Document MQqNyLYmbw5DGKBZ40GN8Jrj

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 CHAPTER 4. HEAT TRANSMISSION COEFFICIENTS j tion and considered as homogeneous for the purpose of calculation, subject to wide fluctuations for different materials and different con whereas they are not really homogeneous but are merely considered so as a ditions. The inside and outside coefficients/i and/0 are in general affected matter of convenience. In general, the thermal conductivity of a material to the same extent by these various factors and test coefficients deter increases directly with the density of the material, increases with the mined for inside surfaces will apply equally well to outside surfaces under amount of moisture present, and increases with the mean temperature at like conditions. Values for _/j in still and moving air at different mean which the coefficient is determined. The rate of increase for these various temperatures have been determined for various building materials3. factors is not the same for all materials, and in assigning proper coef- The surface conductances for different materials at mean temperatures of 20 F are shown in Fig. 1. These values were obtained with air flow parallel to the surface and from other tests in which the angle of incidence between the direction of air flow and the surface was varied from zero to 90 degrees it would appear that values might be lowered approximately 15 per cent for average conditions. While for average building materials there is a difference due to mean temperature, the greatest variation in these coefficients is caused by the character of the surface and the wind velocity. If other surfaces, such as aluminum foil with low emissivity coefficients were substituted, a large part of the radiant heat would be eliminated. This would reduce the total coefficient for all wind veloci ties by about 0.7 Btu and would make but very little difference for the higher wind velocities. In many cases in building construction the heat resistance of the internal parts of the wall is high as compared with the surface resistance and the surface factors become of small importance; In other cases such as single glass windows the surface resistances con stitute practically the entire resistance of the structure, and therefore become important factors. Due to the wide variation in surface co efficients for different conditions their selection for a practical building becomes a matter of judgment. In calculating the over-all coefficients for the walls of Tables 4 to 16, 1.65 has been selected as an average inside coefficient and 6.0 as an average outside coefficient for a 15-mile wind velocity. In special cases where surface coefficients become important factors in the over-all rate of heat transfer more selective coefficients may be required. The surface conductance values given in Table 1, Section A are based Fig. 1. Curves Showing Relation Between Surface Conductances for Different Surfaces at 20 F Mean Temperature on recent tests and are for still air conditions and emissivities of 0.83 and 0.05 respectively, and may be used where it is desirable to differentiate between vertical and horizontal surfaces dr where coefficients applicable to low-emissivity surfaces are required. fidents one should make certain that they apply for the conditions under which the material is to be used in a wall. Failure to do this may result in serious errors in the final coefficients. Surface Conductance Coefficients Heat is transmitted to or from the surface of a wall by a combination of radiation, convection and conduction. The coefficient will be affected by any factor which has an influence on any one of these three methods of transfer. The amount of heat by radiation is controlled by the character of the surface and the temperature difference between it and the sur rounding objects. The amount of heat by conduction and convection is controlled largely by the roughness of the surface, by the air movement over the surface and by. the temperature difference between the air and the surface. Because of these variables the surface coefficients may be 90 Air Space Conductance Heat is conducted across an air space by a combination of radiation, conduction and convection. The amount of heat by radiation is governed largely by the nature of the surface and the temperature difference between the boundary surfaces of the air space. Conduction and con vection are controlled largely by the width and shape of the air space and the roughness 'of the boundary surfaces. The conductances of vertical air spaces bounded by such materials as papier, wood, plaster, etc., are given in Table 1, Section B, having emis sivity coefficients of 0.8 or higher, and with extended parallel surfaces perpendicular to the direction of heat flow. A conductance of 1.10 Btu 'A.S.H.V.E. Research Report No. 869--Surface Conductances as Affected by Air Velocity. Tempera ture and Character of Surface, by F. B. Rowley, A. B. Algren and J. L. Blackshaw (A.SIH.V.E. Transac tions. Vol. 36, 1930, p. 429); 91