This research considers engineering design, operation, functional performance and economic analysis of a heated-air tray dryer designed for medicinal and aromatic plants. In investigating the performance of the dryer, six drying tests with varying loading density were conducted with Mentha piperita and Hypericum perforatum. The drying process which reduced the product moisture contents from 59 to 80% (w.b.) to moisture content below 15% (w.b.) took 6-9 h depending on the material being dried and loading density. To obtain the high-quality dried products in terms of flavour and colour, the temperature of the drying air was controlled at 46 +/- 4 degreesC during drying experiments. This dryer can be successfully used to dry 145 kg of M. piperita and 120 kg of H. perforatum in each drying batch. The specific heat energy consumption of the dryer for M. piperita and H. perforatum were determined as 4840 and 7694 kJkg(-1) [water], respectively, when the dryer was operated at maximum capacity. The payback period of the dryer is estimated to be less than 2.0 months for M. piperita drying and less than 0.5 months for H. perforatum drying. (C) 2001 Silsoe Research Institute.