Document Lgg9pE4oYr9XrL4y6On3Zeyv7

rr^~r-~r ~/r-s? - American Society 0/ Heating and Ventilating Engineers Guide, The next step is to determine the size of the main, which varies11 diameter, increasing from its dead end towards the fan, as necessitate by the branch pipes entering it. Its exact diameter at any point u determined according to the following rule: .- The area of the main at any point should be 20 per cent to 2$ per cent ini excess of the sum ofike areas of the branches entering it between ike point in] question and the dead end of the main. Floor sweeps, if equipped ^1 efficient blast gates, need not be included in computing the area of the main.'? According to Table 3, 2 in. suction at the hoods is suitable for exhaust-. ing from wood-working machinery where the duty is light, as would be the case in this problem. The volume of air to be handled by the system is determined from Table 4. 1 to 5 in. connection, 2 in, suction, ' 1 to 4 in. connection, 2 in. suction, 1 to 5 in. connection, 2 in. suction, 2 to 6 in. connection, 2 in. suction, (from Table 4)605 X 1 equals 605 c.f.m. (from Table 4)386 X I equals 386 c.f.m. (from Table 4)605 X 1 equals 605 c.f.m. (from Table 4)867 X 2 equals 1,734 c.f.m. Total------------------------------------------ -------------------------------------- 3,330 c.f.m. The next step in the calculations is to determine the maintained resistance of the system, which corresponds to the static pressure that the exhaust fan must produce to obtain the proper volume of air and suction at the hoods. This maintained resistance is composed of the following elements : 1. Loss through the hoods. This is usually assumed as equal to the suction main tained at the hoods, in this problem 2 in. 2. Frictional resistance or loss in piping system. This is a function 0/ the pipe diameter and the velocity of flow and accordingly it is necessary to compute the friction for each section and to add the components together. 390 Chapter 25t-Exhaust Systems for Dust and Refuse MiCHIN* ySaw.--................................... t-off Saw.--............................ nd Saw--............................ tner Top Connection.... .......... ner Bottom Connection____ Diameter or Branch Pipes + %Area So. Inches Total Load Area So- Inches Total i iqad Are. JalNCHE 25 Section of Main Diameter of Maim w Even Inches 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 Tin. 65 D 9 in. 100 135 E & F 13 in. I' oneg.u' Pndreesrsucroendsirdoepraotirolnosist tihsrocuusgthomthaerycotollecchtooro.seFaorcaonlleecxtohrauhsatvsinygsteamn sinulcent aosf mthee same, or slightly greater, diameter than the main discharge pipe leading to the collector. ,The friction per 100 ft. of length for various pipe diameters and veTlohceitiefrsicctaionnbfeorreiandtefrrmomediTaateblpeip5e, sizes not given in the table can be computed from the table values, from the relation that the friction in two pipes, the velocity remaining constant, is inversely proportional to the ratio of the diameters. This can be written: **-*>d!,, (3) > z, = friction in pipe of d, diameter, z, = friction in pipe of <fj diameter. The total pipe friction is calculated to be 3.49 in. of water, The individual steps in the calculation are given in Table 7. , The effective length of pipe is determined by adding to the actual length a sum to compensate for the effect of elbows as given in the note .a/tInthethbiosttpormobolefmTatbhele m5.ain discharge pipe is 13 in. in diameter and a collector having a 14 in. inlet is selected. The area of the collector inlet 7. D,a. , F...o sm. ----------- Section op Piping A B C D E F ErpBcnvB Length op Pipe Taken As: 20 ft. 10 ft. 10 ft. 15 ft. 20 ft. 25 ft. Diameter op Pipe in Inches 5 6 7 9 13 13 Area or Pip IN Bo. P*.' 0.136 0.195 0.267 0.442 0.922 0.922 C. F. M. 605 605 991 1,596 3,330 3,330. VZLOCITT OP Air Flow, Feet per Minute 4450 3100 3700 3600 3600 3600 Friction per 100 PT. OP Length 7.4 3.1 3.8 2.8 2.0 2.0 Pipe FrictiOn 1.48 0.31 0.38 0.42 0:40 0.50 Total pipe motion....... ........................... *** 3.49