Document 0J4GKQz1ZMXwwa4d1JpnyBqkb

American Society of Heating and Ventilating Engineers Guide, 1932 Volume of Air The volume of air of standard density taken into the system at each connection is given by the formula: Q = 4,000 A(1) where Q = cubic feet of air per minute. A = area of connection in square feet. / = orifice or restriction coefficient. ": i = static suction measured in inches of water. The orifice coefficient / is dependent upon the shape and construction of the hood and will range from 60 to 90 per cent. An average value is 70 per cent. Table 4. Cubic Feet of Air Handled Per Minute Through Average Collecting Hoods Based on Coefficient of Orifice of 0.71 with 10 Per Cent Added for Leakage Diameter of Connection Pipe In. 1 Maintained Suction--In. Water Gage 1M 2 2M 3 4 5 -38 47 54 61 67 76 86 2 68 84 97 108 118 136 153 2H 107 131 161 168 185 214 238 3 153 188 217 243 266 306 343 3H 209 256 296 330 362 418 466 4 273 334 386 431 473 546 609 345 423 488 546 598 690 775 5 427 523 605 676 741 854 955 .6 614 751 867 970 1062 1228 1373 7 835 1023 1181 1322 1448 1670 1870 8 1092 1337 1546 1727 1892 2184 2440 9 1381 1694 1953 2184 2387 2762 3091 10 1705 2090 2409 2695 2959 3410 3806 Knowing the suction at each hood and the diameter of each connection, the volume of air passing up each branch can be taken from Table 4. The sum of all these volumes gives the total volume to be handled by the exhaust fan. Size of Pipes Common practice is to provide a main suction pipe having an area 20 to 25 per cent in excess of the sum of the areas of the branches entering it between the point in question and the dead end of the main. Similarly the discharge pipe leading from the fan outlet to the collector is frequently made the same diameter as the large end of the main suction pipe. The reason for this increase in size is that a considerable power saving results from the lower air velocity. However, there is no -technical reason why mains should be a certain percentage greater area than the sum of the 350 Chapter 25--Ventilation of Industrial Buildings; Exhaust Systems connections, and still lower power consumption can be obtained by using larger branches and mains of equal area. While the rule of thumb method of determining size of mains works very well in many cases, yet it is Table 5. Frictional Resistance of Straight Conveyor Pipe To Flow of Air Per 100 Feet of Pipe Vbl. of Air per Min. 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4800 . 5200 5600 6000 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4800 5200 5600 6000 Loss of Pressure in Inches for Given Diameter Pipe 4' 1.92 2.32 2.77 3.26 3.76 4.33 4.9? 5.56 6.23 6.95 7.69 8.48 9.26 11.05 13.00 15.25 17.30 . 5' 1.53. 1.85 2.22 2.60 3.01 3.46 3.94 ,4.45 4.98 5.55 6.15 6.78 7.41 8.85 10.50 12.05 13.85 6' 1.28 1.55 1.84 2.17 2.52 2.88 3\28 3.71 4.15 4.62 5.13 5.65 6.18 7.38 8.66 10.05 11.52 7' 1.09 1.32 1.58 1.86 2.15 2.47 2.82 3.18 3.56 3.97 4.40 4.85 5.30 6.32 7.44 8.61 9.89 8' 0.962 1.16 1.39 1.63 1.89 2.08 2.47 2.78 3.12 3.48 3.85 4.25 4.63 5.55 6.50 7.55 8.66 10' 0.770 0.932 1.01 1.30 1.51 1.73 1.97 2.22 2.49 2.78 3.08 3.49 3.71 4.43 5.21 6.03 6.92 12' 0.640 0.778 0.924 1.08 1.26 1.44 1.64 1.85 2.08 2.32 2.57 2.83 3.09 3.69 4.34 5.05 5.76 14* 16' 18' 20' 22' 24' 30' 0.550 0.655 0.790 0.930 1.07 1.24 1.41 1.59 1.78 1.99 2.20 ,2.43 2.66 3.17 3.72 4.32 4.95 0.482 0.582 0.693 0.810 0.932 1.08 1.23 1.43 1.56 1.74 1.92 2.12 2.33 2.77 3.25 3.78 4.33 0.428 0.578 0.617 0.722 0.838 0.961 1.09 1.24 1.38 1.54 1.71 1.88 2.06 2.46 2.89 3.35 3.85 0.385 0.465 0.553 0.6S0 0.754 0.865 0.985 1.11 1.25 1.39 1.54 1.70 1.85 2.22 2.61 3.02 3.46 0.350 0.423 0.504 0.590 0.685 0.788 0.895 1.01 1.13 1.26 1.40 1.54 1.68 2.02 2.36 2.74 3.14 0.320 0.388 0.462 0.542 0.628 0.722 0.820 0.925 1.04 1.16 1.28 1.42 1.54 1.85 2.16 2.52 2.89 0.257 0.310 0.369 0.434 0.503 0.577 0.657 0.742 0.832 0.926 1.03 1.13 . 1.24 1.48 1.75 2.01 2.31 FRICTIONAL RESISTANCE OF ELBOWS Elbows having a throat radius equal to the pipe diameter set up a resistance equivalent to a section of straight pipe approximately 10 diameters long. With a throat radius of 1H times the diameter the resistance is about the same as seven diameters of straight pipe. always desirable to figure the mains and branches of the proper size to give-the velocity which has been found best suited to the work to be done. In certain special cases where explosive"or poisonous dusts such as aluminum buffings, grain dust, powdered sugar, or lead dust are handled, 1 increasing the size of the mains unduly would introduce a serious hazard. 351