Lumber drying is a multi-faceted process, and it often involves kiln drying. Those within the industry know that key to effectively drying lumber is doing it s-l-o-w-l-y. Both the heat-up and the cool-down times need to be long, no matter what the species is. Depending on the species, though, the maximum temperature and precise rate will vary. Many kilns have schedules which lumber professionals follow, as a result of their own experience throughout the years.
While there is variation among kilns and species, thicker lumber poses a unique challenge because complete drying often causes significant damage to the wood. The solution? Radio frequency (or RF) vacuum kiln technology is changing how heavy timbers are dried.
Cracks in large timbers are inevitable. Large timbers take an especially long time to dry, and as the outer layers do dry, they act as insulators that keep the interior portions from drying at a similar rate. If the temperature is raised, one of two results will occur: either the outer fibers will be damaged, or the moist core will separate, creating “honeycomb lumber.”
In order to reduce the opportunity for either scenario, the outer 2 inches of timbers can be dried, leaving the core at a moisture level between 15 and 20%. The outer dry fibers will typically be enough to maintain the timber’s stability, but when the lumber is planed, the outer dry shell is much thinner, decreasing the stability or possibly even exposing the wet fibers. An additional potential problem occurs each time the timber is cross cut, exposing that moist core and possibly causing checking.
Because of this lose-lose scenario, a newly patented RF vacuum kiln revolutionizes the process of drying large timbers by eliminating the potential of damaging wood fibers while producing thoroughly dry timbers. The result is wood that actually acts like an air-dried product: It’s easier to work and demonstrates no cell wall damage.
The two-step process starts with the lumber being loaded into the kiln and the pressure being dropped. The boiling point of water is lowered, as a result, and locked in the lumber to 90 degrees Fahrenheit. The vacuum of air allows the water to be drawn out of the wood the same way it circulated within the live tree — via the end grain — further preventing damage to the cell wall. The next step is heating with radio frequency waves. In their purest form, radio waves are simply vibration of air. The heat caused by vibration causes the molecules to bounce off one another, resulting in volumetric heating. The uniformly rising temperature allows the moisture to boil off all at once, without the potential checking caused by uneven stresses of outer wood fibers shrinking.
These RF Dried Timbers are products called Tru-Dry timbers. The only species currently coming out of this mill are those in highest demand for large timber construction: Douglas Fir, Western Red Cedar, and White Oak. Maybe someday all lumber will be dried this way. If you are interested in purchasing Tru-Dry timbers, please contact us.