Principles and principles of wood drying
Fundamentals of Wood Drying
Through the above introduction, we already know that the moisture in wood is mainly composed of two kinds of moisture, that is, free water and absorbed water. The moisture existing in the large capillary system composed of cell lumens is called free water, and its increase or decrease only affects the weight of wood, but not the properties of wood; the moisture existing in the microcapillary system composed of cell walls is called absorbed water. Its changes not only cause the wood to shrink and expand, but also affect other physical and mechanical properties of the wood.
The purpose of wood drying is to get rid of free water and absorbed water in wood, so as to adapt to different uses and quality requirements. Although there are many ways to dry wood, the basic principle is the same, which is to use the moisture content gradient along the thickness of the wood and the large internal and external water vapor partial pressure difference formed after the wood is heated to promote the moisture in the wood. Moisture moves continuously from the inside to the outside in two forms of liquid and vapor, and evaporates outward through the wood surface; the internal moisture movement intensity should be coordinated with the surface moisture evaporation intensity, so that the wood dries evenly from the surface to the inside .
Taking steam drying in conventional drying as an example, its drying process is: first, the moist air with high temperature (below 100°C) and high humidity (saturated or nearly saturated) is continuously passed through the pile of wood in a circular flow, and the wood warm up. When the wood and its moisture are heated to a certain extent, according to the drying standard, the temperature and relative humidity of the medium are reduced to force the moisture in the wood to evaporate from the surface, which is the initial stage of drying. Then, according to the procedure specified in the drying standard, gradually increase the temperature of the medium and reduce the relative humidity, so that the evaporation surface of the water in the wood gradually moves to the inside until the end of drying. During the drying process, care should be taken to eliminate or reduce the occurrence of drying internal stress, cracking and deformation, without reducing the physical and mechanical properties of wood, so as to ensure the drying quality.
Basic principles of wood drying
The basic principle of wood drying is to increase the drying speed on the premise of ensuring the drying quality, save energy consumption and reduce drying costs. Drying quality means: the moisture content and drying uniformity of the wood to be dried must meet the requirements of the processing technology; the integrity of the wood must also be guaranteed, and no defects that are not allowed by the process specifications will occur, and the quality of the wood and its products must not be weakened. Some properties. The drying speed refers to the degree to which the moisture content of wood decreases within a unit time. The drying speed is fast, the less drying equipment and investment required, the higher the productivity and the lower the drying cost.
When selecting drying equipment in production, it should be based on the tree species, specifications, quantity, use and actual conditions of the production unit of the wood to be dried. The requirement for a modern wood drying kiln is that the temperature, humidity and airflow velocity of the drying medium can be guaranteed in terms of technology, and the external conditions of the stacked dried wood are basically the same, so as to achieve the purpose of uniform drying.
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