REFRIGERATION AND AIR CONDITIONING

Transcription

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2 REFRIGERATION AND AIR CONDITIONING

3 About the Author Late (Dr.) Ramesh Chandra Arora served as Professor of Mechanical Engineering at Indian Institute of Technology Kharagpur from 1987 to He received his B.Tech. and M.Tech. degrees in mechanical engineering from Indian Institute of Technology Kanpur in 1967 and 1969, followed by a Ph.D. (Fluid Mechanics and Heat Transfer) in 1975 from Case Western Reserve University, Cleveland, Ohio, USA. His research areas included fluid mechanics, heat transfer, refrigeration and air conditioning, alternative refrigerants and thermodynamic cycles to highlight a few. Professor Arora had patents pending for Design of facility for generation of mono-disperse test aerosols with size control, Design of low temperature drying system for seed grains, and A novel methodology of energy optimization for variable-air-volume air conditioning systems. Professor Arora also held several administrative assignments during his 30 years of tenure at Indian Institute of Technology Kharagpur ( ). These included Deputy Director (Acting); Founder member and Head of Department, School of Medical Science and Technology; Dean of Students Affairs; Chairman, Hall Management Committee; Coordinator, Master s programme in Medical Science and Technology; Professor-in-Charge, Refrigeration and Air Conditioning Laboratory of Mechanical Engineering Department; Chairman, Undergraduate Programme Evaluation Committee Mechanical Engineering Department; Coordinator, Postgraduate and Research Committee of Mechanical Engineering Department; Faculty Advisor, Mechanical Engineering Society of Mechanical Engineering Department. Professor Arora was a highly respected subject matter expert in academia and industry alike. A few of his responsibilities and associations included: Member, National Advisory Committee of ISHTM-ASME, Joint Heat and Mass Transfer Conference; Member, Engineering Science Committee of Council of Scientific and Industrial Research, New Delhi, Govt. of India; Member, Cryogenics Committee of the Department of Science and Technology, New Delhi, Govt. of India; Reviewer: J. of Institution of Engineers; Reviewer: Indian J. of Engineering and Material Science, CSIR, New Delhi; Member, Board of Governors, Regional Engineering College, Durgapur; Member, Expert Committee to suggest improvement in operation of cold storages, Govt. of West Bengal. Professor Arora was also the recipient of several outstanding teacher awards, national scholarships, merit awards, graduate fellowship. He is credited with 91 research publications, 8 technical reports, 19 industrial projects and 3 pending patents. He guided 12 researchers towards their Ph.D. degrees, and 29 students towards their M.Tech. degrees, apart from being a constant anchor and mentor for numerous undergraduate students across departments.

4 REFRIGERATION AND AIR CONDITIONING RAMESH CHANDRA ARORA Formerly Professor Department of Mechanical Engineering Indian Institute of Technology Kharagpur New Delhi

5 REFRIGERATION AND AIR CONDITIONING Ramesh Chandra Arora 2010 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN The export rights of this book are vested solely with the publisher. Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi and Printed by Rajkamal Electric Press, Plot No. 2, Phase IV, HSIDC, Kundli , Sonepat, Haryana.

6 Contents Preface Acknowledgements 1 History of Refrigeration Introduction Natural Cooling Processes Mechanical Cooling Processes 3 References 17 Review Questions 17 2 Thermal Principles A Review of Fundamentals Introduction Thermodynamic Properties Closed and Open Systems Units The Four Laws of Thermodynamics Zeroth Law of Thermodynamics First Law of Thermodynamics First Law of Thermodynamics for Open Systems Second Law of Thermodynamics Fundamental Relations of Thermodynamics Third Law of Thermodynamics Perfect Gas Mixture of Ideal Gases Real Gas and Vapours Dry Air Properties of Pure Substance 43 v xiii xv

7 vi Contents 2.17 Correlations for Properties of Refrigerants Heat Transfer Conduction Fick s Law of Diffusion Thermal Radiation Convection Condensation Heat Transfer Boiling Heat Transfer Reynolds Analogy Analogy between Heat, Mass and Momentum Transfer Heat Transfer through Composite Walls and Cylinder Heat Exchangers Fluid Flow Cooling Processes 85 References 95 Review Questions 95 3 Mechanical Vapour Compression Cycles Introduction Vapour Compression Cycle Refrigeration Capacity Coefficient of Performance Reversed Carnot Cycle or Carnot Refrigeration Cycle External Regime and Internal Regime Gas as Refrigerant Pure Substance as Refrigerant Standard Vapour Compression Cycle or Vapour compression Cycle or Single Stage Saturation (SSS) Cycle Representation of Work as Areas on the T s Diagram Comparison of Standard Refrigeration Cycle with Reversed Carnot Cycle Refrigerant Tables Thermodynamic Properties Subcooling and Superheating Performance of Single Stage Saturation Cycle Effect of Refrigerant Properties Suction State for Optimum COP, Ewing s Construction Actual Cycle Diagram 148 Review Questions Compressors Introduction Thermodynamics of Compression Reciprocating Compressors Hermetic Compressors Rotary Compressors Centrifugal Compressors Comparison with Reciprocating Compressor Capacity Control 238

