Document 5LMyvYkRw9d58Xgj4e8mm9Ma5

HEATINC VENTILATINO AIR CONDITIONING CUIDE 1944 Subtracting this from the total available resistance: 0.35 in. -- 0.163 in. = 0.187 in. available for duct resistance. Known length of run___ ...._________________ ______________ 150 ft The duct width is then estimated for the following elbow calculations: Four 150% ratio elbows, 4 x 13 x 3.5 ft 182 ft Three 75% ratio elbows, 3 x 35 x 1.5 ft________ ___ _______ 158 ft Total estimated length___ 490 ft The duct friction per 100 ft is then 0.187 4- 4.90 = 0.0382 in. and the mains and branches are sized from the 0.038 in. friction line in Fig. 2. If.it is desired to size each branch for equal resistance, the total resistance back to the point of juncture is calculated and the branch is then sized in a manner similar to that outlined in Example 3. DUCT CONSTRUCTION DETAILS Straight sections of round duct are usually formed by rolling the sheets' to the proper radius and grooving the longitudinal seam. Rectangular' ducts are generally constructed by breaking the corners and grooving the longitudinal seam, although some fabricators still use the standing seam due to lack of equipment. Elbows and transformation sections are gener-: . ally, formed with Pittsburgh comer seams because this seam is easier to' loci in place than the,double seam, but complicated fittings such as double compounded elbows are usually constructed with double seam comers. 610 CHAPTER 32. AIR DUCT DESIGN The construction of these various seams as well as the types of girth connections are shown in Fig. 7. The application of the various slips and connections is outlined in Table 5. The end slip may be used wherever S slips are recommended. Where drive slips are used the end slip may be applied on the narrow side of the duct and only the drive slips on the maximum side. Ducts 25 to 30 in. in size should be reinforced between the joints, but not necessarily at the joint. Ducts 31 in. and up should be reinforced at the joint and between the joints; if drive slips are used the Table 5. Recommended Sheet Metal Gages for Rectangular Duct Construction2 s.u. Std. Maximum Side, Gags Inches 26 Up to 12 Type op Transverse Joint Connections^ S, Drive, Pocket or Bar Slips, on 7 ft 10 in. centers None Bracing 13 to 24 S, Drive, Pocket or Bar Slips, on 7 ft 10 in. centers None 24 25 to 30 S, Drive, 1 in. Pocket or 1 in. Bar 1 x 1 x )4_in- angles Slips, on 7 ft 10 in.centers2 4 ft from joint 31 to 40 Drive, 1 in. Pocket or 1 in. Bar 1 x 1 x 54 in. angles , Slips, on 7 ft 10 in. centers2 4 ft from joint 22 41 to 60 1 54 in. Angle Connections, or 154 154 X 154 X Y% in. angles in. Pocket or 1)4 in. Bar Slips 4 ft from joint with 1 Y% in. x 54 in. bar rein forcing on 7 ft 10 in. centers2 20 61 to 90 1)4 in. Angle Connections, or 1)4 154 X l)4.x 54 inin. Pocket or 1)4 in. Bar .Slips diagonal angles, or 3 ft 9 in. maximum centers with 154 x 154 x 54 !n- angles 1J4 x )4 in. bar reinforcing 2 ft from joint 18 91 and up 2 in. Angle Connections or 1)4 in. Pocket or 1)4 in. Bar Slips 3 ft 154 x 154 x 54 indiagonal-angles, or 9 in. maximum centers with 1)4 x 154 X 154 K 54 in. angles )4 in. bar reinforcing11 2 ft from joint *For normal pressures and velocities (see Table 3) utilized in typical ventilating and air conditioning systems. Where special rigidity or stiffness is required, ducts should be constructed of metal two gages heavier. AU 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. oDuct 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. angles are usually riveted to the duct about 2 in. from the slips. It is good practice'to cross-break or kink all flat surfaces to prevent vibration or buckling, due to the air flow and accompanying variations in internal pressure. Round ducts are sometimes swedged 1.5 in. from the ends so that the larger end will butt against the swedge and are held in place with sheet metal screws. Where swedges are not used it is general practice to paste the joint with asbestos paper to insure a tight joint. The construction of elbows and changes of shape cannot be definitely outlined because of the varied conditions encountered in the field, but in 611