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. 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. Site erection 131 12 Liquid chillers. 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 vi Contents 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. Training 338 34 Efficiency and economy in operation 351 35 Catalogue selection 357 Appendix Units of measurement 367 References 369 Index 373 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. 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.) On the Celsius scale, absolute zero is – 273. 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 + 373.2 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 P v 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. 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. Sensible heat of gas Latent heat of melting Latent heat of boiling Temperature 373.15 K Sensible heat of liquid 273.16 K Sensible heat of soild 334 kJ 419 kJ 2257 kJ Enthalpy Figure 1.1 Change of temperature (K) and state of water with enthalpy Example 1.1 For water, the latent heat of freezing is 334 kJ/kg and the specific heat capacity averages 4. 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: 4.2 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 4.3 The specific enthalpy of water at 80°C, taken from 0°C base, is 334. What is the average specific heat capacity through the range 0–80°C? 334.3 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. This pressure–temperature property can be shown graphically (see Figure 1.0 Critical temperature Liquid Pressure Solid Gas Triple point Temperature Figure 1.2 Change of state with pressure and temperature 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.4 General gas laws Many gases at low pressure, i. 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 2V 2 V1 =2 V2 so p 2 = 2 × p1 = 2 × 1.) Charles’ Law states that, for an ideal gas, the volume at constant pressure is proportional to the absolute temperature: Fundamentals 5 V = constant T 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? T2 V2 = V 1 × T1 = 0.93 m 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(273.5 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.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 = 0.15 = 78 881 Pa For the oxygen pO = 0.15 = 21 189 Pa For the argon pA = 0.15 = 915 Pa ————— Total pressure = 100 985 Pa (1.