Vacuum Kiln

[phil harold]

I am curios if any one has ever tried to use a vacuum kiln for drying green bowls.

I found this plan and figured since a lot of turners already have a vacuum pump if this would be a good idea and if anyone has already what were the results

http://pacificwoodworkersguild.com/Docs/News2003/May2003web.pdf

(it is after the pictures and awards)

[Mauricio Ulloa]

Cool! Thanks! I have an electric kiln from Freight Harbor. It is for dust painting, but I'm using it to dry wood..... Small but practical, and really inexpensive...!

[Chris Rae]

Thanks for posting this, it is an intriguing concept. I may have to experiment with it. I have a small vacuum pump, gauge, etc. and a pressure canner is easy to come by.

[Kim Ford]

Phil;

I have been working on a vacuum kiln for the last year.
There is a lot of science involved in the process, which the commerical kiln market has already addressed and solved but the basic principle is simple.

There are two type of moisture in wood that needs to be removed. That which is asborbed or is in the space between the cells, and that which is inside the cell walls. Given the nominal thickness bowl blanks the asorbed moisture comes out easily with a vacuum because it is not contained by anything. The moisture that is in the cell wall is different.

Something needs to happen to break the cell wall to allow the moisture to excape. In air drying the cells rupture as they dry and the moisture is then not confined and works its way out of the piece by evaporation.

In the DNA process the alcohol breaks down the cell wall allowing the moisture to exit via evaporation.

Boiling the wood in water does the same thing, the expansion of water splits the cell wall and the moisture can then exit by evaporation.

The vacuum process is very similar to the boiling process except it deals with much lower heat and pulls the water from the wood. At about 72 degrees F in a vacuum of about 29 inches water boils. So if you put a piece of wood in a vacuum and are able to pull a tight enough vacuum the water in the cell boils, ruptures the cell wall and then is literaly sucked out by the vacuum. Great in theory and it works. The challange is building a vacuum chamber large enough to hold the wood and then be able to pull that high of a vacuum. Not difficult just costly.

Most of the commerical units interject heat into the equation, because as the temperature of the wood goes up the inches can come down and accomplish the same goal. The problem is that heat does not transfer very well in a vacuum except by direct contact so you have to create some way to heat the wood to about 95 degrees inside the vacuum while maintaing about a 26 inch vacuum, and remember these are not dimensional boards but bowl roughs. Also you have to deal with the water you are sucking out of the chamber so it doesn't go into you vacuum pump and screw things up there.

Those that are successfull are claiming 1" thick green bowl blanks to 15% dry bowl blanks in 3 days or less. The question is "at what cost". Both for the apparatus and the operating expense.

I am using a Welch pump, and have several hundred dollars in parts and pieces. I'm not done yet either. The chamber is 18" in diameter and 24" tall and will accomodate about 3 bowl blanks. My challange is how to bleed off the moisture before is makes it to my pump and contaminates the pump oil without spending thousands of dollars for a liquid nitrogen separator. If I don't solve this one the cost of oil will probably be about $20 a session which is pricy to dry bowl blanks.

Google "wood vacuum kilns" and you will find a lot good information.

Good luck

[phil harold]

Phil;





The vacuum process is very similar to the boiling process except it deals with much lower heat and pulls the water from the wood. At about 72 degrees F in a vacuum of about 29 inches water boils. So if you put a piece of wood in a vacuum and are able to pull a tight enough vacuum the water in the cell boils, ruptures the cell wall and then is literaly sucked out by the vacuum. Great in theory and it works. The challange is building a vacuum chamber large enough to hold the wood and then be able to pull that high of a vacuum. Not difficult just costly.

Most of the commerical units interject heat into the equation, because as the temperature of the wood goes up the inches can come down and accomplish the same goal. The problem is that heat does not transfer very well in a vacuum except by direct contact so you have to create some way to heat the wood to about 95 degrees inside the vacuum while maintaing about a 26 inch vacuum, and remember these are not dimensional boards but bowl roughs. Also you have to deal with the water you are sucking out of the chamber so it doesn't go into you vacuum pump and screw things up there.

Those that are successfull are claiming 1" thick green bowl blanks to 15% dry bowl blanks in 3 days or less. The question is "at what cost". Both for the apparatus and the operating expense.

I am using a Welch pump, and have several hundred dollars in parts and pieces. I'm not done yet either. The chamber is 18" in diameter and 24" tall and will accomodate about 3 bowl blanks. My challange is how to bleed off the moisture before is makes it to my pump and contaminates the pump oil without spending thousands of dollars for a liquid nitrogen separator. If I don't solve this one the cost of oil will probably be about $20 a session which is pricy to dry bowl blanks.



