THE HIGH-TEMPERATURE DRYING OF SOFTWOOD BOARDS - A KILN-WIDE MODEL

Characteristic drying curves are normally determined from laboratory experimental data to specify the drying behaviour in a commercial-scale dryer. By contrast in this paper, we obtain the drying kinetics from the numerical results of a more rigorous mathematical model. In our example, we use a model developed for the high-temperature drying of a single softwood board to describe the drying behaviour of a stack of boards in a kiln. Analysis of the mechanism of moisture movement has led us to identify three stages in the drying of a sapwood board and two for a heartwood board. The drying kinetics can be represented by a dual characteristic curve covering the two falling-rate periods, to yield simplified expressions for the drying kinetics. These expressions can be coupled with mass and energy balances over a control volume to give equations which can be solved numerically to determine the humidity, temperature and moisture-content profiles in the airflow direction within a timber kiln. This two-step modelling procedure provides a computationally efficient way of analysing conditions in any batch dryer.