This monograph aims at providing understanding of the many ways to employ special drying methods using high frequency electric currents for wood and wood products. It offers a broad coverage of research and development over many decades from the original concept to industrial applications.
The physical process of dielectric heating has advantages of rapid and relatively uniform heat transfer, resulting in high drying rates and avoidance of various drying defects, including any significant casehardening and oxidative discoloration of wood. Heat transfer to and evaporation of moisture from wood is shown in dependence on the dielectric properties of materials. Because wood is 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, the selection of a frequency can be made for heating specific products and the power absorption and feasibility of a system can be estimated.
The text traces the historical roots of using high frequency current for heating and drying of wood. Principles of and formulas describing dielectric heating and the dielectric properties of wood give a background useful for understanding parameters of specific applications, energy transfer, and power consumption. Early equipment development for continuous and batch dryers using radio frequencies and microwaves is reported from work in Russia, USA, Canada, Japan, and Europe. The greatest emphasis is placed on the method of combining radio frequency heating with vacuum drying.
Research during the last decades lead to industrial installations designed mainly for drying of lumber and timbers in vacuum kilns. This development provided a positive picture for higher value products. On the other hand, the continuation of research with microwaves promises to make use of their unique properties. Some ideas are advanced on achieving rapid and economic drying by combining other heat transfer methods with high frequency heating along production lines. Finally, the present understanding of the drying mechanism, the technical feasibility, and economics are considered.
A broad listing of relevant literature provides the reader with a multitude of references. Data are presented in international and American units.