No abstract

The thin-layer drying kinetics of Granny Smith apples is determined by thermogravimetric analysis of the drying process at constant temperatures ranging from 20 C to 50 C, using intervals of 5 C. The experimental drying curves obtained in the TGA were fitted to the Wang-Singh equation, which was found to describe precisely the drying process. A novel model, capable of predicting the evolution of the moisture ratio of Granny Smith apples during the drying process and under variable drying temperatures, was proposed. The model was validated with experimental TGA measurements of the drying of apples at variable temperatures, typical of solar drying, obtaining maximum deviations for the drying time of less than 1.5%. Once validated, the model proposed was also applied to the drying of Granny Smith apples in an indirect solar dryer. The comparison of the model prediction with the experimental measurements of the drying of apples at variable drying conditions conducted in a lab-scale solar dryer showed a proper agreement, with low deviations (less than 10%) due to the thermal inertia of the samples.

A theoretical study on the maximum evaporation rate obtainable in a small-scale indirect solar dryer is presented, considering evaporation of free water. A mathematical model of the evolution of the temperature and the specific humidity of the airflow along the drying chamber is presented. Based on the results, some simplifications are proposed and justified in order to calculate the maximum evaporation rate as a function of a reduced number of parameters, to study their effect. The results show that the effect of the air mass flow rate on the maximum evaporation rate depends on the aspect ratio of the drying chamber, defined as the ratio of the total drying area to the cross section in the drying chamber. Design and operation criteria can be extracted from the results. As a global result, for the typical range of dimensions and air mass flow rates employed in solar dryers, the drying chamber aspect ratio should be between 200 and 300 to obtain a proper evaporation rate. Within that range, doubling the air mass flow rate the maximum evaporation rate obtainable increases around 20%.

Abstract.A model is developed to study the influence of design parameters (dimensions, air mass flow rate) in the performance of an indirect solar drier for batch drying. The aim of the work is to cover a wide range of configurations and sizes with a simple model. The model covers the air heating in the collector and the vapour mass transfer in the drying chamber. Constant rate controlled by convection mass transfer is assumed. The results, still preliminary, show the relevance of using a solar collector and the evolution of the drying process in the drying chamber. The obtainable vapour mass flow rate is calculated for a variety of configurations, showing the incidence of the air mass flow and the dimensions of the collector and the drying chamber. Performance parameters such as temperatures during the process and the efficiency of the drying chamber are also depicted.