Our oak lumber is about 90% Moisture Content (MC) when it is sawn. We take the lumber from the sawmill and immediately sticker the lumber in an open pre-drying chamber in one of our three solar cycle kilns. My design starts with stacking lumber on a concrete floor, under a roof, with the ends of the piles closed off to prevent end checking. We apply two inch wide nylon straps around the top 20 layers of lumber to hold the wood flat as it dries. As we harvest a mixture of species each year, we often dry a mixture of species in any given wood chamber. Different thickness can be dried in the same chamber if necessary also.
The roof overhead accelerates the natural wind flow though the piles of lumber while keeping the sun and rain off the wood. We fully pre dry our lumber, reaching an equilibrium moisture content of 12% MC in this climate, in about 3 months. This means that we remove 93% of the water in the wood for free, with natural air flow. Once the wood in a chamber has reached 12% MC, sliding doors enclose the room - creating the kiln drying chamber.
To use our lumber for flooring or furniture in our climate, we must dry the wood to 6%MC. This requires adding heat to the lumber. We have chosen to use solar energy as our source of heat. Each 600 square foot rooftop solar collector catches about 500,000 BTUs on a sunny day - for FREE. The most important design feature of the solar cycle kiln is that we have isolated the solar collector from the well insulated kiln chamber. A large insulated window, a black metal collector surface, and circulating fans are the other essentials.
Two small 1/3 horse power fans are needed to blow the hot air from the collector room down into the piles of wood in the kiln chamber. One-way air-pressure-actuated valves close when the fans stop, trapping all the heat in the well insulated kiln chamber. With 3,000 bf in the kiln chamber, operating temperatures are about 80 degrees above the outside air temperature. By adjusting the fresh air intake and moist air exhaust, you can control the humidity and temperature, easily reaching 130-160 degrees on a sunny day if needed. Lumber is in the kiln heating cycle for several weeks to lower the wood from 12%MC to 6%MC, depending on the amount of sunshine.
Other kilns use a steady heat cycle and create a moisture gradient between the surface of the board and the center. To get the water out of the center of the board, the surface is over-dried. Then steam is sprayed into the kiln to add water to the surface of the wood. This is called "conditioning" - such a nice name. To me it is like calling a clear cut a "regeneration" harvest. By pre-drying to 12%MC and not having to steam condition - our kilns are very simple and inexpensive to build and operate.
Our lumber in the solar cycle kiln equalizes every time the fans turn off. We never have to steam condition the lumber. Our regular lumber customers testify that our wood is much higher quality than other suppliers using commercial kilns. It is the daily equalization cycle that allows us to dry wood of different species and thicknesses if needed. You just wait until the thickest and densest wood is dry before you unload the kiln. Over drying the other wood has never been a problem.