Description of the wood drying process

Wood drying curve

When wood is dried in a gaseous medium with a certain temperature and humidity, and the change in moisture content of the wood is measured from time to time, what is called the "Drying Curve" is obtained. It is plotted with time as abscissas and moisture content as ordinates.

The drying process can be analyzed in the drying curve. The entire process of drying wood can be divided into three stages:

✓     preheat stage

In the graph, stage AB is the stage of preheating the wood. At this stage, on the one hand, the temperature of the medium in the drying oven must be increased and, at the same time, the humidity must be increased to 90% -100%. The aim is to prevent the moisture in the wood from temporarily evaporating into the air and to raise the temperature to the required value from the surface towards the center. The time required for preheating varies depending on the tree species and thickness. After preheating, the temperature and moisture content of the wood is evenly distributed throughout the section. At this time, the temperature and humidity of the medium should be reduced according to the predetermined drying schedule.

✓ Drying stage at constant speed

The downward sloping straight line BC on the graph indicates the constant rate drying stage. This stage is the free water evaporation period. As long as the temperature, humidity, and air circulation rate in the medium remain unchanged, the rate of reduction of water content also remains unchanged.

The end of the BC leg in the graph does not mean that the free water in the wood has been completely removed. Since free water still exists within the wood after evaporation of free water on the surface.

In the constant speed drying stage, the higher the air temperature and the lower the humidity, the stronger the evaporation of free water. This is called the ideal drying state.

✓ Deceleration drying stage

After the free water evaporates, the adsorbed water begins to evaporate. With the evaporation of adsorbed water, the evaporation process gradually deepens to the micro capillary part. The greater the adsorption force of the micro capillary system on the adsorbed water, the more heat time the water needs to absorb during evaporation, and the longer the drying process. Therefore, slower decreases the water content.

Therefore, the drying step below the saturation point of the fiber is called the slow down drying step, line CD of the graph indicates this step. In the slowdown drying stage, in order to increase the rate of water evaporation, the average temperature must be increased, the humidity must be reduced, and a higher air circulation rate must be maintained. However, the rate of water evaporation is restricted by the rate of internal water conduction of the wood, and it is the rate of internal water conduction that determines the total rate of drying.

Movement of internal moisture during wood drying

Moisture in wood can move under certain conditions. Water can move along the grain direction and discharge from both ends. For most boards, the length is much greater than the thickness and width. Therefore, the side area of the plate is much larger than the end area. However, conduction of moisture along the grain is easier than through the grain, but conduction through the grain plays a decisive role in drying the wood.

In the wood drying process, the main driving forces for the movement of water in the wood are the water content gradient and the temperature gradient.

Evaporation of moisture on the surface of the wood

The evaporation of moisture on the surface of the wood always takes place at a certain temperature, humidity and air circulation speed. Generally, the air humidity in the drying oven is always less than 100%, while the air temperature is much higher than normal temperature. Therefore, the equilibrium moisture content of the air is less than that of the wood surface, then the water on the wood surface will evaporate into the air.

The rate of evaporation of water on the surface of the wood depends on the temperature and humidity of the air. When the air temperature increases or the humidity decreases, the rate of evaporation of moisture on the surface of the wood increases. Conversely, the lower the air temperature, the higher the humidity and the slower the rate of evaporation of surface moisture, and the longer it takes to dry.

The speed of the air that circulates on the surface of the wood also has an important influence on the evaporation of water. If the air at the surface of the wood does not flow, a film of saturated, non-flowing water vapor, called the boundary layer, will soon appear, surrounding the surface of the wood at 100% relative humidity, and the moisture at the surface of wood can only enter the air through the boundary layer by slow diffusion. At the same time, the boundary layer also hinders heat transfer. This slows down the drying rate. Therefore, it is necessary to increase the speed of air circulation on the surface of the wood, in order to remove the saturated layer of water vapor and keep the evaporation rate within an appropriate range.

During this period, if the air temperature and humidity remain unchanged, the intensity of water evaporation will remain unchanged. When the evaporation of free water is completed and the absorbed water has begun to evaporate, the evaporation of water gradually moves from the inside of the wood to the surface, changing to the evaporation of absorbed water. The amount of heat required for the evaporation of water per unit mass increases, while the intensity of evaporation tends to decrease.

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