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Visavale - Principles, Classification and Selection of Solar Dryers 1.2. DRYING PRINCIPLES Drying is basically a phenomena of removal of liquid by evaporation from a solid. Mechanical methods for separating a liquid from a solid are not generally considered drying. In the following section an attempt is made to provide a concise overview of the fundamental principles of drying process for agricultural products. These principles are applied, in general, to mechanical conventional drying and here concerned mainly with solar drying. However in general, must be noted that conventional drying principles and phenomena are independent of the type of energy used. Ekechukwu and Norton (1999) and Mujumdar, 2007 gives a comprehensive review of fundamental principles and theories governing the drying process. A major part of energy consumption during drying is for the evaporation of liquid water in to its vapour (2258 kJ/kg at 101.3 kPa). The water may be contained in the solid in various forms like free moisture or bound form which directly affects the drying rate. The commonly encountered terminologies in psychrometry and drying are briefly tabulated in Table 1.1. Moisture content is expressed either on dry or wet basis, e.g. moisture content in wet (X w ) basis is the weight of moisture per unit of wet material. X w = m w m w +m d , kg per kg of mixture (1.1) and on dry basis (X d ), is expressed as the ratio of water content to the weight of dry material: X d = m w m d (1.2) Figure 1.1. Relationship between wet-weight and dry-weight basis (Perry 2007) Although the most convenient way to express moisture for mathematical calculations is on dry basis but for agricultural products moisture content normally is expressed in wet basis. Figure 1.1 shows the plot of relationship between the dry and wetweight basis. 4 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers Table 1.1. Commonly encountered terms in psychrometry and drying (Perry 2007) Sr no. Term Meaning/ Definition 1. Bound moisture is that in a solid liquid, which exerts a vapor pressure less than that of the pure liquid at the given temperature. 2. Capillary flow is the flow of liquid through the interstices and over the surface of a solid, caused by liquid-solid molecular attraction. 3. Constant-rate period is that drying period during which the rate of water removal per unit of drying surface is constant. 4. Critical moisture content is the average moisture content when the constant-rate period ends. 5. Dry-weight basis expresses the moisture content of wet solid as kilograms of water per kilogram of bone-dry solid. 6. is the limiting moisture to which a given material can be Equilibrium moisture dried under specific conditions of air temperature and humidity. content 7. Falling-rate period is a drying period during which the instantaneous drying rate continually decreases. 8. Free-moisture content is that liquid which is removable at a given temperature and humidity, may include bound/unbound moisture. 9. Funicular state is that condition in drying a porous body when capillary suction results in air being sucked into the pores. 10. Hygroscopic material is material that may contain bound moisture. 11. Initial moisture distribution start of drying. refers to the moisture distribution throughout a solid at the 12. Internal diffusion may be defined as the movement of liquid or vapor through a solid as the result of a concentration difference. 13. Moisture content of a solid is usually expressed as moisture quantity per unit weight of the dry or wet solid. 14. Moisture gradient refers to the distribution of water in a solid at a given moment in the drying process. 15. Non-hygroscopic material is material that can contain no bound moisture. is that state of a liquid in a porous solid when a continuous Pendular state 16. film of liquid no longer exists around and between discrete particles so that flow by capillary cannot occur. 17. Unaccomplished moisture change initially present. is the ratio of the free moisture present at any time to that in a hygroscopic material, is that moisture in excess of the Unbound moisture 18. equilibrium moisture content corresponding to saturation humidity. 19. Wet-weight basis expresses the moisture in a material as a percentage of the weight of the wet solid. 1.2.1. Psychrometry in Drying In drying phenomena the psychrometry is of importance as it refers to the properties of air-vapor mixture that controls the rate of drying. When an adequate supply of heat is provided for drying, the temperature and rate at which the liquid vaporization occurs will depend on the vapour concentration in the surrounding atmosphere. When a Drying of Foods, Vegetables and Fruits 5
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