Document 500aqqEYw3KJ5k5xKzVLDpV8J

1 American Society of Heating and Ventilating Engineers Guide>1935"' Chapter 20--Air Duct Design mum velocities with good construction and design may be as high ma* o 0r 2200 fpm in main ducts, with suitable reduction in branches 35 utiets. With these velocities first-class duct construction is essential. .and Proportioning the Sire for Friction Rv means of Tigs. 4 and 5 the diameter of branch pipes necessary to i rrv a given percentage of the total air in the main pipe and to maintain canJ\ 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 ! i Shat the coefficient of friction varies inversely as the 1/7 power of the D iam eter o f B ranch Pipe za sj oo fa. Z^' ' Hy tU jO "ZKofw) 06 CQ Q .z < -'s<2 o capacity- -ample Suppose a 60-in. main pipe is to be used,.and it is desired to know the of branch pipe required to carry 50 per cent of the total air in the main. Find 50 ^ t at the left of the chart, move right to the 60-in. diagonal line and note directly bove 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 She 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 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 just twice that of a 40 x 12-in. duct, or 2 X 23.3 =* 46.6 in. DUCTS FOR PUBLIC BUILDINGS , 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 800 fpm in the risers. This duct system may be designed so that the loss of pressure in the branches is.equalized in a manner similar to that previously described. ; f Equal Friction Method Examples. Fig. 6 shows a typical layout of an air distribution system which is applicable for ventilation.of hotel.dining rooms andoffices................. ... .......... .. 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 example shown, the room ventilated is a hotel dining room 135 ft x 85 ft x 15 ft. A734-minute air change (8 air 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 22,935 1000 332 333