Document Edd94N7YB9v74bLzKke4Y448N

American Society of Heating and Ventilating Engineers Guide, 1929 at the same time, the velocity in the main might easily be lowered to the point where it would not be sufficient to carry the material from the machines still in operation, and thus result in clogging the pipes. Ac cordingly, it is sometimes desirable to use velocities higher than the mini mum to allow a factor of safety to cover this contingency. Table 1. . Size of Connections for Wood-Working Machinery Type of Machine Diameter of Connections in Inches Band Saws, Blade 3-4 in. wide.--................................... .................................. Planers, Matchers, Moulders, Stickers, Jointers, etc.-- Belt Sander, Belt 10-14 in. wide.;......................................................... ............ Drum Sander, 24 in------------- ----- -------- ------------------------- :.......... .................... Drum Sander, 48 in.--............................................................................ ........... 4 5 6. 4 5 6 7 .8 6 6 7 10 5-6 6-8 6-10 4-5 8 5 6 7 5 6 7 8 10 5 6 7 4 In some states codes have been issued specifying suctions to be main tained for the more common dusts. j A suction standard should always be considered in conjunction with the shape of hood, and size connection, as these factors together determine the volume of air exhausted and its velocity which in turn are a measure of the effectiveness of the exhaust system. The cubic feet of air of standard density taken into the system at each connection is given by the formula: Q = iOOOAf^~i where , Q= cubic feet of air per minute. A -= area of connection in square feet. / *= orifice or restriction coefficient. * = static suction measured in inches of water. 464 Chapter XXXIf--Dust, Exhaust and Collecting Systems Table 2. Size of Connections for Grinding and Buffing Wheels Diameter of Wheels Grinding-- 6 in. or less, not over 1 in. thick 7 in. to .9 in., inclusive, not over 1J4 in. thick......... 10 in. to 16 in., " " " 2 in. a 17 in. to 19 in., " " " 3 in. a 20 in. to 24 in., " " " 4 in. u 25 in. to 30 in., " " " 5 in. u Buffing-- 6 in. or less, not over 1 in. thick. 7 in. to 12 in., inclusive, not over 1^ in. thick......... 13 in. to 16 in., " " " 2 in. a 17 in. to 20 in., " " 3 in. u 21 in. to 27 in., " " " 4 in. u 27 in. to 33 in., " " " 5 in. -- Max. Grinding . Surface Sq. In. 19 43 101 180 302 472 19 57 101 189 338 518 Min. DiAm. of Branch Pipes in Inches 3 3H 4 4H 5 6 3H 4 4H 5 6 7 Table 3. Suctions Required at Hoods For Connections of Usual _______________ Proportions Work Static Suction in In. of Water Exhaustine from minding and huffing wheels Exhausting from tumbling harrels Exhausting from wood-working machinery--light dutv Exhausting from wood-working machinery--heavy duty Shoe machinery exhaust. ............. FExlihnat ugsrtinindginfgroemxhrauubsbte..r...m....a...n..u..f.1a.c..tu..r.ing processes. Exhausting from DOtterv orocesses..... ............. Lead dust and fume exhaust Fur and felt machinery exhaust. ............. Exhausting from textile machinerv..... Exhausting from elevatinv and crushing machinerv Conveying bulky and heavy materials 1-2 1-2 1-2 2-4 2-3 1-2 1-2 1-2 1-4 2-3 1-3 1-2 3-5 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. . Knowing the suction at each hood and the diameter of each connection', the volume of air passing up each branch can be taken from the accom panying Table 4. The sum of all these volumes gives.the total volume to be handled by the exhaust fan. 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 enter ing it between the point in question and the dead end of the main. Similarly the discharge pipe leading from the fan outlet to 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 465