Document zQNp57x9mzLJzQv2ej5eomZG0

712 . CHAPTER 32 , 1953 Guide dampers and other .fire protective details, see Pamphlet No. 90 . of the National 'Board, ,of Fire Underwriters.s .......... The'recommended gages for steel (or iron) and aluminum sheet metal rectangular ducts are given in Table 8. Steel or iron sheets are' specified according to the Manufacturers or U; S. Standard Gage System;;. Alu minum sheets are specified according to the American or Brown & Sharpe Gage System. Weights of black and galvanized steel and iron sheets per square foot of surface for various gages are given in Table 9. Similar data for 2S aluminum sheets will be found in Table 10. Weights of standard Tabus 8. Recommended Sheet Metau Gages for Rectangular Duct Construction* Alu minum B.&S. Gage Steel U. 8. Sn>. Gaos' Maximum Side, Inches Type, ov Transverse Joint Connections Bracinq 24 26 Up to 12 S, Drive, Pocket or Bar Slips, on None 7 ft 10 in. centers 13 to 24 S, Drive, Pocket or Bar Slips, on None 7 ft 10 in. centers 22 24 25 to 30 S, Drive, 1 in. Pocket or i in. Bar 1 x 1 x J in', angles Slips, on 7 ft 10 in. centers* 4 ft from joint 31 to 40 Drive, 1 in. Pocket or 1 in. Bar 1 x 1 x i in. angles 20 22 , Slips, on 7 ft 10 in. centers" 4 ft from joint 41 to 60 1) in. Angle Connections, or 1$ in. 1} x 1) x ,, in. angles Pocket or li in. Bar Slips with 4 ft from joint. If in. x J in. bar reinforcing on 7 ft 10 in. centers" 18 20 61 to 90 li in. Angle Connections, or 1) 1) x lj x | in. diagonal in. Pocket or li in. Bar Slips angles, or 1} x 1} x i 3 ft 9 in. maximum centers with in. angles If x i in. bar reinforcing 2 ft from joint 16 18 91 and up '2 in. Angle Connections or 1$ in. li x 1J x J in. diagonal Pocket or 1) in. Bar Slips 3 ft angles, or li x -li x i 9 in. maximum centers with 1} x in. angles i in. bar reinforcing*1 2 ft from joint " For normal pressures and velocities (6ee Table 4) utilized in typical ventilating and air conditioning sys tems. Where special rigidity or stiffness is required, ducts should be constructed.of metal two gages heavier. All uninsulated ducts 18 in/and larger should be cross-broken. Cross-breaking may be omitted on uninsu lated ducts if two gages of heavier metal are used. Other joint connections of equivalent mechanical strength and air tightness may be used. * Duct sections of 3 ft 9 in. may be used with bracing angles omitted, instead of 7 ft 10 in. lengths with joints indicated. d Ducts 91 in. and larger require special field study for hanging and supporting methods. copper sheets are given in Table 11. In calculating the total weight of a given length of duct work from these tables, it is customary to add 20 percent for the weight of joints and bracings. Aluminum sheets of the 2S and 3S type alloy and % hard temper are readily workable, and can be used for practically all auct work. The 2S, type (commercially pure aluminum) is suitable for all, except very large ducts. For large ducts, where more strength is desired, the 3S alloy with ? or M hard temper is frequently used. The higher tempers, particularly full hard, do not have the formability of the lower tempers. For very' large ducts, where considerable strength is required, aluminum sheets Air Duct esign Table 9. Weights of Black and Galvanized Sheets 713 ii US. Gage 28 26 20 16 14 11 10 Black Sheets Galvanized Sheets* Thickness, In. Steel 0.0123 0.0153 0.0184 0.0245 10X1306 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 99 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. Weight'Per Square Foot 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 Ounces Pounds 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 0.656 12.5. ' 0.781 14.5 " 0.906 18.5 1.156 22.5 . 1.406 26.5 1.656 34;5 2.156 42.5 2.656 - 52.5 3.281 72.5 4.531 82.5 - 5.156 92.5. . . 5.781 Galvanized sheets are gaged before galvanizing and are therefore approximately 0.004 in. thicker. should be 2 gages heavier than indicated in Table 8, and should be amply stiffened. Joints can be of any of the standard designs, and can be fabri cated in the same manner as iron. Repeated sharp bending and rebending 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 npt 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 FRQM 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 unconditioned 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; U = - * ^ (20) Table 10. Weights and Thicknesses of 2S Aluminum (Density 0.098 lb/cu in.) B. & S. Gage 28 26. 24 - 22 20 18 16 14 Thickness, Inches Decimal - Nearest Fraction 0.012 0.016 0.020 0.025 : 0.032 0.040 0.051 0.064 1/64 1/64 1/64 1/32 ,1/32 3/64 3/64 1/16 Weight per Square Foot Ounces Pounds ' 2.7' 3.6 4.5 5.4 7.2 9.0 11.5 14.4 0.169 0.226 0.282 0.353 , 0.452 0.563 0.720 0.903 ,