Convective drying process of wood in gaseous medium - wood drying curve
wood drying curve
When the wood is dried in a gaseous medium with a certain temperature and humidity, if the change of the moisture content of the wood is measured at regular intervals, and the time is taken as the abscissa and the moisture content is drawn on the ordinate, the curve is called the drying curve. (Figure 1-2).
The drying process can be analyzed on the drying curve diagram. The whole process of wood drying can be divided into three stages.
Preheating stage The AB stage on the graph is the preheating stage of the wood. In this stage, on the one hand, increase the temperature of the medium in the drying kiln, and at the same time increase its humidity to 90-100%. The purpose is to temporarily prevent the moisture in the wood from evaporating into the air, and raise the temperature evenly from the surface to the center to the value required on the drying basis. The time required for preheating will vary depending on the species and thickness of the lumber.
After preheating, the temperature and moisture content of the wood are evenly distributed along the cross section. At this time, the temperature and humidity of the medium can be reduced according to the predetermined drying standard, and the drying process can be started.
Constant drying stage The downward sloping straight line BC in the graph represents the constant drying stage. This stage is the period of free water evaporation. As long as the temperature, humidity and circulation rate of the medium remain constant, the rate of reduction of the moisture content also remains constant. Since the free water on the surface of the wood has evaporated, there is still free water inside, so the end of the downward sloping straight line segment on the graph does not mean that the free water in the wood has been completely eliminated.
In the constant-speed drying stage, the higher the air temperature and the lower the humidity, the stronger the evaporation of free water will be. If the airflow blows away at a higher speed and destroys the saturated vapor boundary layer on the wood surface, the evaporation rate will increase accordingly.
After the free water evaporates completely in the deceleration drying stage, the adsorbed water begins to evaporate. With the evaporation of adsorbed water, the evaporation process gradually penetrates into the microcapillary part. More, the longer the drying process, the slower the moisture content will decrease. Therefore, the drying stage below the fiber saturation point is called the deceleration drying stage, and the CD line segment on the graph indicates this stage.
In the deceleration drying stage, in order to increase the evaporation rate of water, the temperature of the medium must be increased, the humidity must be reduced and the air circulation rate must be maintained at a high level. However, the rate of water evaporation is limited by the rate of moisture conduction within the wood, and the rate of internal moisture conduction determines the overall wood drying rate.
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