Principles of heating ventilating and air conditioning  with worked examples

xi Contents Chapter 1 Introduction to Heating, Ventilation and Air Conditioning 1 1.2.1 HVAC system using air as the energy transport medium 4 1.2.2 HVAC system using water as the en

9562hc_9789814667760_tp.indd 1 9/10/15 3:42 PM

Tai ngay!!! Ban co the xoa dong chu nay!!!

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British Library Cataloguing-in-Publication Data

v

To my grandchildren

Emiko Chrisanthi, Sunil Hitoshi, Isabella Anjali, Amali Satomi, and Helina Maya

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vii

Preface

Courses in Heating, Ventilation and Air Conditioning (HVAC) are usually offered in departments of mechanical engineering, civil engineering, architecture and building science This book is written mainly with the interests of students and instructors in these departments

in mind However, a significant part of the contents may be used in courses such as, thermal systems and heat transfer, especially the worked examples Practicing engineers could use this book to clarify the fundamental principles behind various design procedures recommended

in professional handbooks

A number of professional societies like the American Society of

Heating, Refrigeration and Air Conditioning Engineers (ASHRAE)

publish comprehensive handbooks and design guides for use by HVAC engineers These handbooks are updated regularly to include the most recent design procedures, developed through sponsored research

projects

One of the main challenges for instructors in HVAC courses is to distill the materials available in professional handbooks, to a concise form to be included in regular undergraduate courses This is often a time

consuming task because the handbooks are intended for practicing

engineers This book tries to make the task easier for instructors by presenting the material in a directly useable format For students the contents should appear as extensions and applications of the material covered in basic courses on thermodynamics, heat transfer and fluid

mechanics

Every effort is made to include simple derivations for most of the

design parameters used in practice, without making the mathematical details unduly complicated For instance, in chapter 9 a simple one-dimensional thermal network approach is used to derive the fenestration design parameter called the ‘solar heat gain coefficient (SHGC)’ Likewise, in chapter 10 a lumped-capacity transient thermal model is

used to clarify the physical meaning of ‘the radiant time series (RTS)’, and its application in cooling load estimation

In chapter 9 a ‘vector approach’ is introduced to analyze complex three-dimensional geometrical design problems These situations are encountered in computing incident angles of solar beams on inclined surfaces, and in determining the effectiveness of shading devices like

overhangs

Included in this book are the most up-to-date empirical models

available in the ASHRAE Handbook - 2013 Fundamentals, that are

relevant for design In particular, in chapter 9, for computing the solar radiation absorption and transmission in building envelopes, the latest two-parameter model is used to estimate the ‘clear-sky radiation’ at

different locations

In design oriented courses such as HVAC, it is important for students

to understand the fundamentals behind the recommended design procedures Comprehensive worked examples provide an ideal means to present design concepts in a practically useful manner With this objective in mind, about 15 worked examples are included in each chapter, carefully chosen to expose students to diverse design situations

encountered in HVAC practice

Computations required in worked examples illustrating basic principles are performed using a calculator Worked examples involving more realistic design situations are done using MATLAB programs,

included in the book

At the end of each chapter there are additional problems for which numerical answers are provided The format of the worked examples and problems is a novel feature of this book For instructors, this should provide a useful source for problems to be included in courses, tutorials

and examinations

MATLAB programming is now taught routinely in most engineering and science courses Therefore a number of MATLAB codes, for solving HVAC design problems requiring extensive computations, are also included Computer codes are included for the following applications: (i) computation of psychrometric properties, (ii) design of cooling towers, (iii) design of wet-coil heat exchangers, (iv) computation of hourly diffuse and direct solar radiation intensities, (v) computation of sol-air

temperature, (vi) estimation of hourly cooling load due to people, lights, roofs, and walls, (vii) design of overhangs, and (viii) design of duct and

pipe systems

I wish to thank ASHRAE for granting permission to extract

representative design data from the ASHRAE Handbook - 2013

Fundamentals, for inclusion in this book

I was fortunate to have had the opportunity to teach a number of courses in refrigeration, air conditioning, and thermal systems at the Department of Mechanical Engineering, National University of Singapore (NUS) The notes developed for these courses provided the framework and much of the material for this book I am thankful to my colleagues in the energy and bio-thermal division at NUS, with whom I shared the teaching of these courses, for many valuable discussions on HVAC systems

I am thankful to Dr Raisul Islam for fruitful discussions on a number

of practical design aspects of chiller systems and their energy efficiency

I wish to thank Dr A H Jahangeer for providing valuable technical support on many occasions and Mr Mahipala D Fernando for fruitful discussions on heat pump systems

Thanks are due to my sons Duminda and Harindra, and my in-law Sindhu and Sophia, for their constant encouragement

daughters-Finally, my heartfelt thanks are given to my wife Kamani for her encouragement and generous support towards the completion of this project

