basic requirement of the air supply point is that the air stream should not strike the occupants before it has lost its velocity.

These grills when provided with dampers are termed registers.

The upper wall side arrangement is the most suitable location, but the disadvantage is that the air velocity required would be higher if the room width is quite large.

Floor to ceiling air distribution provides excellent heating conditions to large spaces like theatres and auditoriums. The disadvantage of this arrangement is that air can pick up dust while flowing over and around the floor.

Ceiling to floor air distribution is suitable for cooling large spaces like theatres and auditoriums. This arrangement gives an advantage of natural downward flow of conditioned air.

Ceiling to ceiling air distribution is best suited for and provides proper airflow for both, cooling and heating applications.

9.5.1 Design of a grill

To determine the size of the grill for a particular room it is necessary to consider the required throw and velocity of air through the grill.

Throw through a grill is defined as the distance from the outlet face of the grill to the point where the average velocity of the free air becomes 0.25 m/sec.

Throw can be calculated using the following equation.

L = ( K × Q ) / √ab

Where:

L is the throw in meters

Q is the quantity of airflow through the grill in m3 per minute.

a is the length of the grill

b is the width of the grill.

K is a constant.

The values of K are different for the different vane angles.

9.5.2 Factors affecting the grill performance

Vanes

Straight vanes produce an included angle of 14 to 24oC. Converging vanes spread the air similar to the straight vanes, but the throw of air is greater than the straight vanes. Diverging vanes produce an angular spread, which has an effect on the direction of airflow and the distance traveled. The throw in this case is smaller than the other two.

Outlet types

The type of outlet has a prominent effect on the induction of air and the throw achieved. The different types of outlets used are perforated, fixed bar grill, adjustable bar grill and slotted outlets.


10


Refrigeration

Objectives

After reading this chapter the student will be able to

  • Understand Refrigeration
  • Explain the various refrigeration systems
  • Know the various refrigerants used for Refrigeration
  • Describe the arrangement of various Refrigeration Equipment
  • Work out piping and pumping systems of Refrigeration

10.1 General

Refrigeration is the removal of heat from a space at a temperature lower than the surrounding temperature. The American Society of Refrigeration Engineers defines refrigeration as the science of providing and maintaining temperature below the surrounding temperatures.

The science of refrigeration utilizes several methods of providing this temperature differential. In various methods of refrigeration, some physical property of matter is used for producing the chilling effect.

Let us briefly discuss the different methods of refrigeration below.

10.2 Methods of refrigeration

10.2.1 Evaporative refrigeration

When a liquid evaporates, heat is absorbed from that liquid. This principle is used by evaporative methods of refrigeration.

The following example will help us to better understand this method of evaporation. Artificial snow-making involves evaporative refrigeration methods. The snow-making device consists of a water nozzle through which a high-pressure jet of air is passed. Water flows from the nozzle and the high-pressure air flowing with the water breaks it into tiny droplets.

When the surrounding atmosphere is at a temperature close to freezing or below freezing, the droplets of water tend to evaporate from their surface and rapidly cool to form small drops of ice. When the Relative Humidity (RH) is low, this artificial snow can be made with an atmospheric temperature as high as 2 deg.C. This is because of the rapid evaporation and evaporative cooling which is caused by the low relative humidity

Figure 10.1
Artificial snow maker

10.2.2 Refrigeration by the expansion of air

The temperature of gas or air can be reduced by an adiabatic expansion of the gas or air. This principle is used in the Bell-Coleman air-refrigeration system.

The effect of expansion for producing cold can be explained by the following example. Let’s assume the atmospheric temperature is 27 oC (300 Kelvin). When it is compressed isentropically with the pressure of 5, then the final temperature of compressed air (T2) will be,

T2 = T1 (P2 / P1) 0.286

= 300 (5) 0.286 = 475 K

This high pressure, high temperature air is then cooled in the heat exchanger. Consider the temperature and pressure of air has been brought back to 300 K. Now the air is expanded isentropically until the pressure falls to atmospheric pressure. The temperature at this point (T3) will be

T3 = 300 / 5 0.286

= 189.5 K

The final temperature has dropped below the original temperature i.e. 300 K.

This principle is universally used for producing low temperatures in all refrigeration systems. A simple arrangement is shown below which works on the above principle.

Figure 10.2
Air-refrigerator

10.2.3 Refrigeration by throttling of gas

The adiabatic throttling process is a constant enthalpy process. Since enthalpy is a function of temperature, the temperature of the perfect gas remains constant after and before throttling. However, with actual gases, the temperature may increase, decrease or remain constant.

The term, which indicates the magnitude of the change in temperature, is called “Joule Thomson Coefficient”. The pressure versus temperature lines are drawn as shown in the figure below taking enthalpy as a parameter.

Figure 10.3
Constant enthalpy

In the throttling process, the pressure of gas decreases along the curve shown in the above figure. When the throttling occurs from point ‘a’ to ‘b’, the temperature of gas increases and at point ‘b’ it is maximum. If the gas is further throttled from point ‘b’ to ‘c’, the temperature drops. The amount of temperature drop depends on the Joule Thomson Coefficient, the pressure drop and the original state of gas. In any event, the resulting temperature will be too high for refrigeration purposes unless the original temperature is low.

10.2.4 Vapor refrigeration systems

In a vapor refrigeration system, instead of air, vapors like ammonia, carbon dioxide and sulfur dioxide are used as working fluids.

Figure 10.4
Vapor compression refrigeration system

The heat carried away by the vapor in the evaporator is in the form of latent heat of the refrigerant. The vapors are then compressed and passed through the condenser. The condensed vapor is again throttled and refrigerated.

The vapor refrigeration system is subdivided into two types:

  • Vapor compression refrigeration
  • Vapor absorption refrigeration.

In the case of vapor compression, the refrigerant is sucked into the compressor and is compressed by adding energy in the form of work to increase its thermal level above atmosphere.

In the absorption system, the liquid refrigerant has a high affinity to dissolve in water. This refrigerant is heated by an external source to generate vapor and its temperature is increased above atmosphere.

10.2.5 Steam jet refrigeration

This system uses the principle of boiling the water below 100 oC. This is achieved by reducing the pressure on the surface of water below atmospheric. The low negative pressure or vacuum is maintained by throttling steam through the jets or steam nozzles.

A typical arrangement of this system is shown below.

Figure 10.5
Steam jet refrigeration

Consider a flash chamber that contains 100 kg of water. As steam is throttled through the steam nozzle creating a negative pressure on the surface of water – say if one kg of water is removed by boiling due to negative pressure, approximately 2394 kJ of heat is removed, which is required to evaporate one kg of water.

The fall in temperature of the remaining water will be:

= 2394 / (4.2 × 99)

= 5.7 oC (ignoring the heat going through the walls)

This shows that the boiling water at lower temperature and pressure takes away the heat from the water as a result of its evaporation. This reduces the temperature of the remaining water. A continuous process will create refrigeration.

Unit of refrigeration and coefficient of performance

For all purposes, refrigeration is expressed as “tons of refrigeration”. (3.5 kw of refrigeration) A ton of refrigeration is defined as the quantity of heat required to be removed from one ton of ice within 24 hours when the initial condition of water is zero deg. C.

NB This is the US ton = 2000 lbs or 907 kg.

The performance of a refrigeration system is the ratio of energy extracted to the work done (Q/W). This is known as the coefficient of performance (C.O.P)

C.O.P = Q (energy) / W (work)

In an ideal Carnot cycle, we can relate this to the temperature by dQ=ds/T

and W=Qu – Ql

Hence COP = T2/(T2 –T1)

For a typical A/c system T1 = 5+273 T2 = 45+273

Hence COP = (45+273)/45 – 273 = 7.95

10.3 Air refrigeration system

In this system, air is used as the working medium. The air refrigeration system, one of the earliest forms of cooling developed, became obsolete for many years because of its low coefficient of performance and high operating cost. Therefore, we shall not study this system in detail.

The basic elements of air refrigeration system are:

  • Compressor
  • Condensator
  • Expander
  • Evaporator

As shown in the following figure, the ideal reversed Carnot cycle uses evaporating(Te) and condensing(Tc) temperatures, assuming that there are no thermal or mechanical losses in the system.

Figure 10.6
Ideal Carnot cycle

Bell Coleman modified the reverse Carnot cycle to make it practical in which the isothermal processes were changed to constant pressure processes. This is known as the Bell Coleman refrigeration cycle.

Figure 10.7a
Closed cycle air-refrigerator working on Bell-Coleman cycle
Figure 10.7b
Bell-Coleman cycle

In the Bell Coleman refrigeration cycle, air from the refrigerator is drawn into the compressor cylinder during the suction stroke 1-2. It is then compressed isentropically to 3 during the first part of the compression stroke. This raises its temperature. During the remaining stroke, the warm air is forced into the heat exchanger at constant pressure. Here the air is cooled at constant pressure. Due to this, the volume is reduced from 4-3 to 4-5 and the temperature falls to that of cooling water under ideal conditions.

The cold air is then drawn into the expansion cylinder during its suction stroke 4-5 and then expanded isentropically to 6. The isentropic expansion cools the air. The cold air is now returned to the refrigerator where it absorbs heat at a constant pressure. This process continues to create refrigeration.

Advantages of an air refrigeration system:

  • Air is more readily available, compared to other refrigerants.
  • There is no risk of fire as with ammonia machines, since air is non-flammable.
  • The weight of this system per ton of refrigeration is quite low compared to other systems.

Disadvantages of an air refrigeration system:

  • As the heat is carried by air from the refrigerator in the form of sensible heat only, the weight of air required to be circulated is more than other refrigerants.
  • The C.O.P of this system is low.
  • The major disadvantage of this system is the freezing of moisture in the air.

10.4 Vapor compression refrigeration system

The major difference in the theory and treatment of the vapor refrigeration system as compared to the air refrigeration system is that vapor alternately undergoes a change of phase from vapor to liquid and liquid to vapor during the completion of a cycle. The latent heat of vaporization is utilized for carrying heat from the refrigerator, which is quite high compared to the air cycle, which depends only on the sensible heat of the air. The schematic view of a typical vapour compression system is shown in the following figure.

Figure 10.8
Vapor compression refrigeration system

The pressure is maintained at different levels at two parts of the system by an expansion valve. The function of the expansion valve is to allow the liquid refrigerant under high pressure to pass at a controlled rate to the low-pressure part of the system.

Some of the liquid evaporates passing through the expansion valve but a greater portion is evaporated in the evaporator at low pressure. The liquid refrigerant absorbs the latent heat of vaporization from air, water or other material that is being cooled. The function of the compressor is to increase the pressure and temperature of the refrigerant above atmosphere, which will dissipate its latent heat in the condenser. While passing through the condenser, the refrigerant gives up the heat that is absorbed in the evaporator. This heat is transferred to the water or air used as the cooling medium in the condenser.

Advantages:

  • The working cycle being close to the Carnot cycle, the C.O.P is high.
  • The operating cost is 20 % less than that of the air refrigeration system.
  • The evaporator size is smaller since the flow required is less per ton of refrigeration compared to other systems.

Disadvantages:

  • Capital cost is high.
  • Maximum maintenance is required to prevent leakage of refrigerant.

In all vapor compression systems, the air leakage at all the points should be prevented for the following reasons.

  • Air affects the C.O.P as it does not take part in the refrigeration process.
  • It absorbs unnecessary power in the compressor.
  • It reduces the heat transfer coefficient by preventing the refrigerant to be exposed to the inner surface of the condenser and evaporator. This also increases the load on the condenser.
  • Due to air leakage in the evaporator, maintaining a vacuum becomes difficult and the required temperature will not be maintained.

10.5 Absorption refrigeration system

All the cycles of refrigeration used in the systems discussed above use energy in the form of mechanical work for the operation of the cycle. The major drawback of the vapor compression system is that it requires more power to compress the large volume of vapor refrigerant. A French Scientist, Ferdinand Cane, developed the first absorption refrigeration system in early 1860. The absorption system fundamentally differs from the vapor compression system only in the method employed for compressing the refrigerant. In the absorption system, an absorber, generator and a pump replace the compressor.

We shall study the two basic absorption systems widely used:

  • Ammonia absorption system
  • Lithium bromide absorption system.
Figure 10.9
Ammonia absorption system

This system uses a heat-operated generator to produce the pressure differential. Instead of compressing the low-pressure refrigerant, it is first absorbed by the weak solution of refrigerant in water, which is sprayed into the absorber. Absorption of ammonia lowers the pressure in the absorber, which in turn draws more ammonia vapor from the evaporator. The strong ammonia solution thus formed is then pumped into the generator. The pump increases the pressure of the solution by 10 bar. This solution is heated by an external source and in this heating process, the refrigerant vapor is driven out of the solution and the same heat is given to the condenser where it condenses to high-pressure liquid ammonia. The weak solution of ammonia left in the generator is first throttled to a low-pressure level by an expansion valve and then returned to the absorber. The high-pressure liquid ammonia is throttled and passed to the evaporator where it absorbs its latent heat of vaporization and maintains cold. The dry ammonia vapor coming out of the evaporator is allowed to mix with the weak solution of ammonia sprayed in the absorber. This completes the cycle.

In this system, energy is supplied to the system in the form of heat in the generator and in the form of work W1 and W2 to the pumps P1 and P2.

C.O.P of this system is given by:

C.O.P = Qa { Qs + ( W1 + W2 ) / J }

Where

Qa is the heat absorbed in the evaporator

Qs is the heat supplied in the generator.

Selection of absorbent and refrigerant

It is very important to know the thermal, chemical and physical properties of the absorbent, refrigerant and their combination, since these play an important role in the refrigeration cycle.

We shall be studying the various properties of refrigerants and their significance in detail in the next session.

Lithium Bromide absorption system

Lithium Bromide (LiBr) is a chemical similar to common salt (NaCl). It is soluble in water. The LiBr water solution has the property to absorb water due to its chemical affinity. As the concentration of LiBr increases, its affinity towards water increases.

Let us briefly study the cooling cycle in a LiBr absorption system.

Evaporator

The evaporator consists of a tube bundle, an outer shell, distribution tray and a refrigerant pan. The chilled water flows inside the tubes. The refrigerant pump circulates the refrigerant from the pan to the distribution trays. The shell pressure is about 6 mm of Hg at which the refrigerant evaporates at a low temperature (3.7 oC) and extracts the latent heat of evaporation from the water being circulated through the evaporator tubes. Thus, the water being circulated through the tubes is cooled.

Example below:

Absorber

The absorber consists of a tube bank, an outer shell (common with evaporator), distribution trays and an absorbent collection sump. Concentrated absorbent solution (LiBr) from the low temperature generator is circulated through the distribution trays. The solution sprays on the absorber tubes. Concentrated absorbent has an affinity towards water. Hence, vaporized refrigerant from the evaporator is absorbed. Due to this absorption, the vacuum in the shell is maintained at a low pressure and ensures correct chilled water temperature.

Example below:

Heat Exchanger

The absorbent pump pumps the cool diluted absorbent from the absorber to the high temperature generator. It first passes through the low temperature heat exchanger where it absorbs heat from the concentrated absorbent. It next flows through the heat reclaimer where it absorbs the heat from the steam condensate. The solution then enters the high temperature generator. The heat exchangers serve the purpose of preheating the cool absorbent solution thereby reducing the heat input required by the high temperature generator.

High Temperature Generator (HTG)

The HTG consists of a tube bank, outer shell and a set of eliminators. As the steam passes through the tubes, the dilute absorbent surrounding the tubes heats up to its boiling point. The absorbed refrigerant evaporates and the concentration of the absorbent increases.

Example below:

Condenser

The low temperature generator (LTG) and condenser tubes are enclosed in a shell and are separated by an insulation plate. The vaporized refrigerant flows into the LTG tubes. It heats the intermediate absorbent outside the tubes and condenses. The condensed refrigerant flows into the condenser. Here it is cooled by the water provided for cooling. The refrigerant vapor condenses outside the condenser tubes and collects at the bottom of the condenser. The condensed refrigerant from the LTG and condenser mixes and flows to the evaporator. The absorbent, which is concentrated in the LTG, drains to the absorber to start a new absorbent cycle.

Example below:

Let us now study the various refrigerants available and their properties.

10.6 Refrigerants

10.6.1 General

Any substance that is capable of absorbing heat from another substance can be used as a refrigerant. A mechanical refrigerant is a refrigerant, which absorbs the heat from a source at lower temperature and dissipates it to the sink having higher temperature than the source, in the form of latent heat or sensible heat. The refrigerants of the first group should have physical properties, which will enable them to repeat continuously a liquid to gas and gas to liquid transformation.

In selecting a refrigerant for a particular purpose, their thermodynamic, chemical and safety characteristics must be considered in addition to their physical properties.

10.6.2 Classification of refrigerants

The refrigerants are classified into two groups:

  • Primary refrigerants
  • Secondary refrigerants

Primary refrigerants

These directly take part in the refrigeration system where secondary refrigerants are first cooled with the help of the primary refrigerants and are further used for cooling.

Primary refrigerants are classified into the following different groups:

Halocarbon Compound

Charles Kettering and Dr. Thomas invented this group of refrigerants in 1928. These are available commercially in the market under trade names such as Freon, Genetron, Isotron and Arcton. Most of the refrigerants used for domestic, commercial and industrial purposes are selected from this group due to their outstanding advantages over the refrigerants from other groups.

Azeotropes

The refrigerants under this group consist of mixtures of different refrigerants, which do not separate into their components with the changes in their pressure or temperature. They have fixed thermodynamic properties.

Hydrocarbons

Most of the organic compounds are considered as refrigerants under this group. Many hydrocarbons are successfully used as refrigerants in industrial and commercial installations. Most of them possess satisfactory thermodynamic properties but are highly flammable. A few examples of these types of refrigerants are Methane, Ethane and Propane.

Inorganic Compounds

The refrigerants under this group were universally used for all purposes before the introduction of the halocarbon group. A few examples of the refrigerants under this group are ammonia, water, air, carbon dioxide, and sulfur dioxide.

10.6.3 Desirable properties of an ideal refrigerant

The desirable properties of a refrigerant are subdivided into three main groups:

  • Thermodynamic properties
  • Physical properties
  • Safe working properties

10.6.4 Thermodynamic properties

Boiling point

An efficient refrigerant requires a low boiling point at atmospheric pressure. A higher boiling point of the refrigerant reduces the capacity of the system and lowers the operating cost.

The boiling point of different refrigerants commonly used is:

Table 10.1
Boiling point of different refrigerants
Refrigerant Boiling point at 760 mm of Hg Refrigerant Boiling point at 760 mm of Hg
NH3 - 33.3 oC F-22 - 41.3 oC
CH2 - 73.6 oC Carren-1 40.5 oC
SO3 - 10 oC Carren-7 - 33.3 oC
F-11 23.3 oC F-113 47.5 oC
F-12 - 29.8 oC    

Freezing point

A low freezing point of the refrigerant is necessary because refrigerant should not freeze under the required evaporator temperature. The refrigerant must have a freezing point well below operating evaporating temperature.

Generally, all the refrigerants have freezing points below –30 oC.

Evaporator and condenser pressure

It is always desirable to have a positive pressure for the required temperature in the evaporator and condenser, but the pressures should not be too high above atmosphere. Positive pressures are required to prevent the ingress of air into the refrigeration system.

The operating pressure range is one of the major considerations in the selection of refrigerant for the economical working of the refrigeration. The operating pressure ranges for different refrigerants are given in the following table:

Table 10.2
Operating pressure of different refrigerants

A larger difference between the evaporator and condenser pressure results in higher compression ratio. The power required for the compressor increases with the increase in compression ratio.

Critical temperature and pressure

The critical temperature of vapor is defined as the temperature above which vapor cannot be condensed irrespective of any high pressure. The critical temperature of the refrigerant should be higher than the temperature occurring in the condenser for easy condensation of vapor.

The critical temperature and pressure of a few refrigerants are:

Table 10.3
Critical pressure and temperature of refrigerants
Refrigerant Critical temperature in oC Critical pressure in bar
NH3 132.8 112
CO2 30.5 72.8
SO2 157 77.5
F-11 197.5 43.5
F-12 112.1 40.6
F-22 95.4 48.7
Carren-1 216 33.6
Carren-7 105 43.5
F-113 214 42.9

Latent heat of refrigerant

The latent heat of the refrigerant at the evaporator temperature should be high to increase the refrigerating effect per kilogram of refrigerant.

High latent heat also reduces the weight of refrigerant to be circulated in the system per ton of refrigeration.

10.6.5 Physical properties

Specific volume

Low specific volume of the refrigerant is always desirable as it reduces the size of the compressor for the same refrigeration capacity.

Specific heat of liquid and vapor

In order to increase the refrigerating effect per kilogram of refrigerant the specific heat of liquid should be low and that of vapor should be high. This also helps in sub-cooling of the liquid and in decreasing the superheating of the vapor.

Thermal conductivity

The thermal conductivities of liquid refrigerants are required for finding the heat transfer coefficients in evaporators and condensers. High thermal conductivities of refrigerants are desirable.

Viscosity

This property of the refrigerant helps in designing the pumping units of the system. A refrigerant should have low viscosity for better heat transfer and low pumping power.

10.6.6 Safe Working Properties

In the selection of a refrigerant, the safe working properties are the prime considerations. There are a few refrigerants, which are highly desirable from the thermodynamic point of view, but they find limited use due to unsafe properties. Ammonia is one of them.

The safe properties of a refrigerant include the following:

  • It should be chemically inert
  • It should be non-flammable, non-explosive and non-toxic.
  • It should not react with lubrication oil or any other material used in refrigeration.

A few of the safe working properties are discussed below.

Toxic Nature

The effect of refrigerant on the human body is also considered in the selection of a refrigerant. High toxic nature of a refrigerant may cause injury to human body and cause substantial suffocation.

Some refrigerants, which are non-toxic in nature, become toxic when mixed with air in a certain percentage. This is particularly true for fluorocarbon refrigerants.

ASHRAE Standard 34 has adopted a matrix that indicates the relative levels of toxicity and flammability

Class A signifies refrigerants for which toxicity has not been identified at concentrations less than or equal to 400 ppm by volume.

Class B signifies refrigerants for which there is evidence of toxicity at concentrations below 400 ppm by volume.

Class 1 indicates refrigerants that do not show flame propagation when tested in air at 1 bar and 21°C.

Class 2 signifies refrigerants having a lower flammability limit (LFL) of more than 0.10 kg/m3 at 1.01bar and 21°C and the heat of combustion (HOC) less than 4542 cal/g.

Class 3 refrigerants are highly flammable. They have a lower flammability limit (LFL) of less than 0.10 kg/m3 at 1.01 bar and 21°C and the heat of combustion greater than or equal to 4542 cal/g.

Flammability

An ideal refrigerant should not have any danger of explosion in the presence of air or in association with lubricating oil.

Flammability is another key parameter in evaluating the safety level of a refrigerant. It is the second parameter that ASHRAE uses in Standard 34 when classifying refrigerant safety. Like toxicity, flammability is not as simple to evaluate as it may first appear. Most would consider water (R-718) to be non-flammable while Propane (R-290) is flammable. However, there are many substances that will burn given the right circumstances. What is required to make a substance combust also varies. Paper can be made to burn at room temperature by exposing it to open flame in standard air. Paper will also spontaneously combust at 2321°C without the presence of open flame.

Flammability Consideration - Consider a refrigerator using propane (R-290) as a refrigerant. The refrigerator has a charge of approximately 200 gms in a kitchen that is 8 ft x 8 ft x 8 ft high. Propane is a gas at atmospheric pressure but is heavier than air. If a leak forms, the propane will diffuse throughout the space as a gas but will have heavier concentrations near the floor. The LFL (lower flammability limit) for propane is 2.1%. To reach a flammable concentration in this example, 400 gms of propane would have to be released, which is more than twice what is in the refrigerator. However, the concentration will be higher near the floor, quite possibly high enough to cause an explosion if ignited. The point here is that flammability is not clear-cut. Even with a flammable refrigerant such as propane, a hazardous situation is not certain.

Corrosive

The chemical reaction of the refrigerant on the materials is not the criteria for the selection of a refrigerant, but the selected refrigerant decides the material to be used for the construction of the refrigeration system. The refrigerant must be non-corrosive in order to use more common materials.

Refrigerants should be chemically inert with materials and in the presence of air and moisture. Freon refrigerants are non-corrosive with all materials such as brass, copper, zinc, iron, tin, lead and aluminum.

Iron and steel are used with an ammonia refrigerant.

Chemical stability

An ideal refrigerant should not decompose at temperatures normally encountered in the system.

10.7 Refrigerant nomenclature

The refrigerants of the methane and ethane series are known by their numbers instead of chemical names. The number system was introduced only to simplify the terminology. Each refrigerant is known by a specific number, which is preceded by “refrigerant” or by the manufacturer’s trade names like Freon, Genetron.

All two digit numbers are derived form methane base and three digit numbers are derived form ethane base.

Let us now see how to find the chemical formula of refrigerants when the number is known or how to find the number when its chemical formula is known.

1. Find the chemical formula of F-12

As the number is a two-digit number, it is derived from methane base.

The digit ‘2’ represents the number of fluorine atoms.

The number of hydrogen atoms = 1 – 1 = 0

To balance the methane, four mono-atoms are required; therefore the number of chlorine atoms will be 4 – 2 = 2

Therefore the Chemical formula = C2Cl2F2

2. Find the chemical formula of F114

As the number is a three-digit number, it is derived from ethane base.

The digit ‘4’ represents the number of fluorine atoms.

The number of hydrogen atoms = 1 – 1 = 0

To balance the ethane, six mono-atoms are required; therefore the number of chlorine atoms will be 6 – 4 = 2

Therefore the Chemical formula = C2Cl2F4

The General formula used to designate the number for methane or ethane base refrigerant is,

C n-1 H n+1 F n

Where n is the number of carbon or hydrogen or fluorine atoms.

3. Find the number for the methane based refrigerant CCl 2F2 2

First Digit Number = 1 – 1 = 0 as n = 1

Second digit number = 1 +1 = 2 as n = 1 as there is no hydrogen atom.

Third digit = number of fluorine atoms = 4 as n=4 (number of fluorine atoms)

Therefore, the number is 114

10.8 Important refrigerants

10.8.1 Ammonia

It is the only refrigerant from an inorganic group, which was used universally for many applications. It possesses many properties required for an ideal refrigerant. It has a wide application because of its low volumetric displacement, low cost and low weight of liquid refrigerant per ton of refrigeration.

Some properties of ammonia are listed below:

  • It is toxic and flammable.
  • Anhydrous ammonia has no effect on lubricants but in the presence of moisture, ammonia forms an emulsion with oil that causes operating difficulties.
  • Ammonia attacks non-ferrous metals in the presence of water.
  • It is highly volatile and becomes explosive when mixed with air and is compressed.
  • Ammonia can be economically used for –70 oC evaporator temperature.

The disadvantage of NH3 is its high discharge temperature, which requires separators that are more efficient.

10.8.2 Carbon Dioxide

Carbon dioxide is odorless, non-toxic, non-flammable, non-explosive and non-corrosive. It is widely used for air-conditioning of hospitals, theatres, hotels and marine services. Nowadays it is replaced by the Freon group refrigerants.

Since this refrigerant is used very rarely, let us not discuss this in detail.

10.8.3 Freon –11 (CCl3F)

It is a fluorocarbon methane series. Due to low operating pressures, centrifugal compressors are used to handle the large volume at low pressure. It is non-corrosive, non-toxic and non-flammable. It is mainly used for air-conditioning of office buildings, factories, theatres etc.

10.8.4 Freon –12 (CCl2F2)

This is the most widely used and popular refrigerant of this group. It is a colorless and odorless liquid. It is a non-toxic, non-flammable, non-explosive and non-corrosive liquid. It condenses at moderate pressure and normal atmospheric conditions and boils at – 29.5 oC. This property makes it suitable for all purpose refrigeration. As a rule of thumb, 0.7 kg of refrigerant is required in the refrigeration system per cu.m of air-conditioned space.

10.8.5 Freon –22 (CHClF 2)

This refrigerant is commonly used in fast freezing units where the temperature requirement is –40 oC. It is miscible with oil at condenser temperature but tries to separate at evaporator temperatures when the system is used for low temperature applications.

The solubility in water is three times greater than Freon –12; therefore special dryers must be provided to remove water.

The advantages of Freon-22 over Freon-12 are:

  • The compressor gives about 60 % more refrigerating effect than F-12.
  • The pressure in the evaporator is above atmosphere for evaporator temperatures between –30deg.C to 10 oC.

10.8.6 Refrigerant – 500 (Carren-7)

Refrigerant-500 is commercially known as Carren-7. It is an azeotropic mixture of F-12 and F-152d in the proportion of 73.2 % and 26.2 % by weight respectively. (An azeotropic fluid is a mixture of two of more different liquids, which acts as a compound.). The solubility of this refrigerant in water is very critical; hence special arrangements should be made to remove moisture from the system.

The principle advantage of this refrigerant is that it gives 18% more refrigeration effect than F-12 with the same compressor. The Freon family refrigerants are one of the major factors responsible for the tremendous growth in the refrigeration and air-conditioning industries.

A brief survey of the applications of these refrigerants is given in the following table.

Table 10.4
Application of refrigerants
Refrigerant Compressor Type Application
Freon -11 Centrifugal Air-conditioning system ranging from 200 to 2000 TR. Cooling industries process water lines. It is used where low freezing point and non-corrosive properties are important.
Freon –12 Reciprocating, centrifugal, Rotary Air-conditioning plants, frozen food and ice-cream cabinets, water coolers, window air-conditioners.
Freon –22 Reciprocating, Centrifugal. Commercial air-conditioning, frozen food plants and storages. For low temperature applications.
Freon –502 Reciprocating Frozen foods and ice-cream display cases. Warehouses, truck refrigeration, heat pumps.
Freon –113 Centrifugal. Small to medium air-conditioning systems and industrial cooling.
Freon –114 Centrifugal, Rotary Used in household refrigerators, rotary compressors used in process cooling.
Freon –13b1 Reciprocating Medium low temperature application up to –50 oC
Freon –13 Reciprocating For low temperature refrigeration up to –90 oC in cascade systems.
Freon –14 Reciprocating For low temperatures up to –130 oC for triple cascade systems.
Freon –500 Reciprocating Commercial air-conditioning and household refrigeration.
Freon –503 Reciprocating For low temperature applications up to –90 oC.

10.8.7 Cryogenic refrigerants

The temperature range from –157 oC to –273 oC is known as the cryogenic range and the refrigerants used for producing these temperatures are known as cryogenic refrigerants. These temperatures may be produced by evaporating cryogenic liquids, which have very low boiling points at atmospheric pressure. The common cryogenic refrigerants are O2, Air, N2, H2 and He.

10.8.8 Selection of refrigerant

Before selecting a refrigerant for a particular purpose, the thermodynamic, physical and the safe working properties should be taken into account. No single refrigerant can be used for all purposes. Different applications require different characteristics.

