High Temperature Drying . . . Research, Application, and Experience in Germany

In 1949 curves for the equilibrium moisture content of wood in a pure superheated steam atmosphere were first developed. Because the equilibrium moisture content of wood falls very quickly with the temperature in superheated steam, the drying process can be regulated by regulating the temperature only. In comparison to kilns with steam-air mixtures below 212?F, the control becomes relatively simple. Superheated steam driers offer several advantages they are simple to operate and can be easily automated; the drying period is up to one-third shorter; they are movable, and their double-walled construction provides optimal insulation; the hygroscopicity of the wood is favorably affected, and slits and checks can be avoided. The process also has disadvantages: the wood develops a harmless yellow stain with increase in temperature and the surface resin crystals which appear may blunt machine knives; loose knots drop out, and the presence of acid necessitates use of non-corrosive finishes in the drier. High temperature driers are double-walled housings with vapor-proof inner sheet mantles, and corrosive resistant linings. Uniform distribution of temperature and flow speed in the kiln are important. High temperature driers are now widely used. They can use either pure superheated steam or a mixture of superheated steam and air. Above the fiber saturation point the wood temperature rarely exceeds the temperature of the wet-bulb thermometer. Since the moisture conductivity and the diffusion resistance coefficient increase with temperature, drying with superheated steam is faster than drying with a steam-air mixture.

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