Solar Cycle Kilns - Drying Lumber with Natural Energy
Lumber contains water in two forms; in the cell cavity and the cell wall. Many species are about half water by weight - a 6" wide one inch thick oak board 8 feet long holds has a total of about one gallon. The water in the center of the cell is fairly easy to remove, but getting out the water in the cell walls is the important factor in drying lumber.
We predry out lumber under a roof to get maximum quality. A roof keeps nearly all the rain and sunshine off the lumber as it dries, minimizing the weathering of the boards. A roof overhead creates increased airflow through the piles of lumber - like an airplane wing accelerates the air under it. The ends of the boards can be treated in one of several ways to minimize end checking of the lumber. A concrete floor gives a flat foundation for the lumber piles. Careful stickering of the lumber keeps the lumber supported evenly.
The top 20 layers of the pile are strapped with 2" nylon straps to keep the entire stack flat at is dries. We use 4 fifteen foot long straps on a 8 or 10 foot long pile to apply over 4 tons of pressure to prevent cupping of the board during the drying process.
We predry our oak lumber in 10-12 weeks from about 90% MC to 12%MC with natural ventilation. Enough air flow is needed to dry the boards fast enough to prevent mold from growing on the lumber. Additional air flow can be created with creative baffling or fans. Too high an air flow and/or too high a temperature will create surface checking of the boards - mesh screens or shade cloths can be used to restrict air flow through the piles.
In our climate, 12% MC is the driest we can achieve with ambient air temperatures. Removing all water possible in the predrying stage minimizes the time and expense of the kiln drying cycle. We must add heat to drive more water from the wood fibers to reach 6%MC - the level needed for flooring and furniture in our region. We use free solar energy to supply the needed heat. Because we remove just the last 6% MC, humidity and corrosion are not a problem in our kilns. We use common building material for our kiln chambers, instead of the expensive aluminum or stainless steel in standard kilns
A solar cycle kiln has a insulated window facing the mid day sun. The angle of the collector surface is set at the latitude of your location = 45 degrees here in Wisconsin. Behind the window is a black layer of used metal roofing - with a space on each side of the metal to allow air to flow along both surfaces. The sun's energy passes through the window, hits the black metal, is converted to heat - that is transferred to the air - which is then blown into the stacks of lumber. The solar collector should be as low in mass as possible - energy needed to heat up the collector is not available for drying wood. As much of the sun's energy as possible is changed into heated air and immediately transferred to the lumber pile. The goal is to heat the boards, and keep the heat working in the lumber chamber of the kiln.