Document 93O1exzwk9YpOrGp141o26a43

D iam eter of Branch Pipe F ig. 5. . M a in and Branch Pipes for Equal Friction per F oot of Length (20 to 100 Per Cent Capacity) Heating Ventilating Air Conditioning Guide 1938 570 Chapter 29. Air Duct Design maximum velocities with good construction and design may be as high as 2000 or 2200 fpm in main ducts, with suitable reduction in branches and outlets. With these velocities first-class duct construction is essential. Proportioning the Size ior Friction By means of Figs. 4 and 5 the diameter of branch pipes necessary to carry a given percentage of the total air in the main pipe and to maintain equal friction per foot of the length through the entire system may be determined. These charts, as well as Fig. 3, are based on the assumption that the coefficient of friction varies inversely as the 1/7 power of the capacity. Example B. Suppose a 60-in. main pipe is to be used, and it is desired to know the size of branch pipe required to carry 50 per cent of the total air in the main. Find 50 per cent at the left' of the chart, move right to the 60-in. diagonal line and note directly above at the top of the chart that the branch pipe will be 46.5 in. in diameter. Where rectangular ducts are used it is frequently desirable to know the equivalent diameter of round pipe to carry the same capacity and have the same friction per foot of length. Table 1 gives directly the circular equivalents of rectangular ducts for equal friction and capacity, which are based on values determined from Formula 6: d = 1.265 J (a (6) where a = one side of rectangular pipe, feet or inches. b ~ other side of rectangular pipe, feet or inches. d = equivalent diameter of round pipe for equal friction per foot of length to carry the same capacity, feet or inches. To obtain the size of rectangular ducts for different capacities, but of the same friction per foot of length, first obtain the equivalent round pipe for equal friction. Thus, if a branch of sufficient size to carry 30 per cent of a 12 x 36-in. pipe is desired, it is found from Table 1 that the main :is equivalent to a 22.2-in. diameter round pipe. From Fig. 5, 30 per cent Of this is a pipe 14.3 in. in diameter, and referring again to- Table 1, the rectangular equivalent branch is a 12 x 14 in., 10 x 17J in., or any other desirable combination. Multiplying or dividing the length of each side of a pipe 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 a 40 x 12-in. duct, or 2 X 23.3 = 46.6 in. : Table 1. Circular Equivalents of Rectangular Ducts for Equal Friction Sn>B . Rhctakgulab Duct 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 IS.5 16 3 5.2 5.4 5.5 5.7 5.8 5.9 6.0 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0 7.1 3.5 5.7 5.9 6.0 6.2 6.3 6.5 6.6 6.7 6.9 7.0 7.1 7.3 7.4 7.5 7.6 7.7 7.8 4 6.1 6.3 6.5 6.7 6.8 7.0 7.1 7.2 7.4 7.5 7.7 7.8 7.9 8.1 8.2 8.3 8.4 4,5 6.5 6.7 6.9 7.1 7.2 7.4 7.6 7.7 7.9 8.0 8.2 8.4 8.5- 8.6 8.7 8.9 9.0 5 6.9 7.1 7.3 7.5 7.7 7.8 8.0 8.2 8.3 8.5 8.7 8.8 8.9 9.1 9.2 9.4 9.5 5.5 7.3 7.5 7.7 7.8 8.1 8.3 8.S 8.6 8.8 9.0 9.2 9.4 9.S 9.6 9.8 9.9 10.1 571