Document KJbxpDN5m0Vd8j3rkQzxnrwnX

262 CHAPTER 21 * P. Drinker and T. Hatch: Industrial that* (McGraw-Hill Book Co., New York, 1936). 'J. L. Alden: Design of Industrial Exhaust Systems (In dustrial Press, New York, 1948). *W. N. Witheridge: Ventilation {Industrial Hygiene and Toxicology, Vol. 1, Chapter 10, ed. F. A. Patty, Lnteracience Publishers, New York, 1948). " A. C. Stem et al: Transport velocities for industrial dusts (American Industrial Hygiene Association Quarterly, December 1948). " A. D. Brandt: Industrial Health Engineering (John Wiley and Sons, New York, 1947). "L. Silverman: Velocity characteristics of narrow exhaust dots (Journal of Industrial Hygiene and Toxicology, November 1942, p. 267). " W. C. L. Hemeon: Plant and Process Ventilation (The In dustrial Press, New York, 1965). ** Code of Recommended Good Practice for Metal Cleaning Sanitation (American Foundrymen's Association). u Industrial Ventilation Manual (American ' Conference of Governmental Industrial Hygienists, 1960, 6th ed.). " W. C. Dressen et al: A Study of Asbestosis in the Asbestos Textile Industry (U. S. Public Health Sendee Bulletin No. 241, 1938). a R. T. Page and J. J. Bloomfield: A Study of Dust Control Methods in on Asbestos Fabricating Plant (if. S. Public Health Reports, Reprint No. 18S3, November 1937). M Rules Relating to the Control of Stftca Dust tn Stone Crushing Operations (New York State Department of Labor, Industrial Code Rule No. 34, July 1942). "J. M. Kane: Design of exhaust systems (Heating and Ventilating, November 1945, p. 68). * J. M. Kane: Foundry ventilation (The Foundry, February, March 1946). W. M. Oddie: Pottery dusts: their collection n<l removal (Pottery Gazette, Vol. 53,1928, p. 1280). BJ. M. Kane: Scrapbook of exhaust hood design (Heating and Ventilating, July, August, November 1950 and February, March, April, June 1951). "R. T. Pring et al: Design of exhaust ventilation for solid material handling (Industrial and Engineering Chemistry, No vember 1949). " Harttell Blotters: Engineering Data A Installation (Hartsell Propeller Fan Company Bulletin 1001). "J. M. Kane: The application of local exhaust ventilation to electric melting furnaces (American Foundrymen's Associa tion Transactions, Vol. 52, 1945, p. 1351). " T. Hatch et al: Control of silicosis hazard in the hard rock industries. II. An investigation of the Kelley dust trap for use with pneumatic rock drills of the jackhammer type (Journal of Industrial Hygiene, February 1932, p. 69). 1962 Guide And Data Book " P. S. Hay: Modified design of Hay dust trap (Journal of Industrial Hygiene, January 1930, p. 28). "American Standard for Grinding, Polishing and Buffing Equipment Sanitation (American Standards Association Z 431941). "J. M. Kane: Swing frame grinder dust control (The Foundry, August 1944). " J. M. DallaValle: Design of kitchen range hoods (Heating and Ventilating, August 1953, p. 95). " What We Make (B. P. Sturtevant Co. Catalog No. 600). "P. Drinker and J. R. Snell: Ventilation of motion picture booths (Journal of Industrial Hygiene and Toxicology, April 1938, p. 321). "P. J. Maraehall: How to justify an industrial air conditioning investment (Heating, Piping and Air Conditioning, February 1952, p. 71). * E. C. Riley et a]: How to design exhaust hoods for quartsfusing operations (Heating and Ventilating, April 1940, p. 23). " E. C. Riley and J. M. DallaValle: A study of qu&rts-fusing operations with reference to measurement and control of silica fumes (U. S. Public Health Reports, 1939, Vol. 54, p. 532). "A. C. Stem: Ventilation of open tasks (Industrial Hygiene Quarterly, September 1950). "I. Kingsley: Tabulation of atmospheric contaminants re leased by open surface tanks (New York State Department of Labor. Division of Industrial Hygiene, Engineering Unit Plates 161, 162, 163). **K. M. Morse and L. Goldberg: Chlorinated solvent ex posures at degreasing operations (Industrial Medicine, October 1943, p. 706). *W. N. Witheridge and H. T. Walworth: Ventilation of a trichlorethylene degreaaer (Journal of Industrial Hygiene and Toxicology, May 1940, p. 175). " American Standard for Safety in Electric and Gas Welding and Cutting Operations (American Standards Association, A 49.1). 4 Rules Relating to the Removal of Dust, Gases, and Fumes (New York State Department of Labor, Industrial Code Rule No. 12, January 1931). T. F. Hatch and D. Barron-Oomzco: Air flow in free con vection over heated bodies (ASHAE TBamsacitons. Vol. 63, 1957, p. 275). "A. D. Brandt and R. J. Steffy: Energy losses at suction hoods (ASHVE Transactions, Vol. 52,1946, p. 205). ** A. Nutting: Resistance test on pipe (Mechanical Engineer ing, May 1938). **G. E. McElroy: Design of Injector for Low Pressure Air Flow (U. S. Bureau of Mines Technical Paper 678). m American Standard Safety Code for Ventilation and Op eration of Open Surface Tanks (American Standards Associa tion, Z 9.1-1951). CHAPTER 22 INDUSTRIAL DRYING SYSTEMS Drying Terminology, Mechanism of Drying, Infernal and External Conditions, Period's of Drying, Equilibrium Moisture Content, Application of Hygrometry to Drying, Determination of Drying Time, Commercial Drying Time, Dryer Cafarfations, Drying Methods and Equipment; Radiant, Conduction, and Convection Drying} Agricultural Drying, Solution of Drying Problem THE term drying, in a broad sense, encompasses the re and from the dryer. Tbe operating conditions are independent moval of water, and occasionally other liquids, from of time. gya-g, liquids, or solids. However, the common usage of the Critical moisture content is that obtaining when the constant- word confines the meaning principally to the removal of rate period ends and the falling-rate period begins. water or solvent from solids by thermal means. Dehxtmidifica- Eouilibrium moisture content is that to which a given material can oe dried under specific conditions of air temperature and tion is tire term that is commonly assigned to the drying of humidity. gaggg This is usually accomplished by condensation or ad sorption by various drying agents (see Chapter 21 of the Evaporative load is the total amount of water evaporated per hour in the dryer. 