DEVELOPMENT AND EVALUATION OF HOT-AIR SUPPLEMENTED SOLAR DRYER FOR WHITE YAM (Dioscorea rotundata) SLICES

Abstract

Solar food dryers represent a major improvement upon the ancient method of dehydrating foods. It reduces the problem of contamination. Drying is completed more quickly, so there is less chance of spoilage. In this study, a hot-air supplemented solar dryer was fabricated for drying white yam slices Dioscorea rotundata. The capacity of the designed hit-air supplemented solar dryer is 14 kg. The equipment was tested in Federal University of Technology Akure (FUTA) using white yam Dioscorea rotundata to establish the effect of incorporating the hot-air section into the solar dryer. Drying experiments were conducted according to 3 by 2 by 1 factorial design which represents 3 temperatures (40, 60, 70 °C), 2 yam slice thicknesses (2 and 4 mm) and 1 drying air velocity (0.8 m/s) for the hot-air supplemented solar dryer and also an indirect solar dryer was also used to carry out a solar drying test on 2 thicknesses (2 and 4 mm) and also 0.8 m/s drying air velocity. The moisture loss was determined every hour and the differences was established graphically and analyzed. The initial moisture content of the white yam used from harvest was determined before the commencement of the experimental process. The initial moisture content of the white yam was found to be 73.1 % w.b. the yam used for the experiment was washed, peeled and sliced to required thicknesses 2 mm and 4 mm after which the sliced yam was spread on the tray and placed inside the drying chamber for drying. The reading was taken and recorded hourly. The readings taken include ambient temperature, solar collector inlet and outlet temperature, drying chamber temperature, drying chamber outlet vent temperature, solar radiation and mass of the yam slices. Two methods were used to evaluate the efficiency of the machine. The solar drying part was used alone and also the hot-air supplemented solar dryer was used in evaluating the dryer and the effect on incorporating hot-air into a solar drying system. After the experiment, it was deduced that the total drying time used to reduce the moisture in the white yam slices to safe storage moisture content (SSMC) differs for the two different drying conditions giving a total drying time of 18 hours for solar dryer and 13 hours for hot-air supplemented solar dryer. In both methods, drying rate increases with increasing temperature and decreases with the increases yam thickness. The machine efficiency was calculated for different drying conditions. Also the incorporation of the hot-air section aids the stability of the temperature in the drying chamber throughout the drying period. The average thermal efficiencies were 31.45 %, 13.97 %, 42.10 % and 55.91 % for solar drying, solar/mechanical drying at 40 °C, solar/mechanical drying at 60 °C and solar/mechanical drying at 70 °C respectively and also the solar collector highest efficiency was calculated to be 83.28 % at solar radiation intensity 1199.46 W/m2 and lowest efficiency of the solar collector was 23.89 % at solar radiation intensity of 300.40 W/m2.