Wood is dried to remove a certain percentage of moisture from it. It is carried out in order to ensure stable linear dimensions of the material. This is very important in construction and some other areas. Note that wood drying consists of two processes: moisture transfer and moisture exchange. The first is the movement of water inside the tree, the second is its evaporation into the environment. In order to get the desired result, the same intensity of processes is needed. Otherwise, internal stress may occur, which will eventually cause deformations and cracks.
The main problem that wood drying technology should solve is the acceleration of moisture transfer from the inner layers to the outer ones. For moisture removal, there are many different devices. The movement of water inside the tree is influenced by the following factors: humidity, temperature, pressure. To be more precise, their different values at different points in the material.
Wood drying should be carried out using equipment that can provide the solution to the following problems: minimize cracks and deformations, minimize energy consumption and time. One of the technologies used for this process is drying in special chambers. Vacuum drying of wood consists of preparing a stack of moist forest, its preliminary heating, the main process of water removal, intermediate and final moisture heat treatment and conditioning.
Depending on the further purpose of using the material, four categories of wood processing are provided: I, II, III, 0. The highest quality is wood drying of the first category of quality. It is produced for lumber, which is later used for the manufacture of things, which are subject to increased operational requirements. Category zero involves the removal of moisture to a level that allows the transportation of material both for export and for domestic use.
Drying quality is determined by several parameters. Among them are the main ones: the average moisture of the tree in the stack and its deviation from a certain target value, the residual stress in the processed lumber, the comparison of the moisture content in individual boards with the indicator as a whole for a uniformly folded batch. In addition, the differences in the percentage of liquid content over the thickness are important.
Not always the drying process can achieve the desired parameters, with any deviations, various defects can occur. The main ones are: mold, sinks, lack of stacks of the material as a whole, strong warpage of the boards, uneven removal of moisture along the thickness of the assortment or the volume of the evenly batch of wood, collapse, cracking. The causes of defects are often: non-compliance with the technology, poor control over the process, uneven heating, distribution and stagnant air circulation, poor stacking of the batch of material, excessive drying force, wood species (in this case, additional preparation is required).