Drying wood by vacuum?

[eranox]

I don't have a vacuum generator or stabilizing setup (yet!), but I was wondering if it would be possible to speed the drying of green wood by holding it under vacuum. When stabilizing, the vacuum draws atmosphere out of the wood, so it occurred to me that you might be able to draw out moisture with it. Just bare wood under vacuum, no liquids, etc. Can this work, does it cause cracks, or would it do nothing?

 

[plantman]

It probably wouldn't do to much because as soon as you release the vaccum, air will fill the voids again. That is the reason for the stabilizor, to replace the air in the wood. A microwave or toaster oven will speed up the drying of blanks. Go to Goodwill and find one that works, don't use the one in the kitchen!! A cardboard box with a 25 Watt bulb placed inside will speed up moisture release. Placing blanks on top of your heating ducts, furnace, or hot waer heater will help, just turn them every few days to keep the moisture even. Any of these will help dry out the wood and remove moisture, but they won't stabilize the wood, and may even make it crack. Drying needs to be done slowly!! I am sure others here have their own methods. Jim S

 

[Curly]

Vacuum will work nicely with little degradation. Joshua Salesin, an accomplished Ornamental Turner, wrote a little book on making small vacuum kilns for drying bowl blanks and the like.

 

[Gary Beasley]

I have experimented with heating wood and placing it directly in my vacuum tank without resin and-watching as it pulls moisture from the wood. Works best when doing really wet wood, you still need to bake it a while to get all the residuals out before stabilizing.

 

[low_48]

Vacuum kilns have been around for some time. The advantage to the kiln operation, is that water boils at a much lower temp when in a vacuum. Something like 95 degrees or so. That leads to much lower degrade of the wood. The tricks are to get heat to the wood, and then get the water vapor out of the kiln. Radiant heat does not work in a vacuum, so the production kilns use aluminum extrusions that circulate warm water through them. Each layer of wood has a plate above it and a plate below, so the wood has full contact with a warm surface. Then there is the problem of getting rid of the water from the wood so it does not go through your pump. They run the vapor through a chiller in front of the pump. Some homemade deals involve pulling the vacuum and then releasing it to pull out the water, but they let it dissipate to the atmosphere. You really have to be careful to not pull the water vapor through your pump. Some woods are very high acid, well you get the point......

 

[Gary Beasley]

The first time I ever heard of a vacuum kiln the one shown was large and used microwaves to heat the wood.

 

[eranox]

Interesting. Thank you all for the info! I was hoping it would be as simple as applying a vacuum and watching the water boil out of the ends of the wood. Apparently, I need vacuum AND heat. Well, I have neither yet, so I'm not really out anything. Perhaps when I get my stabilizing setup, I'll give this a try. If water will boil in a vacuum at 95 degrees (Farenheit, I hope), then perhaps I can heat some wood in a toaster oven and then get it under vacuum before it cools. Somehow, I suspect it won't be that easy. If nothing else, I'll probably lose too much heat too rapidly from the phase change from water to vapor.

 

[Gary Beasley]

From a practical standpoint it's simply easier to leave the blanks in the oven overnight than mess with swapping the blanks back and forth from the oven to the tank.

 

[MesquiteMan]

Actually, water will boil at much lower than 95° F in a really deep vacuum. I can fill a chamber with 75° F water and pull full vacuum and bring it up to a rolling boil. As a matter of fact, at a 10,000 micron vacuum which is easily obtainable with the right chamber and a good rotary vane pump, water will boil at 53° F. See this chart for more information: http://www.jbind.com/pdf/Cross-Reference-of-Boiling-Temps.pdf

As for keeping water from your pump...it depends on the kind of pump you are running. If you are running an oil filled rotary vane pump, water vapor WILL NOT hurt it at all. They are made for this purpose. I am talking about the kinds of pumps used for HVAC work. The primary reason to pull a vacuum on an HVAC system is to boil off the water in the system before introducing freon. The water in an HVAC system also contains acids and such. It is recommended when doing this king of work to change the oil after every use but this is primarily because the water vapor contaminates the oil make the pump less efficient, not because it will damage the pump. By the way, I got the HVAC info directly from a licensed HVAC contractor who used to do HVAC work for me when I was building houses. The pump info I got directly from the head technician at JB Industries which is one of the premier manufacturers of quality vacuum pumps in the US.

