What factors affect wood drying rate
I. External factors
Temperature is the main factor affecting wood drying speed. With the increase of temperature, the water pressure in wood increases and the viscosity of liquid free water decreases, which is conducive to promoting the flow and diffusion of water in wood. The moisture solubility of copper wire drying medium increases and the evaporation rate of water on wood surface accelerates. But it is worth noting that if the temperature is too high, it will cause wood cracking and deformation, reduce mechanical strength, discoloration and so on, which should be properly controlled.
Relative humidity is an important factor affecting wood drying speed. Under the same conditions of temperature and air velocity, the higher the relative humidity, the greater the partial pressure of water vapor in the medium, the more difficult the water on the wood surface evaporates into the medium, and the slower the drying speed is; when the relative humidity is low, the surface water evaporates faster, the surface water content decreases, the gradient of water content increases, the diffusion of water increases, and the drying speed is fast. However, low relative humidity will cause cracking and honeycomb drying defects, and even aggravate them.
3. Air circulation velocity
Air circulation rate is another factor affecting wood drying rate. High-speed air flow can destroy the saturated vapor boundary layer on the wood surface, thus improving the heat and mass transfer conditions between the medium and wood, and speeding up drying speed. For hard-to-dry wood or when the moisture content of wood is low, the moisture movement inside the wood determines the drying speed. It is not practical to speed up the evaporation rate of surface moisture by increasing the flow rate of large medium, but it will increase the gradient of moisture content and increase the risk of drying defects. Therefore, hard-to-dry wood does not need a large medium circulation speed.
II. Internal factors
1. Wood Species and Structural Characteristics
Wood of different tree species has different structure. The size and number of pits and the size of micropore on pit film are very different. Therefore, the difficulty of water moving along the above path is different, that is, wood species is the main internal factor affecting drying speed. The drying speed of annular hardwood (e.g. acid branch) is obviously lower than that of scattered hardwood because of its large fillings in ducts and pits and small diameter of micropore on pit membrane. In the same tree species, with the increase of density, the resistance of water flow in large capillaries increases, and the water diffusion path in cell wall prolongs, which makes it difficult to dry.
2. Wood thickness
The conventional wood drying process can be approximated as one-dimensional heat and mass transfer process along the thickness direction. With the increase of thickness, the distance of heat and mass transfer becomes longer, the resistance increases, and the drying speed decreases obviously.
3. Wood moisture content
Under the fiber saturation point, with the decrease of water content, the transverse diffusion coefficient of water absorption decreases, while the diffusion coefficient of water vapor in the cell chamber increases. Because the proportion of water vapor diffusion in the cell chamber is small during drying process, the lower the water content, the longer the water diffusion path, the more difficult it is to dry.
4. Wood heartwood
There are more inclusions in the heartwood cells of broadleaved trees, and most of the pits in the heartwood of coniferous trees are blocked, so the heartwood is harder to dry than sapwood.
5. Tree Texture Direction
Wood ray is beneficial to water conduction. Water conduction along wood radial direction is about 15%-20% larger than that along chord direction. Therefore, chord cutting board usually dries faster than radius cutting board.