1 Preparation before drying
First of all, the operator must understand the basic knowledge of drying the floor blanks and the general knowledge of drying, drying characteristics and drying operation points. And prepare a certain length, width, thickness of the floor blanks, size can be determined according to the different specifications of the species, such as the length, width and thickness of beech: 960,800,660 × 115 × 25mm; Betula virginiana, Pentagon maple can be: 960 , 800, 660 × 108 × 24mm; cone chestnut wood: 950,800,650 × 108 × 24mm and so on, made of blank material quality requirements of the board surface, length, width, thickness and geometric requirements of the same. After preparation of 25×25mm small fir or Komatsu side, it is required to be uniform in width and thickness, smooth, not twisted, rot, and deformed. The amount is determined according to the floor blank to be dried, and the dry kiln is inspected strictly according to requirements. All facilities, including power supplies, wiring, motors, and blowers, are functioning properly and the dry and wet thermometers are intact.
2 floor blanks into the kiln
The key to entering the kiln floor blank is whether the blank can be stacked properly. The stacking quality directly affects the output rate of the bad slab, so the stacking must be done correctly as required. The commonly used stacking method is to stack wood, and the blanks are classified according to different wood species, thickness, quality, length and moisture content before stacking. Floor blanks or closest blanks of the same nature are stacked together for simultaneous drying. The spacing between the blanks depends on the thickness of the sheet. For floor blanks, small wooden square strips of 25×25 mm are usually used. The interval between stacking codes is 200-250 mm for thin plates and 300-350 mm for thick plates. Each row of square strips must be purchased as a straight line and perpendicular to the ground. Both sides of the billet must be flush with the billet. Stacked into a square, both sides should be flat and perpendicular to the ground. After the stacking of the blank is completed, the top must be pressed with a heavy object, otherwise deformation may occur. In general, a weight of 250-500 kg/m2 is applied at the top of the stack, which can effectively prevent warpage. Weights can be made of special cement blocks or cast iron.
3 Floor Blank Drying
The drying process of the floor blanks can be divided into four stages, namely preheating, drying, conditioning and cooling.
(1) Preheating of Floor Blanks Floor blanks are stacked in the correct way and then placed in a drying kiln. Due to the different wood densities, the moisture in the wood moves at different rates and the drying time varies. The drying step is to first fully preheat the blank. Preheating requires that the temperature in the kiln reach 55-80°C, the heating time is 6-8 hours, and the temperature can be raised quickly before 45°C. When the kiln temperature reaches 45°C, the heating rate can be appropriately slowed, that is, the temperature is increased by 2 to 3°C per hour. At this stage, the temperature rise and steam injection must be performed at the same time. When steam is injected, the two valves of the steam leakage valve and the steam injection valve are first required to be closed. When there is a certain pressure, the steam injection valve is opened to spray steam having a certain humidity into the drying kiln to increase the humidity in the kiln.
The kiln requires a dry-wet temperature difference of 1 to 115°C, so that the relative humidity of the air is controlled at 90% to 95%. The warm-up time refers to the time required to reach the specified number of times from the kiln temperature. The operation characteristics of this stage vary according to the season and the thickness of the sheet varies. Under normal circumstances, each 1cm thick material preheating in summer takes about 1 ~ 115h, floor blanks need about 215 ~ 4h. Winter 1cm thick material is about half an hour than summer, that is 115 ~ 2h. Floor blanks need to keep warm 6 ~ 8h At this stage, the changes in the wood should be carefully observed in the kiln every half-day. Everything can be done in accordance with the above requirements. If there is a slight change, we must adjust the temperature and humidity, reduce the temperature 3 ~ 4 °C, increase the relative humidity.
(2) The drying stage of the floor blank When the central part of the wood reaches the predetermined temperature, it is generally required to control the temperature in the kiln at 55-60° C. This stage is the free evaporation time, which indicates that the blank has entered the constant velocity drying stage. Both moisture gradient and temperature gradient are in the same direction, which can promote the smooth flow of wood moisture from the inside to the outside. At this time, the humidity of the air should be reduced, and the difference between wet bulbs should be gradually improved. The dryness and humidity in the kiln should be adjusted according to the drying reference table. There are three aspects to consider.
First, when the wood moisture content reaches the fiber saturation point, ie, the moisture content is about 25% (depending on the wood fiber saturation point of different wood species, generally 25% to 30%, the cell wall of the wood no longer absorbs water. Through practice, it was found that the temperature difference between dry and wet bulbs in the kiln should be controlled at about 2°C, and the relative humidity should reach 90%.
Second, when the wood moisture content reaches the fiber saturation point, ie the moisture content is about 25%, the air humidity must be reduced slightly. The temperature difference between dry and wet bulbs can be controlled at 3 to 4°C, and the relative humidity can be reduced to 87% to 83%.
Thirdly, when the moisture content of wood is lower than the fiber saturation point, it can be known from the moisture meter that the temperature difference between the wet and dry bulbs in the kiln should be increased and the relative humidity gradually reduced. Finally, the wood is dried to the required moisture content.
(3) Moisture-conditioning phase of the floor blank When the moisture content in the center or 1/3 thickness of the wood reaches the final moisture content of about 12%, the temperature in the drying kiln is reduced, and the temperature difference between the dry and wet balls is reduced, that is, to the kiln. Spray steam, so that the kiln air humidity reaches the standard value of% ~ 5%. The time required for this stage differs depending on the moisture content gradient inside the wood, the type of wood, and the thickness of the plate. It is usually twice the warm-up time, ie ~6.
(4) Cooling stage of floor blanks In this stage, the temperature in the kiln should be gradually reduced to be consistent with the outside temperature, and the air humidity should be consistent with the humidity conditioning stage. At this time, the speed of cooling and dehumidification should not be too fast. Under normal circumstances, after 24 hours of ceasefire, open the effluent hole, and then wait for 24 hours to see the temperature and humidity conditions in the kiln before you can get out of the kiln.
After the wood is out of the kiln, it must be properly stacked and placed at the processing site 2 to 3D before it can be processed. This prevents the wood from being deformed after processing.