Document 3Jd3YYGq6aQbd06EDbQLw6eQ3

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 Cu. Ft. of A ir per M in* CHAPTER 30. AIR DUCT DESIGN For all practical purposes C varies only with the nature of the pipe surface: C = 60 for perfectly smooth pipe; = 55 for pipe as used in planning mill exhaust systems; = 50 for heating and ventilating ducts; = 45 for smooth and 40 for rough conduits of tile, brick or concrete. However, Fritzsche states (and numerous tests check very closely) that f varies inversely as the 2/7 power 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 3 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: ``-"T&rWrr The preceding formulae are based on standard air, and for other con ditions the friction varies directly as the air density or inversely (approxi mately) as the absolute temperature. The increase of friction due to increase of air viscosity with increased temperature is small arid 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 sizes. 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 velocity. 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. Although this chart is laid out for a value of C equivalent to 50, it may be used for other values of C by varying the friction inversely as this constant. For ex ample, if a rougher pipe is used with 40 as the value of C, the friction loss as read from the chart should be multiplied by 50 Example l. Assume that it is desired to pass 10,000 cfm of air through 75 ft of 24 ini 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.; then for 75 ft the friction will be 0.75 X 0.59 = 0.44 in. In a like marir 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 occur at the entrance to the duct, through the heating units, at the air washer and at air filters. In ordinary practice in ventilation work it is usual to keep the sum of the duct losses }/ to ^ and the loss through the heating units at less than Yi 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 velocities proportioned so that the resistance will be practically equal in all ducts regardless of length. ' 547