A mathematical model for the semi-batch operation of industrial dryers with trucks and trays is presented and analysed. Design aspects are discussed concerning problems involving both single dryer and systems of parallel dryers. In both cases, optimum flowsheet configuration and operation conditions are sought and verified by appropriate formulation of design and optimization strategies. The optimization objective is the total annual cost of the plant, subjected to constraints imposed by the operation of the dryer thermodynamics, and construction reasoning. The decision variables were the number of trucks and the drying air stream humidity for each dryer involved as well as the total number of dryers. The MINLP nature of the design problem required mathematical programming techniques for its solution. The optimization was carried out for a wide range of production capacities, and the optimal points, where a new truck or a new dryer is introduced, were evaluated. The effect of market economic figures on the design results is illustrated. The analysis focused on the design of two commercial agricultural products - namely, raisins and currants. A characteristic case study is presented in order to demonstrate the effectiveness of the proposed approach. (C) 1997 Elsevier Science Limited.