Refrigeration and Air-Conditioning pdf

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

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 withinlimits determined by the requirements of the air conditionedenclosure

BS 5643: 1984

Refrigeration and

Air-Conditioning

Third edition

A R Trott and T Welch

OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

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

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 drinksindustries 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

Refrigeration and its application is met in almost every branch ofindustry, so that practitioners in other fields find that they have tobecome aware of its principles, uses and limitations This book aims

to introduce students and professionals in other disciplines to thefundamentals of the subject, without involving the reader too deeply

in theory The subject matter is laid out in logical order and coversthe main uses and types of equipment In the ten years since the lastedition there have been major changes in the choice of refrigerantsdue to environmental factors and an additional chapter is introduced

to reflect this This issue is on-going and new developments willappear over the next ten years This issue has also affected servicingand maintenance of refrigeration equipment and there is an increasedpressure to improve efficiency in the reduction of energy use Thisedition reflects these issues, whilst maintaining links with the pastfor 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 manypractical applications and shows options that are available for thesolution of problems in mechanical cooling systems It is not possiblefor these pages to contain enough information to design a completerefrigeration 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 newrefrigerants

T.C WelchOctober 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, thetriple point of ice is 0.01°C at a pressure of 6.1 mbar.) On theCelsius 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 degreeintervals as the Celsius scale, so that ice melts at +273.16K andwater at atmospheric pressure boils at +373.15K

1.2 Heat

Refrigeration is the process of removing heat, and the practicalapplication is to produce or maintain temperatures below theambient The basic principles are those of thermodynamics, andthese principles as relevant to the general uses of refrigeration areoutlined in this opening chapter

Heat is one of the many forms of energy and mainly arises fromchemical sources The heat of a body is its thermal or internalenergy, and a change in this energy may show as a change oftemperature or a change between the solid, liquid and gaseousstates

Matter may also have other forms of energy, potential or kinetic,depending on pressure, position and movement Enthalpy is thesum of its internal energy and flow work and is given by:

H = u + P v

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

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 anyother base which is convenient Tabulated enthalpies found inreference works are often shown above a base temperature of–40°C, since this is also –40° on the old Fahrenheit scale In anycalculation, this base condition should always be checked to avoidthe 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, inkJ/(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 Theresulting total changes can be shown on a temperature–enthalpydiagram (Figure 1.1)

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 ofheat to be removed from 1 kg of water at 30°C in order to turn itinto 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 capacitythrough 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 varieswith the pressure Thus, while the boiling point of water is commonlytaken as 100°C, this is only true at a pressure of one standardatmosphere (1.013 bar) and, by varying the pressure, the boilingpoint can be changed (Table 1.1) This pressure–temperatureproperty can be shown graphically (see Figure 1.2)

Triple point

Gas

Critical temperature Liquid

Temperature Boiling point curve

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 Betweenthese two limits, if the liquid is at a pressure higher than its boilingpressure, it will remain a liquid and will be subcooled below thesaturation condition, while if the temperature is higher thansaturation, it will be a gas and superheated If both liquid andvapour are at rest in the same enclosure, and no other volatilesubstance is present, the condition must lie on the saturation line

At a pressure below the triple point pressure, the solid can changedirectly to a gas (sublimation) and the gas can change directly to asolid, as in the formation of carbon dioxide snow from the releasedgas

The liquid zone to the left of the boiling point line is subcooledliquid The gas under this line is superheated gas

1.4 General gas laws

Many gases at low pressure, i.e atmospheric pressure and below forwater vapour and up to several bar for gases such as nitrogen, oxygenand argon, obey simple relations between their pressure, volumeand 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

atmospheric pressure is compressed to half the volume at constanttemperature What is the new pressure?

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?

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 standardatmosphere and at 25°C?

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 specificgas constant 260 J/(kg K) and 0.015 kg of argon of specific gasconstant 208 J/(kg K) What will be the total pressure at 20°C?

pV = mRT

V = 1 m3

so p = mRT

For the nitrogen pN = 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

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 anintermediate fluid

Conduction through a homogeneous material is expressed directly

by its area, thickness and a conduction coefficient For a large planesurface, ignoring heat transfer near the edges:

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 outsidefaces of 25 K What is the rate of heat conduction?

as it expands, or ‘forced’ by fans or pumps Other parameters arethe density, specific heat capacity and viscosity of the fluid and theshape of the interacting surface.

