Document QJJgwQQ7N2N46X91DJ1zjp2m8

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 considered. Thus thermal capacity in addition to resistance effects are. significant. The vector sum of the thermal capacitance and resistance is the thermal impedance. It is not within the scope of this chapter to deal with many of these problems. These are, however, solutions available in graphical form for certain special cases. Also a general approximate method may be employed which is analogous to the treatment of capacityresistance lumped parameter electrical circuits. REFERENCES Heat and Thermodynamics, by Mark W. Zemansky (McGraw-Hill Co., 1937, p. 48) Heat Transmission, by W. H. McAdams (McGraw-Hill Co., 1937). The Calculation of Heat Transmission, by Margaret Fishenden and Owen A. Saunders (His Majesty's Stationery Office, London). Industrial Heat Transfer, by A. Schack, translated from the German by H. Gold schmidt and E. P. Partridge (John Wiley and Sons, Inc., 1933). The Metallic State, by W. Hume-Rothery (Oxford Press, 1931). Absorption and Extraction, by T. K. Sherwood (McGraw-Hill Co., 1937). is }- ij r j| I 82 Chapter 4 HEAT TRANSMISSION COEFFICIENTS AND TABLES Methods of Heat Transfer, Coefficients, Conductivity of Homogeneous Materials, Surface Conductance Coefficients, Air Space Conductance, Practical Coefficients, Table of Con ductivities and Conductances, Tables of Over-all Coefficients of Heat Transferfor Typical Building Construction, Combined Coefficients of Transmission IN order to maintain comfortable living temperatures within a building it is necessary to supply heat at the same rate that it is lost from the building. The loss of heat occurs in two ways, by direct transmission through the various parts of the structure and by air leakage or filtration between the inside and outside of the building. The purpose of this chapter is to show methods of calculation and to give practical trans mission coefficients which may be applied to various structures to deter mine the heat loss by direct transmission. The amount lost by air filtration is determined by different methods, as outlined in Chapter 5, and must be added to that lost by direct transmission to obtain the total heating plant requirements. METHODS OF HEAT TRANSFER Heat transmission between the air on the two sides of a structure takes place by three methods, namely,, radiation, convection and conduction. In a simple wall built up of two layers of homogeneous materials separated to give an air space between them, heat will be received from the high temperature surface by radiation, convection and conduction. It will then be conducted through the homogeneous interior section by con duction and carried across to the opposite surface of the air space by radiation, conduction and convection. From here it will be carried by conduction through to the outer surface and leave the outer surface by radiation, convection and conduction. The process'-'of heat transfer through a built-up wall section is complicated in theory, but in practice it is simplified by dividing a wall into its component parts and considering the. transmission through each part separately. Thus the average wall may be divided into external surfaces, homogeneous materials and interior air spaces. Practical heat transmission coefficients may be derived which will give the total heat transferred by. radiation, conduction and convec tion through any of these component parts arid if the selection and method of applying these individual coefficients is thoroughly understood it is usually a comparatively simple matter to calculate the over-all heat transmission coefficient for any combination of materials. 83