In this chapter, we have discussed all the properties required to be considered while selecting a refrigerant. Although all the above properties will give sufficient insight to the engineers in selecting a refrigerant, engineers need to give more attention to the following while selecting a refrigerant:

  • Working pressure range and pressure ratio.
  • Corrosiveness and flammability.
  • Space limitations.
  • Temperature required in the evaporator.
  • Oil miscibility.

The list of various applications is given in the following table, as a ready reference:

Table 10.5
Application examples of refrigerants
Pressure and Temperature Range Utility Refrigerants Used
Low pressure and high temperature range Air conditioning of theatres, offices, factories, auditoriums, and cold water supply used for equipment cooling and distillation. F-11, F-113, H2O, CH2Cl2
Medium pressure and medium temperature range. For household refrigerators, water coolers, small air-conditioners, refrigerated carriers and chemical storages. F-12, SO2, F-500, CH-2Cl,
High pressure and low temperature range. For domestic and commercial freezers, cold storage plants, ice-cream factories and breweries. NH3, F-12 , F-22 , CO2
High pressure and ultra low temperature range. For liquification of gases, metallurgical operations, in aerodynamic wind tunnels. F-13 , F-14 , C2H6

10.9 Refrigeration equipment

10.9.1 Compressors

The compressor is the heart of the refrigeration system as it pumps the refrigerant throughout the system. There are three types of compressors commonly used:

  • Reciprocating compressor
  • Rotary compressor
  • Centrifugal compressor

Reciprocating compressor

The reciprocating compressor works on the principle where the piston moves in a cylinder creating suction and a delivery stroke. Refrigerant at low pressure and low temperature enters the compressor during its suction stroke and is discharged at high pressure and temperature during is delivery stroke, after being compressed. The reciprocating compressors are successfully used for refrigerants such as NH3, SO2, CH3Cl and most of the Freon group. There are two types of compressors in general use.

  • Single acting vertical compressors.
  • Double acting horizontal compressors.

Hermetically sealed compressors

In an ordinary compressor, the crankshaft extends out of the housing and is coupled to the drive. A proper sealing needs to be provided at the place where the crankshaft projects out of the housing. Normally any kind of sealing tends to give away over a period. In order to overcome this operational difficulty, the compressor and motor are enclosed in the same housing, which is known as a hermetically sealed compressor. The motor in the housing is exposed to the low vapor refrigerant, which also provides cooling to the motor.

Multistage compression

When the compression ratio required is high as in low temperature refrigeration systems, the single stage for compression is uneconomical for the following reasons:

  • The volumetric efficiency is low.
  • The frictional losses are high.
  • Leakage problem due to high pressure potential increases.
  • Operating cost is high.

Hence, for a high compression ratio, multistage compression is adopted.

Rotary compressors

There are mainly two types of rotary compressors:

  • Rotary compressor with one stationery sealing blade and eccentric rotor.
  • Rotary compressor with sealing blades, which rotate with eccentric shaft.

In a single blade rotary compressor, a cylinder roller rotates on an eccentric shaft, which is mounted concentrically in the cylinder. The roller touches the cylinder at a point of minimum clearance as the roller is eccentric to the cylinder. It always touches the cylinder wall as it rotates on the shaft.

Figure 10.10
Rotary compressor with eccentric rotor

The advantages of rotary compressors over reciprocating compressors are:

  • Rotary compressors are more silent in operation and free from vibrations.
  • These compressors are successfully used with refrigerants having high specific volume at low suction pressure.
  • It is preferred for low temperature applications.

Centrifugal Compressors

Dr. Willis carrier first used the centrifugal compressor in refrigeration applications in 1920.

The refrigerant vapor is drawn into the compressor and discharged with a high velocity at the outer edge of the impeller. The high velocity head is converted into a pressure head by passing through the diffuser. The rise in the pressure per stage of the compressor is quite small and hence, when high compression ratio is required, a multistage system is used.

A three stage system with a flash chamber is shown in the following figure.

Figure 10.11
Refrigeration system with centrifugal

The following points must be noted with a centrifugal compressor refrigeration system:

  • The baffles must be provided in the suction path of refrigerant to prevent flow of liquid droplets with vapor, which may damage the impeller.
  • The power requirement of the compressor increases with an increase in the pressure of the system.

Advantage of centrifugal compressor over reciprocating

  • It handles larger volumes of refrigerant vapor per unit time compared to the reciprocating type.
  • The efficiency of the centrifugal compressor is as high as 80 %.
  • The part load efficiency is also high compared to the reciprocating compressor.
  • Centrifugal compressors are used in refrigeration systems ranging from 50 to 5000 TR with a temperature range of 10 0C to –100 0C.
  • There are no difficulties of oil separation or fouling of heat transfer elements.
  • As the moving parts are less, the operating life is longer.
  • It is silent in operation and free from vibrations.

Disadvantages

  • Surging is the main disadvantage of this type of compressor. This is caused by the decreased load to a surging point below 50 % of the handling capacity of the compressor.
  • The pressure increase per stage is smaller compared to the reciprocating compressors. These compressors are not feasible for smaller capacity systems and are generally preferred above 50 tons of refrigeration.

10.9.2 Condensers

This is an important component to be considered while designing a refrigeration system. The condenser removes the heat from the refrigerant carried from the evaporator and added by the compressor. It also converts the vapor refrigerant into liquid refrigerant.

The heat transfer takes place from the high temperature vapor to low temperature air or water used as the cooling medium in a heat exchanger. For an effective functioning of the condenser, two factors need to be considered:

  • Effective temperature differential
  • High heat transfer coefficient

The condensers are classified based on the cooling medium used:

  • Air cooled condensers
  • Water cooled condensers

Air cooled condenser

Air cooled condensers are designed for condensing a temperature of 150C to 200C above the temperature of entering air. The quantity of air required for cooling is 30 to 35 cu.m / min/ ton of refrigeration.

The air cooled condenser was not considered as an economical method, but the use of Freon as refrigerants has allowed the use of air as a coolant for condensing temperatures above 300C for normal condensing pressures.

The arrangement of natural and forced convection condensers is shown in the following figures.

Figure 10.12(a)
Natural convection air cooled condenser
Figure 10.12(b)
Forced convection air-cooled condenser

Advantages of Air-cooled Condensers

  • Simple in construction.
  • No handling problems.
  • Piping arrangement for carrying the air is not required.
  • There is no problem of disposal of used air.
  • Fouling effects are much less compared to water.
  • Installation and maintenance costs are less.
  • High flexibility.

Disadvantages of air-cooled Condensers

  • Poor heat carrying capacity.
  • It requires a very large quantity of air (@ 300 cu.m per ton of refrigeration).
  • These condensers are seldom used for capacities above 5 TR as the power required for the fan is very high.

Water cooled condensers

In larger refrigeration systems, air-cooled condensers are not economical due to the high amount of air they have to handle. In such cases water cooled condensers are most suited. The added advantage of the water cooled condensers is that the water coming out of the condenser can also be used as a heating medium as it has a temperature of about 50 to 60oC.

The water-cooled condensers are divided into two groups:

  • Water waste system
  • Water re-circulation system

When ample supply of water is available, it is circulated through the condenser for cooling and then disposed to the sewer. This system is known as the water waste system.

A line diagram explaining this system is shown below.

Figure 10.13
Waste water system for condenser

In places where water is scarce, it is re-cooled and used again in the condenser. This system is known as the water re-circulation system.

Figure 10.14
Water re-circulation system.

10.9.3 Cooling towers

The cooling tower in an air-conditioning system is the means by which the heat from the condenser of the refrigeration system is rejected to the atmosphere. Cooling towers are used for refrigeration systems above 100 TR as below 100 TR, evaporative type condensers are more economical.

The principle of cooling water in a cooling tower is similar to that of an evaporative condenser. The water is sprayed in the air, which takes away the latent heat and cools the water. The rate of evaporation of water in the cooling water depends on:

  • Amount of exposed water surface area
  • The time of exposure
  • The relative velocity of air passing over the water droplets formed in the tower
  • The relative humidity of air

The cooling towers are divided into two types:

  • Natural draft cooling towers
  • Forced or induced draft cooling towers.

Natural draft cooling towers

This type of cooling tower is generally installed in an open space where there is free movement of air. The capacity of this cooling tower is nearly 30 to 75 liters per sq.m of base area depending upon the air velocity. This tower works economically for load under 200 TR.

A typical construction of this type of cooling tower is shown below.

Figure 10.15
Spray-type cooling tower

Forced draft or induced draft cooling towers

In this type of cooling tower, water from the condenser is sprayed at the top of the tower and the blower from the bottom of the tower force air. The air velocity of 2 m/sec is recommended with a flow of 100 to 130 cu.m per min. per ton of refrigeration.

The only difference between a forced draft and an induced draft cooling tower is its method of air supply. The amount of water lost with both the cooling towers ranges form 1 to 2 % by evaporation and 0.5 to 2 % by drift losses.

A typical construction of both these types of cooling towers is shown below.

Figure 10.16a
Forced- draft cooling tower
Figure 10.16b
Induced- draft cooling tower
Table 10.6
Comparison of Forced Draft Induced Draft Cooking Towers

10.9.4 Evaporators

This is also a very important part of the refrigeration system. The refrigerant from the expansion valve comes into the evaporator below the temperature required to be maintained in the evaporator and carries heat from it. The evaporator can also be termed a cooler or a freezer.

The following factors need to be considered while designing an evaporator:

  • Heat Transfer
    The heat is carried by the refrigerant from the air or water according to the medium used for circulation. The heat transfer coefficient on the refrigerant side should be high while it should be low on the other side.
  • Materials
    Evaporators should be constructed of material having good heat conductivity. The selection of material also depends on the type of refrigerant used. Brass and copper, which are good conductors of heat, are used with all refrigerants except ammonia.
  • Velocity
    With an increase in the velocity of the refrigerant, the heat transfer coefficient also increases but increased velocity also causes higher-pressure drops.

The evaporators are mostly divided into two groups as per the mode of heat transfer:

  • Natural convection evaporator
    These evaporators are used where low air velocities and minimum dehydration of the products are desired. The air circulation by natural convection depends on the temperature difference between the evaporator and the space to be cooled. Higher temperature differentials produce higher circulation rates, which also depend on size, shape and location of the evaporator.
  • Forced convection evaporator
    Forced air circulation is used in these evaporators for cooling. These evaporators are more efficient than natural convection, since these require less heat transfer surface area. Air velocity of 1.5 to 3 m/sec is recommended in this type of evaporator.

Secondary evaporators

When the chilled water or brine is used to carry out heat from the refrigerated space and this heat is given to the refrigerant in the evaporator, this system is known as a secondary evaporator.

The advantages of secondary evaporators are:

  • This is economical when the place to be cooled is far away from the refrigeration system.
  • Leaks are more severe in refrigerant piping than in water and brine piping used in secondary evaporators.
  • Long length refrigeration lines create the problems in oil return and cause excessive pressure loss.
  • Secondary evaporators are more commonly used in packing plants and in cold storages where there is every possibility of contamination of NH3 with stored foods.

10.9.5 Expansion Devices

This is one of the basic components of the refrigeration systems. The expansion devices perform the following functions in a refrigeration system.

  • It reduces the pressure of refrigerant coming from the condenser.
  • It regulates the flow of refrigerant as per the load on the evaporator.
  • Two types of expansion devices used for refrigeration systems are:

Capillary tube

A capillary tube is universally used as a refrigerant control device in refrigeration systems that use hermetic compressors. Its resistance to flow allows the capillary to be used as a pressure reducing device to meter the flow of refrigerant given to the evaporator.

It is a small diameter tube connected between the condenser and the evaporator. The required pressure drop is caused due to the heavy frictional losses offered by the small diameter tube. The resistance is directly proportional to the length and inversely proportional to the diameter. The flow rate for a selected capillary tube is the function of the pressure drop between the condenser and the evaporator. However the use of a capillary tube is limited to a maximum capacity of one ton of refrigeration only.

The advantages and disadvantages of this device are:

  • It is simple in construction and no maintenance required
  • System using this device does not require a receiver
  • The cost of a capillary tube is low compared to other devices

The main disadvantage of this system is that the refrigerant must be free from moisture and dirt, which can choke the tube and stop the refrigerant flow.

Automatic expansion valve

An automatic expansion valve works in response to the pressure changes in the evaporator, which take place due to the increase or decrease in the load. This valve maintains a constant pressure throughout the varying load on the evaporator by regulating the flow of the refrigerant.

The arrangement of this device is shown in the following figure.

Figure 10.17
Automatic or constant pressure expansion valve

It consists of a needle valve, a seat, a diaphragm and a spring as shown above. The initial spring tension can be set by a screw. The spring tension and the pressure of the evaporator acting on the diaphragm control the opening of the valve. This can be further explained with the following example.

Assume the spring is adjusted initially to maintain a pressure of 1.5 bar in the evaporator at a given load. If the pressure falls below 1.5 bar due to the decrease in the load, the spring pressure exceeds the evaporator pressure and causes the valve to open more. This increases the flow rate of refrigerant in the system, which increases the rate of evaporation and builds up the pressure until equilibrium is reached with the spring tension.

The major disadvantage of this device is the poor efficiency of operation compared to other expansion devices.

10.9.6 Piping

The main purpose of piping in a refrigeration system is to carry different fluids from one point to another. The following points must be considered while selecting a pipe size and material:

  • Refrigerant, brine or any other fluid to be carried through the pipe should not react with pipe material.
  • The pipe size should be sufficient to avoid any excessive frictional losses.
  • It should be able to withstand the system pressure.

Design Consideration

Engineers need to take care of the following while designing a piping system:

  • Keep the frictional losses to a minimum
  • The piping should ensure the supply of refrigerant to all evaporators in all loads in a multi evaporator system
  • Avoid unnecessary bends in the piping system.

Pipe Schedules

All pipes are classified according to the schedule in order to standardize the wide variation in the sizes of the pipes available today. The most common schedule numbers are 40, 80, 120 and 160. These schedule numbers are the thickness of the pipe for that particular group.

Maintenance Considerations

The maintenance of a metal pipe is determined by the characteristics of the metal. The major considerations are:

The effect of temperature: Metal pipes when subjected to high temperature expand and subsequently contract when the temperature reduces. This constant expansion and contraction of metal due to a varying temperature on the piping can lead to fatigue.

Pipe Supports: Pipe supports are exposed to strain due to the vibrations caused in the piping because of high velocities of the flowing fluids. Supports can also be loosened due to the expansion and contraction of the pipes. While designing the pipe supports all these factors need to be taken care of.

Corrosion: All metals are subjected to corrosion. Many materials react chemically with the piping metal to produce scale, rust and other oxides. The selection of the metal piping should be such that these are reduced.

Piping of the Suction line

Very careful design of the piping connecting the evaporator and the compressor is necessary as it affects the capacity of the compressor and its power requirement. The pressure loss in the suction line should be minimum, such that the pressure in the evaporator should be equal to the suction pressure of the compressor. This pressure drop is generally limited to 50 to 100 mmHG.

Piping of the discharge line

The discharge line is the piping required to carry the hot gases from the compressor to the condenser. This line should be able to maintain the discharge pressure and temperature. The pressure drop in the discharge line is generally limited to 100 to 200 mmHG. The recommended velocity in the discharge line is 5 to 8 m/sec.

Liquid refrigerant line

This is the line used to carry the liquid refrigerant from the receiver to the expansion valve. The frictional loss of this line causes the evaporation of the refrigerant if the liquid is saturated which increases the pressure losses.

The allowable pressure loss in this line is 10 mmHg per meter length of the piping. If the total pressure drop exceeds 250 mmHG, then sub-cooling of the liquid refrigerant is required to avoid vapour formation.

Arrangement of Piping in a Refrigeration system

Suction Pipe Arrangements

Single Evaporator with Compressor

The suction connections from the evaporator to the compressor for the given position of the condenser are shown in the following figure.

Figure 10.18
Evaporator is below the condenser and suction at the bottom of the evaporator

Multiple evaporators with multiple compressors

The following precaution should be taken while designing a suction line for a multiple compressor installation:

  • Suction header should be at a point higher than the compressor suction nozzle. This is necessary to avoid trapping of oil in the suction header and to allow draining of oil into the compressor by gravity.
  • All the suction lines should be terminated at the common suction header.
  • Horizontal take-off from the suction header should be of same rise as suction header. This also eliminates possibility of oil trapping in the suction header.
  • There should be no reduction in the line size to the compressor unless there is a vertical drop.
  • A yoke type suction header should be provided for equal distribution of the flow in all the compressors as shown in the figure below.
Figure 10.19
Connections from Single Suction Main to Parallel Compressors
  • On J-line compressors, it is necessary to provide an inverted loop as shown in the following figure otherwise the oil would drain into the idle compressor causing valve damage during start up.
Figure 10.20
Discharge Piping Arrangement

Engineers should consider the following points while designing discharge piping:

  • It is necessary to avoid oil traps in order to eliminate oil collection during partial loads.
  • Connections to the compressor should be such that the refrigerant should not condense near the idle compressor nor any oil should drain back into the compressor head. This can be easily done by taking all the discharge lines from the compressor vertically downwards into a common header at a lower elevation.
  • When the condenser is above the compressor and the distance between the two is greater, the hot gas lines should loop to the floor near the compressor before rising to the condenser. This prevents the oil and refrigerant draining back to the compressor head during the off cycle.
  • When the discharge line is considerably high, loops at every 5-7 meters should be provided as shown in the figure below.
Figure 10.21
Compressor
  • When more than one condenser is used, it is necessary to provide a balancing line of the same size as that of the individual discharge line. This is shown in the following figure. Absence of a balancing line would create unequal pressures at the discharge of each condenser reducing the capacity of the condenser.
Figure 10.22
Refrigerant Piping Arrangement

In the liquid refrigerant lines from the receiver to the expansion valve, it is essential that flash gas should not be formed as it has the following bad effects on the system:

  • It increases the pressure drop in the lines causing more flash gas.
  • It reduces the expansion valve capacity causing the cooling coil starvation and creates noise in the expansion valve.
  • Leads to erratic control of the refrigerant entering the evaporator.

The correct way of taking the liquid lines from the receiver to the evaporator is illustrated in the following figure. The dotted line in the figure shows the wrong way of connection.

Figure 10.23
Correct liquid line layout from receiver to Evaporators

Piping Material

Materials used for piping in refrigeration systems should be able to withstand the pressure and have the ability to resist corrosion. The common materials used in these systems are copper, brass, steel, wrought iron and zinc.

Copper and brass are light in weight, more resistant to corrosion and easy to install. Copper tubes of a maximum size of 100 mm diameter are economical. The pipelines greater than 100 mm are made of steel. The wrought iron pipes are preferred where the chances of corrosion are greater.

If sea water is used as a cooling medium for the condenser, the metals used for the piping of the condenser are yellow brass, admiralty alloy, aluminum brass or super nickel as all of these have high resistance to corrosion with variable degree.

The material used for pipe lines to carry liquid or vapor refrigerant should not be attacked by the refrigerant. Copper or brass lines should not be used with ammonia. Any suitable material can be used of CO2, SO2 or Freon in the absence of moisture, provided it withstands the pressure of the system.

Recommended velocities

It is always necessary to limit the velocities of the refrigerant flowing in the various lines of the refrigeration system.

It is recommended to maintain the following velocities while designing the piping system.


11


Controls and Instrumentation

11.1 Objectives

At the conclusion of this chapter, student should be able to

  • Understand the various terms used in control and instrumentation
  • Understand the elements and types of controls
  • Know various methods of controls used in HVAC systems
  • Understand the basics of PID tuning
  • Prepare control specifications

11.2 Introduction

What is Control? Control in general terms can be defined as a state of maintaining the specified or required parameters within the required range. HVAC broadly can be defined as the simultaneous control of temperature, humidity, air distribution and pressure.

The basic functions of control in HVAC are to:

  • Ensure proper and trouble free operation of the system
  • Safeguard the plant against accidents
  • Ensure that the plant operates efficiently.

Since the load on an air conditioning plant keeps changing continuously depending on the climatic conditions, a steady state condition is rare. A state maintained in a conditioned space will not remain constant if the plant capacity is uncontrolled.

As an example, consider the plenum ventilation system as illustrated in the figure below.

Figure 11.1
Closed Loop System

When the outside temperature rises above its design winter value, the fabric heat losses diminish and the heater battery capacity will increase. The room, temperature will rise constantly.

To keep this at a nominally constant value four things must be done:

  • The temperature must be measured
  • The change in temperature must be used to send a signal to the heater battery
  • The strength of the signal must produce a matching change in the battery output
  • The time taken for this action must not be so high that further load changes occur in the meantime and the battery output becomes significantly out of phase with the load.

Appropriate controls are used in order to make all the above happen. The above example has been cited just to make you understand the importance of control and instrumentation in HVAC.
Controls can be classified into two:

  • Operational controls
    These controls regulate the functions of the plant in order to ensure that the plant meets the demands for which it is designed.
  • Safety Controls
    These controls ensure that the functions do not deviate from the design parameters. Safety controls are a vital part of any control system and omission of these can cause serious damage to the plant.

11.3 Definitions

To understand the terms used in any control system, a glossary of terms is provided in the British Standards BS 1523: Part 1: 1967. We shall discuss below a few of them useful for us in understanding control and instrumentation.

Controlled variable

It is the quantity or physical property measured and controlled, e.g. room air temperature

Desired value

It is the value of the controlled variable, which it is desired a control system should maintain.

Set point

It is the value of the controlled variable set on the scale of the controller, e.g. 210 C set of the thermostat scale.

Control point

It is the value of the controlled variable that the controller is trying to maintain. This is a function of the mode of control, e.g. with a proportional control and a set point of 210C + 20 C, the control point will be 23o C at full cooling load, 210 C at 50 % load and 190 C at zero load.

Deviation

The difference between the set value and the measured value of the controlled variable at any instant is termed as deviation, e.g. with a set point of 210 C and an instantaneous measured value of 220 C, the deviation is + 10 C.

Offset

A sustained deviation caused by an inherent characteristic of the control system.

Primary element (sensor)

It is that part of the controller which responds to the value of the controlled variable in order to give a measurement, e.g. a bimetallic strip in a thermostat.

Final control element

The mechanism that alters the plant capacity in response to a signal initiated at the primary element e.g. motorized valve.

Automatic Controller

It is a device, which compares a signal from the primary element with the set point and initiates corrective action to counter the deviation, e.g. a room thermostat.

Differential

This refers to a two position control and is the smallest range of values through which the controlled variable must pass for the final control element to move between its two possible extreme positions, e.g. if a two position controller has a set point of 210 + 2 0 C the differential gap is 4 0 C.

Figure 11.2
Two Position Control

Proportional band

It is also known as the throttling range. This refers to proportional control and is the range of values of the controlled variable corresponding to the movement of the final control element between its extreme positions

Figure 11.3
Proportional Control for a Cooling Application

Cycling

This is also known as hunting. It is persistent, self induced, periodic change in the value of the controlled variable.

Open loop system

A control system without a feedback loop is termed an open loop system.

Closed loop system

It is a control system with a feedback loop so that the deviation is used to control the action of the final control element to reduce the deviation.

Dead time

The time between the signal change and the initiation of the response to change is called the dead time.

Accuracy

The ability of a control system to maintain the control point that closely approaches the set point is the measure of its accuracy. This is expressed as a percentage of the full scale of the instrument. An instrument is said to be accurate when the deviation is zero.

Sensitivity

Instruments do not respond instantaneously. The largest variation in the measured variable that occurs before an instrument starts to respond is an indication of its sensitivity.

Lag

It is the delay in the response of the control system to a change in the controlled variable.

Direct acting

A controller is said to be direct acting when it responds in the direction of the control signal . e.g. In a system comprising of a sensor, controller and actuator, an increase in the control signal results in an increase in the output signal of the controller to the actuator.

Reverse acting

It is the opposite of direct acting. An increase in the control signal results in a decrease in the output signal and vice versa.

11.4 Elements of Control

The most common elements of a control system are controlling element, actuating element, limit control system and equipment, which is to be controlled.

The basic elements of a control system are shown below:

Figure 11.4
Basic Component of Automatic Control System

The controlling element takes the signal from the room as per the requirement (either temperature or humidity) and actuates the actuating element for providing the required conditions. The control elements are used to control the temperature, humidity and pressure.

The most common measuring elements of a control system are:

  • Temperature measuring element
  • Humidity measuring element

11.4.1 Temperature Measurement

The control element that is used for measuring temperature is known as a thermostat. A thermostat is a controlling element, which gives direct response to the change in temperature.

The different types of temperature measuring elements used in practice are discussed below.

Bi-metallic type element

A pair of dissimilar metals; Invar (Low coefficient of thermal expansion) and brass (high coefficient of thermal expansion) are joined firmly together for making a bimetallic sensing element. Increasing the temperature of the element causes the bimetal to wrap in the direction of invar and vice versa. This change in the configuration of the bimetal with the change in the temperature can be utilized to open or close electrical contacts in an electrically operated control.

The following figures illustrate the position of the bimetal strips according the temperature they are exposed to.

Figure 11.5
Position of Bimetal Type Thermostat

Bi-metals can be produced in various shapes in order to achieve the control action as required. The two most common types used other than the straight ones are, the coiled element and the helix type element.

Figure 11.6
Type of Bi-Metal Elements

Any bimetallic strip or an element has a certain heat capacity so it requires a certain time to cool down or heat up from a particular temperature. During this time, the space temperature continues to change and with it, the element temperature changes. The resultant effect is a swing from one side to other side of required temperature. These two positioned thermostats are extensively used in controlling heating and cooling systems used for homes and small commercial buildings.

Sealed bellow-type sensing element (Thermostat)

The basic working principle of these elements is based on the expansion of gases with temperature.

The simplest form of this sensing element consists of a vessel containing the expansible gas. When the temperature changes, the volume of this gas increases causing the mercury in the column to rise. This rise in the mercury column makes or breaks an electric circuit either directly or through relays.

A typical arrangement is shown in the figure below:

Figure 11.7
Sealed Bellow-Type Thermostat

A and B shown in the figure below are two platinum wires which lead the current into and out of the mercury respectively. The mercury is in series with the electrical circuit. The circuit breaks at the junction of the gas and mercury at the point A. In these thermostats, the vessel containing gas should have a larger surface area and sufficient volume to replace the effect of room temperature variation on the capillary.

The range of temperature values that can be controlled by this type of thermostat is higher but its sensitivity is low.

The figure below illustrates one more type of thermostat using this principle.

Figure 11.8
Bellows Type Thermostat

In this type of thermostat, the force due to expansion is used to operate a linkage mechanism, which further makes or breaks the electrical circuit.

Some bellow-operated thermostats use a remote bulb or a capsule to feel the temperature at a location away from the bellows. Bellow operated thermostats are accurate and have advantages to assure positive switching action.

Thermocouples

A set of two wires of dissimilar metals soldered together at one end and free at the other end is known as a thermocouple. If the soldered junction is subjected to higher or lower temperature than the free end an e.m.f is generated in the circuit due to the temperature difference. The current generated is used as a feedback variable and depending on the materials adopted, the application is from –250 0 C to 2600 0C.

Resistance Temperature Detector (RTD)

RTD, as it is commonly known operates on the principle that the electrical resistance of a material changes with temperature. This change in the resistance is used to operate the actuating element. This sensing element is used for temperature ranging from –250 C to 10000 C. Accuracy is about 0.25 %. It is simple, accurate, and stable and has a virtually linear response.

Thermistors

The measuring elements are oxides of metals with an inverse, exponential relation between temperature and electrical resistance. The response is rapid although it is non linear. It is possible to reduce the non-linearity by using resistors. However the application of thermistors is limited to a maximum temperature of 200 0C.

11.4.2 Humidity Measurement

Control of humidity is the most required condition of comfort or industrial air conditioning systems. Humidity control can be highly critical in some areas as hospital operation theatres, electronic data processing equipments and many printing operations.

The element used for controlling humidity is known as a humidistat. It is a device, which is sensitive to moisture changes and operates the equipment to maintain the desired humidity in the air-conditioned space.

The most common element generally used for measuring humidity is the human hair. The humidistats are used through relays to supply water to a spray or heat supply to a pan type humidifier. These can also be used to operate fresh air intake ducts to regulate humid air intake.

The different types of humidistats used for different purposes and ranges are:

  • Hair type Humidistat
  • Electrolytic water analyzer (absorption type humidistat)
  • Water vapor recorder
  • Automatic dew point recorder
  • Industrial hygrometer

Let us discuss each of these below

Hair type humidistat

In this type of a humidistat, the human hair or animal membrane is used as water vapour absorbing material. The quantity of water vapour absorbed depends on the R.H of the air. The human hair is suitably connected to actuate an indicating or recording instrument as shown in the figure below.

Figure 11.9
Hair-type Humidostat

This type of humidistat is used up to 600C and a range of 15 to 95 % humidity.

Electrolytic water analyzer

Figure 11.10
Electrolytic Water Analyzer

As illustrated in the figure above, a measured quantity of air is passed through a cell of hygroscopic material such as silica gel, phosphorous pentoxide or calcium chloride. Moisture is absorbed quantitatively and continuously and then decomposed by electrolysis between platinum electrodes.

According to Faraday’s law, the decomposition current is directly proportional to the rate of decomposition of water and moisture content in the sample as known flow rate.

Water vapor recorder

In this type of humidistat, the air stream is divided into two equal parts, one of which is thoroughly dried. The moist and dried air streams are passed through two absorbers as shown in the figure below.

Figure 11.11
Water Vapour Recorder

The temperature difference between the absorbers is detected by thermocouples and measured by voltmeters. This system is generally used for low moisture streams.

Dew point recorder

In systems, where low moisture content is to be determined and when an absolute humidity measurement independent of temperature is desired, hygrometers are not suitable.

The General Electric dew point measuring instrument uses a mirror surface, which is cooled to a temperature at which, a layer of dew first forms. The temperature at which this occurs is recorded and corresponds to the dew point temperature. A typical arrangement of the system is shown below.

Figure 11.12
General Electric Dew-Point Recorder

The instrument consists of four components, A two stage refrigeration system to maintain temperature to –70 0 C., a gas chamber with dual photoelectric system, electric power unit and amplifier and a temperature recorder calibrated in dew point.

Industrial hygrometer

Certain chemicals are hygroscopic and change their electrical conductivity with changes in the moisture content.

In a typical industrial hygrometer as shown below, the humidity cell consists of a temperature sensing resistance element enveloped with glass wool impregnated with lithium chloride solution. The element is also wound with a heating coil terminating in silver connecting wires. A 24 volts AC current passed through the coil increases the temperature of the lithium chloride solution by evaporating water from the salt until the salt has crystallized.

