ANALYSIS AND DESIGN OF TUNNEL DRYERS FOR AGRICULTURAL PRODUCTS

Abstract

A methodology to design purpose-built tunnel dryers for agricultural products is presented. The methodology incorporates a material model and an equipment model and integrate them to produce a dryer model. While the material model describes the drying behaviour of the product based on the "receding evaporation front" theory, the equipment model describes the transfer processes in the tunnel dryer. The material model is validated with experimental data obtained from two dryers of different sizes. Equations in the dryer model are then cast in finite-difference form for simulation studies on the performance of tunnel dryers under different operating conditions. The resulting simulation algorithm becomes an engineering tool for designers of tunnel dryers, enabling the estimation of dryer dimensions and the drying time to reach desired moisture content under various drying conditions. To demonstrate the existence of the receding evaporation front during the drying of agricultural products, the temperature distribution within the product is measured. The material model, which takes into account the material shrinkage and considers the variation of moisture transfer coefficient during drying of hygroscopic products, is validated by comparing the predicted values with experimental results. A parametric analysis involving the three basic parameters, namely the air velocity, temperature and humidity, is conducted to study the drying transients in the dryer. The effect of variation of each parameter is then related to the drying time, the dryer dimensions and the drying energy requirement. Multi-stage drying, where dryers are cascaded, is recommended when saturation of the air in the main dryer occurs before the desired moisture content of the product is attained. A performance parameter, known as the contact factor, commonly used in the design of spray and air-conditioning equipment, is proposed as a performance parameter to simplify the dryer design process. The elegance and simplicity of this performance parameter is demonstrated as it links two other performance parameters, namely, the COP (Coefficient of Performance) and the SEC (Specific Energy Consumption), in the production of performance charts for designing closed-loop heat pump dryers.