Document xjEnZNJ73V39j3V5bvbMyZv7G

1118 CHAPTER 45 1958 Guide the operation and then provide necessary access and working openings. The familiar hoods, such as booths, side- or down-draft hoods (with or with out side shields) have been developed from this complete enclosure concept. Openings in hoods are kept to a minimum size and are placed away from the natural path of the contaminant travel when possible. Doors should* be provided for inspection and maintenance openings when practicable. Capture Velocities and Air Volume Exhausted Only after the hood design has been determined can the exhaust volume requirements be calculated. With enclosures, volumes are calculated from the known open area of the hood and the selected capture or indraft veloc ity sufficient to prevent outward escape. Usual capture velocities for typical operations are listed in Table 1 and refer in the case of remote hoods to the air movement required at the zone of air. contaminant generation. Required capture velocities for any opera tion will vary with the magnitude of the air volume handled, with uncon- Table 1. Minimum Aib Velocities Required at Point of Origin to Capture Contaminant Effectively : Condition of Generation of Contaminant Minimum Capture Velocity, FPM Released without noticeable movement Released with low veloc ity 50-100 100-200 Active generation 200-500 Released with great force 500-2000 PBOCE88 Evaporation of vapors, exhaust from pick ling, washing, degreasing, plating, weld ing, etc. Paint spraying in booth; inspection, sort ing, weighing, packaging, low speed (less than 200 fpm) conveyor transfer points, blending, mixing, barrel filling. Foundry shakeout, high speed (over 200 fpm) conveyor transfer points, crushers, screens. Grinding, tumbling mills, abrasive cleaning. trolled air movement in the area, and often with the location of the process or operation and size of the workroom. Large remote hoods exhausting large air volumes will provide effective control at lower maintained capture velocities than will small remote hoods handling lower exhaust volumes. A hood at one end of a small narrow room with air supply at opposite end will provide control with a lower capture velocity than that required from the same hood in a large room where no perceptible air flow will be ob tained except in the immediate area of the hood. Exterior Hoods Where enclosure of the process is impracticable, the air flow pattern in front of the hood must be such that capture velocities required to convey the contaminant to the hood opening will be maintained in the area of contaminant generation. The method for determining, approximately, the quantity of air that must be exhausted from an unobstructed hood, without flanges, to produce design capture velocities at the point of origin, is given in Equation 1: Q = ExflOA' + A) (D Industrial Exhaust Systems 1119 where - ......... Q = quantity of air exhausted, cubic feet per minute. ' Vx => air velocity at X distance in feet from the hood and on the centerline of the hood, feet per minute. X = distance along the hood centerline, from the face of the hood to the point where the air velocity is Vx feet per minute, feet. A = area of the hood opening, square feet. Fig. 1 shows lines of equal velocities (velocity contours) for a rectangular hood opening with a side ratio of one-half. The velocities are expressed as Pig. 1. Velocity Contours for Rectangular Opening with a Side Ratio of One-Half. . Contours are Expressed as Percentages of the . Velocity at the Opening percentages of the velocity at the opening. Studies have established the principle of similarity of contours which states that the positions of the ve locity contours for any hood (when the contours are expressed in terms of the average velocity at the hood opening) are purely functions of the shape of the hood. Extensive studies2 11 12 have revealed variations in values of such velocity contours for long narrow slots and for hoods with one or more planes shielded against air flow. . For smaller hoods, flanges which are usually 3 to 6 in. wide surroundmg the hood opening usually will improve the air flow in front of the hood and will reduce the air volume required to provide desired capture velocity oy as much as 25 percent. The exhaust volume calculated for an exterior hood by different de signers may vary greatly due to their selection of different empirical re-