Document 03VpDJ38jgRMMNa5gXbR08wM

706 Chapter 40___________ 1945 Guide AIR CLEANING EQUIPMENT Primary dust separators depend for their air cleaning action on centri fuging out the dust particles or on imparting to the transporting air' stream a quick change in direction, accompanied by a sudden reduction in velocity. They may be applied to exhaust systems from polishing and buffing wheels and, in some cases, as a preliminary separator ahead of a final dust arrester. - Dust particles which cannot be caught in inertia type separators are usually removed by one of the following methods: 1. Filtration. Of the filter arresters the cloth type, either of the screen, envelope, or bag construction, is most generally used for industrial ex haust systems. Cloth arresters properly engineered for this job and handling dust for which they are suited operate at a good efficiency. 2. Wet Collectors. Wet collectors have been in use for many years, but only recently have been introduced for collection of industrial dust, from exhaust systems. They are particularly applicable where moisture is present in the air being exhausted. The resistance is relatively, low in wet collectors suitable for dust removal and remains constant for a given exhaust system. 3. High-tension Electrical Precipitation. High-tension electrical preci pitation has been applied to removal of dust, fumes, and mists from air and other gases. (See section on Electric Precipitators in Chapter 28). 4. Dynamic Precipitators. This equipment effects removal of entrained dusts by means of a specially designed fan in which the rotating element imparts a centrifugal force to the dust particles. Originally these preci pitators were intended for the collection of dry dust, but recently the construction has-been modified to permit the introduction of water into the dirty air intake, thus increasing its application to more kinds of dust. Consideration must be given to the type of dust handled13. For additional information on dust and cinders, see Chapter 28, Air Cleaning Devices. RESISTANCE OF SYSTEM The maintained resistance of the exhaust system is composed of three factors: (1) loss through the hoods, (2) collector drop, and (3) friction drop in the duct system. The loss through the hoods is usually assumed to be equal to the suction maintained at the hoods. Where possible the resistance of the particular collector to be used should be obtained from the manufacturer. Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. The velocities should be found in each section of pipe starting with the branch most remote from the fan. The friction drop for these sections can be determined by reference to Table 8 in this chapter and Fig. 2, Chapter 31. Total friction loss in the piping system is the friction drop in the most remote branch plus the drop in the various sections of the main, plus the drop in the discharge pipe. EFFICIENCY OF EXHAUST SYSTEMS The efficiency of an exhaust system depends upon its effectiveness in "The National Silicosis Conference Report {Bulletin No. IS. U. S. Department of Labor, February 3, 1937). ; Industrial Exhaust Systems 707 Table 9. Accepted Standards for Toxic Concentration of Fumes, Dusts and Misrsa Fumes Pentachloronaphthalene. _______ Dusts ___ Silica (less than 10 per cent free silicon-dioxide)________ :____ Mists Milligrams per Cubic Meter 0.1 1.0 0.15 0.1 0.5 5.0 15.0 Million Particles per Cubic Foot 5 100 100 0.15c 6C 100 5 10 100 Milligrams per Cubic Meter 0.1 5.0 ^Adapted from Safe Concentrations of Certain Common Toxic Substances Used in Industry, by M. Bowditcb. C. K. Drinker, P. Drinker. H. A. Haggard, and H. Hamilton (Journal of Industrial Hygiene and Toxicology, VoL 22, No. 6, June, 1940). Industrial Code Bulletin No. 35 (New York State Labor Depart ment). Study of Asbestosfs in Asbestos Textile Industry (U.S. Public Health Bulletin No. 241, 1938). Chronic Manganese Poisoning in an OreCrushing Mill (U. S. Public Health Bulletin No. 247, 1940). ^Determined by light field or equivalent technique. " cMiliigrams per cubic meter. reducing the concentration of dusts, fumes, vapors and gases below the safe or. threshold limits14. Too much emphasis cannot be placed on the necessity of testing exhaust systems frequently by determining the concentration of atmospheric con tamination at the worker's breathing level18. Commonly accepted values of threshold limits for usual atmospheric, contaminants, such as fumes, dusts and mists are given in Table 9. - Similar data covering gases and vapor will be found in Chapter 27. AIR FLOW PRODUCING EQUIPMENT In any type of exhaust system some form of air flow producing equip ment should be required to create the pressure necessary to cause the air ' to flow through the system to the discharge stack. Natural draft should not be depended on to remove harmful 'or dangerous gases, fumes, mists, dusts, or other matter when it is imperative that such matter be removed from the work place or room atmosphere. The principal types of air moving equipment are chimney exhausts, "Criteria for Industrial Exhaust Systems, by J. J. Bloomfield (AJ5.H.V.E. Transactions, Vol. 40, 1934, p. 353). "Keeping Dust Under Control, by John M. Kane (presented to the 31st National Safety Congress, October 28, 1942, and reprinted in part in National Safety News, January, 1943).