Praxis und Fortschritte der Holztrocknung

Summary

As an introduction, a brief review is given on scientific and practical problems in wood-drying reported on in literature during the past years. It shows that no striking innovations have occurred. In the field of fundamentals, the knowledge on the course of shrinkage, on the development of shrinkage stresses in the cellwall and in the drying board have been improved. For controlling wood-drying, a special method is being tested to demonstrate the state of maximum surface tensions in the board. Since the effect of the velocity of flow of the drying-medium could be determined at high temperatures as well, the corresponding economic air velocity can be chosen depending on wood moisture content and temperature. High drying-temperatures and long dryingperiods can cause a decrease in the shear strength of plywood. The destruction of insect larvae in wood is effected by correspondingly high temperatures of the wet bulb. Different pretreatment methods prior to drying can improve biological wood protection, prevent staining, reduce cross-sectional and surface checks and allow the application of more severe drying conditions. The problems of accelerated open-air drying and ils economy are being dealt with. When choosing the drying kiln, the effect of drying conditions and drying period on the type of construction and on the size of kiln is to be regarded. In contrast to timber-drying, in veneer-drying the diffusion resistances in the wood are small as against the resistance of the boundary layer of flow. Therefore, high air velocities or contact heat are economical. Special procedures of wooddrying are discussed brietly. Important drying problems arising in practice are pointed out.