With so many variables, expressions for convective heat flow cannot

be as simple as those for conduction The interpretation of observeddata has been made possible by the use of a number of groupswhich combine the variables and which can then be used to estimateconvective 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 thesefactors will be found in standard reference works, expressed interms of heat transfer coefficients under different conditions offluid 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

Table 1.3

Density of fluidViscosity of fluidDimension of surface

Density of fluidViscosity of fluidForce of gravityTemperature differenceDimension of surface

Dimension of surfaceHeat transfer coefficient

Viscosity of fluidThermal 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 betweentwo fluids, there will be the convective resistances at the surfaces.The overall heat transfer must take all of these resistances intoaccount, and the unit transmittance, or ‘U’ factor, is given by:

R t = R i + R c + R o

U = 1/R t

where R t = total thermal resistance

R i = inside convective resistance

R c = conductive resistance

R o = 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?

R t = R i + R c + R o

=

0.3 + 12.8 + 0.05

= 0.707

Fundamentals 9

U = 1.414 W/(m2 K)

Typical overall thermal transmittances are:

Insulated cavity brick wall, 260 mm thick,

Chilled water inside copper tube, forced

Condensing ammonia gas inside steel

Special note should be taken of the influence of geometrical shape,where other than plain surfaces are involved

The overall thermal transmittance, U, is used to calculate the total heat flow For a plane surface of area A and a steady temperature

any one point, the space–temperature curve will be exponential In

a case where the cooling medium is an evaporating liquid, thetemperature of this liquid will remain substantially constantthroughout the process, since it is absorbing latent heat, and thecooling curve will be as shown in Figure 1.3

Cooled m edium

∆ T

Cooling medium

Providing that the flow rates are steady, the heat transfer coefficients

do not vary and the specific heat capacities are constant throughoutthe working range, the average temperature difference over thelength of the curve is given by:

the logarithmic mean temperature difference (ln MTD) and can be used

the cooling range, or an average figure is known, giving

(b) Fluid cooling refrigerant (c) Two fluids

Example 1.11 A fluid evaporates at 3°C and cools water from11.5°C to 6.4°C What is the logarithmic mean temperature differenceand what is the heat transfer if it has a surface area of 420 m2 andthe thermal transmittance is 110 W/(m2 K)?

Fundamentals 11

saturation temperature With some liquids, the heat transfer valueswill change with temperature For these reasons, the ln MTD formuladoes not apply accurately to all heat transfer applications

If the heat exchanger was of infinite size, the space–temperaturecurves would eventually meet and no further heat could be trans-ferred The fluid in Example 1.11 would cool the water down to

3°C The effectiveness of a heat exchanger can be expressed as the

ratio of heat actually transferred to the ideal maximum:

Rough surfaces such as brick, concrete,

The metals used in refrigeration and air-conditioning systems, such

as steel, copper and aluminium, quickly oxidize or tarnish in air,and the emissivity figure will increase to a value nearer 0.50.Surfaces will absorb radiant heat and this factor is expressed also

as the ratio to the absorptivity of a perfectly black body Within the

range of temperatures in refrigeration systems, i.e –70°C to +50°C(203–323 K), the effect of radiation is small compared with theconductive and convective heat transfer, and the overall heat transferfactors in use include the radiation component Within thistemperature range, the emissivity and absorptivity factors are aboutequal

The exception to this is the effect of solar radiation whenconsidered as a cooling load, such as the air-conditioning of a buildingwhich is subject to the sun’s rays At the wavelength of sunlight theabsorptivity figures change and calculations for such loads usetabulated factors for the heating effect of sunlight Glass, glazedtiles and clean white-painted surfaces have a lower absorptivity, whilethe metals are higher.

1.7 Transient heat flow

A special case of heat flow arises when the temperatures throughthe thickness of a solid body are changing as heat is added or

removed This non-steady or transient heat flow will occur, for example,

when a thick slab of meat is to be cooled, or when sunlight strikes

on a roof and heats the surface When this happens, some of theheat changes the temperature of the first layer of the solid, and theremaining heat passes on to the next layer, and so on Calculationsfor heating or cooling times of thick solids consider the slab as anumber of finite layers, each of which is both conducting andabsorbing heat over successive periods of time Original methods ofsolving transient heat flow were graphical [1, 5], but could noteasily take into account any change in the conductivity or specificheat capacity or any latent heat of the solid as the temperaturechanged

Complicated problems of transient heat flow can be resolved bycomputer Typical time–temperature curves for non-steady coolingare shown in Figures 16.1 and 16.2, and the subject is met again inSection 26.2

