Agriculture Research Papers - Academia.edu (original) (raw)

So far, drying systems have not been classified systematically. However, drying methods can be broadly classified on the basis of either the mode of heat transfer to the wet solid or the handling characteristics and physical properties of the wet material. The first method of classification reveals differences in dryer design and operation, while the second method is most useful in the selection of a group of dryers for preliminary consideration in a given drying problem. According to the mode of heat transfer, drying methods can be divided into: (a) conduction drying, (b) convection drying, and (c) radiation drying. There are other methods of drying also, namely, dielectric drying, chemical or sorption drying, vacuum dryi ng, freeze drying, etc. Of them, convection drying is commonly used for drying of all types of grain and conduction drying can be employed for drying of parboiled grain. Conduction drying When the heat for drying is transferred to the wet solid mainly by conduction through a solid surface (usually metallic) the phenomenon is known as conduction or contact drying. In this method, conduction is the principal mode of heat transfer and the vaporised moisture is removed independently of the heating media. Conduction drying is characterised by: (a) Heat transfer to the wet solid takes place by conduction through a solid surface, usually metallic. The source of heat may be hot water, steam, flue gases, hot oil, etc.; (b) Surface temperatures may vary widely; (c) Contact dryers can be operated under low pressure and in inert atmosphere; (d) Dust and dusty materials can be removed very effectively; and (e) When agitation is done, more uniform dried product and increased drying rate are achieved by using conduction drying. Conduction drying can be carried out either continuously or batch-wise. Cylinder dryers, drum dryers, steam tube rotary dryers are some of the continuous conduction dryers. Vacuum tray dryers, freeze dryers, agitated pan dryers are the examples of batc h conduction dryers. Convection drying In convection drying, the drying agent (hot gases) in contact with the wet solid is used to supply heat and carry away the va porised moisture and the heat is transferred to the wet solid mainly by convection. The characteristics of convection drying are: (a) Drying is dependent upon the heat transfer from the drying agent to the wet material, the former being the carrier of vaporis ed moisture; (b) Steam heated air, direct flue gases of agricultural waste, etc., can be used as drying agents; (c) Drying temperature varies widely; (d) At gas temperatures below the boiling point, the vapour content of the gas affects the drying rate and the final moisture content of the solid; (e) If the atmospheric humidities are high, natural air drying needs dehumidification; and (f) Fuel consumption per kg of moisture evaporated is always higher than that of conduction drying. Convection drying is most popular in grain drying. It can be carried out either continuously or batch-wise. Continuous tray dryers, continuous sheeting dryers, pneumatic conveying dryers, rotary dryers, tunnel dryers come under the continuous system, wherea s tray and compartment dryers, batch through circulation dryers are the batch dryers. Convection drying can be further classified as follows: Pneumatic or fluidised bed drying: When the hot gas (drying agent) is supplied at a velocity higher than the terminal velocity of the wet solid, the drying of the wet solid occurs in a suspended or fluidised state. This phenomenon is known as fluidised bed drying. Drying may be carried out in a semi-suspended state or spouted bed condition also. Generally, the convection drying is conducted under ordinary state, i.e., drying agent is supplied at a velocity much lower than the terminal velocity of the wet m aterial. In natural air drying, the unheated air as supplied by the nature is utilized. In drying with supplemental heat just sufficient amount of heat (temperature rise within 5 to 10°C) only, is supplied to the drying air to reduce its relative humidity so that drying can ta ke place. In heated air drying, the drying air is heated to a considerable extent. The natural air drying and drying with supplemental heat methods which may require one to four weeks or even more to reduce the grain moisture content to safe levels, are generally used to dry grain for short-term storage in the farm. Heated air drying is most useful when large quantity of grain is to be dried within a short time and marketed at once. It is used for both short and long-term storage. Comparative advantages and disadvantages of the three convective drying methods are given as follows: NATURAL AIR DRYING: Advantages: (1) Lowest initial investment and maintenance cost. (2) No fuel cost. (3) No fire hazard. (4) Least supervision. (5) Least mould growth compared to supplemental heat. Disadvantages: (1) Very slow drying rate, drying period may be extended to several weeks. (2) Weather dependent. (3) More drying space necessary in comparison to heated air drying. (4) Useful particularly for short-term storage in the farm. (5) Not useful for humid tropics. SUPPLIMENTAL HEAT DRYING: Advantages: (1) Lower cost of equipment and maintenance. (2) Independent of weather. (3) Requires less supervision. (4) Most Convection drying Drying under Fluidised state Drying under Spouted bed condition Drying under ordinary state Natural / unheated air drying Heated air drying Air drying with supplemental heat