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Wood drying usually refers to the process of reducing the moisture content of wood by evaporation or other means under the action of heat energy.
To remove moisture from wood, heat must be present in its surroundings. At a certain temperature, the moisture in the wood evaporates or boils into the surrounding air, drying the wood. Once the moisture content of the wood has dropped to a certain level, it can be processed and manufactured into desired products.
Intrinsic factors affecting wood drying
In addition to external temperature, humidity, and air circulation speed, the internal factors that affect wood drying are mainly the following:
Wood species: Wood of different species has different structures. The size and number of its pits, as well as the size of the micropores on the pit membrane vary greatly. Therefore, the difficulty of water moving along the above-mentioned path varies. In other words, the wood species is the main internal factor affecting the drying speed.
Wood thickness: The conventional wood drying process can be approximately regarded as a one-dimensional heat transfer process along the thickness direction of the wood. As the thickness increases, the heat transfer distance becomes longer, the resistance increases, and the drying speed decreases significantly.
Heartwood and sapwood: The heartwood cells of broad-leaved trees contain more contents, while the pores in the heartwood of coniferous trees are mostly closed, so the heartwood is more difficult to dry than the sapwood.
Wood grain direction: Wood rays are conducive to moisture conduction, and the moisture conduction speed along the axis of the wood is faster. Therefore, tangentially-sawn boards usually dry faster than quarter-sawn boards.
What is the purpose of wood drying?
First, prevent wood from cracking and warping: As moisture in wood is lost to the air, especially when the moisture content falls below the wood's fiber saturation point, it causes the wood to shrink. If this shrinkage is uneven, the wood will crack or warp. Drying wood to a degree suitable for its intended use or to the desired condition will maintain a relatively stable size and ensure long-lasting durability.
Second, it improves the mechanical strength of wood, and improves the physical properties and processing conditions of wood: when the moisture content of wood is below the fiber saturation point, the physical and mechanical strength of wood will increase as it decreases; at the same time, wood is also easy to saw and plan, reducing losses to woodworking machinery.
Third, it prevents wood from mildew, decay, and insect infestation: When the moisture content of wood is between 20% and 150%, mold is very likely to form, causing the wood to mildew, decay, and beetle infestation. If the moisture content of wood is dried to below 20%, the conditions for mold growth are destroyed, thereby enhancing the wood's resistance to mildew, decay, and insect infestation, and preserving its original properties.
Fourth, reducing wood weight and improving transportability: After drying, wood weight can be reduced by 30% to 40%. If lumber is harvested and dried centrally in the wood supply area, transportability can be greatly improved.
Is it better to dry wood the drier it is?
When wood is placed in a certain environment for a sufficient period of 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 the wood is higher than the equilibrium moisture content of the environment, the wood will shrink due to moisture loss, and will expand due to moisture absorption.
For example, the average annual equilibrium moisture content in Guangzhou is 15.1%, while in Beijing it's 11.4%. Wood dried to 11% is suitable for use in Beijing, but will absorb moisture and swell in Guangzhou, leading to deformation. Therefore, proper drying of wood is crucial, not just drier. Different regions and applications require different moisture content levels.