1.8 Two-phase heat transfer

Where heat transfer is taking place at the saturation temperature of

a fluid, evaporation or condensation (mass transfer) will occur atthe interface, depending on the direction of heat flow In suchcases, the convective heat transfer of the fluid is accompanied byconduction at the surface to or from a thin layer in the liquid state.Since the latent heat and density of fluids are much greater thanthe sensible heat and density of the vapour, the rates of heat transferare considerably higher The process can be improved by shapingthe heat exchanger face (where this is a solid) to improve the drainage

of condensate or the escape of bubbles of vapour The total heattransfer will be the sum of the two components

Rates of two-phase heat transfer depend on properties of thevolatile fluid, dimensions of the interface, velocities of flow and the

Fundamentals 13

extent to which the transfer interface is blanketed by fluid Thedriving force for evaporation or condensation is the difference ofvapour pressures at the saturation and interface temperatures.Equations for specific fluids are based on the interpretation ofexperimental data, as with convective heat transfer

Mass transfer may take place from a mixture of gases, such as thecondensation of water from moist air In this instance, the watervapour has to diffuse through the air, and the rate of mass transferwill depend also on the concentration of vapour in the air In theair–water vapour mixture, the rate of mass transfer is roughlyproportional to the rate of heat transfer at the interface and thissimplifies predictions of the performance of air-conditioning coils[1, 5, 9]

2 The refrigeration cycle

2.1 Basic vapour compression cycle

A liquid boils and condenses – the change between the liquid andgaseous states – at a temperature which depends on its pressure,within the limits of its freezing point and critical temperature Inboiling it must obtain the latent heat of evaporation and in condensingthe latent heat must be given up again

The basic refrigeration cycle (Figure 2.1) makes use of the boilingand condensing of a working fluid at different temperatures and,therefore, at different pressures

Pe

Pc

Te TcTemperature

curve

Saturation

Heat is put into the fluid at the lower temperature and pressureand provides the latent heat to make it boil and change to a vapour.This vapour is then mechanically compressed to a higher pressureand a corresponding saturation temperature at which its latent heatcan be rejected so that it changes back to a liquid

The refrigeration cycle 15

The total cooling effect will be the heat transferred to the workingfluid in the boiling or evaporating vessel, i.e the change in enthalpiesbetween the fluid entering and the vapour leaving the evaporator.For a typical circuit, using the working fluid Refrigerant 22,evaporating at – 5°C and condensing at 35°C, the pressures andenthalpies will be as shown in Figure 2.2

Gas at 12.54 bar Dry saturated gas

– 5 ° C 3.21 bar

249.9 kJ/kg

Compressor

35 ° C Heat out Heat in

Fluid in 91.4 kJ/kg

Liquid out

35 ° C 91.4 kJ/kg

Enthalpy of fluid entering evaporator = 91.4 kJ/kg

Enthalpy of saturated gas leaving evaporator = 249.9 kJ/kg

pressure-of the fluid will flash pressure-off into vapour to remove the energy for thiscooling The volume of the working fluid therefore increases at thevalve by this amount of flash gas, and gives rise to its name, the

expansion valve (Figure 2.3.)

ideal reversible cycle, and based on the two temperatures of thesystem, assuming that all heat is transferred at constant temperature(see Figure 2.4) Since there are mechanical and thermal losses in

a real circuit, the coefficient of performance (COP) will always beless than the ideal Carnot figure For practical purposes in working

(liquid and flash gas)

Condensation

Evaporation

Compression Expansion

The refrigeration cycle 17

systems, it is the ratio of the cooling effect to the input compressorpower

At the conditions shown in Figure 2.2, evaporating at –5°C andcondensing at 35°C (268.15 K and 308.15 K), the Carnot coefficient

of performance is 6.7

Transfer of heat through the walls of the evaporator and condenserrequires a temperature difference This is shown on the modifiedreversed Carnot cycle (Figure 2.5) For temperature differences of

5 K on both the evaporator and condenser, the fluid operatingtemperatures would be 263.15 K and 313.15 K, and the coefficient

A more informative diagram is the pressure–enthalpy chart whichshows the liquid and vapour states of the fluid (Figure 2.6) In thisdiagram, a fluid being heated passes from the subcooled state (a),reaches boiling point (b) and is finally completely evaporated (c)and then superheated (d) The distance along the sector b–c showsthe proportion which has been evaporated at any enthalpy value

