What is the equilibrium moisture content of wood?
The moisture content of wet wood placed in the atmosphere will gradually decrease over time. When the moisture in the wood and the moisture in the atmosphere no longer exchange and reach an equilibrium state, that is, when the moisture is in a static state, the moisture content of the wood is the equilibrium moisture content under the temperature and humidity conditions, expressed in W balance.
The equilibrium moisture content varies with the state of the air the wood is in. When the relative humidity of the air increases, the equilibrium moisture content also increases; when the relative humidity of the air decreases, the equilibrium moisture content also decreases; contrary to the above, when the air temperature increases, the equilibrium moisture content decreases. This means that as the temperature increases, the hygroscopicity of the wood will decrease.
In the process of changing from wet to dry and from dry to wet, wood tends to the equilibrium moisture content gradually under certain air conditions. In general, the moisture content from wet to dry is often slightly greater than the moisture content from dry to wet. This is because some of the gaps in the microcapillary system of wood have been occupied by the infiltrated air, which hinders the wood from absorbing water. This phenomenon is called hygroscopic hysteresis or absorption hysteresis and is represented by ΔW.
The value of absorption hysteresis of fine wood such as veneer, sawdust, shavings is extremely small and can be ignored.
The value of the absorption hysteresis of the air-dried material is not large, and it can be ignored in actual production.
The absorption hysteresis value of room-dried lumber is larger. The higher the temperature of the medium during drying, the greater the absorption hysteresis of the finished product. Absorption hysteresis values varied from 1-5%, with a mean of 2.5%.
The equilibrium moisture content of wood has certain significance in actual production. When the wood in a certain area is drying, it must reach the wood equilibrium moisture content range of the area. Otherwise, it will affect the quality of wood products.