Good luck

There seems to be alot of science and maybe some solutions for you

1. radiant heat will travel thru a vacuum, this is how the sun warms the earth

2. worried about moisture maybe your pump type, is wrong Welch makes oil rotary vane vacuum pumps, dry piston and diaphragm vacuum pumps so I assume you have oil rotary vane pump, could you possibly use ethylene glycol instead of oil? and/or build a condensation coil before it reaches the pump (chill coil with ice if need be)

3. at what cost?
many turners already have a pump
time is money if woodturner can dry bowls in three days instead of 6 months thay cut down thier inventory and get product out faster with less damage to the wood it may be worth the effort

There is plenty of info out on the net, maybe to much
I have a pump and would like to be able to use it for more than just vacuum chucking.
If making a small kiln works, I am game to give it a try as soon as I get the "honey-do list" completed

thanks for all your info!
I did not understand how the moisture gets out of the cells, thanks for clearing that up for me

[Thom Sturgill]

Phil;

I have been working on a vacuum kiln for the last year.
There is a lot of science involved in the process, which the commerical kiln market has already addressed and solved but the basic principle is simple.

There are two type of moisture in wood that needs to be removed. That which is asborbed or is in the space between the cells, and that which is inside the cell walls. Given the nominal thickness bowl blanks the asorbed moisture comes out easily with a vacuum because it is not contained by anything. The moisture that is in the cell wall is different.

Something needs to happen to break the cell wall to allow the moisture to excape. In air drying the cells rupture as they dry and the moisture is then not confined and works its way out of the piece by evaporation.

In the DNA process the alcohol breaks down the cell wall allowing the moisture to exit via evaporation.

<snip>
Good luck


Why not combine the two processes? Could you do a DNA soak or boil and then vacuum kiln to speed the drying while getting the 'drying from the inside out' effect? This might let you use a lower vacuum level, and be easier to achieve.

[Julian Nicks]

Instead of using a vacuum pump, why not use a venturi vacuum system? I have used my venturi seup for years now, and it's a great way to get by the problem you face with moisture since the water in the line will just blow right out of the venturi nozzle, and into the air. www.joewoodworker.com (http://www.joewoodworker.com) is where I bought my kit which cost under $200 to build, and it's a great place for ideas.

[Reed Gray]

I did read a while back about a turner who was making a vacuum kiln. Never got the final results. I have worked a lot of lumber from a vacuum kiln, and love it. It works like air dried lumber, as in you rip a board on the table saw, and you get shavings, not dust, and you can rip a thick board on your bandsaw, and get no spring. The lumber kilns have extruded aluminum sheets between lifts, and they run antifreeze through it, which is heated by wood fire. Worked really well. It would be nice to be able to do bowls this way, but my guess it that it would be more work than it is worth.
robo hippy

[Ralph Lindberg]

Reed

I recall Fred Holder talking about someone doing that a couple years ago, but like you I don't recall hearing anything about the results

[Kim Ford]

Phil;

Thanks for your reply.


You are right about heat traveling through a vacuum it just doesn't do it very well, which means it takes longer.
With some woods the moisture comming out is not nice stuff. With all of the acid's and natural occuring chemicals in can be a bit nasty and gum things up a bit. I have in mind making a separater by using copper tubing inside an old refer or freezer, and using a house hold pressure cooker as the collection point. But remember you have to pull a very tight vacuum which means every connection point and seal must be perfect.
I'm not using my Gast pump because it is only rated for about 24" (some will pull more I know) which is great for vacuum chucking only if you put a relief valve on the setup allowing some air to pass through the system to cool the veins. But holding even a 24" vacuum on a Gast for three days without relief would probably burn it up.

This is a link to a science paper by a university student that might be worth looking at: http://scholar.lib.vt.edu/theses/available/etd-02198-185538/unrestricted/DISS.PDF

I'll keep you informed as things develop. Please let us know how it goes with you.

Thanks

Kim

[Julian Nicks]

As I posted earlier, I am surprised nobody has thought about a venturi vacuum system for this. There is no vacuum pump to worry about, just an aluminum bypass(venturi) valve hooked up to an air compressor. I've had epoxy get in my lines, and the venturi wasn't even ruined, just the lines were.

[phil harold]

As I posted earlier, I am surprised nobody has thought about a venturi vacuum system for this. There is no vacuum pump to worry about, just an aluminum bypass(venturi) valve hooked up to an air compressor. I've had epoxy get in my lines, and the venturi wasn't even ruined, just the lines were.

how much vacuum does you venturi pull (hg) and cfm
also how big of a compressor do you have?

[dirk martin]

I just found this thread, and wish it didn't die.
I am pulling a vacuum, using an air compressor and a venturri valve, in a medium sized container. Seems to work quite well. as Julian suspected.

I just need to figure out a heat source, now, since I can't seem to get below 28 inches of vacuum.

[Josh Bowman]

Ken,
I'm a plant operator at a power plant and we use vacuum pumps for our main condensor. The type most power plants use is called a liquid ring vacuum pump. I havn't a clue if they are available on a small scale. But they solve two problems. The pumps want water, so the moisture coming out of the bowl container will help with make up and the water passing through cools the impeller.
Now to my question. As was mentioned, would DNA not speed up the process? The DNA will rupture the cells and displaces some of the woods water. The DNA boiling point is far less than water so the vacuum pump will remove the DNA in a few hours, I would think. I would be curious what the moisture content left was, just after the DNA escapes.
The logistics of soaking a bowl for a few days in DNA is simple, then if the vacuum system could just boil that off with some of the water in a few hours, this would be great.
Josh