Document Dv4YYbROLv63NXObM8q4ZXQwN

746 CHAPTER 32 1955 Guide Section Table 7: Tabulation op Results (Example 6) Air Volume cfm - Equiva lent Length ' Velocity 1 ft - fpm Rectangular Duct in. Diam. in. Friction Peb 100 FT in. HsO Net Pbes* subs Loss' in. HO`v .A B C D E: F G 8000 6000 4500 3000 1500 2000 1000 40 25 15 26* 15 22.5 15 1500 1500 1300 1040 860 1200 900 48 x 16 36 x 16 31 x 16 26 x 16 16 x 16 15 x 16 10 x 16 29.2 ----- . -- -- " 17. -- 0.13 -- ' ----- -- -- 0.13 -- .05 .03 0 0 o .03 0 * Includes additional equivalent length of elbow between outlets 2 and 3, which is assumed as 5.5 W, or 11 ft equivalent length of duct. (Based on 3000 dm at estimated velocity of 1100 fpm). 5. Total pressure loss of the system is the loss in Section A, plus the loss in Section F (or B), plus the loss in the outlet as follows: 40 Loss in Section A = 0.13 in. X -- = 0.05 100 Loss in Section F (or B) =0.03 Outlet Loss = 0.12 Total Pressure Loss = 0.20 in. DUCT CONSTRUCTION DETAILS Straight sections of found duct are usually formed from sheets, rolled to the proper radius with a longitudinal grooved seam. Each section is swaged 1.5 in. from each end and assembled with the larger end of the adjoining section butting against the swage. The sections are held in place by rivets, sheet metal screws, or by soldering. Rectangular ducts are generally constructed by breaking the corners and grooving the longitudinal seam, although some fabricators still use the standing seam. Elbows and transformation sections are generally formed with Pittsburgh comer seams because this seam is easier to lock in place than the double seam, but complicated fittings such as double compounded elbows are usually constructed with double seam comers: The construc tions of these various seams, as well as the types of girth connections, are shown in Fig. 15. The application of the various slips and connec tions is outlined in Table 8. The end slip may be used wherever S slips are recommended. Where drive slips are used, the end slip may be ap plied on the narrow side of the duct, and the drive slips on only the maxi mum side. Designs M to P of Fig. 15 are for flush type seams on ductwork where joints are to be concealed. For smooth appearance the seams may be soldered flush or filled with auto body filler. Screws or rivets used should be of flat-head type. 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 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. The construction of elbows and changes of shape cannot be definitely outlined, because of the varied conditions encountered in the field, but m general, long radius elbows and gradual changes in shape tend to maintain Air Duct Design GROOVED SEAM STANDING SEAM S SLIP; 0 i=r<=i drive SUP 747 end SLIP DOUBLE SEAM PITTSBURGH SEAM BAIT SLIP ANGLE SLIP REINFORCED BAR SLIP . - POCKET SUP .........CONNECTION N - ... O FLUSH SEAM GOOD '30' '45 FAJR POOR HEATER, FILTER, AND WASHER CONNECTIONS Fig. 15. Sheet Metal Duct and Arrangement Details