Document qmkGmqGkaMLKn3ddaVL9jxKJE

724 CHAPTER 32 1954. Guide dampers and other fire protective details, see Pamphlet No. 90, of the National Board of'Fire Underwriters.a 'The recommended gages for steei (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. AIu: minuin 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 Table 8. Recommended Sheet Metal Gages fob Rectangular Duct Construction* Alu minum B.&S. Gage Steel' U. S. Std. Gage Maximum Side, Inches Type op . Transverse Joint Connections0 Bracing 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 1 in. Bar Slips, on 7 ft 10 in. centers" l x 1 x J in. angles 4 ft from joint 31 to 40 Drive, 1 in. Pocket or 1 in. Bar ...< Slips, on 7 ft 10 in. centers0 1 x 1 x J in. angles 4 ft from joint 20 22 41 to 60 11 in. Angle Connections, or 1J in. Pocket or 1J in. Bar Slips with 11 x'l} x J in. angles 4 ft from joint If in. x in. bar reinforcing on 7 ft 10 in. centers" 18 20 , 61 to 90 1$ in. Angle Connections, or 1$ in. Pocket or 1J in. Bar Slips 1J x 11 x i in. diagonal angles, or >11 ilji) 3 ft 9 in. maximum centers with in. angles 11 x 1 in. bar reinforcing 2 ft from joint 16 18 91 aod up 2 in. Angle Connections or 1J in. Pocket or 1J in. Bar Slips 3 ft 1* x H x 1 in. diagonal angles, or 11 x 1J x { 9 in. maximum centers with If x in. angles I in. bar reinforcing1* 2 ft from joint " For normal pressures and velocities (see Table 4) utilised in typical ventilating and air conditioning sys tems. _ Where special rigidity or stiffness is required, ducts should be constructed of metal two gages bearaer* 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. b Other joint connections of equivalent mechanical strength and air tightness may be used. 6 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 duct work. The 2S type (commercially pure aluminum) is suitable for all, except very larg ducts. For large ducts, where more strength is desired, the 3S alloy with % or 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 Design 725 Table 9. Weights of Black And Galvanized Sheets u.s. Sn>. Oaws , black Sheets Approximate . inickaess, in. Weight Per Square Foot Galvanized Sheets* Thickness, In. weiffht Per Square Foot . Steel Iron Ounces Pounds 30 0.0123 0.0125 8 28 0.0153 0.0156 10 26 0.0184 0.0188 12 24 0.0245 0.0250 16 22 0i0308 . 0.0313 20 20 0.0368 0.0375 24 18 0.0490 0.0500 32 16 0.0613 0.0625 40 14 0.0766 0.0781 - 50 12 0.1072 0.1094 70 11 0.1225 0.1250 80 10 0.1379 0.1406 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 Steel 0.0163 0.0193 0.0224 0.0285 0.0346 0.0408 0.0530 0.0653 0.0806 0.1112 0.1265 0.1410 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 Ounces 92.5 Pounds 1.156 1.406 4.531 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 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 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 = j--^ ^ (20) 7+7 rn le 10. Weights and Thicknesses of 2S Aluminum (Density 0.098 lb/cu in.) ^ 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