Heating Services Design PDF
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Heating Services Design R o n a ld K. M c L a u g h l i n, BSc, mcibs Senior Lecturer in Building Services, Glasgow College of Building and Printing R. Craig M c L e a n, BSc, mcibs Lecturer in Environmental Engineering, University of Strathclyde, Glasgow W. J o hn B o n t h r o n, BSc, CEng, MIMechE, MCIBS, MASHRAE Associate, Hulley and Κ irk wood, Consulting Engineers, Glasgow BUTTERWORTHS London · Boston · Sydney · Wellington · Durban · Toronto All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording without the written permission of the copyright holder, application for which should be addressed to the publishers. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. This book is sold subject to the Standard Conditions of Sale of Net Books and may not be resold in the UK below the net price given by the Publishers in their current price list. First published 1981 © Butterworth & Co (Publishers) Ltd, 1981 British Library Cataloguing in Publication Data McLaughlin, R Κ Heating services design. 1. Heating I. Title II. McLean, RC III. Bonthron,W J 697 TH7222 79-42812 ISBN 0-408-00380-4 Typeset by Reproduction Drawings Ltd., Sutton, Surrey. Printed in Scotland by Thomson Litho Ltd., East Kilbride. Preface The 1970 edition of the I.H.V.E. Guide heralded not only a change to the S.I. system of units but more importantly, a signifi- cant step towards the strengthening of the technological basis of design methods and calculations for building services. This reflected a trend in which standards at all levels of education and training throughout the industry were being raised. More recently, the granting of a Royal Charter to the C.I.B.S. (I.H.V.E.) has led to the recognition of the building services engineer as a Chartered Technologist, educated to degree level. These developments have led to the growth of university and polytechnic departments teaching courses in building services and environmental engineer- ing. Research activities in these departments now supplement the work done by B.R.E., B.S.R.I.A. (H.V.R.A.) and others. During this period of rapid development in industry, education and research, there has been, unfortunately, little parallel develop- ment in the availability of higher-level books to meet the needs of the designer. This has aggravated a situation in which, compared to other branches of engineering, there was already a lack of suit- able books. The aim of this book is to go part of the way towards rectify- ing this situation within the topic area of heating services design. It has been written primarily for professional engineers and undergraduate students in building services and environmental engineering and in architecture. It should also prove useful to building services technicians and students in related disciplines. The early chapters are concerned with a rigorous treatment of fundamentals which are used subsequently in the presentation of the design methods and calculation routines set out in later chapters. These methods and routines have been presented deliberately in a form suitable for manual solution because the authors believe that this is a necessary pre-requisite to the applica- tion of computer-based methods. It is evident from Chapters 5 and 6 that many of the new design methods are intrinsically computer orientated and it is hoped that the discussion and explanations given will prove useful to those who wish to employ computer-based design methods. It is important to note that this book is not intended to be a self-contained design manual but has been written for use primarily with the C.I.B.S. Guide and the other sources referenced. Indeed, it should be considered as complementing rather than replacing existing textbooks. The authors wish to express their thanks to Bill Boyle, Dr Joe Clarke, Professor Tom Maver, Don Stewart and Roy Veitch, Preface all of the University of Strathclyde, for their help and encourage- ment; to Mrs Pat Gray for her patience in typing the text; and to the members of three families for their fortitude. Finally, thanks are due to all those who permitted the repro- duction of material in the book, in particular the C.I.B.S. R.K. McL. R.C. McL. W.J.B. 1 The Fundamentals of Fluid Flow 1.1 INTRODUCTION A highly significant aspect of the work undertaken by the heating services engineer is concerned with the design of the fluid distri- bution systems for both heating and water supply purposes. It is undoubtedly the case that in many straightforward situations workable flow systems may be devised with little awareness on the part of the 'designer' of the fundamental engineering princi- ples involved. However, the problems which can be treated suc- cessfully at such a level are limited and if the engineer is to be equipped to tackle competently the whole range of situations likely to be encountered in practice (and indeed be involved in the initiation of design improvements and new design techniques), then a firm understanding of fluid flow and the nature of its associated mechanisms is required. The aim of this chapter is to go part way towards meeting this requirement. It is by no means intended as an all-embracing intro- duction to fluid mechanics but seeks rather to provide a suitable treatment of the fundamentals which are relevant to the design problems considered in the later chapters. Thus, although some of the material covered in the earlier part of the chapter has applica- tions in many areas of fluid mechanics, the subject matter of the chapter as a whole is concentrated principally on the study of incompressible fluid flow in pipes. 