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

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Visavale - Principles, Classification and Selection of Solar Dryers free liquid or wetted surface is present, drying will occur at the saturation temperature, just as free water at 101.325 kPa vaporizes in a 100 percent steam atmosphere at 100 °C. On the other hand, when evolved vapor is purged from the dryer environment by using a second (inert) gas, the temperature at which vaporization occurs will depend on the concentration of vapor in the surrounding gas. Figure 1.2. Psychrometric chart: properties of air and water-vapor mixtures In effect, the liquid must be heated to a temperature at which its vapor pressure equals or exceeds the partial pressure of vapor in the purge gas, while in the reverse situation condensation occurs. In most drying operations, water is the liquid evaporated and air is the normally employed purge gas. For drying purposes, a psychrometric chart found very useful is that reproduced in Figure 1.2 and can be explained as follows (Perry 2007 and Treybal 1980): a. In the psychrometric chart the wet-bulb or saturation temperature line gives the maximum weight of water vapor that 1 kg of dry air can carry at the intersecting drybulb temperature shown on the abscissa at saturation humidity. The partial pressure of water in air equals the water vapor pressure at that temperature and the saturation humidity is defined by H s = 18 ∙ p s (1.3) 28.9 (P−p s ) b. The percent relative humidity is defined by H R = 100 ∙ p (1.4) p s c. Humid volumes are given by the curves entitled “volume m 3 /kg dry air.” The difference between dry-air specific volume and humid-air volume at a given temperature is the volume of water vapor. d. Enthalpy data are given on the basis of kilojoules per kilogram of dry air. Enthalpy-atsaturation data are accurate only at the saturation temperature and humidity. 6 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers e. There are no lines for humid heats on Figure 1.2. and can be calculated by C s = 1.0 + 1.87H (1.5) The wet-bulb-temperature lines also represent the adiabatic saturation lines for air and water vapor only, and are based on the relationship H s − H = C s λ (T − T s) (1.6) The slope of the adiabatic saturation curve is (C s /λ). These lines show the relationship between the temperature and humidity of air passing through a continuous dryer operating adiabatically. The wet-bulb temperature is established by a dynamic equilibrium between heat and mass transfer when liquid evaporates from a small mass, such as the wet bulb of a thermometer, into a very large mass of gas such that the latter undergoes no temperature or humidity change, and is expressed by the relationship h c (T − T w ) = k g ′ λ(H w − H a ) (1.7) A given humidity chart is precise only at the pressure for which it is evaluated and most air–water-vapor charts are based on a pressure of 1 atm. Humidities read from these charts for given values of wet-bulb and dry-bulb temperature apply only at an atmospheric pressure of 760 mmHg. If the total pressure is different, the humidity at a given wet-bulb and dry-bulb temperature must be corrected according to the following relationship. H a = H o + 0.622p w 1 − 1 (1.8) P−p w 760−p w 1.3. MECHANISM OF DRYING Drying basically comprises of two fundamental and simultaneous processes: (i) heat is transferred to evaporate liquid, and (ii) mass is transferred as a liquid or vapor within the solid and as a vapor from the surface. The factors governing the rates of these processes determine the drying rate. The different dryers may utilize heat transfer by convection, conduction, radiation, or a combination of these. However in almost all solar dryers and other conventional dryers heat must flow to the outer surface first and then into the interior of the solid, with exception for dielectric and microwave drying. 1.3.1. Internal Mechanism of Liquid Flow The movement of moisture within the solid result from a concentration gradient which is dependent on the characteristics of the solid, that may be porous or nonporous. Thus the structure of the solid determines the mechanism for which internal liquid flow may occur and these mechanisms can include (a) diffusion in continuous, homogeneous solids, (b) capillary flow in granular and porous solids, (c) flow caused by shrinkage and pressure gradients, (d) flow caused by gravity, and (e) flow caused by a vaporization-condensation sequence. In general, one mechanism predominates at any given time in a solid during drying, but it is not uncommon to find different mechanisms predominating at different times during the drying cycle. In the plots shown in Figure 1.3, the curves indicate that capil- Drying of Foods, Vegetables and Fruits 7
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