Is there a homebuilt vacuum kiln going?

[low_48]

I've been reading and reading archives, and I'm confused. ??? Have you guys gotten a vacuum kiln going yet? Here's a question on the same subject. What if I just want to dry a few big slabs, maybe some bowl blanks? Can I build a really small pressure vessel and put guts in it like a dehumidification kiln? Don't laugh, I wonder what a Sears dehumidifier would do in a vacuum? Should I ask "myth busters"? Will she pop off like a bomb? If not, I would have the heat of the dehumidifier and the chiller section in place. I could add some electric baseboard units for aux. heat.

Hope this doesn't get me laughed off the site. Just a city bound farm boy, sitting at my day job desk, bored to death, wishing he was slabbing soft maple instead of pushing the computer. ;)

[Den Socling]

The easiest way to dry a few big slabs is in a plastic bag. There's a thread around here with a guy name Taylor (first name) who dried a slab to make a table top for Jerry Seinfeld using basic instructions that we gave him. He is stopping in here on Monday because he is ready to graduate to a chamber.

If you managed to build a chamber that didn't collapse under vacuum, your DH wouldn't do anything. The extra pressure differential of 14 PSI wouldn't hurt the compressor and there wouldn't be any vapor to condense because there wouldn't be any way to get the heat to the wood. A vacuum = no air = no heat transfer.  :

[GaS]

hmm, what about a discontinuous DH kiln?  Put a floor fan in, a milkhouse heater, and a dehumidifier.  Heat it up, run the DH for a while, then pull vacuum for the heck of it, wash rinse repeat?

[FeltzE]

Den,

How much vacum do you apply to drying in vac kilns?

Eric

[Den Socling]

One atmosphere at sea level is about 760 mmHg (760 torr). We run around 50 torr.

[footer]

One atmosphere at sea level is about 760 mmHg (760 torr). We run around 50 torr.

How does that compare to Inches of mercury? 
Is it possible to dry a slab in a chamber without the heat. I undestand that heat does not conduct through a vacuum, but water will boil at room temp at a vacuum, so what is the need of the heat?

[Den Socling]

Our process pressure is around 28" Hg when compared to one atmosphere or 2" Hg absolute.

When water changes from liquid to vapor, thermal energy is used. With reduced pressure, the temperature is reduced but the required amount of energy is nearly the same. So, if you put a wet slab in a vacuum chamber without heat, it would get so cold that drying would stop.

[footer]

Thanks Den! I see now,
I have been wondering why you just could't suck the moiture out.

[low_48]

You're a very patient man, Den, thanks for all the answers. Let's see how patient you really are. Now suppose I had some very tight bags, loaded them with a slab each, kept them pumped down, then put them in an insulated tank of hot water?

Just for curiosity, how much does your smallest kiln cost? Do you know of any in Illinois? Closest to West Central IL?

Thanks a million! Not dollars you understand, just thanks! ;D

[FeltzE]

How do you maintain vac kiln heat as the evaporation of water effectively cools the kiln? And low pressure means less air which transfers heat even less..

Eric

[Den Socling]

footer,

If it was that easy, I would need a new job.  :D

low_48,

The idea with the bag does work. It helps to put a couple tubes in the bag along side the slab to equalize vacuum from end to end.

Our smallest kiln used to be $65k. My business partners want to charge more. I want to charge less. We will be discussing prices in the next couple weeks.

Eric,

A long time ago, I decided to use aluminum plates with warm water circulating through them. This was the only way I could see to get the entire kiln charge heated evenly. But there is still a major problem with heat transfer. We have one semi-competitor in the US. They get over the heat transfer problem by using way too much heat. This would turn warp the wood and they overcome that with a flexible top that puts atmospheric pressure down on top of the load. That creates a major problem. The chamber always needs to be filled and you can't have gaps in the load. In Europe, most companies gave up on plates and are now using the 'steam vac' design. As for our kilns, I have developed a technique that I'm not allowed to talk about.  :D

Den

[footer]

Where is a source of these bags large enough for a slab, and how do you connect the vacuum pump to them?

[Larry]

Try http://www.vacupress.com/ or

http://www.joewoodworker.com/veneering/makingbags.htm

Used a waterbed for my vinyl source when I built one.

[GF]

This question maybe totally off the wall, but I will aski it anyway.  Would it be possible to take large air rams and attach hydraulics or some type or a crankshaft type of device to these and attach the one connection (suction) to the air chamber, this would require a couple of check valves once from releasing the vacuum from the chamber, and the other to release (exhaust) the pressure and the ram retracts?

In general it would be like a vauum pump, but the cylinders would be hydraulic rams.  Not being familer with large vacumm pumps and what kind of pressures they incur not sure if the air rams would be a possibility. 

Another idea I was thinking about was to take a vacuum pump to pull down a vacuum as much as possible.  Take these rams and use them to draw pressure down even more, since a ram can handle 3000PSI, would the same go for reversing the cycle of a ram to create -3000PSI pressure? 

[footer]

A hydraulic cylender would not pull that kind of vacuum. It will make some, but I don't know how much.
Hydraulic, and air cylenders use the pressure created in the cylender to force the seals out against the wall to create a tighter seal as the pressure increases. pulling a vacuum would decrease the effectiveness of the seals.