Document MGZ3nYOQ9E5Vq40xnvxrGqM0y

1930American Society of Heating and Ventilating Engineers Guide, 'JI .... ; convection, provided, of course, that equilibrium has been established :3 and all four temperatures are constant. ? The amount of heat reaching or entering the wall per hour depends on t and h and the coefficient fi varying with the character of the wall material. The symbol fi 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 t,. Hence, the heat received by the inner surface of the wall per hour by both radiation and convection is: H, =/!-/,) 5 (1) 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 Hi represents heat emitted from outer surface, Hi = H% - fa (ti -- lo) (2) Now f0 may not equal f, in which case (U -- tQ) will not equal (t -- ti)'. Usually, in an actual wall exposed to wind on the outside, f0 (Table 5) is greater than/i and (h -- t,,) must be less than (/ -- /,). Moreover, the heat Hc passing through the wall by conduction is equal to H, 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 = Hi = Hc = A (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/i,/0 and k for actual wall materials by test. They cannot be used for computing heat losses in an actual building, since the surface temperatures /, and ta 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 temperatures. Values of U for many common types of construction are given in Tables 12 to 36, inclusive. The heat H transmitted per hour froixi air inside to air outside is then computed as follows: V H = U(t-to) S (4) and since H = Hi = Hi = 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 fhfo and k are known. By proper substitution in the four equations, the unknown temperatures L and h can be eliminated and the value of the transmission coefficient for a simple wall x inches thick is: U' - -L + -L + jl fi fo + k (5) 16 Chapter 2--Heat Losses from Buildings and for a compound wall of several materials having thicknesses in inches of X,, xi, etc., the coefficient is: 1 U = J_+JL fi + /o + Jkir\ .+ T-+J*rz + etc- (6) As in the case of the simple wall, f\ and fQ, 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/; and/Q are the same, and fi = fa', but, if the outside air is in motion, then fa is always greater than fi and will increase as the wind velocity increases. Values for f, in still air, as determined by various investigators, are given in Table 4. (fi)Table 4. Surface Coefficients for Various Building Materials under Still Air (No Wind) Conditions The values in the Table are in B.t.u. per square foot of wall surface per hour per 1 deg. fahr. difference between the MEAN AIR temperature in the room and the inside surface temperature of THE WALL. Building Material Surface Coefficient fi (Still Am) Harding and Willard Wood Asbestos (sheet)..........-......................................... -........................ (blocks)--_--................................. 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. tlUct and for 70 deg. fahr total temperature difference from air to air with moving air on on side. Probable value for still air on both sides l.oU. Values for k and C, the conductivity and conductance of building ma terials and insulations, are given in Tables 7, 8, 9, 10 and 11, and are taken from the published values of various investigators. It should be noted that values of k and C as well as V are dependent on the tempera ture range, and it is therefore desirable that the investigator determine heat-transmission values under conditions approximating those existing under actual conditions. In the case of air-space construction, an air-space coefficient for each air space must be inserted in either equation (5) or (6). Thus for a simple wall with one air space, 1 V = JL fo X T (7) and for a simple'wall of several air spaces having conductances of at, a,i, at, etc., the coefficient is: 17