T e c h n o l o g y
Optimum Moisture Gradients Increase
Lumber Grade and Speed up Drying Time!
A brief description of Moisture Content Gradients:
The difference between moisture content inside a board of wood and the moisture content on the board surface creates the driving force during drying. It is the object of a correct drying process to adjust this wood moisture content gradient so that the drying process proceeds at an optimum. It is disadvantageous if the wood moisture content gradient is too great, since the moisture content flowing from the inside of the board to the surface is disturbed. Case hardening is the result. If the moisture content gradient is too small during drying, then the resulting drying times are uneconomical. The fully automatic dynamic FC Controller controls the climate inside the kiln so that an optimum wood moisture content gradient is present.
Moisture tries to achieve equilibrium in wood. It tends to distribute itself equally throughout the piece by moving from areas of high to areas of low moisture content. When green lumber begins to dry, the evaporation of moisture lowers the surface moisture content below that of the interior. As drying progresses the moisture from the interior moves outward to the area of lower moisture near the surface. This difference in moisture content between the interior and the surface of a board is termed the moisture gradient. The FC Controller controls the surface moisture content by the temperature, relative humidity, and circulation of the surrounding air.
Figure 1 Moisture Gradient of Dried Lumber with a Target of 14%
Figure 1 is an example of the moisture gradient in a piece of lumber right after being pushed from the kiln. The moisture gradient is roughly linear so the formula can be used to calculate the average MC.
The outside MC + 8%
The highest inside reading = 24%
Actual MC% = 8 + (24-8) / 3 = 8 + 5.3 = 13.3% or 14%
If the lumber is left outside to sit eventually the moisture gradient will balance out to 14% from the surface to the inside. This happens because over time moisture tries to achieve equilibrium in wood. This is why using longer needles when checking moisture content gives a much more accurate value.