Document 3J5v5NjkpRGr2MovBJ3JXrZ33

American Society of Heating and Ventilating Engineers Guide, 1936 l*7~w f. ,.4. -C * l _i 4 SECTION `TOP SHEET, SIDE SHEET THESE CROSSBREAKS ><^ 'A-NREEVER ,ra SSHHOOWT N OH A PUN H ELEVATION REINFORCED CROSS SEAMS SEAMS BETWEEN ADJACENT PANELS OR PUIN CROSS SEAMS Fig. 9. Details of Seams kt-- : z h* a lu X Fig. 10. Method of Installing Heating Unit Fig. 11. Installation of Easement in Duct Around Obstruction Chapter 20--Air Duct Design To this must be added the resistance on the discharge side of the fan. A fan outlet velocity of approximately 1500 to 1600 fpm may be used. Assuming the fan outlet to be equivalent in area to a 45-in. pipe, the velocity is 1525 fpm. Loss on discharge (15 ft from fan outlet to discharge): 15 X 12 45 4 diameters of 45-in. pipe.' The velocity head corresponding to a velocity of 1525 fpm is 0.145 and the dischargeside loss is 0----14^5 X----4 = 0.012 in. The total static pressure loss of the system is then: 0.012 + 0.325 = 0.337 in. The fan will be selected to handle 16,800 cfm at a static pressure of 0.337 in. and to have an outlet velocity of 1525 fpm. Outlet area 11 sq ft. Where there are one or more ducts with branches, the velocity of air in the ducts may be either chosen arbitrarily or calculated for friction losses. When arbitrary values are assigned, a certain amount of dampering should be provided for; this will be small when the method chosen permits a drop in velocity as the quantity of air is reduced. After the total air quantity and the size of fan are ascertained, the main duct is usually fixed eis being at least equal in area to the fan outlet, or perhaps 10 per cent greater. From this main pipe all others are propor tioned. For example, if the main duct is 30 in. in diameter, a branch, to carry 10 per cent of the total capacity should be 12.7 in. in diameter (see, Fig. 4) in order to have the same friction per foot of length, while one carrying one-half the total capacity of a 30-in. main with the same friction loss per foot would be 23.4 in. in diameter.! By this method of equalizing friction it is unnecessary to consider the resistance of each section of pipe independently, but only to know the distance from the fan outlet to the end of the longest run of pipe, the number and size of elbows, and the diameter and velocity in the largest pipe. Example 5. If the greatest length of piping in a system is 130 ft with a 26-in. diameter main pipe and one 20-in. elbow, the piping having been designed for equal friction per foot of length, the friction would be the same as for 130 linear feet of 26-in. pipe, or 60 diameters; To this should be added the friction loss in elbows, in this case one 20-in. elbow, which has a loss equivalent to 8.5 diameters of 20-in. pipe. This in turn is 20 ks 2o X 8.5 = 6.6 diameters of 26-in. pipe. The total equivalent length of the system will then be 60 -f- 6.6, or 66.6 diameters. Since 50 diameters is equivalent to one velocity head, the loss is = 1.33 times the velocity head. If the velocity is, for example, 2200 fpm, corresponding to 0.3-in. pressure, the friction loss of the system will be 1.33 X 0.3 = 0.399 in. Frequently the prevention of sound in a heating or ventilating system imposes more severe restrictions than the prevention of excessive pressure drop. This question is highly involved and requires consideration of cinany factors. The air velocities to be used will vary with the standard of construction used in the ducts themselves as well as with the nature of the occupancy and the construction of the building. In general, architects and engineers who leave the details of duct construction to the contractor must, of necessity, design for lower velocities than might be required for quiet operation if proper construction details were always followed. The 367