Figure 11.13
Electric Circuit for Hygrometric Using Lithium Chloride Element

Equilibrium is reached where the partial vapour pressure of salt equals that of the surrounding atmosphere. The cell temperature is therefore maintained automatically at a point, which corresponds to the DPT of air.

11.5 Types of Control System

There are four types of controls used in common practice.

  • Self acting control
  • Pneumatic control
  • Electric control
  • Electronic control

11.5.1 Self-acting control system

The pressure, force and displacement produced as a signal by the measuring element is used directly as the power source at the final control element. A good example is the thermostatic expansion valve.

Figure 11.14
The Thermostatic Expansion Valve

The vapor pressure generated in the bulb by the temperature measured is transmitted along the capillary and acts above the diaphragm of the valve. This causes the diaphragm to move and regulate the flow of refrigerant. This is a simple system where no external power is needed. The action is proportional in nature but has a wide proportional band as it needs to produce enough power to move the final control element between its extreme positions.

11.5.2 Pneumatic control system

As the name suggests, compressed air pressure is used to operate the final control element.

Compressed air is piped to each controller, which by bleeding some air to waste reduces the air pressure to a value related to the measured value of the controlled variable. This reduced pressure is transmitted to the control element.

Figure 11.15
Simple Pneumatic System

Sizing of the pressure-reducing orifice shown in the inlet line in the figure is critical. If it is too big, an excessive amount of air will be bled to waste and if it is too small airflow, above the valve diaphragm will be slow and the control element response will be sluggish.

Advantages:

  • Simple and easy to maintain
  • Faster response from pneumatic control devices
  • Low-pressure air is safe and does not require specialists to work on it
  • Overall cost is less

Disadvantages:

  • Performance depends highly on the compressed air quality
  • Regular recalibration required
  • Commissioning can be complicated

11.5.3 Electrical control system

This provides control by starting or stopping the flow of electricity or by varying the current and voltage by means of a rheostat or bridge circuits.

This can be illustrated by a typical electric system.

Figure 11.16
Simple Electrical System

Advantages:

  • Low initial cost
  • Very simple installation and easy to understand
  • Wide range of controls available
  • Low voltage controls are safe.

Disadvantages:

  • Accuracy and sensitivity is limited.
  • Noise or interference possible with some systems.

11.5.4 Electronic control system

The basic control function of electronic controllers is derived from a Wheatstone bridge. In this, a change in resistance on any side will cause an imbalance and create a flow of current. Even small flows of current can be converted into variable outputs of 0-10 volts DC with the help of amplifiers.

Electronic controls have entirely replaced pneumatic controls in commercial applications. With electronic controls using low cost integrated circuits, complex functions can be achieved.

Advantages:

  • Low initial cost
  • It is very simple to commission
  • It is intrinsically safe as the voltages handled are very low
  • Complex functions can be easily achieved
  • Any electronic control system is compatible to computerized building management systems

Disadvantages:

  • Certain control systems can get affected due to noise and any other interference
  • Fail-safe operation using spring return on actuators becomes expensive
  • Troubleshooting and repairs can become difficult.

11.6 Methods of Control

A control mode is used to indicate the way the controller acts on the signal it receives and what kind of movement it produces on the actuator.

Various methods of control are:

  • Simple two position control
  • Timed two position control
  • Floating control
  • Simple proportional control
  • Proportional plus integral derivative (P+I+D) control.

11.6.1 Simple Two Positions Control

This is the simplest form of control also known as “ON-OFF” control.

There are only two values of manipulated variables; maximum and zero. The sensing element switches on full capacity when the temperature falls to the lower value of the differential and switches the capacity to zero when the upper value of the differential is reached.

The following figure illustrates the way a controlled variable alters with respect to time when an air heater battery is switched on and off.

Figure 11.17
Simple Electrical Holding Circuit

Overshoot and undershoot occur because of the total lag. In on-off controllers, if the lag is large, the control is poor. For a correct application, it is an excellent, simple and cheap form of control. Typically ON-OFF controls are used where, overshoot and undershoot do not affect the process a great deal.

11.6.2 Timed Two-Position Control

In a two-position control load and capacity do not match. For example, if the load is 50%, the plant will be ON for half the time and OFF for half the time. In such a case, in each hour the plant can either be ON for say 30 minutes and OFF for 30 minutes or it can switch between ON and OFF every alternate minute. However, it is clear that, a 30-minute ON-OFF cycle would provide a larger variance in the controlled variable than a one-minute cycle. The lower the value of the room air temperature, the longer the period of the plant remaining ON, which is desirable.

To achieve this the two positions control can be provided with a timed variation of capacity, which gives a smaller differential gap.

11.6.3 Floating Control

Unlike ON-OFF control in a floating control action, the final control element floats in a fixed position as long as the value of the controlled variable lies between the two chosen limits.

When the value of the controlled variable reaches the upper limit, the final control element is actuated to open at a constant rate. During this movement, if the value of the controlled variable starts to fall in response to the control element, the movement of the control element stops and it remains in the new position, partly open. It remains in this position until the controlled condition again reaches a value equal to the limit.

Thus, the final control element is energized to move in a direction, which depends on the deviation: a positive deviation gives the movement of the element in one direction and a negative deviation causes the movement in the reverse direction.

11.6.4 Proportional Control

A control action is said to be simple proportional when the output signal from the controller is directly proportional to the deviation. If this output signal is used to vary the position of the modulating valve, then there is only one position of the valve for each value of the controlled variable. The following figure indicates this.

Figure 11.18
Proportional Control Graph

Offset is an inherent feature of a proportional control. The terms direct action and reverse action are used to denote the manner in which the final control element moves in response to the signals it receives from the sensor.

For example, consider a room suffers a heat gain and is offset by means of a fan coil unit fed with chilled water; output being regulated by means of an electrically actuated, motorized valve. When the room temperature rises the controlling, thermostat sends an increasing signal to the motorized valve and this is termed as direct acting. If the controller drives the motorized valve to close, chilled water flow is reduced which is undesired at this condition. A reversing relay is added to the control circuit to drive to the motorized valve to open. Thus, the control action becomes a reverse action.

11.6.5 PID Control

PID (Proportional-Integral-Derivative) control action allows the process control to accurately maintain the set point by adjusting the control outputs.

Proportioning control continuously adjusts the output dependent on the relative positions of the process temperature and the set point. PID (Proportioning/Integral/Derivative) are control functions commonly used together in today's controls. These functions when used properly allow for the precise control of difficult processes. It allows the output to be a value other than 100% or 0%. The temperature can be controlled without oscillations around the set point.

Proportioning band

It is the area around the set point where the controller is actually controlling the process. The output is at some level other than 100% or 0%. The band is generally centered on the set point (on single output controls) causing the output to be at 50% when the set point and the temperature are equal.

In the illustration below two output controls (i.e.: heat/cool) there are two proportioning bands. One is for heating and one is for cooling. In this case, the bands generally end at the set point as shown below.

Proportioning bands are normally expressed in one of three ways:

  • As a percentage of full scale
  • As a number of degrees (or other process variable units)
  • Gain which equals 100%/proportioning band% (example PB% = 5; Gain = 20)

If the proportioning band is to narrow, an oscillation around the set point will result. If the proportioning band is to wide the control will respond in a sluggish manner, could take a long time to settle out at the set point and may not respond adequately to upsets.

Manual reset

Virtually no process requires precisely 50% output on single output controls or 0% output on two output controls. Because of this, many older control designs incorporated an adjustment called manual reset (also called offset on some controls). This adjustment allows the user to redefine the output requirement at the set point. A proportioning control without manual or automatic reset (defined below) will settle out somewhere within the proportioning band but likely not on the set point.

Automatic reset (Integral)

The integral function corrects for any offset (between set point and process variable) automatically over time by shifting the proportioning band. Reset redefines the output requirements at the set point until the process variable (temperature) and the set point are equal. Most current controls allow the user to adjust how fast reset attempts to correct for the temperature offset. Control manufacturers display the reset value as minutes, minutes/repeat (m/r) or repeats per minute (r/m). This difference is extremely important to note for repeats/ minute is the inverse of minutes or minutes/ repeat). The reset time constant must be greater (slower larger number m/r smaller number r/m) than the process responds. If the reset value (in minutes/repeat) is small a continuous oscillation will result (reset will over respond to any offset causing this oscillation). If the reset value is too long (in minutes/ repeat) the process will take too long to settle out at set point. Automatic reset is disabled any time the temperature is outside the proportioning band to prevent problems during startup.

Given below is an example of a single output (heat only temperature control) with a 10% proportioning band and a set point of 400. Note how reset shifts the proportioning band when the temperature (PV) enters the proportioning band.

Reset stops moving the proportioning band as soon as the set point and PV are equal. In the above example reset determined approximately, 38% output is required to maintain set point. Stable control is achieved and the temperature matches the set point of 400.

Rate (Derivative)

It shifts the proportioning band on a slope change of the process variable. Rate in effect applies the "brakes" in an attempt to prevent overshoot (or undershoot) on process upsets or startup. Unlike reset, derivative operates anywhere within the range of the instrument. Derivative usually has an adjustable time constant and should be set much shorter than reset. The larger the time constant the more effect derivative will have. Too large a derivative time constant will cause the process to heat too slowly. Too short and the control will be slow to respond to upsets.

Tuning

Many control manufactures provide various facilities in their controls that allow the user to more easily tune (adjust) the PID parameters to their process.

Tuning on demand with upset

This facility typically determines the PID parameters by inducing an upset in the process. The controls proportioning is shut off (on-off mode) and the control is allowed to oscillate around a set point. This allows the control to measure the response of the process when heat is applied and removed (or cooling is applied). From this data, the control can calculate and load appropriate PID parameters. Some manufactures perform this procedure at set point while others perform it at other values.

Adaptive tuning

This mode tunes the PID parameters without introducing any upsets. When a control is utilizing this function, it is constantly monitoring the process variable for any oscillation around the set point. If there is an oscillation, the control adjusts the PID parameters in an attempt to eliminate it. This type of tuning is ideal for processes where load characteristics change drastically while the process is running. It cannot be used effectively if the process has externally induced upsets for which the control could not possibly tune out.

11.7 Selection of a Control System

We have now studied the different types of controls in practice in an HVAC system. In every system, it is very important to select the appropriate control mode.

The following needs to be considered while selecting a control mode:

  • The time taken by the sensor to detect the change and initiate an action.
  • The time taken by the controlling media to travel from its source to the point of control action.
  • The time taken to transfer energy to the controlled variable.
  • The accuracy required.

Generally, a system, which is slow to change because of its large demand side capacity, requires simple control. A fast reacting system needs a complex control.

11.7.1 Variables to be controlled

It is also very important to know what are the variable that are needed to be controlled before we go ahead in selecting the correct type of control mode.

The most common variables used in an air conditioning system are:

  • The temperature of air inside the air-conditioned space.
  • Supply air temperature to a system.
  • Return air temperature from the air-conditioned space.
  • Dew point temperature.
  • Water temperature in a heating or cooling system.
  • Duct pressures
  • Evaporator pressure
  • Condenser pressure
  • Relative humidity of air entering or leaving the space.
  • Flow of water or air

11.7.2 Applications

Two position Control

This control, also known as ON-OFF control, is generally suitable for processes and applications where there is a large demand side capacity. In such applications big differences can be tolerated.

For example; Hot water storage tanks, space heating of rooms by radiators or room air conditioners.

Floating control

Floating controls can be adjusted to give stable conditions in applications where the load changes are slow and relatively constant. The stability of a floating action can increase to a great extent by adding integral action to it.

For floating control to be effective, the sensor needs to be close to the energy input so that it can detect the change quickly and take action immediately.

Proportional control

This mode of control is the most versatile of the control industry and is ideal for the majority of applications.

Proportional control is suitable in applications where the load changes are not so fast and a sustained offset from set point is acceptable.

The proportional band when added to this control helps in matching the speed of the control with the load change. The bigger the proportional band the bigger will be the offset.

An integral action added to this will eliminate the offset. Thus for closer control and accuracy, integral action is necessary.

PID control combines all the three controls in one enabling the control to remove sudden deviations quickly by a large derivative correction.

PID control can be used where a wide variation in load occurs and an accurate control is required. The best example can be the control of duct pressure in a VAV system.

11.8 Typical Control Systems

In this section, let us briefly discuss how these controls work in an HVAC system.

We shall look at the following systems:

  • Preheat and humidification control (Winter Air-conditioning)
  • Cooling, dehumidification and reheat control. (Summer Air-conditioning).
  • Face and by-pass control
  • All year round air-conditioning system
  • Zone control system.

11.8.1 Preheat and Humidification Control

The arrangement of this system is shown in the following figure:

Figure 11.19
Preheat and Humidification Control

This system typically has a thermostat and humidistat located in the duct that circulates the conditioned air through the space to be conditioned. The humidistat controls the movement of the motorized valve located on the sprayer of the humidifier, while the thermostat controls the motorized valve located on the steam line of the heating coil.

As the latent heat load or the sensible heat load changes in the room, the temperature or humidity change will be corrected by the steam flow in the heating coil or spray quantity in the humidifier, which are controlled by the thermostat and humidistat.

11.8.2 Cooling, Dehumidification and Reheat Control

This arrangement is illustrated in the following figure.

Figure 11.20
Cooling, De-Humidification and Reheat Control

In this particular case, the thermostat controls the movement of the motorized valve located in the chilled water line entering the cooling and dehumidifying coil. The humidistat controls the motorized valve on the steam line entering the heating coil.

As the latent heat load or the sensible heat load changes in the room, the temperature change will be corrected by correcting the chilled water flow through the cooling and dehumidifying coil. Similarly, the humidity change will be corrected by correcting the steam flow through the reheat coil.

11.8.3 Face and by-pass control system

Figure 11.21
Face and Bypass Control System

A typical arrangement of this system is illustrated above.

Say outside conditions are fixed and the room load conditions are changed. For example, when the sensible heat load in the room reduces, the temperature of air leaving the room (temperature of re-circulated air) will increase. This temperature rise will be sensed by the sensor (S), which will give signal the motor (M1) to start. The motor is mechanically connected through levers to the dampers (Db). The motor will increase the air bypass by opening these dampers and decrease the airflow through dampers (Df).

It is also possible to arrange the system in such a manner that, when the dampers (Df) close completely the heating medium is shut off by the operation of the motorized valve. This system can be effectively used for wide range of conditions. A similar arrangement can be used for cooling application also.

11.8.4 All Year Round Air-Conditioning Control System

The arrangement of this system is shown below.

Figure 11.22
All Year Round Air-Conditioning Control System

The quantity of outdoor and recirculated air is controlled by the thermostats (TO1) and (TO2). These are designed in such a way that when the outdoor air temperature is either above or below the set point, the quantity of outdoor air is changed by the action of the damper motor (M1), until the minimum quantity set is reached. Thermostats Tc1 and Tc 2 perform similar functions as discussed in the previous systems. The temperature of air leaving the cooling coil is controlled by the motor (M2), which controls the face and bypass dampers. Humidity is controlled separately by motor (M3) by changing the quantity of cold fluid passing through the motorized valve at the inlet of the cooling coil. The signal to the motor is received by the humidistat (H). Under conditions of high humidity, the humidistat (H) keeps the valve open to provide high dehumidification.

11.8.5 Zone Control System

Zone control system is used in buildings, which possess a number of exterior aspects of some considerable area, each of which is thermally affected differently according to its exposure to atmospheric changes. Changes in the direction of the sun, wind and rain result in varying temperatures of rooms. The heat input to these rooms should be different as per the conditions. This is done by sectionalizing the circulating mains as shown in the figure below. The regulating valve introduced in each section is controlled by its respective thermostats. A separate thermostat is used to control the heat given by the boiler to the mains.

Figure 11.23
Zone Control System

11.9 Control Specifications

A system design engineer must always ensure that all the specifications he provides for any control are accurate. This is very important in selecting the heating and cooling equipment. The control specification is essential for the commissioning engineer as well. Any commissioning cannot be complete unless all the controls and the instruments are tested and made to operate at their intended conditions. A typical control system specification can be as follows.

Objective

A departmental store and the adjacent administration buildings are to be air-conditioned.

Requirement

  • Cooling to be provided by means of chilled water.
  • Heating to be provided by means of electric heater.
  • The temperature and humidity are to be controlled from sensors installed in supply air ducts.
  • Humidification to be done by means of steam type evaporative humidifier.
  • All usual interlocks and safeties to be provided.

Conditions to be maintained

  • Temperature --- 22 0 C + 2 K
  • Humidity ---55 % + 5 %
  • Minimum supply air temperature --- 12 0 C.
  • Maximum supply air humidity –--75 %
  • Operating Hours – Monday to Saturday – 8.00 a.m to 5.00 p.m
  • Sunday – 8.00 a.m to 1.00 p.m

Control specifications

Starting and stopping the plant

The plant shall start and stop automatically by means of a timed clock. The air conditioning system should start one hour prior to the store opening hours in order to pre-cool or pre-heat the conditioned spaces. There shall be an ON-OFF selector switch with auto, OFF and manual positions.

Energizing the control system

The control system shall be energized when the supply air fan is started and should be de-energized when it is switched off.

Temperature control

The cooling and heating shall be controlled by a control system having a single temperature sensor, which is to be positioned near the return air grill, at a height of 1.5 meters from the floor.

Any rise in temperature in the conditioned space shall activate the chilled water control valve to open in proportion to the deviation. The valve shall not open further once the supply air temperature has reached a minimum of 12 0 C. Any drop in temperature shall switch ON the electric heaters.

The electric heaters should not start unless a certain pressure has been established in the supply air duct, which is an indication that the supply air fan is ON. This shall be done providing an interlock of the heater contactors with the supply air fan starter or provide a pressure switch in the supply air duct to sense the pressure and activate the heaters accordingly.

The temperature control shall be adjusted such that heating and cooling does not operate simultaneously.

Humidity control

The humidity sensor shall be positioned adjacent to the temperature sensor. No set point adjustment shall be provided at the humidity sensor.

On the rise in humidity, the humidity control system shall override the temperature control and open the chilled water valve in proportion to the humidity deviation.

If the temperature drops due to the dehumidification action, the temperature control shall switch on the heaters to provide reheating.

If there is a drop in the humidity, the control system shall modulate the dry steam humidifier in proportion to the deviation. The humidifier shall be interlocked with the supply air fan and shall not operate if this fan is off.

Outside air damper control

The outside air dampers shall be closed during non-working hours. The opening of the outside air damper during working hours shall be set by means of a remote control.

Instruments required

  • Temperature: Outside air
  • Return Air
  • Chilled water supply and return
  • Humidity: Supply and return air.
  • Pressure: Pressure loss across the filter
  • Supply air

11.10 Conclusion

There are different air-conditioning systems in use; today and all system demand a different kind of control system. As we have stated in the beginning, in this section we have tried to cover all the general aspects needed in a control system.

This section has aimed at developing an understanding of controls and control systems. One must keep in mind that the control system must be kept as simple as possible and should be user friendly.


12


Installation, commissioning operation, testing and maintenance

12.1 Objectives

After reading this chapter the student will be able to:

  • Supervise the installation activity of a refrigeration unit.
  • Understand the various Service Operations required for Refrigeration
  • Understand the charging method
  • Assist in commissioning activities of an air conditioning system
  • Carry out maintenance activities on a Refrigeration plant

12.2 Installation

Points to be considered while installing the refrigeration unit of the Air-conditioning system.

The troubles in the refrigeration unit after installation fall under various categories such as no refrigeration, not in continuous operation, higher electricity consumption, poor refrigeration temperature, frosted suction lines and so on. All such woes can be avoided if the refrigeration unit is installed correctly. Following point should be considered while installing a refrigeration unit.

  • In a multiple installation system, the condensing unit should be close to the cabinets. It is always advisable to install the condenser at a location where it is exposed to very low temperatures. It should be mounted on a concrete base to avoid bad effect of moisture and vibrations.
  • The cooling coils should be firmly fastened. These are normally mounted on the ceiling of the cabinet.
  • The tubing of the installation is generally run along the walls or ceilings and well supported to ensure the tubes run straight. The tubing should not run near any sort of heat source. Such sources of heat can cause poor refrigeration and low efficiency.
  • The compressor should be placed at such a location that its noise does not disturb the occupants. It should be close to the suction line to avoid superheat of the refrigerant along the tubing.
  • The expansion valve should be located very close to the cooling coil to avoid the cooling losses.

12.3 Charging the refrigeration unit

12.3.1 Charging a small refrigeration unit

There are four main components in a refrigeration system:

  • The Compressor
  • The Condensing Coil
  • The Metering Device
  • The Evaporator

Two different pressures exist in the refrigeration cycle. The evaporator or low pressure, in the "low side" and the condenser, or high pressure, in the "high side". These pressure areas are divided by the other two components. On one end, is the metering device which controls the refrigerant flow, and on the other end, is the compressor.

Figure 12.1
Charging schematic in refrigeration cycle

Basic Refrigeration Cycle

  • Starting at the compressor;
  • Low pressure vapor refrigerant is compressed and discharged out of the compressor.
  • The refrigerant at this point is a high temperature, high pressure, “superheated” vapor.
  • The high pressure refrigerant flows to the condenser by way of the "Discharge Line".
  • The condenser changes the high pressure refrigerant from a high temperature vapor to a low temperature, high pressure liquid and leaves through the "Liquid Line".
  • The high pressure refrigerant then flows through a filter dryer to the Thermal Expansion valve or TXV.
  • The TXV meters the correct amount of liquid refrigerant into the evaporator.
  • As the TXV meters the refrigerant, the high pressure liquid changes to a low pressure, low temperature, saturated liquid/vapor.
  • This saturated liquid/vapor enters the evaporator and is changed to a low pressure, dry vapor.
  • The low pressure, dry vapor is then returned to the compressor in the "Suction line".
  • The cycle then starts over.
Figure 12.2
Service ports locations

Refrigerant Vapor Charging

For charging refrigerant vapor into the system, a standard manifold is being used as shown in figure below. The manifold consists of two pressure gauges mounted on the top side and three external hose connections at the bottom. On either side of the manifold we can see, there are shut-off valves.

These valves seat to a position at each side of the centre hose connection, and when closed (turned fully clockwise) will prevent passage of vapor to the centre hose. A colour code has been introduced for the pressure gauges, hoses and shutoff valves.

Figure 12.3
Service Gauge Manifold operating principle

The left hand pressure gauge is known as a compound gauge and is calibrated to read zero at atmospheric pressure. The gauge pressure range is from 30 in Hg to 0 psi (0.9 to 0 bar) for pressures below atmospheric pressure, and from 0 to 250 psi (0 to 10.7 bar) or more for those above. This gauge is color coded as BLUE.

The right hand gauge is called a pressure gauge. It only records pressures above atmospheric, from 0 to 500 psi (0 to 35 bar). This gauge is color coded as RED.

The hoses are also color coded to correspond to the gauges and shut-off valves. The blue hose should be connected to the compound gauge, the yellow to the centre connection and the red to the pressure gauge.

Figure 12.4
Service ports hose details

When the manifold is assembled it is not necessary to open the valves. Pressure will be recorded as soon as the system pressure is passed to the hose after setting the compressor service valve. When either valve is opened, and assuming pressure is available from the system, it will pass to the centre hose. It is advisable to keep the centre hose plugged at all times, or the centre connection capped when the hose is removed.

With the centre connection capped and both valves open, it will be seen that the pressures will equalize on both gauges. When both valves are closed and the centre connection is capped, it will be seen that both negative and positive pressures can be recorded when the compressor service valves are set to operating positions, assuming that the system is designed to operate in such a manner.

The procedure for refrigerant vapor charging is as follows:

Figure 12.5
Refrigerator Charging Schematic
  1. Fit gauges, set the service valves to operating positions and operate the plant.
  2. Obtain a service cylinder of the correct refrigerant: this can be verified from the equipment log, the compressor nameplate or the label on the expansion valve.
  3. Connect the yellow hose to the centre connection on the manifold and to the service cylinder.
  4. Open the valve on the service cylinder, loosen the connection on the centre hose on the manifold and purge air from the hose.
  5. Tighten the hose connection and set the suction service valve to the midway position
  6. Open the compound gauge valve on the manifold slowly, and regulate the refrigerant into the system at an approximately average suction pressure (e.g. 30 psi or 2 bar for R12).
  7. Observe the liquid-indicating sight glass and, when the bubbles cease, close the compound gauge valve on the manifold. If bubbles return intermittently after a short time, add more refrigerant. When bubbles have ceased completely the operating pressures will have returned to normal and the evaporator will be fully frosted.

When vapour charging a system, cylinder must always be kept in a vertical position to prevent the possibility of liquid refrigerant from entering the compressor. This can create a dynamic pressure when the compressor starts, causing damage to valves or may even break piston connecting rods and damage pistons. The liquid refrigerant will also flush lubricating oil from bearing surfaces.

12.3.2 Purging

Purging a Refrigeration System

Air at atmospheric pressure is very much needed for life for all living things, but air in a refrigeration system is detrimental to the proper functioning of that system, meaning a loss of efficiency, increase in energy cost and even can destroy the equipment if not eliminated in time.

The Effect of air in the system

The Non-Condensable air in the system can result in excess wear and tear on bearings and drive motors and can result in reducing the life of seals and belts. The added head pressure can also result in premature failure of gaskets. For every 2 bar excess head pressure, the cost of power to operate the compressor will increase by 2% and the compressor capacity will be reduced by 1%.

How the air get into the refrigeration system?

Inspite of adequate measures taken to prevent air getting into the system, the air will be there and accumulate on the innner surface of the heat exchanger, acting as a insulation barrier. Some of the ways are illustrated below:

  • The Refrigerant itself contain about 1.5% noncondensable air, when delivered.
  • Before charging the refrigerant, the system was not adequately air evacuated.
  • Air ingress takes place, when the equipment is taken out for repair.
  • Breakdown of the refrigerant or lubricating oil or both will result in air accumulation
  • When there is a leakage, the air will get into the system through valve packing, seals etc, especially when the suction pressure is below atmospheric

How to test the presence of air in the system?

The Table from ASHRAE’s 1997-Fundamental Handbook, shows the data on temperature-Pressure.Check the condenser pressure and temperature of the refrigerant gas leaving the condenser and then compare the values with the table.We can also refer table B9-in appendix-B. If we find the pressure in excess of the given value in table at a particular temperature, then there is air in the system.

Purging Method

There are two methods of Purging:

  • Manual Purging
  • Automatic Purging

Manual Purging:

Purging of noncondensable gases from ammonia refrigeration systems was done by simply opening a hand valve at the condenser outlet. By this method, the undesirable gases to escape to the atmosphere. When the visible vapor became pure white, indicating a high concentration of ammonia, the system seems to have purged and the hand valve closed.

Automatic Purging:

The figure below, shows an automatic purging equipment installed in a refrigeration system.The schematic includes a Compressor, Condenser, Receiver, Evaporator and the purger equipment and pipings.

The purger equipments available in:

  1. Non-electrical Mechanical Purger
  2. Automatic electronic purgers

The figure represented is an illustrative only and there are many other manufacturers and purging methods are available.

Figure 12.6
Refrigerator Purging Schematic

12.3.3 Pump Down of the Refrigeration System

When the refrigeration system is to be repaired or overhauled, the refrigerant must be removed from the system. This is done by pumping the refrigerant into the receiver as a temporary storage.

To do this, close the liquid line shut off valve V4 and start the compressor. The compressor pumps the entire refrigerant into the receiver. The receiver inlet valve V2 can now be closed after pumping out the entire refrigerant from the system. During the pumping down process, a rapid decrease in the crankcase pressure causes the refrigerant in the oil to vaporize. This causes foaming, which will result in slugging of oil through the valves of the compressor. This produces knocking and if allowed to continue may damage the compressor. If while pumping down knocking is heard from the compressor, it should be stopped for some time and when the oil settles down pumping can be restarted.

While pumping down, the refrigeration system should never be open while in vacuum as air, dust and moisture would quickly force their way into the system. It is always advisable to break the vacuum with refrigerant vapour.

12.3.4 Leakage test

Once the system is charged with the refrigerant, it is necessary to check all the joints for any leakages. The methods for testing the leakage in the system vary with different refrigerants used.

Test for SO2

Ammonia is used to test leakage of SO2. A small piece of ammonia soaked cloth is fastened to one end of a stick. It is then placed adjacent to the joints. If there is a leak, it is noticeable by a thick white smoke forming at the place of the leak.

Test for NH3

Two methods are used for testing the leaks of NH3 in either compression system or absorption system. A sulphur candle flame gives a thick white smoke if it is exposed to ammonia. When a phenolphthalein paper is bought near the leaking, joint its colour changes instantly indicating there is a leak. Both these methods are accurate and fast.

Test Freon, Carrene and Ethyl chloride

The leaks of the above refrigerants are tested with a halide ( alcohol ) torch. If the intake tube of the halide torch is brought near the leaking joint, the leaking gas enters the tube and gives a green hue, which is a sure indication of a gas leak.

Electronic leak detector

An electronic leak detector measures the electronic resistance of the gas samples and if air containing the refrigerant vapour is tested the flow of current changes. The change in the current flow is indicated on a milli-ammeter.

12.4 Adding Oil to the Compressor

Following is the methodology adopted for adding oil to the compressor

  • Put the correct grade oil into a clean dry glass container.
  • Connect the oil charging line to the suction service gauge port through a hand shut off valve and put the other end into the oil.
  • With a shut off valve closed, operate the compressor unit until the time a vacuum of 500 mm is obtained. The vacuum should be maintained after the compressor is stopped.
  • Now stop the compressor and open shut off valve to allow the desirable amount of oil into the crankcase.
  • Close the shut off valve and observe the suction pressure. It should be above atmospheric, if not, open the service valve to obtain a pressure slightly above atmosphere.
  • Now close the shut off valve, back seat the service valves and remove the oil suction connections.

12.5 Commissioning

Any system or equipment installed in industry, hotels, hospitals, residential complexes or commercial complexes has to be commissioned properly to ensure it performs satisfactory to its rated capacity.

Commissioning a system is defined by ASHRAE as the process of checking, adjusting and testing all the equipment to produce its design objectives.

Commissioning in an HVAC system includes testing, adjusting and balancing of:

  • The Refrigeration system
  • The water circulation system.
  • Air distribution system
  • Control and electrical system

12.5.1 Pre-requisites of commissioning

Before handing over the system for commissioning to the commissioning engineer, the design engineer must provide the following details.

  • All relevant as built drawings such as the P & I Diagram ( Piping and instrumentation diagram), GATP ( General Arrangement and terminal points), a complete electrical diagram ( power and control )
  • A complete description of the intended mode of operation of the entire system and the equipment thereof.
  • Operation and maintenance instructions of the manufacturer of all bought equipment.
  • Design data of all flows, pressures, temperatures in ducts and terminals.
  • Design specifications of each equipment of the system.
  • Any other data on the operating parameters, as felt necessary by the design Engineer or the commissioning engineer.

