Introduction: In a broad sense, “wood drying” refers to the forced evaporation and dissipation of moisture in wood by natural and artificial methods, so that the moisture content of wood can be reduced to the moisture content that meets the requirements of use, so as to avoid the erosion of insects and fungi, wood splitting, and deformation during the storage, transportation, and use of wet wood, improve the quality of wood products, and extend the service life.
Drying treatment
According to the classification of wood drying methods, there are two types summarized.
Manual drying
Manual drying is put the wood into a seal steam drying chamber and use the steam to promote water evaporation to dry the wood. (the drying time based on wood thickness and moisture content. For example 4cm board generally takes one to two week), the minimum water content of the wood only reach 8%, but high temperature evaporation,will make the wood becomes brittle, loses toughness, which is not good for furniture. Usually, the 30% of water content is enough for log.
Natural drying
Natural drying is also called air drying. It is place the wood in a outdoor place (board, talent or round wood) , and lay it in a pile. The bottom of the pile is about 60cm from the ground, and there is a gap in the middle to let the air circulate, take away the water, and the wood will gradually dry.
Air drying generally takes several years or months to finish the wood drying process. That is really slow and obviously can not meet the industry demand. and of course,the surrounding weather environment is highly dependent, and the weather is out of control.
Wood drying purpose
Avoid wood products deformation and cracks during use.
As all we know, wood volume will shrink during the water in the wood is discharged into the air, The cracks and deform happens If the shrinkage is uneven.
But If make the wood moisture content is suitable or close to the use environment humidity the volume and size of the wood products can be stable and durable so that the wood products with a long service life.
Prevent the wood products from decapitating, The wet wood is easy damaged by insects if put it outside for a long time. Mushroom may grows on the wet wood products.
But if make the moisture content of wood lower than 20%, it will reduce the damage by fungi and pests. That is why many furniture manufacturers dried the wood to 8-12%MC before making furniture.
The strength and corrosion resistance of wood will highly improve. Prevent the wood products from decapitating, The wet wood is easy damaged by insects if put it outside for a long time. Mushroom may grows on the wet wood products.
But if make the moisture content of wood lower than 20%, it will reduce the damage by fungi and pests. That is why many furniture manufacturers dried the wood to 8-12%MC before making furniture. The strength and corrosion resistance of wood will highly improve.
Reducing the weight of wood
Compare with the newly cut wood,the weight of wood will reduce more than 30-50% after dried That is conducive to improving the vehicle’s carrying capacity and save transport costs.
In short, dried wood can improve the quality of wood products, extend the service life and save wood. The importance of wood drying has already been reached to consensus in the woodworking industry. and become a specialized discipline in science.
Wood drying equipment
At present, there are many widely used drying equipment in the world. It can be divided into the following types according to the heating mode.
Traditional wood drying kiln, steam heating drying kiln, hot water heating wood drying kiln, furnace gas indirect heating wood drying kiln, dehumidifying wood dryer, vacuum wood dryer, air energy heat pump wood drying, high-frequency and microwave wood drying.
For wood with a thickness of more than 10cm, it must be dried by high frequency or microwave, because its drying principle is permeability drying, which is a drying process from inside to outside.
Wood drying process
For most heat conduction wood drying kilns, the principle is to simulate the natural environment suitable for wood drying. Manual control of temperature, humidity and wind power can achieve the purpose of wood drying. First, put the wood into the furnace and place it flat and not easy to deform; The plates are separated by the same size partition strips, which is convenient for ventilation and drying and improves the drying speed; During the drying process, the heat in the wood drying process is transferred to the wood through the generation of drying medium; At the same time, the water evaporated from the wood is taken away.
High-frequency and microwave drying is a method of using the principle of electric field to directly make water molecules in wood friction to generate heat. It is a direct heating method from the inside to the outside, combined with the characteristics of vacuum, which enables the wood to complete the drying process in a low temperature environment, so it has unique advantages for thicker wood and hard-dried wood.
How dry should the wood be
When wood is placed in a certain environment for a long enough time, its moisture content will tend to an equilibrium value, which is called the equilibrium moisture content of the environment.
When the moisture content of wood is higher than the equilibrium moisture content of the environment, the wood will drain moisture and shrink, and vice versa, it will absorb moisture and expand. For example, the average equilibrium water content of Southeast Asian countries is 15.1%, while that of Russia is 11.4%.
The wood dried to 11% is suitable for use in Russia, which can be used in Southeast Asian countries. It will absorb moisture and expand, causing deformation. Therefore, wood should be properly dried, not the drier the better. Different regions and different uses have different requirements for wood moisture content.
Moisture content measurement methods
Drying method and electrical measurement method are generally used in production. The drying method is a method to measure the moisture content difference of wood samples before and after drying.
This method has high accuracy, but is time-consuming and cumbersome, and is generally applicable to laboratories. According to the relationship between some electrical characteristics of wood and moisture content, the electrical measurement method is designed to measure the moisture content of wood directly with moisture content measuring instrument.
It is fast and convenient, and its accuracy is not as high as that of drying method, but it can meet the requirements of production technology, and is suitable for the application in the production of large quantities of wood and bamboo products.
