Document k9K178KQ5QOmYaYRoZ7m1oG9O

742 CHAPTER 41 1946 Guide. Air Duct Design 743 CU FT OF. AIR PER MINUTE Fig. 2. Friction of Air in Straight Ducts (Based on Standard Air at 29.921 In. of Mercury barometric pressure and 70 F flowing through average: clean, round, galvanized metal ducts having approximately 40 joints per 100 ft.) resistance equivalent to the pressure drop, in the elbows. Curves B and C are based on tests of round and square elbows 6 of ordinary good sheet metal construction. Values obtained from Curve A should be used when there is any doubt as to quality of duct construction. It is suggested that tills curve be used for rectangular elbows and five-piece elbows as it will thus allow an additional factor of safety without seriously affecting the design. As indicated in Fig. 5, long radius elbows will offer much less-re sistance to the flow of air than short radius elbows. Experience has . shown that good results may be expected when the radius to. the center . of the elbow is 1.5 times the pipe diameter or duct width parallel-to the radius. Examination of the curve will indicate that little advantage is to be gained by selecting elbows having a centerline radius of more than two diameters6. Elbows having a,radius of more than three diameters show a ' slightly increased resistance due to the increased length of pipe but, when . used, they reduce the over-all resistance of the system and therefore should / not be avoided. . Where space conditions necessitate the use of short radius or miter elbows in square or rectangular duct work, turning vanes should be used to reduce the pressure losses. Rough or raw edges on the vanes should be avoided to prevent objectionable noise. Typical types of vanes are shown in Table 2 with the total equivalent length of duct that may be used in ' estimating the resistance of each type. . The pressure loss through-elbows of less them 90 deg may be assumed to be directly proportional to the ratio of the angle through which the Table 1. Circular Equivalents of Rectangular Ducts for Equal Friction3 Sms RXCTANOtTLAB 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 Due* 16 3 3.5, 4 4.5 5 ' 5.5 5.2 5.4 5.5 5.7 5.8 5.9 6.0 6.2' 6.3 6.4 6.5 6.6 6.7 6'8 6.9 7.0 5.7 5.9 6.0 6.2 6.3 6.5 -6.6 6.7 6.9 7.0 7.1 7.3 7.4 7.S 7.6 .7.7 6.1 6.3 6.5 6.7 6.8 7.0 7.1 7.2 7.4 7.5 7.7 7.8 7.9 8.1 8.2 8.3 6.5 6.7 6.9 7.1 7.2 7.4 7.6 7.7 7.9 8.0 8.2 8.4 8.5 8.6 8.7 8.9 6.9 7.1 7.3 7.5 7.7 7.8- 8.0 8.2 8i3 8.5 8.7 8.8 8.9 9.1 9.2 9.4 7.3 7.5 7.7 7.8 8.1 8.3 8.5 8.6' 8.8 9.0 9.2 9.4 9.5 9.6 9.8 9.9 7.1 7.8 8.4 9.0 9.5 10. i "Additional dza: 4 X S - 4.9; 4X6 = 5.4; 4X7 = 6.8; 5X5 = 5.5; 5X6 = 6.3; 5 X 7 = 6J. '