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Wood drying is known to be a complicated heat and mass transfer process. Although many scientific publications, textbooks, manuals and monographs are devoted to many processes of drying veneer, chopped wood and lumber, convective drying of profile workpieces is not sufficiently investigated yet in literature sources, because profile workpieces such as shoe pads, profile elements of joiner's and furniture and furniture products and firewood have an alternating cross-section, which also varies along the length of the workpieces. Therefore, the process of wood drying, including profile billets (wood) from massive wood is considered. It consists of such thermal technological operations as initial heating, steaming and drying, and intermediate heat-treatment and conditioning in the process of drying of firewood are not considered. The regularities of the external and internal movement of heat and moisture in the processes of heating and drying of wood are studied. As a result, the basic equation of the kinetics of the drying process is defined to establishe the relationship between heat exchange and moisture exchange in the form of the ratio of the amount of heat consumed to heating the wood and the evaporation of moisture and the drying rate. The differential equation of convective diffusion together with the equation of mass transfer of the stationary process of moisture transfer due to the difference in the concentration of the diffusion substance (in this case, the moisture) and the equation of the first Fick law, which allowed deriving the values of the mass transfer criteria of Nusselt, Fourier, Peckel and Prandtl, were used. The authors have also determined that the necessary prerequisite for the similarity of the processes of mass deducing is the observance of aerodynamic similarity, which is described by the Reynold and Froude criteria. Finally, we have synthesized on the basis of the above-described physical-mathematical model of the drying process in the form of dependencies between the Nusselt criterion and the Fourier, Reynold and geometric similarity criteria. The obtained dependence can be described by all thermal process operations of the drying process.

The authors have considered some issues of influence of chemical and physical-chemical bonding of moisture with wood on the processes of moistening and drying of wood. We have found that the most complex physical and chemical phenomena occur in wood when changing its initial moisture content, and the subsequent saturation of the capillaries of wood walls with water leads to the formation of bound moisture. Wood has a complex chemical composition and structure, and water has a specific structure. In the process of interaction, the properties of both components change: the wood substance passes from the glassy to the colloidal state, and the water from the free state to bound state. In addition, the laws of capillary phenomena, hydrostatics and hydrodynamics cannot be applied to the bound moisture in colloidal materials because it is in adsorption with wood. The process of binding moisture (fluid) to a colloidal body at the first stage is similar to mixing of two fluids of different molecular weights. During the absorption of liquid by a colloidal body, a certain amount of heat (heat of swelling) is released, similarly to the heat release when dissolving alcohol or sulphuric acid in water. Using the law of conservation of energy and mass, one can establish the relationship of mean integral values of moisture content and temperature with the intensity of heat-and-mass transfer, and, accordingly, with the rate of drying in the form of a heat balance. According to the law of energy conservation, the amount of heat that is supplied to the body is equal to the amount of heat that goes to the evaporation of moisture plus the heat going to heat the body itself. Removal of the bonded moisture occurs during the deceleration of drying. The intensity of drying and the intensity of heat transfer continuously decrease over time. The change in the intensity of drying occurs in a complicated fashion, which is determined by the form of moisture bonding with the material and by the mechanism of movement of heat and moisture within the material. Thus, the main solution of kinetics is derived – the rate of the process of drying bound moisture, which can be applied to any wood material and any method of drying. The resolution of the issue of the influence of the forms of moisture bonding with wood on the technological processes depends on two aspects: the complexity of chemical structure of the wood material and the complex unique properties of water in the wood.