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764 CHAPTER 41 1948 Guide HEAT LOSSES FROM DUCTS The thermal transmittance U for ducts can be found as follows: For uninsulated metal duct, V = J_+_L Si + So For uninsulated non-metallic ducts, V = h where U, Si, Jo, * and k are as defined on page 769. k + /,, (8) (9) Where x is small and k is large, however, this factor -Xy is of little im portance and may be neglected. Film conductance f\ 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 : h = jy>jt *(10') Air DuctDesif'n 765 Table 7. Weights of Sheet Metal Used for Duct Construction .: Black Sheets Galvanized Sheets u. s. Std. Gage '.30 28 26 24 22 20 18 16 14 12 11 10 . Approximate Thickness, In. Steel 0.0123 0.0153 0.0184 0.0245 0.0306 0.0368 .0.0490 0.0613 0.0766 0.1072 0.1225 0.1379 Iron 0.0125 0.0156 0.0188 0.0250 0.0313 0.0375 0.0500 0.0625 0.0781 0.1094 0.1250 0.1406 'Weight Per Square Foot , Ounces 8 10 12 16 20 24 32 40 50 70 80 90 Pounds 0.500 0.625 0.750 1.000 1.250 1.500 2.000 2.500 3.125 4.375 5.000 5.625 Approximate Thickness. In. Steel 0.0163 0.0193 0.0224 0.0285 0.0346 0.0408 0.0530 0.0653 0.0806 0.1112 0.1265 0.1419 Iron 0.0165 0.0196 0.0228 0.0290 0.0353 0.0415 0.0540 0.0665 0.0821 . 0.1134 0.1290 0.1446 Weight Per Square Foot Ounces 10.5 12.5 14.5 18.5 22.5 26.5 34.5 42.5 52.5 72.5 82.5 92.5 Pounds 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 axe gaged before galvanizing and are therefore approximately 0.004 in. thicker. 'where Va = velocity of air in duct, feet per second. D = inside diameter of duct, feet. Film conductance /0 depends on a number of variables including tem perature, diameter,- and emissivity of the outer surface and can be calcu lated from data in Chapter 5. 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: . Q = UPI [ - t. ] (ID The heat given up by the air in the duct is: Q = 0.24 W (fi - h) = 14.4 AVrafr, (/, - h) (12) Equating (11) and (12): /i tt -- 2/j 28.8 A Vm py h - it = UPl _Let y = 2--8-.-8--jAjpVpm ~ cfor recAtangular duct,s, -and,7--.-2--D- ^Vm--&--, (for round, Table 8. Weights and Thicknesses of Standard Copper Sheets3 Rolled to Weight Weight per Square Foot Ounces 10 12 14 16 18 20 24 28 32 36 40 48 64 Pounds 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 Thickness, Inches Equivalent 0.0135 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 Nearest ' Fraction '4> Ut *44 XA& lA XA A % *4 'A '& A % Nearest Gage No. B. & S. 27 26 25 23 22 21 20 19 17 16 15 15 14 13 11 Stubs 29 27 26 24 23 22 21 20 19 18 17 17 16 15 14 U. S. Std. 29 28 26 25 24 23 22 20 19 18 17 17 16 ' 14 13 Variations from these weights must be expected in practice.