Document 3ea0BkdDZaeo0eMDR5R6zQXGO

748 CHAPTER 32 1955 Guide uniform velocities accompanied by decreased turbulence, lower resistance and a minimum of noise. . Heavy canvas connections (asbestos cloth if there is a fire hazard) are recommended on both the inlet and outlet to -alLfans. Self-vulcanizing adhesive tapes are available for this purpose and for sealing joints in duct work. The fan discharge connections shown in Fig. 15 are marked good, fair, and poor ini the order of the amount of turbulence produced. An DTable 8. Recommended Sheet Metal Gages fob Rectangular uct Construction* ALU MINUM BAS. Gage Steel U.8. Srb. Gage Mattmttm Side, Inches Type of Transverse 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 None 7 ft 10 in. centers 22 25 to 30 S, Drive, I in. Pocket or i in. Bar 1 x 1 x f in. angles . Slips, on 7 ft 10 in. centers0 4 ft from joint ;31 to 40 Drive, 1 in. Pocket or 1 in. Bar 1 x 1 x i in. angles Slips, on 7 ft 10 in. centers" 4 ft from joint 41 to.60 11 in. Angle Connections, or If in. " Pocket or 1} in. Bar Slips with If in. x I in. bar reinforcing on 7 ft 10 in. centers0 li x 1^ x | in. angles 4 ft from joint 18 20 61 to 90 If in. Angle Connections, or If If x If x f in. diagonal " in. Pocket or If in. Bar Slips angles, or If x If x f 3 ft 9 in. maximum centers with in. angles If x f in. bar reinforcing . 2 ft from joint 16 18 91 and up 2 in. Angle Connections or If in. If x If x f in. diagonal Pocket or If in. Bar Slips 3 ft angles, or If x If x f 9 in. maximum centers with If x in. angles f in. bar reinforcingd 2 ft from joint . For normal pressures and velocities (see 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. AITuninsulated 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. d Ducts 91 in. and larger require special field study for hanging and supporting methods. inspection of the heater connections shown in Fig. 15 will readily show that uniform velocity through the heater cannot be expected in the diagram noted poor. When obstructions cannot be avoided, the duct area should never be decreared more than 10 percent, and then a streamlined collar should be used.' Larger obstructions require an increase in the duct size in order to maintain as nearly uniform velocity as possible. Branch take offs should always be arranged to cut or slice into the air stream in order to reduce as far as possible the losses in velocity head. Wherever ducts pass through fire walls or connect two fire areas of a building, automatic fire dampers should be provided. For design of such Air Duct Design 749 Table 9. Weights of Black and Galvanized Sheets 0. s. Gage Black Sheets Appro xi mote Thickness, In. Steel Iron Weight Per Square Foot Ounces Pounds Galvanized Sheets Appro simate Thickness, In. Steel Iron Weish* Paw Square Foot Ounces Pounds 30 0.0123 0.0125 8 28 0.0153 0.0156 10 28 0.0184 0.0188 12 24 0.0245 0.0250 16 22 0.0306 0.0313 20 20 0.0368 0.0375 24 0.500 0.625 0.750 1.000 1.250 1.500 0.0163 0.0193 0.0224 0.0285 0.0346 0.0408 0.0165 0.0196 0.0228 0.0290 ; 0.0353 0.0415 10.5 12.5 14.5 18.5 22.5 26.5 0.656 0.781 0.906 1.156 1.406 1.656 18 0.0490 0.0500 32 2.000 0.0530. 0.0540 34.5 2.156 16 0.0613 0.0625 40 2.500 0.0653 0.0665 42.5 ; 2.656 14 0.0766 0.0781 50 3.125 0.0806 0.0821 : 52.5 3.28! 12 0.1072 0.1094 70 4.375 0.1112 0.1134 72.5 4.531 . 11 0.1225 0.1250 80 5.000 0.1265 0.1290 82.5 5.156 10 0.1379 0.1406 90 5.625 0.1419 0.1446 92.5 5.781 a Galvanized sheets are gaged before galvanizing and are therefore approximately 0.004 in. thicker. dampers and other fire protective details, see Pamphlet No. 90 of the National Board of Fire Underwriters,n 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 copper Sheets are given-in Table 11. In calculating the total weight of a given length of duct work from there 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 large aucts. For large ducts, where more strength is desired, the 3S alloy with fnr ^ 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 should be 2 gages heavier than indicated in Table 8, and should be amply Table 10. Weights and Thicknesses of 2S Aluminum (Density 0.098 lb/cu in.) S- & 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