Document e7R647n5Ke31oVOojOKnoM1Dp

668 CHAPTER 31 1950 Guide Table 9. Weights of Black and Galvanized Sheets u.s. Std. Gags Black Sheets Approximate Thickness, In. Weight Per Square Foot Galvanized Sheets0 Approximate Thickness. .In. Weight'Per ' Square Foot 50 - 28 26 24 22 20 18 16 14 1121 10 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 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 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 0.0106 0.0228 . 0.0290 0.0353 0.0415 0.0540 0.0665 .0.0821 0.1134 0.1290 0.1446 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 are gaged before galvanizing and are therefore approximately 0.004 in. thicker. should be avoided, as aluminum has a tendency to crack under such treat ment. Aluminum of 16 B-. & S. gage or heavier can readily be welded by the metallic arc or acetylene process. Soldering is difficult and is not gen erally recommended. Riveting is done in the same manner as in iron or steel sheet. Self-tapping screws tend to loosen because of the softness of aluminum. HEAT LOSSES FROM DUCTS In designing duct systems, the heat gains or losses of ducts should not be neglected. Heat gains in large duct systems can be quite considerable, not only if the duct passes through unconditional space, but also on long duct runs within conditioned space. Proper insulation will remedy this situation considerably, but sometimes a redistribution of the supply air to the various' supply outlets is necessary in order to compensate for the heating effect of the duct surface. The thermal transmittance U for ducts can be found as follows: For uninsulated metal duct, V -- 1 1+1 fv U For uninsulated non-metallic ducts, U 1 l.i.l F+~k + fa (20) (21) Table 10. Weights and Thicknesses of 2S Aluminum (Density 0.098 lb/cu in.) B. A 8. Gaob - Thickness, Inches Decimal Nearest Fraction Weight peb 8quabe foot Ounces Pounds 28 26 24 . 22. 20 18. ... .. i6 - 14 0.012 0.016 0.020 0.026 0.032 0.040 0.051 0.064 . 1/64 1/64 1/64 1/32 .1/32 3/64 3/64 1/16 2.7 3.6 4.5 5.4 7.2 9.0 . . 11.6 14.4 - 0.169 0.226 0.282 0.353 0.452 0.563 0.720 0.903 Air Duct "Design 669 where - V = overall coefficient of heat transfer, Btu per (hour) (square foot) (degree Fahrenheit). / = surface conductance (inside) Btu per (hour) (square foot) (degree Fahrenheit). fo = surface conductance (outside) Btu per (hour) (square foot) (degree Fahrenheit). x = thickness, inches. k = unit conductivity of material, Btu per (hour) (square foot) (degree Fah renheit per inch thickness). Where x is small and k is large, however, the factor r is of little impor tance 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 Table 11. Weights and Thicknesses of Standabd Cofpeb Sheets* Rolled to Weight Weight per Square Foot Thickness, Inches Ounces 10 12 14 16 18 20 24 28 32 36 40 .44 48 56 - 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 Decimal 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 . l4 U%* *4 4 '4 , 16 *4 >4 -% % Variations from these weights must be Expected in practice. 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. Sro. 29 28' 26 25 24 23 22 20 19 18 17 17 16 14 13 inside coefficient can be calculated from Schultz's modified equation- 0.32 (22) where u = velocity of air in duct, feet per second: D = inside diameter of duct, feet. ` Film conductance /,, 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: V vpl: / (23)