Refrigeration and Air-Conditioning pdf

Refrigeration and

Air-Conditioning

Refrigeration: The process of removing heat.

Air-conditioning: A form of air treatment whereby temperature,

humidity, ventilation, and air cleanliness are all controlled within

limits determined by the requirements of the air conditioned

enclosure.

BS 5643: 1984

Refrigeration and

Air-Conditioning

Third edition

A. R. Trott and T. Welch

OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

Butterworth-Heinemann

Linacre House, Jordan Hill, Oxford OX2 8DP

225 Wildwood Avenue, Woburn, MA 01801-2041

A division of Reed Educational and Professional Publishing Ltd

A member of the Reed Elsevier plc group

First published by McGraw-Hill Book Company (UK) Ltd 1981

Second edition by Butterworths 1989

Third edition by Butterworth-Heinemann 2000

© Reed Educational and Professional Publishing Ltd 2000

All rights reserved. No part of this publication

may be reproduced in any material form (including

photocopying or storing in any medium by electronic

means and whether or not transiently or incidentally

to some other use of this publication) without the

written permission of the copyright holder except

in accordance with the provisions of the Copyright,

Designs and Patents Act 1988 or under the terms of a

licence issued by the Copyright Licensing Agency Ltd,

90 Tottenham Court Road, London, England W1P 9HE.

Applications for the copyright holder’s written permission

to reproduce any part of this publication should be

addressed to the publisher

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloguing in Publication Data

A catalogue record for this book is available from the Library of Congress

ISBN 0 7506 4219 X

Typeset in India at Replika Press Pvt Ltd, Delhi 110 040, India

Printed and bound in Great Britain

Contents

1 Fundamentals 1

2 The refrigeration cycle 14

3 Refrigerants 28

4 Compressors 36

5 Oil in refrigerant circuits 57

6 Condensers and water towers 63

7 Evaporators 83

8 Expansion valves 93

9 Controls and other circuit components 104

10 Selection and balancing of components 121

11 Materials. Construction. Site erection 131

12 Liquid chillers. Ice. Brines. Thermal storage 144

13 Packaged units 154

14 Refrigeration of foods. Cold storage practice 162

15 Cold store construction 170

16 Refrigeration in the food trades – meats and fish 188

17 Refrigeration for the dairy, brewing and soft drinks

industries 193

18 Refrigeration for fruit, vegetables and other foods 201

19 Food freezing. Freeze-drying 205

20 Refrigerated transport, handling and distribution 208

21 Refrigeration load estimation 214

22 Industrial uses of refrigeration 223

23 Air and water vapour mixtures 227

24 Air treatment cycles 240

25 Practical air treatment cycles 255

26 Air-conditioning load estimation 263

27 Air movement 273

28 Air-conditioning methods 297

29 Dehumidifiers and air drying 316

30 Heat pumps. Heat recovery 320

31 Control systems 324

32 Commissioning 333

33 Operation. Maintenance. Service. Fault-finding. Training 338

34 Efficiency and economy in operation 351

35 Catalogue selection 357

Appendix Units of measurement 367

References 369

Index 373

vi Contents

Preface

Refrigeration and its application is met in almost every branch of

industry, so that practitioners in other fields find that they have to

become aware of its principles, uses and limitations. This book aims

to introduce students and professionals in other disciplines to the

fundamentals of the subject, without involving the reader too deeply

in theory. The subject matter is laid out in logical order and covers

the main uses and types of equipment. In the ten years since the last

edition there have been major changes in the choice of refrigerants

due to environmental factors and an additional chapter is introduced

to reflect this. This issue is on-going and new developments will

appear over the next ten years. This issue has also affected servicing

and maintenance of refrigeration equipment and there is an increased

pressure to improve efficiency in the reduction of energy use. This

edition reflects these issues, whilst maintaining links with the past

for users of existing plant and systems. There have also been changes

in packaged air-conditioning equipment and this has been introduced

to the relevant sections. The book gives worked examples of many

practical applications and shows options that are available for the

solution of problems in mechanical cooling systems. It is not possible

for these pages to contain enough information to design a complete

refrigeration system. The design principles are outlined. Finally,

the author wishes to acknowledge help and guidance from colleagues

in the industry, in particular to Bitzer for the information on new

refrigerants.