1961 Guide And Data Book). Distillation, and particularly fractional distillation, is associated with the drying of liquids. It is usually more economical to employ, whenever posa ble, means of separating as much water as is practicable from the solid materials before undertaking dry Falling-rate period is that drying' period during which the instantaneous drying rate continually decreases. Fiber-saturation point a the moisture content of cellular ma terials (wood, etc.) at which the cell walls are completely satu rated while the cavities are liquid-free. It may be defined as tbe equilibrium moisture content as the humidity of the surround ing or dehydration steps. These mechanical methods such as ing atmosphere approaches saturation. filtration, screening, presang, centrifuging, or settling usually require much less power, and frequently less capital outlay, thereby making tire operation cheaper in terms of cost per pound of water removed. Final moisture content is the percentage by weight of moisture remaining in the solid at tbe end of tbe drying operation. Free moisture content is that liquid content which is remova ble at a given temperature and humidity. Free moisture may include both bound and unbound moisture. DRYING TERMINOLOGY1 Fuel economy is the number of units of fuel (cubic feet of gas, pounds of steam, gallons of oil, etc.) required to remove 1 lb of water from the solid being dried. The generally accepted definitions of terms used in drying Humidity denotes the amount of water vapor actually pres technology follow: ent in a gas, and is generally expressed as weight of vapor per Adiabatic drying occurs in a dryer when all of the sensible beat given up by the air in cooling is used to evaporate water from the wet stock. unit weight of any gas. Hygroscopic material is material that may contain bound moisture. Batch drying is that type of drying operation in which the material is fed to and discharged from tne drying chamber in batches at definite intervals of time. Bone-dry basis indicates the moisture content of a wet solid as pounds of water per pound of bone-dry solid. The advantage of using this basis is that the absolute amount of moisture loss u obtained simply by subtracting the moisture contents before and after drying. (See definition of Wet Basis.) Bound moisture refers to liquid (held by a solid) which exerts a vapor pressure less than that of the pure liquid at the same temperature. liquid may become bound by retention in small capillaries, by solution in cell or fiber walls, by homogeneous solution throughout the solid, and by chemical or physical Adsorption on solid surfaces. Bound moisture can be removed from a solid only under specific conditions of humidity in the Initial moisture content is the percentage by weight of mois ture in the solid at the start of the drying operation. Initial moisture distribution refers to the moisture distribu tion throughout a solid when drying begins. .-- Moisture gradient refers to the internal distribution of water in a solid at a given moment in the drying process, the nature of which depends on the characteristics of the solid involved. Non-hygroscopic material ia material that can contain no bound moisture. Test drying time is the drying time of a relatively small and representative sample of the material which is dried in a labora tory dryer under conditions simulating commercial operation. Through-circulation drying is that method of drying in which the air flow is directed through a permeable bed of solids. external surroundings. Unbound moisture in a hygroscopic material is that moisture Capillary flow refers to the flow of liquid through the inter stices and over the surface of a solid. It is caused by liquidsolid molecular attraction. in excess of tbe equilibrium moisture content corresponding to saturation humidity. All water in a non-hygroscopic material is unbound water. Commercial dry hosts expresses the moisture content of a product as pounds of water per pound of solid as it leaves the dryer, i.e., per pound of commercially dry solid. Commercial drying time is the residence time of the material being dried in a commercial dryer. Constant-rate period is that drying period during which the rate of water removal per unit of drying surface is constant. Continuous drying is that type of drying operation in which the material to be dried is fed and discharged continuously to Wet basis expresses the moisture in a material as a percentage of the weight of the wet solid. This basis is less satisfactory thin the dry-weight basis on which the percentage change of mois ture is constant for all moisture contents. MECHANISM OF DRYING1 When a solid dries, two fundamental processes are involved: (1) the transfer of heat to evaporate the liquid, and (2) the 263