12.5.2 Air Volume measurements

Measurement of air volume is one of the most important aspects during commissioning to verify the designed capacity of the system.

Various methods and techniques used by engineers all over the world to measure the airflow in ducts and terminal devices are different and there is no single universally accepted standard. It is always difficult to measure air velocity accurately in the field.

In this section let us study the general practices followed for measuring the air volume at different point in the system.

Air Terminals

Air terminal manufacturers generally prescribe the use of special velocity pressure probe or the deflecting vane anemometer to measure the air velocity. This is then multiplied with the cross section area of the terminal to get the airflow rate. The accuracy largely depends how the measurement is done.

In order to overcome the problems encountered during measuring, it is recommended to use one or two alternative methods such as:

  • A flow measuring cone is used of sufficient size to cover the entire air outlets. The measuring cone is especially useful where a large number of grills of different sizes are to be measured. The cone dimensions used are 316 × 316 mm2 or a 357 mm diameter round exit. The air quantity can be easily obtained by dividing the measured velocity by 10, as the cross section area of this cone is 0.1 m2. Before using the cone, it should be calibrated against measured Pitot tube readings to establish correction factors for the cone with the anemometer.
  • An alternative method is to measure the air quantity in a duct feeding two or more terminals and then adjusting the dampers on the outlets in order to maintain the same velocities.

Duct Flows

Usually flow rates measured in the ducts are more accurate and reliable than those measured at the terminals. Generally for testing and balancing of the air handling systems the airflow measured in the ducts are considered.

The most preferred method to measure the volumetric flow rates in the ducts is by taking the average of readings measured with the help of the Pitot tube. Care should be taken that the measuring point of the Pitot tube has a distance of at least one meter as straight length of the duct.

In round ducts, at least 20 readings should be taken at two diameters as centers of equal area. For rectangular ducts, the number of spaces for taking the readings should not be less than 16 and not more than 64.

The following method can be used to determine the velocity in the ducts after obtaining the readings with the above methods.

The velocity at each point can be calculated from the formula:

V = √ P / 0.5 ρ

Where:

V is the velocity in m/sec.

P is the velocity pressure as measured with the Pitot tube in Pascal

ρ is the density of air in kg/cm2

The average velocity can be calculated by adding the individual velocities and dividing the total with the number of readings.

This average velocity when multiplied with the cross section area of the duct gives air flow rate through the respective duct.

Mixture plenums

It is the area where the outside air is taken in the system. It is important to measure this air intake as it has a significant effect on the economic running of the plant.

Since the total air here comprises of the return air and the fresh air intake it become difficult to accurately determine the quantity of fresh air intake. General method adopted in by considering the temperature of the mixture. The temperature signifies the proportions of the two air streams.

The fresh air percentage can be calculated as follows:

% Outdoor air = {( t mix - tr ) / ( to - tr ) } * 100

Where:

t mix is the temperature of the air mixture.

tr is the temperature of the return air

to is the temperature of outside air

12.5.3 Balancing Procedures

Testing and balancing the air distribution system is also a very important activity to be performed during commissioning. The instruments required for an air balancing are:

  • An inclined manometer or an electronic one.
  • A combination manometer (inclined and vertical with a range of 0 to 2500 Pascal)
  • Pitot tubes ( 500 mm long or 1200 mm long ).
  • A tachometer
  • Clamp-on current tester
  • A deflecting vane anemometer with different measuring heads.
  • A rotating vane anemometer
  • Thermometers

Preliminary procedure

The following steps should be followed before the system is put into operation.

  • Obtain all the relevant drawings, specifications and control circuit diagrams.
  • Obtain all as built drawings of the entire system.
  • Check all the layout and installed equipment matches with the designed specifications and drawings.
  • Note down all the variations observed compared to the design specifications.
  • Check whether the filters, air dampers , grill are positioned correctly.
  • Check completeness of the control system.
  • Obtain the recommended test procedures of all bought out equipments of the system.
  • Identify the test points for carrying out any measurements during commissioning.

System Check

In order to put the system into operation the following checks need to be made on the installed equipments after you have completed with the preliminary procedure.

  • Check for leakages on all joints, in the casings, around the coils and filter frames. All leakages should be sealed
  • Ensure all the vibration dampeners are installed correctly and adjust if required.
  • Ensure all the flexible connections are fastened properly.
  • Check all the drains and traps in the system.
  • Carry out the settings on the controllers as per the procedure stated in their respective instructions.
  • Set the overload relays of all the motors at a slightly lower value than the full load current of the motors.
  • With humidifiers in circuit, check for proper water supply.
  • Position all the valves in the chilled water piping as desired.
  • Check alignment of all the pumps with their drives. For belt driven motors, check for tightness of the belt.
  • Carry out the lubrications as needed for all the equipment according to the requirement of the respective manufacturers.

Start Up

After we have completed the two sets of activities mentioned above, it can be assumed that it is now safe to start the equipment.

As air supply is the most critical without which most of the Air-conditioning equipments cannot be started, it is important that the fan is started first. Carry out the following activities for the start up of any rotating equipment like the fan.

  • Check the direction of rotation of the motors, and correct it if the motor rotates in the wrong direction.
  • Check the incoming voltage to the equipment.
  • Start the motor and check its starting current;
    For DOL motors – the starting current should not exceed 6 times the full load current.
    For start-delta starters – the starting current should not exceed 3 times the full load current.
  • Run the motor and measure the current drawn in all the three phases. It should not exceed 75 % of the full load current.
  • Check for any unusual noise or vibrations.
  • Check for working of all the safeties and interlocks by simulating the interlocks and safeties.

Air balancing

Air balancing is another important activity that needs to be carried out during commissioning an air-conditioning system. The associated air balance council has set a standard procedure for air balancing. The procedure is as follows:

  • Position all supply and return air dampers to the full open position.
  • Set all dampers in the air terminals to fully open positions.
  • Prepare the measurement point for the static pressure readings and the velocity pressure readings.
  • Measure air flow rates in all main and branch ducts. This can be done with a pitot tube traverse or any other accurate method.
  • Carry out the proportioning of air in the required quantities to various branches and main ducts.
  • Circulate air throughout the system to check whether the proportioning has been achieved as desired.
  • Using a pitot tube set all the main dampers by measuring the velocity pressure.
  • Measure the air volumes at each zone outlet and adjust if required.

Cooling, heating and humidification

After completing the air balancing and once we are sure that the required amount of air is being circulated through all the ducts and the equipments, heating and cooling can be tested.

This requires commissioning of major out sourced equipment such as compressors, refrigeration equipment and boilers. Since these are specialist’s work it is recommended that the manufacturers or suppliers or certified agents do the commissioning. However one must follow a set of procedures listed below to test the heating and cooling capacities of the system installed.

Cooling

  • Set the control set point to maximum cooling.
  • Check and record the average wet and dry bulb temperature entering the cooling coil.
  • Check and record the average wet and dry bulb temperature leaving the cooling coil.
  • Check the airflow rate over the coil.
  • Calculate the cooling capacity of the coil and compare with design capacity. ( We have already studied the calculations in the earlier chapters )

Heating

  • Set the control set point to maximum heating.
  • Check and record the average wet and dry bulb temperature entering the heating coil.
  • Check and record the average wet and dry bulb temperature leaving the heating coil.
  • Check the airflow rate over the coil.
  • Calculate the heating capacity of the coil and compare with design capacity. ( We have already studied the calculations in the earlier chapters )

Humidification

  • Set the control set point to maximum humidification.
  • Check and record the average wet and dry bulb temperature entering the humidifier.
  • Check and record the average wet and dry bulb temperature leaving the humidifier.
  • Check the airflow rate over the coil.
  • Calculate the humidifying capacity after obtaining the moisture content entering and leaving the humidifier from the psychrometric chart.

12.5.4 Commissioning Report

A commissioning report is not just a report that says “The system has been commissioned and handed over to the customer for regular use.”. It is all about the system, its equipment schedule, the operating parameters, the testing data, operating instructions, spares list, maintenance instructions. All these in a commissioning reports helps in the following ways:

  • The report remains as a master data reference through the life of the system.
  • It is useful for the customer’s maintenance department.
  • It is very useful data for a trouble-shooting engineer at a later date.

A few guidelines on what needs to be included in a Commissioning report have been laid down here:

  • The make, model and serial numbers of each of the equipment installed in the system.
  • The duty point for which the equipment was selected.
  • All name plate details of the equipment.
  • Design data on the air quantities in various zones.
  • Design data on the temperature, humidity and the variation allowed.
  • The amount of outdoor air to be introduced.
  • The amount of air to be exhausted.
  • A full description of the control system and how it is required to be operated.
  • Complete as built drawings of the system along with their technical specifications.
  • All technical details of electrical equipment.
  • Data recorded at the time of commissioning, like, air velocities measured at various points in the system.
  • Air pressure and temperature recorded at various points.
  • The amperages drawn by the drives.
  • Pressure losses across the filters, coils, dampers.
  • The sound level measured data of all moving equipments.
  • The total and static pressures measured at the fan suction and delivery.
  • Humidity measured during the trials

Operating instructions of the entire system.

  • Starting and stopping procedures in the form of a flow chart.
  • Recommended settings for all controls and instrumentation.
  • Normal operating pressures and temperatures.
  • Maximum allowable operating pressures and temperatures.
  • Complete maintenance instruction of each equipment installed that should include:
    • Frequency of maintenance
    • Operations to be carried out.
    • Parts to be replaced.
    • Troubleshooting tips in the form of FTA’s (Fault tree analysis).

12.6 Other service operations

12.6.1 Changing the compressor

The following process must be followed for removing the compressor from a system.

  • Close the suction service Valve.
  • Operate the compressor until a pressure of 0.125 bar is achieved.
  • Close the discharge service valve and loosen the gauge port plug to release the pressure in the head.
  • Remove the suction and discharge valves from the compressor.
  • Slide the motor forward and remove the belt. Unbolt the compressor and take off the flywheel.
  • Attach the flywheel to the new compressor and bolt it to the frame.
  • Connect the service valves.
  • Start the compressor and operate until air stops coming from the discharge valve port.

12.6.2 Replacing the air cooled condenser

The following steps should be followed before disconnecting the condenser from the system.

  • The entire charge must be removed when there is no valve between the condenser and the receiver.
  • When the refrigerant has been removed and the pressure balances the atmospheric pressure, the condenser can be disconnected.
  • Install and connect the new condenser.
  • Evacuate the system.
  • Charge the required amount of refrigerant and test for leaks.

The above outlined procedure can also be followed for a water-cooled condenser.

Note: If there is a valve in between the condenser and the receiver then this valve and the compressor discharge valve may be closed. A small amount of refrigerant left in the system is allowed to escape to the atmosphere. The removal of the entire charge from the system is not necessary.

12.6.3 Changing the Evaporator

The following procedure should be adopted for replacing the evaporator from the system when low side float valve with liquid and suction valves at the evaporator are used.

  • Attach a pressure gauge to the compressor service valves.
  • Close the liquid valve at the receiver and all other valves are left open.
  • Operate the compressor until the liquid line becomes cold and then warms up. Then close the liquid and suction valves at the evaporator.
  • If necessary operate the compressor again to balance the pressure in the suction line after the evaporator valves have been closed. Close the suction valve.
  • Disconnect the lines at the evaporator and plug these.
  • Then remove the faulty evaporator, put the new one in place, and then connect the liquid and the suction lines.
  • Open the receiver-liquid valve slightly and purge the air from the line by cracking the flare nut at the evaporator.
  • Open the two valves at the evaporator and purge the suction line by cracking the flare nut at the compressor.
  • Open all valves and keep the system in operation.

The following procedure should be adopted for replacing the evaporator from the system when there are no valves at the evaporator:

  • Attach gauges to the suction and the discharge service valves at the compressor.
  • Close the receiver liquid valve going to evaporator and operate the compressor until the entire refrigerant has been removed from the evaporator. This can be determined by observing that the suction pressure does not rise above atmospheric after the unit has been shut off for about ten minutes.
  • Open the receiver valve and allow the refrigerant to flow from the lines and evaporator until the suction pressure reads 0.125 bar gauge.
  • Close the suction valve and loosen the flare nut at the compressor to relieve the vapour remaining in the lines.
  • Install the new evaporator and connect the lines.
  • Open the receiver valve and purge the air by cracking the flare nut at the compressor suction valve.
  • Open the suction valve wide, start the compressor and then remove the gauges.

12.6.4 Maintaining a Household Refrigerator

The following points should be remembered for trouble-free operation of the refrigerator:

  • The refrigerator must be properly leveled. It is essential for automatic closing of door, which is done by a magnetic switch mounted in the refrigerator cabinet. The proper leveling should be achieved with the use of 4-screws provided with the refrigerator. Open the door by hand and leave free; if it closes properly without any gap, it is an indication of proper leveling.
  • Refrigerator must be operated on 230 V and 50Hz supply through a voltage stabilizer.
  • Refrigerator should be connected to the electrical point, which is nearest to the refrigerator.
  • It should not be exposed to direct sun rays as it adds to the load on the refrigerator.
  • It should be kept a minimum of 25cm away from the wall for proper air circulation over the condenser.
  • It should be operated without load for two hours before loading it.
  • The condenser should be cleaned from its outer surface periodically otherwise, it increases the load on the refrigerator.
  • If you need urgent ice, turn the control knob of thermostat to position five or 6 and turn the chill tray deflector to the out position. Don’t forget to change these to their normal positions after the need is over.
  • Defrosting of the freezer should be done periodically (once in two days minimum), otherwise it increases load on the refrigerator as frost adds thermal resistance for heat flow.
  • Never use harsh cleansers or striking or chipping for removing ice from the freezer coils as it may result in leakage of refrigerant gas.
  • Deflector of the chill tray should be kept out during defrosting to collect the water.
  • Moist food should be placed in wrapped polythene bags to avoid dehydration and giving out its smell to other foods.
  • The bottom most compartment is provided with a tray, which should be used for storing green vegetables and fruits.
  • Do not start and stop the refrigerator immediately. There must be half an hour gap between start and stop.
  • Hot fluids should not be stored in the refrigerants otherwise hot vapours may damage the refrigerator cabinet.
  • The refrigerator should be properly earthed to safeguard against electric shock. Improper earthing may result in a fatal accident.

12.7 Operational Activities

The activities are:

  • Air purging
  • Blow down of refrigerant
  • Maintenance of operating log

These should be carried out as per the Operational Activity Schedule.

12.7.1 Air Purging

Air purging is the activity by which non-condensable gases are removed from the machine. This is done by operating the vacuum pump and opening the manual air purge valves. After purging is completed, the valves are closed and the vacuum pump is stopped.

The air purging system consists of:

  • Purge unit
    Non condensable gases and water vapour are drawn from the absorber into the purge unit by a venturi action using absorbent bled off the absorbent pump discharge. The gases are bubbled through LiBr collected at the bottom of the storage tank to remove water vapour. The non-condensable gases collect to a maximum pressure of 50mmHg.
  • Valve assembly
    This consists of an arrangement of three diaphragm valves that enable the operator to connect the vacuum manometer to different parts of the machine by closing and opening the valves.
  • Manometer
    A mercury-in-glass vacuum manometer capable of reading vacuum from 0 mmHg absolute to 100 mmHg absolute.
  • Service valves
    One service valve provides access for the manometer to the machine and the other provides access for maintenance procedures like N2 charging and sampling.

Purge pump

A double stage oil sealed vacuum pump for evacuating the machine and carrying out maintenance procedures.

Measuring Vacuum

Three different vacuum measurements are carried out. The positions of the manual purge valves for theses measurements are described in the table below.

Table 12.1
Measuring Vacuum
  Position of Manual purge valves
Vacuum reading Valve no. 1 Valve no. 2 Valve no. 3
Ultimate (No load) Vacuum Open Closed Closed
Pressure in the storage tank Closed Open Closed
Pressure in the shell Closed Closed Open

Reading the manometer

The manometer consists of a glass tube fitted into a casing with a graduated scale. One end of the glass tube is sealed and the other end is connected to the vessel whose degree of vacuum is to be measured. Mercury fills the glass tube from the sealed end to the bottom of the graduated scale.

When the manometer is connected to a vessel at a vacuum of 100 mmHg or greater, the mercury drops in the sealed leg of the glass tube. The degree of vacuum is the difference in the level of mercury in the two legs. This is read from the graduated scale.

The level of mercury in the sealed leg of the manometer should always be higher than the other leg. Otherwise, the reading is wrong and the manometer has to be repaired.

When the manometer is connected to the atmosphere the mercury should rise to the top of the sealed leg. If there is a gap between the mercury and the top of the sealed leg of the manometer, it is faulty and should be repaired.

Kinds of air purging

The two types of air purging are:

  • Purging from the storage tank, and
  • Purging from the shell.

The procedure followed for the two types of purging are the same except for the position of the manual purge valves (shown in the earlier table).

Air purging procedure

  • Power ON the machine panel if it is off.
  • Start the purge pump.
  • Check that the purge pump is running well.
  • Open Manual purge valve no.1.
  • Check that the ultimate (no load) vacuum developed by the vacuum pump is 4 mmHg or less.
  • Open Manual purge valve No.2 – for purging storage tank and Manual purge valve No.3 – for purging from shell.
  • Continue operation of purge pump until purging is completed.
  • Close Manual purge valve No.2 (When purging storage tank) and Manual purge valve No.3 (When purging from shell).
  • Continue operation of purge pump for 30 minutes to remove water vapour from pump oil.
  • Close Manual purge Valve No.1.
  • Stop the purge pump.

Precautions

  • Check that the ultimate vacuum of the purge pump is 4mmHg or less before air purging.
  • Carry out air purging at least once a week. Purging may be required at more frequent intervals.
  • Always keep the gas ballast valve open during purging to prevent water from contaminating the purge pump oil.
  • Carry out purging from storage tank before the storage tank pressure reaches 50mmHg.

12.7.2 Blow down Of Refrigerant

During operation, a small amount of absorbent disperses in the refrigerant and accumulates over time, resulting in a reduction in cooling capacity. Blowdown is carried out to purify the refrigerant. Contaminated refrigerant collected in the refrigerant pan is transferred to the absorber sump. Pure, fresh refrigerant collects in the refrigerant pan.

Procedure

  • Put the refrigerant pump switch into “auto” mode if it is not so already.
  • Confirm that the refrigerant pump is running and refrigerant is visible in the slight glass.
  • Open the refrigerant blow down valve completely. Refrigerant starts being pumped to the absorber sump.
  • Wait until the refrigerant pump stops due to low level signal from the refrigerant level relay (about 15min.), and close the refrigerant blow down valve completely.
  • Refrigerant will start building up and the refrigerant pump will start when the refrigerant level reaches the center of the evaporator sight glass (About 20 min.).
  • Repeat the procedure two or three times for complete purification of refrigerant.

Precautions

  • Outlet chilled water temperature goes up during the blow down procedure and will go down when the refrigerant starts. Blow down operation is carried out when chilling requirements are not critical.
  • Carry out refrigerant blow down once a week.

12.8 Do’s And Don’ts

Do’s

  • Blow down refrigerant once a week.
  • Purge non condensable gases from the purge unit, it the storage tank vacuum is more than 15 mm Hg.
  • Close the main steam isolation valve after switching off the machine.
  • Maintain inlet steam pressure constant at rated value.
  • Check the presence of Octyl alcohol and corrosion inhibitor periodically.
  • Analyze chilled and hot/cooling water every three months and maintain the quality of chilled and hot/cooling water.
  • Ensure that hot/cooling water flow stops one minute after the stopping the machine.
  • Change the vacuum pump oil, if it is contaminated (white misty colour).
  • Charge nitrogen gas upto 0.3kg/cm2 (g) in the system for long shutdown of the machine (i.e. more than a month). If the shutdown is for a shorter period, purge the machine for 20 minutes, twice a week.
  • Ensure that the rated electric supply (3 phase) is available.
  • Always start the chilled water pump first and then the hot / Cooling water pump. Ensure that the rated chilled water and hot / cooling water flows are passing through the machine. Ensure that the steam is available at rated inlet condition.

Don’ts

  • Don’t open any of the valves of the purge unit without running the vacuum pump.
  • Don’t disturb the setting of any instrument or safety provided on the machine.
  • Don’t put the refrigerant pump switch or the steam control valve operational switch in MANUAL , during normal operation of the machine.
  • Don’t increase the overload setting of any motor, even if the overload alarm does not trip.
  • Do not run the vacuum pump continuously.
  • Do not let the temperature of hot /cooling water fall below 22 0C. Maintain it by stopping the cooling tower fan or by bypassing the cooling water into the tower basin by installing a 3 way valve.
  • Don’t decrease the chilled water flow below the specified value.
  • Don’t remove the bulb of the antifreezes thermostat from the thermo well provided. Ensure that the thermo well is always full of oil.
  • Don’t run the hot / cooling water pump if the chilled water pump is not running.
  • Do not by pass or change settings of any safety devices or instruments.

12.9 Maintenance

12.9.1 General

The maintenance engineer is responsible for operating and maintaining the air conditioning equipments so that it provides the intended services at the lowest downtime and cost.

Preventive maintenance

A thorough preventive maintenance program should be followed so that the equipment undergoes the least downtime. It also extends the life of the equipment. The fix it when it fails approach will only lead to longer downtimes and hence increase the overall operating cost of air conditioning equipment. A proper checklist should be developed for all equipments showing each inspection activity and its frequency. The manufacturer’s operating and maintenance instructions should be used in developing these checklists. A sample checklist for a centrifugal machine is illustrated below.

Table 12.2
Preventive Maintenance-Inspection Checklist

Operating log

Record readings in accordance with the operating log at frequent intervals. These aid the operator to recognize both normal and abnormal machine conditions and also aids in planning the preventive maintenance schedule and in diagnosing machine problems. A typical operating log sheet is shown below.

Table 12.3
Operating Log Sheet
Re. Items Unit Date : / /
1 Time :      
2 Ambient temperature. °C      
3 Chilled water flow rate m3/hr      
4 Chilled water inlet & outlet temperature. °C ( / ) / / /
5 Chilled water inlet & outlet pressure kg/cm3g / / /
6 Hot/Cooling water flow rate m3/hr      
7 Hot/Cooling water inlet and outlet temperature. °C ( / ) / / /
8 Hot/Cooling water inlet and outlet pressure kg/cm3g / / /
9 Generator temperature. °C      
10 Generator pressure mmHg      
11 Absorbent pump pressure kg/cm3g      
12 Generator fluid level O      
13 Absorber fluid level O      
14 Evaporator fluid level O      
15 Supply steam pressure kg/cm3g      
16 Steam flow rate kg/hr      
17 Steam control valve opening %      
18 Pressure in storage tank mmHg      
19 Pressure in shell mmHg      
20 Attained vacuum of purge pump mmHg      
21 Concentration of diluted absorbent %      
22 Concentration of intermediate absorbent %      
23 Concentration of concentrated absorbent %      
24 Concentration of refrigerant %      

12.9.2 Absorption Machine

Leak tightness

In vapor absorption machines, as we have studied in the earlier chapters, the refrigeration effect is produced by maintaining a vacuum in the evaporator section. Because of the high vacuum that exists in the absorber-evaporator section of the machine, it is very important that a high degree of leak tightness is maintained. Even small leaks will allow air and other non-condensable gases to enter the machine and disrupt the cycle. This disruptive effect can be determined by taking samples of lithium bromide solution and water to measure the specific gravity of each. This should be compared with the standard conditions to decide whether the solution should be discarded.

Pumps

In a vapor absorption machine, pumps are used to circulate lithium bromide and refrigerant solution within the machine. These pumps are provided with mechanical seals, which should be replaced once in two years. The motors that drive these pumps should be suitably lubricated every year.

Service valves

The diaphragms of the service valves located on the machine should be replaced every two years.

Safeties

Various controls and safeties such as

  • Refrigerant low temperature cut off
  • Chilled water low temperature cut off
  • Chilled water flow switch
  • Condenser water flow switch should be checked for proper operation once in three months.

Leak test

While leak testing an absorption machine, the machine vacuum should be broken with nitrogen. The machine should be pressurized by using a combination of nitrogen and the refrigerant. Never use air.

12.9.3 Coils

Cooling coils and the heating coils are made with prime or extended surface tubing. Maintenance in either case is essentially the same, and involves two features – tightness and cleanliness.

Tightness

In the case of coils handling refrigerant for direct cooling, a tightness check should include inspection of all joints in the piping connections to the coils. All leaks should be repaired promptly.

Coils handling water should be observed periodically for leakage and should be given careful attention to protect damages due to freezing during severe weather. Positive protection against damage to coils when not in use is done by using anti freeze solutions in the coil or draining of the coil. Many factors contribute to make complete drainage of the coil difficult. In order to completely remove water from the coils adequate air blowing should be done. About 150 cfm of air at a pressure of 0.2 kg/cm2 is sufficient enough to drain a 3 m long coil completely.

Cleaning

The face of any coils over which flows an air stream containing lint, fly, rug nap etc will require frequent cleaning of the entering side. All such coils should be checked weekly until a suitable cleaning schedule is established.

Coil surfaces from which water evaporates get coated with chemicals and scales left behind by the water during evaporation. Evaporative condenser coils are particularly subjected to such problems. These surfaces can be cleaned by the process of descaling in which the coil needs to be kept in an acidic solution to dissolve the coated scales. However, the concentration of the acidic solution should not be high enough to damage the coil tubes.

The inside surfaces of the coils that carry refrigerant generally do not require cleaning unless these are operated under abnormal conditions.

12.9.4 Condensers

Air-cooled condenser

The maintenance procedures should include

  • 1. Inspection
    • The condenser must be inspected once in a year.
    • The coil should be inspected for physical damage and any air-flow restrictions.
    • Examine fan for bent blades and their alignment.
    • Belt drives should be inspected for wear and proper belt tension.
    • Motor power and control connections must be checked for tight connections.
  • 2. Cleaning
    The condenser must be cleaned at the beginning of each cooling season and at regular intervals during the season. Air borne dirt may be removed from the air inlet screen, coil face and fan by brushing, vacuum cleaning or by spraying low-pressure water. For non-greasy and dry coils it is preferred to blow compressed air from the outlet face and then the vacuum inlet face.

Evaporative condensers

The following maintenance procedures can reduce repair costs and improve efficiency.

  • Cleaning
    Clean equipment when the equipment is shut down for the season. Year round systems should be cleaned annually. Airborne dirt should be removed form the coil surface by washing down with a high velocity jet of water or steam. Caution: If steam is used the coil should be pumped down first. Dirt form the coil surface can also be removed simply by a wire brush. As we have seen above, the evaporative condensers are subjected to chemical deposits. It is preferred to treat the water used here to prevent such deposits. Inspect the coil periodically to detect the presence of scales over the surface. Evaporative condenser surfaces are also prone to rust which should be immediately cleaned with a wire brush and painted with anti corrosion paints.
  • Fan section
    • The current drawn by the fan motor should be checked periodically.
    • Check the condition of the fan belts twice a year.
    • The belts should be replaced only with matched sets.
    • Check Fan alignment and bearings periodically.
  • If the pump motor has grease fittings or cups check bearings twice a year and grease if required.
  • Freezing precautions
    Evaporative condensers should not be operated with a wet coil when the air circulated is below zero deg. C. If the unit is subjected to freezing temperatures take precaution to prevent damage to the pump and piping. When the unit is shut down, drain all water from the pump and the coil. Blow out piping and coil and add anti freeze solution to ensure complete protection.

12.9.5 Cooling towers

A forced or induced draft cooling tower is an assembly of various functional components, which include a fan, spray nozzles and motor drive with starter. Maintenance of these parts is essential and the following procedures are adopted.

Exposure

Cooling towers are outdoor installation and use outside air. Necessary protection should be provided to the electrical components such as the motor, starter and disconnect drive. The structural part also should be protected suitably to avoid any corrosion due to the oxygen present in the air. The structure should be painted with anti rust paint.

Fans

The fans should rotate freely. Gear driven fans should be checked for alignment annually. Oil level in the gearbox should be checked weekly. The oil should be replaced annually or after 3000 hours of operation. Belted drives should be checked for belt tension and alignment. Belts should not be tight enough to impose undue load on the bearings and should be tight enough to avoid slipping.

Water distribution

Water distribution should be checked periodically and kept uniform. The spray nozzles should be cleaned regularly. The nozzles should be checked for wear and tear and replaced if necessary as a worn out nozzle may affect the spray and the cooling efficiency.

Eliminators

Eliminators should be kept free of algae growth. Metal eliminators should be painted annually. Algae growth should be cleaned in all the parts of tower and water treatment put in use to prevent regrowth.

Cleaning

The water basin should be drained and hosed out weekly. The strainer should be checked and cleaned periodically.

12.9.6 Fans

Fans in an air conditioning system are designed to give a rated quantity of air at rated pressure. Any increase in the operating pressure reduces the flow produced by the fan due to its centrifugal action. The fan would handle more than the rated air if the pressure decreases. This also draws excess power. Hence it is important to operate the fan on its rated conditions. The following maintenance procedures should be thus adapted to avoid any undue load on the fan.

Cleaning

In many locations, fine dust particles find their way into the fan and stick to the blade surfaces. This should never be allowed to build up as it reduces the efficiency. This also disturbs the balancing of the fan creating excess noise. The fan impeller should be checked and cleaned periodically.

Bearings

Generally larger housed fans are provided with pillow block, sleeve or ball bearings. These bearings are normally self-aligning type bearings. The sleeve bearings are ring oiled. Ball bearings are grease packed and are provided with greasing arrangement. Good bearings of any type, properly selected and installed will give long satisfactory service if the following precautions are taken.

Do not lubricate too little or too much.

  • Do not use wrong lubricant
  • Prevent bearings from dust, moisture and corrosive atmosphere.
  • Avoid heat radiation or conduction to the bearings from adjacent surfaces.
  • Do not load the bearings excessively due to improper V-belt tension, misalignment, impeller imbalance, vibration transmitted from another source.
  • Do not operate the fan at a higher speed than it is designed for.

A bearing that feels hot is not necessarily too hot to operate. Bearings on high speed fans operate safely at as high as 23oC above the ambient temperature. Use the following as an approximate guide for the maximum operating temperature of the bearings

  • Ball or roller bearings 75oC
  • Ring oiled sleeve bearings 65oC
  • Water-cooled sleeve bearings 45oC

Lubrication

Sleeve bearings in normal room temperature use high-grade automobile engine oil, SAE 40. Detergent oils should not be used to lubricate the bearings. When handling air at temperature above 65oC special considerations must be given for the election of oil. In ring oiled sleeve bearings the oil should be drained and replaced after 2000 hours of operation. If the drained oil is dirty, the bearing and the well should be flushed with machine oil before adding new oil.

Grease lubricated bearings should be filled with a good grade of soda base grease at fixed intervals. Under normal circumstances grease should be added after every 1500 hours of operation.