Nihal E Wijeysundera

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xi

Contents

Chapter 1

Introduction to Heating, Ventilation and Air Conditioning 1

1.2.1 HVAC system using air as the energy transport medium 4

1.2.2 HVAC system using water as the energy transport medium 5

1.2.3 HVAC system using water and air as energy transport media 7

References 15

Chapter 2

2.7.2 Correlations for the heat transfer coefficient 32

2.8.5 Absorption, transmission and reflection 37

2.8.8 Radiation exchange between a black surface and a gray surface 39

2.8.10 Radiation exchange between a curved surface and a flat surface 42

Problems 59 References 62

Chapter 3

Refrigeration Cycles for Air Conditioning Applications 65

3.8 Vapor Compression Systems for Air Conditioning Applications 78

3.8.2 Central air conditioning systems using chilled water 79

Problems 114 References 117

Chapter 4

4.3.1 Relative humidity, humidity ratio and degree of saturation 121

4.3.4 Adiabatic saturation and wet-bulb temperature 126

4.4.2 Saturation curve and constant relative humidity lines 132

Problems 153 References 156

Chapter 5

Psychrometric Processes for Heating and Air Conditioning 159

5.4.3 Summer air conditioning systems with bypass paths 177

5.4.5 Air conditioning systems using evaporative cooling 179

5.5.2 Dual-duct multi-zone air conditioning systems 181

Problems 212 References 216

Chapter 6

Direct-Contact Transfer Processes and Equipment 217

6.3 Simplified Model for Simultaneous Heat and Mass Transfer 221

6.4.2 Efficiency and number of transfer units (NTU) 228

6.5.1 Analysis of cooling towers 230

6.5.2 Enthalpy potential based model for cooling towers 232

Problems 258 References 260

Chapter 7

7.2.8 Overall heat transfer coefficient for finned tubes 282

Problems 331 References 335 Appendix A7.1 - MATLAB Code for Design of Chilled Water Coils 335

Chapter 8

Steady Heat and Moisture Transfer Processes in Buildings 339

8.2 Steady Heat Transfer through Multi-Layered Structures 340

8.4.3 Heat transfer through surfaces at grade level 355

8.5 Infiltration in Buildings 355

Problems 388 References 392

Chapter 9

Solar Radiation Transfer Through Building Envelopes 395

9.2.3 Angle of incidence of beam radiation on a surface 400

9.2.4 Total radiation incident on an inclined surface 402

9.2.5 Clear-sky model of direct and diffuse solar radiation 404

9.4.2 Transmittance of a multi-layered fenestration 410

9.4.3 Radiation absorption in multi-layered fenestrations 411

Problems 443 References 446

Chapter 10

10.6.1 Heat balance method (HBM) 468

10.6.3 Application of the RTS method and the CTS method 475

Problems 507 References 513 Appendix A10.1 - MATLAB Code for Cooling Load due to People 514

Appendix A10.2 - MATLAB Code for Cooling Load due to Wall Conduction 515

Appendix A10.3 - MATLAB Code for Cooling Load due to Windows 519

Chapter 11

Problems 582 References 585 Appendix A11.1 - MATLAB Code for Pressure Loss in Circular Ducts 586

Appendix A11.2 - MATLAB Code for Equal Friction Design Method 587

Appendix A11.3 - MATLAB Code for Static Regain Design Method 588

Chapter 12

12.3.2 Dynamic head loss in fittings 595

Problems 625 References 630 Appendix A12.1 - MATLAB Code for Head Loss in Pipe-Sections 630

Chapter 13

Building Energy Estimating and Modeling Methods 633

13.4.3 Simulation of water-loop heat pump system (WLHPS) 654

Problems 685 References 690

Index 693

1

Chapter 1

Introduction to Heating, Ventilation

and Air Conditioning

1.1 An Overview of HVAC Systems

The commonly used acronym for heating, ventilation and air conditioning is HVAC In its broadest sense, HVAC encompasses the means, the processes, and the technology used to maintain an indoor space at a desired set of physical conditions Typically these conditions depend on the type of activity for which the space is used For instance,

if the space is the inside of a building occupied by people, the most pertinent conditions are the temperature, the humidity level, and the cleanliness of the air

The indoor conditions for a health facility like a hospital or a clinic would be similar, but they would have to be controlled within more stringent limits compared to those of an ordinary home In the case of a process plant, such as a food preparation facility or a paint-shop, the indoor conditions would have to satisfy those mandated by the industrial guidelines for these processes

