DEVELOPMENT AND EVALUATION OF AIR RECIRCULATION SYSTEM FOR CONVECTIVE TRAY DRYER
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Date
2005
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MPUAT, Udaipur
Abstract
In India mechanical drying is one of the important unit operation of post harvest technology of cereals, pulses, oilseeds, fruits and vegetables. There are many types of dryers in use for dehydration of foods. A majority of industrial dryers use steam, diesel, LPG, kerosene, electricity for heating. The financial value of energy varies enormously with its grading and electrical energy has highest energy grading of all. It is therefore imperative that electricity should be utilized efficiently and the areas of where the energy is wastefully used are to be identified and corrective measures are searched for adoption. Hence research has been carried out to achieve cost-effectiveness of drying process to conserve energy and to study product quality by recycling of exhaust air.
Dryer under study is classified as the air convection batch type tray dryer. Principal components of recirculatory system consists of exit air diversion tee, flow control gate, air recirculation duct and air mixing chamber. Tomatoes were used for testing of recirculatory type convective tray dryer. Tomatoes pretreated with freezing mixture of crushed ice and salt in the ratio 10:1 for 5 min followed by that dipping in 2 percent sodium chloride at 600C for 5 min was cut into 1 cm thick slice and subsequently dried in the recirculatory dryer. Drying experiment was carried out at 70/600C (700C for first one hour and 600C thereafter) and 70/650C under three recirculation rates (50, 75 and 90%). One experiment was carried out without any recirculation of exhaust air for comparative study. The moisture loss data was used to determine drying characteristics such as drying rate, diffusivity. Dryer performance was measured with respect to heat utilization factor (H.U.F.), thermal heat efficiency (T.H.E).
The initial moisture content of tomato slices was 93 - 94 per cent which reduced to 6 to 6.5 percent on dry basis and it took 8 - 11 h under different drying air conditions. Dehydration of tomato slices took place in falling rate of drying period only. Diffusivity values for tomato dehydration process was observed to be vary between 4.55 x 10-10 to 6.43 x 10-10 m2/s. Tomato colour index (T.C.I.) of tomato powder varied in between 18.63 - 27.02. Highest retention of ascorbic acid was 10.67 mg/100g of sample dried at drying air temperature 70/600C under 50% recirculation. Rehydration ratio was maximum for test carried under 75% recirculation rate. Maximum saving of energy with recirculation of exhaust air as compared to drying without any recirculation was 43 % when dried at 70/600C and 90% recirculation. Thermal efficiency and heat utilization factor of the dryer increased with recirculation rate.
Description
DEVELOPMENT AND EVALUATION OF AIR RECIRCULATION SYSTEM FOR CONVECTIVE TRAY DRYER
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Citation
Patil and Champawat, 2005