Thermodynamic Design Data for Heat Pump Systems. A Comprehensive Data Base and Design Manual PDF
Preview Thermodynamic Design Data for Heat Pump Systems. A Comprehensive Data Base and Design Manual
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A. HOLLAND F. A. WATSON and S. DEVOTTA University of Salford, England PERGAMON PRESS OXFORD · NEW YORK · TORONTO · SYDNEY · PARIS · FRANKFURT Ü.K. Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 OBW, England U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. CANADA Pergamon Press Canada Ltd., Suite 104, 150 Consumers Road, Willowdale, Ontario M2J 1P9, Canada AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., P.O. Box 544, Potts Point, N.S.W. 2011, Australia FRANCE Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg-Taunus, OF GERMANY Hammerweg 6, Federal Republic of Germany Copyright © 1982 Pergamon Press Ltd. All Rights Reserved. No part of this publication may be reproduced, stored !!T a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. First edition 1982 Library of Congress Cataloging in Publication Data Holland, F. A. Thermodynamic design data for heat pump systems. Includes index. 1. Heat pumps—Thermodynamics. I. Watson. F. A. (Frank Alfred) II. Devotta. s' III. Title TJ262.H64 1982 621.4025 81-23536 AACR2 British Library Cataloguing in Publication Data Holland. F. A. Thermodynamic design data for heat pump systems 1. Heat pumps—Design and construction I. Title II. Watson. F. A. III. Devotta. S. 621.4025 TJ262 ISBN 0-08-028727-1 In order to make this volume available as economically and as rapidly as possible the typescript has been reproduced in its original form. This method unfortunately has its typographical limitations but it is hoped that they in no way distract the reader. Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter PREFACE The rapidly escalating cost of energy has led to a growing interest in the use of heat pumps since these are the only heat recovery systems capable of increasing the temperature of recovered heat. The aim of this book is to provide a comprehensive data base for the design of vapour compression heat pump systems, particularly in industrial appli cations where careful matching is essential. Heat pumps are amplifiers of useful heat and the theoretical Rankine coeff icient of performance (COP)R is the best that can be expected for a partic ular working fluid. For a working fluid condensing at a temperature TQQ and pressure ?QQ and evaporating at a temperature TEV and pressure PEV> the gross or theoretical maximum temperature lift is (Tco " TEV) and the compression ratio (CR) = PCO/PEV. The values of (CR) and Tco are fixed by the capability of the compressor and the required temperature of the delivered heat respectively. These values automatically determine the values of both (TQQ " TEV) an<3 (COP)R for a particular working fluid. This fact is of crucial importance in the design of vapour compression heat pump systems. The book consists of two chapters and 21 appendices. The latter present the required design data for 21 materials, presently available, which are likely to be used as heat pump working fluids. The first chapter describes how the data in the graphs and tables in the appendices have been derived and the second chapter gives examples of how the data can be used. Thermodynamic considerations provide essential guidelines both for equipment design and for areas of future development. It is hoped that the publica tion of this comprehensive data bank will facilitate the development of heat pump systems and widen the range of potential applications. v ACKNOWLEDGEMENTS The incentive to analyse and assemble the data in this book arose from the planning of a joint research programme in heat energy recycling and heat pumps between the Department of Chemical Engineering at the University of Salford, U.K. and the National Chemical Laboratory, Pune, India. This research programme is concerned with heat energy recycling in industrial processes where a systems approach involving precise matching is essential. The successful start to this programme has been possible through the help and enthusiasm of the Director of NCL, Pune, Dr L.K.Doraiswamy