The conditions of all indoor environments are dynamic because they are subject to various time varying inputs, some of which are predictable while others are random or accidental In the case of a residential building or a home, the indoor temperature, humidity, and cleanliness of the indoor air change due to a number of inputs which can be internally

or externally generated

The temperature difference between the indoor air and the ambient air causes heat to flow across the building envelope During winter, when the temperature outside is lower than the indoor temperature, heat is lost

to the outside This makes the inside air colder, and therefore uncomfortable for occupants The HVAC system needs to balance this

heat loss by supplying the necessary heat input, using an external energy source In addition, cold air leaking through any openings and cracks in the building envelope has to be heated to the indoor temperature by the HVAC system If the indoor air is too dry and therefore uncomfortable for the occupants, moisture has to be introduced artificially The total amount of energy supplied by the HVAC system per unit time, to maintain the space at the desired temperature and humidity, is referred to

as the winter heating load of the building

In the summer, the air outdoors is usually hotter and more humid than the typical indoor comfort conditions stipulated In this case the heat flow across the building envelope occurs in the opposite direction Moreover, the indoor air is heated indirectly by the solar radiation entering through the glass surfaces of the building envelope, such as, windows, glass doors, and skylights The transmitted solar radiation is first absorbed by the interior surfaces of the building like the walls, the floor, and other items, such as furniture This absorbed energy is later released to the indoor air when the latter surfaces get warmer

People occupying the building, the indoor lights, and appliances, such

as, computers and coffee makers, also release heat and moisture, which increases the temperature and humidity of the indoor air If comfortable indoor conditions are to be maintained steadily, then all the aforementioned heat and moisture flows have to be balanced by the HVAC system The amount of energy that needs to be removed by

HVAC system per unit time is called the cooling load of the building It

is interesting to note that in the winter, the energy inputs, like absorbed solar radiation, energy from people, lights and equipment, tend to heat

the indoor air, and thereby reduce the heating load of the building

In section 1.2 below we shall present a few optional designs of HVAC systems including some of the equipment used But first, it is

instructive to highlight the interactions between the conditioned space

and the HVAC system by referring to the conceptual diagram depicted in Fig 1.1

Fig 1.1 Conceptual diagram of HVAC system

Within the indoor conditioned space shown in Fig 1.1 energy is

released by people depending on the type of activities they are engaged

in People also add moisture to the air and thereby increase the humidity

of the air The artificial lights inside the space and equipment, such as, computers, fax machines, coffee makers, and others produce heat that flows into the air Solar radiation transmitted through the transparent sections of the enclosure, like the glass windows of a building, contributes indirectly to the heating of the air inside Sensible heat is gained or lost through walls of the enclosure due to the inside-to-outside temperature difference There may also be unwanted leakage of air and moisture through cracks and openings in the enclosure As a consequence of these energy flows the conditions of the air inside the enclosure, such as, the temperature and humidity, change continuously The conditions of the space are monitored by sensors located inside, and the data is transmitted to a controller This could be a simple thermostat in the case of a HVAC system serving a residence In the case

of a modern commercial building, the control system could be a computer based building management system (BMS)

The controller compares the actual conditions of the space with the desired conditions supplied to it by the designers of the HVAC system or its operators Based on the discrepancy between the actual and desired conditions, the controller activates the necessary hardware items of the HVAC system to reestablish the desired conditions within the space The HVAC system does this by either supplying or removing the appropriate amount of energy and moisture from the space Careful control of the inside conditions, at the desired values, contributes both to the comfort of

Solar

radiation

Lights People Equipment

Heat flow Air, moisture

flow

Controller

HVAC System

Desired conditions Actual

Conditions Conditioned

space

the occupants of the space and to the overall energy efficiency of the system

1.2 Some Optional Designs of HVAC Systems

We shall now consider a few optional designs of HVAC systems that are being used for building related applications However, these same designs could be modified for industrial and transportation applications where the requirements may be somewhat different

1.2.1 HVAC system using air as the energy transport medium

Shown schematically in Fig 1.2 is an HVAC system, commonly used for heating and cooling residences and small commercial buildings

Fig 1.2 Typical forced air heating and cooling system for homes

Air from the rooms or the conditioned spaces of the building is drawn by

a fan, through a filter, to be processed by the HVAC system In the heating mode of operation, the air passes over the tubes of a heat exchanger through which hot combustion gases flow in the opposite or

‘cross-flow’ direction The combustion process occurs in a chamber that

is usually supplied with a fuel, such as, natural gas The heated air then

Refrigeration

Unit

Furnace

Fan compartment

Air Cleaner

Gas Line

Humidifier Evaporator

Coil

Exhaust pipe

Conditioned air

Return air

Heat exchanger

Return Air duct

Room

flows through the supply duct network to the various spaces of the building If necessary, the moisture content of the air can be increased by activating the humidifier located in the return air duct

