Document oKaKQ49Kpgv9bmDGZVzvoNo

1116 CHAPTER 44 1958'Guide :A apecial caae occars where local concentrations of-solvent vapors at the breathing zone*.resulting from concentrated sources of contamination, are intolerably higher than. the average, design .concentration when-using, dilution methods. Data are available for calculations, blit involve many assumptions regarding boundary condi tions, such as convection area and random air movement in the vicinity. . 2. Dusts and Fumes. Maximum allowable concentration-of various dusts', fumes and mists are also tabulated in Chapter 8, However, the dilution method as a means of treating particulate contaminating agents should be used with care, since the allowable air movement in spaces will ordinarily be lower than the capture velocity required for such particles. Exhausting at the source (see Chapter 46) will generally be the recommended treatment for these particulate contaminants. REFERENCES 1 Encyclopedia of Instrumentation for Industrial, Hygiene (Publications Distribu tion Service, University of Michigan, Ann Arbor, Michigan). , * Industrial Ventilation published by American Conference of Governmental In dustrial Hygienists. 1 Index for Evaluating Heat Stress in Terms of Resulting Physiological Strains by H. S. Belding and T. F. Hatch (ASHAE Transactions, Vol. 62, 1956, p. 213). `Industrial Heat Exposure-Evaluation by G. F. Haines, Jr. andJT. F. Hatch (Heating and Ventilating, November 1952, p. 93). ` Heat Relief in Industry by B. R. Small (Iron and Steel Engineers Magazine, April 1952). - Evaporative Cooling--A Symposium (ASHAE Journal Section Healing, Pip ing and Air Conditioning, August, 1955, p. 141). ..X - CHAPTER 45 1 INDUSTRIAL EXHAUST SYSTEMS Elements of Exhaust Systems, Hoods or Enclosures,.Capture Velocities; Air Volume, Exterior Hoods, Special Exhaust Requirements, 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 ' IN industrial plants, some type of exhaust system designed 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 con taminants, their particle sizes, maximum allowable concentrations, and upper and lower explosion limits are included in Chapter 8, Air .Con taminants. Exhaust systems are extensively used for control of contaminants from: 1. Mechanical cutting and abrading operations including abrasive 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 obtain more positive control, as well as a great reduction in exhaust volume handled, thereby reducing the cost of air cleaning equipment required. ELEMENTS OF EXHAUST SYSTEMS An exhaust system consists of:(l) hoods or enclosures at all sources, of air contamination, (2) branch and main ducts through which an air stream transports the contaminant 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 equipment when required. . See, .Chapter 33, Part II, for discussion of types, applications, and principles of operation, HOODS OR ENCLOSURES11010 The most effective hood or enclosure is one that will require the minimum exhaust volume for effective contaminant control. The design must thereore be based upon a knowledge of the process or operation for which conrf ^ust be obtained. The more complete the enclosure, the more ec- nomical and effective will be the installation. Many designers give first consideration to a hood completely enclosing 1117