Document Ranr8ZqG2zK73GGeMrJambVxz

American Society of Heating and Ventilating Engineers Guide, 1931 t and t, and the coefficient f% varying with the character of the wall material. The symbol f\ may be defined as the B.t.u. per hour entering each square-foot of wall surface per degree difference between the inside air temperature t and the inside surface temperature h. Hence, the heat received by the inner surface of the wall per hour by both radiation and convection is: H. =/>-OS (l) where S is the inner-wall surface area in square feet and the other terms are as heretofore indicated. Whatever amount of heat Hi enters the inner-wall surface must be given off from the outer-wall surface, so that if H, represents heat emitted from outer surface, Hi = H,= f0 (/. - to) S (2) Now/o may not equal /,, in which case (l, -- t0) will not equal (l -- h). Usually, in an actual wall exposed to wind on the outside, f0 (Table 2) A represents warm surfaces at temperature / of inside air; ^represents cold surfaces at temperature /<> of outside air. Fig. 1. Temperature Curve or Gradient from Air Inside to and through Wall to Air Outside, Wall Material Assumed Air-Tight is greater than/; and {U -- t0) must be less than (1 -- h). Moreover, the heat Hc passing through the wall by conduction is equal to Hi and Hi, and if k is the thermal conductivity expressed in B.t.u. transmitted per hour per square foot of material per 1 in. thickness per degree difference between the surface temperatures, then Hi = H2 = Hc = -j- {h - Q S (3) where x = wall thickness in inches. These equations (1), (2) and (3) are fundamental and are used for determining values for f\, fQ and k for actual wall materials by test. They cannot be used for computing heat losses in an actual building, since the . surface temperatures h and ts are seldom known, although these surface temperatures can be determined in a test by means of thermocouples. Hence, for actual-conditions where the only temperatures known are the. . inside and outside air temperatures t and t0, it is necessary to use the transmission coefficient U = B.t.u. transmitted per hour per square foot of wall surface per degree difference between the inside and outside air . 26 V' - /f V Chapter 3--Heat Losses by Transmission temperatures. Values of U for many common types of construction are given in Tables 9 to 33, inclusive. The heat H transmitted per hour from air inside to air outside is then computed as follows: H = U(t - t0) S (4) and since H = Hi = H, = Hc, the right-hand members of equations (1), (2), (3) and (4) are all equal. The coefficient U may be computed for any wall provided values for fi,f0 and k are known. By proper substitution in the four equations, the unknown temperatures /, and U can be eliminated and the value of the transmission coefficient for a simple wall x inches thick is: U= x T (5) Table 1. Surface Coefficients {/;) for Various Building Materials under Still Air (No Wind) Conditions ~ ...PC .vtheTaBI.EAEE in B.t.u. per square foot of wall surface per hour per 1 DEC. FAHR. TfIE ^Yfference `between the mean air temperature in the room and the inside surface Ulf temperature of the wall. _______________________ Building Material Surface Coefficient fi (Still Air) Harding and Willard Wood Asbestos (sheet)-............... Brickwork (ordinary)........ Cement Plaster (finished). Concrete..................... .'........ Corkboard........................... Glass (window)_______ __ Magnesia (blocks)---_____ Wood (finished surface)--. Buildinj ; paper------------- ------ .verage of all values. 1.40 1.40 0.93 1.30 1.25 1.50 1.45 1.40 1.34 ....... ----------....... 1.20 1.90a ....... 1.40 .Average of both sides of glass 0.12 in. thick and for 70 deg. fahr. total temperature difference from air to air with moving air on one side. Probable value for still air on both aides 1.60. and for a compound wall of several materials having thicknesses in inches of Xi, x,, x,, etc., the coefficient is: U= 4fi -+4fo~+JkiLi +ir+ir + e`c- (0) As in the case of the simple wall, fi and /0, are always the inside and outside surface coefficients for the two materials in contact with air. If the air is still (no wind), then for the same material/i and/,, are the same, and fi = but, if the outside air is in motion, then f0 is always greater than fi and will increase as the wind velocity .increases. Values for fi in still air, as determined by various investigators, are given in Table 1. Values for k and C, the conductivity and conductance of building ma terials and insulations, are given in Tables 4, 5, 6, 7 and 8, and are taken from the published values of various investigators. It should be noted 27