Typically, the temperature of the building is controlled by a thermostat located in one of the rooms If the temperature of the room exceeds the preset temperature the thermostat switches off the combustion process, thus shutting down the furnace The operation is reversed when the space temperature falls below the preset value

In the cooling mode of operation, the furnace is shut down, and the compressor of the refrigerator is switched on In most HVAC systems, the evaporator coil of the refrigerator is located in the supply air duct, as shown in Fig 1.2 The condensing unit of the refrigerator, including the compressor, is usually placed outdoors The air flowing over the finned tubes of the evaporator coil is cooled and dehumidified, and the condensate from the air is drained by gravity, to a sump in the plant room When the temperature of the space falls below the preset temperature, the thermostat switches off the compressor In this type of simple system the thermostat has to be manually set to either the heating

or cooling mode of operation, depending on the outdoor conditions

1.2.2 HVAC system using water as the energy transport medium

An HVAC system using water as the energy transfer medium is shown schematically in Fig 1.3

This system is equipped with a boiler, for producing hot water to be used in the heating mode of operation, and a water chiller, to be used in the cooling mode of operation The water chiller is essentially a refrigerator, where the evaporator coil is used to produce chilled water, typically at a temperature of about 3 to 5°C The system depicted in Fig

1.3 is called a two-pipe arrangement [2] As in the case of the HVAC system described in section 1.2.1, the all-water, two-pipe system either

operates in the heating mode or the cooling mode at any one time

In the heating mode of operation, the two header pipes that circulate water are connected to the boiler through valves A and B The hot water from the supply header pipe flows through heat exchanger tubes in the fan coil units Air from the room is circulated over the finned tubes by

means of a fan located inside the fan coil unit The flow rate of water through the fan coil is usually controlled by valves at the inlet, which could either be operated manually, or by means of a thermostat, to maintain the desired room temperature

Fig 1.3 All-water, two-pipe heating and cooling system

In the cooling mode of operation, the same header pipes are connected to the water chiller by repositioning the valves A and B The chilled water flowing through the fan coils, from the supply header pipe, cools and dehumidifies the room air circulated through them by the fans Any condensate produced within the fan coil units is piped out to a sink The two-pipe system shown in Fig 1.3 cannot handle simultaneous heating and cooling needs of different rooms If some rooms served by the system have high heat loads, and therefore require cooling, while others require heating, then the two-pipe system is not a suitable design option However, two-pipe systems are less complicated and require fewer pipes, fan coils, valves, and controls

A more versatile all-water system is the four-pipe system [2,4] It has

fan coil units which incorporate separate heating and cooling coils These

are connected to separate hot water and chilled water header pipes, similar to those in shown Fig 1.3 The two sets of header pipes, in turn, are connected separately to the boiler and the chiller When the boiler and the chiller are both operating, a fan coil unit can be rapidly switched between heating and cooling modes, by means of the valves at the inlets

of the heating and cooling coils Therefore these systems can provide heating to some rooms, while simultaneously providing cooling to other rooms of the building The disadvantage, however, is that they requires more pipes, heat exchangers, and controls

1.2.3 HVAC system using water and air as energy transport media

A central HVAC system using both air and water as energy transport media is shown schematically in Fig 1.4 Return air from the

conditioned space is drawn into the air handling unit (AHU) by the

return air fan As the air passes through the AHU a fraction of it is discharged to the outside ambient through the exhaust port EA, and replaced with an equal amount of fresh ambient air drawn through the inlet port OA, for hygienic reasons Dampers are used to control this process The mixture of return air and fresh air then passes through a filter before entering the cooling and dehumidifying coil

In the cooling mode of operation, the air passing over the cooling coil

is cooled and dehumidified, and the condensate produced is drained out from the AHU The supply air fan then distributes the cold air through the supply duct network to the conditioned space

Finally, the desired quantity of air is discharged to each conditioned

space or room through flexible ducts connected to ceiling diffusers in the

space

In the heating mode of operation, the cooling coil is inactive, while the air is heated by water flowing through the hot water coil, and hot air

is distributed to the different spaces as described above

The air handling units (AHU), the air distribution system, the heating and cooling coils, and the liquid distribution network are commonly

called secondary components of the HVAC system as indicated in Fig

1.4

Fig 1.4 Central heating and air conditioning system using air and water

The cooling coil receives chilled water pumped from the chiller, which is essentially a refrigerator, where the evaporator cools water to a temperature of about 3 to 6°C The heat rejected by the condenser of the refrigerator is carried away by cooling water, pumped through the tubes

of the condenser This cooling water finally discharges heat to the atmosphere in a cooling tower, before being circulated back to the

condenser, by the cooling tower pump

The heating coil of the AHU receives hot water pumped from a fired boiler The boiler and the chiller that convert fuel or electrical energy to heating and cooling effects respectively, are usually called the

fuel-primary components of the system Since the hot water and chilled water

circuits are independent, they could serve a number of separate AHUs, similar to that depicted in Fig 1.4, some supplying hot air, and others supplying cold air, at the same time

