Document QMRn5G2M9L78N7VRrBBzvqxv

American Society of Heating and Ventilating Engineers Guide, 1934 n ri v in' co 800000 600000 500 000 400 000 300000 4n ia (p oo * Friction in Inches of Water per 100 Ft. n Fig. 3. Friction of Air in Pipes 258 ri e w <s. Chapter 19--Air Duct Design The proceding 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 lo?s 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 50 as 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 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 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 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 veloci ties proportioned so that the resistance will be practically equal in all ducts regardless of length. SIZES OF DUCTS The sizes of ducts and flues for gravity or mechanical circulation ,pf 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 mechanical ventilation is that derived from the fan, while in gravity ventilation the aspirating effect due to the temperature and height of the column of heated air causes the pressure difference. General Rules /. ,- The general rules to be followed in the design of a duct system are: 1. The air should be conveyed as directly as possible at reasonable velocities to obtain the results desired with greatest economy of power, material and space. 2. Sharp elbows and bends should be avoided. 259