Document NE8dMd9E091Vdo4DQRjEXmxob

TABLE 29-B MINIMUM CONTROL VELOCITIES (Minimum Air Velocities Recommended for the Capture of Dusts, Fumes, Smokes, Mists, Gases, and Vapors Released at Vanous Types of Operations) Conditions of Generation, Dispersion, or Release of Contaminant Minimum Control Velocity (fpm) Examples of Processes or Operations Released with no significant velocity into relatively quiet air Released with low initial velocity into moderately quiet air Released with considerable velocity or into zone of rapid air movement Released with high velocity or into zone of very rapid air movement 100 100-200 200-500 500-2,000 Evaporation or escape of vapors, gases, or fumes from open vessels, degreasing, pickling, plating Spray paint booths, cabinets, and rooms, intermittent dumping of dry materials into containers, welding Some spray painting in small booths and with high pressure. active barrel or container filling, conveyor loading Grinding, abrasive blasting, surfacing operations on rock From Allen D Brandi A Summary of Design Data for Ezhaust Systems " Heating and Ventilating (May 1945) For circular or square openings, b is es sentially 0 1 and the equation becomes 01(5 Suppose the duct is 6 in in diameter, and the velocity of air in it is 4000 fpm--a com mon situation in dust exhaust systems Since a 6-in diameter circle is 0 196 sq ft in area of cross section, Q = 0 196 X 4000 or 780 cfm (In ducts, 0=AoDuct ) Two inches out from the duct end, t> has fallen from 4000 fpm to 1650 At 4 m from the duct, the velocity is 600, at 6 in away, it is only 290 fpm --barely enough to be felt by the hand Where x is very large compared with A, equation (2) becomes See Table 29-A Flanged hood. If a flange is placed around the duct opening, as shown in Fig 29-1 (right), it will reduce entrance or turbulence loss by keeping the hood from drawing air from in back of the hood face For the same total amount of air exhausted, a larger portion will come from in front of the duct This is beneficial, since air that moves from the back side of the hood does not help control the contaminant out m front A flange will in crease the useful air by 20 to 30 per cent, for the same total volume of air handled The flange width can equal the hood diameter or side, but generally need not exceed 6 m Face velocity os mass air movement The preceding equations also show that the cap ture velocity, v, depends on total air flow entering the hood This is frequently over looked A high face velocity or, m the case of a slot-shaped hood, a high slot velocity is not the important factor The capture of air contaminants depends on mass air move ment, Q, not on mere face velocity Regardless of face velocity, a source of suction has a woefully poor ability to ``reach out" in a certain direction and induce an in flowing stream of air even a few inches from the usual hood face Yet, not infrequently. 857