1.2 FLUID PROPERTIES 1.2.1 Introduction Whilst the ideal fluid, which constitutes the simplest model for flow analysis, is considered to be incompressible and to offer no resistance to deformation, all real fluids display to varying degrees both compressibility and viscous effects. In addition, vaporisa- tion effects are displayed in liquids which have a free surface. These three important characteristics will now be reviewed briefly. 1.2.2 Compressibility All fluids may be compressed by the application of pressure forces. The degree of compressibility of a fluid is characterised by 1 2 The Fundamentals of Fluid Flow defining the bulk modulus V K = -Ap — (1.1) E ΔΙ/' Here Ap represents the increase in pressure necessary to decrease a given volume V' by the amount-ΔΙ/'. All liquids have a high value of bulk modulus and are compressible only to a small extent. For example, the bulk modulus of water is quoted as 20.085 x 105kN/m2, and thus a decrease of only 0.2% in a given volume requires a pressure increase of ΔΙ/' 20.685 χ 105 Ap = K = 41.37 χ 102 kN/m2 E V' 500 In the study of fluid mechanics it is necessary to make the dis- tinction between flows in which compressibility effects may be ignored and flows in which they require to be taken into account. As the change in the density of a liquid with an increase in pres- sure is small even for very large pressure changes, the density of a liquid is consequently taken as constant in most flow situations. The analysis of problems involving liquids is thereby greatly sim- plified. Some exceptions to this general simplification do exist however, and in certain special flow problems the compressibility of liquids is an important factor—as in the case of water-hammer, where the fluid is subjected to a very high rate of velocity change. Unlike liquids, gases are highly compressible. However, in flows where a gas is subjected to relatively small changes in pres- sure (e.g. ventilation and air conditioning systems), the corres- pondingly small density variations are generally ignored and the gas is treated as an incompressible fluid. On the other hand, in high-speed flows, where the fluid velocity approaches that at which sound is propagated through the medium, compressibility effects become important and must be taken into account. Theory developed on the assumption of incompressibility would lead to very serious errors if applied to such situations. 1.2.3 Viscosity Fluids resist any force which tends to cause the motion of one layer of fluid relative to another. This fractional phenomenon is attributed to the viscosity of the fluid and is manifested as a shearing stress between the layers acting to oppose the relative motion. Consider the two-dimensional flow of a fluid adjacent to a solid boundary as described in Figure 1.1. Two infinitesimally thin fluid layers are shown in relative motion. A frictional stress r will exist at the interface of these two layers, such as to retard the faster moving layer and accelerate the slower moving one. This shear stress is defined by Newton's viscosity law 3 The Fundamentals of Fluid Flow Figure 1.1 y υ-υ{ y) υ dv τ = μ- (1.2) where dv/dy is the velocity gradient and μ is known as the coeffi- cient of viscosity (also termed the absolute viscosity, dynamic viscosity, or simply, the viscosity of the fluid). This coefficient, being the ratio of a shear stress to a velocity gradient, is dimen- sionally given by 2 Since [F] = [ML/T ], it is evident that this dimensional combina- tion is equivalent to The basic S.I. unit of viscosity may therefore be quoted as Ns/m2 or kg/ms. As we shall see in 1.5.1. and 2.3.4, the second dimen- sional form of μ given above is the one which is employed usually in dimensional analysis procedures. The c.g.s. units of viscosity, the poise (dyne s/cm2) and the cent/poise, are still widely used 1 Ns/m2 = 10 poise = 103 centipoise The viscosity of a fluid has its origins in two fundamental molecular mechanisms, both of which are highly dependent on temperature-intermolecular attraction and molecular momentum exchange between fluid layers. In gases, the dominant mechanism is the momentum exchange which results from the thermal agi- tation of molecules normal to the direction of motion. As this activity increases with temperature, it therefore follows that the viscosity of gases will also increase with temperature (Figure 1.2a). In liquids, the effects of molecular momentum exchange between layers are insignificant compared with the forces of