Solar Drying: Fundamentals,Applications and Innovations - National ...

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Visavale - Principles, Classification and Selection of Solar Dryers lary flow is typified by a moisture gradient involving a double curvature and point of inflection (Figure 1.3a) while diffusional flow is a smooth curve, concave downwards (Figure 1.3b). They also showed that the liquid-diffusion coefficient is usually a function of moisture content which decreases with decreasing moisture (Keey, 1978 and Treybal 1980). Figure 1.3. Two types of internal moisture gradients in solid drying (Perry 2007) 1.3.2. Periods of Drying Figure 1.4 shows a plot of moisture content (X) versus time (θ) generally obtained by experimentally drying a solid. This curve represents a typical case when a wet solid loses moisture initially by evaporation from a saturated surface on a solid, followed by a period of evaporation from a saturated surface on a solid, followed by a period of evaporation from a saturated surface of gradually decreasing area and finally when the latter evaporated in the interior of the solid. Figure 1.4a indicates that the drying rate is subject to variation with time or moisture content, further better illustrated by graphically or numerically differentiating the curve and plotting dX/dθ versus W, as shown in Figure 1.4b, or as dX/dθ versus θ, as shown in Figure 1.4c. These rate curves illustrate that the drying process is not a smooth, continuous one in which a single mechanism controls throughout. Figure 1.4c has the advantage of showing how long each drying period lasts. The section AB on each curve represents a warming-up period of the solids. Section BC on each curve represents the constant-rate period. Point C, where the constant rate ends and the drying rate begins falling, is termed the critical-moisture content. The curved portion CD on Figure 1.4a is termed the falling-rate period and, as shown in Figure 1.4b and c, is typified by a continuously changing rate throughout the remainder of the drying cycle. Point E (Figure 1.4b) represents the point at which all the exposed surface becomes completely unsaturated and marks the start of that portion of the drying cycle during which the rate of internal moisture movement controls the drying rate. Portion CE in Figure 1.4b is usually defined as the first falling-rate drying period; and portion DE, as the second falling-rate period (Perry 2007). 8 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers 1.3.2.1. Constant-Rate Period Figure 1.4. Drying period curves (Perry 2007) In the constant-rate period moisture movement within the solid is rapid enough to maintain a saturated condition at the surface, and the rate of drying is controlled by the rate of heat transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated surface of the material across a stagnant air film into the environment and as the rate of mass transfer balances heat transfer, the temperature of saturated surface remains constant. If the heat supplied for drying is solely by convection, the surface temperature approaches the boiling point temperature rather than the wet bulb temperature. This mode of heat transfer is typically seen in indirect dryers. Radiation is an effective mode of heat transfer as it increases the constant rate by augmenting the convection heat transfer and raising the surface temperature above the wet bulb temperature. The magnitude of the constant rate depends upon three factors: • The heat or mass transfer coefficient • The area exposed to the drying medium • The difference in temperature or humidity between the gas stream and the wet surface of the solid. All these factors are the external variables. The internal mechanism of liquid flow does not affect the constant rate (Keey, 1980). 1.3.2.2. Falling-Rate Period The falling-rate period begins at the critical moisture content when the constantrate period ends. This is generally divided into two zones viz., (i) the zone of unsaturated surface drying and (ii) the zone where internal moisture movement controls. In the first zone, the entire evaporating surface can no longer be maintained and saturated by moisture movement within the solid. The drying rate decreases from the unsaturated portion, and hence the rate for the total surface decreases. Generally, the drying rate depends on factors affecting the diffusion of moisture away from the evaporating surface and those affecting the rate of internal moisture movement. As drying proceeds, the point is reached where the evaporating surface is unsaturated. The point of evaporation moves into the solid, and the dry process enters the second falling-rate period. The drying rate is now governed by the rate of internal moisture movement; the influence of external variables diminishes. This period usually predominates in determining the overall drying time to lower moisture content (Treybal 1980). Drying of Foods, Vegetables and Fruits 9
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