Document gDdzJOD33B2LMQK4GyenVmEeL

1022 CHAPTER 46 1954 Guide "A Study of Quartz-Fusing Operations with Reference to Measurement and Control of Silica Fum by E. C. Riley and J. M. DallaValle (U. S. Public Health Reports, Vol. 54, 1939, p; 632). ** Ventilation of Open Tanks, by A. C- Stern (American Industrial Hygiene Association, Industrial Hy giene Quarterly, Sept. I960). n American Standard for Safety in Electric and Gas Welding and-Cutting Operations (American Stand ards Association, Z 49.1). u Rules Relating to the Removal of Dust, Gases, and Fumes (New York State Department 'of Labor, In dustrial Code Rule No. IS, Jan. 1931). n Energy Losses at Suction Hoods, by D. Brandt and R. J. Steffy (A.S.H.V.E. Transactions, VoL _^62, 1946, p. 2Q5). ** Resistance Test on Pipe, by A. Nutting (Mechanical Engineering, May 1938). " Design of Injector for Low Pressure Air Flow, by G. E. McElroy (U. S. Bureau of Mines, Technical Pa per 678). American Standard Safety Code for Ventilation and Operation of Open Surface Tanks (American Stand ards Association, Z 9.1-1951). CHAPTER 47 INDUSTRIAL DRYING SYSTEMS Drying Terminology, Mechanism of Drying, Internal and External Conditions, Periods of Drying, Approximate Equations for Estimating Drying Time, Equi librium Moisture Content, Applications of Hygrometry to Drying, Dryer Calculations, Drying Methods and Equipment; Radiant, Conduction and Convection Drying; Solution of Drying 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 by thermal means. Dehumidificaiion 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. DRYING TERMINOLOGY1 The generally accepted definitions of terms used in drying technology follow: 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 sur faces. Bound moisture can be removed from a solid only under specific conditions of humidity in the external surroundings. Capillary flow refers to the flow of liquid through the interstices and over the sur face of a solid. It is caused by liquid-solid molecular attraction. Commercial dry basis expresses the moisture content of a product as pounds of water per pound of solid as it leaves the dryer, '.e., per pound of commercially dry aohd. The constant-rale period is that drying period during which the rate of water re moval per unit of drying surface is constant. The critical moisture content is that obtaining when the constant-rate period ends and the falling-rate period begins. Cry basis indicates the moisture content of a wet solid as pounds of water per pound of bone-dry solid. The advantage of -using this baBis is that the absolute amount of moisture loss is obtained simply by subtracting the moisture contents oefore and after drying. (See definition of Wet Basis.) Dryer efficiency is that fraction of the total heat, supplied by fuel, used to evaporate . air Overall efficiency is sometimes used to distinguish overall system efficiency rotn the efficiency of the drying space or evaporative efficiency. Equilibrium moisture content is that to which a given material can be dried under Pecific conditions of air temperature and humidity. .Evaporative efficiency compares the amount of evaporation actually obtained in a uryr with that which would obtain by saturation of the air. falling-rale period is that drying period during which the instantaneous drying te continually decreases. 1023