intermole- cular cohesion and the viscosity depends primarily on the magni- tude of these forces. Since they decrease rapidly with temperature, the viscosity of liquids decreases as the temperature rises (Figure 1.2b). in order to maintain relative motion in a fluid, it is obvious 4 The Fundamentals of Fluid Flow Figure 1.2a Viscosities of some common gases 20 40 60 80 100 120 140 Temperature °C Figure 1.2b Viscosities of some I8r common liquids I6 I4 Mercury Ι2 Ο χ ΙΟ 1 > 8 ο S*r C ar bon tetr' achloric e ω Water δ Petrol 20 40 60 80 100 120 140 Temperature °C that work must continuously be done in overcoming the viscous forces opposing the motion. This work is manifested wholly as an irreversible addition of heat to the fluid. The maintenance of the relative motion is therefore accompanied by a unidirectional flow of energy from the mechanical forms generating the motion to 5 The Fundamentals of Fluid Flow the internal energy of the fluid. Most fluids, including those en- countered in heating services applications, behave in the manner described by equation 1.2, with shear stress being proportional to velocity gradient. These are known as Newtonian fluids. Some other fluids, such as plastics, are not characterised by this propor- tionality. Such fluids are described as non-Newtonian (Figure 1.3). Figure 1.3 du/dy du/dy Newtonian fluid Non-Newtonian fluid In many fluid flow problems it is frequently convenient to use the ratio of the viscosity to the density. This ratio, Ρ is known as the kinematic viscosity. Dimensionally, ν is given by 3 \M/LT]I[M/L}=[LVT], and the basic S.I. unit of ν is therefore m2/s. More generally, however, the mm2/s (= 10"6 m2/s) is employed. The c.g.s. units 2 of kinematic viscosity are the stoke (cm/s) and the cent/stoke, thus 1 centistoke = 10"2 stokes = 1 mm2/s 1.2.4 Vaporisation All liquids having a free surface tend to vaporise because of the continuous projection of molecules through the free surface and out of the body of the liquid. Some of these molecules re-enter the liquid so that there is, in fact, a constant interchange of mole- cules between the liquid and the space above it. The molecules which are free of the liquid, being gaseous, exert their own partial pressure, known as the vapour pressure. This pressure, together with the pressure of any other gases present, make up the total pressure above the liquid. For a liquid with an enclosed space above it, the vapour pres- sure will increase until a maximum value is attained. At this equilibrium condition the rate of molecules leaving the liquid is equal to the rate at which molecules are returning to it. This maximum value of vapour pressure is termed the saturation vapour pressure and any gas above the liquid is said to be satu- rated with the vapour. Saturation vapour pressure depends only 6 The Fundamentals of Fluid Flow on temperature (Table 1.1) and is independent of the presence of any other gas(es). If the total pressure above the liquid becomes less than the saturation vapour pressure then bubbles of vapour develop inside the liquid and rise to the surface. This is the pheno- menon of boiling and it is, of course, associated with an extremely rapid escape of molecules from the liquid. Table 1.1 Saturation Vapour Pressure of Water _ ,o ~s Saturation Vapour Temperature ( C) Pressure {tym*) 0 611 10 1227 20 2337 40 7375 60 19920 80 47360 100 101325 Even in the absence of a free surface, the vaporisation charac- teristics of liquids can still be of great importance. If a liquid enters a region of a flow system where the pressure falls locally to a value below the prevailing saturation vapour pressure, bubbles of vapour form within the fluid and are carried along by the fluid stream. It is interesting to note that a similar process may occur at pressures greater than the saturation vapour pressure if dissolved gases are present in the liquid, because as the pressure is reduced these gases come out of solution as bubbles. The production of bubbles (whether of vapour or gas) in a flow system under low pressure conditions, is known as cavitation. When the fluid stream enters a region of higher pressure, cavitation bubbles undergo a sudden collapse. As the liquid rushes in to fill the cavities, ex- tremely high pressures and temperatures are generated and if the collapse occurs adjacent to a solid surface, serious mechani- cal damage can result. Anti-cavitation precautions will be con- sidered in Chapter 9. 1.3 VISCOUS FLUIDS IN MOTION 1.3.1 The two regimes of flow In regions of flow adjacent to solid surfaces (pipes, boundary layers), velocity gradients are large and fluid viscosity is an impor- tant parameter. The effects of viscosity allow the possibility of two physically different types of flow, known as flow regimes. The essential characteristics of these regimes were first clearly demonstrated by Reynolds^1 * in 1883. It is instructive to con- sider briefly the nature of the experiments which Reynolds performed. Reynolds' apparatus is schematically illustrated in Figure 1.4.
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