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Author Topic: Vac Kilns?  (Read 1384 times)

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Offline RickV

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Vac Kilns?
« on: April 29, 2016, 05:18:14 pm »
I am contemplating of starting a small lumber business and have been researching kilns. Could someone explain the theory and operation of vacuum kilns? I was reading that the wood doesn't need stickered, this confuses me, how does the moisture get out of the center of the stack? Does a heating platen go between each layer of the stack? What are the ball park costs for a 1k to 3k bft kiln?

Offline Den Socling

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Re: Vac Kilns?
« Reply #1 on: April 29, 2016, 07:06:00 pm »
Vacuum kilns are completely different than conventional kilns. With conventional kilns you create  a humidity differential. The air is drier than the wood. The air flow carries away the water. Only off the surface. In a vac kiln you use a pressure differential. The vapor pressure of the water in the wood is raised to about the chamber pressure. Water is boiled away from within the wood. The difference in the moisture gradient makes it possible to dry thick wood without degrade in our kilns because it is so small. No air flow is needed so no stickers.

Online Ianab

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Re: Vac Kilns?
« Reply #2 on: April 30, 2016, 01:39:31 am »
Like Dens says, as you drop the air pressure the boiling point of water falls. If you can suck the pressure down to 1 psi, then water boils off at 102F rather than 212. At 0.5 psi it's boiling at 80F.

But the water boiling off takes energy (heat) from the wood. Eventually the drying would slow down as the wood got colder. Evaporative cooling, which is how swamp cooler works.

Hence needing some sort of heating plate, usually between each layer of wood, to pump some heat back into the wood, and so keep the water boiling off.

Airflow isn't an issue, because you are in an almost complete vacuum. Practically no air anyway. What vapor is coming out of the wood can easily escape the layers of wood through the tiny gaps that are there, and be sucked away by the vacuum pump.

Because you are setting up conditions inside the wood where the water turns to a gas, and can then seep out of the wood, rather than slowly seeping to the surface as a liguid, and then evaporating, you can dry thicker pieces of wood a lot faster than other methods, with less risk of degrade.
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Offline Den Socling

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Re: Vac Kilns?
« Reply #3 on: April 30, 2016, 12:19:41 pm »
I'll tell you a little more about this heating process because I think it's so neat. Because of the boiling water, the temperature of the wood can not rise as long as it's mostly free water in the wood. The heat that you add is used for evaporation and doesn't heat the wood. More heat means faster drying, less means slower drying. Wood temperature doesn't change. At our normal process pressure, water boils at 104'F but we do a little more. The chamber pressure is controlled across a dead band. The vacuum pump pulls the pressure down to the bottom of the band. Water evaporates quickly and the wood cools. At the bottom of the band, the vac pump turns off and the pressure starts up. Drying is slow and the wood warms as the boiling point goes up. Water in the wood migrates from wetter to drier regions. I always think of it as a conditioning process. At the top of the band, the vac pump turns on and the cycle is repeated. This can occur two or three times an hour. When you approach FSP, the temperature of the wood starts to rise. When you are at 7% MC, wood temperature is very close to heating temperature. Now isn't that neat?  :D

Offline RickV

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Re: Vac Kilns?
« Reply #4 on: April 30, 2016, 06:10:49 pm »
Thanks for the replies everyone. That's some really neat technology. Now that you have explained it makes a lot of since. A vac kiln would be the way to go if it was more cost effective. Pay back looks to be approximately 4 years .minimum.

Offline Den Socling

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Re: Vac Kilns?
« Reply #5 on: April 30, 2016, 06:18:42 pm »
Maybe four years but maybe less than a year if you have a market for 12/4 and slabs. I did an analysis one time and even with 8/4 Hard Maple payback is less than a year.

Offline BLink

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Re: Vac Kilns?
« Reply #6 on: November 01, 2017, 09:55:58 pm »
So how expensive are they?
Is it feasible to make my own?
Are there dealers for them?
There are a lot of Old Loggers.
There are a lot of Bold Loggers.
But there ain't a lot of Old, Bold Loggers!

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Offline dougan2469

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Re: Vac Kilns?
« Reply #7 on: November 02, 2017, 03:47:53 pm »
Hello Rick,

I had the EXACT same idea of building a vacuum kiln. Spent about 3 months putting things on paper and figuring out what I would need to make it operational. I did all this before finding out they are available through Den. What little savings would come from building it yourself quickly vanishes when you add your time and labor up. Plus the drying schedules Den has figured out over the years is priceless.

Den's kilns are definitely at the top of my wish list and I WILL HAVE ONE!!! .... Eventually ... LOL

For the time being I built the ugliest dehydration kiln known to man  ;D

Offline TKehl

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Re: Vac Kilns?
« Reply #8 on: November 02, 2017, 04:49:05 pm »
*** I have no hands on experience, but have done a lot of research into this topic, have experience with vacuum pumps, vacuum furnaces, and a piece of paper that says I can BS you about Physics.   :D ***

Biggest question is what do you want it to do?  Dry one or two slabs occasionally?  A vac bag would be great for that.  Want to go commercial?  Get a commercial unit as destroying a customers wood is not worth the risk.  Drying substantial quantities of your own wood?  Maybe homebuilt if you don't want to hire it out.

