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840 CHAPTER 46 ,1948.,Guide "to flow through the system to the discharge stack. Natural draft should hot 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 having an abrasive character are conveyed, the fan blades arid 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 thatmetals 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 ducti, 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 I Tentative Recommended Good Practice Code and Handbook on the Fnnrlampntala of Design, Con struction. Operation and Maintenance of Exhaust Systems, Page 21, Industrial Hygiene Codes Committee, American Foundrymtris Association. *--How to Design Exhaust Hoods, by J. M. DallaValle (Heating and Ventilating, -Series of 12 articles March, 1943 to February, 1944). ....... s--Industrial Exhaust Ventilation in Industrial Hygiene, by Allen D. Brandt (A.S.H.V.E. Trans actions, Vol. 60, 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). Principles of Exhaust Hood Design, by J. M. DallaValle (U. 5. Public Health Service, 1939). ""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. Xll, 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 s M.ft. 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). l0--Tank Ventilating Power Costs Cut by Low Velocity Systems, by William B. Harris (Heating and Ventilating, July, 1942, p. 42). II Health Hazards in Chromium Plating, by j: J. Bloomfield and William Blum (7. 5. 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 'The National Silicosis Conference Report (Bulletin No. IS, U. S. Department of Labor, February 3, 1937). ,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 28, 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). - le--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). Chapter 47 DRYING SYSTEMS Mechanism of Drying, Omissions in Drying Cycle, Factors Influencing Drying Rates, Drying Methods and Equipment, Radiant Drying, Conduction Drying, Convection Drying, Calculation and Design, Estimating Methods HE term drying in a broad sense encompasses the removal of water, and occasionally other liquids, from gases, liquids, or solids. How ever, the common usage of the word confines the meaning principally to the removal of water or solvent from solids. Dehumidification is the term that is commonly assigned to the drying of gases. This is usually accomplished by condensation or adsorption by various drying agents and is treated in Chapter 38. Distillation and more particularly fractional distillation is associated with the drying of liquids. It is usually more economical to employ, whenever possible, mechanical means of separating as much water as is practicable from the solid materials before undertaking drying or dehydration steps. These me chanical methods such as filtration, screening, pressing, centrifuging, or settling usually require, much less power and frequently less capital out lay, thereby making the operation cheaper in terms of cost per pound of water removed. MECHANISM OF DRYING The course of drying of any substance under constant conditions of temperature, humidity, and air velocity and distribution, follows a definite pattern which can be represented by a graph such as Fig. 1, which is that for a slab of whiting, wherein the rate of drying is plotted against the moisture content. The drying cycle may then be divided into three components: (1) Constant rate period. (2) Uniform falling rate period. (3) Varying falling rate period. One or more of these components may be missing for a substance under a particular set of- conditions. During the constant rate period, water is being evaporated from the surface of the material at a rate comparable to that of a free water surface and moisture is being supplied by diffusion or capillarity to the surface at a rate equal to or greater than the evaporative rate. However, a point is reached at which the moisture reaches the surface at a rate less than the potential evaporative rate. The moisture content of the substance at which this occurs is known as the critical moisture. Up to this point the material is generally considered to assume the wet bulb temperature of the air. This is not strictly true as in many cases radiant heat is absorbed or some heat is supplied by conduction. During the uniform falling rate period, moisture is still evaporating from the surface, but the area of wet surface is constantly diminishing. During the varying falling rate period the water is vaporized beneath the surface and escapes as the vapor. As the thickness of the dried material increases, it becomes more difficult for the vapors to escape. Finally all drying stops at the equilibrium moisture content, where the vapor pressure of water in the air is equal to the vapor pressure of water in the material. The equilibrium moisture content varies with the humidity 841