Document K2d5w6yv1XQ2YojMLpgvg35K

American Society of Heating and Ventilating Engineers Guide, 1934 266 F ig . 6. T ypic al L ayout of A ir D is tr ib u tio n System Chapter 19--Air Duct Design Table 2. Pipe Sizes for Example 3 Volume or A.ib (cm) 22,935 12,510 10,425 8,340 6,255 4,170 2,085 Per 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 29H 23 . [Velocity through diffuses (not shown) to be approximately 300 fpm]. Equivalent Size or Rectangular Duct (Inches) 60x44 58x30 50x30 42x30 42x24 30 x 24 30 x 15 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 58 in.-by-30 in. and 50 in.-by-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 equ.iva,len_t to-1-5--0-^X--12 ............................. ................................... ____ _________32.2 dia. Two 45-in., 90-deg elbows (2 X-X 10).............. ....................... .............. ,___ 16.1 dia. 23 Two 23-in., 90-deg elbows (2X r00rX 10)............... -- ------------ ___ _________ 8.2 dia. Two 23-in., 90-deg elbows in riser (2 X 23 jgX 30)....... ................. ........... ...........24.7 dia. (Two bad elbows in riser, each equivalent to 30 diameters of duct). Total diameter of 56-in. pipe.,--........--......... ...................... :.................... . 81.2 (1340 \2 4005 / = ln" Taking 50 diameters as one head loss, then -81'2- X 0.112 = 0.182 in. static loss in duct. Where the connection pieces are made with long easy slopes and the general work manship is good, a regain in static pressure may be deducted from the foregoing pressure loss. This can be taken as approximately two-thirds the difference in velocity pressures at the fan outlet and the last run of pipe. The velocity in the riser is 667 fpm with a corresponding velocity pressure of 0.033 in. The fan outlet velocity is 1400 fpm with a corresponding velocity pressure of 0.122 in. The regain equals two-thirds (0.122 -- 0.033) = 0.059 in. ........... 267