T.C. Welch

October 1999

1 Fundamentals

1.1 Basic physics – temperature

The general temperature scale now in use is the Celsius scale, based

nominally on the melting point of ice at 0°C and the boiling point

of water at atmospheric pressure at 100°C. (By strict definition, the

triple point of ice is 0.01°C at a pressure of 6.1 mbar.) On the

Celsius scale, absolute zero is – 273.15°C.

In the study of refrigeration, the Kelvin or absolute temperature scale

is also used. This starts at absolute zero and has the same degree

intervals as the Celsius scale, so that ice melts at + 273.16 K and

water at atmospheric pressure boils at + 373.15 K.

1.2 Heat

Refrigeration is the process of removing heat, and the practical

application is to produce or maintain temperatures below the

ambient. The basic principles are those of thermodynamics, and

these principles as relevant to the general uses of refrigeration are

outlined in this opening chapter.

Heat is one of the many forms of energy and mainly arises from

chemical sources. The heat of a body is its thermal or internal

energy, and a change in this energy may show as a change of

temperature or a change between the solid, liquid and gaseous

states.

Matter may also have other forms of energy, potential or kinetic,

depending on pressure, position and movement. Enthalpy is the

sum of its internal energy and flow work and is given by:

H = u + Pv

In the process where there is steady flow, the factor Pv will not

2 Refrigeration and Air-Conditioning

change appreciably and the difference in enthalpy will be the quantity

of heat gained or lost.

Enthalpy may be expressed as a total above absolute zero, or any

other base which is convenient. Tabulated enthalpies found in

reference works are often shown above a base temperature of

– 40°C, since this is also – 40° on the old Fahrenheit scale. In any

calculation, this base condition should always be checked to avoid

the errors which will arise if two different bases are used.

If a change of enthalpy can be sensed as a change of temperature,

it is called sensible heat. This is expressed as specific heat capacity,

i.e. the change in enthalpy per degree of temperature change, in

kJ/(kg K). If there is no change of temperature but a change of

state (solid to liquid, liquid to gas, or vice versa) it is called latent

heat. This is expressed as kJ/kg but it varies with the boiling

temperature, and so is usually qualified by this condition. The

resulting total changes can be shown on a temperature–enthalpy

diagram (Figure 1.1).

Figure 1.1 Change of temperature (K) and state of water with enthalpy

373.15 K

273.16 K

Temperature

Latent

heat of

melting

Sensible heat of gas

Latent heat of boiling

Sensible heat of liquid

Sensible heat of soild

334 kJ 419 kJ 2257 kJ

Enthalpy

Example 1.1 For water, the latent heat of freezing is 334 kJ/kg and

the specific heat capacity averages 4.19 kJ/(kg K). The quantity of

heat to be removed from 1 kg of water at 30°C in order to turn it

into ice at 0°C is:

4.19(30 – 0) + 334 = 459.7 kJ

Example 1.2 If the latent heat of boiling water at 1.013 bar is 2257

kJ/kg, the quantity of heat which must be added to 1 kg of water at

30°C in order to boil it is:

Fundamentals 3

4.19(100 – 30) + 2257 = 2550.3 kJ

Example 1.3 The specific enthalpy of water at 80°C, taken from

0°C base, is 334.91 kJ/kg. What is the average specific heat capacity

through the range 0–80°C?

334.91/(80 – 0) = 4.186 kJ/(kg K)

1.3 Boiling point

The temperature at which a liquid boils is not constant, but varies

with the pressure. Thus, while the boiling point of water is commonly

taken as 100°C, this is only true at a pressure of one standard

atmosphere (1.013 bar) and, by varying the pressure, the boiling

point can be changed (Table 1.1). This pressure–temperature

property can be shown graphically (see Figure 1.2).