12.10 Economics

Economics is the first consideration in designing and using the air-conditioning plant. This is more important in industrial air-conditioning than comfort conditioning because the profit is the primary consideration in industry. The designer should not overlook the importance of complete economic study.

In considering the cost of an air-conditioning plant, the factors encountered are:

  • Annual fixed charges
  • Annual operating charges.

12.10.1 Annual Fixed Charges

In considering the annual fixed charges, depreciation should be taken into account. Depreciation is based on deterioration and obsolescence. The deterioration period is widely variable and depends upon proper operation and adequate maintenance. Adequate maintenance decreases the deterioration but also adds charges for maintenance.

Obsolescence depends upon the future developments, which are very difficult to assess. It depends upon the developed system which has lower annual charges or newer systems which are capable of earning greater income through better results.

The total annual fixed charges are the sum of annual interest charge on the capital investments plus annual rate of depreciation.

12.10.2 Annual Operating Cost

This includes:

  • Energy cost
  • Labor cost
  • Maintenance cost

The assessment of labor and maintenance costs is very difficult which depends only on actual calculations. It also differs from place to place as per the type of installation.

The following formulae suggested by different scientists for calculating energy cost are listed below:

C = (24 RQa P / Qt)/( D / Tm – 21)

Where:

C = cost of operating the unit in US Dollars.

R = power cost in USD/kW hr.

Qa = Average hourly cooling load calculated on 24 hours basis (kJ/hr).

Qt = Total cooling capacity of equipment (kJ/hr).

P = Average power input to unit in kW.

D = Number of summer degree-days based on 21°C outside dry-bulb temperature.

Tm = Summer outside design DBT – ½ the average summer daily range.

For calculating the operation cost of the refrigeration plant the following formulae are used:

Electrical cost

This is given by following formula:

C = ( 0.746 PTHe R / η)

Where:

C = Electrical cost during the considered period in US dollars.

P = power consumption rate in kW/Ton.

T = Refrigeration at maximum design load in tons.

He = Equivalent full load refrigeration operation in hours.

R = Power cost in USD/kWhr.

η = Motor efficiency.

Water costs

C = 0.06QwTHeRw

Where:

Qw = Water consumption rate in liters/ton

Rw = Water cost in USD/1000 liters.

T and He have the same units as mentioned above.

Steam cost

The cost of steam when used in an absorption refrigeration system is given by:

C = WTHeR / 1000

Where:

W = Steam used in absorption system in Kg/Ton/hr.

R = Steam cost in US dollars per 1000 kg.

Installation cost

The installation cost of air-conditioning includes the cost of the following equipment:

  • Heat producing equipment as boilers, burners and auxiliaries.
  • Air-handling equipment such as fans, blowers, filters, controls, air heaters and air-conditioners.
  • Air distribution system such as ducts, grills and registers.
  • Refrigeration equipment such as piping and pumps, compressors, condenser, evaporator and receiver etc.
  • Water-conserving equipment such as cooling tower, evaporator, condensers and auxiliaries.

The cost of the air-conditioning system varies widely according to the type of system and equipment.


13


Fault finding and troubleshooting

13.1 Objectives

At the conclusion of this chapter, the student should be able to

  • Identify various types of faults occurring in an air-conditioning system.
  • Understand the importance of troubleshooting.
  • Develop basic troubleshooting skills.

13.2 Introduction

We all know the fact that “Prevention is better than cure”. We have in the previous chapters studied how we can take care of the HVAC system in order to prevent a breakdown.

The reliability of modern air-conditioning systems is quite high. However, one cannot rule out the possibility of a sudden breakdown in the system. Inspite of the design and commissioning engineers successfully completing their part of activities in giving us a trouble free and efficient system, it can breakdown or cause a sudden failure.

The maintenance engineers should be geared up for such emergencies. This chapter is aimed at giving general guidance to the maintenance engineers on how to troubleshoot quickly to avoid higher downtime.

Troubleshooting is all about quickly diagnosing the cause of the malfunction and correcting the situation at the quickest possible time. One of the most important skills to be learned by a technician is the art of faultfinding and trouble-shooting.

13.3 Faults

When equipment on its own does not perform as desired to, it is said to have developed some fault. In most of the cases, faults develop due to the influence of an external factor or due to increased wear and tear of a particular component.

However, faults can be segregated into the following categories.

  • Improper Adjustments and settings
  • Poor design and installation
  • Equipment failure
  • Limitations in operation

13.3.1 Improper Adjustment and settings

In many cases it has been found that, equipment does not perform properly due to improper settings or adjustment done at the time of commissioning or during maintenance. This can also cause great dissatisfaction to the occupant. During such time, it is more irritating because the performance of the equipment gradually deteriorates testing the patience of the occupant.

A few typical faults developed due to improper settings or adjustments are:

  • The cooling or heating capacity starts to decrease.
  • The cooling and heating sometimes can be uneven.
  • The humidity may be high but the temperature is normal.
  • The temperature may vary but the humidity may remain constant.
  • The noise level would rise.
  • The operating costs would increase.

A simple example of how a bad adjustment can increase the operating cost is, if the fresh air damper is not set properly. Ideally it would be open such that only a sufficient amount of fresh air is taken into the system. However if it is open too far it allows excessive fresh air into the system. As a result the fans would be excessively loaded rendering higher power bills. This condition would only be detected over a period of time when the heating or cooling becomes insufficient. In the meantime, many hours of inefficient operation would have occurred resulting in higher cost of operation than required.

13.3.2 Poor Design and Installation

Let us now look at how a poor design or a poor installation can affect the performance of a system.

Theoretically the designer should ensure each and every part is selected and specified correctly and with complete details. He should also take part in the supervision of the installation to ensure all his ideas have been implemented.

However, in actual practice, the designer passes on a good deal of responsibility to the installation engineer assuming that the installation engineer is experienced and knows the practical nitty-gritty’s of the system such as, routing the ducting, locating the equipment etc.

This can result in certain errors and no field correction at a latter date will add to the unit capacity if it was wrongly sized during design.

13.3.3 Equipment Failure

Any equipment having moving parts can fail abruptly. However, an equipment failure is the easiest fault for the maintenance technician to detect and rectify. Once the cause of failure has been detected it becomes quite easy to replace the parts and put the equipment back into service.

It is also important that, once the equipment has been put back into operation, the technician must further investigate the cause of the failure of the particular part and take remedial action to prevent re-occurrence of the part failure.

We can attribute a few causes for such part failures in equipment.

  • A manufacturing defect.
  • Overloading of the part.
  • Improper material used in the manufacture of the part.
  • Wear and tear of the part.
  • Wrong installation.
  • Ingress of moisture/water or dirt into the system.

13.3.4 Limitations in Operation

Every system is designed to operate within a certain limit, which is beyond the normal operating conditions. However, at many instances, the heating or cooling load increases and the user does not really tend to look at this and overloads the existing air-conditioning system. This is what we mean by limitations of operation. These could be of the following nature.

  • Increases heating or cooling load
    After the system has been installed and operated for quite some time, the owner might add a room to be conditioned and expect the same system to give the air-conditioning effect to the new room added along with the existing load.
  • Unit operating beyond design conditions.
    Every plant as we have seen above is designed for certain maximum and minimum ambient conditions. If ambient conditions are above these limits, the unit is of insufficient capacity. For example; if the ambient temperature is higher than that considered during design, the room temperature may not be maintained or may take time.
  • Change of application
    If the application for which the equipment has been selected is changed, the equipment may not perform as per the design parameters. For example, equipment selected to provide air-conditioning to a computer room might not perform effectively if it has been installed in a storeroom or a cool room as the load conditions for both applications would not be same.

13.4 Troubleshooting

Troubleshooting is the skill of applying knowledge, experience and logic in order to solve a problem.

In order to develop troubleshooting skills, a maintenance technician should have the basic knowledge of how a plant operates and the function of each piece of equipment in the plant. Mostly, troubleshooting is done by the trial and error method. However, there have been certain tools devised recently in order to facilitate troubleshooting such as:

  • DOE (Design of Experiments)
  • FTA (Fault tree analysis)
  • Cause and Effect diagrams

We shall briefly study what these tools are, later in this session.

13.4.1 Troubleshooting guide to electrical faults

Generally other than equipment failure, most of the faults generated in a system are due to electrical faults and hence let us study a few guidelines of troubleshooting electrical faults.

An electrical system actually can be segregated into two, viz. Power cabling and control cabling.

Power system provides electricity/power to run the drives in the system. This can be 220 VAC, single phase or 380 to 415 VAC, 3-phase.

Control system is necessarily single phase and can be of 110 VAC, 220 VAC or 24 volts. It provides electricity to operate the contactors, relays, solenoid valves, control valve actuators etc.

Any electrical fault generated in a system renders the entire system to trip as the electrical safeties are generally interlocked with the operation of the critical equipments. It is very important that the electrical circuit diagram of individual equipment or the entire system should be kept at a location, which is easily accessible to the electrician or should be held by the electrician.

Finding Electrical Faults

Check power supply

For a 3-phase power circuit, the incoming voltage across the phases should be checked. Generally the 3-phse voltages should lie within the voltage limits specified by the manufacturer, say 380 to 415 volts. Voltage lower than these limits may cause a drive to draw excessive current and trip the drive.

For single-phase circuit, voltage should be checked across the phase and neutral. This should be 220 VAC + 10 %. A variation in the single voltage generally is related to the variation on the 3-phase voltage. A lesser single-phase voltage may cause a solenoid coil or a contactor coil to chatter.

Once a fault has been detected, it becomes very easy to rectify it. Say for example; if there is a substantial drop in the voltage between the supply and the unit, the cable needs to be changed to one size higher.

Check fuses and circuit breakers

A fuse or a circuit breaker is a necessity in all-electrical circuits, which protects the system from over current. If there is an excess current drawn in the system, the fuse wire melts and disconnects the circuit from the main supply. The fuse can only be replaced when it blows whereas a circuit break would trip and can be reset.

After checking the voltages, if it is found that there is no voltage at any of the points in the circuit, the fuse is the first thing to be checked. The most common short cut method applied by technicians, is to use any available wire strand as a replacement fuse. This is highly risky, as the current carrying capacity of the wire strand is not known and in all probability it would withstand the high current drawn and damage the equipment.

Hence it is recommended that, a fuse has to be replaced with the same-size fuse only.

  • Check earthing
    All equipment necessarily should be ground or earthed. Earthing provides a least resistance path for the current to travel in case of any hazard or leakage of current. The maximum resistance to the ground is 0.1 ohm at 6 volts.
  • Check overload relays
    An overload relay is a device installed in the single-phase control circuit, which breaks the control circuit when there is high current drawn in the circuit.

Overload relays can be of magnetic or thermal type. All the overload relays need certain time to reset. This time is generally in seconds. When an electrical fault occurs in a control system, it is important to check at what amperage the overload relay has been set. This should never be set more than the full load current of the motor that is specified on the motor nameplate. It should be set to 90% of the full load current. Say for example the full load current of a motor is specified as 22 amps, the overload relay should be set at 19.8 amps.

  • Check insulation resistance
    All wires carrying voltage must be properly insulated or protected from touching the equipment. In the event of an electrical fault it is also important to check for the insulation resistance. This should be a minimum of 0.5 Megohm at 500 Volts.
  • Check all connections
    In an electrical circuit, any loose wires not terminated tightly at the terminals points can also lead to a fault. This can also generate sparks, which are dangerous. Hence it is very important that the connections are tightened regularly as a maintenance practice.

Above all this, it is very important to note here that:

  • ONLY A COMPETENT PERSON SHOULD CARRY OUT FAULT FINDING ON ELECTRICAL CIRCUITS.
  • BEFORE CARRYING OUT FAULT FINDING ENSURE THAT THE MAIN CIRCUITS ARE SWITCHED OFF.
  • ALWAYS WORK WITH INSULATED TOOLS.

13.4.2 Troubleshooting Skills

Troubleshooting is the art of following a planned procedure to locate a malfunction instead of u sing a trial and error approach.

Troubleshooting skills cannot be developed by mere classroom training; however, classroom training can guide a technician on how to develop troubleshooting skills. A technician gains expertise in troubleshooting by handling more and more jobs on troubleshooting and continuously updating himself with the maintenance practices being followed.

A simple guideline on the basic steps to be followed in troubleshooting is given below.

  • Study the operation of the equipment.
  • Compare the present observed parameters with those recorded at the time of commissioning.
  • List down the malfunctions observed.
  • Look out for any changes done on the equipment, for example, any part replaced or any settings changed.
  • List down all possible causes and their effects.
  • Eliminate each probable cause one by one and observe the malfunction to get to the root cause.
  • Once the root cause is identified, take necessary action to rectify.
  • Check if the problem is eliminated.

As we have seen above in recent years certain troubleshooting tools have been devised to facilitate troubleshooting. A few of them are:

  • DOE (Design of Experiments)
    This tool is a systematic approach towards solving a problem. You can call it systematic trial and error. This tool can be used to troubleshoot in plants where there is more than one piece of similar equipment and say, one of the pieces is malfunctioning.

The simple steps to be followed are:

  • List down the components, which can affect the malfunction of equipment.
  • Select another piece of equipment of the same kind that is functioning correctly
  • Replace one of the identified parts from the working machine on to the non-working machine. See the effect on the non-working machine. If there is a change in the performance of the machine, than it has been proved that the replaced part is the culprit.
  • If the machine continues to malfunction, replace the next part and check for malfunction. In this way, step by step, check each identified part.
  • Once the defective part has been identified, replace the part from the non working machine on to the working machine to see if it causes the malfunction in the working machine or not. If it causes a malfunction then you are sure it is the culprit.
  • If this does not cause a malfunction on the working machine, than there is one more part related to this one, which is also causing the malfunction. Carry out the same exercise as above to identify this sub-component.

Although this is a very effective tool for troubleshooting, it has the following limitations:

  • It is possible only on multiple installations.
  • The failed part may cause accidental damage to the working machine.
  • One should be able to afford to use the working machine for trials.

Cause and Effect diagram

This is also a very effective tool in diagnosing the cause of a malfunction. The cause and effect diagram can also be called a fish bone diagram. In this particular method the technician or a group of technicians sit together and carry out a brainstorming session.

The major causes of a malfunction are written down first, such as:

  • Manufacturing defect
  • Setting parameters
  • Material defect
  • Human error
  • Others

These are further brainstormed to list down sub-causes for each of the above major causes. These are put on a piece of a paper in the form of a fish bone structure.

Having done this, each of these causes are than eliminated one by one. This is a very effective tool in diagnosing a cause of failure. Every cause and effect diagram can then be stored in a separate file to help other technicians with troubleshooting.

13.4.3 Troubleshooting Tools

No technician can carry out successful maintenance or a troubleshooting activity without the use of appropriate tools. There are certain basic tools and instruments required to service an air-conditioning system.

Since an air-conditioning system consists of electrical and mechanical components a variety of tools are needed. A few basic tools needed are:

Electrical Tools

  • Multimeter
  • Clamp on Ammeter
  • Neon test lamp
  • A capacitor tester
  • Suitable screwdrivers, pliers, wire cutters.
  • Jumpers

Mechanical tools

  • Psychrometer
  • Psychrometric chart
  • Pitot tube
  • Stopwatch
  • Manometer
  • Thermometer
  • Belt tension tester
  • Vibration measuring instrument
  • Refrigerant service cylinders
  • Weigh scale
  • Leak detectors
  • Vacuum pumps
  • Soldering or brazing tool
  • Tube cutters
  • Tube benders
  • Tube flaring tool
  • Set of standard spanners, hammers, mallet, drills, pliers etc.

Hope you will find this useful in your day-to-day activities, while troubleshooting or carrying our any maintenance activities on any equipment in an HVAC system.

13.4.4 Troubleshooting guide for fans

Table 13.1
Troubleshooting guide for fans
Problems Likely causes
Fan developing pressure below rated Total system resistance less than assumed
  Fan speed low
  Dampers or variable inlet vanes not adjusted properly
  Poor fan inlet and outlet conditions
  Air leaks in the system
  Fan impeller imbalanced
  Incorrect direction of rotation of the fan
Fan generating abnormal vibration and noise Misalignment of the bearings, couplings or V-belt
  Foundation bolts loose
  Foreign material in the fan causing imbalance
  Worn out bearings
  Impeller imbalanced
  Fan delivering more than rated capacity
  Loose dampers
  Speed of the fan higher than rated
Bearings getting overheated Too much grease in the bearings
  Poor alignment
  Impeller imbalance
  Dirt in the bearings
  Excessive belt tension
  Heat conducted or radiated from another source
Motor getting overloaded Motor speed too high
  Air flow too high due to reduced system resistance
  Motor winding weak
  Poor alignment of the fan with the motor
  Bearings not lubricated properly
  Wrong or loose cable connections

13.4.5 Trouble-shooting Compressors

Table 13.2
Troubleshooting guide for Compressors

14


Greenhouse effect and future refrigerants

14.1 Objectives

At the conclusion of this chapter, the student should be able to

  • Understand the effect of a Greenhouse gas on the environment
  • Understand the ill effects of CFCs
  • Know about various new CFC free Refrigerants

14.2 General

The year 1998 has set a new record for economic losses from weather-related disasters. The storms, floods, droughts and fires have cost $89 billion in economic losses worldwide. The losses represent 48% increase over the previous record of $60 billion in 1996 and far exceed $55 billion in losses for the entire decade of the 1980s as per the report of the World Watch Institute. The most damaging events were unnatural disasters in which the hand of man could be clearly seen, such as reclaiming of forests and wetlands and intense population pressure.

The man made activities on earth for increasing comfort have created a big problem for his survival it-self. Power generation and its consumption per capita is the measure of human comfort. This has reached the level of saturation in developed countries but is fast growing in the developing countries. In addition, to this the population is also increasing at a rapid rate and need for power even with the same standard of comfort is doubling every 10 years.

The major activity of power generation throughout the world has increased the level of CO2 in the atmosphere and destruction of forests has reduced the CO2 absorption capacity of the earth. The overall effect is, the level of CO2 in the atmosphere is increasing and presently creates a threat to the existence of human life.

Another major industry that is responsible for human comfort is refrigeration and air-conditioning. The use of refrigerators and air-conditioners even in developing countries is increasing day by day as living standards continue to grow. The refrigerants (CFC’s), which are universally used, have created a threat to the earth as their leakage can destroy the Ozone layer. The percentage of CFCs in the atmosphere compared with CO2 is negligible but its damaging effect on earth is 100 to 1000 times more than CO2. Therefore, serious thought has been given to replacing CFCs with HFCs and HCs within the coming 20 years throughout the world.

The Montreal Protocol on substances that deplete the Ozone layer is the international response to the dangers posed by CFCs. As a result of this agreement, CFCs are being replaced by HFC’s and HC’s. While this replacement begins, we must assure that the substitutes are environmentally acceptable. Although HC’s and HFC’s have no ozone depleting characteristics, HFC’s have a relatively high global warming potential and HC’s are flammable. Therefore, in balancing these risks it is necessary to select refrigerants that have high-energy efficiency, can be safely used and can be recycled.

14.3 The Greenhouse Effect

In many countries, where the climate has a low number of sunlight hours, cultivation of fruits and vegetables, is made possible through the green house effect. The term Greenhouse was first brought into use by Swedish Scientist Swante Arthensum in 1896.

A greenhouse is a house made completely of glass where vegetables are cultivated. In this house, the solar energy at the short-wave radiation of visible light enters through the glass, as glass is almost (80-90%) transparent to short wave radiations in the visible spectrum. This short-wave radiation when it strikes the inner earth surface of the green house, converts into heat or long-wave radiation. This long-wave or infra-red radiation is only partially reflected back into the atmosphere since glass is a good thermal insulator at these wavelengths. This trapped radiation contributes to the warming of the glass house and provides energy for the growth of plants. The trapped energy essential for the growth of plants keeps the plants green. Therefore, this effect is known as the Green House effect. The earth’s surrounding atmosphere behaves just like glass and keeps the earth green. Therefore, this effect is also popularly known as the Greenhouse effect.

Warming of the atmosphere and subsequently of the earth’s surface takes place because of the green house effect. The atmosphere contains radiatively active substances like CO2, water vapour and other trace gases that contribute towards the warming.

In bodies without an atmosphere, such as the moon, the sun facing side is unbearably hot because the radiation from the sun strikes it directly and heats its surface. The dark side is bitterly cold because the heat it catches during the ‘day; is lost in ‘night’. But on earth, the thick blanket of atmosphere (nearly 200km) prevents heat from escaping to space. Actually, the atmosphere is almost transparent to short-wave radiation and therefore all short wave solar energy (95% lies in between 0.4 to 8µ wavelength) reaches the earth. This radiation is converted into heat long-wave radiation which is then re-radiated. This long-wave radiation cannot get through the atmosphere as easily as short-wave radiation. It remains trapped by the air and this trapped heat, which contributes to the general warming of the earth and creates temperature conditions conducive to human and biological life. The earth green house effect can be illustrated in the following figure.

Figure 14.1
Greenhouse Gases Trap long Wavelength Energy From The Earth's Surface, Heating The Atmosphere, Which In Turn, Heats The Earth. (Earth As A Greenhouse)

In the greenhouse process, visible light comes in, as it would have, if there were no atmosphere. However, the atmospheric gases transparent in the visible part of the solar spectrum tend to be opaque in the longer infrared part of the spectrum. The thermal radiation in the infrared is impeded from getting out, resulting in the blanketing of the earth in the infrared spectrum but not in the visible part of the spectrum.

As a result, the earth surface temperature has to go up until the radiation, which is leaking out in the infrared just, balances the visible radiation that is coming in.

However, if too much heat is held-back, life can be endangered. Such a runaway “Greenhouse Effect” occurs on Venus, which has an extremely dense CO2 rich atmosphere. The surface temperature of Venus is 477 °C. It is not because it is close to the sun, but it is because of the dense surrounding layer of CO2 in its atmosphere.

If no greenhouse effect existed on earth, the average temperature would be –19°C well below the freezing point of water and hence, incompatible with any form of life as we know it. Most of the heat entering the earth’s atmosphere from the sun is re-radiated but in the process, a delicate balance is maintained which keeps the earth surface at an average temperature of 15°C. It is important to understand how crucial the green house effect is in keeping us warm and comfortable. Too much of a good thing can have the opposite effect, such as on the planet Venus.

The green house effect has been working from the very birth of the earth. It has helped in the development of the environment over the earth. The animals, birds and vegetation are all tied up with each other as an ecosystem, which tends to be self-balancing when not interfered with by human activity. However, excessive rise in population, excessive felling of forested areas and unlimited rise in power generation causes an imbalance of the ecosystem. Nature itself has proven to be competent enough to deal with imbalances brought about by unpredictable events such volcanic eruptions and floods. However there is a growing body of evidence that suggests that nature cannot deal with the imbalances in the ecosystem produced by indutrialised society.

The CO2 gas in the atmosphere is solely responsible for the green house effect on the earth as it is transparent to short wavelength and opaque to long wave radiations. Its percentage in the atmosphere definitely decides how much energy will be trapped in the surrounding atmosphere, which is responsible for the earth’s surface temperature. Higher percentages of CO2 will trap more radiation and increase the earths surface temperature slowly. Its immediate effect on increase in temperature may be as low as 5°C within the coming 100 years, but its long term effects are serious and destructive and therefore, it is necessary to take due care before the atmosphere is damaged by human activities.

CO2 alters the earth’s heat balance by acting as a one way screen as it is transparent to short-wave solar radiation and stops the back flow of heat from long wave radiations emitted from earth’s surface. This increases the earths surface temperature and this phenomenon is known as the Green House Effect.

14.3.1 Green House Effect from CO2 and CFCs

The earth is designed in such a way that it keeps every thing in equilibrium. The Earth is known as a green house because it looks green due to trees and plantation. This is essential to maintain the earths temperature and oxygen level. As we know, all living beings on earth emit CO2, which is absorbed by the trees, and inhale O2 emitted by the trees. Once the mass balance of O2 and CO2 is disturbed, it affects the atmospheric conditions. Large generating thermal power stations and use of large numbers of automobiles causes emission of CO2 on a large scale. With a growth in living standard, the power consumption per capita (in terms of burning of fossil fuels) is continuously increasing. In addition to this, the large reductions of the forests have increased CO2 content in the atmosphere (CO2 absorption by plants is reduced) during the last decade.

The level of CO2 in atmosphere is maintained steady by photosynthesis, which reduces CO2 and increases O2 in the atmospheric air. Photosynthesis is a process, which is carried out by the green plants by absorbing CO2 and moisture in the air and converting them into O2 and organic matter in the presence of sunlight.

There must be sufficient green plants to absorb generated CO2 and convert it into O2 . When the CO2 emitted rate is higher and the number of green plants is decreased, the CO2 steady state condition is reached at higher concentrations in the air. This is what is happening because of increasing demand for power worldwide and the large destruction of forests.

The present CO2 emission into atmosphere is 5700 x 106 tons per year from manmade causes. Therefore, there is a move to reduce power requirement and increase the forest area by artificial plantation.

Presently, there is general acceptance by scientists and engineers that the average earth temperature is increasing because of the Greenhouse effect. Certain gases (CO2, CFC’s, CI) in the upper atmosphere trap infrared radiation (preventing it from being rejected into the space) and these gases consequently enhance a radiative forcing effect that contributes to future global warming. The earth will be warmer with these gases in the upper atmosphere than it would be without them. The warming effects of these gases over time depend on measurable quantities such as atmospheric lifetime and the spectral absorption of the molecules.

The cooling provided by an air conditioning system requires power to run the refrigeration system and any power generation system that burns fuel produces CO2. The power system supplying 1kWh energy to a refrigeration system produces CO2 as shown in Figure 14.2.

Figure 14.2
Emission of CO2 when 1kWh of Electricity is used by Refrigeration System

It is obvious that the emission of CFCs into the atmosphere (30% of the total used refrigerants due to leakages and other sources) creates global warming because of greenhouse effects. The time taken to cause this effect depends upon the quantity, their chemical and radiative properties, their life time in the atmosphere, the time required to ascend into the stratosphere, their distribution in the atmosphere and chances of bombardment by high energy photons which will further destroy the O3 layer (once energized) is too large compared with CO2.

It is necessary to replace present conventional CFC refrigerants by some other refrigerants, which will have less greenhouse effect.

14.4 History of CFCs

The Belgian chemist Frederic Swarts (1892-1907) describes the properties of synthetically produced groups of molecules in which chlorine and fluorine are grouped around the carbon atom-CFCs. Thomas Midgley (1929) of General Motors patented the compounds and technical applications began. Midgley (1930) suggested the use of R-11 and R-12 as a substitute for the cooling agents SO2 and NH3, R-11 and R-12 entered into the market in 1936. US Army in Indochina to protect soldiers against a bothersome insect plague in 1942 used the first Aerosol insecticide. Aerosol production started in 1953-54 and CFCs were used in hair and deodorant sprays, shaving creams, perfumes, glues, varnishes and paints. CFC applications experienced a boom in 1970. They are even now used as cleaning agents in electrical and textiles industries. Halons are used in fire extinguishers. In 1970 only, a British scientist James E.Lovelock detected F-11 in the atmosphere. Roland and Molina from California University published a paper in 1974 stating that CFCs could be destroying the protecting Ozone layer of earth in the stratosphere. A ban was imposed in USA in 1978 on the use of R-11 and R-12 in aerosols except in pharmaceutical applications. An Ozone hole over the Antarctic in 1985 was discovered. The Vienna Convention on the protection of the ozone layer was agreed. In 1987, the Montreal Protocol was adapted as a completion of the Vienna conventions. Forty six countries signed an agreement aimed at a step-by-step reduction of the production and use of fully halogenized CFC to 50% of the 1986 level by 1999. The Enquete Commission of the German Federal Republic submitted a report in 1988 containing demands that are more far-reaching than the requirements stipulated in the Montreal Protocol. In 1989, German parliament approved the recommendations of the Enquete Committee and US Congress passed a CFC taxation scheme at $1.37 per kg of CFC released.

Invented in the late 1920s, CFCs are nontoxic, non-corrosive, non-flammable and non-reactive with most other substances. They have been widely used in refrigerators and air-conditioners, as a blowing agent for foam products, as aerosol propellants in spray cans and solvents.

Lacking understanding of the threat posed by CFC’s to the Ozone layer, they remained a popular industrial substance that improved the comfort and ease of life. Before 1970, scientists had not worried about how CFC’s affect the atmosphere. In 1974, Molina and Rowland found that CFCs were extremely stable in the lower atmosphere, they could drift up into the stratosphere where they would break apart when bombarded by the sun’s radiation. They suspected that millions of tons of chlorine atoms released into the stratosphere could severely damage the ozone layer. Destruction of the life protecting ozone layer by CFC’s is one of the most serious global environmental problems. Ozone in the stratosphere absorbs dangerous ultraviolet radiation from the sun, protecting humans from the risk of skin cancer. Ultraviolet light also harms food crops and the natural ecosystem. CFC’s released into the atmosphere damage the O3-layer, threatening global sustainability through the spread of disease and dwindling of harvests.

14.5 Ozone Depletion by CFCs and the Greenhouse Effect

The CFC leakage destroys the Ozone layer and creates holes through which large amounts of harmful heat in the form of ultraviolet rays fall on the earth and increases the earths temperature. The effect of this mode of heating is more serious than the effect caused by CO2.

The danger that man-made chlorinated compounds (CFC’) pose to the stratospheric Ozone layer is now a well-documented fact. Current levels of atmospheric chlorine are 3.5 parts per billion by volume (ppb) compared with naturally occurring levels of 0.7 ppb. These high chlorine levels are the product of the breakdown of CFC's in the stratosphere (15-50km above the earth’s surface).

Most molecules released to the atmosphere are removed by:

  • By absorption of sunlight (photolysis)
  • By dissolution in water (rainout) or
  • By reaction with hydrosol.

However, as CFC’s are transparent, insoluble and unreactive towards atmospheric oxidizing agents, none of the above processes affects them. They remain wandering for a long time in the lower atmosphere. When these wandering CFC molecules are carried in the stratosphere by great storms, at 15 to 20 km altitudes they absorb intense ultraviolet (UV) radiation, which destroys the CFC molecules as shown below:

The reaction releases a CI atom, which attracts O3 as:

CI + O3 = CIO + O2

The reaction further continues as:

CIO+ O = CI + O2

….which regenerates CI atoms, so that a long chain process is involved which conserves CI atoms.

Each chlorine atom can destroy upto 100,000 Ozone molecules before it is washed out of the atmosphere. Chlorine released from CFC takes 50 to 100 years to be reduced by 50% in the atmosphere. The destruction of Ozone is faster than its formation; therefore, the effect is a reduction in the Ozone layer or formation of an Ozone hole.