1.2.4 Packaged and unitary systems

The HVAC systems described above can be adopted to serve different arrangements of conditioned spaces in a building by designing the duct and pipe networks accordingly However, there are HVAC systems,

commonly called packaged systems [4], that incorporate a vapor

compression refrigeration unit, and a fuel-fired or electrical heating unit,

in a single compact package Such packaged systems are usually installed on flat roofs of commercial buildings, and connected through supply and return ducts to the conditioned space below

Smaller air conditioning units, designed to serve a single space like an

office room, are called unitary systems or window units These are

installed in a window or a wall opening, with the controls on the inside Room air is cooled and dehumidified by circulating it across the finned tube coils of the evaporator using a fan The condenser coil of the unit, facing the outside, is cooled by a fan blowing ambient air over it

1.2.5 Reversible heat pumps for heating and cooling

Fig 1.5 Reversible heat pump for heating and cooling

A reversible heat pump is an HVAC system that can be used for cooling

or heating an indoor space A simplified schematic diagram illustrating its principle of operation is shown in Fig 1.5 It is essentially a vapor compression refrigeration system, consisting of a compressor, a reversing

Compressor

Evaporator

Expansion valve

Condenser

Building wall

T am

(Condenser) (Evaporator)

Reversing valve

valve, an expansion valve, and two coils, which are able to function both

as the evaporator and the condenser of the system

During the cooling mode of operation, the coil located inside the conditioned space is the evaporator The evaporating refrigerant absorbs heat from indoor air, thus cooling the space The position of the reversing valve allows the compressor to suck refrigerant from the evaporator and deliver the compressed vapor to the condenser, located outdoors The refrigerant flowing through the condenser rejects heat to the ambient, and then passes through the expansion valve to enter the evaporator, thus completing the cycle

In the heating mode of operation, the outdoor coil is the evaporator, and refrigerant passing through it absorbs heat from outdoor ambient air The reversing valve is repositioned so that the compressor is now able to suck refrigerant from the outdoor evaporator, and deliver it to the condenser coil, located inside the space The condensing refrigerant releases heat to the indoor air, thus heating it

Reversible heat pump systems are being used to heat and cool homes and commercial buildings Several variations of the basic system, described above, are now available commercially In one of these,

commonly called ground-source heat pumps, the refrigerant in the

outdoor unit, shown in Fig.1.5, exchanges heat with a fluid circulating through a coil buried in the ground Compared to the ambient air temperature, the fluctuation of the ground temperature over the seasons

is much smaller, and therefore the variation of the performance of the heat pump is much less

Another heat pump system, more suitable for large buildings like

hotels, is called a water-loop heat pump Here the outdoor coils of the

individual heat pumps, located in different rooms, exchange heat with water passing through a common pipe-loop For the rooms requiring heating, the heat pumps absorb heat from the common water loop and transfer it to the rooms On the other hand, in the rooms being cooled, the heat pumps reject the heat absorbed from the room to the water-loop The water loop temperature is typically maintained between 18°C and 32°C A boiler is used to heat the water in the loop if the net heating demand becomes high, and a cooling tower is used to cool the water if the net cooling demand is high

1.3 Overview of HVAC Design Procedure

The design, installation and commissioning of an HVAC system requires the inputs of specialists from several different areas

Fig 1.6 Overview of HVAC design procedure

These include architects, civil engineers, HVAC-mechanical engineers, control engineers, equipment manufacturers and different contractors The contribution of these specialists and the interactions between them can be best illustrated by referring to the block diagram in Fig 1.6, which is an overview of the HVAC design process

The overall design and construction of a building depends on the intended purpose and use of the building Moreover, the location and the local weather conditions influence the design of the building Hence the historical data on the variation of the solar radiation intensity, the ambient temperature, the humidity, and the wind speed are important inputs to the design This data should also include the extreme weather conditions, such as, the highest and lowest ambient temperatures, and the highest wind speed at the location

Purpose and intended use of building

Location, Weather data , Extreme conditions

Building design Comfort and airquality criteria

Design heating and cooling loads

System selection and component sizing

Operating cost

Initial cost

Life -cycle -cost

People, Lights etc.

