SOLAR DRYER PERFORMANCE STUDY OF SOME CROPS (MINT, OKRA AND GRAPES) I - ASSESSING THE DRYING RATES

Abstract

Three crops were subjected to drying in the passive crop dryer. Crop samples were also open-sun dried as control treatment and the weight losses and temperature were taken. Mint leaves, okra and grapes were dried in the dryer and the same masses of samples were dried in the open–sun (control). The maximum stagnation temperature attained in the dryer is 66 oc and the corresponding values of solar radiation and ambient temperature were 832 W/m2 and 36 oc, respectively. The overall heat loss coefficient of the dryer varied between 28.34 and 21.56 W/m2 oc. All the drying process occurred in the falling rate period, starting from the initial moisture contents which are (83 wb or 488 db % for mint), (88.5 wb or 770 db % for okra) and (78 wb or 354 db for% grapes) to the final moisture content after drying were (1.18 and 19 db % for mint), (43 and 108 db % for okra) and (3.8 and 65.8 db % for grapes), for (solar dryer and natural drying) respectively. The assessment of the drying rate of the crops in the solar dryer and in open-sun gives an average of 7.68 g/h and 7.40 g/h for mint leaves, 7.08 g/h and 5.91 g/h for okra and 7.17 g/h and 5.92 g/h for grapes respectively. For mint, drying efficiencies during different days of drying for the dryer and natural drying were (60.92, 7.11 and 2.84%) and (55.51, 9.82 and 2.98%) on (first, second and third drying days), respectively and the averages were 23.62 and 22.77%. For okra drying efficiencies during different days of drying for the dryer and natural drying were (42.93, 17.01 and 2.62%) and (37.09, 15.36 and 4.49%)on (first, second and third drying days), respectively and the averages were 20.85 and 18.98%.For grapes, drying efficiencies during different days of drying for the dryer and natural drying were (34.27, 20.32 and 11.65%) and (40.31, 8.88 and 5.46%) on (first, second and third drying days), respectively and the average values were 22.05 and 18.21%.