Document 2qVYybg9BEMXxbKd1ndZJVLMa

American Society of Heating and Ventilating Engineers Guide, 1928 Since actual temperature differences vary widely in different parts of the country, it is desirable to adopt some standard basis for testing, such as 70 deg. inside and 0 deg. outside, and in very precise work make a correction for other temperatures. It has been found that the absolute mean temperature of the wall affects the coefficient materially. The coefficient increases with the absolute mean temperature. Tests are usually run under still air conditions, which means there was no wind movement, during the test, over the surfaces of the wall. In practice, some wind movement over the exterior surface of the wall should always be allowed for; hence still air coefficients cannot be used in actual work as they do not provide for the normal wind movement over the outside of the building in the locality in question' during the heating season. Moreover, still air transmission coefficients cannot be corrected to provide for moving air conditions by multiplying by a single constant factor.1 H A represents warm surfaces at temperature t of inside air; B represents cold surfaces at temperature <o of outside air. For an actual temperature gradient curve see Fig. 2. Fig. 1. Temperature Curve or Gradient from Air Inside to and through Wall to Air Outside, Wall Material Assumed Air-Tight ~ The coefficient of heat transmission of various building materials and types of construction as given in Table 6, are for still air and for a wind movement of 15 miles and are generally applicable to heat trans mission computations using equation (9). Such heat transmission co efficients are always based on the difference between the air temperatures on the inside and the outside of the wall. \ Transmission Coefficients by Computation If heat transmission coefficients are to be computed, and in many, if not most cases, they should be computed, the following analysis of the transmission of heat through a simple, solid wall is used as the basis for such computations. The diagram in Fig. 1 exhibits four important temperatures: First the air temperature t inside of the building; second, the temperature t, of the inside surface of the wall; third, the temperature U of the outside surface of the wall, and fourth, the air temperature t,, outside of the build- ^'Effect of Wind on Heat Transmission Coefficients" in Appendix to Section III, Code of Minimum Requirements of tbe American Society of Heating and Ventilating Engineers. 8 Chapter I--Calculating the Heat Losses from Buildings ing. Heat reaches or enters the inside surface of the wall by radiation and convection, since the air and objects A within the building are always warmer than the inside surface of the wall, when the inside air tempera ture t is greater than the outside air temperature t,,- This heat must then pass through the material of the wall from inside to outside surface by conduction, and is finally given off from the outside surface by radiation and convection, provided, of course, that equilibrium has been established and all four temperatures are constant. The amount of heat reaching or entering the wall per hour depends on t and t, and a coefficient K, varying with the character of the wall Fig. 2. Temperature Gradient Curves for Glass (Taken from Bulletin No. 24, Engrg. Exp. Sta., Pennsylvania State College) material. K, may be defined as the B.t.u. per hour entering each square foot of wall surface per degree difference between the inside air tempera ture t and the inside surface temperature U. Hence the heat received by inner surface of the wall per hour by both radiation and convection is H, = K, (t - <,) 5 (1) where 5 is the inner wall surface area in square feet and the other terms are as heretofore indicated. Whatever amount of heat H. enters the inner wall surface must be, given off from the outer wall surface, so that if H, represents heat emitted from outer surface H, = H, = K, (t, - to) S. (2) Now K, may not equal K,, in which case (/Mo) will not equal (Mi). 9