Solar radiation, Ambient temperature Building

Envelope

The orientation of the building, the types of construction material used, and the fraction of the building envelope area with transparent surfaces, are usually decided by architects and civil engineers Although the aforementioned inputs are not within the purview of the HVAC

engineer, they do affect the heating and cooling loads to a large extent

For example, the thermal properties and thicknesses of the construction materials used in the building envelope has a direct bearing on the

heating load of the building Moreover, the area of transparent surfaces

in the envelope, and their thermal and optical properties, determine the

contribution of the transmitted solar radiation to the cooling load The

materials used for items like doors and windows of the building, and the

quality of their installation, influence the air infiltration rate, which

affects the heating and cooling loads

The cooling load also depends on the energy and moisture release

rates by people occupying a building (see Fig 1.1), which is a function

of the type of activities they are engaged in, and their occupancy schedule in the building These energy inputs to the cooling load would

be very different for an office building, a sports hall, and a church

Typically, the first step in the design of an HVAC system is to determine the design heating and cooling loads, based on the extreme weather conditions expected for the location The heating and cooling loads are then used to select the appropriate system, and the sizes of the individual equipment As we discussed in section 1.2, there are usually a number of design configurations to choose from These include all-air systems (Fig 1.2), all-water systems (Fig 1.3), air–water systems (Fig 1.4), packaged-systems, and others

Once the type of HVAC system is selected, it is necessary to size the specific components that constitute the system These sizes depend on the specific design parameters of the component which satisfy the intended performance requirements For instance, for a chiller the

appropriate performance parameters are the cooling capacity and the

chilled water temperature range For a water circulating pump or an air circulating fan (see Fig 1.4), these are the pressure rise and the fluid flow rate

Once the performance parameters have been determined, manufacturer’s catalogues have to be consulted to select the appropriate

unit that satisfies the performance requirements The availability of units with similar capabilities from different equipment manufacturers makes this step of the design process somewhat complicated Although most available equipment may satisfy the performance requirements, their indices of merit, like the efficiency, the coefficient of performance (COP), and others, could be quite different More importantly, there could be considerable variation in the cost of the equipment offered by different manufacturers

For instance, the first cost of a chiller with a high COP will usually be higher than that of a low COP chiller However, the operating cost of the former chiller could be significantly lower than that of the latter An appropriate life-cycle-cost (LCC) analysis may be needed to resolve the conflicting trade-offs with the two chillers This situation could arise for most of the required equipment Therefore the final decision on the selection of an equipment has to be made by an iterative process, as indicated in Fig 1.6, where the LCC of owning and operating equipment from different manufacturers are compared

1.4 Aims and Organization of the Book

In the preceding sections we presented a brief overview of the aims of HVAC, a few examples of specific HVAC systems, and a summary of the design and component selection procedure for HVAC systems From this discussion it is clear that much of the fundamental knowledge required to understand the design and operation of HVAC systems is covered in basic courses in engineering These courses include: thermodynamics, heat transfer, mass transfer, fluid mechanics, and control engineering

A number of professional societies, like the American Society of

Heating, Refrigeration and Air Conditioning Engineers (ASHRAE)

publish comprehensive handbooks [1], and design guides, for use by HVAC engineers These handbooks are updated regularly, to include the most recent design procedures, usually developed through sponsored research projects One of the main challenges for students in HVAC courses is to distill the detailed content available in professional handbooks, to a concise form that could be easily understood and used

The main aim of this book is to present the information available in professional handbooks as extensions and applications of the material covered in basic engineering courses mentioned above

With the above goal in mind, we have tried to avoid presenting HVAC design procedures as simple recipes Wherever possible we have included simple derivations of the various design parameters used in practice, without making the mathematical details unduly complicated For instance, in chapter 9 we use a simple one-dimensional thermal network approach to derive the fenestration design parameter called the

‘solar heat gain coefficient (SHGC)’

Likewise, in chapter 10 we use a lumped capacity transient thermal model to clarify the physical meaning of ‘the radiation time series (RTS)’ and its application to cooling load estimation We have introduced the ‘vector approach’ to analyze complex three-dimensional geometrical design problems in chapter 9 These situations are encountered in computing incident angles of solar beams on inclined surfaces, and in determining the effectiveness of shading devices like overhangs

The book includes the most up-to-date empirical models available in

the ASHRAE Handbook - 2013 Fundamentals [1], where these are relevant for design In chapter 9, for computing the solar radiation

absorption and transmission in building envelopes, the latest parameter model is used to estimate the ‘clear-sky radiation’ at different locations

two-We have included in each chapter about 15 worked examples, carefully chosen to expose students to diverse design problems encountered in HVAC practice The examples designed to illustrate the application of basic physical principles are solved using a calculator The more comprehensive HVAC design examples are solved using the MATLAB codes included in Appendices in the different chapters Comments are included in the computer codes to clarify the main steps

of the computation procedure

Using these codes judiciously, students could explore realistic design scenarios without having to perform tedious hand calculations Computer codes are included for the following applications: (i) design of cooling towers, (ii) design of wet-coil heat exchangers using chilled water, (iii)

