Wood drying refers to the natural and artificial method of forcing the evaporation of water in the wood to reduce the moisture content of the wood to the moisture content of the use requirements, to avoid the erosion of the wet materials during the storage, transportation and use. Wood splitting and deformation, improve the quality of wood products and extend the service life. One: drying treatment 1. Artificial drying: The wood is sealed in a steam drying chamber, and steam is used to promote evaporation of water to make the wood dry. (According to the size and thickness of the wood, such as 4cm plate drying time generally takes one week), the highest degree of drying can make the wood moisture content only 3%, but after the high temperature evaporation, the wood becomes brittle and loses toughness and is easily damaged. Conducive to carving furniture. Usually the degree of drying of the logs should be maintained at about 30% water content. 2, natural drying: the wood is placed in a ventilated place (plate, square or log), placed in a raft, the bottom of the raft is about 60cm from the ground, leaving a gap in the middle, so that the air circulates, take away the moisture, and the wood gradually dry. Natural drying usually takes several years or months to achieve certain drying requirements. Second, the purpose of wood drying 1. Prevent deformation and cracking of parts. When the moisture in the wood is removed into the air, 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. 2. Prevent deterioration and decay of wood and prolong the service life of wood products. If wet wood is piled up in the open air for a long time, it will often cause decay or insect damage if appropriate measures are not taken. When the moisture content of wood is reduced to less than 20%, the damage and destruction of fungi and pests can be greatly reduced. Therefore, generally in the production unit, the wood is dried to a moisture content of about 8-15%. This not only ensures the inherent properties and strength of the wood, but also improves the corrosion resistance of the wood. 3. Improve the mechanical strength of wood and wood products and improve the physical properties of wood. When the moisture content of the wood is below the fiber saturation point, the physical and mechanical strength of the wood increases with its reduction; at the same time, the wood is also easy to be sawed and planed, reducing the loss of woodworking machinery. 4. Reducing the weight of the wood is conducive to improving the carrying capacity of the vehicle. The newly cut wood has a water content that exceeds its own weight. After short-term storage and natural drying, its water content is still high. After the wood is dried through the chamber, its weight can be reduced by about 30-50%, which is beneficial to improve the carrying capacity of the vehicle. In short, dried wood can guarantee the quality of wood products, improve the performance of wood and extend the service life, thus saving wood. Practice over the years has proven that wood drying is an indispensable process in production and has become a specialized discipline in science. Three: wood drying equipment According to the principle of wood drying, the company uses electric heating to humidify and humidify. It has been carried out in four stages: preheating stage, drying dehumidification stage, intermediate processing drying stage and final treatment stage. The drying process is the drying process from the surface to the inside. For wood with a thickness of more than 10CM, it must be treated by strong microwave drying. The drying principle is osmotic drying, which is a drying process from the inside to the outside. Four: wood drying process Firstly, the wood is put into the furnace, and it is required to be placed flat and not easily deformed; The plates are separated by spacers of uniform size to facilitate ventilation and drying, and to increase the drying speed; During the drying process, the heat in the drying process of the wood is transferred to the wood by the drying medium; at the same time, the water evaporated from the wood is taken away. Five: How much should the wood be dried? 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. Six: method of moisture content measurement Drying and electrical testing are generally used in production. The drying method is a method for measuring the moisture content before and after drying (dry) of the wood test piece, and the method has high precision, but it is time-consuming and cumbersome, and is generally suitable for use in a laboratory. The electrical measurement method is based on the relationship between certain electrical properties of wood and water content. It is designed as a method for directly measuring the moisture content of wood by water content measuring instrument. It is fast and convenient, and the precision is not as good as the drying method, but it can meet the requirements of production process and is suitable for large The application of bulk wood bamboo products and other aspects. 1) Drying method measurement (dry test method) First, a representative moisture content test piece of about 20 mm X 20 mm X 20 mm size was sawn in the wood to be tested. The so-called representativeness is that the dry and wet degree of this test piece is consistent with the whole piece of wood, and there are no defects such as skin, thrift, decay, and insects. Generally, it should be taken at a distance of 250~300mm from the end of the sawing material. After scraping the burr and sawdust from the moisture content test piece, the mass should be immediately weighed on a balance with an accuracy of 0.01 g and a range of not less than 200 g. The mass is recorded as M, and then the test piece is placed at a temperature of 103 ± Bake in a 2 ° C incubator for about 6 h, then take out the weighed mass and record it, then put it back into the oven to continue drying. It is then weighed every 2 hours until the mass of the last two weighings is constant, which is the absolute dry mass, recorded as Mo. Thus, the water content can be calculated by the following formula: W = (M - Mo) / Mo × 100% Note: Because the thin test piece is exposed to the air, its moisture is easy to change. Therefore, the measurement should be done immediately after the test piece is taken or after the oven is taken out. If the quality cannot be called immediately, it must be packed in a plastic bag immediately to prevent it. Water evaporation. 2) Electrical measurement The electrical measurement method is generally based on a DC resistance type (pin type) and an alternating current type (inductive type). The pin type wood moisture meter is a probe inserted into the inner layer of wood to measure the electrical resistance between the two electrodes. This measuring instrument is better for determining the moisture content of wood and wood-based panels. Pin type notes: a) The temperature of the measured object is 0~40°C, and the relative humidity of the environment is <85%. b) The gear should be adjusted according to the type of wood and the table should be adjusted to “zeroâ€. c) The measuring point should generally be longer than 200mm from the end of the sawn timber d) Insert the measuring instrument into the wood about 6 mm deep e) Generally, the horizontal grain direction display value should be small, and the detection should generally be based on the horizontal grain direction. The inductive wood moisture meter is designed according to the principle that the electromagnetic wave acts on the polarization of water molecules, which affects the charge and discharge voltage of the probe equivalent capacitance of wood and air. It is corrected by probe, display, indicator light and tree species. The dial, the start switch and the internal digital circuit can measure the moisture content of various woods. The probe is placed close to the surface of the wood during measurement. This instrument is quick to measure and easy to operate, because it does not damage the surface of the product during measurement. It can be used for the determination of finished products, but its measurement accuracy is poor. Inductive notes: f) The temperature of the measured object is 0～40°C, and the relative humidity of the environment is <85%. g) When measuring, adjust the type according to the type of wood and the thickness of the test object (refer to the instruction manual) h) The measuring point should generally be longer than 200mm from the end of the sawn timber i) Because the induction test is penetrating, the test object is required to be suspended so as not to affect its true value. Seven: sampling method of wood moisture content When measuring, it should be carried out at the sampling site (or a region). Take the test piece 100mm or more from the ground as the test site. Select any three different parts, and select each of the three points for each part. For the range of selection points mentioned above, the average values ​​measured on the three parts were respectively determined, and the largest average value was used as the measured value of the wood moisture content of the test piece. When the battery is low, it has an effect on the test results. When the battery voltage is less than 70~80% of the rated voltage, the measured value will become smaller. If the measured value is 14% at the rated voltage, when the voltage is less than 70% of the rated voltage, the same object is detected, indicating that the battery is low. At this time, although the meter can still work, the displayed reading is inaccurate and needs to be replaced. The battery can guarantee the accuracy of the measurement. Eight: annual average moisture balance of timber in major cities of China Nine: annual average moisture balance of wood around the world
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