Application of Dielectric Heating to the Seasoning of Wood

Placing an insulating material such as wood in a rapidly alternating electric field causes molecular friction which generates heat within the material. The temperature rise within the wood is not dependent on its thermal conductivity. Dielectric heating thus provides an ideal theoretical method of drying wood, from the inside out, with a minimum of degrade. The “boiling technique” involves applying a high power concentration, 15 to 20 watts of radio-frequency power per cubic inch of wood; the moisture is driven out as steam as the internal heat creates a vapor; pressure which accelerates the movement of moisture from the center to the surface. The internal temperature does not rise much above 212?F., and the surface of the wood remains cool and moist until the drying is almost completed. Though this technique is successful with small pieces of wood, in larger pieces having a cross-sectional dimension of 2 inches or more, the movement of the moisture through the wood is slower, and the build up of vapor pressure within the wood may cause it to crack or explode. With soft- woods, overheating may take place at the center and near knots and pitch pockets, causing burning. To control these problems, the “temperature gradient method” keeps the temperature within the wood below 212?F.; however, this results in increased seasoning defects, because as the moisture on the surface is evaporated faster than the moisture from the center can move out to replace it, the surface becomes excessively dry. Tests at the Forest Products Laboratories of Canada have shown that this excessive surface drying can be retarded by surrounding the drying set-up with a low temperature and a high relative humidity. This work has confirmed that radio-frequency power selectively heats the portions of the wood with a high moisture content, thus adjusting the drying rate to produce uniform moisture content. Tests using beech specimens have revealed that checking occurs until the moisture content drops below 30 percent. Drying green or very wet impervious hardwoods by dielectric heating appears impractical, because the drying rate is limited to the rate at which free moisture can diffuse through the wood. However, once the moisture content falls below the fiber saturation point, the tendency to excessive surface drying can be controlled by high surrounding relative humidity. Radio frequency heating may thus provide a practical method for final seasoning of high-grade hardwoods, when the amount of moisture to be evaporated is relatively small.

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