Document z3Xm9KwQzOO2vvDqpMYzN5Ma

( 650 CHAPTER 31 i 1950 Guide les3, customary to express the dynamic and friction losses in duct sections or elements in equivalent length of duct in feet, or in diameters to facilitate their computation as friction losses. Formulas have been developed for expressing these relations.* PRESSURE LOSSES IN ELBOWS It is conveninent to express the dynamic and friction losses in elbows as equal to a number of diameters of round pipe, or a number of-widths of. rectangular pipe, or equivalent length of duct.9 The curves in Fig. 5 give the number of diameters or widths of pipe which have a frictional, resistance equivalent, to the pressure drop in the: elbows. Curves B and C are based on tests.of round and square elbows10 of ordinary good sheet'metal..con-: Air-Duct Design 651 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 depend ing upon the aspect ratio and the length of straight pipe between the elbows but, for practical purposes, it may be assumed that the ratio 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 indicated in Fig. 5 should he doubled for the second elbow. Addi tional tests are needed on the loss of pressure in elbows and other types of duct fittings in order to reconcile the difference between values shown in Table 3. Effect of Vanes on Pressure Loss' of 7-inch Square Ventilating Duct* ' Expressed in feel of total equivalent length of. duct {ELD) ... COCnMJNE RADIUS IN PHI CENT. OF PIPE.DIAMETER OR WIDTH . Fig. 5. Loss of Pressure in Elbows , . .' 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 addi-i tional factor of safety without seriously affecting the design. -j . .- As indicated in Fig. 5, 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 du6t width parallel ta the radius. Exami nation of Fig. 5 will indicate that little advantage is to be gained by select ing elbows having a centerline radius of more than two diameters.11 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 overall resistance of the system and'therefore should not be avoided. Where space conditions necetoitate 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 of, vanes and gives the resistance, expressed in equivalent length of straight; pipe, for a 7-in. x.7rin. elbow.of.each,type. . .; . .. . , . ; ^ * For more oomplete data see A.S.H.V.E.- Research Repost No. 1216--Effect of .Vanes in Pressure Loss in Elbows in 7-Inch Square Ventilating Duct, by M; C. Stuart. C: F. Warnerand W. C. Roberta (AJ3.H.VJ3. Transactions, Vol. 48,1942, p. 409). Note A: Vane A made up of a large number of smallsplitters; B made up of a number of large split* ten bent on a large radius, C hollow vanee having differentoutside and inside curvature; and'2) four splitters with R/W a 0.4. Elbow same as D except 2 in. trading edge on the end of each splitter, ELD in feet a 174). Fig. 5 and results of more recent tests shown in Table 3. A study- of thin subject is in progress at the, A.S.H.V.E. Research Laboratory for the purpose of obtaining such data.; ; LOSSES DUE TO AREA CHANGES .v Area changes in ducts, generally .unavoidable, are necessitated fre quently by the building construction, or changes in, the -volume, of air .car ried. Experimental investigations121* of pressure changes, and,'pressure losses at changes of the area of duct cross-sections,'indicate thiit^the excess pressure loss. over , the "normal friction loss is a shock .loss due to a faster stream' expanding into a. slower stream,'as'determined1 by the actual areas occupied by the flow rather than the areas of the duct. No perceptible