Heating and Cooling Load Calculations PDF
Preview Heating and Cooling Load Calculations
OTHER TITLES IN THE SERIES IN HEATING, VENTILATION AND REFRIGERATION VOL. 1. OsBoRNE-Fans VOL. 2. Em-An Introduction to Heat Transfer Principles and Calculations VOL. 3. Kur-Heating and Hot Water Services in Buildings VOL. 4. ANcus-The Control of Indoor Climate HEATING AND COOLING LOAD CALCULATIONS P. G. DOWN, M.i.H.v.E. Associate, Oscar Faber and Partners PERGAMON PRESS OXFORD · LONDON · EDINBURGH · NEW YORK TORONTO · SYDNEY · PARIS · BRAUNSCHWEIG Pergamon Press Ltd., Headington Hill Hall, Oxford 4 & 5 Fitzroy Square, London W.l Pergamon Press (Scotland) Ltd., 2 & 3 Teviot Place, Edinburgh 1 Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523 Pergamon of Canada Ltd., 207 Queen's Quay West, Toronto 1 Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street, Rushcutters Bay, N.S.W. 2011, Australia Pergamon Press S.A.R.L., 24 rue des Écoles, Paris 5e Vieweg & Sohn GmbH, Burgplatz 1, Braunschweig Copyright © 1969 Pergamon Press Ltd First edition 1969 Library of Congress Catalog Card No. 68-57885 Printed in Great Britain by A. Wheaton & Co., Exeter 08 013001 1 EDITORS' PREFACE MODERN industrial civilization depends for its existence on man's control of his environment. Simple comfort requires that in most parts of the world buildings must be artificially heated or cooled during some part of the year. Rising standards of living have made people intolerant of the conditions of yesteryear in factories, offices and the home, and manufacturing processes themselves are requiring ever closer control of environment. Present-day air travel would be impossible without the air conditioning of aircraft. Heating and air conditioning, then, has an essential contribution to make to the life of everyone—in the home, at work, while travel ling or during recreation. These engineering services can account for between one-tenth and one half of the total cost of a building, depending on their complexity and sophistication. They require expert design; and the number of skilled personnel is, almost every where, too small. These, then, are the justifications for a series of textbooks dealing with the design of heating and air conditioning plant and equipment. The series is planned to include the following subjects: Basic principles of heating and ventilating Heating and cooling load calculation Heating and hot-water supply Ventilation and air conditioning of buildings Industrial ventilation Fuels and boilerhouse practice Heat and mass transfer Fans Dust and air cleaning Refrigeration technology Each volume in the series is complete and self-contained in so far as the technical and practical engineering applications of its main theme are concerned, but for a more detailed discussion of the underlying principles of certain subsidiary subjects and for derivation of the ix X EDITORS' PREFACE formulae and equations quoted reference to the other volumes may be necessary. For example, heat transfer formulae must be quoted and used in more than one of the books but their derivation is given in all necessary detail in the specialist volume on heat transfer. Similarly for heating and cooling load calculations which concern not only ventilation and heating but also refrigeration. This treatment has allowed more detailed consideration of the subject than is possible in an omnibus volume of manageable size. Another books that should be consulted when more detail is required is The Measurement of Air Flow by Ower and Pankhurst (Pergamon Press, 1966), which does not form part of the series because it covers a considerably wider area. The authors have taken as their starting-point a basic training in general engineering such as may be acquired during the first years of apprenticeship. On this foundation, the specialist treatment is built and carried to a level approximating to that of a first degree. The graduate engineer or physicist who wishes to enter this field will also find the series useful, since he is introduced to new disciplines (for example, human physiology or climatology) and new applications of his fundamental knowledge, while some parts of his undergraduate course work are taken to much greater depth. Throughout the whole series, the practical applications are stressed. The volumes do not pretend to cover the whole range of problems encountered in design, though a student who has mastered the basic principles embodied therein should be a competent engineer capable of handling a majority of the tasks he will meet. For the rest, practical experience backed by further study of more advanced texts will be essential. N.S.B. E.O. PREFACE AN ABILITY to calculate the heating or cooling load requirements of a structure is basic to the design of the system. The aims of this book are to provide a logical study of the physical and engineering factors which affect the load and to give sufficient examples to show their practical application. An attempt has been made to define or derive all expressions and formulae used in the book. The book