An increased exposure to ultraviolet rays, at low levels has a serious impact on human health and the global ecosystem. The potential effects of Ozone reduction and resulting greater exposure to solar ultraviolet radiation produces skin cancer and increased cataracts and blindness. One% increase in ultraviolet radiation will lead to 3% increase in skin cancer and 0.6% increase in cataracts. There is approximate 2% increase in destructive ultraviolet radiation for every 1% decrease in Ozone.

Depletion of 20% or more could make it difficult or even impossible for animals to graze and humans to work outdoors and have catastrophic impacts on good production, economy and society in general.

A large amount of Ozone is consumed as CI chain regenerated which eats Ozone continuously. This can create an Ozone hole in the stratosphere and allow large amounts of UV-rays to enter into the lower earth atmosphere. An Ozone hole over Antarctica was the largest ever measured and lasted a month in 1998. The area covered was 10 million square kilometer and it remained more than 100 days. Average amount of O3 during Sept. to Nov. (in 1998) showed a shortfall of close to 40% compared to average values before 1976. That is considered the base year for measuring atmospheric O3. The global warming has speeded the melting of two Antarctic ice shelves, which lost nearly 3000 square kilometers of their total area in 1998. The problem of an O3 hole lasting longer over the Antarctic is that it extends the influence of ozone-poor air into January, which could result in an increase of ultraviolet (UV) radiation into Southern Brazil.

14.6 Global Warming Potential (GWP) and Ozone Depleting Potential (ODP)

The effect of gases (like CO2, CH4) in the atmosphere on the earths temperature and effects of CFC’s on the destruction of the Ozone layer, are both the effects that are undesirable and harmful to human life as well crop life in the long term. Therefore, the safe gases which will be used in future for refrigeration purposes must have low green house forming and low ozone destruction tendencies.

The commonly used parameters that describe the potential of refrigerant on global warming are ODP and GWP. These parameters not only reflect the capacity of individual molecules for depleting O3 or global warming but also represent the atmospheric life span of the chemicals.

The GWP of CO2 is included in the table to show the comparison with CFC’s. The residence time of 100 to 500 years are listed in the table for different refrigerants. Lashof and Ahuja used 230 years for CO2 for comparison only. At this 230 years atmospheric life, the CFC’s accounted for only 0.004% of the total mass of greenhouse gases emitted compared to 93.6% CO2. Therefore, their relative contributions to global warming potential were 9.5% and 71.5% respectively. However, the relative GWP of a refrigerant would be reduced by a factor of two if the effective residence time of CO2 is at the end of the potential range. In any case, greater quantities of CO2 are released in the atmosphere but the CFC’s have must higher potential as global warming gases per unit mass (one kg of R-11 would have the same impact as 4500 kg CO2 during first 20 years, 3500 kg of CO2 over 100 years and 1500 kg of CO2 over 500 years).

The following table gives the values of ODP and GWP estimated atmospheric life for presently used CFCs and alternative refrigerants for future. The ODP is taken as one for R-11, the most commonly used CFC for cooling and GWP is taken as one for CO2, the most abundant greenhouse gas.

Table 14.1
Ozone Depleting (ODP) and Global Warming (GWP) Potentials

The AFEAS (Alternative Fluorocarbon Environmental Acceptability Study) report provided a set of data for atmospheric lifetimes, ODP’s and GWP’s for different present and future refrigerants.

14.7 Montreal Protocol (1987)

The greenhouse effect has existed on earth long before life. Presently, the production of CO2, CH4 and CFCs the most important of greenhouse gases, has drastically increased and as a result, the earth is warming up. Since the middle of last century, the mean temperature of the earth’s surface has risen by 0.8°C. If greenhouse gases continue to be emitted at the current rate, it is predicted that the global temperature will rise by 1.5 to 4.5°C by the year 2050. This increase in temperature would cause a rise in sea level upto 5 meters leading to flooding, crop damage and serve disruption of life systems.

Looking into the danger of green house effect, in 1987, more than 150 countries have signed a protocol on curtailing the production and phasing out of CFC’s on a global scale. As per Montreal Protocol, HFC’s have also been included as controlled substances with virtual elimination by 2020 and total phase-out by 2030.

The original protocol has been amended several times over the years, adding substances, changing dates and so on. As well, the protocol differentiates between Annex 1 (developed countries) and Annex 2 (developing countries) providing different timelines for each group. The following is a summary on the original requirements and subsequent changes made over the years.

Annex 1 (Developed) Countries 22

1987 Montreal Protocol: Requires 50% reduction in CFC production by 2000. Other compounds were also controlled.

1990 London Amendment: Requires 100% phaseout of CFC production by 2000.

1992 Copenhagen Amendment: Advances CFC phaseout to 1996. Set HCFC consumption cap in 1996 with a ratcheted phaseout as follows.

! 65% of Cap in 2004

! 35% of Cap in 2010

! 10% of Cap in 2015

! And 0.5% of Cap in 2020

The cap was set as the ozone depletion potential weighted consumption for HCFC plus 3.1% of the CFC ODP weighted consumption with a base year of 1989.

1995 The CFC portion of the HCFC cap was reduced from 3.1% to 2.8%.

1997 Montreal Protocol Amendments: HCFC consumption from 2020 to 2030 only be used for service on existing equipment

1999 Beijing Amendments: HCFC production caps introduced to deal with export issues to developing countries.

14.8 Kyoto Protocol

In 1990, the United Nations established a committee to form a framework on Climate Change. In May 1992, the countries at the Rio de Janeiro Earth Summit adopted the agreement. Since then, over 176 countries (including the United States) have ratified it. The agreement established a process where countries could meet and discuss the formation of an international legal agreement or Protocol. The first such meeting was held in Berlin. In 1997, the third meeting was held in Kyoto, Japan where the Kyoto Protocol was approved.

The Kyoto Protocol enters into force when at least 55% of the countries representing at least 55% of the annex 1(1990) carbon dioxide equivalent emissions have ratified it. To date, 13 countries have ratified the Protocol.

Kyoto Protocol Requirements

The Kyoto Protocol is intended to reduce the Annex 1 (developed countries) carbon dioxide emissions by 5.2% from the 1990 levels based on the emissions from 2008 to 2012. Each Annex 1 country has it’s own target as shown in the following table.

Table 14.2
Carbon Dioxide Emissions Reductions by Country
Country Percent Reduction Country Percent Reduction
Australia +8 Liechtenstein 8
Austria 8 Lithuania 8
Belgium 8 Luxembourg 8
Bulgaria 8 Monaco 8
Canada 6 Netherlands 8
Croatia 5 New Zealand 0
Czech Republic 8 Norway +1
Denmark 8 Poland 6
Estonia 8 Portugal 8
European Community 8 Romania 8
Finland 8 Russian Federation 0
France 8 Slovakia 8
Greece 8 Slovenia 8
Hungary 6 Spain 8
Iceland +10 Sweden 8
Ireland 8 Switzerland 8
Italy 8 Ukraine 0
Japan 6 United Kingdom 8
Latvia 8 United States 7

Targeted Gases

There are 6 greenhouse gases covered by the Kyoto Protocol. These are carbon dioxide, methane (CH4), nitrous oxide (N2O), HFCs, perfluorocarbons (PFCs) and sulfur hexafluoride (SF6) (with a GWP of 23,900). Although CFCs and HCFCs are greenhouse gases, they are not included because they are being phased out by the Montreal Protocol. The baseline year each country is to use for carbon dioxide, methane and nitrogen oxide is 1990. Each country has the choice of setting the base level for HFCs, PFCs and SF6 on either 1990 or 1995 levels.

The emissions for all these gases are added up for each country in what is referred to as a “basket”. There are no requirements by the Kyoto Protocol to stop using any of these substances. The only requirement is to reduce their combined emissions to the country’s target. This is a very important difference between the Kyoto Protocol and the Montreal Protocol where specific substances were to be phased out of production and use.

14.9 Future Refrigerants to replace CFCs

In the mid-nineties, the HVAC industry went through a huge change as the requirements of the Montreal Protocol phased out CFCs. Many then unheard of refrigerants (R-134a and R-123) came to the forefront as replacements for old standbys like R-12 and R-11. The Industry is on the cusp of another change as the HCFCs start to be phased out (R-22 will be phased out by 2010 in the United States) and the effects of the Kyoto Protocol start being felt.

Despite the wide range of refrigerants available, only a few are under serious consideration.

Water (R-718)

Water is the refrigerant in absorption chillers. It is non toxic, abundant, non-flammable and has no ODP or GWP. The main drawback is the efficiency on absorption chillers. The current commercial double effect, direct fired (operates on natural gas) chiller has a COP=1. Compare this with centrifugal chillers with COPs of 6.4 or better.

In markets such as Japan, absorption chillers are the norm and centrifugal chillers are the exception. In North America, absorption chillers cost approximately twice as much as electric centrifugal chillers and often cost more to operate, making them difficult to justify.

It is predicted that absorption chillers will continue to see niche applications (Cogen and hybrid plants. If energy rates change enough (a possibility with deregulation) then absorption chillers may take on a larger role.

Ammonia (R-717)

Ammonia has excellent performance, no ODP and a small GWP. However, the health and flammability issues surrounding ammonia have limited it to industrial and controlled commercial applications.

It is predicted that ammonia will continue to see industrial and controlled commercial applications.

Carbon Dioxide (R-744)

Carbon dioxide is non-toxic, non-flammable and has no ODP and a low GWP. Its low critical point, however, makes it a poor performer at typical commercial operating conditions. As well, the operating pressures are very high (900 psi). Research is underway to study carbon dioxide at trans-critical conditions. As well, it has been used successfully in cascade refrigeration systems.

It is predicted as the possible future applications in automotive air conditioning and cascade refrigeration plants.

Propane (R-290) and Isobutane (R-600a)

Propane and isobutane have low toxicity, good performance, no ODP and low GWP. However, they are flammable. Northern Europe has accepted them in refrigerators.

It is predicted that these flammable refrigerants will most likely continue to see use in the domestic and small system market.

R-134a

R-134a is classified as an A1 (lower toxicity – no flame propagation) refrigerant by ASHRAE, it has no ODP but the GWP is 1300. It has very good performance, heat transfer properties and is a good candidate for screw and centrifugal compressor applications. Its acceptance in the automotive industry makes it very abundant.

R-134a appears to be caught between the Montreal and Kyoto Protocols. The Montreal Protocol is systematically removing from service common refrigerant with ODP> 0. By default, the protocol is driving the market to refrigerants with ODP=0 such as R-134a. On the other hand, the Kyoto protocol has put HFCs in the basket of targeted gases.

R-134a is one of the best solutions available. It has 0 ODP and therefore has no phaseout date from the Montreal Protocol. Although HFCs are in the Kyoto Basket, only its emissions are regulated. There is no phase out date for HFCs. While CFCs and HCFCs are major contributors to ozone depletion (28% of anthropogenic ozone depletion), HFCs, CFCs and HCFCs direct effect is only a minor player in climate change (4% of anthropogenic global warming). All second generation centrifugal and screw chillers in the market place have been designed around R-134a.

It is predicted that R-134a will continue to be the main large capacity refrigerant in the HVAC industry for the foreseeable future.

Replacements for R-22

R-22 is classified as an A1 (Lower toxicity – no flame propagation) refrigerant by ASHRAE. It is the most popular refrigerant in the world. R-22 is also an HCFC and as such is being phased out. In the United States, R-22 is already capped and will be completely phased out in 2010 except for a small amount for service. R-22 is extremely versatile. It is used in supermarket refrigeration, skating rinks, chillers with all types of compressors, packaged rooftop units and most residential air conditioning.

There are no direct replacements for R-22. Different refrigerants will end up replacing various applications of R-22. The use of R-22’s in centrifugal chillers has always been limited and for the most part that sector has moved to R-134a. R-22’s use in screw chillers (water and air-cooled) is now moving to R-134a for the most part. All second-generation screw compressors have been developed to work with R-134a. R-404A and R-507 are being used in refrigeration applications previously performed by R-22. This leaves all the small compressor applications including package rooftop units, small air and water cooled chillers (under 200 tons) and the entire residential air conditioning (multi-billion dollar) industry. The two main candidates are R-407C and R-410A. Propane (R-290) can be used in the residential market, but there are concerns about flammability.

R-407C

R-407C is a zeotropic blend of HFC-32, HFC-125 and HFC-134a. Its properties have been “tuned” to be very close to R-22 . R-407C can be “dropped” into an existing R-22 refrigeration system and work although often with some performance loss. In many applications, the performance can be improved with minor changes to the refrigeration system sub-components (for example adding condenser surface area).

R-407C is often considered a “replacement drop in” refrigerant to be used in upgrading existing systems to an HFC refrigerant. There is limited new generation product development based on R-407C.

It is predicted that R-407C will be used as a drop in replacement for R-22 in existing systems and as an interim solution for existing product lines until new lines are developed.

R-410A

R-410A is a zeotropic blend of HFC-32 and HFC-125. It has a very low volume flowrate (1.5 cfm/ton). It also operates at higher pressures than R-22 (450 psi). It cannot be dropped into an existing R-22 system but instead must be used in a new generation design.

There is a lot of interest in R-410A as the new generation replacement for R-22 with smaller systems. Compressor manufacturers have started to offer small (1/2 to 5 ton) compressors designed for R-410A, which has led to its introduction in the residential market (Carrier refers to residential R-410A systems as Pureon). As larger compressors become available, R-410A will spread into more commercial products. With the higher operating pressures associated with R-410A, all sub-components (valves, sight glasses, filter–driers etc) need to be re-designed, which has slowed R-410A’s launch into the commercial market place. One issue with R-410A is the low critical point in applications in high ambient conditions. This significantly reduces the performance of air-cooled equipment in hot locations. Water-cooled equipment is generally unaffected because of the lower condensing temperatures.

As more refrigeration sub components become available, next generation residential and light commercial products will utilize it. Some technology will be implemented to address the low critical point issue.

Replacements for R-123

ASHRAE Standard 34 classifies R-123 as a B1 refrigerant (higher toxicity – no flame propagation). It is the HCFC replacement for CFC-11 and as such is being phased out. In the United States, R-123 is already capped and will be reduced to 0.5% production in 2020 for service only until 2030. R-123 is used almost exclusively in negative pressure centrifugal chillers.

There is no clear replacement for R-123. Possible use of R-601 (n-pentane) or R-601a (isopentane) is unlikely. These are both highly flammable. The required charge levels used in centrifugal chillers would make an explosion possible. As well, negative pressure chillers draw in air allowing an explosive mixture to be present in the chiller.

To refrigerant manufacturers, R-123 use in negative pressure centrifugal chillers is a small market. Fortunately, R-123 can be manufactured as a byproduct from manufacturing other more common refrigerants. R-245fa is a more expensive refrigerant to manufacture. Its realistic introduction into the market place would require other applications for the substance (e.g. foam blowing) to get the production quantities large enough to get the cost down.

R-123 will be used in negative pressure chillers until the Montreal Protocol phases it out. There is some speculation that there may be an exemption for R-123 for refrigeration use, but that is unlikely. It would require a majority vote of the Montreal Protocol members. With its limited use outside the United States and other technologies available a change in the Protocol is extremely unlikely.

Possible retrofit kits for existing R-123 chillers with R-245fa may be available but only if the shells are ASME rated.

Comparison of Physical properties of Conventional and Future Refrigerants

Table 14.3
Comparison of Physical Properties of Conventional and Future Refrigerants
  R-123 R134a R-22 R-502 R-717
Formula CHCI2F CH2FCF3 CHCIF2 CHCIF2+
CF3+CCIF2
NH3
Molecular weight 153 102 86.5 112 17
Boiling point (°C) 27.1 -26.22 -40.8 -45.6 -33.3
Γ = Cp/Cv 1.11 1.116 1.18 1.135 1.29
Flammability (Vol.% in air) Nil Nil Nil Nil 16 to 25
Toxicity (TLV) ?
(5-10)
No
(1000)
No
(1000)
No
-
Yes
(25)
Latent heat (kJ/kg) at 15°C 175 205 217 156 1314
ODP 0.02 0.00 0.05 0.23 0.00
HGWP 0.02 0.285 0.365 5.1 0.00
Availability Yes Yes Yes Yes Yes
Cost ($ 1 kg) 6.05 10.45 1.98 - 0.88
Ideal COP 7.63 6.77 7.06 - 7.28

The refrigerants discussed are direct alternatives for conventional refrigerants which are less environmentally friendly and are suitable for a wide range of commercial applications including beer chillers, milk tanks, cold storages, display cabinets, refrigerated transport and pack systems.

HVAC is a vast field, which has no boundaries for learning. With all the chapters in this manual we have tried to cover all the practical aspects in HVAC which people dealing with HVAC should be familiar with. We hope this will be a successful tool to all of you in your career in the HVAC industry.


APPENDIX A


Psychrometry

A1. Pstychrometric Tables

A2- Absolute Steam Table

A3-Desirable conditions for various industries

A4-Desirable conditions for different buildings (Summer Conditions)

A5-Recommended F.H and D.B.T for Various industries

A6-Cold Storage conditions for animal Products

A7-Cold Storage conditions for Fruits and Vegetables

A8-Specific Heats of Food Stuffs


APPENDIX B


Properties of refrigerants


APPENDIX C


Conversions And Tables

CONVERSION TABLES

LENGTH CONVERSIONS

1 inch = 25.4 millimeters

1 foot = 304.8 millimeters

1 meter = 39.37 inches

1 meter = 3.28 feet

1 foot = 0.3048 meters

1 Micron = 10-6

1 Micron = 0.000039 inches

AREA CONVERSIONS

1 square inch (In2) = 645.16 square millimeter (mm2)

1 square foot (ft2) = 92903.04 square millimeter (mm2)

1 square foot (ft2) = 0.09290 square meter (m2)

1 square millimeter = 0.00155 square inch

1 square meter = 1550 square inch

1 square meter = 10.7639 square foot

VOLUME CONVERSIONS

1 cubic inch (In3) = 16387 cubic millimeters (mm3)

1 cubic inch = 0.000163 cubic meters (m3)

1 cubic inch = 0.0164 Liters (L)

1 cubic inch = 0.003604 UK gallon

1 cubic inch = 0.0043 US gallon

1 cubic foot (ft3) = 1728 cubic inches

1 cubic foot = 28316846.6 cubic millimeters

1 cubic foot = 0.02831 cubic meters

1 cubic foot = 28.317 Liters

1 cubic foot = 6.229 UK Gallon

1 cubic foot = 7.5 US Gallon

1 cubic millimeter = 0.00061 cubic inch

1 cubic millimeter = 3.5e-8 cubic foot

1 cubic millimeter = 0.000001 Liters

1 cubic millimeter = 2.2e-7 UK Gallon

1 cubic millimeter = 2.64e-7 US Gallon

1 cubic meter = 62574.5 cubic inch

1 cubic meter = 35.314 cubic feet

1 cubic meter = 1000 Liters

1 cubic meter = 220 UK Gallon

1 cubic meter = 264.172 US Gallon

1 USGallon = 8 pints

MASS CONVERSIONS

1 Pound = 7000 Grains

1 Pound = 0.4536 Kg

1 Ounce (oz) = 28.35 Grams

1 Kg = 2.20462 Pounds

1 Gram = 0.22046 Pounds

1 Gram = 15.432 Grains

1 Gram = 0.03527 Ounce (oz)

1 Grain = 0.0648 Grams

WEIGHT CONVERSIONS

1 pound (lb) = 0.453 Kilogram (Kg)

1 pound = 4.448 Newton (N)

1 kilogram = 2.204 pounds

1 kilogram = 9.81 Newton

1 Newton = 0.222 lb

1 Newton = 0.101 kilogram

FORCE CONVERSION

1 Pound-force (lbf) = 0.4536 Kilogram-force

1 Pound-force = 4.44822 Newton

1 Kilogram-force = 2.2046 Pound-force

1 Kilogram-force = 9.81 Newton

1 Newton = 0.10197 Kilogram-force

1 Newton = 0.2248 Pound-force

ENTHALPY CONVERSION

1 Btu/lb = 0.5555 Kcal/kg

1 Btu/lb = 2.3244 Kilojoules/kg (KJ/Kg)

1 kcal/kg = 1.8 Btu/lb

1 kcal/kg = 4.184 KJ/kg

1 KJ/kg = 0.4302 Btu/lb

1KJ/kg = 0.239 Kcal/kg

THERMAL CONDUCTIVITY

1 Btu/h/ft2/in/°F = 1.9224 Kcal/h/m2/°C

COEFFICIENT OF HEAT TRANSFER

1 Btu/h/ft2/°F = 4.8823 Kcal/h/m2/°C

1 Btu/h/ft2/°F = 5.6783 W/m2/°C

1 Kcal/h/m2/°C = 0.2048 Btu/h/ft2/°F

1 Kcal/h/m2/°C = 1.163 W/m2/°C

1 W/m2/°C = 1761.1 Btu/h/ft2/°F

1 W/m2/°C = 8598 Kcal/h/m2/°C

PRESSURE CONVERSIONS

1 Atmosphere (atm) = 1.013 bar

1 Atmosphere = 101.33 kPa

1 Atmosphere = 14.696 psi

1 Atmosphere = 33.9 ft.hd

1 Atmosphere = 406.84 inch of water (In.wg)

1 atmosphere = 76 cm Hg

1 bar = 0.9869 atm

1 bar = 100 kPa

1 bar = 14.51 psi

1 bar = 33.44 ft.hd

1 bar = 406.84 inch of water (In.wg)

1 bar = 76 cm Hg

1 Kilopascal (kPa) = 0.009869 atm

1 Kilopascal = 0.01 bar

1 kilopascal = 0.145 psi

1 kilopascal = 0.3346 ft.hd

1 kilopascal = 4.01 inch of water (In.wg)

1 Kilopascal = 0.7501 cm.Hg

1 psi = 0.06805 atm

1 psi = 0.06893 bar

1 psi = 6.895 kPa

1 psi = 2.307 ft.hd

1 psi = 27.7 In.wg

1 psi = 5.171 cm.Hg

1 ft.hd = 0.0295 atm

1 ft.hd = 0.0299 bar

1 ft.hd = 2.989 kPa

1 ft.hd = 0.4335 psi

1 ft.hd = 12 In.wg

1 ft.hd = 2.242 cm.Hg

1 In.wg = 0.002458 atm

1 In.wg = 0.002491 bar

1 In.wg = 0.249 kPa

1 In.wg = 0.0361 psi

1 In.wg = 0.833 ft.hd

1 In.wg = 0.1868 cm.Hg

1 cm.Hg = 0.01316 atm

1 cm.Hg = 0.01333 bar

1 cm.Hg = 1.333 kPa

1 cm.Hg = 0.1934 psi

1 cm.Hg = 0.446 ft.hd

1 cm.Hg = 5.353 In.wg

TEMPERATURE CONVERSIONS

1 Degree Celcius = 33.8 Farenheit

1 Degree Farenheit = -17.2 Celcius

Farenheit to Celsius conversion = F-32/1.8

Celsius to Farenheit conversion = 1.8C +32

Farenheit to Kelvin (K) conversion = TF+460/1.8

Kelvin to Celsius conversion = TK-273

Rankine to Farenheit conversion = TR+460

MASS FLOW RATE

1. Pounds / min = 0.4536 Kg/min

1 Kg/min = 2.20462 Pounds / min

MOISTURE CONTENT IN AIR

1 grains per pound = 0.1428 Gram/Kilogram (g/kg)

1 grams/kilogram = 7.002 Grains/pound (Gr/lb)

ENERGY AND HEAT CONVERSIONS

1 Btu = 1055.06 Joules

1 Btu = 0.252 calories

1 Joule = 0.0009478 Btu

1 Joule = 0.000239 Calories

1 Calorie = 3.968 Btu

1 Calorie = 4184 Joules

1 Btu/hr = 0.000293 kw

1 Btu/hr = 0.07 Calories/second

1 Btu/hr = 1.0548 KJ/hr

1 Btu/hr = 0.0003929 hp

1 Btu/hr = 0.252 Kcal/hr

1 Kw = 3602 KJ/hr

1 Kw = 239.8 Cal/s

1 Kw = 3413 Btu/hr

1 Kw = 1.341 hp

1 Kw = 860 Kcal/hr

1 hp = 33000 Ft.lb

1 hp = 0.7457 Kw

1 hp = 2545 Bru/hr

1 hp = 178.2 cal/s

1 hp = 2685 KJ/hr

1 hp = 641.5 Kcal/hr

1 cal/s = 15.073 KJ/hr

1 Cal/s = 0.004186 Kw

1 Cal/s = 14.29 Btu/hr

1 Cal/s = 0.005613 hp

1 Cal/s = 3.6 Kcal/hr

1 Kcal/hr = 0.0011628 Kw

1 Kcal/hr = 3.968 Btu/hr

1 Kcal/hr = 0.2778 Cal/s

1 Kcal/hr = 0.001559 hp

1 Kcal/hr = 4.185 KJ/hr

1 KJ/hr = 0.0002776 Kw

1 KJ/hr = 0.948 Btu/hr

1 KJ/hr = 0.0663 cal/s

1 KJ/hr = 0.0003724 hp

1 KJ/hr = 0.239 Kcal/hr

1 Btu/hr/ft2F = 5.68 W/m2C

1 W/m2C = 0.176 Btu/hr/Ft2F

1 Watt hour (Wh) = 2655 Ft.lb

1 watt hour = 3.413 Btu/h

1 watt hour = 0.8698 Kcal/h

1 Ton Refrigeration = 12000 Btu/h

1 Ton Refrigeration = 3024 Kcal/h

1 Ton Refrigeration = 3.516 Kilowatt

1 Watt = 3.412 Btu

1 Watt = 0.8598 kcal

1 Watt/s = 1.0 Joule (J)

DENSITY CONVERSIONS

1 Pound/Cubic feet = 16.02 Kilogram/cubic meter (kg/cu.m)

1 kg/cu.m = 0.06247 Lb /cft

1 Lb/cft = 0.13368 Lb/US gallon

1 Gram/cc = 62.43 Lb/cft

SPEED CONVERSIONS

1 Ft/s = 60 Ft/min

1 Ft/s = 0.3048 m/s

1Ft/s = 30.48 m/s

1 Ft/min = 0.1667 ft/s

1Ft/min = 0.00508 m/s

1Ft/min = 0.508 cm/s

1 m/s = 196.85 ft/min

1 m/s = 3.281 ft/s

1 m/s = 100 cm/s

1 cm/s = 1.9685 ft/min

1 cm/s = 0.03281 ft/s

1 cm/s = 0.01 m/s

FLOW RATE CONVERSIONS

1 cu.ft/min = 0.000472 cu.m/s

1 cu.ft/min = 1.7 Cu.m/hr

1 Cu.ft/min = 7.479 US Gallons

1Cu.ft/min = 6.233 UK Gallons

1 Cu.ft/min = 0.472 Liters/S

1 Cu.m/s = 2118.6 Cu.ft/min

1Cu.m/s = 3600 Cu.min/hr

1Cu.m/s = 15845 USGPM

1Cu.m/s = 13204 UKGPM

1Cu.m/s = 1000 Liters/s

1Cu.m/hr = 0.0002778 Cu.m/s

1Cu.m/hr = 0.5882 Cu.ft/min

1Cu.m/hr = 4.402 USGPM

1Cu.m/hr = 3.668 UKGPM

1Cu.m/hr = 0.2778 Liters/s

1USGPM = 0.00006311Cu.m/s

1USGPM = 0.2272 Cu.m/hr

1USGPM = 0.1337 Cu.ft/min

1USGPM = 0.833 UKGPM

1USGPM = 0.0631 Liters/s

1 UKGPM = 0.000007583 Cu.m/s

1 UKGPM = 0.0273 Cu.m/hr

1 UKGPM = 1.2 USGPM

1 UKGPM = 0.1604 Cu.ft/min

1 UKGPM = 0.07572 Liters/s

1 Liters/s = 0.001 cu.m/s

1 Liters/s = 3.6 cu.m/hr

1 Liters/s = 7.479 USGPM

1 Liters/s = 13.21 UKGPM

1 Liters/s = 2.119 Cu.ft/min

Source: AIRAH Application Manual No. DA9 – Air Conditioning Load Estimation and Psychrometrics


APPENDIX D


Psychrometric Charts Plotting


APPENDIX E


Testing, adjusting and balancing in HVAC systems (TAB)

General Requirements

A person, or a Contractor, who does the TAB for a running HVAC system must have at least few years of experience in working with HVAC system installed in their plants.

A complete drawing showing the locations of HVAC equipments, their specifications, PID diagram, related software installed computers, a complete documentation on Building Automation System (BAS) are essentials for performing TAB.

A little bit of mathematical equation used in HVAC system designing, fluid flow properties on both Air and water, General knowledge on how the system built in their factory for air and water system, Electrical knowledge with controls schematic, measuring instruments used,

Most importantly, all the measuring points identified, incorporated in the various systems, easy accessibility for a tab technician to take measurements.

A set of forms for each equipment and accessories to be measured and recorded, a set of TAB procedures are vital for performing the TAB.

HVAC systems identification for performing TAB

Basically, there are three main HVAC systems:

Air distribution systems:

  • Know the purpose of each component in an air system and how these components interact as related to system operating pressure.
  • Know the effect of duct leakage on balancing.
  • Know the function of the fan laws, system effect and V-belt drives
  • Understand the function of the components in various types (constant volume, VAV, dual-duct, terminal reheat, etc) of air distribution systems.

Hydronic systems:

  • Know the purpose of each component in a hydronic system.
  • Know the effect on balancing of system resistance.
  • Be able to apply Pump Laws, the effect on pumps when balancing, and pump curves.

Outdoor Air Ventilation system:

  • Direct measurement of outdoor air conditions
  • Mixed air temperature measurement

Preliminary system procedures for Air Distribution Systems

The following TAB procedures are basic to all types of air systems:

  1. Verify that the construction team responsibilities for system installation and startup aare complete.
  2. Record unit nameplate data .
  3. Confirm that every item affecting the airflow of a duct system is ready for the TAB work, such as doors and windows being closed, ceiling tiles (return air plenums) in place, etc.
  4. Confirm that the automatic control devices will not adversely affect TAB operations.
  5. The control systems shall be installed and commissioned by others prior to starting the TAB work.
  6. Establish the conditions for design maximum system requirements.
  7. Verify that all dampers are open or set, all related systems (supply, return, exhaust, etc.), are operating, motors are operating at or below full load amperage ratings, and rotation is correct.
  8. Positive and negative pressurization zones should be identified at this time.

Basic air system balancing procedures

Balancing air systems may be accomplished in various ways. Two acceptable methods for balancing systems are presented. These methods are appropriate for supply, return and exhaust systems.

  1. Proportional Method (Ratio Method)
  2. Stepwise Method

Proportional method (ratio method)

This technique is initially described for a basic constant volume supply system without branch ducts.

