Document Z86z6X5zBaVM00Gn12LkxBZJ

7.54 CHAPTER 41 1948 Guide. Air Duct Design 755 resistance equivalent to the pressure drop in the elbows. Curves B and C are based on tests of round and square elbows 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 this 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. 6, long radius elbows will offer much less re sistance to the flow of air than short radius elbows. Experience has Table 3. Effect of Vanes on Pressure Loss of 7-inch Square Ventilating Duct3 Expressed in feet of total equivalent length of duct (ELD) T able 2. Circular Equivalents of Rectangular D ucts for Equal Friction-- (Concluded) splitters with R/W = 0.4. Elbow same as D except 2 in. trailing edge on the end of each splitter, ELD In feet = 17.0. Note B: The air velocity has no effect :on the loss of elbows when the loss Is expressed as equivalent length of duct. 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 diameters7. 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 pf 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. Table 3 shows typical types