Document 4JjKZyR785e0y7a7gLrBDEd4N

of andAmerican Society Heating Ventilating Engineers Guide, 1932 These equations 1, 2 and 3 are fundamental and are used for determin ing values for/i,/o 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 U are.seldom known, although these surface tem peratures 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 tQ, it is necessary to use the trans mission coefficient U = Btu 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 9 to 33, inclusive. The heat H transmitted per hour from air inside to air outside is then computed as follows: H -- U {t -- to) S (4) Table 1. Surface Coefficients (J-,) for Various Building Materials under Still Air (No Wind) Conditions The values in the Table are in Btu per square root op wall surface per hour per 1 deg F 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).. ................... . Brickwork (ordinary).-......... Cement Plaster (finished).... Concrete.... ............................... Corkboard............................... . Glass (window)....................... Magnesia (blocks)...... ........... Wood (finished surface)--..... Building paper...... .................. Average of all values. 1.40 1.40 0.93 1.30 1.25 1.50 1.45 1.40 1.34 1.20 1.90b 1.40 Average of both aides of glass 0.12 in. thick and for 70 F total temperature difference from air to air with moving air on one side. Probable value for still air on both sides 1.60. and since H = H, = H, = Hc, the right-hand members of equations 1, 2, 3 and 4 are all equal. The coefficient 'V may be computed for any wall provided values for. fi, Jo and k are known. By proper substitution in the four equations, the unknown temperatures /, and Is can be eliminated and the value of the transmission coefficient for a simple wall x inches thick is: U= T + To + T. (5) and for a compound wall of several materials having thicknesses in inches of x,, Xz, etc., the coefficient is: U = J_+_L + /i So *i kt + T-+X + ttc. 26 (6) Chapter 3--Heat Transfer Through Materials and Constructions As in the case of the simple wall, fi and f0, 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/0 are the same, and fi = f0; 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 that values of k and C as well as U are dependent on the temperature range, and it is therefore desirable that the investigator determine heattransmission values under conditions approximating those existing under actual conditions. X. OOL.C. rACiUKS to BE USED IN. ^DETERMINING VALUES OF OUTSIDE SURFACE Coefficients (/0) under Moving Air Conditions In bach case, the moving air factor is based on still air coefficient fi for same material. For conditions where wind velocity is not shown use the factor (3) or TAKE /o AS 3 ft FOR SAME MATERIAL. ` Wind Velocity in Miles feb Houb Brickwork MtJLTIPLrERS OF fia Wood Average 5 10 15 20 2.38 3.20 3.76 4.22 2.19 2.71 2.95 3.02 2.28 2.96 3.36b 3.62 Additional Values--Smooth Surface 10 20 Above 20 ........ 2.20 2.60 3.00 -- ..gui engineering experiment station Bulletin No. 102, of the University of Illinois AddiVTw,,i?!? Engl.neci?ng Experiment Station. Pennsylvania State CoUege. reported by Professor tthhaannmmaatteerriiaa}i ooff jsuuSrfacTMe. TM Stat' College lndlcate character of surface, rough or smooth, more important bThis is usually taken as 3 even. 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, U= (7) and for a simple wall of several air spaces having conductances of a,, a,, a,, etc., the coefficient is: 1 U= 1T+7-+4-+-J-+--+ -- +ete. 'i Jo B a, r ___a, a. (8) With certain special forms of materials which have irregular air spaces (such as hollow tile) or are otherwise non-homogeneous, it is necessary 27