Document 6b3k1O0ObLo1qJd583QZZMBL1

700 CHAPTER 51 1959 Guide gas or vapor will occupy approximately 358 cu ft at standard pressure and temperature. Thus, cfm (vapor) = -- X 359 X -- (10) where W = rate of generation of contaminant, pounds of liquid solvent per minute. 2f,, = molecular weight. t " air temperature, Fahrenheit. A special ctw* occurs where local concentrations of solvent vapors at the breathing tone, resulting from concentrated sources of contamination, are intolerably higher than the aver age damgn concentration when using dilution methods. Data are available for calculations, but involve many assumptions re garding boundary conditions, such as convection area and ran dom air movement in the vicinity. 2. Dveta and Fumes. Maximum allowable concentration of various dusts, fumes, and mists are also tabulated in Chapter 7. However, the dilution method as a means of treating particulate contaminating agents should be used with care, since the allow able air movement in spaces will ordinarily be lower than the capture velocity required for such particles. Exhausting at the source (see Chapter 52) will generally be the recommended treatment for these particulate contaminants. REFERENCES 1 Encyclopedia of Instrumentation for Industrial Hygiene (Publications Distribution Service, University of Michigan). * Industrial Ventilation (American Conference of Govern mental Industrial Hygienists). ' H. S. Belding and T. F. Hatch: Index for evaluating heat stress in terms of resulting physiological strains (ASHAE Trans actions, Vol. 62, 1956, p. 213). 4 G. F. Haines, Jr. and T. F. Hatch: Industrial heat exposureevaluation (Heating and Ventilating, November 1952, p. 93). *B. R. Small: Heat relief in industry (Iron and Steel Engi neers Magazine, April 1952). 'Evaporative cooling--a symposium (ASHAE Jouhnal Sec tion, Heating, Piping and Air Conditioning, August 1955, p. 141). 11 CHAPTER 52 INDUSTRIAL EXHAUST SYSTEMS Elements of Exhaust Systems/ Hoods or Enclosures, Capture Velocities, Air Volume, Exterior Hoods, Special Exhaust Require ments, Exhaust of Hot Processes, Induced Air Flow,- Duct System Design/ Calculations,- Construction Specifications, Materials, Details/ Air Flow Producing Equipment; Air Cleaning Equipment/ Make-up Air, Maintenance of Performance/ Materials for Corrosion Resistance N. industrial plants, some type of exhaust system de I signed to collect and remove dusts, fumes, mists, vapors, and gases is installed to protect health and safety of industrial personnel, promote worker efficiency, salvage usable material, or improve plant housekeeping. This chapter will not include consideration of systems similar in design, but used to convey heavy loadings (100 or more grains per cu ft) of materials. Definitions of various air contaminants, their particle sizes, maximum allowable con centrations, and upper and lower explosion limits are included in Chapter 7, Air Contaminants. Exhaust systems are 'extensively used for control of contaminants from: 1. Mechanical cutting and abrading operations including abra sive blasting and rock cutting. 2. Fuel-burning and exhaust gas producing operations. 3. Molten materials handling operations. 4. Welding, burning, and soldering operations. 5. Fiber handling operations. 6. Volatile and gaseous material handling operations. 7. Chemical processes. Local exhaust systems should be considered whenever possible rather than. general ventilation methods which allow contaminants to be dispersed within the workroom. The former provide more positive control, as well as a great reduction in exhaust volume handled, thereby re ducing the cost of air cleaning equipment required. booths, sidedraft or downdraft hoods (with or without side shields) have been developed from this complete enclosure concept. Openings in hoods are kept to a min imum size and are placed away from the natural path of the contaminant travel when possible. Doors should be provided for inspection and maintenance when needed. Capture Velocities and Air Volume Exhausted Only after the hood design has been determined can the exhaust volume requirements be calculated. With en closures, volumes are calculated from the known open area of the hood and the selected capture or indraft velocity suffi cient to prevent outward escape. Usual capture velocities for typical operations are listed in Table I 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 operation will vary with the magnitude of the air volume handled, with uncontrolled 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 the opposite end will provide control with a lower capture velocity than that required from the same hood in a large room where no ELEMENTS OF EXHAUST SYSTEMS An exhaust system consists of (1) hoods or enclosures at sources of air contamination, (2) branch and main ducts through which an air stream transports the con taminant to air cleaning devices or to the atmosphere, (3) air moving equipment to produce the required air flow into hoods or enclosures, and (4) air cleaning equip ment when required. See Chapter 24, Part II, for discus sion of types, applications, and principles of operation. HOODS OR ENCLOSURES 1 The most effective hood or enclosure is one that will require the minimum exhaust volume for effective con taminant control. The design must therefore be based upon a knowledge of the process or operation for which control must be obtained. The more complete the en closure, the more economical and effective will be the installation. Many designers give first consideration to a hood com pletely enclosing the operation and then provide necessary access and working openings. The familiar hoods, such as Table 1 .... Minimum Air Velocities Required at Point of Origin to Capture Contaminant Effectively Minimum Condition of Generation Capture of Contaminant Veiodty, Fpm Process Released without 50-100 noticeable move ment Released with low 100-200 velocity Evaporation of vapors, exhaust from pickling, washing, de greasing, plating, welding, etc. Paint spraying in booth; inspec tion, sorting, weighing, pack- - aging, low speed (less than 200 fpm) conveyor transfer points, trending, mixing, barrel fill ing. Active generation 200-500 Foundry shakeout, high speed (over 200 fpm) ' conveyor transfer points, crushers, screens. Released with great 500-2000 Grinding, tumbling mills, abra force sive cleaning. 701