Document xjY0EnKmoaVg8m761rjLMRwjb

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 out air spaces in concrete and tile blocks, and walls in which there are parallel paths for heat flow through materials having different heat resistances. In such cases it is necessary to resort to test methods to check the calculations, and in practically all cases it has been necessary to determine fundamental coefficients by test methods. Conductivity of Homogeneous Materials The thermal conductivity of homogeneous materials is affected by several factors. Among these are the density of the material, the amount of moisture present, the mean temperature at which the coefficient is determined, and for fiberous materials the arrangement of fiber in the material. There are many fiberous materials used in building construc tion and considered as homogeneous for the purpose of calculation, whereas they are not really homogeneous but are merely considered so as a matter of convenience. In general, the thermal conductivity of a material increases directly with the density of the material, increases with the amount of moisture present, and increases with the mean temperature at which the coefficient is determined. The rate of increase for these various factors is not the same for all materials, and in assigning proper coef ficients 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 subject to wide fluctuations for different materials and different con ditions. The inside and outside coefficients/i and/0 are in general affected to the same extent by these various factors and test coefficients deter mined for inside surfaces will apply equally well to outside surfaces under like conditions. Values for /i in still and moving air at different mean temperatures have been determined for various building materials'. The relation obtained between surface conductances for different materials at mean temperatures of 20 F is 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 F it would appear that these 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 lA.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.S.H.V.E. Transac tions. Vol. 36. 1930. p. 429). 86 CHAPTER 4. HEAT TRANSMISSION COEFFICIENTS AND TABLES part of the radiant heat would be eliminated. This would reduce the total coefficient for all wind velocities 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 constitute practically the entire resistance of the structure, and therefore become important factors. Due to the wide variation in surface coefficients for different conditions their selection for a practical building becomes a matter of judgment. In calculating the Fig. 1. Curves Showing Relation Between Surface Conductances for Different Surfaces at 20 F Mean Temperature over-all coefficients for the walls of Tables 3 to 12, 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 on recent tests and are for still air conditions and emissivities of 0.83 and 0.05, respectively. 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 87