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American Society of Heating and Ventilating Engineers Guide, 1931 Table 2. Factors to be Used in Determining Values of Outside Surface Coefficients (Jo) under Moving Air Conditions In each case, the moving air factor is based on still ais coefficient / for same material. For conditions where wind velocity is not known use the factor (3) or TAKE /a AS 3 fi FOR RAMff **AirtTT.v Wind Velocity in Miles per Hour Brickwork Multipliers op /ja 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 10 20 Above 20 Additional Values--Smooth Surface -- 2.20 2.60 3.00 -- tional values from Engineering Experiment Station N)* the University of Illinois. Addi- Wood. Tests at Penns^vaAia^tifte^oItegeted^ttTclraj^tM r? sifrff*te C(lege' reBorted by Professor than material of surface. K ' cnaracter of surface, rough or smooth, more important bThis is usually taken as 3 even. 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. In the case of air-space construction, an air-space coefficient for eachair space must be inserted in either equation (5) or (6). Thus for a ' simple wall with one air space, Table 3. Conductances of Air Spaces a at Various Mean Temperatures Deg. Fahr. ___________ Conductances ot Are Swcra fob ViRiom Wioths m Inches 0-12S 0.250 0.364 0.493 0.713 i.oo 1-500 20 30 40 50 60 70 80 . 90 100 110 120 130 140 150 2.300 2.385 2.470 2.560. 2.650 2.730 2.819 2.908 2.990 3.078 3.167 3.250 3.340 3.425 Air 1.370 1.425 1.480 1.535 1.590 1.648 1.702 1.757 1.813 1.870 1.928. 1.980 2.035 2.090 1.180 1.234 1.288 1.340 1.390 1.440 1.492 1.547 1.600 1.650 1.700 1.750 1.800 1.852 1.100 1.148 1.193 1.242 1.295 1.340 1.390 1.433 1.486 1.534 1.580 1.630 1.680 1.728 1.040 1.080 1.125 1.168 1.210 1.250 1.295 : 1.340 1.380 1.425 1.467 1.510 1.550 1.592 1.030 1.070 1.112 1.152 1.195 1.240 1.280 1.320 1.362 1.402 1.445 1.485, 1.530 1.569 1.022 1.065 1.105 1.149 1.188 1.228 1.270 1.310 1.350 1.392 1.435 1.475. 1.519 1.559 . byTTn^;;d A. B; Algren (A.S.H:V.e! Transactions: 28 f Chapter 3^Heat Dosses by Transmission U = (7) and for a simple wall of several air spaces having conductances of o, a, a*, etc., the coefficient is: V _L _L _i_ J- JL -f. fi + fo + k + 1. 1 , 1 , . ------- 1-------- 1----------- b etc. d\ Q>2 O3 (8) With certain special forms of materials which have irregular air spaces (such as hollow tile) or are otherwise non-homogeneous, it is necessary to use the conductance (C) for the unit construction, in which case is replaced by 4r. kg Air Spaces and Surface Coefficients The following from Bulletin No. 102 of the' Engineering Experiment Station of the University of Illinois, is pertinent: In making calculations for heat transmission coefficients of compound walls, an air space may be treated in either of the following ways: the air space may be regarded as a solid insulating material through which the heat passes according to the so-called conductivity theory or considering the transfer by the three methods, radiation, con vection, and conduction, the radiation and convection action may be combined into a single surface coefficient and the true conductivity of the air neglected. For every air space two surface coefficients, accordingly, would be considered. If different surfaces enclosed the air space, different surface coefficients would be used for the two walls. Because reliable air-space conductance values have not been available until recently, it has been the practice in the past, when calculating the heat transmission through constructions containing air spaces, to assign still-air surface values to the surfaces enclosing the air spaces, rather than to consider the air spaces as solid insulating materials. In the 1928 and 1929 editions of The Guide the heat transmission tables were based on the average value of 1.34 for surfaces in still air taken from Table 1, which is equivalent to an air-space conductance of 0.67 B.t.u. per hour per square foot per degree fahrenheit difference in temperature between the two sides. According to tests conducted at the University of Minnesota, under the direction of Prof. F. B. Rowley, conductances of air spaces for various widths and mean temperatures average somewhat higher than 0.67 (Table 3). These tests indicate that there is practically no increase in the conductance of an air space beyond about 1 in. in width, and that the average conductance of air spaces of this width or greater at a mean tem perature of 40 deg. fahr. is about 1.10 B.t.u. per hour per square foot per degree fahrenheit difference in temperature, although this value is probably sufficiently accurate for all air spaces of J4 in. or more in width. Surface coefficients increase with the velocity of air passing over tne surface. Factors for determining conductances of outside surfaces under moving-air conditions are given in Table 2. It is the practice to increase the still-air surface coefficient by the factor 3 to allow for moving-air conditions, which is approximately equivalent to a wind exposure of 15 miles per hour. Thus, the conductance of the average surface, for still 28