The working principle of wood drying equipment is to increase the drying speed, save energy consumption and reduce drying costs while ensuring the drying quality. Drying quality means: the moisture content and drying uniformity of the dried wood must meet the requirements of the processing technology; the integrity of the wood must be maintained, no defects that are not allowed by the process specifications must occur, and the properties of the wood and its products must not be weakened. Drying speed refers to: the degree to which the moisture content of the wood decreases per unit time. The faster the drying speed (or the shorter the drying cycle), the less wood drying equipment and investment are required, the higher the productivity and the lower the drying cost.

When selecting wood drying equipment in production, it is based on the tree species, specifications, quantity, purpose and actual conditions of the production unit of the dried wood. For modern wood drying rooms, it is required to ensure that the temperature, humidity and air flow velocity of the drying medium and the external conditions of the stacked dried wood are basically the same in terms of technology to achieve the purpose of uniform drying. The operation should be safe and reliable, flexible in control, and the labor intensity of workers should be low. The equipment has low investment, small footprint, quick results, and energy conservation. At the same time, it should also minimize pollution to the surrounding environment.

The moisture in wood mainly includes free water and absorbed water. The moisture in the large capillary system composed of cell cavities is called free water. Its increase or decrease only affects the weight of wood, but not the properties of wood; the moisture in the microcapillary system composed of cell walls is called absorbed water. Its changes can not only cause wood to shrink and expand, but also affect other physical and mechanical properties of wood.

Wood drying is to remove free water and absorbed water in wood to adapt to different uses and quality requirements. Although there are many methods for drying wood, the basic principle is the same, that is, using the moisture content gradient along the thickness of the wood, and the water vapor partial pressure difference formed after heating, which is large inside and small outside, to promote the continuous movement of moisture from the inside to the outside in the form of liquid and vapor, and evaporate to the outside through the wood surface; the internal moisture movement intensity should be coordinated with the surface moisture evaporation intensity, so that the wood is evenly dried from the outside to the inside.

The traditional steam kiln drying process is: first, high temperature (below 100 ℃) and high temperature (saturated or nearly saturated) wet air is continuously passed through the wood pile in a circulation flow to preheat the wood. When the wood and its moisture are heated to a certain degree, the temperature and relative humidity of the medium are lowered according to the provisions of the drying standard, forcing the moisture in the wood to evaporate from the surface, which is the beginning of drying. Then, according to the procedures specified in the drying standard, the temperature of the medium is gradually increased and the relative humidity is lowered, so that the evaporation surface of the moisture in the wood gradually moves inward until the drying is completed. During the drying process, internal stress, cracking and deformation should be eliminated or reduced, and the physical and mechanical properties of the wood should not be reduced to ensure the drying quality.

1. When drying wood with wood drying equipment, if conditions permit, a spray device can be equipped to make the wood achieve the effect of hot steaming, and the moisture of the wood is discharged from the inside to the outside in stages. The dried wood will not deform, crack, or crack at the end.

2. A hot air recovery system can be added to recycle the exhausted hot air, accelerate the running speed of the airflow in the drying room, and speed up the drying time to improve the utilization of heat energy.

3. Wood drying equipment Wood drying is a matter of haste makes waste, so the temperature cannot be raised too fast, otherwise the wood will crack. In addition, attention should be paid to the stacking of wood, pay attention to uniform heating, and also consider the direction of wind flow so that hot air can flow through the stacked wood.

4. Using a wood drying kiln, it can be dried to any required final moisture content, without being restricted by natural conditions; it has a more mature drying theory and process technology, and can ensure the drying quality; compared with air drying, it has a short drying cycle, high production efficiency, and a small footprint; the equipment is not too complicated, has a long service life, is safe and reliable, and has good economic effects.

It takes a few minutes or more than ten minutes to cut down a tree, but it takes several years, decades or even hundreds of years to plant a tree and grow it into a useful tree. Now the world is facing the problem of continuous reduction of forests. How to effectively use limited wood resources is becoming increasingly important.

When it comes to effectively using limited wood resources, it is inseparable from improving the technology of wood drying equipment. More than 80% of the commonly used wood drying equipment belongs to conventional drying technology, which uses wet air at normal pressure as the drying medium, steam or hot oil as the heat source, first heats the air, and then heats the wood. The general temperature can only be below 100℃. The energy consumption is relatively large, and when dealing with exhaust gas, it will also cause pollution to the environment, making the operating environment bad.

In fact, when dealing with wood drying, it is ideal to use solar energy or heat pump dehumidification and drying. These are new energy-saving and environmentally friendly energy sources, which can not only effectively improve thermal efficiency, but also not pollute the environment. These are the future development directions of wood drying equipment.

2.Solar drying

Due to the increasingly tight energy supply, the development and utilization of solar energy has attracted more and more attention. Solar drying is to use solar energy as the heat source of the drying kiln to dry wood.

The advantages of solar drying are: it can use the infinite energy of the sun, save non-renewable energy such as coal, and reduce production costs. The drying cycle is shorter than air drying.

The disadvantages of solar drying: it is highly dependent on climate, the use of solar energy is restricted by the sunshine time, the drying humidity is lower than conventional kiln drying, so the drying cycle is longer.

Solar drying is suitable for areas with long sunshine time and many sunny days and has broad development prospects.

3.Infrared drying

Infrared drying is a process that uses the infrared radiation (electromagnetic waves with a wavelength of 0.72-1000um) to absorb infrared radiation, generate heat, increase the temperature of the wood, and evaporate moisture for drying. In industry, infrared in the 0.72-4um band is collectively referred to as near infrared, and infrared in the 4-1000um band is called far infrared. Since far infrared is easier to absorb than near infrared when radiating to wet materials, the thermal effect is positive, so it is widely used. The penetration ability of infrared is limited (generally 2-8mm) and is more suitable for drying stable wood and veneer. For the drying of finished wood, the drying kiln combining infrared and convection heating is more effective.

Common far-infrared radiation elements include electric heating, gas heating and steam heating.

Advantages of far-infrared drying: simple equipment, no boiler is required, and easy to put into production.

Disadvantages of far infrared drying: If electricity is used as energy, the power consumption is large. If there is no steam spraying and humidity control measures, the drying quality of difficult-to-dry thick boards cannot be guaranteed.

4.High frequency and microwave drying

High frequency generally refers to electromagnetic waves with a wavelength of 1000-7.5m and a frequency of 0.3040 MH z, and microwaves refer to electromagnetic waves with a wavelength of 1-1000mm and a frequency of 300 to 300000 MH z. Wood is a dielectric. When it is placed in a high-frequency or microwave electromagnetic field, the wood becomes strongly heated due to the loss of the dielectric, and the moisture in the wood is vaporized and evaporated and discharged, so that the wood is dried.

Both high-frequency drying and microwave drying belong to dielectric drying. Their characteristics are that heat is not transmitted from the outside of the wood, but occurs directly inside the wood. The temperature inside the wood is higher than the surface temperature of the wood. The moisture inside the wood moves outward quickly. Its drying speed is more than 10 times faster than conventional kiln drying. Under the same electromagnetic field strength, the energy absorbed by microwave radiation is 20 times higher than that of high-frequency radiation, so microwaves are more suitable for wood drying.

Advantages of high-frequency drying and microwave drying: very fast drying speed, uniform drying, can solve the problem of drying wood that is difficult to dry with conventional kiln drying.

Disadvantages of high-frequency drying and microwave drying : complex drying equipment technology, large investment, high power consumption, high drying cost, and only suitable for drying a small amount of precious wood.