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1026 CHAPTER 47 1954 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 drying 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 dW/d3, which becomes a horizontal line on the rate curves in Figs. 3 and 4. Moisture Content (Ory Sersis) dW Fio. 3. Rate of Drying vs. Moisture Content IP (W Industrial Drying Systems 1027 The curved 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 warming-up period, and it may, or may not, be a significant item depending on the total time involved. 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 drying 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 the surface 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 emperature in turn produces a higher constant, rate of evaporation. th^n ^?ose dryers in which heat is transferred to a wet solid by conduction '"rough hot surfaces, and heat transfer by convection is not a factor, the 'et surfaces approach the boiling point temperature rather than a wetmo temperature. When all the heat for evaporation in the constant-rate period is supplied tra.a f ^as' a dynamlc equilibrium is established between the rate of heat ThL 61 the material and the rate of vapor removal from the surface, folio ^rdidorinm between heat and mass transfer rates can be expressed as where dw hiAat de = x = k9A&p dw dd ^ drying rate, pounds of water per hour. (1)