Document v6anNyga6yw3pNYpM31mkbgvY

American Society of Heating and Ventilating Engineers Guide, 1930 mains should be a certain percentage greater area than the sum of the 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 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, increasing the size of the mains unduly would introduce a serious hazard. 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 Addedfor Leakage Diameter op Pipe In. 1 Maintained Suction--In. Water Gage IX 2 2M 3 4 5 2 2H 3 3M 4 4H 5 6 7 8 9 10 38 68 107 153 209 273 345 427 614 835 1092 1381 1705 47 84 131 188 256 334 423 523 751 1023 1337 1694 2090 54 97 161 217 296 386 488 605 867 1181 1546 1953 2409 61 108 168 243 330 431 546 676 970 1322 1727 2184 2695 67 118 185 266 362 473 598 741 1062 1448 1892 2387 2959 76 136 214 306 418 546 690 854 1228 1670 2184 2762 3410 86 153 238 343 466 609 775 955 1373 1870 2440 3091 3806 The maintained resistance of the exhaust system is composed of three factors: (1) loss through the hoods; (2) collector drop; and (3) friction drop in the pipes. The collector drop in inches of water is given roughly by equation (2) but where possible the resistance of the particular collector to be used should be ascertained from the manufacturer as these resistances differ quite widely. where c(w)'Drop = (2) C -- a constant which depends upon the type of collector and ranges from 0.065 to 0.145. V = velocity in feet per minute of air entering the collector. Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. Find the velocities in each section of pipe starting with the branch most remote from the fan. The friction drop for these sections can be determined by reference to Table 5. Total friction loss in the piping system is the friction drop in the most remote branch plus the drop in the various sections of the main, plus the drop in the discharge pipe. 462 Chapter 28--Pneumatic Exhaust Systems 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 Loss of Pressure in Inches for Given Diameter Pipe 4' 1.92 2.32 2.77 3.26 3.76 4.33 4.93 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 6.92 140 16' 18' 20' 22' 24' 2000 2200 2400 . 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4800 5200 5600 6000 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 0.650 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 6.320 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 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 30' 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 athroat 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. Table 6. Diameters of Branch Pipes and Calculations of Main for Typical ______ Exhaust System Illustrated in Fig. 1 Machine DiAmcraa or Branch Pipes Arba So. Inches Total Load Area Sq. Inches Total Load Area So. Inches + 25% Section or Main DlAMETER or Main in Even Inches . Rip Saw.................................................. Cut-off Saw Band Saw.............................................. Planer Top Connection.. ............... Planer Bottom Connection.... ........... 5 in. 4 in. 5 in. 6 in. 6 in. 19.6 12.6 19.6 28.3 28.3 19.6 32.2 51.8 80.1 108.4 25 B '6 in. 40 C 7 in. 65 D 9 in. 100 135 E&r F 13 in. 463