Lumber Convective Drying Based on Controlled Moisture Transfer

Abstract

Wood drying (in particular lumber drying), which almost entirely determines the quality of the wooden products, is one of the most complicated and energy-consuming processes in wood processing. Convective drying of lumber, in all its varieties, remains the most common by far. Computer modeling of wood drying processes is usually based on solving systems of differential heat and mass transfer equations. The methods for solving such systems, both analytical and numerical, are well researched and developed. However, the most important methodological issue is the correct formulation of the boundary conditions that determine the process of interrelated heat and mass transfer at the interface (wood – moist air). The convective drying has traditionally used the boundary conditions of type III by Academician A.V. Lykov, which are characterized by a sufficiently close correspondence between the mass flows moving from the wood depth and at the interface. This correspondence is described by the value of the so-called Bio mass transfer criterion. A computational experiment was carried out to verify these assumptions. It enabled to determine the possible controllability of the moisture removal during low-temperature convective drying of conventional lumber using modes of three-step and stepless structure. Besides this, the influence of the moisture removal on the dynamics of the internal stresses in wood was also studied. The results have shown that the stepless drying modes, on the one hand, offer a significant improvement in drying quality with practically no loss of drying capacity, and on the other hand, have better controllability. The results of the previously conducted and mentioned studies made it possible to propose general principles and then to patent a system for automatic control of moisture transfer during wood drying, which is fundamentally different from the previously known wood drying control systems. The system controls the drying process by adjusting the ratio of external to internal moisture transfer, rather than the value of the media parameters in the chamber. The moisture transfer control system for convective drying of lumber is of limited control: it cannot fully stabilize the values of the Bio mass transfer criterion. However, its use maintains a certain balance between internal and external moisture transfer, ensures the required drying quality and almost completely eliminates the occurrence of defects. For citation: Gorokhovsky A.G., Shishkina E.E., Agafonov A.S. Lumber Convective Drying Based on Controlled Moisture Transfer. Lesnoy Zhurnal [Russian Forestry Journal], 2022, no. 1, pp. 166–172. DOI: 10.37482/0536-1036-2022-1-166-172