Wood can have various degrees of humidity. This is due to the fact that it has the ability to absorb water vapor from the air. This property of wood is called hygroscopicity.
Depending on the humidity level there are the following types of wood:
Wood drying allows to protect the wood from fungus and biological damage.
In the woodworking industry there are used different types of wood drying: atmospheric drying, contact drying, drying in liquids, induction drying, rotary drying and radiation drying.
The most ancient and the simplest way of wood drying is an atmospheric or natural drying. It is produced in the open air or under a tent. This type of drying allows to reduce the moisture content to 18-22%. The duration depends on the temperature and humidity, time of year, type and cross section of the raw material, the initial and final moisture content, the method of laying.
The advantages of air drying are cost savings for heating of air and raw materials and the simplicity of its implementation. Disadvantages are: the inability to dry the material moisture below the 18-22%; the long duration of the process (8-70 days); the inability to control the drying process (temperature, humidity).
Chamber drying is the most common method of drying wood. It is carried out in special equipment — drying chambers. The source of heat for drying chambers can be steam coming from the steam boiler, or flue gases derived from fuel combustion in special furnaces.
Chamber drying occurs regardless of external weather and climate conditions and it is characterized by a much shorter duration compared to atmospheric drying. The process of chamber drying can be controlled and regulated, and allows to dry the material to the required moisture content (below 18-20%). Other advantages are saving of time for the preparation of wood before processing and the reduction of production space.
Contact drying is carried out due to the heat transfer to the dried material, which is directly in contact with heated surfaces. This method is used in the drying of thin layers of wood and sawdust. The main advantage is that drying occurs within a few minutes. The disadvantage of this method is the relative darkening of the wood on the outside in case of long staying.
Drying of wood in liquids is used as an additional step before impregnation. Liquid drying agents are: hydrophobic liquids, i.e. liquids that do not dissolve in water (sulfur, paraffin, molten metals, oil) and aqueous solutions of mineral salts. This method allows to accelerate the drying process by almost 5-7 times in comparison with traditional chamber drying, and thus provides a high quality of dried wood.
The wood drying by different methods can lead to the appearance of cracks and timber warping. The main reason for their occurrence is that the wood dries in the direction from the periphery to the center.
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