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 [...]... removed in the condenser is seen to be the refrigerating effect plus the heat of compression 2.3 Heat exchanger size Transfer of heat through the walls of the evaporator and condenser requires a temperature difference, and the larger these heat exchangers are, the lower will be the temperature differences and so the closer the fluid temperatures will be to those of the load and condensing medium The closer... superheated and the liquid leaving the condenser subcooled The gas leaving the evaporator is superheated to point A1 and the liquid subcooled to C1 Also, pressure losses will occur across the gas inlet and outlet, and there will be pressure drops through the heat exchangers and piping The final temperature at the end of compression will depend on the working limits and the refrigerant Taking these... cases, the convective heat transfer of the fluid is accompanied by conduction at the surface to or from a thin layer in the liquid state Since the latent heat and density of fluids are much greater than the sensible heat and density of the vapour, the rates of heat transfer are considerably higher The process can be improved by shaping the heat exchanger face (where this is a solid) to improve the drainage... 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 the heat changes the temperature of the first layer of the solid, and the remaining heat passes on to the next layer, and so on Calculations for heating... drainage of condensate or the escape of bubbles of vapour The total heat transfer will be the sum of the two components Rates of two-phase heat transfer depend on properties of the volatile fluid, dimensions of the interface, velocities of flow and the Fundamentals 13 extent to which the transfer interface is blanketed by fluid The driving force for evaporation or condensation is the difference of vapour... account, the refrigerating effect (A1 – D1) and the compressor energy (B1 – A1) may be read off directly in terms of enthalpy of the fluid The distance of D1 between the two parts of the curve indicates the proportion of flash gas at that point The condenser receives the high-pressure superheated gas, cools it down to saturation temperature, condenses it to liquid, and finally subcools it slightly The energy... 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 and pressurization of aircraft The turbines used for compression and expansion turn at very high speeds to obtain the necessary pressure ratios and, consequently, are noisy The COP is lower... [10], Courtesy of the Chartered Institution of Building Services Engineers) 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 of equal 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... 3°C The effectiveness of a heat exchanger can be expressed as the ratio of heat actually transferred to the ideal maximum: Σ= T A in – T A out T A in – TB in Taking the heat exchanger in Example 1.11: Σ = 11.5 – 6.4 11.5 – 3.0 = 0.6 or 60% Radiation of heat was shown by Boltzman and Stefan to be proportional to the fourth power of the absolute temperature and to depend on the colour, material and texture... Refrigeration and Air-Conditioning Table 1.3 Number Sign Parameters Reynolds Re Grashof Gr Nusselt Nu Prandtl Pr Velocity of fluid Density of fluid Viscosity of fluid Dimension of surface Coefficient of expansion of fluid Density of fluid Viscosity of fluid Force of gravity Temperature difference Dimension of surface Thermal conductivity of fluid Dimension of surface Heat transfer coefficient Specific heat . slab of meat is to be cooled, or when sunlight strikes on a roof and heats the surface. When this happens, some of the heat changes the temperature of the first layer of the solid, and the remaining. liquid state. Since the latent heat and density of fluids are much greater than the sensible heat and density of the vapour, the rates of heat transfer are considerably higher. The process can be. Refrigeration and Air-Conditioning Refrigeration: The process of removing heat. Air-conditioning: A form of air treatment whereby temperature, humidity, ventilation, and air cleanliness