Rocky Mountain Kiln Dry
CSWoods is tackling wet lumber head on. Kiln drying is our specialty. Improperly dried wood can shrink, crack, twist and warp-devastating many a craft persons careful work. In Colorado’s dry climate, working with thoroughly dry lumber is crucial. The drying process includes everyone from the sawyer to the trucker to the kiln loader and operator. We operate a 7000 board foot de-humidification kiln and a state of the art vacuum kiln with 2500 board foot capacity.
About the Kilns
CSWoods is running a state-of-the-art AirVac kiln, made by Vacutherm, allowing us to dry material in a very short time frame - from green to dry in a week and a half for most species, and much less on material already in stock. This programmable machine has a low defect rate, and is able to actually flatten lumber. The computer controlled system is able to recondition the skin on material so that it releases moisture at a controlled rate, keeping the tension in the lumber low. This 2500 board foot kiln (5' wide by 18' long) has been running consistently for the last few years, and we have dried a wide variety of species and thicknesses. We are thrilled with the results! Do you need dry slabs right away for a special project? We are happy to answer questions and schedule dry times. Please call.
The original solar kiln at Collector's Specialty Woods has been converted to a 10,000 board foot DH Kiln. This included the installation of fans, a dehumidification box, drainage system, and power storage area. Dehumidification kilns use the continuous cycling of heat and water condensation to dry lumber. Instead of heating the lumber until it releases moisture into the air, and then venting the saturated heated air, water is condensed on dehumidifier coils and then removed as liquid. When the hot air passes over the cold refrigeration coils, the evaporated moisture from the wood condenses to liquid and drains away. This recycling of heated air is more efficient, and although these kilns use electricity to both heat and cycle the air, a set of four large electric thermal storage units, heated during off peak hours, are used to store and disperse heat when needed, adding to the energy efficiency of our kiln drying operation.
CSWoods Dehumidification Kiln
A DH kiln is loaded from wall to wall so that the air current is moved through the stacks of lumber. Here you see the kiln full of Douglas Fir lumber.
Kiln Schedules: the minimization of drying stress
Kiln schedules are developed for each species and the thickness being dried. This schedule is essentially a plan outlining the temperature and humidity cycles needed to dry the specific material with the least stress on the wood. Drying stress can result in a wide variety of defects from the tension and compression of wood as the water moves from the inside out. Wood will dry faster on the exterior of the board, thus the exterior shrinks at a faster rate than the interior creating tensile stress on the outside and compression stress on the inside. If this stress exceeds the level that the specific species of lumber can handle, surface and end checking and even cell collapse can occur. It is also possible to cause what is referred to as case hardening. Case hardened wood has surface compression stress because the outer layer dried and set but the center of the board continued to shrink. This leads to over stretched fibers on the outer surface and a strong tendency toward cupping. It is possible to relieve case hardening using a kiln and controlling the atmosphere, but with a good kiln schedule, case hardening can be avoided completely.
Claro Walnut Kiln Drop
Live edge slabs take about four months in the DH kiln to dry to the 6% to 8% moisture content our Rocky Mountain climate requires.
The importance of dry lumber - history and progress
The wood world is full of anecdotal claims from wood suppliers and woodworkers about the process of drying lumber, and which process yields a better, more stable product. Although some of the claims can be chalked up as folklore, the discussion is one of the most important in the lumber industry. This issue of moisture content weighs heavily on the minds of everyone involved with wood.
Before kilns, all wood was air dried and, because it is the 'old' way, this process is usually highly respected. Although it sounds simple, air drying lumber properly is a long arduous process involving sticker stacking, waxing, moving, constant checking and restacking. The invention of kilns and the ability to dry lumber more quickly was of great benefit to the wood world, but there is a question about what the process of kiln treating does to the lumber and how stable the resulting product is. The drier the wood, the stronger but also the more brittle it is. Although some believe that it does not matter how the lumber got that way, air or kiln, there is research to suggest that air drying can result in a stronger and more elastic product. The Encyclopedia of Wood cites a 40% greater modulus of rupture (strength/hardness) without heat and or moisture from kiln treating and up to 15% greater modulus of elasticity (can expand and contract with changing equilibrium moisture content conditions without splitting or cracking at glue joints). This doesn’t mean that kiln drying should be avoided, but it is a reminder that kiln operators must be careful, constantly managing the load and monitoring the temperature and humidity. In order to yield the best product, the kiln operator needs to be experienced with the species of lumber and ratio of heat and time needed to reach the desired moisture content, without forcing it to conform to their latest delivery schedule.
At CSWoods, because of our dry climate, we can air dry lumber to around 7%. This process takes years of attentiveness, tying up capital and inventory. On site, we use a dehumidification kiln, and a vacuum kiln to speed things up. This will allow us to carefully dry wood for customers in as little as a week, ensuring that they are getting the most stable product possible. We are constantly working to educate ourselves, our suppliers and our clients about drying lumber, and the right moisture content for their region.