Empirical correlations for drying kinetics of some fruits and vegetables

An indirect type natural convection locally made solar dryer, consisting of a flat-plate solar air heater connected to a drying chamber, has been investigated experimentally and theoretically. The system is used for drying fruits and vegetables such as seedless grapes, figs, green peas, tomatoes and onions. The experimental data are used to calculate the drying constants k for the selected products. The obtained values of k are then correlated with the drying product temperature T-dp. Linear correlations between k and T-dp are found to satisfactorily describe the drying curves of the materials under study. Furthermore, the characteristics constants c and n of Henderson's equation are determined for the selected crops using the experimentally measured relative humidity of the drying air. A simple mathematical model is presented for the drying chamber based on the energy and mass balance equations and employing the proposed correlations for k as well as the values of the constants c and n. Numerical calculations are performed on typical summer and winter days in Tanta. Comparisons between experimental and theoretical results showed that good agreement has been achieved. The effect of the mass of the drying product m(dp) on the dryer performance is also investigated by computer simulation. (C) 2002 Elsevier Science Ltd. All rights reserved.