AIR CONDITIONING SYSTEM DESIGN AIR CONDITIONING SYSTEM DESIGN ROGER LEGG Retired, previously senior lecturer at London South Bank University Butterworth-Heinemann is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States © 2017 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.
This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein.
In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-101123-2 For information on all Butterworth-Heinemann publications visit our website at https://www.com/books-and-journals Publisher: Matthew Deans Acquisition Editor: Brian Guerin Editorial Project Manager: Edward Payne Production Project Manager: Anusha Sambamoorthy Cover Designer: Mark Rogers Typeset by SPi Global, India DEDICATION To staff and students, past and present, of the ‘National College’. v The general antiphlogistic remedies are … free admission of pure cool air.
John Alikin, ‘Elements of Surgery’, 1779 … the dreadful consequences which have been experienced from breathing air in situations either altogether confined or ill ventilated … if others are in the same apartment, the breath from each person passes from one to another, and it is fre- quently in this way that diseases are communicated. The Marquis de Chabannes, 1818 The very first rule of nursing … is this: to keep the air he breathes as pure as the external air, without chilling him. Florence Nightingale, 1863 vii FOREWORD Air conditioning is no longer regarded as the luxury that it once was, and there is now an increasing demand for applications ranging through domes- tic, commercial, industrial, and transport and for specialized installations such as hospitals, research facilities, data centres, and clean rooms. The engi- neering systems in modern buildings and installations make a significant con- tribution to the overall building performance in terms of energy use.
Systems need to be increasingly sophisticated in their design, installation, operation, control, and maintenance at a time when there is increasing pressure for greater energy efficiency. This has led to a demand for more qualified engineers and other profes- sionals involved in building design. All those involved need to understand the underlying principles of the topics covered in this volume. The book, which is a complete revision of Roger’s previous work published by Batsford in 1991, contains new chapters on unitary systems and chilled beams.
It provides a good technical foundation of building service engineering and covers significant proportions of the syllabus requirements of academic courses in this discipline. The theoretical coverage is backed with relevant worked examples and the use of data from the latest editions of CIBSE and ASHRAE publications, which should make this text appeal to students and practising professionals in both Europe and North America. The author is well qualified in this discipline having taught the subject for more than 30 years at the Institute of Environmental Engineering (formerly the National College for Heating, Ventilation, Refrigeration and Fan Engineering, South Bank University, London). In addition, he has used contributions from key specialists to support specific areas; these included Associate Prof.
Risto Kosonen, Prof. Tim Dwyer, Mr. Terry Welch, Prof. Ron James, Prof.
John Missenden, and Mr. Farrell London 2017 (Retired, previously principle lecturer at London South Bank University and head of the Institute of Environmental Engineering) xv ACKNOWLEDGMENTS I am indebted to my ex-colleagues at South Bank University for much prac- tical help, encouragement, and advice in the writing of this book. In particular, I am most grateful to Mr. Terry Welch for VRV systems and the discussion in Chapter 7; to Prof.
Ron James and Terry Welch for Chapter 9, on refrig- eration and heat-pump systems; to Stan Marchant for the text on cooling towers in Chapter 10; to Prof. Tim Dwyer who contributed the overview of control systems in Chapter 17; and to Prof. John Missenden who pro- vided the text for control valves in Chapter 17. My thanks are also due to Prof.
Risto Kosonen of Aalto University, Sweden, for writing Chapter 8. My son Mark gave me a great deal of help with word processing. Lastly, my thanks are due to Brian Guerin, Edward Payne, and other members of Elsevier for their dedication in bringing this book to its completion. BROMLEY 2017 RCL The author and publishers thank the following for permission to use certain material from books and articles and to use illustrations as a basis for figures in this volume: Tables 1.1 from the CIBSE Guide by permission of the Chartered Institute of Building Services Engineers.4 courtesy of the FISCHER company.2 (redrawn) by permission of McGraw Hill Book Co.4 warm temperatures in the United Kingdom, CIBSE Guide A.16 VAV Redrawn from Fig.27 of the C1BSE Guide B, by permission of the Chartered Institute of Building Services Engineers.7 based on illustrations, courtesy of Trox Brothers Ltd.4 courtesy of ICI Chemicals and Polymers Ltd.6 drawing of jacketed steam humidifier based on Armstrong via website.8 supplied by Thermal Technology Ltd.4 by permission of Fl€akt Woods Limited.4 supplied by Vokes Ltd.4 courtesy of Flaxt Woods—the United Kingdom.
xvii xviii Acknowledgments Fig.7 Moody chart from D S Miller Internal Flow Systems, Second edition, 1990, BHRA, Cranfield, the United Kingdom, with permission (note that the chart has some additional information that has been removed).13 (based on figures in Internal Flow Systems (Second Edition) 1990, BHRA, Cranfield, the United Kingdom) by permission of DS Miller.8 hooded vane anemometer, courtesy of Inlec the United Kingdom Ltd.9A courtesy of Holmes Valves Ltd.13 courtesy Crane Fluid Systems.15 courtesy of Crane Ltd.1 by permission of the Building Services Research and Informa- tion Association. CHAPTER 1 Properties of Humid Air Air is the working fluid for air conditioning systems. It is therefore important for the engineer to have a thorough understanding of the properties of air, before going on to consider the processes that occur when air passes through the various plant items that make up systems. The word psychrometry is often used for the science that investigates the properties of humid air, and the chart that shows these properties graphically is known as the psychrometric chart.