The refrigeration cycle is shown by the process lines ABCD (Figure

2.7) Compression is assumed to be adiabatic, but this will alter

–60

–20 0 20 40 60

Courtesy of the Chartered Institution of Building Services Engineers)

C1C 20 0 –20

The refrigeration cycle 19

according to the type of compressor Since there is no energy input

or loss within the expansion valve, these two points lie on a line ofequal enthalpy The pressure–enthalpy chart can give a direct measure

of the energy transferred in the process

In a working circuit, the vapour leaving the evaporator will probably

be slightly superheated and the liquid leaving the condensersubcooled The gas leaving the evaporator is superheated to point

A1 and the liquid subcooled to C1 Also, pressure losses will occuracross the gas inlet and outlet, and there will be pressure dropsthrough the heat exchangers and piping The final temperature atthe end of compression will depend on the working limits and therefrigerant Taking these many factors into account, the refrigerating

effect (A1 – D1) and the compressor energy (B1 – A1) may be readoff directly in terms of enthalpy of the fluid

The distance of D1 between the two parts of the curve indicatesthe proportion of flash gas at that point The condenser receivesthe high-pressure superheated gas, cools it down to saturationtemperature, condenses it to liquid, and finally subcools it slightly.The energy removed in the condenser is seen to be the refrigeratingeffect plus the heat of compression

2.3 Heat exchanger size

Transfer of heat through the walls of the evaporator and condenserrequires a temperature difference, and the larger these heatexchangers are, the lower will be the temperature differences and

so the closer the fluid temperatures will be to those of the load andcondensing medium The closer this approach, the nearer the cyclewill be to the ideal reversed Carnot cycle (See Table 2.1.)

These effects can be summarized as follows

Larger evaporator 1 Higher suction pressure to give denser gas

entering the compressor and therefore a greater mass of gas for agiven swept volume, and so a higher refrigerating duty; 2 Highersuction pressure, so a lower compression ratio and less power for agiven duty

Larger condenser 1 Lower condensing temperature and colder

liquid entering the expansion valve, giving more cooling effect; 2.Lower discharge pressure, so a lower compression ratio and lesspower

2.4 Volumetric efficiency

In a reciprocating compressor, there will be a small amount of

The refrigeration cycle 21

clearance space at the top of the stroke, arising from gas ports,manufacturing tolerances, and an allowance for thermal expansionand contraction of the components in operation High-pressuregas left in this space at the end of the discharge stroke must re-expand to the suction inlet pressure before a fresh charge of gascan be drawn in This clearance space is usually of the order of 4–7% of the swept volume, but it is possible to design compressorswith less clearance

This loss of useful working stroke will increase with the ratio ofthe suction and discharge absolute pressures, and the compressorefficiency will fall off This effect is termed the volumetric efficiency[11] Typical figures are shown in Figure 2.8

Compound systems use the same refrigerant throughout a common

circuit, compressing in two or more stages (Figure 2.9) Dischargegas from the first compression stage will be too hot to pass directly

to the high-stage compressor, so it is cooled in an intercooler, usingsome of the available refrigerant from the condenser The opportunity

is also taken to subcool liquid passing to the evaporator Smallcompound systems may cool the interstage gas by direct injection

of liquid refrigerant into the pipe

The cascade cycle has two separate refrigeration systems, one acting

as a condenser to the other (see Figure 2.10) This arrangementpermits the use of different refrigerants in the two systems, and high-pressure refrigerants such as R.13 are common in the lower stage.The Mollier diagrams for compound and cascade systems (Figures2.9 and 2.10) indicate the enthalpy change per kilogram of circulatedrefrigerant, but it should be borne in mind that the mass flows aredifferent for the low and high stages

Intercooler

Expansion valve

Condenser Evaporator

Expansion

valve

Enthalpy (b) –60

–40

–20

0 20 40 60 80 (a)

(compound)

The refrigeration cycle 23

Expansion valve

High-temperature compressor

Condenser

Expansion valve

Enthalpy (b) (a)

(cascade)

2.6 Refrigerants for vapour compression cycles

The requirements for the working fluid are as follows:

