Document O3OXeBp54a905E7wy4omEq8eM

Cl 120 CHAPTER 45 1958- Guide quired capture velocities. Usually capture velocity design values are in the 50 to 200 fpm range. ... The exhaust rates.calculated for large exterior hoods may become need lessly large if the capture velocity is selected blindly. Whether or not an .individual particle or a small .wisp of gas or smoke in motion away from the hood is captured before it escapes from the zone of influence created by the hood is determined not only by the specific air. velocity, (design capture velocity) at the immediate point of contaminant release but also by the depth of the moving air curtain it must traverse before it gets beyond the influence of the hood. The capturing force of the air flow in front of any exhaust hood is the summation of all the separate velocities beyond (away from the hood) the point of contaminant release multiplied by the distance through which each velocity acts; i.e., force times distance, the force being a function of the velocity. For large hoods, or more accurately for large exhaust rates, the depth of the zone of influence is much greater than for small exhaust rates and, also for the former, the distance between successive velocity contours is greater. Further effectiveness for larger volumes can be obtained where the air supply source produces a sweeping flow toward the zone of contaminant release. Table 2 presents design data by Hemeon11' 2ap3p4lic5a6bl*e to exterior hoods employed for exhausting contaminants originating in a cold process only. These values are based on Equation 1 wherein varying values of Fx have been employed according to a scale that takes account of velocity depth factors discussed in the preceding paragraph, and where X-distance is taken as the distance from the hood face to the point where any high velocity air currents engendered by the contaminating process itself have expended their energy. In addition, the ranges of exhaust volumes which have been found satisfactory in practice for many operations are reported in Table 3. . These data provide a means of checking the calculated rates for specific applications. 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 by the thermal, or stack effect from sources of ex treme heat. 2. Induced air flow caused by falling granular material in large quantities through considerable height, or by internal rotating parts such as some types of crushers, knives, or macerators. (See discussion in later section Induced Air Flow and Table 4.) 3. Exhaust volumes insufficient to dilute mixtures of combustible vapor and air to less than 20 percent of the lower explosive limit of the combustible. 4. Room air currents caused by cross drafts, spot cooling, motion of machinery or operators. Cross drafts are of great significance in the exhaust of hot processes with high canopy hoods. 5. Design volumes, especially in the case of excess heat control and solvent vapors, may be based on dilution systems where exhaust volumes are selected to keep con taminant concentrations below design levels. See Reference 13, Chapter 10. 6. Local or State regulations may specify larger exhaust volumes for specific opera tions. Exhaust volume requirements for many specific operations have been listed in Tables 3, 5, 6 and 7. i jIndustrial Exhaust Systems ,1121 Table 2. Summary op Exhaust Rates Required with Common Types op Exterior Hoods Applied to Cold Processes Only11 X-Distance inches Simple Opening, with or without Flange or Taper, but no Plane Flanking Openings Simple Rectangular Opening Flanked bt Plane Parallel to Axis Exhaust Rate-rCfm Exhaust Rate--Cfm 2 25-75 25-50 4 50-100 50-76 6 100-200 75-150 8 200-400 150-300 10 300-600 200-400 12 400-800 300-600 15 600-1200 400-800 18 s 900-1500 600-900 21 1200-1800 700-1000 24 1500-2200 800-1200 30 2500-3500 1200-1800 36 3000-4500 1500-2200 42 4000-6000 2000-3000 48 5000-7000 2500-3500 X-Distance Single Slots.Flanked bt Parallel Plane on Which Contamination Process Occurs Fox X-distance (.., width of plane) always leas than H length of Less than 2 ft More than 2 ft Exhaust rate 125 to 150 cfm per sq ft of plane area. Exhaust rate 75 to 125 cfm per sq ft of plane area. * Refers to simple hood, opening that'performs in accordance with Equation 1. Refera to hood having a complete flanking plane parallel to axis which prevents flow of air from half of normal air supply zone, and performs in accordance with the equation, Q = Vx (5X* -f A). Example; reoH^foUength opening resting on bench top. Includes single slots, so flanked, for X distance greater th*n . -^-distance is greater than M slot length, consider it as a single opening flanked by a plane (see upoer ngnt column). With exhaust rate divided between 2 slots, an additional safety factor is provided. From Plant and Process Ventilation, by W. C. L. Hemeon (Industrial Press, New York). Exhaust of Hot Processes In designing local exhaust hoods for hot processes, it is necessary to esti mate the rate of delivery of hot air to the hood by the convection column. 1 he exhaust capacity should exceed this by an amount sufficient to create a velocity in the excess air flowing into the envelope surrounding the con- co^umn that wiU prevent escape of the heated air at the edges of the hood. Unless distances from the heat source to the receiving howl are ?ail (possibly under 3 ft), the quantity of heated air entering the hood is increased substantially by the induction and turbulent mixing of large quantities of room air as the distance above the heat source increases. Uness the exhaust hood can handle this total volume, spillage of contaminated mixture must occur. The exhaust rate for satisfactory control of hot processes can be kept at minimum by enclosing the hot processes as completely as possible and xoausting from the top of the enclosure. Low canopy hoods rank next to enclosures in economy of air flow. High thD ^oods must handle greatly increased amounts of air over that in st,re?,m itself, in order to receive and dispose of the air entrained y tne convection column from far below. Where lateral exhaust ventila-