Performance of a Recirculating-Type Solar Dryer

Abstract

A novel recirculation-type integrated solar collector drying chamber solar dryer has been designed, constructed, and tested. The solar dryer comprises a feed hopper, a centrifugal blower, a pneumatic conveyor, and a transparent structure acting as drying chamber containing a hopper with a vortex at the top. The transparent structure has a dimension of 3 m in diameter and height of 3 m. The blower used has a capacity of maximum 1 kW, 220 V, and a supplying air velocity of 30.3/s. A series of tests were conducted outdoors where solar radiation was available. A test with 104 kg of rough rice indicated that the drying time required to reduce the moisture content (m.c.) of rough rice from 28.4 % wb to the final m.c. of 14.3 % wb was 5 h. During the test, the drying temperature was kept constant at 50.1 °C and the relative humidity (RH) was 21.73 %. The required power for the pneumatic conveyor was 581 W, with a total energy input of 210.7 MJ, including LPG and solar radiation. The resulting drying efficiency was 22.4 % with specific energy of 15.2 MJ/kg of water evaporated. Another test using 200 kg with initial m.c. of 27.6 % wb of rough rice has shown that the required drying time to achieve the final m.c. of 14.3 % was 8 h. During the test, the drying temperature was kept constant at 46.9 °C, and the RH was 21.7 %. The drying efficiency was 31.7 % with specific energy of 10.7 MJ/kg of water evaporated. By knowing the amount of rough rice retained in the receiving hopper and the rate of recirculation, it was possible to measure the drying time of each cycle until the drying process completed. By using the sphere model, we were able to estimate the drying curve, where it was found that a good agreement exists between the theoretical and experimental data using the value of drying constant k = 2.75 (1/hr) and M _e of 7 % db. As the drying time increased toward the end of drying process, it was observed that the variation in rough rice m.c. became smaller and smaller and was better than that found in the previous study.