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1004 CHAPTER 47 riv.Mi'iJZ j 51953 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 ip Fig. 4. These rale 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/dSj-which becomes-'a horizontal line on the rate curves in Figs. 3 and 4. - ` 1 Industrial Drying Systems : : 1005 , The curved portion of Fig. 2 is termed the falling-rate period, andj as shown in Figs. 3 and 4, it is typified by a continuously changing rate. Point A, where the constant rate ends arid the drying rate begins to de crease, is termed the critical moisture content. The portion of the curves designated by GB represents a wanriing-up period, and it may, or may not, be a significant .iteiri deperidipg on the total time involved. Constant-Rate Period. Drying during the constant-rate period is equiv alent to evaporation from a free-water surface op the; surface of the solid. The rate of drying iri 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 pir. If heat, flowsVfq the surface of evaporation by radiation,and conduction, or both.inaddition to convection, the surface ternperature mil be constant at some value between the air temperature arid 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. ; Whep all the heat for evaporation in the constant-rate period is supplied by a hot gas, a dynarriic equilibrium is established between the rate of heat transfer to the material and the fate of vapor removal from tbe surface. This equilibriuin between heat and mass transfer rates can be expressed as follows: '1 dw htAat d$ ~= Map (l) where ` dw - ./jjg =* drying rate, pounds of water per hour. ..