Figure 1.2 Change of state with pressure and temperature Pressure Solid

Triple

point

Gas

Critical

temperature

Liquid

Temperature

Boiling point curve

Table 1.1

Pressure (bar) Boiling point (°C)

0.006 0

0.04 29

0.08 41.5

0.2 60.1

0.5 81.4

1.013 100.0

4 Refrigeration and Air-Conditioning

The boiling point is limited by the critical temperature at the upper

end, beyond which it cannot exist as a liquid, and by the triple point

at the lower end, which is at the freezing temperature. Between

these two limits, if the liquid is at a pressure higher than its boiling

pressure, it will remain a liquid and will be subcooled below the

saturation condition, while if the temperature is higher than

saturation, it will be a gas and superheated. If both liquid and

vapour are at rest in the same enclosure, and no other volatile

substance is present, the condition must lie on the saturation line.

At a pressure below the triple point pressure, the solid can change

directly to a gas (sublimation) and the gas can change directly to a

solid, as in the formation of carbon dioxide snow from the released

gas.

The liquid zone to the left of the boiling point line is subcooled

liquid. The gas under this line is superheated gas.

1.4 General gas laws

Many gases at low pressure, i.e. atmospheric pressure and below for

water vapour and up to several bar for gases such as nitrogen, oxygen

and argon, obey simple relations between their pressure, volume

and temperature, with sufficient accuracy for engineering purposes.

Such gases are called ‘ideal’.

Boyle’s Law states that, for an ideal gas, the product of pressure

and volume at constant temperature is a constant:

pV = constant

Example 1.4 A volume of an ideal gas in a cylinder and at

atmospheric pressure is compressed to half the volume at constant

temperature. What is the new pressure?

p1V1 = constant

= p 2V2

V

V

1

2

= 2

so p2 = 2 × p1

= 2 × 1.013 25 bar (101 325 Pa)

= 2.026 5 bar (abs.)

Charles’ Law states that, for an ideal gas, the volume at constant

pressure is proportional to the absolute temperature:

Fundamentals 5

V

T = constant

Example 1.5 A mass of an ideal gas occupies 0.75 m3

at 20°C and

is heated at constant pressure to 90°C. What is the final volume?

V2 =

V T

T 1

2

1

×

=

0.75 273.15 + 90

273.15 + 20 ×

= 0.93 m3

Boyle’s and Charles’ laws can be combined into the ideal gas

equation:

pV = (a constant) × T

The constant is mass × R, where R is the specific gas constant, so:

pV = mRT

Example 1.6 What is the volume of 5 kg of an ideal gas, having a

specific gas constant of 287 J/(kg K), at a pressure of one standard

atmosphere and at 25°C?

pV = mRT

V =

mRT

p

=

5 287(273.15 + 25)

101 325

×

= 4.22 m3

1.5 Dalton’s law

Dalton’s Law of partial pressures considers a mixture of two or

more gases, and states that the total pressure of the mixture is equal

to the sum of the individual pressures, if each gas separately occupied

the space.

Example 1.7 A cubic metre of air contains 0.906 kg of nitrogen of

specific gas constant 297 J/(kg K), 0.278 kg of oxygen of specific

gas constant 260 J/(kg K) and 0.015 kg of argon of specific gas

constant 208 J/(kg K). What will be the total pressure at 20°C?

6 Refrigeration and Air-Conditioning

pV = mRT

V = 1 m3

so p = mRT

For the nitrogen p N = 0.906 × 297 × 293.15 = 78 881 Pa

For the oxygen pO = 0.278 × 260 × 293.15 = 21 189 Pa

For the argon pA = 0.015 × 208 × 293.15 = 915 Pa

—————

Total pressure = 100 985 Pa

(1.009 85 bar)

1.6 Heat transfer

Heat will move from a hot body to a colder one, and can do so by

the following methods:

1. Conduction. Direct from one body touching the other, or through

a continuous mass

2. Convection. By means of a heat-carrying fluid moving between

one and the other

3. Radiation. Mainly by infrared waves (but also in the visible band,

e.g. solar radiation), which are independent of contact or an

intermediate fluid.