8 Contents vii 4.9 Selection of Compressors 239 References 240 Review Questions Performance of Single Stage Saturation Cycle with Reciprocating Compressor Introduction Volumetric Efficiency and Mass Flow Rate Work Requirement and HP/TR Specific Refrigeration Effect and Refrigeration Capacity Swept Flow Rate per TR Adiabatic Discharge Temperature Coefficient of Performance Methods of Improving COP Choice of Intermediate Pressure Optimum Intermediate Pressure for Ideal Gas Compressor with Ideal Intercooling Optimum Intermediate Pressure if Intercooling is Done Up to Temperature T w Optimum Intermediate Pressures for Three-Stage Compression 259 Reference 267 Review Questions Multistage Refrigeration Systems Introduction Two-stage NH 3 Cycle Recommended Temperature Ranges for Multistage Systems Multi-evaporator Systems Two-stage Reversed Carnot Cycle Limitations of Multistage Systems Cascade Refrigeration System Dry Ice Manufacture Auto-cascade System 347 References 348 Review Questions Absorption Refrigeration Systems Introduction Absorption Cycle of Operation Maximum COP Properties of Solutions Aqua Ammonia Solution Simple Absorption System h x Diagram for Simple Absorption System Drawbacks of Presence of Water Vapour in Evaporator and Condenser Ammonia Enrichment Process Water Lithum Bromide Absorption Refrigeration System The Platen Munters System 404

9 viii Contents 7.12 Properties of Refrigerant Pairs for Absorption Systems Comparison of Absorption System with Mechanical Vapour Compression Refrigeration System 408 References 408 Review Questions Refrigerants Introduction Designation of Refrigerants Some Commonly Used Refrigerants Desirable Properties of Refrigerants Reaction with Lubricating Oil Reaction with Moisture Thermodynamic Properties Alternative Refrigerants Mixtures Alternatives to Various Popular Refrigerants Natural Refrigerants Secondary Refrigerants 465 References 468 Review Questions Expansion Valves Introduction Capillary Tube Automatic Expansion Valve Thermostatic Expansion Valve Float Type Expansion Valve Electronic Type Expansion Valve Some Practical Problems in Operation of Expansion Valves 502 References 503 Review Questions Condensers Introduction Heat Rejection Ratio Types of Condensers Comparison of Water-cooled and Air-cooled Condensers Comparison of Water-cooled and Evaporative Condensers Air-cooled Condenser Mean Temperature Difference for Crossflow Heat Exchanger Fin Efficiency Heat Transfer Areas Overall Heat Transfer Coefficient Heat Transfer Coefficients Water Cooled Condensers 530 References 547 Review Questions 548

10 Contents ix 11 Evaporators Introduction Classification of Evaporators Natural Convection Coils Flooded Evaporator Shell-and-Tube Liquid Chillers Direct Expansion Coil Plate Surface Evaporators Finned Evaporators Boiling Heat Transfer Coefficients 567 Reference 570 Review Questions Complete Vapour Compression System Introduction Reciprocating Compressor Performance Characteristics Condenser Performance Characteristics Evaporator Performance Characteristics Expansion Valve Characteristics Condensing Unit characteristics Performance of Complete System Condensing Unit and Evaporator Effect of Expansion Valve Conclusion 581 Reference 582 Review Questions Gas Cycle Refrigeration Introduction Ideal Gas Behaviour Temperature Drop Due to Work Output Temperature Drop in Steady Flow Due to Change in Kinetic Energy Temperature Drop in Closed System Due to Change in Kinetic Energy Reversed Carnot and Joule Cycles for Gas Refrigeration Aircraft Refrigeration Cycles Vortex Tube Refrigeration Pulse Tube Stirling Cycle Air Liquefaction Cycles 648 Review Questions Water Steam Ejector Refrigeration System and Thermoelectric Refrigeration System Introduction Principle of Operation Centrifugal Compressor-Based System Steam-Jet Ejector System 664

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