There are two kinds of moisture in wood: free water and absorbed water. The water in the large capillary system consisting of cell walls is called free water, and its increase or decrease only affects the weight of wood, but does not affect the properties of wood. Other physical and mechanical properties of wood.
Wood drying is to exclude free water from wood and absorb water to adapt to different uses and quality requirements. Although there are many methods of drying wood, the basic principles are common, that is, the use of moisture gradients along the thickness of the wood, as well as the formation of internal and external small vapor pressure difference after heating, to promote water in liquid and vapor form from the internal to external continuous movement, and through the surface of the wood outward. Boundary evaporation; internal moisture movement intensity should be consistent with the surface moisture evaporation intensity, so that the wood from the surface to the interior of a balanced drying. The traditional steam drying chamber, its drying process is: first of all, the high temperature (below 100 degrees centigrade) and high temperature (saturated or nearly saturated) humid air in the circulating flow through the stack, the wood preheating.
When the wood and its moisture are heated to a certain extent, the temperature and relative humidity of the medium are lowered according to the drying criteria, forcing moisture in the wood to evaporate from the surface, which is the beginning of drying. Then, in accordance with the drying standard procedures, gradually increase the temperature of the medium and reduce the relative humidity, so that the evaporation surface of moisture in the wood gradually moved to the interior until the end of drying. In the drying process, internal stress, cracking and deformation should be eliminated or reduced, and the physical and mechanical properties of wood should not be reduced to ensure the drying quality.