It is also appropriate for exhaust or return duct systems.

a) Verify that all Grille, register and diffuser (GRD) dampers are wide open.

b) Set air outlet deflections as specified.

c) Determine total system airflow by the most appropriate method.

d) Calculate the percentage of actual airflow to design airflow.

e) Adjust the fan to approximately 110% of design airflow, if possible.

f) Measure the airflow at all GRD's.

g) Compute the ratio of measured airflow to design airflow for each GRD.

h) The damper serving the GRD at the lowest percentage of design flow is not adjusted in this procedure.

i) Adjust the damper serving the GRD with the next (second) lowest percentage of design until both GRD's are the same percentage of design. These GRD's are now in balance.

j) Adjust the damper serving the GRD with the next (third) lowest percentage of design until all three GRD's are at the same percentage of design, and in balance.

k) Continue this procedure until all remaining GRD's have been adjusted to be in balance at approximately the same percentage of design airflow.

l) If necessary, adjust the fan speed to set all GRD's at design airflow, ± 10%.

m) Re-measure all GRD's and record final values.

n) Mark all GRD's with felt markers, spray paint, or in some other manner that is permanent, so that adjustment may be restored if necessary.

Where a basic constant volume supply system has branch ducts, the procedure is:

o) Follow above steps a) through f) for the GRD's on each branch.

p) Compute the ratio of measured branch flow to design branch flow.

q) The damper serving the branch at the lowest percentage of design flow is not adjusted in this procedure.

r) Adjust the damper serving the branch with the next (second) lowest percentage of design until both branches are the same percentage of design. These branches are now in balance.

s) Adjust the damper serving the branch with the next (third) lowest percentage of design until all three branches are at the same percentage of design, and in balance.

t) Continue this procedure until all remaining branches have been adjusted to be in balance at approximately the same percentage of design airflow.

u) If necessary, adjust the fan speed to set all branches at design airflow, ± 10%.

v) Perform the proportioning techniques specified in above steps a) through m) for the diffusers on each branch.

w) Re-measure all GRD's and record final airflow values.

x) Mark all dampers, with felt markers, spray paint, or other permanent technique, so that the adjustment may be restored if necessary.

Stepwise method

This technique is initially described for a basic constant volume supply system without branch ducts.

It is also appropriate for exhaust or return duct systems.

a) Verify that all GRD dampers are wide open.

b) Set air outlet deflections as specified.

c) Determine total system volume by the most appropriate method.

d) Calculate the percentage of actual airflow to design airflow.

e) Adjust the fan to approximately 110% of design airflow if possible.

f) Measure the airflow at all GRD's.

g) Starting at the fan, as the GRD's closest to the fan will typically be the highest, adjust the GRD volume dampers to a value approximately 10% below design airflow requirements.

h) As the adjustment proceeds to the end of the system, the remaining GRD airflow values will increase.

i) Repeat the adjustment passes through the system until all GRD's are within ± 10% of design airflow requirements and at least one GRD volume damper is wide open.

j) If necessary, adjust the fan speed to set all GRD's at design airflow, ± 10%.

k) Re-measure all diffusers and record final airflow values.

l) Mark all dampers, with felt markers, spray paint, or other permanent technique, so that adjustment may be restored if necessary.

Where a basic constant volume supply system has branch ducts, the procedure is:

m) Follow above steps a) through e) for the GRD’s on each branch.

n) Compute the ratio of measured branch flow to design branch flow.

o) Starting at the fan, as the branches closest to the fan will typically be the highest, adjust the branch volume dampers to a value approximately 10% below design airflow requirements.

p) As the adjustment proceeds to the end of the system, the remaining branch airflow values will increase.

q) If necessary, adjust the fan speed to set all branches at design airflow, ± 10%.

r) Balance the GRD's on each branch as described in steps e) through i) above

s) Re-measure all GRD's and record final values.

t) Mark all dampers with felt markers, spray paint, or other permanent technique, so that adjustment may be restored if necessary.

Basic air system balancing

The following systemS are identified for air system balancing procedure:

  1. Fan Total Airflow
  2. Constant Volume Supply Systems
  3. Multizone Systems
  4. Induction Unit Systems
  5. Variable Volume System
  6. Dual Duct Systems
  7. Variable Volume Terminal Unit
  8. Underfloor Plenum Supply Air Systems
  9. Return Air Systems
  10. Exhaust Air Systems

Units

  1. Laboratory Fume Hoods
  2. Biosafety Cabinets
  3. Industrial Exhaust Hoods and Equipment
  4. Building Static Control Methods
  5. Stairwell Pressurization Testing
  6. Elevator Pressurization Testing

Basic Hydronic System Balancing Procedures

  1. Proportional Balancing Method (Ratio Method)
  2. Stepwise Balancing Method
  3. Systems With Self Adjusting Valves

Proportional balancing method (ratio method)

The Proportional Balancing Method initially is described for a hydronic system without branch circuits:

a) Verify that all balancing, control, and isolation valves are wide open.

b) Determine total system volume by the most appropriate method.

c) Calculate the percentage of actual hydronic flow to design flow requirements.

d) Adjust the pump to approximately 110% of design flow, if possible.

e) Measure the flow at all balancing valves.

f) Compute the ratio of measured flow to design flow for each terminal unit.

g) The balancing valve serving the terminal unit at the lowest percentage of design flow is not adjusted in this procedure.

h) Adjust the balancing valve serving the terminal unit with the next (second) lowest percentage of design until both terminal units are the same percentage of design. These terminal units are now in balance.

i) Adjust the balancing valve serving the terminal unit with the next (third) lowest percentage of design until all three terminal units are at the same percentage of design, and in balance.

j) Continue this procedure until all remaining terminals have been adjusted to be in balance at approximately the same percentage of design flow.

k) If necessary, adjust the pump volume to set all terminals at design flow ±10%.

l) Re-measure all terminal units and record final values.

m) Mark or set all memory stops on all of the balancing valves so that the adjustment may be restored if necessary.

Where a hydronic system has branch circuits with branch balancing valves, the proportional balancing procedure is:

n) Follow above steps a) through f) for the terminals on each branch.

o) Compute the ratio of measured branch flow to design branch flow.

p) The balancing valve serving the branch at the lowest percentage of design flow is not adjusted in this procedure.

q) Adjust the balancing valve serving the branch with the next (second) lowest percentage of design until both branches are the same percentage of design and in balance.

r) Adjust the balancing valve serving the branch with the next (third) lowest percentage of design until all three branches are at the same percentage of design, and in balance.

s) Continue this procedure until all remaining branches have been adjusted to be in balance at approximately the same percentage of design flow.

t) If necessary, adjust the pump volume to set all branches at design flow, ±10%.

u) Perform the proportioning techniques specified in above steps a) through m) for the terminal units on each branch.

v) Re-measure all terminal units and record final values.

w) Mark or set all memory stops on all of the balancing valves so that the adjustment may be restored if necessary.

Stepwise balancing method

The Stepwise Method initially is described for a hydronic system without branch circuits:

a) Verify that all balancing, control, and isolation valves are wide open.

b) Determine total system volume by the most appropriate method.

c) Calculate the percentage of actual hydronic flow to design hydronic flow.

d) Adjust the pump volume to approximately 110% of design flow if possible.

e) Measure the flow at all balancing valves.

f) Starting at the pump, as the terminal units closest to the pump will typically be the highest, adjust the balancing valves to a value approximately 10% below design flow requirements.

g) As the adjustment proceeds to the end of the system the remaining terminal unit flow values will increase.

h) Repeat the adjustment passes through the system until all terminal units are within ±10% of design flow requirements and at least one balancing valve is wide open.

i) If necessary, adjust the pump volume to set all terminal units at design flow, ±10%.

j) Re-measure all terminal units and record final values.

k) Mark or set all memory stops on all of the balancing valves so that the adjustment may be restored if necessary.

Where a hydronic system has branch circuits with branch balancing valves, the Stepwise procedure is:

l) Follow above steps a) through e) above for the terminal units on each branch.

m) Compute the ratio of measured branch flow to design branch flow.

n) Starting at the pump, as the branches closest to the pump will typically be the highest, adjust

the branch balancing valves to a value approximately 10% below design requirements.

o) As the adjustment proceeds to the end of the system the remaining branch flow values will increase.

p) If necessary, adjust the pump volume to set all branches at design flow, +/- 10%.

q) Balance the terminal units on each branch as described in above steps e) through i) above.

r) Re-measure all terminal units and record final values.

s) Mark or set all memory stops on all of the balancing valves so that the adjustment may be restored if necessary.

Systems with self adjusting valves

a) Verify that all balancing, control, and isolation valves are wide open.

b) Determine total system flow by the most appropriate method.

c) Calculate the percentage of actual hydronic flow to design hydronic flow.

d) Measure the differential pressure at each self adjusting balancing valve

Basic hydronic system balancing

The following system are identified for hydronic system balancing procedure

  1. Preliminary System Procedures
  2. Hydronic System Measurement Methods
  3. Basic Hydronic System Procedures
  4. Hydronic System Balancing Procedures
    1. Basic Procedures
    2. Bypass Valves
    3. Variable Volume Hydronic Systems
    4. Primary – Secondary Hydronic Systems
  5. Balancing Specific Systems
    1. Cooling Tower (Condenser Water) Systems
    2. Chilled Water Systems
    3. Heat Exchangers and Boiler Systems
    4. Heat Transfer Components

Outdoor Air Ventilation Procedures

Direct measurement method

The preferred method of outdoor air measurement is direct, which may include but is not limited to, Pitot tube traverse, velocity averaging grid, and airflow measuring station. When direct measurement of the outdoor air path is not an option, then a Pitot tube traverse of the total supply minus the total return air quantities is deemed acceptable.

Mixed air temperature method

The mixed air temperature method may be used for setting outdoor air dampers to yield the specified percentage of outdoor air. Quite often, the mixed air temperature is very difficult to measure accurately. With regard to this method, it is important to note that air stratification within HVAC units may inhibit accurate airflow temperature measurement, which may prevent its use. Mixed air temperatures may vary considerably depending on where the readings are taken. If it is determined that air stratification is present, it will be necessary to take several temperature readings by performing a weighted average temperature traverse. This can be a time consuming process and a quick reading digital thermometer may speed up the process. Accurate readings and large differentials between outdoor air and return air temperatures [over 20°F (12°C) t] are essential to this method.

Sample pre-tab-Checklist

This is a sample checklist to be provided by the TAB Firm to the installing contractor for the contractor’s use in verifying system readiness prior to balancing.

Table 1
Minimum Instrument required for AIR-TAB-SI Units
Table 2
Minimum Instrument required for Hydronic TAB-SI Units

Generalized Balancing For Air Distribution Systems

Before Starting, obtain up-to-date plans, drawing and or shop drawings of the complete mechanical system. Compare installed equipment to design and check for completeness of installation. Obtain the manufactures’ outlet factors and recommended procedure for testing air outlets.

Pre-balance Equipment and System Check

Equipment Check

  1. Check fan housings, coils, louvers, etc., to ensure they are clean and free of foreign material.
  2. Check filters to ensure that they are clean and in place.
  3. Check adjustment of vibration eliminators.
  4. Examine drives for proper belt tension and alignment.
  5. Check fan and motor lubrication.
  6. Check fan overload proctors or heaters for proper size – check motor amperage to guard against overload.
  7. Check automatic dampers for proper operation and position.
  8. Check fan for proper rotation.

System Check

  1. Check for installation of all required balance dampers.
  2. Turn off the air handler unit (AHU).
  3. Set all system dampers in their open position. This includes all volume dampers, fire dampers, outlet dampers, etc.
  4. Turn on the AHU. (Caution: Check fan amperages, in some cases the AHU motor may be overloaded when the system is turned on when all system dampers are opened).
  5. Check for air leaks at the fan and the system ductwork.
  6. Position all doors and windows to their normal position.
  7. Check air temperature to ensure required air temperature delivery.

Air Handling Equipment Balance

  1. Check motor amperage and voltage to ensure motor is not being overloaded.
  2. Set minimum outdoor air quality using the temperature ratio method.
    Tm = (% O.A.)(To) + (% R.A.)(Tr)
    % O.A. = 100 (Tm-Tr) / (To-Tr)
    % R.A. = 100 (To-Tm) / (To-Tr)
    Xo = Percent outdoor air
    Xr = Percent return air
    To = Outside air temperature
    Tr = Return air temperature
    Tm = Mixed air temperature
  3. Determine the volume of the air being delivered by the fan. Adjust the fan speed to increase or decrease the flow if required. If the speed is increased, ensure the motor is not overloaded. Check total flow with the dampers set to their minimum outside air and again for 100 percent outside air; variation should be within 10 percent.
  4. Check fan motor speed, operating amperage and voltage. Calculate break horsepower.
  5. Take fan static pressure readings and static pressure across the fan system components; i.e., filters, coils, etc.

System Balancing

  1. Using the duct velocity traverse method, adjust the volume dampers to deliver the design airflow in each main, zone, and branch duct
  2. Adjust the individual supply outlets both for air volume and distribution pattern. Follow the manufactures’ recommended procedure, using the proper factor. Use the proportional (ratio) method or other appropriate systematic procedure for outlet balancing. Compare the outlet total flow to the duct traverse previously made; variation should be within 10 percent.
  3. Using the same basic procedures for the supply side, balance the return and exhaust systems.
  4. Recheck speed, amperage and pressure readings at the fan.
  5. Submit Reports.

Duct Air Leakage

If duct work is not properly sealed, the air will leak out of it as it flows from the HVAC unit to the HVAC registers. These systems become largely inefficient, driving up energy costs significantly. When you compute duct air leakage, you will find out whether the HVAC system is inefficient, and you will be able to correct the problem easily by sealing the duct. The few dollars spent in caulking the duct air may save you hundreds in energy costs.

Instruments needed:

  • Pressure gauge
  • Static pressure tip
  • Drill
  • Air balancing hood
  • Calculator
  1. Calculate the airflow of the HVAC unit's blower. Cubic feet per minute (CFM) is the value used to measure airflow. Drill a hole in the supply air duct right above the HVAC unit and insert the static pressure tip of the pressure gauge. Record the reading that your pressure gauge displays. This is the static pressure of the duct work. Static pressure is measured in inches of water column.
  2. Pull out the engineering fan curve data sheet that came with your HVAC unit and use it to figure out the CFM of your blower fan. The static pressure value that you calculated will directly match up with the CFM of your blower fan.
  3. Take your air balancing hood and measure the air flow out of each of your supply air registers. Add up each of these values to get the total CFM moving from your HVAC unit to your supply air registers.
  4. Subtract the total air making it to the registers, which you calculated in Step 3, from your blower fan's calculated output from Step 2. For example, if your blower fan output was 1800 CFM and you measured 1700 CFM at the registers, 100 CFM would be your total leakage in the duct system.

Typical Balancing of Variable-Air-Volume System

A variable-air-volume (VAV) air-conditioning system varies the volume of constant-temperature air that is supplied to meet the changing load conditions of the space.

To understand further about VAV A/C system, first let us explain what is the Constant Volume Variable Temperature System.

Constant Volume (CV), Variable Air Temperature System

This system delivers a constant volume of air to the space and, to maintain the required space temperature at all load (full load or Part load) conditions, varies the temperature of this air. In this example, the temperature of the air is varied by controlling the capacity of the central cooling coil.

Constant volume - Full Load

Since the supply air from the AHU is only at 12.8°C and the required constant volume, varying temperature at the space cooling, require either

Therefore, as the space sensible load drops from 40,000 Btu/hr to 20,000 Btu/hr(11724 W to 5862 W), this system modulates the temperature of the constant volume 1840 cfm (0.873 m3/s) supply air from 55°F to 65°F (12.8°C to 18.3°C)

The refrigeration energy savings are realized at part load conditions, but the humidity level suffers due to the warmer air supply conditions in the space.

How much humidity in the space or room? Calculate using Psychrometric chart.

The terminal reheat system uses a central air handler and cooling coil to deliver cool primary air to all the spaces. Each space has its own heating coil to temper the air to satisfy the space load. Of course, any heat added to meet the part-load requirements of a space becomes a cooling load that the refrigeration system must overcome. This can result in a nearly constant refrigeration load, even when the building is at part-load conditions.

Therefore, reheating cooled air to achieve part-load supply air temperature control is not very energy efficient and is used only in special constant-volume applications, or when there is a “free” source of heat (i.e. heat recovery).

A variable-air-volume (VAV) system delivers the primary air at a constant temperature and varies the airflow to maintain the required space temperature at all load conditions.

Reference:

  1. National Environment Balancing Beuro, USA
  2. National Science Foundation, USA
  3. TRANE Balancing of air distribution system

APPENDIX F


Practical Exercises

CHAPTER 2 - PSYCHROMETRY

EXERCISE 1

On a particular day the weather forecast states that the dry bulb temperature is 37oC, while the relative humidity is 50% and the barometric pressure is 101.325 kPa. Find the humidity ratio, and enthalpy of moist air on this day.

EXERCISE 2

Moist air at 1 atm. pressure has a dry bulb temperature of 32oC and a wet bulb temperature of 26oC. Calculate using perfect gas law model and psychrometric equations.

a) The partial pressure of water vapour,

b) Humidity ratio,

c) Relative humidity,

d) Dew point temperature,

e) Density of dry air in the mixture,

f) Density of water vapour in the mixture

g) Enthalpy of moist air

EXERCISE 3

A large warehouse located at an altitude of 1500 m has to be maintained at a DBT of 27oC and a relative humidity of 50% using a direct evaporative cooling system. The outdoor conditions are 33oC (DBT) and 15oC (WBT). The cooling load on the warehouse is 352 kW. A supply fan located in the downstream of the evaporative cooler adds 15 kW of heat. Find the required mass flow rate of air. Assume the process in evaporative cooler to follow a constant WBT.

EXERCISE 4

A winter air conditioning system maintains a building at 21oC and 40% RH. The outdoor conditions are 0oC (DBT) and 100% RH. The sensible load on the building is 100 kW, while the latent heating load is 25 kW. In the air conditioning system, 50% of the outdoor air (by mass) is mixed with 50% of the room air. The mixed air is heated in a pre-heater to 25oC and then required amount of dry saturated steam at 1 atm. pressure is added to the pre-heated air in a humidifier. The humidified air is then heated to supply temperature of 45oC and is then supplied to the room.

Find:

a) The required mass flow rate of supply air,

b) Required amount of steam to be added, and

c) Required heat input in pre-heater and re-heater. Barometric pressure = 1atm.

EXERCISE 5

A split A/C unit performance is represented in the following process; the AC unit delivers 600L/s of air to the room.

Point (1) represents air entering the unit and (2) leaving the unit.

If (1) = 30°C dbt, (2) = 15°C dbt; moisture in air at (1) = 18.0 g/kg at (2) = 9.0 g/kg

Find capacity of the A/C unit by way of:

  1. Sensible capacity
  2. Total capacity

EXERCISE 6

Calculate the moisture content of 1kg of dry air at 20°C mixed with saturated steam for barometric pressures of (a) 101.325 kPa and (b) 95 kPa.

EXERCISE 7

Calculate the approximate enthalpy of moist air at a dry bulb temperature of -10°C, 50 per cent saturation and a barometric pressure of 101.325 kPa. Use psychrometric tables or a psychrometric chart to establish the moisture content.

EXERCISE 8

Moist air at a state of 60°C dry-bulb, 32.1°C wet-bulb (sling) and 101.325 kPa barometric pressure mixes adiabatically with moist air at 5°C dry-bulb, 0.5°C wet-bulb (sling) and 101.325 kPa barometric pressure. If the masses of dry air are 3 kg and 2 kg, respectively, calculate the moisture content, enthalpy and dry-bulb temperature of the mixture.

CHAPTER 3 - COMFORT AIR-CONDITIONING

EXERCISE 1

A ducted ventilation system is needed to remove stale, moist air from the changing room of a village hall after footballers have showered. The recommended ventilation rate is 15 to 20 air changes per hour, which, for a room volume 41 m3, gives a minimum target air flow of 615 m3/h. To keep the mean air velocity in the fan duct below the preferred figure of 8 m/s the duct diameter must satisfy:

Unfortunately, this is larger than the recommended value for extraction terminals, so the flow needs to be split between two separate extraction terminals, and then joined by a ‘Y’-piece just prior to the fan.

Plan and evaluate a ventilation duct with two extraction branches

CHAPTER 4 - HEATING & COOLING LOAD CALCULATIONS

EXERCISE 1

A winter air conditioning system maintains a building at 21oC and 40% RH. The outdoor conditions are 0oC (DBT) and 100% RH. The sensible load on the building is 100 kW, while the latent heating load is 25 kW. In the air conditioning system, 50% of the outdoor air (by mass) is mixed with 50% of the room air. The mixed air is heated in a pre-heater to 25oC and then required amount of dry saturated steam at 1 atm. pressure is added to the pre-heated air in a humidifier. The humidified air is then heated to supply temperature of 45oC and is then supplied to the room.

Find:

a) The required mass flow rate of supply air,

b) Required amount of steam to be added, and

c) Required heat input in pre-heater and re-heater. Barometric pressure = 1atm.

EXERCISE 2

A building has a U-value of 0.5 W/m2.K and a total exposed surface area of 384 m2 The building is subjected to an external load (only sensible) of 2 kW and an internal load of 1.2 kW (sensible). If the required internal temperature is 25oC, state whether a cooling system is required or a heating system is required when the external temperature is 3oC. How the results will change, if the U-value of the building is reduced to 0.36 W/m.K?

EXERCISE 3

An air conditioned room that stands on a well ventilated basement measures 3 m wide, 3 m high and 6 m deep. One of the two 3 m walls faces west and contains a double glazed glass window of size 1.5 m by 1.5 m, mounted flush with the wall with no external shading. There are no heat gains through the walls other than the one facing west. Calculate the sensible, latent and total heat gains on the room, room sensible heat factor from the following information. What is the required cooling capacity?

Inside conditions : 25oC dry bulb, 50 percent RH

Outside conditions : 43oC dry bulb, 24oC wet bulb

U-value for wall : 1.78 W/m2.K

U-value for roof : 1.316 W/m2.K

U-value for floor : 1.2 W/m2.K

Effective Temp. Difference (ETD) for wall: 25oC

Effective Temp. Difference (ETD) for roof: 30oC

U-value for glass ; 3.12 W/m2.K

Solar Heat Gain (SHG) of glass ; 300 W/m2

Internal Shading Coefficient (SC) of glass: 0.86

Occupancy : 4 (90 W sensible heat/person)

(40 W latent heat/person)

Lighting load : 33 W/m2 of floor area

Appliance load : 600 W (Sensible) + 300 W(latent)

Infiltration : 0.5 Air Changes per Hour

Barometric pressure : 101 kPa

EXERCISE 4

A school classroom is 6 m long, 6 m wide and 3 m high. There is a 2.5 m x 4 m window in the east wall. Only the east wall/window is exterior. Assume the thermal conditions in adjacent spaces (west, south, and north, above and below) are the same as those of the classroom. Determine the cooling load at 9:00 am, 12:00 noon on July 21.

Other known conditions include:

Latitude = 40o N

Ground reflectance = 0.2

Clear sky with a clearness number = 1.0

Overall window heat transmission coefficient = 7.0 W/m2K

Room dry-bulb temperature = 25.5°C

Permissible temperature exceeded = 2.5%

Schedule of occupancy: 20 people enter at 8:00 am and stay for 8 hours

Lighting schedule: 300 W on at 8:00 AM for 8 hours

Exterior wall structure:

• Outside surface, A0

• Face brick (100 mm), A2

• Insulation (50 mm), B3

• Concrete block (100 mm), C3

• Inside surface, E0

Exterior window:

• Single glazing, 3 mm

• No exterior shading, SC = 1.0

CHAPTER 5 - HVAC SYSTEMS

PUMP DESIGN

PUMP AFFINITY LAWS

  1. Flow (Volume) varies directly as the change in speed or diameter of the Impeller
  2. Head varies as the square of the change in speed or diameter of the impeleer
  3. Brake power varies as the cube of the change in speed or diameter of the impeller

PUMP EQUATION (Based on affinity laws)

FLOW Q2 / Q1 = RPM2 / RPM1 = D2 / D1

HEAD H2 / H1 =(Q2 / Q1)2 = (RPM2 / RPM1 )2= (D2 / D1)2

POWER BHP2 / BHP1 = H2 / H1 =(Q2 / Q1)3 = (RPM2 / RPM1 )3= (D2 / D1)3

Where Q = Flow ( gpm or L/s)

RPM = Revolutions per minute

D = Impeller diameter (Inches or mm)(

H = Head ( ft.wg, or In.wg, or Pa)

BHP = Brake power ( HP or KW)

EXERCISE 1

(English unit)

A pump is required with 7.5” impeller to produce a flow of 360 gpm at 36 ft.wg and 5 BHP. The existing pump is delivering only 247 gpm at 48 ft.wg and 4.2 BHP

Calculate the required head,(H) Impeller size (D) and BHP to obtain full flow

(Metric unit)

A pump is required with 190 mm impeller to produce a flow of 22.7 L/s at 108kPa and 3.8 KW BHP. The existing pump is delivering only 15.6 L/s at 144 kPa and 3.15 KW BHP

Calculate the required head,(H) Impeller size (D) and BHP to obtain full flow

EXERCISE 2

(English unit)

A pump is delivering 450 gpm of water at a 90 ft. head. Calculate the motor brake horse power.

EXERCISE 3

(English unit)

A pump installed as shown in the fig. below.

Data given:

Pump flow = 200 gpm

Impeller diameter = 6 inches

Pump speed = 3500

Suction pipe diameter = 4 inches (4.026 in ID)

Water temperature = 60 deg F

Calculate NPSHA and compare with NPSHR.

Pump Schematic-1

EXERCISE 4

Pump capacity = 100 gpm

Temperature of water = 60°F

Positive suction head = 5 ft

Tank open to atmosphere = 14.7 psi

Static discharge head = 10 ft

Discharge pipe size = 2” + 90 deg.elbow

Net positive suction head required NPSHR = 9 ft

Calculate the Net positive suction head available (NPSHA)

Pump Schematic-2

EXERCISE 5

This exercise shows the pumping from a tank under vacuum.

Data given:

Pump capacity = 100 gpm

Temperature of water = 60°F

Positive suction head = 5 ft

Tank gauge pressure = - 20” Hg

Static discharge head = 10 ft

Discharge pipe size = 2” + 90 deg.elbow

Net positive suction head required NPSHR = 9 ft

Calculate the Net positive suction head available ( NPSHA)

Pump Schematic-3

PIPING DESIGN

EXERCISE 1

The piping system shown is a typical example for calculating the pressures at various points.

Data given:

All pipes are same diameters.

Pressure at point 1 20 psig

Pump flow rate 100 gpm (6.3 L/s)

Head loss between 1 & 2 (l f 1-2) 0

Head loss between 2 & 3 (l f 2-3) 20 ft (6m)

Head loss between 3 & 4 (l f 3-4) 15 ft (4.6m)

Pump head (HP) 80 ft (24m)

Calculate the pressures at point 2,3 and 4

The energy equation is used to determine the pressures at various points in the system

Piping design-1 Layout

EXERCISE 2

Size the pipe for the open cooling tower circuit shown in the figure below. What is the head requirement of the pump?

Data given :

Water flow rate = 475 gpm

Total equivalent length of pipes and fittings = 656 feet

Pressure loss in the condenser coil = 5 psi

Cv value for the strainer = 300

Piping Design-2-Layout

From the pipe friction chart shown below, we can compute the pipe size for handling 475 gpm.

Pipe friction chart

EXERCISE 3

A chiller piping circuit with piping layout shown in figure below.

The Pipes are commercial steel material schedule 40 and all the elbows and fittings are as shown. Additional data is given in table below: Size the piping for the layout. Assume that fittings are as shown. Specify the pump requirements.

Typical Chiller piping layout

Additional Data given

Unit Flow rate (gpm) Head (ft) 3-way valve CV
Coil a 30 15 12
Coil b 40 12 18
Coil c 50 10 24
Chiller 120 20 --

CHAPTER 7 - DUCT DESIGN, AIRFLOW AND ITS DISTRIBUTION

EXERCISE 1

Compute the equivalent lengths for the fittings in the duct system as shown in the figure below.

The fittings are (1) an entrance, (2) a 45 deg. WYE, (3) a straight through section of the WYE fitting, (4) a 45 deg. Elbow, and (5) a 90 deg. elbow

Flow rate = 400 cfm (0.19 m3/sec.)

EXERCISE 2

The figure below shows a branch of a duct system detailed out of a complete air-distribution system. The total pressure available in the plenum is 0.13 in.w.g. However, 0.04 in.w.g is required by the diffuser at the end of duct section 3. The pressure available for airflow in the duct system is 0.09 in.w.g.

Size the duct system using round steel duct.

Perimeter type Low Velocity Duct Layout

EXERCISE 3

Design the duct system shown in the figure below.

The system is supplied air by a roof top unit that develops 0.25 in.wg total pressure external to the unit. The return air system requires 0.10 in.wg. The duct construction shall be round and the maximum velocity is 850 fpm in the main run and 650 fpm in branch runs.

Compute the total pressure loss for the system

Size the ducts using the (a) Equal friction method (b) balance-capacity method

Duct Design 3-Layout

EXERCISE 4

Size the duct system shown in the fig. below using the Balanced-Capacity method.

The total pressure available for the duct system is 0.12 in.w.g (30 Pa) , and the loss in total pressure at each diffuser at the specified flow rate is 0.02 in.w.g (5Pa). Plenum flow rate is 500 cfm with a velocity of 650 fpm.

Duct Design 4-Layout

EXERCISE 5

High velocity duct design

Design the duct system shown in the figure below using static regain method.

Each outlet has a terminal box that requires a minimum of 0.5 in.wg total pressure. The plenum entrance section is assumed to be of a smooth converging bell-mouth with end wall.

High Velocity Duct Design layout

CHAPTER 9 - FANS AND BLOWERS (Air-Conditioning equipment)

EXERCISE 1

A duct system require a fan with 9 m3/s of air at 1.2 kPa total pressure. Estimate the speed, shaft power and total efficiency of the fan. Assume the fan is a forward-curved blade type.

EXERCISE 2

A centrifugal fan is delivering 1700 cfm (0.8 m3/s) of air at a total pressure differential across the fan of 1.4 in.wg (350 Pa).. The fan has an outlet area of 0.71 ft2 (0.07 m2) and requires 0.7 hp (0.5 kw) shaft output. The fan operating at 1200 rpm.

Compute (a) The total power (b) The static efficiency (c) The total efficiency, and (d) the fan static pressure.

If the fan speed is increased to 1500 RPM, Compute (a) the capacity, (b) the static and total pressure, and (c) the shaft power at the higher speed.

EXERCISE 3

A Single Wheel Single Inlet (SWSI) backward curved blade fan is operating with both inlet and outlet duct elbows. The outlet duct elbow is in position C and located one duct diameter from the fan outlet. The average velocity in the duct is 4000 ft/min ( 20 m/s). The fan inlet is configured as shown in fig with a duct length ratio of 2 and R/H of 0.75. Calculate the system effect factor.

