Document xdRQGd43EyeBeBzL2EqMG89b

674 CHAPTER 31 1952 Guide -lengths and mean'velocities of flow as a circular duct of the same hydraulic diameter. When duct sizes are. expressed in tertns of hydraulic diameter, and when equations for friction loss in round and rectangular ducts are equated for equal capacity and equal length, an equation giving the circu lar equivalent of a rectangular duct is Obtained6 (Equation 3). where d. 130 (o + 6)* = 1.30 (ab)5 (a+ 6)*. (3) a = length of one side of rectangular duct, inches. (Other side is 6.) _ 6 = length of one side of rectangular duct, inches. (Other side is o.): , d0 = circular equivalent of a rectangular duct for. equal friction and capacity, inches. . Table 2 gives, the circular equivalents of rectangular ducts for equal friction'and capacity for aspect ratios not. greater than 11.7:1'based on Equation 3.6 ! . Multiplying or dividing, the length Of each'side of aductiby a constant is the same as multiplying or dividing the equivalent round size by the same constant.Thus, if the circular equivalent of an 80 x 24 in. duct is required, it will be twice that of 40 x 12 in. duct, or 2 x 23.0 = 46.0 in. DYNAMIC LOSSES Wherever eddying flow is present, brought about by sudden changes in the direction or magnitude of the velocity of the air flowing, a greater loss in pressure takes place than would occur in a steady flow through a similar length of straight duct having a uniform cross-section. The amount of this loss, in excess of straight duct friction, is termed dynamic loss. Dy namic losses generally are greater with decelerating, flow in duct;enlarge ments than with accelerating flow in reducing fittings.. Although dynamic Table 2. Circular Equivalents of Rectangular Ducts for .Equal,Friction and Capacity ' ;" , . Dimensions in Inches. 6 8: Side ' RectAN 4JO *5 5j0 ' 5.5 . J0 6-5 7j0 7& is 8,5' DjO 9$ - 10J> GUISH Duct 3:0 3.8: 4.0 4.2' 4.4 4.6 4.8 4.9 5:1 5.2 5:4- 6.5 5:6: 5:7 3.5 4.1 4.3 4.6 4.8 5.0 5.2 5.3 5.5 5.7 5.8' 6:0 ,6.1 6.3 4.0 4.4 4.6 4.9 5.1 5.3 6.5 5.7 5.9 6.1 6.3 6.4 6.6 6.8 4.5 4:6 4.9' 5.2 5.4 5.6 5.9 6.1 6.3 6.5 6.7 6.9 7.0 7.2 5.0 4.9 5.2 5.5 5.7 6.0 6.2 6.4 6.7 6.9 7.1 7.3 7.4 7.6 5.5 5.1 5.4 5.7 6.0 6.3 6.5 6.8 7.0 7.2 7.4 7.6 7.8 8.0 Side Rectan- 10.0 105 in) 115 12J0 125 13J0 135 145 145 155 155 155 gui-ab Duct 3.0 5.7 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0 3.5 6.3 6.4 6.5 6.7 6.8 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 4.0 6.8 6.9 7.1 7.2 7.3 7.5 7.6 7.7 7.8 7.9 8.1 8.2 8.3 4.5 7.2 7.4 7.5 7.7 7.8 8.0 8.1 8.2 8.4 8.5 8.6 8:7- 8.9 5.0 7.6 7.8 8.0 8.1 8.3 8.4 8.6 8.7 8.9 9.0 9.1 9.3 9.4 5.5 8.0 8.2 8.4 8.6 8.7 8.8 9.0 9.2 9.4 9.5 9.6 9.8 9.8 Air Duct Design 675 Table 2. Circular Equivalents of Rectangular Ducts for Equal Friction and Capacity (Continued) Dimensions in Inches - Side Reo- TAN- 6 7 8 9 10 11 12 13 14 15 16 17 ; 18 18 'GULAB Duct 6 6.6 7 7.1 7.7 8 7.5 8.2 8.8 9 8.0 8.6 9.3 9.9 10 8.4 9.1 9.8 10.4 10.9 11 8.8 9.5 10.2 10.8 11.4 12.0 12 9.1 9.9 10.7 11.3 11.9 12.5 13.1 13 9.5 10.3 11.1 11.8 12.4 13.0 13.6 14.2 14 9.8 10.7 11.5 12.2 12.9 13.5 14.2 14.7 15.3 15 10.1 11.0 11.8 12.6 13.3 14.0 14.6 15.3 15.8 16.4 ii. 1167 10.4 11.4 12.2 13.0 13.7 14.4 15.1 15.7 16.3 16.9 17.5 10.7 11.7 12:5 13.4 14.1 14.9 15.5 16.1 16.8 17.4 18.0 18.6 18 11.0 11.9 12.9 13.7 14.5 1613 16.0 16.6 17.3 17.9 18.5 19.1 19.7 19 11.2 12.2 13.2 14.1 14.9 15.6 16.4 17.1 17.8 18:4 19.0 19.6 20.2 20.8 20 11.5 12.5 13.5 14.4 15.2 15.9 16.8 17.5 18.2 18.8 19.5 20:1 20.7 21.3 22 12.0 13.1 14.1 15.0 15.9 16.7 17.6 18.3 19.1 19.7 20.4 21.0 21.7 22.3 24 12.4 13.6 14.6 15.6 16.6 17.5 18.3 19.1 19.8 20.6 21.3 21.9 22.6 23.2 26 12.8 14.1 15.2 16.2 17.2 18.1 19.0 19.8 20.6 21.4 22.1 22.8 23.5 24.1 28 13.2 14.5 15.6 16.7 17.7 18.7 19.6 20.5 21.3 22.1 22.9 23.6 24.4 25.0 30 13.6 14.9 16.1 17.2 18,3 19.3 20,2 21.1 22.0 22.9 23.7 24.4 25,2 25.9 32 14.0 15.3 16.5 17.7 18.8 19.8 20.8 21.8 22.7 23.6 24.4 25.2 26.0 26.7 34 14.4 15.7 17.0 18.2 19.3 20:4 21.4 22.4 23.3 24.2 25.1 25.9 26.7 27.5 36 14.7 16.1 17.4 18.6 19.8 20.9 21.9 23.0 23.9 24.8 25.8 26.6 27.4 28.3 38 15.0 16.4 17.8 19.0 20.3 21.4 22.5 23,5- 24.5 25.4 26.4 27.3 28.1 29.0 40 42 . - 44 46 15.3 16:8 18.2 19.4 20'7 21.9 23.6 24.0- 25.1 26.0 27.0 27.9 28.8 29.7 15.6 17.1 18.5 19.8 21:1 22.3 23.4 24.5 25:6 26.6 27.6 28.5 29.4 30.4 15.9 17.5 18.9 20.2 21.5 22.7 23.9 25.0- 26.1 27.2 28.2 29.1 30.0 31.0 16.2 17.8 19.2 20.6 21.9 23.2 24.3 25.5 26.7 27.7 28.7 29.7 30.6 31.6 ! -.'.48 1 50 52 .54 16.5 18.1 19.6 20.9 22.3 23.6 24.8 26.0 27.2 28.2 29.2 30.2 31.2 32.2 16.8 18.4 19.9 21.3 22.7 24.0 25.2 26.4 27.6 28.7 29.8 30.8 31.8 32.8 17.0 18.7 20.2 21.6 23.1 24.4 25.6 26.8 28.1 29.2 30.3 31.4 32.4 33.4 17.3 19.0 20.5 22,0 23.4 24.8 26.1 27.3: 28.5 29.7 30.8 31.9 32,9 33.9 56 : 17.6 19:3 20.9 22.4 23.8 25.2 26.5 27.7 28.9 30.1- 31.2 32.4 33.4 34.5 58 17.8 19.5 21.1 22.7 24.2 25.5 26.9 28.2 29.3 30:5 31.7 32.9 33.9 35.0 60 18.1 19.8 21:4 23.0 24.5 25.8 27.3 28.7 29:8 31.0 32.2 33.4 34.5 36.5 62 18.3 20.1 21.7 23.3 24.8 26.2 27.6 29.0 30:2 31:4 32.6 33.8 35.0 36.0 64 18.6 20.3 22.0 23.6 25.2 26.5 27.9 29.3r 30.6 31:8: 33.1 34.2 35.5 36.5 66 18.8 20.6 22.3 23.9 25.5 26.9 28.3 29.7 31.0 32.2. 33.5 34.7 35.9 87.0 68 19.0 20.8 22.5 24.2 25.8 27.3 28.7 30.1 31.4 32.6 33.9 35.1 36.3 37.5 ,, 70 19.2 21. 22.8 24.5 26.1 27.6 29.1 30.4 31.8 33.1 34.3 85.6 36.8 37.9 losses may be assumed to be caused by changes in area actually occupied by the air flow, for convenience they are divided into two general classes: (1) those caused by changes in direction of the duct at bends and branches, and (2) those caused by changes in cross-sectional area of the duct at transi tions.' Dynamic losses vary substantially as the square of the mean velocity of