Maderas. Ciencia y tecnología 8(2): 67-82, 2006
ISSN online 0718-221X
1University of Natural Resources, Vienna, Austria.
Corresponding author: helmuth.resch@.boku.ac.at
Received: November 11, 2005. Accepted: January 23, 2006.
HIGH-FREQUENCY ELECTRIC CURRENT FOR DRYING OF WOOD –
In memoriam of Dr. Walter G. KAUMAN
Among the many interesting topics in the field of Wood Science and Technology is a fascinating
story about research and development on drying wood products with high-frequency electric current.
Historically, it can be traced back over decades.
Heat transfer to and evaporation of moisture from wood may be accomplished with high frequency
current depending on its dielectric properties. Because wood is generally heterogeneous, these properties
vary not only with the frequency of the current and the field orientation, but also with the moisture
content, temperature, and density of wood. Considering these parameters and the specific heat of the
material, estimates of power absorption can be made.
In an attempt to develop this technology, research covered many products from paper and veneer to
lumber and heavy timbers. Much emphasis, however, has been placed on wood species and/or products
with larger dimensions that are difficult or impossible to dry when using conventional drying methods.
The advantages of employing dielectric heating were found to be rapid and fairly uniform heat transfer
often to solidly stacked timbers, very high drying rates, and avoidance of various drying defects including
any significant case-hardening and oxidative discoloration of the wood.
During the last two decades, the development focused mainly on drying lumber in vacuum kilns
using dielectric heating, often termed high-frequency/vacuum drying. It has been justified economically
on the basis of increased throughput and higher quality. Existing industrial installations provide a positive
picture for higher value products. The economics should improve with advances