EXERCISE 4

A SWSI , backward-curved blade fan discharges air into a 0.3 m x 0.45 m rectangular duct at the rate of 2 m3/sec. An elbow located 0.3 m from the fan outlet directs the air flow up. Estimate the system effect factor for the elbow.

The same fan have an inlet duct & elbow as per fig. 1.14.4 type C. The diameter is 0.4 m and the duct length is 0.7 m and R =0.26 m. Calculate the system effect factor of inlet elbow.


APPENDIX G


Practical Exercises - Answers

CHAPTER 2 - PSYCHROMETRY

EXERCISE 1

Answer:

At 37oC the saturation pressure (pS) of water vapour is obtained from steam tables as 6.2795 kPa.

Since the relative humidity is 50%, the vapour pressure of water in air (pWV) is:

PWV= 0.5 x pSaturation = 0.5 x 6.2795 = 3.13975 kPa

The humidity ratio W is given by:

W = 0.622 x pWV/ (pBarpWV) = 0.622 x 3.13975/(101.325?3.13975) = 0.01989 kgw/kgda

The enthalpy of air (h) is given by the equation:

h = 1.006 t + W (1.84 t + 2501)

h = 1.006 x 37 + 0.01989 (1.84 x 37 + 2501)

h = 37.222 + 0.01989 (2569.08)

h = 37.222 + 51.09= 88.312 kJ/Kgda

EXERCISE 2

Answer:

a) Using modified Apjohn equation and the values of DBT, WBT and barometric pressure, the vapor pressure is found to be:

Where,

pV = Vapor pressure, Pa

pv’ = Saturation vapor pressure at wet-bulb temperature, Pa

p = Atmospheric pressure, bar

t = Dry-bulb temperature, oC

t’ = Wet-bulb temperature, oC

pv = 2.956 kPa

b) The humidity ratio W is given by:

W = 0.622 x 2.956/(101.325-2.956) = 0.0187 kgw/kgda

c) Relative humidity RH is given by:

RH = (pV/pS) x 100 = (pV / saturation pressure at 32oC) x 100

From steam tables, the saturation pressure of water at 32oC is 4.7552 kPa, hence,

RH = (2.956 / 4.7552) x 100 = 62.16%

d) Dew point temperature is the saturation temperature of steam at 2.956 kPa. Hence using steam tables we find that:

DPT = TSat (2.956 kPa) = 23.8oC

e) Density of dry air and water vapor

Applying perfect gas law to dry air:

Density of dry air ρS =(pa/RaT)=(pt?pV)/RaT= (101.325?2.956)/(287.035 x 305)x103

= 1.1236 kg/m3 of dry air

f) Similarly the density of water vapor in air is obtained using perfect gas law as:

Density of water vapor ρv = (pV /RV T) = 2.956 x 103/(461.52 x 305) = 0.021 kg/m3

g) Enthalpy of moist air is found from the equation:

h = 1.006 x t + W (2501+1.84 x t)

= 1.006 x 32 + 0.0187(2501+1.84 X 32)

= 80.06 kJ/kg of dry air

EXERCISE 3

Answer:

At 1500m, the barometric pressure is equal to 84.436 kPa. Inlet conditions to the evaporative cooling system are the outdoor conditions:

DBT = 33oC, WBT = 15oC

At these conditions and a barometric pressure of 84.436 kPa, the enthalpy of outdoor air is obtained using psychrometric equations as:

ho= 46.67 kJ/kgda

The above system is shown on the psychrometric chart in the figure above.

Assuming the evaporative process to follow a constant WBT and hence nearly a constant enthalpy line,

ho= ho’= 46.67 kJ/kgda

Applying energy balance for the sensible heating process in the fan (process o’-s) and heating and humidification process through the conditioned space (process s-i), we obtain:

mS(hS – h) = 15 = sensible heat added due to fan (1)

mS(hi– hS) = 352 = cooling load on the room (2)

From psychrometric equations, for the inside condition of the warehouse (DBT=27oC and RH = 50%), the enthalpy hi is found from psychrometric equations as:

hi = 61.38 kJ/kgda

We have two unknowns (mS and hS) and two equations (1 and.2), hence solving the equations simultaneously yields:

mS = 24.94 kJ/kg

hS = 47.27 kJ/kgda

EXERCISE 4

Answer:

From the psychrometric chart the following properties are obtained:

Outdoor conditions: 0oC (DBT) and 100% RH

Wo= 0.00377 kgw/kgda, ho = 9.439 kJ/kgda

Indoor conditions: 21oC (DBT) and 40% RH

Wi = 0.00617 kgw/kgda, hi = 36.66 kJ/kgda

Since equal amounts of outdoor and indoor air are mixed:

tm = 10.5oC, Wm = 0.00497 kgw/kgda, hm = 23.05 kJ/kgda

From sensible energy balance across the room (Process s-i) in the figure below:

a) Required mass flow rate of supply air is:

mS= QS / {cpm (t S – t i) } = 100 / {1.0216 (45 – 21) } = 4.08 kg/s

From latent energy balance for process s-i, the humidity ratio of supply air is found to be:

WS= Wi + Ql / (hfg . mS) = 0.00617 + 25/(2501 x 4.08) = 0.00862 kgw/kgda

b) Required amount of steam to be added m W is obtained from mass balance across the humidifier (process r-h) as:

mw= mS(WS – Wm ) = 4.08 x (0.00862 – 0.00497) = 0.0149 kg/s

c) Heat input to the pre-heater (process m-r) is obtained as:

Qph= mS .Cpm (tr– tm ) = 60.44 kW

Heat input to the re-heater (process h-s) is obtained as:

Qrh= mS .Cpm (tS – tr) = 83.36 kW

In the above example, it is assumed that during addition of steam, the dry bulb temperature of air remains constant. A simple check by using energy balance across the humidifier shows that this assumption is valid.

EXERCISE 5

Answer:

EXERCISE 6

Answer:

(a) By psychrometric tables, the saturation vapor pressure, pWV, is 2.337 kPa.

Hence, using equation W = 0.622 x pWV/ (pBar?pWV)

W = 0.622 x 2.337 / (101.325 – 2.337)

W = 0.01468 kg/kgda

(b) Similarly, because the saturation vapour pressure is independent of barometric pressure pss is still 2.337 kPa,

Hence, using equation W = 0.622 x pWV/ (pBar?pWV)

W = 0.622 x 2.337 / (95.000 – 2.337)

W = 0.01569 kg/kgda

It is often convenient to express moisture content as g/kg dry air. The two above answers would then be 14.68 g/kg and 15.69 g/kg dry air.

EXERCISE 7

Answer:

From tables (or less accurately from a chart)

W = 0.000 804 kg per kg dry air

Using the equation:

h = 1.005 × (–10) + 0.000 804 (2501 = 1.84 × (–10))

= –8.054 kJ per kg dry air

Psychrometric tables quote –8.060 kJ per kg dry air

As in the case of specific volume, the general principle followed by ASHRAE (1997), for determining the enthalpy of moist air is to add to the enthalpy of dry air, ha. This is expressed by the following equation:

Where μ is the percentage saturation

EXERCISE 8

Answer:

From tables of psychrometric data:

W1 = 18.400 g per kg dry air

W2 = 2.061 g per kg dry air

h1 = 108.40 kJ per kg dry air

h2 = 10.20 kJ per kg dry air

The principle of the conservation of mass demands that:

CHAPTER 3 - COMFORT AIR-CONDITIONING

EXERCISE 1

Answer:

With this arrangement, the flow in each branch is half that in the main duct, reducing the associated pressure drops by a factor four. These are indicated by asterisks in the following table, which gives the pressure coefficients for all the components in the system:

Component Pressure drop (Pa) for 1 m length and flow 1000 m3/h in main duct Pressure drop (Pa) for flow 1000 m3/h in main duct
Extract terminal *   156 *
90° bend *   5 *
1.5 m straight duct * 2.5 * 4 *
‘Y’-piece   19
0.5 m straight duct 10 5
90° bend   21
Exhaust terminal   66
Total   276

Pressure coefficients for components of a ventilation duct.

So, for a flow of 615 m3/h , the total pressure drop is 276 x (0.615)2 = 104 Pa.

The performance chart below [1, p.109] shows that the point (615, 104) is inside the performance envelope of the recommended ACM200 fan, which can therefore deliver the target flow.

Performance of Vent-Axia ACM200 mixed-flow fan (with operating curve for duct system)

To determine what actual ventilation can be achieved, write for the normalised flow , so that

CHAPTER 4 - HEATING & COOLING LOAD CALCULATIONS

EXERCISE 1

Answer:

From the psychrometric chart the following properties are obtained:

Outdoor conditions: 0oC (DBT) and 100% RH

Wo= 0.00377 kgw/kgda, ho = 9.439 kJ/kgda

Indoor conditions: 21oC (DBT) and 40% RH

Wi = 0.00617 kgw/kgda, hi = 36.66 kJ/kgda

Since equal amounts of outdoor and indoor air are mixed:

tm = 10.5oC, Wm = 0.00497 kgw/kgda, hm = 23.05 kJ/kgda

From sensible energy balance across the room (Process s-i) in the figure below:

a) Required mass flow rate of supply air is:

mS= QS / {cpm (t S – t i) } = 100 / {1.0216 (45 – 21) } = 4.08 kg/s

From latent energy balance for process s-i, the humidity ratio of supply air is found to be:

WS= Wi + Ql / (hfg . mS) = 0.00617 + 25/(2501 x 4.08) = 0.00862 kgw/kgda

b) Required amount of steam to be added m W is obtained from mass balance across the humidifier (process r-h) as:

mw= mS(WS – Wm ) = 4.08 x (0.00862 – 0.00497) = 0.0149 kg/s

c) Heat input to the pre-heater (process m-r) is obtained as:

Qph= mS .Cpm (tr– tm ) = 60.44 kW

Heat input to the re-heater (process h-s) is obtained as:

Qrh= mS .Cpm (tS – tr) = 83.36 kW

In the above example, it is assumed that during addition of steam, the dry bulb temperature of air remains constant. A simple check by using energy balance across the humidifier shows that this assumption is valid.

EXERCISE 2

EXERCISE 3

Answer:

From the psychrometric chart:

For the inside conditions of 25oC dry bulb, 50 percent RH:

Wi = 9,9167 x 10-3 kgw/kgda

For the outside conditions of 43oC dry bulb, 24oC wet bulb:

Wo = 0.0107 kgw/kgda, density of dry air = 1.095 kg/m3

External loads:

a) Heat transfer rate through the walls: Since only west wall measuring 3m x 3m with a glass windows of 1.5m x 1.5m is exposed; the heat transfer rate through this wall is given by:

Qwall = UwallAwallETDwall = 1.78 x (9-2.25) x 25 = 300.38 W (Sensible)

b) Heat transfer rate through roof:

Qroof = UroofAroofETDroof = 1.316 x 18 x 30 = 710.6 W (Sensible)

c) Heat transfer rate through floor: Since the room stands on a well-ventilated basement, we can assume the conditions in the basement to be same as that of the outside (i.e., 43oC dry bulb and 24oC wet bulb), since the floor is not exposed to solar radiation, the driving temperature difference for the roof is the temperature difference between the outdoor and indoor, hence:

Qfloor = UfloorAfloorETDfloor = 1.2 x 18 x 18 = 388.8 W (Sensible)

d) Heat transfer rate through glass: This consists of the radiative as well as conductive components. Since no information is available on the value of CLF, it is taken as 1.0. Hence the total heat transfer rate through the glass window is given by:

Qglass = Aglass [Uglass(To?Ti)+SHGFmaxSC] = 2.25[3.12 x 18 + 300 x 0.86] = 706.9 W (Sensible)

e) Heat transfer due to infiltration: The infiltration rate is 0.5 ACH, converting this into mass flow rate, the infiltration rate in kg/s is given by:

minf = density of air x (ACH x volume of the room)/3600 = 1.095 x (0.5 x 3x3x6)/3600

minf = 8.2125 x 10-3 kg/s

Sensible heat transfer rate due to infiltration,Qs,inf;

Qs,inf = minfcpm(To?Ti) = 8.2125 x 10-3 x 1021.6 x (43 – 25) = 151 W (Sensible)

Latent heat transfer rate due to infiltration, Ql,inf:

Ql,inf = minfhfg(Wo?Wi) = 8.8125x10-3 x 2501x103(0.0107?0.0099)=16.4 W (sensible)

Internal loads:

a) Load due to occupants: The sensible and latent load due to occupants are:

Qs,occ = no.of occupants x SHG = 4 x 90 = 360 W

Ql,occ = no.of occupants x LHG = 4 x 40 = 160 W

b) Load due to lighting: Assuming a CLF value of 1.0, the load due to lighting is:

Qlights = 33 x floor area = 33 x 18 = 594 W (Sensible)

c) Load due to appliance:

Qs,app = 600 W (Sensible)

Ql,app = 300 W (Latent)

Total sensible and latent loads are obtained by summing-up all the sensible and latent load components (both external as well as internal) as:

Qs,total = 300.38+710.6+388.8+706.9+151+360+594+600 = 3811.68 W (Ans.)

Ql,total = 16.4+160+300 = 476.4 W (Ans.)

Total load on the building is:

Qtotal = Qs,total + Ql,total = 3811.68 + 476.4 = 4288.08 W (Ans.)

Room Sensible Heat Factor (RSHF) is given by:

RSHF = Qs,total/Qtotal = 3811.68/4288.08 = 0.889 (Ans.)

To calculate the required cooling capacity, one has to know the losses in return air ducts. Ventilation may be neglected as the infiltration can take care of the small ventilation requirement. Hence using a safety factor of 1.25, the required cooling capacity is:

Required cooling capacity = 4288.08 x 1.25 = 5360.1 W 1.5 TR (Ans.)

EXERCISE 4

Answer:

Cooling Load due to Exterior wall:

U =1/R = 0.643 W/m2 K

From Table A28-33A, find wall type 13.

From Table A28-32, CLTD9:00 = 9 and CLTD12:00 = 14 for east wall.

CLTD Corrected = CLTD + (25.5 - Ti) + (Tm - 29.4)

Ti = inside temperature

Tm = mean outdoor temperature

Tm = (maximum outdoor temperature) - (daily range)/2

At 9:00 am CLTD Corrected = 9 + (25.5 - 25.5) + (31 -9/2 - 29.4) = 6.1 K

At 12:00 am CLTD Corrected = 14 + (25.5 - 25.5) + (31 -9/2 - 29.4) = 11.1 K

Q = U A (CLTD)

Q = 0.643 (W/m2 K) x (6 x 3 - 4 x 2.5) m2 x 6.1 = 34 W (at 9 am)

Q = 0.643 (W/m2 K) x (6 x 3 - 4 x 2.5) m2 x 11.1 = 62 W (at 12 noon)

Window conduction:

From Table A28-34, CLTD9:00 = 1 and CLTD12:00 = 5

CLTD Corrected = CLTD + (25.5 - Ti) + (Tm - 29.4)

CLTD corrected = 1 + 0 + (31 - 9/2 - 29.4) = -1.9 K (at 9 am)

CLTD Corrected = 5 + 0 + (31 - 9/2 - 29.4) = 2.1 K (at 12 noon)

Q = U A (CLTD)

Q = 7.0 W/m2 K x 4 x 2.5 m2 x (-1.9 K) = -133 W (at 9 am)

Q = 7.0 W/m2 K x 4 x 2.5 m2 x 2.1 K = 147 W (at 12 noon)

Solar load through glass:

From Table A28-35B, find zone type is A

From Table A28-36, find CLF = 576 at 9 am and CLF = 211 at 12 noon

Q = A (SC) (SCL)

Q = (2.5 x 4 m2) x 1.0 x 576 = 5760 W (at 9 am)

Q = (2.5 x 4 m2) x 1.0 x 211 =2110 W (at 12 noon)

Cooling load from partitions, ceiling, floors: Q = 0

People:

From Table A28-35B, find zone type is B

From Table A28-3, find sensible/latent heat gain = 70 W

From Table A28-37, find CLF9:00 (1) = 0.65 and CLF12:00 (3) =0.85

Q Sensible = N (sensible heat gain) CLF

Q Sensible = 20 x 70 W x 0.65 = 910 W (at 9 am)

Q Sensible = 20 x 70 x 0.85 = 1190 W (at 12 noon)

Q Latent = N (latent heat gain)

Q Latent = 20 x 45 = 900 W (at 9 am and 12 noon)

Lighting:

From Table A28-35B, find zone type is B

From Table A28-38, find CLF9:00 (1) = 0.75 and CLF12:00 (3) = 0.93

Q Lighting = W Ful Fsa (CLF)

Q Lighting = 300 W x 1 x 1 x 0.75 = 225 W (at 9 am)

Q Lighting = 300 x 1 x 1 x 0.93 = 279 W (at 12 noon)

Appliances:

Q Sensible = 0

Q Latent = 0

Infiltration:

Since room air pressure is positive, we have:

Q Sensible = _0

Q Latent = 0

Total cooling load:

CHAPTER 5 - HVAC SYSTEMS

EXERCISE 1

EXERCISE 2

EXERCISE 3

Answer:

Available net positive suction head NPSHA

(Ps x gc )/ ρ g +( V2 /2 g) – (Pv x gc)/ ρ g

Where Ps gc / ρ g = Static head at the pump inlet, ft or m absolute

V2 / 2g = Velocity head at the pump inlet, ft or m

Pv gc / ρ g = Static vapor pressure head of the liquid at the pumping temperature, ft or m, absolute

The net positive suction head available must always be greater than the NPSHR or noise and cavitation will result.

Pump Schematic-1

The standard barometric pressure acting on the water level in the tank, which is equivalent to:

Pressure Head = 29.92 in Hg = 29.92 x 1.13 ft H2O = 33.80 ft H2O

NPSHA = (Pressure Head) - (Static Suction Head – Vapor pressure Head – Friction Head)

= (33.80) – (10 – 0.59 – 5) = 33.80 – 15.59 = 18.21 ft of water

= 18.21 ft of water is greater than NPSHR, therefore no cavitation will occur.

Note : Here static vapor pressure head at the pumping temperature is calculated based on 60 deg.F, equal to 0.2562 psi

If the water temperature is increased to 160 deg.F, then vapor pressure becomes

(4.74 x 144) / 61 = 11.2

Then, NPSHA = (33.80) – ( 10-11.2-5) = 7.6 ft which is less than NPSHR, therefore there will be cavitation.

EXERCISE 4

Friction Loss Chart / 100 ft water pipe

EXERCISE 5

PIPING DESIGN

EXERCISE 1

EXERCISE 2

EXERCISE 3

Answer:

Fig.1.8.2
Resistance coefficient chart

The Head loss for elbows, globe valves, gate valves, and control valves are summarized in the following table 1.8.3. The values given in the table are for single components only. If more than one component exists in a pipe section, the head loss must be multiplied by the number of identical components. The head loss for tee is considered separately.

Table 1.8.3
Summary-Head loss values for fittings and valves
Section gpm Size lf (ft/100 ft) L (ft) lf (ft)
1-2 120 3 3.8 33 1.25
2-3 70 2 ½ 3.5 10 0.35
2-4 50 2 4.6 28 1.29
3-4 40 2 3.2 18 0.58
3-5 30 1 ½ 6.0 28 1.68
4-5 90 2 ½ 5.5 10 0.55
5-1 120 3 3.8 35 1.33
Table 1.8.4
Head loss in fittings-1
Fig.1.8.3
Head loss for Tee

The head loss associated with water flowing through the branch will be calculated using a 2-inch standard tee with a flow rate of 50 gpm. The head loss associated with water flowing through the run (straight through) will be calculated using a 2 ½ inch standard tee with a flow rate of 70 gpm. The same approach will be used when flow rates converge in a tee such as between point 4 and 5.

LOOP ANALYSIS

Fig.1.8.1
Typical Chiller piping layout
Table 1.8.5
Head Loss in fittings-2

CHAPTER 7 - DUCT DESIGN, AIRFLOW AND ITS DISTRIBUTION

EXERCISE 1

Answer:

(1) Entrance—A conical converging bell mouth with end wall, round and rectangular

Table 1
Total pressure loss coefficients for Duct Entrances

Rectangular : D = 2HW / (H+W)
Co
L/D θ =0° 10° 20° 30° 40° 60° 100° 140° 180°
0.025 0.50 0.47 0.45 0.43 0.41 0.40 0.42 0.45 0.5
0.05 0.50 0.45 0.41 0.36 0.33 0.30 0.35 0.42 0.5
0.075 0.50 0.42 0.35 0.30 0.26 0.23 0.30 0.40 0.5
0.10 0.50 0.39 0.32 0.25 0.22 0.18 0.27 0.38 0.5
0.15 0.50 0.37 0.27 0.20 0.16 0.15 0.25 0.37 0.5
0.60 0.50 0.27 0.18 0.13 0.11 0.12 0.23 0.36 0.5

In this case, the entrance bell mouth is a straight opening without any elbow, the angle θ is taken as zero and assuming L = 1, and D is given as 10 in.,

Then, L/D ratio = 1/10 = 0.10

Referring to table 1, the L/D ratio on the left hand side of the table, the loss coefficient C0 = 0.5

Table 2
Friction factors for various Galvanized Steel ducts
Diameter Darcy Friction Factor
Inches Millimeter Friction factor
4 100 0.035
6 150 0.028
8 200 0.023
10 250 0.022
12 300 0.019
14 360 0.017
16 400 0.016
20 500 0.015
24 600 0.014

Referring to table 2, the friction factor for 10” dia duct is 0.022

(2) A 45 degree, WYE fitting ( Diverging Wye, round 45 degree)

45 degree Elbow

(3) A straight through section of the WYE

Considering the line 1-a-2,

Qs/Qc = 280 cfm/400 cfm = 0.7 and (As/Ac)2 = (9/10)2 = 0.81

Then, Cs = 0.13 from table 3 and 0.0225 for 9” duct dia.

Ls/D = 0.13/0.0225 = 5.8

Ls = 5.8 (9”/12) = 4.4 ft (English unit)

Ls = 5.8 (0.225m) = 1.35 m (Metric unit)

(4) A 45 deg. elbow

A 45 degree elbow will have about one-half the equivalent length of a 90 degree elbow. See calculation below for 90 degree elbow = 3.85 ft = 1.17 m

(5) A 90 deg. elbow

Assume that the 90 degree elbow is the pleated type with a r/D ratio of 1.5. Then the loss coefficient is 0.43 from table below:

Pleated Elmows

Table 4
Pleated Elbow-90 degrees and 45 degrees
Co at D, in(mm)
Angle 4 (100) 6 (150) 8 (200) 10 (250) 12 (300) 14 (350) 16 (400)
90 0.57 0.43 0.34 0.28 0.26 0.25 0.25
60 0.45 0.34 0.27 0.23 0.20 0.19 0.19
45 0.34 0.26 0.21 0.17 0.16 0.15 0.15

Then L90e/D = 0.43 / 0.028 = 15.4

L90e = 15.4 ( 6” /12) = 7.7 ft ( English unit)

L90e= 15.4 (0.15m) = 2.31 m (Metric unit)

EXERCISE 2

Since we consider the Line 123 as main duct section, the net pressure available in this duct section

Plenum Pressure – Diffuser Pressure

0.13 in.wg – 0.04 in.wg = 0.09 in.wg

The above value taken as the design pressure available for air flow in Line 123

Pressure Loss = ΔP/L123 = 0.09 / 132 ft = 0.068 in.wg / 100ft

Table-1.2.1
Equivalent lengths,Le,ft at diameter inches shown

For sizes other than shown in table, calculate the equivalent length using:

(Le/D) ratio(given in tabe colum 2) x Dia.(ft)) = Le

This value is indicated in the fig 1.2.2 in Dotted line and this has been used to size all duct sections. Each duct section is sized based on the airflow rate in the section and requiring the pressure loss per 100 ft to be less than or equal to the design value. The actual pressure loss per 100 ft for each duct section is determined once the duct size has been established.

Modification of duct line 1-2-5

The line 1 is maintained as 10”, The line 2 is changed to 9” and line 5 is changed to 7”. This is necessitated to bring down the pressure loss per 100 ft less than the design value of 0.068 in.wg per 100 feet.

Fig.1.2.2
Pressure Loss Chart

EXERCISE 3

Answer:

Both the equal friction method and the balance-capacity method require the equivalent lengths of duct fittings, which are to be calculated first.

Line 1-2-3-4-9 is considered as Main Duct line

Line-1 (14”)(a) Entrance (b) 2 nos. 90 degree elbow

Line-2 (12”)(a) Diverging 45 deg Wye, through

Line-3 (7”)(a) Diverging 45 deg.Wye, through

Line-4 (9”)(a) Diverging 45 deg.Wye, through

Line-5 (6”)(a) Diverging 45 deg.Wye, branch (b) Elbow, 45 deg.pleated (c) Transition 90 deg.boot

Line-6 (9”)(a) Diverging 45 deg.Wye, branch (b) Elbow, 45 deg.pleated (c) Transition 90 deg.boot

Line-7 (8”)(a) Diverging 45 deg.Wye, branch (b) Elbow, 45 deg.pleated (c) Transition 90 deg.boot

Line-8 (6”)(a) Diverging 45 deg.Wye, branch (b) Elbow, 45 deg.pleated (c) Transition 90 deg.boot

Line-9 (7”)(a) Diverging 45 deg Wye, through (b) Elbow, 90 deg.pleated (c)Transition 90 deg.boot

Fig.1.3.2
Pressure Loss Chart

This design pressure loss is marked in fig.1.3.2 in dotted line. This line used to size all duct section.

As per fig.1.3.2

The flow rate lines cut the reference dotted line and from there , the nearest pipe size and duct velocity and pressure loss are identified.

Fig.1.3.3
Pressure Loss Chart

EXERCISE 4

Fig.1.4.2
Pressure Loss Chart
Fig.1.4.3
Pressure Loss Chart (Metric)

EXERCISE 5

Answer:

As we can see from figure on pressure loss per 100 ft, a maximum velocity of 2500 fpm is obtained for a duct flow rate of 2000 cfm with 12” duct size. We can also calculate the velocity pressure and the velocity rate based on 12” duct dia.

Fig.1.5.2
Bellmouth entrance
Table: 1.5.1
Smooth converging bell mouth with end wall
R/D Co R/D Co
0. 0.50 0.06 0.20
0.01 0.43 0.08 0.15
0.02 0.36 0.10 0.12
0.03 0.31 0.12 0.09
0.04 0.26 0.16 0.06
0.05 0.22 ≥ 0.20 0.03

Where D = Duct dia,inches

Co = Dynamic loss coefficient-Dimensionless

From the table1.5.1, smooth converging bell mouth with end wall, the loss coefficient is assumed as 0.03 and from the pressure loss per 100 ft of duct chart (ASHRAE), the flow rate 2000cfm, 2546 fpm, 12” plotted and 0.75 in.wg / 100 ft. pressure loss due to friction is obtained.

Fig.1.5.3
Tee Fitting

The total pressure requirement for the system is determined by the run with the maximum cumulative static pressure loss plus the velocity pressure in section 1.

ΔP0 = (ΔP)1237 + Pv1 = (0.392 + 0.405 = 0.797 in.wg

Where ΔP1237 = Σ (Pu – Pd) fron table. However, this does not include the pressure loss for the terminal box. In allowing for this pressure, remember that the total pressure just upstream of the box is equal to the velocity pressure. Section 7 has a velocity pressure of 0.052 in.wg. Because the terminal box requires at least 0.5 in.wg, the system total pressure requirement should be increased to:

ΔP0 = 0.797 + (0.5--0.052) = 1.245 in.wg

The system balance and total pressure required in the plenum may also be checked by summing the lost pressure P0 for each run plus the velocity pressure in the last section of each run.

For run 1-2-3-4

P0p = (0.387+0.136+0.110+0.087) + 0.075 = 0.795 in.wg.

To allow for the terminal box:

P0p = 0.795 + (0.5-0.075) = 1.22 in.wg

For run 1-2-3-7

P0p = 0.387 + 0.136 + 0.110 +0.112 = 0.797 in.wg

And allowing for the terminal box:

P0p = 0.797 + (0.5-0.052) = 1.245 in.wg

The same procedure yields P0p of 1.16 in.wg and 1.087 in.wg for runs 1-2-6 and 1-5 respectively. It is clear that the system is not perfectly balanced, because the loss in total pressure is unequal for the different runs. The balanced-Capacity method could be used to size the branches where a large imbalance may occur. Trial and error will still be required, due to the need to use loss coefficients for the fittings.

The required total pressure for this example is relatively low because of its small size. As high-velocity systems becomes larger, the maximum velocity may be increased according to previously discussed criteria, with a corresponding increase in required total pressure.

CHAPTER 9 - FANS AND BLOWERS (Air-Conditioning equipment)

EXERCISE 1

Answer:

As per the table, the volume required falls between 8.62 and 9.10 m3/s.

To calculate the velocity, we must know the outlet area of the fan.

Assume the outlet area as 0.479 m2 or look at the manufacturers catalogue for the outlet area

Q = A x V

For 8.62 m3/s.flow, the velocity = 8.62 / 0.479 = 18 m/s

For 9.10 m3/s flow, the velocity = 9.10 / 0.479 = 19 m/s

Table-1.12.1
Pressure-Capacity table for a forward curved blade fan

EXERCISE 2

Answer (1):

EXERCISE 3

Answer:

First let us calculate the fan inlet elbow system effect. The inlet duct elbow loss coefficient is given in table 1.14.1 and for the configuration as per fig.1.14.4

The average velocity in the duct is given as 4000 ft/min (20 m/s)

The inlet duct elbow loss coefficient for a duct length ratio of 2 and R/H value of 0.75 can be taken from the table and is equal to 1.2.

The effective duct length is calculated based on the duct velocity and is given in table 1.14.2 as a standard reference

The fan outlet velocity is 4000 ftm and therefore referring to table 1.14.2, the effective length is 4 duct diameter.

The blast area ratio for centrifugal backward-curved fan = 0.70.

As per table 1.14.4, for a blast area ratio and type c outlet elbow position at 25% effective duct length=1.00

Therefore Blast area ratio = Blast area (Ab ) / Outlet area (Ao)

L / Le = ¼ = 0.25 or 25%

Therefore ΔPod = 1.0 x 0.075 (4000/1097)2 = 1.00 in.w.g

Finally, the total pressure loss for inlet and outlet system effect is

ΔPo = ΔPod + ΔPoi

= 1.00 + 1.20 = 2.2 in.w.g

and this must be added to the calculated system total pressure to obtain the actual total pressure that the fan must produce. (See fan curve fig.1.14.2)

Fig.1.14.1
Fan Effective Duct Length
Fig.1.14.2
Fan Curve
Fig.1.14.3
Fan outlet duct position
Fig.1.14.4
Fan Inlet duct position

EXERCISE 4

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