Document 6RNZgqNrXkmppmeQxOeg89NYm

American Society of Heating and Ventilating Engineers Guide, 1929 \ 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 ait 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.* The coefficients of heat transmission of various types of construction as given in Tables 7 to 13, are for still air for inside walls, floors and ceilings and for a wind movement of 15 miles per hour for exterior walls and roofs and are generally applicable to heat transmission computations using equation (9). Such heat transmission coefficients 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 1, of ! the inside surface of the wall; third, the temperature t, of the outside surface of the wall, and fourth, the air temperature U outside of the build 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 t0. 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 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 t,. 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 ate as heretofore indicated. 2"Effect of Wind on Heat Transmission Coefficients'' in Appendix to Section III, Code of Minimum Requirements of the American Society of Heating and Ventilating Engineers. 10 Chapter I--Heat Losses from Buildings 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, = H2 = K, (t, - i,,) 5. (2) Now Ki may not equal K,, in which case (t2-l0) will not equal (t-t,). Usually, in an actual wall exposed to wind on the outside, K> (Table 6) is greater than K, and (t,-t0) must be less than (t-t,). Moreover, the heat Hc passing through the wall by conduction is equal to H, and H,, and if C is the coefficient of conduction = B.t.u. transmitted per hour A represents warm surfaces at temperature t of inside air; B represents cold surfaces at temperature l0 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 Matervil Assumed Air-Tight per square foot of material per 1 in. thickness per degree difference between the surface temperatures, then H, = H2=Hc=-j (t, -t,)S (3) where x = wall thickness in inches. These equations (1), (2) and (3) are fundamental and are used for determining values for K,, K, and C for actual wall materials by test. They cannot be used for computing heat losses in an actual building, since the surface temperatures t, and l, are seldom known, although these surface temperatures can be determined in a test by means of thermo couples. Hence, for actual conditions where the only temperatures known are the inside and outside air temperatures' t and t0, it is necessary to use a transmission coefficient U = B.t.u. transmitted per hour per square foot of wall surface per degree differe'nce between the inside and outside air temperatures. Values of U for many common types of construction are given in' Tables 7 to 13. The heat H transmitted per. hour from air inside to air outside is then computed as follows: H = U(t - to) S (4) and since.H = H, = H, = Hc, the right hand membersJof equations (1), (2), (3) and (4) are all equal. The coefficient U may be determined by test, or it may be computed for any wall provided values for K,, K, and C are known. By proper substitution in the four equations, the unknown temperatures t, and t. 11 y s'