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908 . CHAPTER 46 . 1949 Guide ical corrosion. A list of the materials which may be used to resist the action of certain fumes is given in Table 7. 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 as chlorine and hydrochloric acid. CHAPTER 47 DRYING SYSTEMS REFERENCES Mechanism of Drying, Omissions in Drying Cycle, Factors Influencing Drying Rates, 1 Tentative Recommended Good Practice Code and Handbook on the Funda mentals of Design, Construction, Operation and Maintenance of Exhaust Systems, Page 21, Industrial Hygiene Codes Committee, American Foundrymen's Association. I How to Design Exhaust Hoods, by J..M. DallaValle (Healing and Ventilating, Series of 12 articles March, 1943 to February, 1944). * Industrial Exhaust Ventilation in Industrial Hygiene, by Allen D. Brandt (A.S.H.V.E. Transactions, Vol. 50, 1944, p. 331). * Fan Engineering, 1948, Buffalo Forge Co. * Nature of Air Flow at Suction Openings, by A. D. Brandt, R. J. Steffy and R. G. Huebscher (A.S.H.V.E. Transactions, Vol. 53, 1947). * Energy Losses at Suction Hoods, by A. D. Brandt and R. J. Steffy (A.S.H.V.E. Transactions, Vol. 52, 1946, p. 205). 7 Industrial Health Engineering, by Allen D. Brandt (John Wiley & Sons, Inc., New York). * 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. S. Public Health Servr ice, 1939). * Standards of the N.B.F.U. for Installation of Blower and Exhaust Systems for Dust Stock and Vapor Removal or Conveying, National Board of Fire Underwriters, Drying Methods and Equipment, Radiant Drying, Conduction Drying, Convection Drying, Dryer Calculations, Typical Solution of Dryer Problem THE 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 mate rials before undertaking drying or dehydration steps. These, mechanical methods such as filtration, screening, pressing, centrifuging, or settling usually require much less power and frequently less capital outlay, thereby making the operation cheaper in terms of cost per pound of water removed. 10 National Electrical Code 1947, N.B.F.U. Pamphlet No. 70, National Board of Fire Underwriters. II The Control of Industrial Dust, by J. M. DallaValle (Mechanical Engineering, Vol. 55, No. 10, October, 1933). 17 Studies n the Design of Local Exhaust Hoods, by J. M. DallaValle and Theodore Hatch (A.S.M.E. Transactions, Vol. 54,1932). 11 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 arid Ventilating, October, 1940, p. 27). 1S Tank Ventilating Power Costs Cut by Low Velocity Systems, by William B. Harris (Heeding and Ventilating, July, 1942, p. 42). Health Hazards in Chromium Plating, by J. J. Bloomfield and William Blum (17. . Public Health Report, Vol. 43, No. 26, September 7,1928). ' 17 New Data for Practical Design of Ventilation for Electroplating, by W.. P. Battista, Theodore Hatch and Leonard Greenburg''(//ea(mj, Piping and Air Condi tioning, February,4941, p. 81). Ventilation of Plating Tanks, by Allen D. Brandt (Healing, Piping and Air Conditioning, July, 1941, p. 434). 18 Criteria for Industrial Exhaust Systems, by J. J. Bloomfield (A.S.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 Safely News, January, 1943). The Determination and Control of Industrial Dust, by J. J. Bloom field and J. M. DallaValle (Public Health Bulletin H7, 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). MECHANISM OF DRYING The removal of moisture from solids is largely dependent on the difference in vapor pressure between the moisture in the pores of the material and the surrounding air, and the rate of diffusion of internal moisture to the surface of the material. As long as the vapor pressure of the moisture within the material is greater than the vapor pressure of the surrounding air, moisture will travel outward to the surface of the material and be evaporated. While the magnitude of the vapor pressure difference is the force or po tential that causes the drying, it is not representative of the actual results in terms of time'or removed moisture. . Water existing on the surface or held in the voids of a solid is known as free or unbound moisture and its vapor pressure will be equal to that of water at the same temperature. Bound, hygroscopic or chemically com bined moisture on the other hand is intimately associated with the physical nature of the material and its vapor pressure during drying is less than that of free water at the same temperature. 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 graphs such as Fig. 1 and Fig. 2.1 i Fig. 1 may be considered a typical drying time curve for solid material. Fig. 2 is the drying rate curve derived from Fig. 1, the ordinate in this case being the drying rate, which can be expressed as pounds of water evaporated per hour per square foot or pounds of. water evaporated per Hour per pound of dry stock. . .In Fig. 2 the section CB is attributed to a warming up period during which the surface of the solid is reaching a uniform temperature. This . temperature is maintained constant during the period of a constant drying a 909