Document 2q11QJZ1jQxegjJ9NJ9OGmgMp

American Society of Heating anrf Ventilating Engineers Guide, 1932 454 Chapter 32--tAir Distribution Systems mill exhaust systems = 50 for heating and ventilating ducts = 45 for smooth and 40 for rough conduits of tile, brick or concrete. However, Fritzche states (and numerous tests check very closely) that / varies inversely as the 277-po.wer of the pipe diameter, and inversely as the 1/7 power of the velocity, or inversely as the 1/7 power of capacity, which is the same thing. Thus Formula 1 may be revised as follows, based upon a loss of one velocity head (at 2000 fpm) in a length equal to 50 diameters of 24 in. galvanized swedged pipe: CD,,nAl = 1.1 (4005) (5) The preceding formulae are based on standard air, and for other con ditions the friction varies directly as the air density and inversely (ap proximately) as the absolute temperature. The increase of friction due to increase of air viscosity with increased temperature is small and is generally neglected. Friction Loss Chart Fig. 3 is a convenient chart for determining the friction loss for various air quantities in ducts of different size. The general form of this chart is familiar, but it should be noted that it is corrected for changes in the coefficient of friction based on the rule that the coefficient of friction varies inversely as the 2/7 power of the diameter, and inversely as the 1/7 power of the capacity. Fig. 3 is based on a loss of one velocity head (at a velocity of 2000 fpm) in a length equal to 50 diameters of 24-in. round galvanized-iron duct of the usual construction. Example 1. Assume that it is desired to pass 10,000 cfm of air through 75 ft of 24-in. diameter pipe. Find 10,000 cfm on the right scale of Fig. 3 and move horizontally left..to the diagonal line marked 24-in. The other intersecting diagonal shows that the velocity in the pipe is 3200 fpm. Directly below the intersection it is found that the friction per 100 ft is 0.59 in.; (hen for 75 ft the friction will be 0.75 X 0.59 = 0.44 in. In a like man ner any two variables may be determined by the intersection of the lines representing the other two variables. Proportioning the Losses Other losses of pressure, are at the entrance to the duct, through the heating units, air washer, etc. In ordinary practice in ventilation work it is usual to keep the sum of the duct losses ys to and the loss through the heating units at less than x/i of the static pressure. The remainder is then available for producing velocity. In the design of an ideal duct system, all factors should be taken into consideration and the air veloci ties proportioned so that the resistance will be practically equal in all ducts regardless of length. DESIGN OF DUCTS . ' The sizes of ducts and flues for gravity or mechanical circulation of airare usually based on the losses due to friction, and these losses must be kept'within-the available pressure difference. This pressure difference in: mechanical ventilation is that derived from the fan, while in gravity 455 sI