Up posture: I have to know the knowledge of wood drying!
Wood has its own temper like a human being. If it is too dry, it will lose its vitality. If it is not evenly dried, it will cause physical pain to the wood. So if the wood is to be properly dried, it will be alive and kick!
What does wood dry mean?
Wood drying generally refers to the process of removing wood moisture by evaporation or boiling under the action of heat.
In order to remove moisture from the wood, there must be a thermal energy in its surroundings, and this heat is generally a heat source that generates heat. At a certain temperature, the moisture in the wood is discharged into the air around it by evaporation or boiling, and the wood is dried. When the moisture in the wood drops to a certain level, we can use it to process and manufacture the products we need.
What are the factors that affect the drying of wood?
First, external factors
temperature
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.
humidity
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.
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
Wood species and structural features
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.
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.
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.
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.
Wood texture direction
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.
What is the purpose of drying wood?
First, prevent cracking and deformation of wood. When the moisture in the wood is removed from the air, especially when the moisture content of the wood is below the fiber saturation point of the wood, it causes a shrinkage of the volume of the wood. If the shrinkage is uneven, the wood will crack or deform. If the wood is dried to a level suitable for the environment to be used or the state required for use, the volumetric size of the wood can be kept relatively stable and durable.
Second, improve the mechanical strength of wood, improve the physical properties of wood and processing conditions. When the moisture content of the wood is below the fiber saturation point, the physical and mechanical strength of the wood increases with its decrease; at the same time, the wood is also easy to be sawed and planed, reducing the loss of woodworking machinery.
Third, prevent mold, decay and insects in the wood. When the moisture content in the wood is in the range of 20% to 150%, mold is easily generated, causing mildew, decay and insects in the wood. If the moisture content of the wood is dried below 20%, the conditions for mold production in the wood are destroyed, which enhances the ability of the wood to resist mildew, decay and insects, and maintains the original characteristics of the wood.
Fourth, reduce the weight of wood and improve transportation capacity. After drying, the weight of the wood can be reduced by 30% to 40%. If the wood is concentrated in the supply area of ​​the wood and the concentrated dry wood, the transportation capacity of the wood can be greatly improved; at the same time, the mildew and decay of the wood during transportation can be prevented, and the quality of the wood can be ensured. In short, wood drying is an important technical measure for rational use and saving of wood, and is an indispensable important process in wood processing and production. As a raw material, there are many fields of wood application, and most of them need to be dried. Therefore, wood drying has important practical significance for national economic construction.
Related reading: Graphical wood common sense furniture must know!
The wood is dry, the better it is, the better?
When the wood is placed in a certain environment, after a long enough time, the water 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 and contract, and vice versa. For example, the average annual equilibrium moisture content in Guangzhou is 15.1%, compared with 11.4% in Beijing. Drying to 11% of wood is suitable for Beijing, and it can be used in Guangzhou to absorb moisture and deform. Therefore, wood drying should be appropriate, not as dry as possible. Different areas and different uses have different requirements for wood moisture content.
The main reason for the cracking of solid wood products is that the moisture content of the wood is not balanced with the moisture content of the external environment. The equilibrium moisture content of wood varies with the region in which it is located, about 12% in the north of China, 18% in the south, and 15% in the Yangtze River.
What are the problems and solutions for uneven wood drying?
First, deformation warping:
1. The distance between the spacers is too large, or the thickness is inconsistent;
2. The spacers are not on a vertical line;
3. The temperature is too high, the humidity is too low, and the drying is uneven.
Solution:
1. Material accumulation should be carried out in a reasonable way, and the thickness of the spacers should be uniform;
2. Ensure that the spacers are to be up and down on a vertical line;
3. Control the temperature and humidity according to the operating procedure and make a balanced treatment.
Second, the surface crack:
1. The drying temperature is too high and the humidity is too low;
2. The internal stress of the material is not eliminated in time;
3. Uneven airflow makes the temperature in the kiln uneven;
4, the stress after drying is large, not eliminated;
5. Cracks in air-dried materials have not been treated and developed.
Solution:
1. Choose a softer drying benchmark;
2. Handle in time to eliminate stress;
3, check the fan and raft to achieve even ventilation;
4. Do an initial treatment;
5, do a good post-processing.
Third, the water content is not uniform:
1. The airflow speed is low and cannot be fully covered;
2. Improper accumulation of wood;
3. The heat dissipation of the heating tube is uneven.
Solution:
1. Check equipment, improve ventilation, and increase circulation fan;
2. Stacking rafts according to regulations;
3. Adjust and repair the heating system.
After reading the above knowledge, do you have your own opinions on wood drying?
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