Document Rjy9RzGDoOMy0w3vQ9GxaEjy7

570 . . Chapter 31 1945 Guide drop in the elbows. Curves B and C are based on tests of round and square elbows4 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 eibows and five-piece elbows as it will thus allow an additional factor of safety without seriously affecting the design. As indicated on the chart,, long radius elbows will .offer much less resistance 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 diameters5. Elbows having a radius of more than three diameters show a Air Duct Design . __________ ' ; - .' _________ 571 remains proportional to the angle' through which the turn is made. Reverse 90 deg elbow turns should be avoided wherever possible but, where used, the friction of the elbows indicated in Fig. 5 should be. doubled for the second elbow. PROPORTIONING THE LOSSES The entrance loss through the outside air intake louvers will vary with the design of the louvers and method of connection to the system. The louvers arid connecting duct will have a friction resistance of from 0.25 to 1.00 times the velocity pressure. Therefore, the total entrance loss will Table 2. Effect of Vanes on Pressure Loss of 7-inch Square Ventilating Duct3' Expressed in feet of total equivalent length of duct (ELD) oi------- ------- --------L.___ _____ _____ _____ _____ _____ _____ ___JI 50 75 100 125 150 175 200 225 250 275 300 325 CENTER-ONE RADIUS IN PER CENT OF PIPE DIAMETER OR WIDTH Fig. 5. Loss of Pressure in Elbows 350 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 than 90 deg may be assumed to be directly proportional to . the ratio of the angle through which the turn is made. The resistance will vary widely for the large degree turns depending upon the aspect ratio and the length of straight pipe between the elbows, but for practical purposes, it may be assumed that the ratio 4Loss of Pressure Due to Elbows in tbe Transmission of Air Through Pipes or Ducts, by F. L. Busey (A.S.H.V.E. Transactions. Vol. 19. 1913. p. 366). `Pressure Losses in Rectangular Elbows, by R. D. Madison and J. R. Parker (.Heating, Piping and Air Conditioning, July, p. 365, August, p. 427, September, p. 4S3, 1936). `vr 2,+ $r; For more complete data see A.S.H.V.E. Research Report No. 1216--Effect of Vanes in Reducing Pressure Loss in Elbows in 7-Inch Square Ventilating Duct, by M C. Stuart, C. F. Warner and W. C. Roberts (A.S.H.V.E. Transactions, Vol. 48. 1942. p. 409). .. Note A: Vane A made up of a large number of small splitters; B made up of a small number of large splitters bent on a large radius; C hollow vanes having different outside and inside curvature; and D four splitters with RJW 0.4. Elbow same as D-except 2 in. trailing edge on tbe end of each splitter, ELD in feet = 17.0. Nate B; The air velocity has no effect on the loss of elbows when the loss is expressed as equivalent length of duct. vary from 1.25 to 2.00 b- Common practice is to. use 1.5 b for a 75 per cent free area louver with connecting duct having 15 deg tapered sides. Wherever air passes through a plenum space having a negligible velocity, allowance must be made for the loss in velocity head. This may be taken as the velocity head corresponding to the difference in velocities in the plenum and the duct. Where the ducts are very smooth with long transformation fittings, a regain in static pressure is soriietimes allowed, but generally ordinary construction' does not warrant a consideration of this factor, and it is customary to neglect it. When it is allowed, the regain is estimated at orie-half the difference between the velocity pres sure at the fan outlet and at the last run of pipe. Other losses of pressure occur through the heating units, at the air washer and at air filters. In ordinary practice in ventilation work it is