First, external factors
Temperature is a major factor affecting the drying rate of wood. When the temperature rises, the water pressure in the wood increases, and the viscosity of the liquid free water decreases, which is conducive to promoting the flow and diffusion of moisture in the wood; the drying capacity of the copper wire drying medium is increased, and the evaporation rate of the surface moisture of the wood is accelerated. However, it is worth noting that if the temperature is too high, it will cause cracking and deformation of wood, reduce mechanical strength, discoloration, etc., and should be properly controlled.
Relative humidity is an important factor affecting the drying speed of wood. In the case of the same temperature and airflow speed, the higher the relative humidity, the greater the partial pressure of water vapor in the medium, the more the water on the surface of the wood is less likely to evaporate into the medium, the slower the drying speed; when the relative humidity is low, the surface moisture evaporates quickly. The surface water content is reduced, the water content gradient is increased, the water diffusion is increased, and the drying speed is fast. However, if the relative humidity is too low, cracking and drying defects such as honeycombs may occur or even worsen.
3, air circulation speed
Air circulation speed is another factor that affects the drying speed of wood. The high-speed airflow can destroy the saturated vapor boundary layer on the surface of the wood, thereby improving the heat transfer and mass transfer conditions between the medium and the wood, and accelerating the drying speed. For difficult dry materials or when the moisture content of wood is low, the moisture movement inside the wood determines the drying speed; it is not practical to increase the evaporation rate of surface moisture by increasing the flow rate of large medium, but it will increase the water content gradient and increase the drying. The danger of defects. Therefore, difficult materials do not require a large media circulation speed.
Second, the internal factors
1. Wood species and structural characteristics
The wood of different tree species has different structures, and the size and number of pits and the size of micropores on the pit membrane are very different. Therefore, the difficulty of water movement along the above path is different, that is, the wood species are affected. The main internal cause of drying speed. Due to the large number of fillings in the cavities and pits of the hard-boiled hardwood (such as rosewood) and the small diameter of the micropores in the perforated membrane, the drying speed is significantly smaller than that of the loose-leaved broad-leaved trees; in the same tree species, the density increases. The resistance of water flow in the large capillary tube increases, the water diffusion path in the cell wall is prolonged, and it is difficult to dry.
2, wood thickness
The conventional drying process of wood can be approximated as a one-dimensional heat and mass transfer process along the thickness direction of the material. The thickness increases, the heat and mass transfer distance becomes longer, the resistance increases, and the drying speed decreases significantly.
3, wood moisture content
Under the fiber saturation point, as the water content decreases, the lateral diffusion coefficient of the sorbed water decreases, while the diffusion coefficient of water vapor in the cell cavity increases, due to the proportion of water vapor diffusing in the cell cavity during drying. It is not large, and the lower the water content, the longer the water diffusion path, so the lower the water content, the more difficult it is to dry.
4, wood heart sapwood
The heartwood cells of broad-leaved trees have more inclusions, and the pits in the heartwood of conifers are mostly occluded, so the heartwood is more difficult to dry than the sapwood.
5, the direction of wood texture
Wood ray is good for moisture conduction, and the moisture conduction in the radial direction of the wood is about 15% to 20% larger than the chord direction. Therefore, the string cutting board usually has a faster drying speed than the diameter cutting board.