Document MM32rpQ7z0G9nOy0qyK26xGoj

680 CHAPTER 31 1951 Guide Section Table 7. Tabulation oe Results (Example 6) Aib Volume cfm Equiva- Length ft Velocity fpm Rectanqdlab Duct in. Dum. in. Friction. rr Nett Phe8bubs Loss in. HiO In. HAD A 8000 B 6000 C 4500 D 3000 E 1500 40 1500 25 1500 15 1300 26* 1040 15 860 48 x 16 36 x 16 31 x 16 26 x 16 16 x 16 29.2 -- -- -- -- 0.13 -- -- -- -- .05 .03 0 0 0 F 2000 G 1000 22.5 15 1200 900 15 x 16 10 x 16 17. 0.13 .03 --0 a Includes additional equivalent length of elbow between outlets 2 and 3, which is or 11 ft equivalent length of duct. (Based on 3000 cfzn at estimated velocity of 1100 fpzn). as 54 W 5. Total pressure loss of the system is the loss in Section A, plus the loss in Sec* tion F (or &), plus the loss in the outlet as follows: 40 Loss in Section A = 0.13 in. X 1U1) = 0.05 Loss in Section F (or B) = 0.03 Outlet Loss =0.12 Total Pressure Loss . = 0.20 in. DUCT CONSTRUCTION DETAILS Straight sections of round duct are usually formed from sheets, rolled to the proper radius with a longitudinal grooved seam. Each section is swedged 1.5 in. from each end and assembled with the larger end of the adjoining section butting against the swedge. The sections are held in place by rivets, sheet metal screws, or by soldering. Rectangular ducts are generally constructed by breaking the comers 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 tion- 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. 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 in general, long radius elbows and gradual changes in shape tend to maintain 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 all fans. Self-vulcanizing Air Duct Design 681 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 in the order of the amount of turbulence , produced. An inspection of the heater connections shown in Fig. 15 will readily show that uniform velocity through the heater cannot be expected in the diagram Grooved seam - Standing seam S slip Drive slip End OouNe seam Pittsburgh seam Bar s&p Reinforced bar slip Pocket slip Angle connection | I { i i I i !! Fig. 15. Sheet Metal Duct and Arrangement Details noted poor. When obstructions cannot be avoided, the duct area should' never be decreased 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 i