performance of a flat plate solar air heater for the crop drying in multiple trays. It is also useful for predicting the moisture content crop temperature and drying rate in the different drying trays. They also concluded that the grain temperature increase with the increase of collector length, breadth and tilt angle up to typical value of these parameters. El-Sebaii et al. [34] designed an indirect type natural convection solar dryer. It consists of a flat-plate solar air heater connected to a cabinet acting as a drying chamber. Sand was used as the thermal storage material. The drying parameters such as drying temperature, ambient temperature, relative humidity, solar irradiance and temperature distribution in different parts of the system during drying had been recorded. Grapes, figs, apples, green peas, tomatoes and onions were dried successfully with and without using the storage material. They concluded that the equilibrium moisture content Me for seedless grapes was reached after 60 and 72h when the system was used with and without storage the material, respectively. Therefore, the storage material reduced the drying time by 12 h. In order to accelerate the drying process, the drying products are divided into pieces and then chemically treated by dipping the samples into boiling water containing 0.4% olive oil and 0.3% NaoH for 60 s. The required time to achieve Me for the chemically treated seedless grapes, when the system is used with sand as a storage material, was drastically reduced to 8h