computation of hourly diffuse and direct solar radiation intensities, (iv) computation of sol-air temperature, (v) estimation of hourly cooling load due to people, lights, roofs, walls, (vi) design of overhangs, (vii) design

of air distribution systems, and (viii) design of water distribution systems

References

1 ASHRAE Handbook - 2013 Fundamentals, American Society of

Heating, Refrigeration and Air Conditioning Engineers, Atlanta,

2013

2 Kuehn, Thomas H., Ramsey, James W and Threlkeld, James L.,

Thermal Environmental Engineering, 3rd edition, Prentice-Hall,

Inc., New Jersey, 1998

3 Mitchell, John W and Braun, James E., Heating, Ventilation, and

Air Conditioning in Buildings, John Wiley and Sons, New York,

2013

4 Stoecker, Wilbert F and Jones, Jerold W., Refrigeration and Air

Conditioning, International Edition, McGraw-Hill Book Company,

London, 1982

Heat transfer is a well established engineering discipline on which the published literature is voluminous This literature includes numerous textbooks, handbooks, and research journals In most engineering courses heat transfer is covered as a separate subject Therefore the main purpose of the present chapter is to briefly review the relevant principles

of heat transfer to facilitate their application to air conditioning systems

in later chapters We have listed several text books [1-3] on heat transfer

in the section on references for the benefit of readers who wish to pursue heat transfer in greater detail

Heat is defined as the transfer of energy at the boundary between two systems due to a difference in temperature From this point of view there are two modes of heat transfer: conduction and radiation Heat transfer

by conduction is usually associated with solids where heat is transferred

from a region of higher temperature to a region of lower temperature by molecular vibrations Often, the solid as a whole, remains stationary while heat conduction takes place For example, the heat transfer through the wall of a metal pipe occurs by conduction

All solids, liquids and gases have a tendency to emit energy in the form of electromagnetic waves They also absorb such waves emanating from neighboring bodies For example, the ‘radiant energy’ emitted by a room heater is absorbed by the walls, the ceiling, the floor, and any other

items in the room This type of energy transfer is known as thermal

radiation

Fluids like liquids and gases can transport momentum by virtue of their mass and velocity Unlike the molecular vibrations in a solid, the flow of a fluid moves ‘packets’ of fluid physically from one place to another, thereby transporting energy with it This type of energy transfer,

due to fluid motion, is called convection In engineering practice it is

common to use the term convection broadly to describe heat transfer from a solid surface to a fluid moving over it

Fig 2.1 Modes of energy transfer for wall

The distinction between the different modes of heat transfer is best illustrated by referring to a practical situation where several different modes of heat transfer are involved simultaneously

Consider the external wall of a building, shown schematically in Fig 2.1, where the various heat transfer processes are indicated The outer exposed surface of the wall absorbs a portion of the solar radiation

Thermal radiation

to surroundings Thermal radiation

to room

falling on it As this surface gets warm heat flows through the wall by conduction Heat is also transferred from the warm outer surface to

ambient air due to convection, which is enhanced by wind impinging on

the wall Moreover, the warm surface exchanges thermal radiation with objects in its field of view, including part of the sky

As the inner surface of the wall gets warm due to conduction, it

transfers heat by convection to the air in the room The heat flow into a

thin air layer adjacent to the surface occurs by conduction This causes the temperature of the air layer to increase, which in turn, decreases its density The air circulation resulting from the density variation in the air

layer is called natural convection The inner surface of the wall also exchanges thermal radiation with surfaces inside the room, like the other

walls, the ceiling, and the floor

In general, a medium where heat transfer takes place has three dimensions Depending on the shape of the medium, the temperature inside could vary along all three dimensions Consider the wall, shown in Fig 2.1, whose height and the breadth, are much larger than the thickness Therefore the modes of heat transfer at different points on the surfaces are likely to be similar Moreover, around the central area of the wall, it is reasonable to assume that the temperature varies only along the thickness Therefore the heat flow through the central area of the wall is

called one-dimensional heat transfer However, at the edges and the

corners of the wall the heat flow may depend on all three directions, namely, along the height, the breadth and the thickness Hence, the heat

flow at these locations is called three-dimensional heat transfer

When the temperature distribution in a medium changes with time,

the heat transfer process is called transient heat transfer For example, as

the solar radiation falling on the wall, shown in Fig 2.1, becomes more intense, the temperature at different points inside the wall will increase The temperature distribution is then a function of the three spatial

dimensions as well as time

When the temperature distribution in a medium remains constant with

time, the heat flow in the medium is called steady-state heat transfer For

example, the heat flow to the surroundings from a well-insulated pipe

carrying steam may be treated as a steady-state heat transfer process

Fig 2.2 Steady heat conduction in a slab: (a) the slab, (b) temperature distribution

