What are the drying methods of wood?

Conventional drying is a method that uses atmospheric humid air as the drying medium and steam, hot water, furnace gas or hot oil as the heating medium to indirectly heat the air, and the air heats the wood by convection to achieve the purpose of drying. In conventional drying, most drying rooms use steam as heating medium, which is usually called steam drying.

The difference between high temperature drying and conventional drying is that the temperature of drying medium is higher. Its drying medium can be wet air or superheated steam. The advantages of high-temperature drying are fast drying speed, good dimensional stability and short cycle, but high-temperature drying is easy to produce drying defects, deep color change, hard surface and difficult processing.

Dehumidification drying, like conventional drying, also uses atmospheric humid air as drying medium, and air convection heats wood. It has the advantages of energy saving, good drying quality and no environmental pollution, but dehumidification drying usually has low temperature, long drying period and large power consumption, which affects its popularization and application.

Solar drying uses the heat energy radiated by the sun to heat the air, and uses the hot air to circulate between the heat collector and the woodpile to dry the wood. Solar drying generally has two types: greenhouse type and collector type. The former integrates the collector and the drying chamber, while the latter arranges the collector and the drying chamber separately. The layout of solar drying room with collector is flexible, the area of collector can be large, and the capacity of drying room is larger than that of greenhouse. Although solar energy is a clean and cheap energy source, it is an intermittent energy source greatly influenced by climate, with long drying cycle and large investment per unit volume, so the promotion of solar energy is limited.

Vacuum drying is the drying of wood under atmospheric pressure, and its drying medium can be wet air, but most of it is superheated steam. In the process of vacuum drying, the pressure difference between the internal and external steam of wood increases, which accelerates the water migration speed inside wood, so its drying speed is obviously higher than that of conventional drying, usually 3-7 times faster than that of conventional drying. At the same time, due to the low boiling point of water in vacuum, it can achieve higher drying speed, shorter drying cycle and good drying quality at lower drying temperature, especially suitable for drying thick hardwood. Due to the complexity of vacuum drying system, large investment, high power consumption and generally small vacuum drying capacity, it is difficult to maintain vacuum degree.

Both high frequency drying and microwave drying use wet wood as dielectric. Under the action of alternating electromagnetic field, water molecules in wood rotate at high speed and frequently, and the friction between water molecules generates heat, so that wood is heated and dried from the inside out at the same time. These two drying methods are characterized by fast drying speed, uniform internal temperature field, small residual stress and good drying quality. The difference between high-frequency drying and microwave drying is that the former has low frequency, long wavelength and deep penetration into wood, and is suitable for drying thick wood with large cross section. The frequency of microwave drying is higher than high frequency (also called ultra-high frequency) but the wavelength is shorter, and its drying efficiency is faster than high frequency, but the penetration depth of wood is not as good as high frequency drying.

The advantages of high-frequency and microwave drying are that the drying speed is very fast, usually dozens or even hundreds of times faster than conventional drying, followed by uniform internal temperature of wood, small drying stress and good quality. But the disadvantages of these two drying methods are large investment and high power consumption. At the same time, if the power selection is different, the power is too large or the drying process is not properly controlled, it is easy to produce internal cracking and carbonization. In addition, microwave drying is not ideal for drying wood with large thickness or high moisture content.

Because microwave and high-frequency drying have outstanding advantages in solving the problem of large-section pith cube drying, and the microwave and high-frequency drying equipment are relatively perfect, the drying process has gradually matured, and its industrial application is similar to vacuum drying, usually vacuum-microwave and vacuum-high-frequency combined drying.

Flue gas drying is the primary stage of conventional furnace gas drying, which generally refers to a small drying chamber built by earth method. Its advantages are less investment and low drying cost. Its main disadvantages are that smoke and dust pollute the environment seriously, which is easy to cause fire, and the drying quality is not easy to guarantee, which is easy to cause losses.