Implementation of the drying process


① Start-up of the drying room: close the intake and exh […]

① Start-up of the drying room: close the intake and exhaust ducts; start the fans. For a reversible circulation drying room with multiple fans, start the fans one by one to avoid overloading the circuit; open the valve of the steam trap bypass pipe and slowly turn on the heating to heat up the heating system slowly, and at the same time discharge the air, accumulated water and rust in the piping system. When a large amount of steam is ejected from the bypass pipe, close the valve of the bypass pipe and open the valve of the steam trap to make the water flow device work normally. . When the indoor dry bulb temperature rises to 40~50℃, keep it warm for 0.5h to preheat the inner wall of the drying room and the wood surface, then gradually open the valve of the heater, and spray steam properly to make the dry and wet bulb temperatures at the same time. Rise to the medium state required by preheating treatment. When entering the drying stage after the treatment, the inlet and outlet pipes need to be opened. Then operate according to the process requirements.
②Preheat sawn timber. After the wood drying room is started, the wood is preheated first. The purpose of heat treatment is to heat the wood, and the wet wood is heated through, so that the direction of the moisture content gradient and the temperature gradient are consistent, and the growth stress of the wood is eliminated. For semi-dry wood and air-dry wood, it also has the effect of eliminating surface stress. For green wood And wet wood, preheating can make the wood with high moisture content evaporate part of the water, so that the moisture content tends to be consistent. At the same time, preheating can also reduce the viscosity of moisture at the fiber saturation point, expand the capillary on the wood surface, and increase the speed of moisture movement on the wood surface.
Preheating temperature: 8-10°C higher than the first stage;
Preheating humidity: When the initial moisture content of sawn timber is above 25%, the relative humidity is 98%-100%; when the moisture content is below 25%, the relative humidity is 90%-92%.
Preheating time: 1-1.5h/cm (thickness) in summer; 1.5-2h/cm (thickness) in winter. And from the preheating time to the stage of equivalent moisture content of the drying reference, the time should not be less than 2 hours. The energy consumption in the preheating stage is 1.5-2 times that of the drying stage, so preheating cannot be performed at the same time as drying.
③Adjust the temperature and humidity of the drying room.
After the wood is preheated, it enters the drying stage. The drying stage is strictly carried out according to the drying standard, and the temperature is not allowed to increase sharply and the humidity is reduced.
Pay attention during the drying operation:
——Closing the inlet and exhaust ports can increase the temperature of the medium, and the temperature of the medium can be increased while spraying steam;
——The steam supply pressure of the drying chamber is 0.4Mpa, and it must be stable;
——The control accuracy of dry bulb temperature is ±2°C, and the temperature difference between dry bulb and wet bulb is ±1°C.
——The dry bulb temperature is controlled by the heater valve, and the wet bulb temperature is controlled by the steam spray pipe valve and the inlet and outlet channels;
——In the drying stage, do not spray steam when heating; do not heat when spraying steam, close the intake and exhaust passages; do not heat when the intake and exhaust passages are open.
——For the periodical reversible circulation wood drying room, it is necessary to periodically (4h or 8h) change the direction of the fan to change the direction of the circulating airflow and improve the uniformity of wood drying.
——If the dry bulb temperature and the wet bulb temperature are difficult to reach the value required by the benchmark for a while, the dry bulb temperature should be adjusted first, and then the difference between the dry bulb temperature and the wet bulb temperature should be adjusted.
——In case of power failure or shutdown for some reason, stop heating and steaming immediately, and close the intake and exhaust passages.
——In the drying room with wet and dry bulb thermometers, the gauze should be replaced every time and a certain water level should be maintained.
——For the automatic control drying room, in addition to setting the input parameters correctly, it is also necessary to check the readings of each moisture content measurement point frequently. If there are abnormal readings, they should be canceled immediately.
④ Mid-term treatment.
When the moisture content on the surface of the wood reaches the fiber saturation point, the moisture content inside the wood is much higher than the fiber saturation point. The harder the dry base, the more prominent the phenomenon, and the greater the possibility of wood cracking. Therefore, in the actual drying process, it is necessary to carry out mid-term treatment in time according to the dry state of the wood, that is, to spray and steam the wood to stop the evaporation of water on the surface of the wood, and even to absorb a little moisture, so that the water inside the wood moves to the surface of the wood. Thereby reducing stress in the wood.
The number of mid-term treatments is determined according to the wood species, thickness and wood use (that is, the requirements for dry quality) and the existing stress. The whole process of mid-term treatment requires a stress test board to test the effect of the treatment, and the effect of the treatment can be judged from the tooth shape of the stress test board.
⑤ Finish processing.
The final treatment is to eliminate the uneven distribution of moisture content on the cross-section of the wood and eliminate the residual stress. Sawn timber requiring dry quality grades 1, 2 and 3 must be finished.
 Conditions of drying media during processing:
Temperature: 8-10°C higher than the temperature in the final stage of the drying benchmark;
Relative humidity: the relative humidity of the equilibrium moisture content that is 3%-4% higher than the final moisture content;
Time: Roughly calculated by spraying and steaming for 1 hour and maintaining for 1 hour for every 1 cm thick sawn timber.