is intended to be of use to students, and also to practising engineers, as a handbook of design principles and an introduction to new techniques. Where subjects require involved mathematical treatment this has been extracted and presented as Appendixes, so that the basic theory of the study remains clear. For the same reason design-data tables, where these are available in standard references, have not been included unless required for the design examples. Acknowledgement is given to the authors to whom reference is made where the source is known and an apology is offered to any author whose work has inadvertently not been acknowledged. In particular, thanks are due to the Institution of Heating and Ventilating Engineers and to the American Society of Heating, Refrigeration and Air Conditioning Engineers for permission to use data from their design handbooks. LIST OF SYMBOLS a coefficient c specific heat d solar declination angle, thermal diffusivity / a function g gravitational acceleration h solar hour angle i solar incident angle k thermal conductivity / linear size m material mass n harmonic coefficient p pressure q rate of flow resistivity temperature temperature difference air or ambient temperature sol-air temperature inside air temperature outside air temperature surface temperature v velocity w weight z solar azimuth angle A area B latitude angle C thermal conductance C, surface conductance E total emission F Fahrenheit G gas constant H pressure head or height, rate of heat supply / solar radiation intensity J rate of interest L material thickness M monthly percentage P percentage or ratio Q total quantity R thermal resistance RH relative humidity S percentage or ratio T time U thermal transmittance V volume, ventilation rate W weight XIV LIST OF SYMBOLS a solar altitude angle alpha Δ difference delta € exponential epsilon λ harmonic decrement factor lambda μ viscosity mu V solar reflectivity fraction nu P density rho Σ summation sigma σ solar absorptivity fraction sigma τ solar transmissivity fraction tau Symbols used as constants are defined in the text. CHAPTER 1 HEAT TRANSFER 1.1. BASIC THEORY The calculation of heating and cooling loads requires some know ledge of the mechanics of heat transfer. A formal introduction to the subject is contained in the companion volume Heat and Mass Transfer, and only those aspects which directly affect heating or cooling loads will be briefly discussed in this chapter. Each of the three ways in which heat is transferred : conduction, convection and radiation, is met, either separately or combined, in the calculation of heating or cooling loads, and a clear understanding of the res pective mechanics of transfer is essential to the intelligent use of theoretical and test data (Fig. 1.1). Conduction is the process of heat transfer by which heat passes from the hot to the cold parts of a solid, liquid or gas by an exchange of energy. The manner in which the exchange takes place varies with the nature of the material, but essentially it is the flow of heat from particle to particle and not the flow of the particle intself. A simple illustration of conduction is the way in which the handle of a poker pushed into a fire will become hot. Convection is the process of heat transfer by which heat passes between a fluid or gas and a surface, or within the fluid, by the actual movement of the molecules. The process is known as free convection when the movement is due to gravitational effects resulting from density changes, and forced convection when the movement is due to an external force. Following the illustration above, convection is the way in which warm air is circulated in the room. Radiation is the process of heat transfer by which heat is propa gated by the radiating body by means of electromagnetic waves which are converted back into heat upon encountering matter. The process does not depend on the medium through which the heat 1 2 HEATING AND COOLING LOAD CALCULATIONS passes and can take place across a vacuum. Completing the illus tration, radiation from a hot fire can be shut off by interposing a solid material. 1.2. APPLICATION TO BUILDING HEAT TRANSFER Heat will flow into or out of a building by a combination of conduction, convection and radiation, but it is useful to distinguish the component parts and to define the units of flow. The following definitions are accompanied by the symbol by which they are represented. FIG. 1.1. Types of heat transfer. Conductivity (k). The quantity of heat which will be conducted through unit area of a slab of material of unit thickness with unit difference of temperature between faces in unit time. It will be seen from the definition that steady-state heat transfer by conduction between the faces of a wall varies with the following factors and relationships: conductivity of wall material (Jc) directly area normal to the direction of heat flow (A) directly thickness of material (L) inversely temperature difference across wall (Δί) directly time (Γ) directly
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