Convective drying of wood in gaseous media - factors affecting wood drying speed
Factors Affecting Wood Drying Speed
The main factors affecting the drying speed of wood are medium temperature, humidity, air circulation speed, wood moisture content gradient, and temperature gradient.
1) Medium temperature
The medium temperature is the main external factor that determines the drying speed of wood.
When the wood is surrounded by high-temperature air, the heat required to increase the temperature of the wood and its internal moisture and to evaporate the water is supplied by convection heat transfer.
The higher the temperature of the medium, the higher the temperature of the wood and its internal moisture, which accelerates the thermal movement of water molecules and increases the speed and intensity of water evaporation.
Therefore, the increase of medium temperature can speed up the drying speed of wood, but the temperature is too high will cause the reduction of wood strength and performance.
(2) Medium humidity
When the temperature remains the same, the decrease of the humidity of the medium will make the moisture of the wood evaporate into the air more easily, and the drying speed will be accelerated; on the contrary, the humidity of the medium will increase, and the drying speed will slow down. If the humidity of the medium reaches 100%, the moisture will stop evaporating. The drying speed is zero.
3) Air circulation speed
The airflow velocity of the drying medium directly affects the evaporation of moisture on the wood surface. If the airflow velocity is too small, the drying speed will be reduced, and if the airflow velocity is too large, energy will be consumed too much. Experience shows that when the airflow velocity through the wood surface exceeds 1m/s, the airflow is in a turbulent state, and the air's ability to transfer heat to the wood and absorb water vapor from the wood surface increases, but generally does not exceed 3 m/s.
(4) Gradient of moisture content
The moisture content gradient is the driving force for the movement of moisture within wood.
Moisture in wood moves from places with high moisture content to places with low moisture content. The greater the difference in moisture content, that is, the larger the gradient, the faster the moisture moves and the faster the drying speed.
During the drying process, a moisture content gradient with high inside and low outside should be formed to promote the internal moisture to move outward, but care should be taken not to cause too large a moisture content gradient, and not to form a hardened layer of moisture depletion on the surface of the wood, otherwise not only the moisture will It cannot continue to move outward, and it will also cause drying defects.
(5) Temperature gradient
Temperature gradients are also the driving force for moisture movement within wood.
In the normal drying process, the surface temperature of wood is higher than the internal temperature, because the temperature gradient is low inside and high outside, which hinders the movement of moisture outward. In order to create a temperature gradient that is low on the outside and high on the inside, it is consistent with the moisture content gradient that is high on the inside and low on the outside, and jointly push the moisture inside the wood to move outward quickly. Usually, high-temperature medium is used to treat wood, so that the wood can be heated through without evaporating water, and the temperature inside the wood is increased; then the temperature and humidity of the medium are lowered to reduce the surface temperature of the wood, but at this time, the inside of the wood is still at high temperature. This forms a temperature gradient of high inside and low outside, which is consistent with the moisture content gradient of high inside and low outside, and accelerates the movement of moisture inside the wood.
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