Wood Drying and Processing > Kilns

Discontinuous Vacuum Kiln

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HaroldCR - AKA Fla.-Deadheader:
 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.

furu:
Interesting read and description.  It would have been  nice to see it in operation.  Sorry you never had the chance to make a go of it. 

I did not really understand the door seal design but sure it would have worked for you. 

Since you beefed-up the outer hull to prevent collapse what had you figured was the vacuum point that the tank would deform significantly without the support?

HaroldCR - AKA Fla.-Deadheader:

 That's a good question, furu. Unfortunately, I can't remember the actual  differential between atmosphere pressure and vacuum pressure. The thickness of the tank was nearly 1/4", so, as I tend to overbuild things, the channel was an insurance thing. Back then, 2002, it was not an expensive plan.

 On the door flange-seal question, 2---1/2" thick X 2" wide flat bars was to be rolled edgewise to make a circle that would be the same dia as where the door part was cutaway.  One would be welded to the tank and one to the door so they would mate up. On the tank part there was to be a slight offset as the flange was placed back from the front edge of the tank groove left after the welding, so a special rope type seal could be mashed-glued in and the flat flange of the door would mash it really tight as the lock ring was rotated.

 The ring was shaped like a channel and added to the tank side after the flange was welded on.  Place it on the flange, then weld the ends together, so it would slip around the flange without falling off. There would be notches cut in the door side of this ring and slightly tapered metal blocks welded to the door flange.

 As the door was closed, the ring would slip over the blocks and then the ring rotated to make an ever tightening compression of the seal from the door flange. Once sufficient vacuum was attained, there should be no leakage of air into the chamber, until the drain valve was opened.  We figured to have 13-14 blocks spaced at approximately 10" in the approximately 132 inch circumference of the flange. Actual construction would be more exact. We were not machinists, so, drawings were all approximate.  ;D

 Final plan was, if this worked as we thought, we could always add more tanks. This way, a steady supply of dried lumber would always be coming on, and always protected from the open environment.

 Once we made our first run, we could have actual temp measurements, to see if we could reach pitch set and insect killing temps. If not, we could always devise a heated water system and circulate very hot water to reach the actual high temps needed, through piping placed inside the tank, under the track area. The space was figured at placing the rails at 6-8" or so, above the bottom of the curved tank. The carriage would pass over that plumbing.

 Too bad I didn't keep all the drawings we had made up.
 

furu:
Got it now.  Thanks.

bandmiller2:
Hal, it would have to work, what worries me is how fast its dewatered and the effects on the wood cells. I know little of kiln drying lumber, always used dry winter air and time. You may not need too deep a vacuum. Once you start pulling a vacuum the door will hold itself closed. Frank C.

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