In this chapter, the various air properties are defined, and the appropriate equations are given. In deriving the equations, it is usual to consider the air as consisting of two gases, dry air and water vapour. Even though one of these is strictly a vapour, both are considered to obey the ideal gas laws. Lastly, the tables and chart, from which numerical values of the air properties are obtained for practical calculations, are described and illustrated.
ATMOSPHERIC PRESSURE At any point in the earth’s atmosphere, there exists a pressure due to the mass of air above that point—the atmospheric pressure. Standard atmospheric pressure at sea level is 1013.25 mbar (usually approximated to 1013 mbar), but due to changes in weather conditions, there are variations from this stan- dard pressure. For example, among the minimum and maximum values recorded in London are 948. Atmospheric pressure varies with height above sea level, and for altit- udes at which mankind lives, the rate of decrease (lapse rate) for a stan- dard atmosphere may be taken as a reduction of 0.13 mbar per meter of height above sea level and an increase of 0.13 mbar per meter of depth below sea level.
Air Conditioning System Design © 2017 Elsevier Ltd.00001-7 All rights reserved. 1 2 Air Conditioning System Design Example 1.1 Determine the standard atmospheric pressure for Nairobi, which is at an altitude of 1820 m above sea level. Solution Standard sea-level atmospheric pressure 1013 Lapse rate ¼ 1820 0.13 237 Standard atmospheric pressure for Nairobi 776 mbar Atmospheric pressure may be measured by using a number of instru- ments. In the laboratory, it is usual to use a Fortin barometer, while for site work an aneroid barometer is the most usual instrument.
For continuous recording, a barograph is used. DRY AIR AND WATER VAPOUR Dry air consists of a number of gases but mainly of oxygen and nitrogen. It is necessary to know the molecular mass of the dry air, and this is calculated from the proportion each individual gas makes in the mixture.1 gives this data, together with the calculation. The sum of the molecular mass fractions is 28.97 and this is the value taken as the mean molecular mass of dry air.
Water vapour is said to be associated with the dry air. Its molecular mass is obtained from the masses of its chemical composition H2O, i.1 Determination of molecular mass of dry air Proportion by Molecular Molecular mass volume (%) mass (%) fraction (%) Gas (1) (2) (1) × (2) Nitrogen, N2 78.72 Carbon dioxide, CO2 0.38 Molecular mass fraction 28.97 Properties of Humid Air 3 VAPOUR PRESSURE Saturated Vapour Pressure Consider the vessel shown in Fig. The contents are at temperature 1°C, and the atmosphere above the water contains water vapour that exerts a pressure known as saturated vapour pressure (SVP). When heat is applied to the vessel, more water evaporates, and as the temperature rises, the SVP increases.
Eventually, with heat still being supplied, the water will boil, and this happens when the SVP is equal to atmospheric pressure. The var- iation of saturated vapour pressure against temperature is shown in Fig. Open to atmosphere Mixture of dry air and water vapour at temperature t Water Heat Fig.1 Vessel with saturated vapour. Saturation vapour pressure Temperature Fig.2 Saturation vapour pressure versus temperature.
4 Air Conditioning System Design Table 1.2 Saturation vapour pressures Dry-bulb temperature SVP Dry-bulb temperature SVP (°C) (mbar) (°C) (mbar) 0 6.2 Values of SVP have been determined by experiment and published in the form of steam tables, selected values of which are given in Table 1. There is no simple relationship between temperature and SVP. The following equations are the relevant curve fits published by the National Engineering Laboratory [2]: For water above 0°C, log 10 Pssw ¼ 28:59 8:2 log 10 T + 0:00248T 3142=T where Pssw is the SVP in bar, over water at absolute T (K). For ice below 0°C: log 10 ¼ 10:538 2664=T where Pssi is the SVP in bar, over ice at absolute temperature T (K).
These equations are suitable for use in computer programs in which air property values are required; they are not used in this text. Superheated Vapour If all the water in the vessel shown in Fig.2 evaporates before boiling point has been reached and heat continues to be applied, the water vapour becomes superheated with the vapour pressure remaining constant. There- fore, on Fig.