Consider the heat flow through the slab shown in Fig 2.2, the length and

breadth of which are much larger than the thickness Therefore the heat

flow in the slab may be treated as one-dimensional Also assume that the

heat transfer is steady and therefore the temperature distribution in the

slab is independent of time Under these conditions, the temperature

distribution in the slab may be expressed in the form, T = T(x) where x is

the distance along the thickness

The steady, one-dimensional heat transfer in the slab is governed by

Fourier’s law of heat conduction, which states that the rate of heat flow,

Q is proportional to the area normal to the direction of heat flow, A and

the temperature gradient across the thickness, L of the slab We can

express Fourier’s law in the mathematical form

ܳλ஺ሺ்೚ ି்ಽሻ

where T o and T L are temperatures at x = 0 and x = L respectively, as

indicated in Fig 2.2(b)

We introduce the constant of proportionality k, called the thermal

conductivity, to write the equation of heat conduction as

X L

A

The thermal conductivity of a given substance depends on its

microscopic structure and tends to vary somewhat with temperature

Metals, like copper and aluminum, have a high thermal conductivity, and

are therefore good conductors of heat On the other hand, materials, such

as cork and fiberglass, have a very low thermal conductivity, and are

therefore called thermal insulators These insulating materials are

applied on hot surfaces like the outside of steam and hot water pipes to

reduce the heat loss to the surroundings

We encounter numerous situations involving one-dimensional, steady

heat flow through multi-layered slabs when dealing with air conditioning

design situations In chapters 8 and 10 we shall consider the detailed

analysis of heat flow through actual walls and roofs of buildings

Here we develop a convenient method, based the thermal network

analogy, to solve steady, one-dimensional heat transfer problems

Rearranging Eq (2.2) we have

Compare Eq (2.3) with Eq (2.4), for the steady flow of electric

current through a conductor subject to a voltage difference, given by

Ohm’s law as

Since Eqs (2.3) and (2.4) have the same mathematical form we could

express Fourier’s law in a form analogous to Ohm’s law The heat flow

rate and the temperature difference are analogous to the electric current

and the voltage difference respectively The equivalent thermal

resistance of the slab is then defined as

Hence Eq (2.3) may be written in the form

ܶ௢െ ܶ௅ൌ ቀ௅

Following the practice in electrical circuit analysis we adopt the

equivalent thermal network element shown in Fig 2.3(b), which is

analogous to the electrical circuit element in Fig 2.3(a), to solve

one-dimensional heat transfer problems related to slabs

Fig 2.3 (a) Electrical circuit element, (b) Equivalent thermal network element

2.4.1 Thermal resistances in series

There are many air conditioning design applications where heat flow

occurs through a slab consisting of parallel layers of different materials

as shown schematically in Fig 2.4 For instance, the walls of buildings

usually have a layer of plaster or a siding on the outside The wall itself

could be made of brick or concrete blocks The inside could have a layer

of thermal insulation sandwiched between the wall and a finishing-layer

of gypsum board

Fig 2.4 Multi-layered slab and the corresponding thermal network

We now apply Eq (2.6) to each element of the thermal network

shown in Fig 2.4 to obtain the following equations:

We notice from Eq (2.10) that the total thermal resistance of the

composite slab is equal to the sum of the thermal resistances of the

individual layers

2.4.2 Overall heat transfer coefficient

The rate of heat flow through a series of parallel layers may also be

expressed in terms of the overall heat transfer coefficient, U For the slab

shown in Fig 2.4, this gives

2.4.3 Thermal resistances in parallel

There many applications of air conditioning where conduction heat flow

occurs through parallel paths An example is the common wall section,

shown in Fig 2.5(a), where slabs of thermal insulation material like

fiberglass are placed in cavities between vertical rectangular structural

members, usually called studs A portion of the heat entering through the

surface of the wall section flows through the insulation while the rest

flows through the studs These heat conduction paths are parallel and

therefore the thermal circuit representing overall heat flow includes two

parallel thermal resistors as shown in Fig 2.5(b)

Fig 2.5 (a) Heat flow through wall section, (b) Equivalent thermal network

The overall thermal resistance of two parallel resistors is given by

All heat conduction situations involve boundary conditions These, in

general, are the known or specified values of physical parameters at the

boundary of the medium through which heat conduction occurs We shall

use the heat transfer process in a slab to illustrate some of the more

common boundary conditions encountered in air conditioning

applications

Fig 2.6 Boundary conditions: (a) Specified heat flux, (b) Convection

(i) Specified wall surface temperature:

The simplest boundary condition is where the temperatures of the

external surfaces of the slab are specified For the multi-layered slab

shown in Fig 2.4 this would be the surface temperatures T1 and T4