why do we kiln dry lumber?
Kiln drying lumber removes the moisture from the boards not just to make them more stable but it increases the durability by hardening the cellular walls and even removing some of the resins and other extractives that attract bugs and encourage decay.
Air drying lumber also removes the moisture and brings the boards into equilibrium moisture content (EMC) but without the heating process that kills the bugs or hardens those cellular walls. The upshot is that air dried lumber is more prone to take on moisture as the climate changes and it will more easily move with seasonal changes. Kiln drying lumber makes the boards resistance to absorb moisture from temporary changes in the environment and prevents the boards from expanding and contracting in the first place during these short term weather changes.
3 types of lumber kilns
Over the years there have been a lot of advancements in kiln drying lumber all with the intent of speeding up the process while still producing stable lumber that works well. Today we have 3 (maybe 4) types of lumber kilns:
- Steam kilns
- Dehumidification kilns
- Vacuum kilns
- Radio Frequency Vacuum kilns
The first two, steam and dehumidification rely on an external power source to heat up the kiln chamber and fans that blow air across the lumber. A steam kiln uses a boiler to heat water into steam and that heats the kiln. But the steam can also be used to control the wet bulb temperature of the kiln in order to control the rate of drying and to condition the lumber during the process.
A dehumidification kiln blows warm air over the boards which picks up moisture and then it is blow over a cooling unit which causes the moisture to condense out. Then the dry air is recirculated back into the kiln and the process repeated.
Vacuum kilns suck out the air and thus lower the boiling point of water causing the moisture in the boards to evaporate out while not heating the chamber up dramatically and allowing for a more gentle drying process on the lumber. An RF kiln is the only kiln that uniformly dries the lumber making larger timbers possible to be fully dry all the way through without severe case hardening that can occur when too much heat is applied over too long a time. The biggest drawback being the size of vacuum kilns. These cannot hold very much lumber and must be hand loaded and unloaded making the process labor intensive and slow for the relatively small volume dried.
The one kiln I did not mention in this episode is a solar kiln which works much like a greenhouse. The sun heats the kiln raising the temperature inside. These are much more of a DIY solution and can be hard to control the moisture content and properly condition the lumber. They do work but are the domain of the hobbyist more than anything else because of the lack of consistency.
If there are solar kiln users out there I would love to hear from you about your kiln and your experience with it. Admittedly it is an area I do not know much about outside of theory.