Before entering the wood into the drying chamber, it is important that you follow the following advice:
Wood Stacking Requirements
The ideal case is that the separators are positioned correctly, the pile size is correct, the pile is uniform in the drying room and the airflow is in good condition. The deviation of the thickness of the sawn timber entering the dryer should not be too large; when the thickness deviation is obvious, the same layer of wood should be used to ensure that each board can be pressed by partitions.
Correct placement of divider bars:
➢ The spacing between the divider bars should be appropriate to reduce warping of the wood and ensure smooth airflow.
➢ The dividing bars must be perpendicular to the longitudinal direction of the pier, and in each layer a vertical line is maintained in the direction of the height, ensuring that they will fall on the pier support beam. In this way, normal ventilation and a smooth passage of air are ensured.
➢ The distance between the side of the parting bar and the end of the pile should be within the width of one parting bar (within 2.54 - 3 cm), the length of the parting bar should be consistent with the stack width, and partition bar width should be uniform.
The longitudinal direction of the stacking materials is perpendicular to the direction of the airflow and stacking wood along the direction of the airflow is not allowed. The shape of the stack is a regular hexahedron, and the two sides of the stack should be neat and vertical. When the lengths of lumber are different, the longest should be stacked at the bottom and both sides of the stack, and the shortest should be stacked at the middle and top of the stack to ensure stability.
The stacking of the material must be neat and stable to avoid accidents caused by the collapse of the material stacks during drying. It is suggested to place a heavy object (cement block) on top of the pile to press it, in order to avoid warping layers of wood on top.
For proper drying of the wood, follow the instructions below:
I. Preheat treatment
Purpose: Increase the temperature of wood, achieve overall heat transmission and uniform temperature, promote redistribution of water in wood, improve wood plasticity, prevent cracking, and improve dimensional stability.
The preheat time can be determined based on the species of tree, the thickness of the wood, and the initial temperature. Generally, the preheating time is about 1 - 1.5 h/cm (thickness) in summer and 1.5 - 2 h/cm (thickness) in winter.
If the initial moisture content is greater than the saturation point of the grain, there is no stress in the wood and the relative humidity is selected to be 100% saturated air to promote rapid heat penetration into the wood.
If the initial moisture content is the same as the saturation point of the grain, the selected relative humidity may be higher than 96%, allowing a small amount of moisture absorption at the wood surface to reduce the gradient of the wood. moisture content on the surface of the wood, restore roughness deformation capacity and improve the existing stress state in the wood.
If the initial moisture content is less than the saturation point of the fiber, the relative humidity must be balanced with the moisture content of the wood, since there is residual strain and a state of latent residual stress in the wood.
II. Wood drying and intermediate treatment
For hard-to-dry wood or thick boards, steam spray treatment should be done in due time during the drying process to weaken the moisture content gradient, so that the existing stress tends to decrease and internal cracking caused by the destructive stress of wood. It is avoided.
The timing, frequency and time of intermediate treatment will be determined according to specific conditions. When the temperature is slightly higher than or equal to the specified temperature and the relative humidity is balanced with the moisture content of the wood at that time, the treatment time can be calculated by spraying 1h for every 1cm of thickness and holding it for 1, 5 - 2 p.m.
III. The end of the moisture balance and control stage
In the equilibrium treatment stage, uneven moisture content in the drying of the wood is removed. Treatment is carried out until the moisture content of the wetter wood reaches the required final moisture content. During treatment, the temperature of the medium is equivalent to that of the final drying stage, and the relative humidity is balanced with the moisture content of the driest wood.
IV. The End of Moisture Control Treatment
Eliminates or reduces residual stress and drift in moisture content in the thickness of the wood. During treatment, the temperature of the medium is equivalent to that of the final stage of drying, and the humidity depends on the type of wood. Generally, the moisture content of the medium should be 3% (softwood) or 4% (broadleaf wood) higher than the final moisture content, and the treatment time should be 2 - 6 h/ cm thick.
V. Cooling treatment
At the end of the drying process, the dehumidifier should be turned off, the exhaust window should be opened, and the fans should continue to run. When the wood cools down to around 20 °C or a temperature consistent with the outside, it is advisable to unload the wood from the chamber. Carrying out this action prevents the wood from cracking due to a sudden change in temperature.
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