Document DGG8Vj9G1kNzpnD0Mvqjrr00M

.754 CHAPTER 41 1946 Guide The recommended gages for sheet metal duct construction are given in Table 5. Weights of sheet metal per square foot of surface for different gages are given in Table 6. The weights of various gages and the areas for any length of run of rectangular sheet metal ducts ma^T also be determined from Fig. 8. The bottom scale represents the sum of the two Grooved seam Standing seam $ slip Drive slip End slip Double seam Pittsburgh seam Bar slip Reinforced bar slip . Pocket slip Angle connection Good Fair Heater, filter, and washer connections Poor Fig. 7. Sheet Metal Duct and Arrangement Details sides of the duct and the oblique lines give the length of run in feet. Proceeding horizontally to'the right'from the intersection of. vertical and oblique lines on the chart; the area of the duct may be determined in the first vertical scale. The scales to the right give the weights of the duct run' for different gages of metal'. In calculating the weights of duct, it is considered good practice to allow 20 per cent additional for weights of "joints'and bracings- Various:weights and thicknesses of standard cqpper sheets will be found in Table 7: ' " Air Duct Design Table 5.- Recommended Siiket Metal Gages for Rectangular Duct Construction3* 755 U. S. Sn>. Maximum Side. Gags- Inches 26 Up to 12 Type of Transverse Joint Connections^ S, Driye, Pocket or Bar Slips, on 7 ft 10 in.v centers None Bracing 13 to 24 S, Drive, Pocket or Bar Slips, on 7 ft 10 in. centers None 24 25 to 30 S, Driven 1 in. Pocket or 1 in. Bar 1 x 1 x )4 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 )4 in. angles Slips, on 7 ft. 10 in. centers0 4 ft from joint 22 41 to 60 1)4 in. Angle Connections, or 1)4 IK x 1)4 x K in. angles in. Pocket or 1)4 in-. Bar Slips 4 ft from joint with 1)4 in., x K in* bar rein forcing on 7 ft 10 in. centers0 20 61 to 90 1)4 in. Angle Connections, or 1)4 IK x 1)4*14 in. in. Pocket or 1)4 in. Bar Slips diagonalangles, or ,3 ft 9 in. maximum centers with 1)4 x 1)4 x K in. angles 1 K x K in. bar reinforcing 2 ft from joint 18 91 and up- 2 in. Angle Connections or 1)4 in.- IKx 1)4 X Kin.' Pocket or 1)4 in. Bar Slips 3 ft diagonal angles, or 9 in. maximum centers with 1)4 x 1)4 x IK x K in. angles )4 in. bar reinforcing^ 2 ft from joint For norma! pressures and velocities (see Table 3) utilired in typical ventilating and air conditioning systems. 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 uninsulated ducts if two gages of heavier metal are used. . bother joint connections of equivalent mechanical strength and air tightness may be used. cDuct sections of 3 ft 9 in. may be used with bracing angles omitted, instead of 7 ft 10 in. lengths with joints indicated. ^ dDucts 91 in. and larger require special field study for hanging and supporting methods. .- HEAT LOSSES FROM DUCTS The thermal transmittance U for ducts can be found.as follows: For uninsulated metal duct, V =------ ---1, 1 Si + U For uninsulated non-metallic ducts, U = --------- -------- -- 1x1 T + ir + T where land k are as defined on page 759. (8) (9) '' Where x is small and k is large, however, this factor ~ is of little im-' k portance and may be neglected. Film conductance J\ for air flowing in ducts apparently depends only on the velocity of the air and the diameter of the duct. A fairly reliable inside coefficient can be.calculated from Schultz!s modified equation: where. , 0.32 Ka h >. (10) Va = velocity of air in duct, feet per second. D = inside diameter of duct, feet.