Also, from personal experience...I test every vacuum chamber I make by filling it partially with water and pulling a full vacuum until the water is under a rolling boil. Once my oil level starts to get higher in the pump or I start pulling a lesser quality vacuum, I will change the oil. I have set this oil aside to let it separate and you would be surprised how much water is in it. I have been doing this with the same pump for 3 years of hard use and nothing has happened to my pump.

As for using vacuum to remove the water form wood...it can be done without heat but not very efficiently. If the wood is already air dried, it will take 15-20 hours of the pump running continuously at full vacuum to remove the water. You will not see much, if any water being removed as it will be boiling the water off and it will be coming out of the wood as vapor. Drying green wood with vacuum and no heat will take much longer and is simply not worth it.

 

[low_48]

Isn't boiling water inside wood tougher than a tank of water? I would think the wood will insulate it somehow. Just trying to make sense of it. Otherwise the commercial vacuum wood kilns wouldn't go to the expense of those aluminum plates. Wood-Mizer vacuum kilns basically used electric blankets if I remember correctly.

 

[larryc]

I've used this method with good results.

Microwave drying wood turnings

 

[lrawlins]

One thing to consider is what happens to the moisture?

It ends up in you vacuum pump oil which is not a good thing.

Better to put it in an oven for a time than mess up a perfectly good vacuum pump.

Bye

 

[MesquiteMan]

Isn't boiling water inside wood tougher than a tank of water? I would think the wood will insulate it somehow. Just trying to make sense of it. Otherwise the commercial vacuum wood kilns wouldn't go to the expense of those aluminum plates. Wood-Mizer vacuum kilns basically used electric blankets if I remember correctly.

Rich,

The moisture in the blank does not know it is insulated! As long as the temperature of the water is 53° F, under a 10,000 micron (29.526" Hg at sea level) vacuum, it will boil. Remember, boiling water does not require heat.

Commercial vacuum kilns add heat because they are not pulling the very deep vacuums that are possible in a small vacuum chamber. Most of the information I could find shows that a vacuum kiln usually only pulls a 124,973 (25" Hg) to 74,171 (27" Hg) micron vacuum. At 124,973 microns, water needs to be around 130° F to boil. At 74,171 microns, it needs to be around 110° F to boil. (I did not do the math on this, just guestimating from the chart linked above).

Thus the need for heat!

 

[MesquiteMan]

One thing to consider is what happens to the moisture?

It ends up in you vacuum pump oil which is not a good thing.

Better to put it in an oven for a time than mess up a perfectly good vacuum pump.

Bye

As I mentioned above, the moisture gets pulled into the pump and if you are running a rotary vane, oil filled pump, it WILL NOT hurt the pump. They are made for a very similar application of boiling water out of HVAC systems!

 

[larryc]

One thing to consider is what happens to the moisture?

It ends up in you vacuum pump oil which is not a good thing.

Better to put it in an oven for a time than mess up a perfectly good vacuum pump.

Bye

As I mentioned above, the moisture gets pulled into the pump and if you are running a rotary vane, oil filled pump, it WILL NOT hurt the pump. They are made for a very similar application of boiling water out of HVAC systems!

Curtis

Read your post with interest because I recently had to replace my JB industries pump because the pump froze up. The motor is still good.

I had not used the pump for about a year and I thought that the problem occurred because of water and resulting rust.

I had never changed the oil in the pump.

I've gotten paranoid about replacing the oil in my new (used) JB pump after every use because of my experience, but reading your post tells me I don't have to.

Any thoughts?

 

[MesquiteMan]

I guess I should clarify a little...sucking moisture into an oil filled rotary vane pump will not hurt it at all. HOWEVER, if you suspect that you have sucked a lot of moisture into your pump such as trying to use it for drying wood, then it would be highly advised to change the oil after use.

When stabilizing, you should be drying your wood ahead of time to 0% moisture in the oven for best results. If you are doing that, then there will be no moisture in the pump and no need to change the oil. If I was going to let my pump set for a long period of time without using it, I would change the oil first.

 

[Jim Burr]

Why not just DNA it for a week and dry it for another 2-3 and be done with it. Most professional turners do this every day and a pen blank will take 1/10th the time.