Document baOm7aRN5o5JJ6VJELvZ3a85O

646 CHAPTER 31 1950 Guide . lengths and mean velocities of flow as a circular duct of the same hydraulic diameter. When duct sizes are expressed in terms 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 (o6)`g dc 1.30 (a + 1.30 (oft)* + W* (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 a.) . d = 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 a duct by 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 (SHOCK) LOSSES Any sudden change in the direction or magnitude of the velocity of the air flow causes a greater, loss in pressure than would occur in steady flow through a straight duct of uniform cross.-section and the same length. Although all shock losses may be considered as caused by changes in thq area actually occupied by the air flow, for convenience they are divided into two general classes: those caused by changes in direction of the duct, and those caused by changes in cross-sediorud area of the duct. Conduit Table 2. Circular Equivalents of RectanqulAb Ducts fob Equal Fbiction and. Capacity Dimensions in Inches 40Sida Rxctanoulab Duct 4.5 5.0 . 6.5- J) .6.5 7.0 7JS 8.0 8.5 90 ; 10.0 . 3.0 3.6 4.0 4.5 5.0 5.5 3.8 4.0 4.2 4.4 4.8 4.8 4.9' 5.1 5.2 5.4 6.5 6.6 .6.7 4.1 4.3 4.6 4.8 5.0 6.2 6.3 5.5 8.7 5.8 6.0 6.1 6.3 4.4 4.6 4.9 6.1 5.3 5.5: 5.7 5.9 6.1 6:3 6.4 6.6 618 4.6 4.9 6.2 5.4 5.6 5.9 6.1 6:3 6.5 6.7 6.9 7.0 7.2 4.9 6.2 5.5 5.7 6.0 6.2 6.4 6.7 6.9 7.1 7.3 7.4 7.6 6.1 5.4 6.7 6.0 6.3 6.5 6.8 7.0 7.2 7.4 7.6 7.8 8.0 SlDB Rbctax* 10-0 10.5 . UA 114 124 .124 taja 13.5 14.0 14.5 15j0 ' 154 - ` 16.01. quijlb Doer 3.0 3.5 4.0 4.5 5.0 -6.6 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, 6.3 6.4 6.6 6.7 6.8 6.9 7.0 7.1 7.2 7.3 7.4 7.6 7.6 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 7.2 7.4 7.6 7.7 7.8 8.0 8.1 8.2 8.4 8.5. 8:6 8.7 8.9 7.6 7.8 8.0 8:.T 8.3 8.4 8.6 8.7 8.9 9.0 9.1 9.3 9.4 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 . 647 Table 2. Circular Equivalents of Rectanoulab Ducts fob Equal Fbiction and Capacity (Continued) Dimensions in Inches SlOB Rbo- . TAN- 6 7 8 6 10 11 12 13 14 15 16 17 18 is OULAB Doer 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 16 10.4 11.4 12.2 13.0 13.7 14.4 15.1 15.7 16.3 16.9 17.5 17 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 15.3 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 21.31 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 14.6 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 16.4J 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- 15.3 16.8 18.2 19.'4 20.7 21.9 23.0 24.0 25.1 26.0 27.0 27.9 28'8 29.7 " 42 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 . 44 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 46 16.2 1718 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 16.5 18.1. 19.6 20.9 22.3 23.6 24.8 26.0 27^? 28.2 29.2 30.2 31.2 32.2 50 , 1618 1S."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 52 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 . 54 , 17.3 19,0 20.5 22.0 23.4 24.8 26.1 ; 27.3 28,51 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 85.5 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 29.3 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 37.0 - 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 36.8 37.6 . 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 transitions are representative of changes in cross-sectional area, and bends (elbows) are representative of changes in direction of the duct. Shock losses vary substantially as the square of the velocity of the air, and are:therefore conveniently expressed'as a fraction of the velocity head.