There are three different stages in the wood drying process:

Release time: 2022-12-07

The first stage is to remove water from the surface of wood, which can be performed by dripping or other external mechanical forces (centrifugal force, pressure), but can only remove free water from porous wood. The other form is evaporation, where free water exists in wood cells similar to capillary systems
The second stage: Water moves from the inside of the wood to the outside, such as from the water containing region to the drying region. As the drying continues, the saturation point decreases, and the water moves from one conduit to another, evaporating through the opening. When drying the wood that is not soaked in water, the diffusion of water will occur at the beginning of the second stage. First, the water permeates, and then the water evaporates between the cell walls
Stage 3: When the moisture in the entire wood thickness is diffusing, the wood drying enters the third stage
The purpose of wood drying is to reduce moisture content and achieve the final required moisture content of wood in the shortest possible time. The drying process should be economical and use as little power consumption as possible
Drying speed
The drying rate is determined by the following factors:
1) Wood structure and density
2) Wood thickness (plate length, width)
3) Wood drying temperature
4) Drying moisture content range
5) Drying gradient, which is the ratio of wood moisture content to the equilibrium moisture content in the drying kiln
6) Air flow organization in the wood pile
7) Kiln structure and selected drying criteria
Effects of structure and density
Low density wood (i.e., low density of cellular structure) has a much smaller resistance to water movement than high density wood. In order to properly dry wood of the same density (such as central wood), it is necessary to adjust the drying time based on the density of the wood and the direct exposure to air
Effect of wood thickness
The thicker the board, the more attention should be paid when drying. The moisture content of the wood varies from the center to the outside. Of course, the length and width of the board also affect the drying time, as the edge surface of the wood (such as shorter wood blocks) will release a large amount of water
Effect of temperature
The impact of temperature is the most important factor in the movement of water in wood. As the temperature rises, drying becomes faster, as the movement of water in the wood from the inside to the outside surface accelerates. The higher the wood temperature, the faster the water flows (when the temperature is 80 ℃, the faster the water flow coefficient in the wood is 5 times that of 25 ℃), and the more water is released. As long as there is no adverse impact on future wood use, the temperature can be increased as much as possible
Impact of water content range
"It is a general rule that the longer the drying time of wood, the more moisture to be removed, but wood with a moisture content above the fiber saturation point takes much shorter time to remove the same amount of moisture than wood with a moisture content below the fiber saturation point."
Effect of drying gradient
"If the drying gradient is low, it is much more difficult for highly saturated air to absorb more moisture than for water to escape from the wood. Conversely, if the drying gradient is high, it is much easier for air to extract moisture from the wood surface. Therefore, appropriate drying gradients should be selected based on the wood type and thickness to ensure that there are no drying defects."