Document 06y8qdyYm643xZdoYYNDOdBdM

1098 CHAPTER 45 1957 Guide openings are kept to a minimum size and are placed away from the natural path of the contaminant travel wherever possible. Inspection and main tenance openings are provided with doors whenever practicable. Capture Velocities and Air Volume Exhausted Only after the hood design has been determined can exhaust volume requirements be calculated. With enclosures, volumes are calculated from the known open area of the hood and the selection of the capture or indraft velocity sufficient to prevent outward escapement. Where enclosure of the process is impracticable, the air flow pattern in front of the hood must be such that selected capture velocities will be maintained in the area of generation, conveying the contaminant to the hood opening. Usual capture velocities for typical operations are listed in Table 1 and Table 1. Minimum Air Velocities Required at Point of Origin to Capture Contaminant Effectively Condition or Generation or Contaminant Released without no ticeable movement Released with low veloc ity Minimum Capture Velocity, FPM 60-100 100-200 Active generation 200-500 Released with great force 500-2000 Process 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. 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 trolled air movement in the area, and oftentimes with the location of the process or operation and size of the workroom. Larger remote hoods ex hausting large air volumes will provide effective control at lower maintained capture velocities than in the case of 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 lower capture velocities than that required from the same hood in a large room where no perceptible air flow will be obtained except in the immediate area of the hood. The method for determining, approximately, the quantity of air that must be exhausted from an unobstructed hood, without flanges, to produce these capture velocities at the point of origin, is given in liquation 1: Q = V(10X + A) (1) where Q = quantity of air exhausted, cubic feet per minute. V -- air velocity in feet per minute at X distance in feet from the hood and on the centerline of the hood. X = distance in feet, along the hood centerline, from the face of the hood to the point where the air velocity is V feet per minute. A = area in square feet of the hood opening. Industrial Exhaust Systems 1099 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 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 studies'-11 1! 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 surround ing the hood opening usually will improve the air flow in front of the hood Fig. 1. Velocity Contours for Rectangular Opening with a Side Ratio of One-Half. Contours are Expressed as Percentages of the Velocity at the Opening and will reduce the air volume required to provide desired capture velocity by as much as 25 percent. Special Exhaust Requirements It is important to note that certain operations may require exhaust volumes in excess of the quantities based on design data from Table 1 and Equation 1. Typical reasons for increased ventilation rates include: 1. Induced air flow caused from .falling granular material in large quantities through considerable height, or from internal rotating parts like some types of crushers, knives, macerators. 2. Induced air flow caused by the thermal or . stack effect from sources of ex treme heat.