Document Rj2Eax50VYL48ynOE2GpjGXGB

HEATINC VENTILATING AIR CONDITIONING. GUIDE 1944 For additional information on dust and cinders, , see Chapter 29, 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 ascertained from the manufacturer. Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. Find the velocities 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 6 and Fig. 2, Chapter 32. 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. Table 7. Accepted Standards for Toxic Concentration of Fumes, Dusts and Mists3 Cadmium oxide Fumbs Trichloronaphthalene................ Milligrams per Cubic Meter 0.1 1.0 0.15 0.1 0.5 5.0 15.0 , Dusts --Million Particles per Cubic FooTb Asbestos.. .. Cement Lead or compounds of lead................. ........... Manganese . Marble... Silica (more than 70 per cent free silicon-dioxide)_______________ Silica (more than 10 per cent free silicon-dioxide)_______________ Silica (less than 10 per cent free silicon-dioxide) 5 100 100 0.15c 6C 100 5 10 100 . Mists Milligrams per Cubic Meter Chromic acid........................ Sulphuric acid . 0.1 5.0 ^Adapted from Safe Concentrations of Certain Common Toxic Substances Used in Industry,^by?M. Bowditch. 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 Department). Study of Asbestosis in Asbestos Textile Industry (U. S. Public Health Bulletin No. 241, 1938). Chronic Manganese Poisoning in an Ore Crushing Mill (/. 5. Public Health Bulletin No. 247, 1940). ^Determined by light held or equivalent technique. cMOligrams per cubic meter. 730 CHAPTER 40. INDUSTRIAL EXHAUST SYSTEMS Table 8. Corrosion Resisting Materials for Exhaust Systems3 add. cstainless steel of (24 Cr--10 Ni) fairly resistant at low temperature for HCl and Hi PO*. dUnder most conditions.' eAt room temperatures. 4 EFFICIENCY OF EXHAUST SYSTEMS The efficiency of an exhaust system depends upon its effectiveness in reducing the concentration of dusts, fumes, vapors and gases below the safe or threshold limits10. 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 level. Commonly accepted values of threshold limits for usual atmospheric contaminants, such as fumes, dusts and mists are given in Table 7. Similar data covering gases and vapor will be found in Chapter 28. TYPES OF FANS Manufacturers generally provide special fans for the collection of various industrial wastes. These are available for the collection of coal `Criteria for Industrial Exhaust Systems, by J. J. Bloomfield (A.S.H.V.E. Transactions, VoL 40, 1934, p. 353). / .731