Document nkE4388BNzpxYeJRzD8aOY2Xm

American Society of Heating and Ventilating Engineers Guide, 1936 F ig . 6. T y p ic a l L ayo ut op A ir D is tr ib u tio n System Chapter 20--Air Duct Design 22.94 sq ft. If the air washer is provided with automatic humidity control, the tempering coil should raise the temperature of the entering air to 32 F. The washer with its auto matic control will then raise the temperature from 32 F to 42 F. If the washer is not provided with automatic humidity control, the tempering coil must raise the temperature of the entering air to at least 55 F to allow for some temperature drop in the washer due to evaporation. The reheating coil is selected to raise the temperature of the air from that leaving the air washer to 70 F. The air washer should have a maximum velocity of 500 fpm through the clear area, which, in this case, is 46 sq ft. For more detailed infor mation on tempering coil and air'washer control, see Chapters 14 and 23. Since the plan shows a moderately short run of main duct with no risers near the fan outlet, a fan should be selected which will have the required capacity of 22,935 cfm with a maximum velocity through the fan outlet of 1400 fpm. The outlet area, therefore, should be 16J sq ft. Table 2. Pipe Sizes for Example 3a Volume or Am (cm) 22,935 12,510 10,425 8,340 6,255 4,170 2,085 Peb Cent or Total . Volume 100.0 54.6 45.4 36.3 - 27.2 18.2 9.1 Diameter or Pipe (Inches) . 56 45 42 39 35 29^ . 23 Equivalent Size or Rectangular Duct ... (Inches) 60 x 44: 58x30 50 x 30 42 x 30 42x24 30x24 30 x 15 .Velocity through diffusers (not shown) to be approximately 300 fpm. X, The main pipe size should be selected to give a velocity equal to or less than the velocity at the fan outlet. Choosing a 56-in. pipe with a cross-sectional area of 17.1 sq ft, the velocity in the main pipe will be 1340 fpm. Using the friction pressure loss method this 56-in. main pipe will be taken as the basis of calculation. Fig. 6 shows the amount of air to be handled by each section of pipe. Expressing the volume handled by each section as a percentage of the total volume and using the charts. Figs. 4 and 5, the pipe sizes are as shown in Table 2. . The pressure at the outlets nearest the fan will be greater than at the pipes farther along the run so that the former will tend to deliver more than the calculated amount of air. To remedy this condition, volume regulating dampers should be located at the base of each riser and adjusted for proper distribution, At points where branches leave the main it may be advisable, depending upon the nature of the installation, to install adjustable splitters similar to that shown in Fig. 6 where the main duct divides into the 68 in. X 30 in. and 50 in. X 30 in. branches. The rectangular equivalents are selected from Table 1; the width to depth proportion will be determined by construction requirements and ease of fabrication. The calcu lation of the friction is as follows: The longest run from the fan outlet to diffuser is 150 ft 0 in.; 150 ft of 56-in. pipe is equivalent to----- =------------------------------------------- ________________________ 32.2 dia. Ac Two 45-in., 90-deg elbows (2 X ^ X 8.5)______________ ___ _______________ 13.7 dia. 50 f (Assume each elbow equivalent to 8.5 diameters of duct. Fig. 1.) 09 Two 23-ih., 90-deg elbows (2 X X 8.5)_____________ ________ _________ 7.0 dia. 00 Two 23-in., 90-deg elbows in riser (2 X ~X 30) 24.7 dia. 5o (Two bad elbows in riser, each equivalent to 30 diameters of duct). (Total diameter of 56-in. pipe.. 77.6