1 A high latent heat of vaporization

2 High density of suction gas

3 Non-corrosive, non-toxic and non-flammable

4 Critical temperature and triple point outside the working range

5 Compatibility with materials of construction, with lubricatingoils, and with other materials present in the system

6 Convenient working pressures, i.e not too high and preferablynot below atmospheric pressure

7 High dielectric strength (for compressors having integral electricmotors)

2.7 Total loss refrigerants

Some volatile fluids are used once only, and then escape into theatmosphere Two of these are in general use, carbon dioxide andnitrogen Both are stored as liquids under a combination of pressureand low temperature and then released when the cooling effect isrequired Carbon dioxide is below its critical point at atmosphericpressure and can only exist as ‘snow’ or a gas Since both gasescome from the atmosphere, there is no pollution hazard Thetemperature of carbon dioxide when released will be – 78.4°C.Nitrogen will be at –198.8°C Water ice can also be classified as atotal loss refrigerant

2.8 Absorption cycle

Vapour can be withdrawn from an evaporator by absorption (Figure2.11) into a liquid Two combinations are in use, the absorption ofammonia gas into water and the absorption of water vapour intolithium bromide The latter is non-toxic and so may be used for air-conditioning The use of water as the refrigerant in this combinationrestricts it to systems above its freezing point Refrigerant vapourfrom the evaporator is drawn into the absorber by the liquidabsorbant, which is sprayed into the chamber The resulting solution(or liquor) is then pumped up to condenser pressure and the vapour

is driven off in the generator by direct heating The high-pressurerefrigerant gas given off can then be condensed in the usual wayand passed back through the expansion valve into the evaporator.Weak liquor from the generator is passed through another pressure-reducing valve to the absorber Overall thermal efficiency is improved

The refrigeration cycle 25

Low-pressure refrigerant gas

Absorber

Pressure reducing valve Weak liquor

High-pressure refrigerant gas Generator

Pump Strong liquor

Expansion valve

Condenser

High-pressure refrigerant liquid

Evaporator

Expansion valve

suction-to-(a)

(Solar radiation can also be used.) The overall energy used is greaterthan with the compression cycle, so the COP (coefficient ofperformance) is lower Typical figures are as shown in Table 2.2.

2.9 Steam ejector system

The low pressures (8–22 mbar) required to evaporate water as arefrigerant at 4–7°C for air-conditioning duty can be obtained with

a steam ejector High-pressure steam at 10 bar is commonly used.The COP of this cycle is somewhat less than with the absorptionsystem, so its use is restricted to applications where large volumes ofsteam are available when required (large, steam-driven ships) orwhere water is to be removed along with cooling, as in freeze-dryingand fruit juice concentration

2.10 Air cycle

Any gas, when compressed, rises in temperature Conversely, if it ismade to do work while expanding, the temperature will drop Use

is made of the sensible heat only (although it is, of course, the basis

of the air liquefaction process)

The main application for this cycle is the air-conditioning andpressurization of aircraft The turbines used for compression andexpansion turn at very high speeds to obtain the necessary pressureratios and, consequently, are noisy The COP is lower than withother systems [15]

The normal cycle uses the expansion of the air to drive the firststage of compression, so reclaiming some of the input energy (Figure2.12)

The refrigeration cycle 27 Air

inlet Cooling airout

Compressor

Heat exchanger

Expander

Cold air

to process

Cooling air in Fan

2.11 Thermoelectric cooling

The passage of an electric current through junctions of dissimilarmetals causes a fall in temperature at one junction and a rise at theother, the Peltier effect Improvements in this method of coolinghave been made possible in recent years by the production of suitablesemiconductors Applications are limited in size, owing to the highelectric currents required, and practical uses are small cooling systemsfor military, aerospace and laboratory use (Figure 2.13)

Cooled surface Heat

sink

P type –

+

15 V d.c.

N type

or air-conditioning refrigeration applications because of its toxicity,flammability and attack by copper.

This chapter is about the new refrigerants and the new attitudeneeded in design, maintenance and servicing of refrigerationequipment

3.2 Ideal properties for a refrigerant

It will be useful to remind ourselves of the requirements for a fluidused as a refrigerant

• A high latent heat of vaporization

• A high density of suction gas

• Non-corrosive, non-toxic and non-flammable

• Critical temperature and triple point outside the working range

• Compatibility with component materials and lubricating oil

• Reasonable working pressures (not too high, or belowatmospheric pressure)

• High dielectric strength (for compressors with integral motors)

• Low cost

• Ease of leak detection

• Environmentally friendly

Refrigerants 29

No single fluid has all these properties, and meets the newenvironmental requirements, but this chapter will show thedevelopments that are taking place in influencing the selection andchoice of a refrigerant

