Solar Cycle Kilns - page 2
A simple thermostat in the collector room turns on the circulating fans when the air temperature reaches 90 degree F. We use two 1/3hp fans to circulate the air to dry 3,000 to 4,000 board feet of lumber. In our location, a 600 sqft collector will kiln dry 3,000 bf of red oak lumber from 12%MC to 6%MC in six days of full sunshine. Due to normal weather conditions here, the lumber is usually in the kiln heating cycle about 14 days. The fans run about 8 hours per day, with a cost of about 50 cents per day. This gives a total energy cost for drying 3,000 bf of oak lumber of $4-$10 (depending on the time of year here).
A solar cycle kiln is built to be well insulated and air tight. One-way air pressure-actuated valves open when the fans blow and close off the wood chamber when the fans stop blowing - to isolate the wood room from the solar collector. This keeps the collected energy in the lumber piles, instead of allowing it to radiate out of the collector window.
Under normal operating conditions, the solar cycle kiln will run at 80 to 90 degrees F above the outside temperature. Temperature and Humidity in the solar cycle kilns are controlled by a moist air vent in the kiln chamber, and a fresh air inlet in the solar collector. A six inch opening is appropriate for normal operations. If the vents are closed, the air circulates around and around - becoming more humid and hotter with each cycle. Temperatures of 150 to 180 F are attainable. A sliding door on the vent allows simple control.
A square foot of solar collector area will dry a certain volume of wood per day. We use 600 sqft of collector for 3,000 board feet of lumber as optimum for our operation. Faster or slower rates can be achieved by changing the size of collector in relation to the size of the stack of lumber.
The solar cycle kiln naturally "conditions" or equalizes the MC of the lumber each night, so no steam conditioning is needed at the end of the drying cycle. This provides higher quality lumber than traditional kilns that dry the lumber more quickly but require steam conditioning.
These basic ideas can be adapted to local building methods and the specific needs of a lumber operation.