They are expensive if bought or built.  Building would require a steeper learning curve.

Here's a thread that details a build that may have been successful.  The story ended before they got good results.

http://www.forestryforum.com/board/index.php/topic,66317.0.html

If I were to build one, I think I would start with a agricultural "honey wagon" or the back of a vac truck since the vacuum tank that opens part is done.  A propane truck or semi trailer would be my next best.  A junk vacuum furnace would be great if one could be found that was the right size (Usually the cooling jackets go bad before there are other significant issues).

Dougan, Maybe we should get together since we share a similar attitude and similar geography.  Or if you buy one first, put me down as a customer.  In the meantime, I need to get some slabs air drying.   ;)
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Offline btulloh

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Re: Vac Kilns?
« Reply #9 on: November 06, 2017, 06:22:18 pm »
TKehl, have you dried a slab in vac bag?  What is necessary besides the normal platen used for gluing?  I'd like to give that a go in my vac bag.

I tried a small roughed-in bowl in a paint pot I turned into a vacuum vessel one time.  No results from that.
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Offline TKehl

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Re: Vac Kilns?
« Reply #10 on: November 08, 2017, 08:41:02 am »
I haven't yet, but have seen it done and seems to work well for individual pieces.  I intend to give it a go when I get some slabs cut.

Biggest issue with vacuum is you loose most convection.  Then heat only moves through conduction or radiation. 

You can make the water boil at room temperature by pulling it to about 20 Torr.  However, I suspect that would do bad things to the wood.  It's a pressure to be approached, but not exceeded.
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Offline btulloh

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Re: Vac Kilns?
« Reply #11 on: November 09, 2017, 09:05:12 am »
Seems like it needs some heat at the right times and a water trap in the vacuum line.  Not sure how to get the heat to the wood.  Heat lamps?  But not enough to hurt the bag?
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Offline TKehl

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Re: Vac Kilns?
« Reply #12 on: November 09, 2017, 10:08:28 am »
A hot water bath would work.  Good conduction.

There are are a few methods of removing water in a vac line.  However, using a pump that can tolerate the moisture is simpler.  That's what makes liquid ring pumps ideal.
Lucas 6-13+slabber, Mr. Sawmill bandmill, orange chainsaws, JD SSL, Case Backhoe, farm tractors, trailers, and 150ish acres of trees.  Fledgling woodshop with CNC router, laser engraver, Woodmaster 712, and a Berlin 108 moulder (project).  Oh, and a lovely (patient) wife and four offbearers.

Offline btulloh

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Re: Vac Kilns?
« Reply #13 on: November 09, 2017, 11:44:02 am »
My pump is a Robinair HVAC pump.  I know it will tolerate some water but I need to check and find out how much.  I set it up with a vacuum switch and some tanks.  The tanks would probably end up with some of the water, which would have to be drained at some point.

I assume (or hope) the hot water bath wouldn't be continuous.  That would certainly provide good conduction. 

I know there's a fairly complex set of graph of vacuum, heat, etc.  versus time associated with the process, but don't know the specifics.  Seems like that would be proprietary knowledge with vac kiln manufacturers.  Something I need to look for.  I'd like to try this when I get enough information to have a chance at success.
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Offline btulloh

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Re: Vac Kilns?
« Reply #14 on: November 09, 2017, 06:00:07 pm »
I define "chance of success" initially as attaining target MC in a reasonably short time, even if there was an unacceptable amount of degrade in the piece being dried.  Of course I'd want to do all that with no degrade ultimately.  I certainly don't mind making some firewood during the learning curve.
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Offline TKehl

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Re: Vac Kilns?
« Reply #15 on: November 09, 2017, 08:36:16 pm »
That's where, "the devil is in the details" comes in. 

I think I would try electric blankets before the water bath.  Simpler if it works well enough.
Lucas 6-13+slabber, Mr. Sawmill bandmill, orange chainsaws, JD SSL, Case Backhoe, farm tractors, trailers, and 150ish acres of trees.  Fledgling woodshop with CNC router, laser engraver, Woodmaster 712, and a Berlin 108 moulder (project).  Oh, and a lovely (patient) wife and four offbearers.

Offline btulloh

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Re: Vac Kilns?
« Reply #16 on: November 11, 2017, 07:55:15 am »
Good idea on the electric blanket.  Easier than hot water.

Details, details, details.  I need to work out a few more details before I'm ready to try this.  My platens are MDF.  Probably not the right material for this job.  I'm looking around to see if I can find some more info on the details.
HM126