Document 3Q7LLYnZ0k52XpqoKeNMJK2OE

American Society of Heating and Ventilating Engineers Guide, 1934 employed for standard wood-working machinery and for grinding and buffing wheels. Static pressures which in practice have been found necessary to control and convey various materials, are given in Table 3. It must be remembered, however, that the term suction is merely a rough 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 1 "10 in. to 16 in., " 17 in. to 19 in., " "2 " "3 20 in. to 24 in., " " "4 25 in. to 30 in., " " "5 in. thick__ in. in! in. " " " II!I! in. " I Buffing-- 6 in. or less, not over 1 in. thick._______________ 7 in. to 12 in., inclusive, not over 1V4 in. thick.. 13 in. to 16 in., " " " 2 in. " 17 in. to 20 in., " " " 3 in. " I 21 in. to 27 in., " " " 4 in " 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 5 6 SH 4 4H 5 6 7 Table 3. Suction Pressures Required at Hoods Static Suction in Inches or Wateb Exhausting from grinding and buffing wheels Exhausting from tumbling barrels. Exhausting from wood-working machinerv--light rintv Exhausting trom wood-working: machinerv--h?aw duty Shoe machinerv exhaust-.......... Exhausting from rubber manufacturing processes I* lint grinding exhaust.. ___ __ Exhausting from pottery processes . Lead dust and fume exhaust._ hur and felt machinerv exhaust____ Exhausting from textile machinerv........... Exhausting from elevating and crushing machinery Conveying bulky and heavy materials 1H~5 2 2 2-4 2-3 .2 2 2 2-4 2-3 2-3 2 3-5 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 to regulation by codes. The static suction requirements, which range from to 5 ih. water displacement in a (/-tube, should be followed although in several instances they may appear to be excessive. Frequently, in these operations, a large part of the wheel must be exposed and the dust- 282 Chapter 21--Industrial Exhaust Systems laden air within the hood is thrown outward by the centrifugal action of the wheel, thus counteracting useful inward draft. This tendency may be diminished by locating the connecting duct so as to create an air flow of not less than 200 fpm about the lower rim of the wheel. Exact determinations of hood control velocities are not available, but it is safe to assume that for most dusty operations they should not be less than 200 fpm at the point of origin. For granite dust generated by pneumatic devices, Hatch2 gives velocities from 150 to 200 fpm, depending on the type of hood used, as sufficient for safe control. Considering the character of the industry, air velocities of this order may be extended to. similar dusty operations. The method for approximately determining these velocities in terms of the velocity at the hood opening is given below. DESIGN OF HOODS No set rule can be given regarding the shape of a hood for a particular operation, but it is well to remember that its essential function is to create an adequate velocity distribution. The fact that the zone of greatest effectiveness does not extend laterally from the edges of the opening may frequently be utilized in estimating the size of hood required. Where complete enclosure of a dusty operation is contemplated, it is desirable-to leave enough free space to equal the area of the connecting duct.. Hoods for 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 heavy dusts and articles dropped by the operator. Such provisions are of assistance in keeping the ducts clear. Air volumes used to control many dust discharges may often be reduced by effective baffling or partial enclosure of an operation. This procedure is strongly urged where dusts are 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 axis: Y _ 0.1A 100 - Y x* (1) where Y = per cent of velocity at opening. . A = area of opening in square inches (or square feet), x = distance outward from opening in inches (or feet). It is important to note that the velocity function varies in direct proportion to the area. Hence, under certain conditions, a large opening may function more effectively than a small one for. the same volume of Control of the Silicosis Hazard in the Hard Rock Industries. (Journal of Industrial Hygiene. Vol. XII. No. 3. March. 1930). 283 >. !