Document a4D7V9v6y0rne8EKb42LNqGpy

742 CHAPTER 32 1956 Guide Air Duct Design 743 noted, however, that absolutely reliable dynamic loss coefficients are not yet available for all duct elements and-that; the information available for pressure losses due to area changes is generally restricted to' symmetrical area changes. Fig. 6, which shows the relation of velocity pressure to velocity for stand ard air (V = 4005a/#v), can be conveniently, used to find thetota'l dynamic pressure loss for any duct element with known dynamic loss coefficient C. PRESSURE LOSSES IN ELBOWS Dynamic loss coefficients are nearly independent of the air.velocity and are affected by the roughness of the duct walls only in the. case of bends, for which the dynamic losses are often grouped with the friction losses to facilitate design calculations. An A.S.H.A.E. survey6 of available data has indicated that the method of expressing the combined dynamic and friction losses due to an elbow as equivalent to the loss in a length L of similar straight duct is justified for design purposes, owing to the relation of the loss to the corresponding friction factor, /. Fig. 7 gives the additional equivalent length of duct in terms of widths W for elbows in rectangular ducts; Fig. 8 gives the equivalent length of duct in terms of diameters 2)-for round ducts. When these curves for additional equivalent length are used, the straight lengths of duct between elbows should be measured to the intersection of their center lines. Example 4: (Use of Fig. 7 for the calculation of elbow losses.) Given the portion of a duct system shown in Fig. 9, it is required to determine the ressure loss between points A and D. Air at. nnn/iitmno ;0 ------ W,Solution: Ih 6 For elbow No. 1 the radius ratio is Si IF, =~ .24 = 0.75 and theJ aspect ratio 24 0.25. The additional equivalent length for elbow No. 1 in terms of W Fig^ 7. Loss in 90-Deo Elbows of Rectangular Cross Section Fig. 9. Portion of Duct System, for Example 4