In order to understand the role of relative humidity, we must first appreciate the close relationship between wood and relative humidity. Wood is a hygroscopic material; meaning it loses and gains moisture until the amount of water in the wood is in balance with the surrounding climate. The moisture content at this balance point is called the equilibrium moisture content (EMC). Therefore, when we are drying lumber we are creating a climate in the kiln with a lower EMC than the wood being dried. The lower EMC first begins to remove moisture from the surface of the lumber, after which the moisture in the wetter core will begin to migrate to the drying surface area. The difference between the EMC and the core moisture content is also called the DRYING GRADIENT. Each specie has a different safe drying gradient. Slow drying species (such as oak) have a lower drying gradient than faster drying species (such as Poplar). Simply put, the safe drying gradient of a specie is measured by the amount of drying stress we can safely create between the core and the surface without creating excess degrade.
The EMC and the relative humidity (or depression) are very closely related. We will from hereon only refer to the RH% to understand the effects/changes it has on the drying process.
A decrease in RH% will also dry the lumber faster. This is accomplished because of two factors.
- The drying gradient is increased; thereby forcing the moisture to migrate faster from the wet core to the now dryer surface.
- The dryer air carries more moisture away at the same temperature and air flow.
Rule of thumb: Drying rate = constant * (100 - RH).
*Constant is a specie with a certain moisture content and size.
This rule is a little more complicated than the aforementioned temperature "rule of thumb". For example, if drying lumber at 70 R.H.%, you could increase the drying rate by two if you lower the R.H. to 40%.
One of the problems with drying lumber is that you are trying to create a wick effect from the core to the surface where moisture is removed at the optimum pace. Trying to dry the lumber too fast using lower relative humidity (an increased drying gradient) may well "case harden" the lumber (over dried surface). In addition, the drying process will eventually slow and potentially require not only higher temperatures, but excessive heat, to force the moisture out.
One could conclude that operating the kiln at a higher than needed relative humidity is a safe way to insure the best possible quality. However, with white species the higher relative humidity can cause excessive surface moisture content and actually promote mold and fungi. The consequence of this condition is often stained lumber. As mold and fungi cannot begin growing on lumber below 20% moisture content, the proper use both EMC/RH% and adequate air flow prevent stain by achieving the required moisture removal.
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