The TLM method has been used to model the field distribution within a full-scale pilot plant for a wood drying kiln developed by the Council of Forest Industries, Vancouver, B.C., for drying softwood lumber. The kiln exploits a combination of radio-frequency heating and vacuum drying technologies and offers improvements in drying time and in wood quality over conventional gas-fired kilns. The RF generator oscillates between frequencies of 5 and 10 MHz with a maximum power output of 350 kW and the vacuum chamber has a lumber-holding capacity of 55 cubic metres. By coupling the field simulation with a drying model, the variation in the moisture content of the lumber packages throughout the drying process has been obtained. This has enabled improvements to the existing kiln design to be made so that more uniform drying in the package cross-section could be achieved.
In the original electrode configuration, a stack of three lumber packages was placed between a hot and a ground electrode with air gaps on either side to facilitate material handling and to reduce the change in total load impedance as the wood dried. The moisture content at four stages in the drying cycle obtained by simulation is shown in fig. 1, over the cross-section of the packages. The hot electrode is on the left side and the darker areas indicate drier wood. It is apparent that the drying is non-uniform and the lumber remains wet on the ground electrode side. A set of simulations were performed to investigate how this difference could be minimized within the confines of the existing kiln. Once the recommended improvements had been made to the electrodes a much more satisfactory drying uniformity was achieved, as shown in fig. 2. The experimental results verified the predictions made by the model and this has greatly improved confidence in using the model for future RF/vacuum drying applications.
Further details can be found in:
J. L. Herring, W. J. R. Hoefer and R. Zwick, `TLM Modeling of a RF/Vacuum Wood Drying Kiln', Progress in Electromagnetics Research Symposium, Seattle, Washington, USA, July 24-28, 1995.
Fig. 1 - Moisture content in three packages with the original electrode configuration
Fig. 2 - Moisture content in two packages with the revised electrode configuration
