Document mdM60d11MRV3xXEx8J7kzJk4

664 CHAPTER 31 1950 Guide duct runs containing numerous successive outlets (usually designated the outlet run). An outlet run is typified by Sections C-D-E in Fig. 12. On larger systems several methods of duct sizing may be combined to secure equal or approximately equal pressures at all outlets. Example 5 shows the method of approach as applied to a very small system. As with any other method of duct design, balancing dampers should be installed in each branch, and each outlet should be equipped with means of regulating air volume. Example 6. (Static Regain Method). A duct layout is shown in Fig. 12. The fan delivers 8000 cfm. Outlets 1,2, 3 and 4 deliver 1500 cfm each, and outlets 5 and 6 1000 cfm each. The operating pressure loss at all outlets is 0.12 in. water. Initial trunk velocity is assumed as 1500 fpm; the area of the trunk duct will then be 5.33 sq ft, and the size will be 48 x 16 in. It is assumed that for this example it is de sirable to maintain a 16 in. depth on all duct sections. Determine the sizes of duct sections B, C, D, E, F and G so that substantially the same static pressure will be obtained at each of the outlets, and find the total pressure loss of the system. Fio. 12. Duct Layout fob Example 5 Solution. 1. Size Section F by the equal friction method so that it has the same rate of friction loss as Section A. Section A is equivalent to a 29.2 in. round duct (Table 2) and the pressure loss from Fig. 2 is 0.13 in. per 100 ft. For 2000 cfm flowing at this rate of pressure loss, the indicated round duct diameter for Section F is approxi mately 17 in. (Fig. 2). This is equivalent to a 15 x 16 in. duct, which will be used for Section F. The velocity, in Section F will be 1200 fpm. 2. Determine the pressure loss in Section F. Actual length of duct is 10 ft; equiva lent length of elbow take-off is assumed (Fig. 5) as 10 W, or 12.5 ft. Therefore, total equivalent length is 22.5 ft. The pressure loss in F = 0.13 in. X 22 5 == .0.03 in. water. 3. Using Static Regain Chart, Fig. 11, size Section B for a net.pressure loss equal to the loss in F, or 0.03 in. water as follows: The operation is indicated by arrow heads on the dotted line on Fig. 11. On Fig. 11, start at the velocity in Section A (1500 fpm) at left margin. Proceed horizontally to 6000 cfm ordinate, and then run parallel to the curved lines to intersect the di agonal Base Line. From this point, rise vertically to.the 0.03 net static pressure loss line, and from this intersection proceed horizontally to the Air Velocity Base Line. Proceed parallel to curved lines to intersect ordinate for 25 ft equivalent duct length, and then move horizontally to left margin and read the velocity (1500 fpm). * 6000 Since Section B carries 6000 cfm, the area required will be = 4 sq ft, and the size of duct will be 36 x 16 in. 4. Using Static Regain Charts (Figs. 10 and 11) determine size of Sections C, D, E and G, but instead of allowing 0.03 in. net loss, which was used for Section B, proceed from the diagonal Base Line vertically to the no gain or loss diagonal. The procedure for Section E is shown by the dotted line and arrows on Fig.,10: starting from 1040 fpm velocity, which is the velocity in Section D. ' , _'! Table 7 is a tabulation of the duct sizes.as determined. Air:Duct Design 665 Section .Table 7. Tabulation op Results (Example 5) Air Volume cfm Equita- Length ft Velocity fpm. Rectangular Duct in. Diam. in. Friction' 'ft in. HjO Net Pres* BURE LOSS in. HjO A 8000 B 6000 C 4500 D 3000 E 1500 F 2000 G 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 0 .03 0 * Includes equivalent length of elbow between outlets 2 and 3, which is assumed (see Fig. 5) as 5AW. or 11 ft equivalent length of duct.' (Based on 3000 cfm at estimated velocity of 1100 fpm). 5. Total pressure loss of the system is the loss in Section A, plus the loss in Sec tion F (or B), plus the loss in the outlet. 40 - ' Loss in Section A-= 0.13 in. X = 0.05 " Loss in Section F (or B) Outlet Loss = 0.03 =0.12 Total Pressure Loss = 0.20 in. 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 comers 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 corner 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 construction of thelsefvarious seams, as well as the types of igirth con nections, are shown in Fig: 13. The application of the various slips and connections 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 applied on the narrow side of the duct, and the drive slips on only the maximum side. Ducts .25 to 30 in. in size should be reinforced between the joints, but pot 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 adhesive tapes are available for this purpose and for sealing joints in duct work. The fan discharge connections shown in Fig. 13 are marked good, air, and poor in the order of the amount of . turbulence produced. An