Document vBR6EbOOZM3xMQa5oa95gr68

696 CHAPTER 31 1952, Guide -dampers and other fire protective details, see Pamphlet No. 90 of the Nationai Board of Fire Underwriters.0 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 Table 8. Recommended Sheet Metal Gages fob Rectangular Duct Construction* Alu minum B.&S. Gage Steel U.B. Std. Gage Maximum Side, Inches Type of . TBANSVERBS JOINT CONNECTIONS 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 None7 ft 10 in. centers 22 24 25 to 30 S; Drive, 1 in. Pocket or 1 in. Bar Slips, on 7 ft 10 in. centers0 lxl 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 i in. angles 4 ft from joint 20 22 4i to 60 1J in. Angle Connections, or 1} in. 1J x 1J x ,, in. angles Pocket or 1$ 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 1$ in. Angle Connections, or H 14 x-14 x 4 in. diagonal in. Pocket or 14 in. Bar Slips angles, or 14 x 14 x 4 3 ft 9 in. maximum centers with in. angles If x | in. bar reinforcing 2 ft from joint 16 18 91 and up 2 in. Angle Connections or If in. 14 x 14 x 4 in. diagonal Pocket or 1$ in. Bar Slips 3 ft angles, or 14 x 14 x 4 9 in. maximum centers with If x in. angles. f in. bar reinforcing* 2 ft from joint terns. Where special rigidity or stillness is required, aucis snouia ue wusuuuwu m tucm vnu * .w. 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. * Duct sections of 3 ft 9 in. may be used with bracing angles omitted; instead of 7 ft 10 in. lengths with joints indicated. Ducts 91 in. and larger require special held .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 M 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 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 Table 9. Weights op Black and Galvanized Sheets 697 u.s. Std. Gags Black Sheets Approximate Tbickn ess, In. Weight Per Square Foot Galvanized Sheets* Thickness, In. Weight'Per Square Foot Steel Iron Ounces Pounds Steel Iron Ounces Pounds 30 28 26 24 22 20 18 16 14 12 11 10 0.0123 . 0.0153 0.0184 0.0245 0.0308 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 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 0.656 12.5. 14.5 0.781 0906 18.6 ' 1.156 22.5 -1.406 26.5 l.ARA 34:5 42.5 52.5 : 2.656 -a oai 72.5 4.531 82.5 ' 6.16 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 not generally 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 compensate1 for the heating effect of the duct surface. - The thermal transmittance U for ducts can be found as follows: ' ; ' ! For uninsulated metal duct, U = 7,+To (20) 'J Table 10. Weights and Thicknesses op. 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 peb 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