Document KJkV2xrzrD6GB4jzzRmojkGKw

American Society o/ Heating and Ventilating Engineers Guidi Table 2. Size of Connections for Grinding and Buffing Wheel Diameter op Wheels Grinding-- 6 in. or less, not over 1 in. thick._______ ______ 7 in. to9 in., inclusive, not over 1)4 in. thick- 10 in. to16 in., " " " 2 in. " 17 in. to19 in., " " " 3 in. * _ 20 in. to24 in., " " 4 in. " _ 25 in. to30 in., " " * 5 in. " _ Buffing-- 6 in. or less, not over 1 inrthick.......... .........--:: 7 in. to12 in., inclusive, not over 1)4 in.thick- 13 in. to16 in., " " " 2 in. 0 17 in. to20 in., * " ` 3 in. ' - 21 in. to27 in., " ` * 4 in. ` . 27 in. to33 in., " " " 5 in. " ... Table 3. Suction Pressures Required at'Hoods Exhausting from grinding and buffing wheels______________ Exhausting from tumbling barrels._________ ;__________ :----Exhausting from wood-working machinery--light duty_____ Exhausting from wood-working machinery--heavy duty. Shoe machinery exhaust____ Exhausting from rubber manufacturing processes. Flint grinding exhaust___________ : Exhausting from pottery processes. Lead dust and fume exhaust. Fur and felt machinery exhaust.___ Exhausting from textile machinery.. Exhausting from elevating and crushing machinery... Conveying bulky and heavy materials. measure of the air volume handled and consequently of the air velocity at the opening of the hood. The elimination of any dusty condition requires added information concerning the shape, size and location of the hood used with regard to the operation in question. In some states grinding, polishing and buffing wheels are subject ti regulation by codes. The static suction requirements, which range fron 1% to 5 in-, water displacement in a Z7-tube, should be followed althougl in several instances they may appear to be excessive. Frequently, u these operations, a large part of the wheel must be exposed and the dust laden air within the hood is thrown outward by the centrifugal action o the wheel, thus counteracting useful inward draft. This tendency ma; be diminished by locating the connecting duct so as to create an air floi of not less than 200 fpm about the lower rim of the wheel. Exact determinations of hood control velocities are not available, 348 Chapter 21--Industrial Exhaust Systems f. , t0 assume that for most dusty operations they should not be less fom at the point of origin. For granite dust generated by 30 - - Hatch et als give velocities from 150 to 200 fpm, pe of .hood used, as sufficient for safe control. Con- TM - tne __ it of the industry, air velocities of this order may be |i"ded to similar dusty operations. The method, for approximately Hfermining these velocities in terms of the velocity at the hood opening |!given below. DESIGN OF HOODS ,t` No set rule can be given regarding the shape of a hood for a particular ^vteratiom but it is well to remember that its essential function is to create Mi adequate velocity distribution. The fact that the zone of greatest Ineffectiveness does not extend laterally from the edges of the opening may '`'frequently be utilized in estimating the size of . hood required. . Where femplete enclosure of a dusty operation is contemplated, it is desirable to tjeave enough free space to equal the area of the connecting duct. Hoods Jor grinding, polishing and buffing should fit closely, but at the same time should provide an easy means for changing the wheels. It is advisable to design these hoods with a removable hopper at the base to capture the jjjjavy dusts and articles dropped by the operator. Such*provisions are of jgstance in keeping the ducts clear. Air volumes used to control many ajp discharges may often be reduced by effective baffling or partial enclosure of an operation. This procedure is strongly urged where dusts ayi directed beyond the zone of influence of the hood. . Axial Velocity Formula for Hoods When the normal flow of air into a hood is unobstructed, the following "formula may be used to determine the air velocity at any point along the %is: 0.L4 100 - Y CD -T = per cent of velocity at opening. A.= area of opening, square inches (or square feet). * distance outward from opening, inches (or feet). It is important to note that the velocity function varies in direct groportion to the area. Hence, under certain conditions, a large opening mjy function more effectively than a small one for the same volume of Bqw. The formula, of course, presumes that the air velocity distribution "toss the hood opening is uniform4. 1. A small hood 64 sq in. in area handles 400 cfm. What will be the air SSSpty at a point 5 in. outward along the axis if the flow is unobstructed? gg^Theodore. Drinker Philip, and Choate. Sarah P,, Control of the Silicosis Hazard in the Hard Cnac,,,H"vLr Laboratoiy Study of the Design of Dust Control Systems for Use with Pneumatic wSi tb * T S' ("Iournai fini Hygiene, Vol. XII, No. 3, March. 1930). 27?932)CteraCteri3tic8 of Hoods und Suction. by J- M. DallaVaUe (A.S.H.V.E. Transactions 349