The Construction and Testing of a Combined Solar and Mechanical Cabinet Dryer

The combined solar and mechanical dryer was constructed to use solar energy and electrical energy either separately or in combination to conduct air drying. There was various arrangement of drying condition for mechanical drying system such as using either 1KH-1F or 1KH-2F or 2KH-1F or 2KH-2F. While for solar drying either no fan or one fan or two fans were used. Different drying conditions were applied by changing the heating source and flow of air. The highest temperature was obtained in 2KH-1F condition for mechanical drying and convection gives highest temperature among different solar drying alternatives. The temperature in upper shelf for different solar operational condition ranged from 1.53 to 1.67 times of ambient temperature (250C-310C) at peak time and 1.28 to 1.36 for lower shelf. The air velocity was 0.35 m/sec and 0.70 m/sec for 1-fan and 2-fan respectively. The performance of dryer was evaluated by drying potato slices having thickness 3, 5 and 7 mm. In both methods, drying rate increases with the increasing temperature and decreases with the increasing slice thickness following the power law equation. The values for the index ‘n ’ of the power law equation are less than 2 (range from 0.53 to 1.42) for all the conditions, this indicating that external resistance to mass transfer was significant and internal resistance to mass transfer did not control the drying process under the given conditions. The effect of temperature on Diffusion co-efficient (De) follows Arrhenius type relationship. While analyzing the effect of temperature, the maximum activation energy for diffusion of water from Potato (7 mm thick slices, at upper shelf) was found to be 7.656 Kcal/g-mole and 8.252Kcal/g-mole for mechanical and solar conditions respectively