Document 50bLGpk54o4vMpN6bdqqOwg4

D iam eter ,s>f ' Branch Pipe M a in a n d B ranch Pipes for E q u a l F r ic tio n per F oot of L en g th (20 to 100 Per Ce n t Ca p a c ity ) American Society of Heating and Ventilating Engineers Guide, 1937 8S $ 370 ,id d Chapter 20-^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 for 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 2. 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 iif the main. Find 50 net cent at the left of the chart, move right to the 60-in. diagonal line and note directly jbdve at the top of the chart that the branch pipe will be 46.5 in. in diameter. Where rectangular ducts are iised it is frequently desirable fo 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 equivalent of rectangular ducts for-equal friction and capacity.. 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 173^:in., or any other desirable combination.- . I* 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-iri. duct; is required, it will be just twice, that of a 40 x 12-in. duct, or 2 X 23.3 = 46.6 in. .... MAIN TEUNK DUCTS A main duct with branches is generally used to . convey tempered air for ventilation purposes only; In place of individual; ducts, a compara tively large main duct supplies air by branches to the room or rooms. The, velocities vary according to the nature of the installation and the degree of quietness required. At- the start of-the run a velocity as high as 2000 fpm. may be used, but this is considered the maximum, for. public building work, and-is reduced to from 400 to 8Q0 fpm in the risers. This duct system may be designed so that the loss of pressure in the branches is equalized jti. a manner similar to that previously described. . Equal Friction Method Example s. Fig. 6 shows a'typical layout of an air distribution system which; is applicable for ventilation of hotel dining rooms.and offices............................... .. ...r The volume of air in cubic feet per minute for the room is determined on the basis;of the number of air changes per hour required. In the exampleshown, the room ventilated If 3 dining room 135 ft x 85 ftx 15 ft;-' A 7J^-minute air change (8air changes per hour) is assumed for proper ventilation, giving 22,935 cfm as the air required.; The clear area of the fresh air inlet is based on a velocity of 1000 fpm or = 371