3.3 Ozone depletion potential

The ozone layer in our upper atmosphere provides a filter forultraviolet radiation, which can be harmful to our health Researchhas found that the ozone layer is thinning, due to emissions intothe atmosphere of chlorofluorocarbons (CFCs), halons and bromides.The Montreal Protocol in 1987 agreed that the production of thesechemicals would be phased out by 1995 and alternative fluidsdeveloped From Table 3.1, R11, R12, R114 and R502 are all CFCsused as refrigerants, while R13B1 is a halon They have all ceasedproduction within those countries which are signatories to theMontreal Protocol The situation is not so clear-cut, because thereare countries like Russia, India, China etc who are not signatoriesand who could still be producing these harmful chemicals Table3.2 shows a comparison between old and new refrigerants

Domestic refrigerators and freezers R12

Small retail and supermarkets R12, R22, R502

R22 is an HCFC and now regarded as a transitional refrigerant,

in that it will be completely phased out of production by 2030, asagreed under the Montreal Protocol A separate European Com-munity decision has set the following dates

1/1/2000 CFCs banned for servicing existing plants

1/1/2000 HCFCs banned for new systems with a shaft input power

greater than 150 kW

1/1/2001 HCFCs banned in all new systems except heat pumps

and reversible systems

1/1/2004 HCFCs banned for all systems

1/1/2008 Virgin HCFCs banned for plant servicing

Table 3.2 Comparison of new refrigerants

at 26 at 1 barbar (°C) abs °C

3.4 Global warming potential (GWP)

Global warming is the increasing of the world’s temperatures, whichresults in melting of the polar ice caps and rising sea levels It iscaused by the release into the atmosphere of so-called ‘greenhouse’gases, which form a blanket and reflect heat back to the earth’ssurface, or hold heat in the atmosphere The most infamousgreenhouse gas is carbon dioxide (CO2), which once released remains

in the atmosphere for 500 years, so there is a constant build-up astime progresses

The main cause of CO2 emission is in the generation of electricity

at power stations Each kWh of electricity used in the UK produces

Refrigerants 31

about 0.53 kg of CO2 and it is estimated that refrigeration compressors

in the UK consume 12.5 billion kWh per year

Table 3.3 shows that the newly developed refrigerant gases alsohave a global warming potential if released into the atmosphere.For example, R134a has a GWP of 1300, which means that theemission of 1 kg of R134a is equivalent to 1300 kg of CO2 Thechoice of refrigerant affects the GWP of the plant, but other factorsalso contribute to the overall GWP and this has been represented

by the term total equivalent warming impact (TEWI) This term shows

the overall impact on the global warming effect, and includesrefrigerant leakage, refrigerant recover y losses and energyconsumption It is a term which should be calculated for eachrefrigeration plant Figures 3.1 and 3.2 show the equation used and

an example for a medium temperature R134a plant

TEWI = (GWP × L × n) + (GWP × m [1 – α recovery ] + (n × Eannual× β )

TEWI = TOTAL EQUIVALENT WARMING IMPACT

Leakage Recovery losses Energy consumption direct global warming potential

GWP = Global warming potential [CO2-related]

L = Leakage rate per year [kg]

n = System operating time [Years]

m = Refrigerant charge [kg]

α recovery = Recycling factor

Eannual = Energy consumption per year [kWh]

β = CO2-Emission per kWh (Energy-Mix)

indirect global warming potential

Refrigerants 33

One thing that is certain is that the largest element of the TEWI

is energy consumption, which contributes CO2 emission to theatmosphere The choice of refrigerant is therefore about the efficiency

of the refrigerant and the efficiency of the refrigeration system.The less the amount of energy needed to produce each kW ofcooling, the less will be the effect on global warming

3.5 Ammonia and the hydrocarbons

These fluids have virtually zero ODP and zero GWP when releasedinto the atmosphere and therefore present a very friendly environ-mental picture Ammonia has long been used as a refrigerant forindustrial applications The engineering and servicing requirementsare well established to deal with its high toxicity and flammability.There have been developments to produce packaged liquid chillerswith ammonia as the refrigerant for use in air-conditioning insupermarkets, for example Ammonia cannot be used with copper

or copper alloys, so refrigerant piping and components have to besteel or aluminium This may present difficulties for the air-conditioning market where copper has been the base material forpiping and plant One property that is unique to ammonia compared

to all other refrigerants is that it is less dense than air, so a leakage

of ammonia results in it rising above the plant room and into theatmosphere If the plant room is outside or on the roof of a building,the escaping ammonia will drift away from the refrigeration plant

tcm Mean condensing temperature

tom Mean evaporating temperature

blends