Document vBNdrdKGqgag8ERzLGrE7DqDw

HEATING VENTILATING AIR CONDITIONING CUIDE 1941 Table 2. Size of Connections for Grinding and Buffing Wheels Diameter of Wheels Grinding-- 6 in. or less, not over 1 in. thick------ 7 10 17 20 25 in. in. in. in. in. to to to to to 9 16 19 24 30 in., in., in., in., in., inclusive, " " " " not " " " * over " " " " 1H 2 3 4 5 in. in. in. in. in. thiack___ a a a Buffing-- 6 in. or less, not over 1 in. thick.------- ________ 7 in. to 12 in., inclusive, not over 1J4 in. thick...... 13 in. to 16 in., " " " 2 in. 17 in. to 20 in., 21 in. to 27 in., 27 in. to 33 in., " " " " " " "3 "4 "5 in. in. in. a a 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 4M 5 6 3H 4 5 6 7 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 13^ to 5 in. water displacement in a Z7-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 diistladen air within the hood is thrown outward by the centrifugal action of the wheel, thus counteracting useful inward draft. This tendency may be diminished1 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 Table 3. Suction Pressures Required at Hoods .Type or Installation Static Suction ib Inches or Watzb Shoe machinery exhaust..--------------- ------------------- -------------................ -- 686 1M-5 2 2 2-4 2-3 2 2 2 2-4 2-3 2-3 2 3-5 CHAPTER 39. INDUSTRIAL EXHAUST SYSTEMS than 200 fpm at the point of origin. For granite dust generated by pneumatic devices, Hatch et al2 give velocities from 150 to 200 fpm, depending on the type of hood used, as sufficient for safe control. : Con sidering 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. 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 axis5: ' y -- 01Q x' + Q.lA (1) where V = velocity at point, feet per minute. A = area of opening, square feet. x = distance along axis, feet. Q = volume of air handled, cubic feet per minute. Velocity Contours It is possible by use of a specially constructed Pitot-tube4 to .map contours of equal velocity in any axial plane located in the field of in fluence. It has been found that the positions of these contours for any hood can be expressed as percentages of the velocity at the hood opening and are purely functions of the shape of the hood6. *Control of the Silicosis Hazard in the Hard Rock Industries. I. A Laboratory Study of the Design of"" De control Systems for Use with Pneumatic Granite Cutting Tools, by Theodore Hatch, Philip Drinker and Sarah P. Choate. {Journal of Industrial Hygiene, Vol. XII, No. 3, March, 1930). 1933) k6 ^ontro* Industrial Dust, by J. M, DallaValle (Mechanical Engineering, Vol. 55, No. 10, October `Studies in the Design of LocaL Exhaust Hoods, by J. M. DallaValle and Theodore Hatch (A.S.M.E. Transactions, Vol. 54, .1932). . ,, /Velocity Characteristics of Hoods under Suction, by J. M. DallaValle (A.S.H.V.E. Transactions, Vol. 38, 1932, p. 387). .