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936 CHAPTER 46 1950 Guide Periods of Drying1 A typical drying time curve for a wet solid is shown in Fig. 2. This curve is a plot of the moisture content at any time in a solid undergoing drying. It is the ;usual method of presenting experimental drying data. Although Fig. 2 shows that the moisture content is subject to a continuous variation with time, a more precise illustration of the nature of this varia tion can be obtained by differentiating the curve and plotting the drying rate (pounds of water per. hour per pound of dry material) against the moisture content (pounds of water per pound-of dry material) as shown in Fig. 3, or plotting the rate of (hymg against time as shown in Fig. 4. These rate-curves show that the drying process is not a smooth) continuous one in which a single mechanism controls throughout. The rate curve in Fig. 4 has the advantage of showing how'long each drying period predom inates. ' - ' ': ) ' Section AB on each curve represents a constant-rate period. In Fig: 2, it is'shown by a straight line of constant slope dJF/d0, which becomes a horizontal line on the rate curves in Figs. 3 and 4. Drying-Systems 937; / Theicuryed.portion.of Fig; 2-is termed the falling-rate period, and, as shown in Figs. 3 and 4, it is typified by a continuously changing rate. Point A, where the constant rate ends and the drying rate begins to'de crease, is termed the critical moisture content. .. The portion of the curves designated by CB represents a waiming-up period, and it may, or may not, be a significant item depending on the total time involved. 7\ Constant-Rate Period. Drying during, the constant-rate period-is-equiv- alent to evaporation from a free-water surface on the surface Of the' solid. The rate of (hying in this period is determined by the rate Of diffusion of water vapor through an air film at the wet surface of the solid. A,con stant, rate of.evaporation on the surface of the solid maintains, thelsurface at a constant temperature, which, in the absence of Other heat effects,, is very nearly the wet-bulb temperature of. the air. If heat .flows to the surface of evaporation by radiation and conduction, or both, in addition to Convection,' the surface; temperature' will be constant at some value between the air temperature and the wet-bulb temperature. This higher temperature in turn produces a higher constant rate of evaporation. In those dryers in which heat' is transferred to a wet solid by conduction through hot surfaces, and heat transfer by convection is not a factor,, the wet surfaces approach the boiling point temperature rather than a wet- bulb temperature. ,. . j .-.. . When all the heat for evaporation in the constant-rate period is supplied by a hot'gas; a dynamic equilibrium is established between the rate of heat transfer to the material and the rate of vapor removal from the surface.1 This equilibrium betwCen heat and mass transfer rates can be expressed as follows: /- ` dv> h,Aat da " x k,A&p (1) where -. dw ... de drying rate, pounds,of,waterper hour.