Wood drying is the process of discharging the moisture in the wood from the tree in the form of vaporization or evaporation. The methods of wood drying are mainly divided into two categories, namely “natural drying” and “artificial drying”.

Natural drying is also called natural drying, while atmospheric drying is the main form of natural drying, referred to as air drying.

Atmospheric drying refers to stacking wood in an open space or under a ventilated shed, and using atmospheric heat to evaporate the moisture in the wood for drying. When people use atmospheric drying to dry wood, they do not do it in the case of logs, but according to the requirements of the product, the logs are sawn into sawn timber (boards and square timber) of certain specifications and sizes, and then the wood is dried according to the technical process regulations of natural drying.

The advantages of atmospheric drying are: energy saving, low investment, simple technology, easy operation, low drying cost, and can keep the original color of the wood basically unchanged.

The disadvantages are: large area, uncontrollable drying conditions, relatively long drying time, and wood is prone to drying defects. Because it is dried in the atmosphere, the moisture content in the wood cannot be dried to the required value.

Artificial drying refers to the use of artificial methods to create conditions suitable for wood drying in a certain equipment, such as temperature, relative humidity, air circulation speed, etc., or use other heating and dehumidification methods to make the wood expel moisture under artificial control and dry. Artificial drying can ensure drying quality, shorten drying time, and meet the drying requirements of various purposes, and even some special requirements.

In modern wood drying production, atmospheric drying is only an auxiliary method, and the method of artificial conventional kiln drying is currently the main drying method.

1.Kiln drying

Kiln drying refers to the use of wood drying kilns to dry wood. It can artificially control the drying conditions to dry wood, referred to as room drying or kiln drying. At present, conventional kiln drying accounts for 85%~90% of wood drying production in domestic and foreign wood drying production. The heat source used is a steam heater, which needs to be equipped with a steam boiler.

The advantages of conventional kiln drying: it can ensure the drying quality of wood of any tree species and thickness, can dry the moisture content of wood to any required state, has a short drying cycle, flexible equipment operation, easy to master drying conditions, and is easy to realize the mechanical automation of wood drying production.

The disadvantages of conventional kiln drying: The drying equipment is relatively complex, the one-time investment is large, the energy consumption is relatively high, and the drying cost is relatively high. Conventional room drying can be divided into three types according to the temperature control range of the drying room: low-temperature drying of 45-60℃, room-temperature drying of 60-100℃, and high-temperature drying greater than 100℃.

Generally speaking, a combination of low-temperature drying and room-temperature drying is mostly used for difficult-to-dry wood or easy-to-dry but thick wood; high-temperature drying can be used for easy-to-dry and thin wood. Atmospheric drying and conventional kiln drying are both traditional wood drying methods, which are collectively referred to as conventional drying.

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.