FNA and the Deputy Director and Head of the Chemical Engineering Division, Dr R.A. Mashelkar. This book is a first step in providing the foundations for a comprehensive research and development programme which could eventually lead to a significant reduction in primary energy usage in industrial processes. The authors would like to thank the Editor of the Indian Chemical Engineer, Dr L.K.Doraiswamy FNA for permission to publish the tables in Appendices 7,9 and 16 and the Editor of the Journal of Heat Recovery Systems, Mr D.A. Reay for permission to publish the tables in the rest of the Appendices. The authors are greatly indebted to Mrs J.Hudson and Mrs B.Price for pre paring the camera-ready copy and to Mrs A.Rand and Mr G.Eckersley for help with the diagrams. F.A.HOLLAND F.A.WATSON S.DEVOTTA vi CHAPTER 1 Heat Pump Theory INTRODUCTION Heat pumps have enormous potential for saving energy, particularly in indus trial processes. They are the only heat recovery systems which enable the temperature of waste heat to be raised to more useful levels. Although the principle of the heat pump has been known since the middle of the last cent ury, there was little incentive to develop them in a time of cheap and abun dant energy (Ref. 1.8). A heat pump is essentially a heat engine operating in reverse. Its princi ple is illustrated in Fig. 1.1. delivered heat Q at temperature T A A high grade energy input W net temperature heat lift <T - T ) pump D s λ source heat Q at temperature T »L s FIG.1.1 PRINCIPLE OF HEAT PUMP TDDHPS - A« 1 2 Thermodynamic Design Data for Heat Pump Systems From the first law of thermodynamics, the amount of heat delivered Q at the higher temperature T is related to the amount of heat extracted Q at the low temperature T and the amount of high grade energy input W by the equa tion Q = Q + W (1.1) D s A coefficient of performance (COP) can be defined as (COP) = Q /W (1.2) D A heat engine operating between a higher temperature T and a lower tempera ture Τ has a theoretical maximum thermodynamic efficiency ς η = (T - T )/T (1.3) D S D known as the Carnot efficiency. A heat pump can be considered as a heat en gine operating in reverse. The Carnot coefficient of performance defined by Equation (1.4) (COP) = T /(T - T ) (1.4) c D D s represents the upper theoretical value obtainable in a heat pump system. In practice attainable coefficients of performance are significantly less than (COP) . c All heat pumps must cool and heat at the same time. A refrigerator is a heat pump which is designed to cool at the lower temperature T rather than to heat at the higher temperature T . The coefficient of performance is de fined as (COP) = Q /W (1.5) s Equations(1.1), (1.2) and (1.5) can be combined to show that the coefficient of performance of a heat pump is related to the coefficient of performance of a refrigerator by the equation (COP) heat pump = (COP) refrigerator +1 (1.6) Thermodynamic data for a number of working fluids have been available for many years in the low temperature or refrigeration range. In contrast there has hitherto been a scarcity of corresponding data in the high temperature heat pump range. A conventional mechanical vapour compression heat pump illustrated in Fig. 1.2 consists of two heat exchangers, a compressor, an expansion valve and a working fluid. In the evaporator heat exchanger, the working fluid evapor ates at a temperature T by extracting heat from the source. It is then compressed and gives up its latent heat as it condenses at a higher tempera ture T in the condenser heat exchanger. The condensed liquid is then ex panded through an expansion valve and is returned to the evaporator to com plete the cycle. The difference between the condensing and evaporating temperatures (T - T ) is the gross or maximum possible temperature lift. The net temperature lift (T - T ) is less than the gross temperature lift by the sum of the tempera ture difference driving forces in the evaporator and condenser heat exchang ers. The ratio of the corresponding pressures in the condenser and evapora tor P /P is the compression ratio (CR) . (T ~~ ^ ) » ^^ anc* t^ie con<^en" pn FV sing temperature T are the critical parameters which determine the feasible Heat Pump Theory 3 condenser CO ΛΛ/V * h > I—KVW Π high O net gross expansion/S?