Document LKVGg7Kz531p93V6MqLL5oRwq

580. . Chapter 31 1945 Guide Table 6. Weights of Sheet Metal Used for Duct Construction u. s. STD. Gagb Black Sheets Approximate Thickness, In. Weight Per Square Foot Steel Iron Ounces Pounds Galvanized Sheets8 Approximate Thickness,-In. Weight Per Square Foot Steel Iron Ounces Pounds 30 28 26 24 22 20 18 . 16 14 12 11 10 0.0123 0.0153 0.0184 0.0245 0.0306 0;0368 0.0490 0.0613 0.0766 0.1072 0.1225 0.1379 0.0125 0.0156 0.0188 0.0250 0.0313 0.0375 0.0500 0.0625 0.0781 0.1094 0.1250 0.1406 8 10 12 16 20 24 32 40 , 50 70 80 90 0.500 0.625 0.750 1.000 1.250 1.500 2.000 2.500 3.125 4.375 5.000 5.625 0.0163 0.0193 0.0224 0.0285 0.0346 0.0408 0.0530 0.0653 0.0806 0.1112 0.1265 1 0.1419 0.0165 0.0196 0.0228 0.0290 0.0353 0.0415 0.0540 0.0665 0.0821 0.1134 0.1290 0.1446 10.5 12.5 14.5 18.5 22.5 26.5 34.5 42.5 52.5 72.5 82.5 92.5 0:656 0.781 0.906 1.156 1.406 1.656 2.156 2.656 3.281 4.531 5.156 5.781 "Galvanized sheets are gaged before galvanizing and are therefore appraximately 0.004 in: thicker. run for different gages of metal. In calculating the weights of duct, it is.considered good practice to allow 20 per cent additional for weights of joints and bracings. Various'weights and thicknesses of standard copper sheets will be found in Table 7. HEAT LOSSES FROM DUCTS The thermal transmission coefficient U for an uninsulated metal duct can be obtained from the equation: 1 V= i J_. (8) fi Jo In the case of non-metallic ducts the formula in Equation 8 will become: V= 4fl + -f* + -7JO where U, fi, fo, x and k are as given on pages 81 and 82, Chapter 4. (9) Table 7. Weights and Thicknesses of Standard Copper Sheets3 Rolled to Weight Weight per Square Foot Ounces Pounds Thickness, inches Decimal Equivalent Nearest Fraction 10 12 14 16 . 18 20 24 28 32 36 40 44 48 56 64 0.625 0.750 0.875 1.000 1.125 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.500 4.000 0.0i35 0.0162 0.0189 0.0216 0.0243 0.0270 0.0324 0.0378 0.0432 0.0486 0.0540 0.0594 0.0648 0.0756 0.0864 H* Hi Hi H2 Hi . Hi Hi. Hi Hi Hi Hi Me Me Hi Hi .Variations from these weights must be expected in practice. Nearest Gage No. B. & S. Stubs U. S. Sn>. 27 29 26 27 25 26 23 24 22 23 21 22 20 .21 19 20 17 19 16 18 15 17 15 17 14 16 13 15 11 14 29 28 26 25 24 23 22 20 19 18 17 17 16 14 13 ) A-. .t Air Duct Design 581 Where x is small and k is large, however, this factor ~ is of little im- k portance and may be neglected. Film conductance/i for air flowing in ducts apparently depends only on the velocity of the air and the diameter of the duct. A fairly reliable inside coefficient can be calculated from Schultz's modified equation: where 0.32 Vo0-* 1)0.0 Vo = velocity of air in duct, feet per second. . D -- diameter of duct, feet. (10) Film conductance f0 depends on a number of variables including tem perature, diameter, and emissivity of the outer surface and can readily be calculated from data in Chapter 3. From this explanation, it is seen that it is unwise to recommend a given value of U for all uninsulated metal ducts. The heat loss from a given length of duct can be expressed by: <2= (ii) The heat given up by the air in the duct is: Q = 0.24 M ((, - t,) = 14.4 A Ye (/, - t,) (1?) Equating 11 and 12 enables the determination of the temperature drop in the duct: fi + t, - 2t, _ 28.8 AVp 1,-1, URL j . 28.8 AVp. y = --UPL-- ' solving for t, and k: . ,, 7.2 DVp r ,, rectan?u,ar ducts, = --jjj- - for round ducts, , t, (y + 1) - 21, (13) t, = t, (y-- 1) + 21, (y + 1) (14) For low velocities and long ducts of small cross-section, a somewhat more accurate formula may be used as follows: ` In these equations u = tl ~l> 4- ( .^ / URL \ + e\14.4 ApVj (15) Q = heat loss through duct walls, Btu per hour. V = thermal transmission coefficient, Btu per square foot per hour per degree Fahrenheit. R -- perimeter of duct, feet. L = length of duct, feet. h = temperature of air entering duct, degree Fahrenheit. * = temperature of air leaving duct, degree Fahrenheit. I, = temperature of air surrounding duct, degree Fahrenheit. M = weight of air per hour, through the duct, pounds.