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174 CHAPTER 9 1955 Guide- .> between zero and 140 F can be estimated by considering a straight-line^^ relationship between the two values given. n|j ST Surface Conductance ,\ The surface conductance of a wall is the combined heat transfer to or;? from the wall by radiation, convection and conduction. Each of the three'f portions making up the total may vary, independently of the others, thus. affecting the total conductance. The heat transfer by radiation between? two surfaces is controlled by the character of the surfaces (emissivity),i;S. the temperature difference between them, and the solid angle through which' they see each other. The heat transfer by convection and conduction is-?' Fig. 3. Determining Thermal Conductivity of Soils from Density and Moisture Content controlled by the roughness of the surface, by air movement, and tempera1'# ture difference between the air and the surface. The importance of the effect of temperature of surrounding surfaces|* on the surface conductance, due to the effect on radiation, is illustrated in| Table 4, which applies to a vertical surface at 80 F, with ambient air.'^j 70 F and effective emissivity equal to 0.83.4 ''m In many cases, because the heat resistance of the internal parts of the'.f wall is high compared with the surface resistance, the surface factors are off;? minor importance. In other cases, e.g., single glass windows, the surfacej-; resistances constitute almost the entire resistance and are therefore very0 important. An analysis of various factors affecting surface conductance,^ and the difference between surface and air temperatures will be found Reference 5. (See also Chapter 24.) The convection part of the surface conductance is affected markedly^ by air movement. This is illustrated by Fig. 4, which shows the result^?! of tests6 made on 12 in. square samples of different materials at a meafS? temperature of 20 F, and for wind velocities up to 40 mph. These con#? ductances include the radiation portion of the coefficient which, for tb<|.;i conditions of the tests, was about 0.7 Btu per (hr) (sq ft) (F deg). More,.: recent tests7 oh smooth surfaces show that surface length also affects sig$? -'V I Heat Transmission Coefficients of Building Materials 175 nificantly the convection part of conductance; the average value de creases as the surface length increases. Moreover, observations8 of the magnitude of low temperature radiant energy received from outdoor sur roundings show that only under certain conditions may the out-of-doors be treated as a black body radiating at air temperature. Because of these factors, the selection of surface conductance coefficients for a practical building. becomes a matter of judgment. In calculating the overall heat transmission coefficients for the walls, etc., of Tables 6, 7, 8, 9,15,16,17,18,19, and 21,1.65 has been selected as an average inside surface conductance, and 6.0 as an average outside surface conductance for a 15 mph wind. Both values combine the effects of convection and radiation, and are applicable to ordinary building materials. They should not be used for low emissivity surfaces such as bright metal. Values of TJ for windows in Table 20 have been computed from somewhat different data, as described in a later section, in order to give proper weight to actual surface con ductance. In special cases, where surface conductances become important factors in the overall rates of heat transfer, more selective coefficients may be Table 4. Variation in Surface Conductance Coefficient for Vertical Surfaces with Different Temperatures of Surrounding Surface Surrounding Subfacb Temperature 75 F 70 F 69 F . 60 F 50 F Convection--Btu per (hr) (sq ft).. Radiation-- Btu per (hr) (so ft)... 1 otal--Btu per (hr) (sq ft)............. 6.6 4.4 11.0 6.6 8.6 15.2 6.6 9.6 16.2 6.6 17.0 23.6 6.6 24.9 31.5 required. Data given in Table 1, Section A, and principles and data given in Chapter 5, Heat Transfer, may be applied in such cases. Air Space Conductance The transfer of heat across an air space involves the boundary surfaces as well as the intervening air; consequently, the factors influencing sur face conductance play an important part in determining the conductance of the air space. The coefficients given for air space conductance represent the total conductance from surface to surface. --------------- VA uvuv uviuoo OIAX cbii spate U1VU1VC8 1/liC UUUUUtiry SUITRC6S as well as the intervening air, and depends markedly on the orientation of the air space and the direction of heat flow. The coefficients given for air space conductance represent the total conductance from one surface bounding the air space to the other. The total conductance is the sum of a component due to radiation and a component due to convection and conduction combined. These components may vary independently of each other. The radiation portion of the coefficient is affected by the temperature ot the two boundary surfaces, and by their respective surface emissivities, be combined effect of which is expressed by means of the effective emis- fpiiy of the air space. The radiation component is not affected by the hickness of the space.or by its orientation or direction of heat flow. The eat transfer by convection and conduction combined, however, is markedly eeted by the orientation of the air space and the direction of heat flow, significantly affected by the temperature difference across the space and . ,some cases by the thickness of the space, and is affected to only a small tent by the mean temperature of its surfaces. For air spaces usually