Document 4a1jjq09NKpOqmMkqRkq4VKZa

Chapter 20--Air Duct Design -r j 40 for rough conduits of tile, brick or concrete. However, u'.*?0- he- states (and numerous tests check very closely) that/ varies ]?`;Fritzsc" ^ t^e 2/7 power of the pipe diameter, and inversely as the 1/7 ; /T?''erS^)f the velocity, or inversely as the 1/7 power of capacity, which is P?^me thing. Thus Formula 3 may be revised as follows, based upon a velocity head (at 2000 fpm) in a length equal to 50 diameters galvanized swedged pipe: (w)hL = 1.1 CD'17 13/7 (5) The preceding formulae are based on standard air, and for other conns the friction varies directly as the air density and inversely (ap- " ` rtwdmately) as the absolute temperature. The increase of friction due 7 / increase of air viscosity with increased temperature is small and is generally neglected. friction Loss Chart ' '-` Tie 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 power of the velocity. Fig. 3 is based on a loss of one velocity head ; , ' vat a velocity of 2000 fpm) in a length equal to 50 diameters of 24-in, fe/ mund 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 Is read from the chart should be multiplied by . ?, 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 ' 100ft is0.59 in.; then 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 occur at the entrance to the duct, through the ; heating units, and at the air washer. 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 Yl 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 ithe air velocities proportioned so that the resistance will be practically equal in all ducts regardless of length. * DUCT SIZES The sizes of ducts and flues for gravity or mechanical circulation of air are usually based on the losses due"to friction, and these losses must be kept within the available pressure difference. This pressure difference in 329