Conduction through a homogeneous material is expressed directly

by its area, thickness and a conduction coefficient. For a large plane

surface, ignoring heat transfer near the edges:

Conductance =

area thermal conductivity

thickness

×

=

A k

L

×

and the heat conducted is

Q f = conductance × (T1 – T2)

Example 1.8 A brick wall, 225 mm thick and having a thermal

conductivity of 0.60 W/(m K), measures 10 m long by 3 m high,

and has a temperature difference between the inside and outside

faces of 25 K. What is the rate of heat conduction?

Q f =

10 3 0.60 25

0.225

×× ×

= 2000 W (or 2 kW)

Fundamentals 7

Thermal conductivities, in watts per metre kelvin, for various common

materials are as in Table 1.2. Conductivities for other materials can

be found from standard reference works [1, 2, 3].

Table 1.2

Material Thermal conductivity (W/(m K))

Copper 200

Mild steel 50

Concrete 1.5

Water 0.62

Cork 0.040

Expanded polystyrene 0.034

Polyurethane foam 0.026

Still air 0.026

Convection requires a fluid, either liquid or gaseous, which is

free to move between the hot and cold bodies. This mode of heat

transfer is very complex and depends firstly on whether the flow of

fluid is ‘natural’, i.e. caused by thermal currents set up in the fluid

as it expands, or ‘forced’ by fans or pumps. Other parameters are

the density, specific heat capacity and viscosity of the fluid and the

shape of the interacting surface.

With so many variables, expressions for convective heat flow cannot

be as simple as those for conduction. The interpretation of observed

data has been made possible by the use of a number of groups

which combine the variables and which can then be used to estimate

convective heat flow.

The main groups used in such estimates are as shown in Table 1.3.

A typical combination of these numbers is that for turbulent flow

in pipes:

(Nu) = 0.023 (Re)0.8 (Pr)0.4

The calculation of every heat transfer coefficient for a refrigeration

or air-conditioning system would be a very time-consuming process,

even with modern methods of calculation. Formulas based on these

factors will be found in standard reference works, expressed in

terms of heat transfer coefficients under different conditions of

fluid flow [1, 4–8].

Example 1.9 A formula for the heat transfer coefficient between

forced draught air and a vertical plane surface ([1], Chapter 3,

Table 6) gives:

h′ = 5.6 + 18.6V

8 Refrigeration and Air-Conditioning

Table 1.3

Number Sign Parameters

Reynolds Re Velocity of fluid

Density of fluid

Viscosity of fluid

Dimension of surface

Grashof Gr Coefficient of expansion of fluid

Density of fluid

Viscosity of fluid

Force of gravity

Temperature difference

Dimension of surface

Nusselt Nu Thermal conductivity of fluid

Dimension of surface

Heat transfer coefficient

Prandtl Pr Specific heat capacity of fluid

Viscosity of fluid

Thermal conductivity of fluid

What is the thermal conductance for an air velocity of 3 m/s?

h′ = 5.6 + 18.6 × 3

= 61.4 W/(m2 K)

Where heat is conducted through a plane solid which is between

two fluids, there will be the convective resistances at the surfaces.

The overall heat transfer must take all of these resistances into

account, and the unit transmittance, or ‘U’ factor, is given by:

Rt = Ri + Rc + Ro

U = 1/Rt

where Rt = total thermal resistance

Ri = inside convective resistance

Rc = conductive resistance

Ro = outside convective resistance

Example 1.10 A brick wall, plastered on one face, has a thermal

conductance of 2.8 W/(m2 K), an inside surface resistance of 0.3

(m2 K)/W, and an outside surface resistance of 0.05 (m2 K)/W.

What is the overall transmittance?

Rt

= Ri + Rc + Ro

=

0.3 + 1

2.8

+ 0.05

= 0.707