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848 CHAPTER 46 r 1946 Guide Table 9. Corrosion Resisting Materials for Exhaust Systems1 MATERIAL .Mbtaia ACIEfc Acetic -Chromic Htoro , . Htdro CHLORIC* FLUORIC . Nitric Phos phoric Sul phurous Sul phuric Dil|Conc. Dil.|Conc. Dil.| Cone. DiL Cone. Dil. Cone. Dil.|Cone: DiLjConc. Dil.jConc. Aluminum..___ Good Fair Poor No Data Poor Good Poor Poor Poor Magnesium and Alloys-- No Data Good | Poor No Data Poorj Good No Data.. No Data ; No Data No Date Lead and Lead-Coated__ Poor Good Poor Poor Poor * Poor Good Good| Poor Moly Alloy (60 Ni--2QMo -20 Fe). - Good No Data Fair No Data Poor Monel Metal__ Fair Poor Fairj Poor Goodd Fairj Poor Poor Fair No Data - Good Poor GoodejPoor Bronze___ - Poor Good Silioon Iron___ Fair |Good No Data Fair Poor - Good Good No Date Good Stainless Steele (18 Cr8 Ni) . . Good - Good Enameled Steel_________ No Data No Data MtSCELLANEOUB Asbestos Comp Poor' Good No'Data Poor Good Good Poor Poor Good except against strong acids and alkalies Good PoonGoode No Date Good Wood Rubber. Some woods are decomposed or softened faster than others. | | | | IW' | | Poor In general plastics resist weak adds and are decomposed by concentrated acid. ^Standard Practice Sheet No. 115 ^Division of Industrial Hygiene, New York State Labor Department). bActd mists in air are more corrosive than as liquid in storage tank. Galvanised iron not resistant to acid. cstainless steel of (24 Cr--10 Ni) fairly resistant at low temperature for HCl and Ht POt. dUnder most conditions. At room temperatures. Friction drop in the pipes must be computed for each section wherethere 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 41. 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 reducing the concentration of dusts, fumes, vapors and gases below the safe or threshold limits u. 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 16. Commonly accepted values of threshold limits for usual atmospheric contaminants will be found in Tables 3, 4 and 5, Chapter 10. 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 Exhausting and Conveying Systems 849 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, venturi ejectors, centrifugal exhaust fans, disc or propeller fans, and axial flow fans16. Manufacturers generally provide special fans for the collection of various industrial wastes. These are available for the collection of coal dust, wood shavings, wool, cotton and many other substances. When substances haying ah abrasive character are conveyed, the fan blades and housing should be protected from wear. This may be accomplished by placing a collector on the negative side of the fan or by lining the housing and blades with rubber. PROTECTION AGAINST CORROSION The removal of gases and fumes in many chemical plants requires that metals used in the construction of the exhaust system be resistant to chemical corrosion. A list of the materials which- may be used to resist the action of certain fumes is given in Table 9. Hoods and ducts, when short, may frequently be constructed of wood and be quite effective! Rubberized paints are available and may be applied as protective coatings in handling such gases and fumes as chlorine and hydrochloric acid. REFERENCES Tentative Recommended Good Practice Code and Handbook on the Fundamentals of Design, Con struction, Operation and Maintenance of Exhaust Systems. Page 21, Industrial Hygiene Codes Committee, American Foundrymen's Association. 2--How to Design Exhaust Hoods, by J. `M. DallaValle {Heating and Ventilating, Series of 12 articles March, 1943 to February. 1944). 3~Industrial Exhaust Ventilation in Industrial Hygiene, by Allen D. Brandt (A.S.H.V.E. Trans actions, Vol. 50, 1944, p. 331). ' *--For more detailed requirements refer to Fundamentals Relating to the Design and Operation of Exhaust Systems, Z9-1936 (American Standards Association). Industrial Code-Bulletin Nos. 10 and 12 (New York State Labor Department). Principals of Exhaust Hood Design, by J. M. DallaValle (U. S. Public Health Service, 1939). 5--Control of the Silicosis Hazard in the Hard Rock Industries. I. A Laboratory Study of the Design of Dust Control Systems for Use with Pneumatic Granite Cutting Tools, by Theodore Hatch, Philip Drinker and Sarah P. Choate. (Journal of Industrial Hygiene, Vol. XII, No. 3, March, 1930). --The Control of -Industrial Dust, by J. M. DallaValle (Mechanical Engineering, Vol. 55,- No. 10, October, 1933). - 7--Studies in the Design of Local Exhaust Hoods, by J. M. DallaValle and Theodore Hatch'(A.5.Af.E. Transactions. Vol. 54, 1932). *--Velocity Characteristics of Hoods under Suction, by J. M. DallaValle (A.S.H-.V.E. Transactions, Vol. 38, 1932, p. 387). Low Velocity Exhaust Systems, by Theodore Hatch (Heating and Ventilating, October, 1940, p. 27). ,0--Tank Ventilating Power Costs Cut by Low Velocity Systems, by William B. Harris (Heating and Ventilating, July, 1942, p. 42). 11 Health Hazards in Chromium Plating, by J. J. Bloomfield and William Blum (U. S. Public Health Report, Vol. 43, No. 26, September 7, 1928). 1 *--New Data for Practical Design of Ventilation for Electroplating, by W. P. Battista, Theodore Hatch and Leonard Greenburg (Heating, Piping and Air Conditioning, February. 1941, p. 81). Ventilation of' Plating Tanks, by Allen D. Brandt (Heating, Piping and Air Conditioning, July, 1941, p. 434). 1^3^3~The National Silicosis Conference Report {Bulletin No. IS, U. S. Department of Labor, February 3, ,4--Criteria for Industrial Exhaust Systems, by J. J. Bloomfield (A.S.H.V.E. Transactions, Vol. 40, 1934, p. 353). 1 ^Keeping.Dust.Under Control, by John M. Kane (presented to the 31st National Safety Congress. October 23, 1942, and reprinted in part in National Safety News, January. 1943). The Determination and Control of Industrial Dust, by J. j. Bloomfield and J. M. DallaValle {Public Health Bulletin 17, 1935). Engineering Control of Air Contamination of the Working Environment, by A, D. Brandt (In Manual of Industrial Hygiene,.U. S. Public Health Service, 1943. p. 198-266). / *--The Axial Flow Fan and Its Place in Ventilation, by W, R. Heath and A. E. Criqui (A.S.H.V.E. Transactions, Vol. 50, 1944, p. 197),