\ firade > compressor valve ^? temp temp energy lift lift input isjk porator ι y eva IjaL FIG.1.2 MECHANICAL VAPOUR COMPRESSION HEAT PUMP operating range of a heat pump operating on a particular working fluid. IDEAL RANKINE CYCLE HEAT PUMPS In practice the operation of a mechanical vapour compression heat pump appr oximates more closely to the Rankine heat pump cycle than to the theoretical Carnot cycle (Ref. 1.3). The ideal Rankine heat pump cycle is the reverse of the Rankine power cycle and can be illustrated with reference to the R12 pressure enthalpy diagram shown in Fig.1.3.(Ref.1.4). R12 is one of the most widely used working fluids. Its chemical formula is C Cl F and its ? ? critical temperature and pressure are 112 C and 41.155 bar respectively. The principal disadvantage of R12 is the relatively high condensing pressures P which correspond to relatively modest condensation temperatures T . co Since most heat pump systems are not designed to operate at pressures higher than 17 to 19 bar, a condensing temperature of about 70 C will be about the highest obtainable with most compressors operating on R12. In Fig.1.3, the working fluid at point S2 is in the form of saturated vapour at an evaporation temperature T. of 25°C. It is isentropically compressed EV to point Dl in the superheated vapour region. The superheat (H - H) is ? then removed and it is isothermally condensed from saturated vapour at point D2 to saturated liquid at point D3 at a condensing temperature T of 65 C. From D3 it is isenthalpically expanded to a mixture of liquid and vapour at point SI from which it is isothermally evaporated at a temperature of 25 C to point S2. With reference to Fig.1.3, the theoretical Rankine coefficient of performance of a heat pump can be defined as 4 Thermodynamic Design Data for Heat Pump Systems (COP) = (H -H )/(H (1.7) R D1 D3 D1 HS2> where H is the enthalpy per unit mass. Since the compression from point S2 to point Dl is at a constant entropy, φ„ = φ . where φ is the entropy per ς unit mass. 50 40 30 20 X- o ....7i).0i:. J Dl. 65 c D Dl saturated liquid y v 10 u <d ■a SI« 35°C Xo Λ 20 CJ 10 C CO QLCJ CO 0) u saturated vapour J. 150 200 250 300 enthalpy per unit mass H, kJ kg"1 FIG. 1.3 PRESSURE AGAINST ENTHALPY PER UNIT MASS FOR R12 The enthalpy per unit mass of superheated vapour at point Dl is related to the enthalpy per unit mass at the saturated vapour point D2 by the equation HD1 " HD2 + Cp (TD1 " V ( 1 · 8) where C is the heat capacity per unit mass at constant pressure. Equation (1.8) can be approximated with accuracy sufficient for design pur poses by HD1 " HD2 + (*S2 " *D2> TC0 (1'9) Consider the particular case of T___ = 25 C and T = 65 C. -1 H = 274.407 kJ kg D2 -1 H = 165.309 kJ kg D3 -1 H = 261.677 kJ kg g2 φ = 1.53275 kJ kg"1 K_1 02 φ = 1.54484 kJ kg"1 K_1 32 Heat Pump Theory 5 (*S2 " ())D2) TC0 = ^1·54484 " 1.53275) (338.15) = 4.0882 kJ kg"1 Substitute into Equation (1.9) to give HD1 = 274·407 + 4·088 = 278.495 kJ kg"1 Substitute into Equation (1.7) to give (C0P)R = 278^495 - 261.611 = 6#73 The error involved in using this method can be shown to be less than 1 per cent, which is within the probable error of the data and equations on which the thermodynamic tables in the appendices are based (Ref. 1.4). In general, isentropic compression of saturated vapours results in super heating of the vapour. However, the thermodynamic properties of some working fluids, such as R113, R114, R600a and RC318 imply that partial condensation should result on isentropic compression of the saturated vapour over certain pressure ranges. The ideal Rankine cycle with partial condensation is illus trated in Fig. 1.4, which is a plot of pressure P against enthalpy per unit mass H for R113. 200 250 300 350 -1 enthalpy per unit mass H, kJ kg FIG. 1.4 PRESSURE AGAINST ENTHALPY PER UNIT MASS FOR R113 With reference to Fig. 1.4, the theoretical Rankine coefficient of perfor mance can still be defined by the Equation (1.7). The entropy φ of the two phase mixture at D can be related to the entropies of the saturated
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