Drying of lumber in vacuum using dielectric heating

The feasibility has been investigated of drying lumber in a vacuum kiln using dielectric heating. Emphasis was placed on Austrian wood species and products with large dimensions that would be difficult or impossible to dry when using conventional drying methods. Deviating from the American approach, a dryer construction, based on the design by the Russian Academy of Science in Moscow, was used for the first time in central Europe. This pilot plant with a maximum capacity of 10 m3 was erected at the site of the Wolf-Systembau company in Scharnstein, Austria. Its generator worked at a frequency of 13,57 MHz and a maximum high frequency voltage of 10 kV. The investigation focused on the drying behavior of beech, oak, spruce and white fir as well as on the heat transfer, water evaporation and drying stress development in the wood. The following, expected advantages of this drying methods could be validated: rapid and fairly uniform heating of the lumber piled without stickers; fast drying of the wood in just a few days without the development of significant case-hardening; and avoidance of various drying defects, including oxidative discoloration of the wood.