Haven't seen any discussion of this type Kiln. The reasoning behind the design is, relieving stress as the lumber dries quicker than air-solar-large scale drying. Anyone ever heard of "Freeze drying" ??
Before my world kind of fell apart, we were in the process of building such a kiln. We had made a deal on a used 4' dia X 12' Propane tank. One end was to be cut off and a hinge device attached to make the door swing and land flatly against the stop flange. A sliding locking ring would be built, so it could be rotated encapsulating locking pawls to create a super tight fitting door. We had a metal supply/machine shop a few miles from home, and were planning to have them roll 2 pieces of 3" X 1 1/2" return, channel iron, so we could weld it around the tank to alleviate crushing from air pressure differential. We would build the door mechanism.
The tank would be set in a mostly sunny area and painted black, of course.
A special type of vacuum pump would be incorporated in the system, that would not allow excessive moisture to be pulled into the pump, eventually destroying it. We had bought a couple of these pumps, cheaply, on ebay. We had in fact bought 2 complete vacuum systems, including valves and vacuum tanks which were filters for removing condensation.
2 angle iron rails would be welded inside the tank as tracks for the carriage. On the bottom of the tank, below the tracks and in from the door, would be installed a vacuum sealing valve. This would be for draining condensed water from the wood as the vacuum would draw it out. Heat sensor mounts and bar measuring gauges would be installed, also, to monitor the stages of drying.
We were being coached by a guru of Vacuum Kiln designing. The project fell apart before we could get it built. I had hopes of doing the same down here, in CR, but, the rainy season would not permit satisfactory results.
The way the kiln would function, is, in the morning, the sun would start to heat the tank, and the vacuum pump would draw the heated air out of the wood, carrying moisture with it. A preset bar measurement would be used to shut down the vacuum pump and hold the vacuum at a set level. A timer would be used to start-stop the pump.
What this would be doing, is, "sweating" the moisture out of the lumber. A timer could be incorporated in the system, so, as the sun was setting, the drain valve could be opened and the water drained out, ALSO, allowing fresh air into the chamber, so the lumber had a chance to "relax" from the vacuum draw and relieve tension from the process. It would also allow what moisture was still present in the lumber, to "equalize", further relaxing the fibers of the wood.
This type kiln would solve a few problems. It would do away with excess handling of lumber from the mill, to a forklift or whatever, stacking for air drying and then, restacking later, as the lumber dried. In our case, it would also deter theft, as we were located close to a main road and people knew our lumber was valuable.
So, no air drying, and no harsh drying schedule. Stack on the carriage for the kiln right off the mill and start the drying process. Most of our logs were full of holes, so, drying was not necessary, unlike solid wood. The holes allowed the Pecky Cypress to be dry within a few days so we could then transfer it home to find markets. I found a great market within 6 miles from home and they took all we could produce, wet, right off the mill, so, we stacked it right on the trailer off the mill. We had not yet found a way to locate the solid logs on the river bottom, so, what few we did find, were to be experimented with.
Everyone that has read this far must be thinking, "I saw all day every day, so, this system is too small for me". That would be correct thinking. However, sitting on a few Mdbt of lumber, waiting for it to dry, ties up a LOT of money. The guru and us figured it would be 30 days or so to dry the lumber. Also, I had spotted a Propane delivery truck in a junkyard, with that big tank on it. The tank was over 7' dia. and over 16' long. THAT would hold a bunch of lumber. That would take a LOT of time away from normal drying and even Solar Kilns. Also, this was a way for a small operator to obtain a cash flow.
I DID build a solar kiln here, but, people don't want or need "dry" lumber here. 14-16% is fine in the humid tropics.
