Document JJ32Z8NnQL7V9X2Qg627odB26

854 CHAPTER 47 ' 1946 Guide. surface so rapidly that the surface remains thoroughly wet, and evaporaJ . tion proceeds at a constant rate, precisely as from a free water surface. The material tends to assume a temperature corresponding to the wetbulb temperature of. the surrounding air. But the actual temperature is often slightly higher, due to radiation and conduction from dry surfaces . adjoining the material. The constant rate period continues until a time is reached when the moisture no longer comes to the surface as fast as it is evaporated. This point is called the critical moisture content in the drying process. As the drying proceeds, a period .of uniform falling rate is entered. During this period, the surface of the material is gradually.drying out, and the rate of drying falls as the remaining wet surface decreases in area. This period is also known as unsaturated surface drying. Asldrying continues, the surface is completely dry and. the water from . the interior evaporates and comes through the surface as vapor. As the plane of water recedes, the diffusion of the vapor becomes more difficult and hence the period is known as varying falling rate period, of sub-surface drying. ' Drying ceases when the equilibrium moisture content has been reached. , The final moisture content of the-product depends on the relative humidity of the air in contact with it. Equilibrium is established when the vapor pressure of the moisture in the air and the vapor pressure of the moisture in the material are equal. The equilibrium moisture content varies with the hygroscopic properties of the material, (see-table of Regain of Hygro scopic Materials, Chapter 45). The drying of a slab of whiting is shown in Fig. 5 and illustrates the principles referred to previously. The factors affecting the variations of drying rates during the periods mentioned are outlined in Table 2. . Omissions in the Cycle ' Many solids, such as lumber, are so dry at the beginning of the drying operation that the constant rate period of free surface, evaporation does not occur. Frequently the surface of the material is dry enough so that no surface drying can take place, in which case only the final stage of sub surface drying is involved. In other instances, the critical moisture con- Drying Systems 855 tent of a wet solid is sufficiently low.that sub-surface drying starts almost immediately, after the conclusion of-.the constant rate period. Thus the intermediate state of unsaturated surface drying does not occur and the drying is of the sub-surface type during practically the whole of the falling rate period. With other kinds of material, particularly thin sheets, such as newsprint paper, sub-surface drying may occur at such a low moisture content that it . is not encountered in commercial work, the falling rate period being confined in practice to unsaturated surface drying, Table 2. Factors Influencing Drying Factor. , Temperature Humidity Air Velocity Air Direction Thickness of Material Drying Period - Constant Rate, Uosatorated Surface Sub-Surface Increase in temperature increases drying rate Increase in temperature in creases drying rate, . because with decreased viscosity, capil lary flow is increased. Drying rate increases as humidity is decreased No effect until equilibrium con tent is reached; drying then ceases Drying rate varies approximately as the 0.6 power of the velocity No effect Drying rate increases,; the, more nearly the air blows perpendicular, to surface; for dead air film becomes thinner No effect ... Drying rate is not affected , by. the Drying rate varies inversely as thickness v , :t the square of the thickness GENERAL RULES FOR DRYING Temperature %" - The highest temperature possible should be used because of faster drying and smaller requirements for ventilation. The amount of'moisture that can be carried by- a pound of air increases rapidly with rise in tem- . perature-as shown in the humidity chart of Fig.' 6. Too high a temperature 'may cause spoilage of materials; many materials calcine or change their chemical properties if heated too.hot; gypsum and glauber salts lose some of the chemically combined water, fall'apart, and change their . chemical properties. Too high or rapid rise in temperatures in drying lumber or ceramics may create a liquid vapor tension within the material so high that the cells explode; causing permanent-injury to the fiber. If too high a temperature, is used on some chemicals, they, begin to react exothermally; a temperature, rise and chemical action from within-will 'burn the materials, e.g., bakelite products, gunpowder, etc. /During-the constant rate period of drying, the material heats only to the wet-bulb temperature of the surrounding air, consequently; high temperatures will ; not injure tdie material in this stage. ., r. Humidity .. Moisture in the drying air may be very importaiitr- Many-' materials tend to case-harden, dry on the outside, forming/a skin which retards the moisture flow from/the inside to:the surface, or stops it completely, and so