Wood drying is an important part of ensuring and improving wood quality, reducing wood loss and increasing the important rate of wood. Artificial drying methods include conventional drying, high temperature drying, vacuum drying, high frequency and microwave drying, and flue gas drying. Among all the artificial drying methods, conventional drying has the advantages of long history, mature technology, guaranteed dry quality, and easy realization of large-scale industrial drying.
Conventional drying is a method in which atmospheric pressure humid air is used as a drying medium, steam, hot water, furnace gas or hot oil is used as a heat medium to indirectly heat the air, and the air is heated by convection to achieve the purpose of drying.
The difference between high temperature drying and conventional drying is that the drying medium has a higher temperature. The drying medium may 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, the material color becomes deep, the surface is hardened, and it is difficult to process.
Vacuum drying is the drying of wood under atmospheric conditions. The drying medium can be wet air, but most of it is superheated steam. When vacuum drying, the water vapor pressure difference between the inside and outside of the wood increases, which accelerates the water migration speed in the wood, so the drying speed is significantly higher than the conventional drying. At the same time, due to the low boiling point of water under vacuum, it can achieve higher drying rate, short drying period and good drying quality under low drying temperature, and is especially suitable for drying thick hard hardwood.
Dehumidifying and drying, as well as conventional drying, also use atmospheric air as a drying medium, and air convection heats the wood. The utility model has the advantages of energy saving, good drying quality and no pollution to the environment, but the dehumidifying and drying usually has a low temperature, a long drying cycle, and relies on electric heating and high power consumption, thus affecting its popularization and application.
Solar drying uses the heat of solar radiation to heat the air, using hot air to circulate between the collector and the pile to dry the wood. Solar drying generally has two types of greenhouse type and collector type. The former integrates the collector and the drying chamber, and the latter separates the collector from the drying chamber. The collector type solar drying chamber is flexible in layout, the collector area can be large, and the corresponding drying chamber capacity is larger than the greenhouse type. Although solar energy is a clean and cheap energy source, it is a intermittent energy source that is greatly affected by the climate. The drying cycle is long and the investment in unit volume is large, so the promotion of solar energy is limited.
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