wood drying sort of

[pete00]

Howdy

I've been trying to read up on all the various ways to dry out and stabalized wood. Then i got to wondering.

Instead of trying to force something into the wood can we push or force the water out.

What would happen if you put a wet piece of wood in a container and pressurized it with air.

Would the air push out the water in the wood, or would it just keep everything as is.

What would happen if you released the pressure everyonce in a while, would that force any water out.

Something like a compressed air sprayer. The one you fill up with paint, hit it with pressure pull the triger and spray everything except what you aiming at.

Im sure someone will say great idea it will work or
you should not have fallen asleep in physics class wont work.

pete

 

[Dario]

Problem with this is...water cannot be compressed. What is inside will stay inside unless you blow it out of the wood...which could be tough I imagine.

Air can be compressed and the ones inside the wood will be...what may happen is you'll force the water that is scattered around voids in the wood further in.

Just my thought.

 

[Dario]

I have been experimenting with another drying method/idea and seems to be working great. I will post about it when my test get more conclusive.

 

[gerryr]

From a strictly physics standpoint, and I'm certainly not an expert, putting the wood under pressure doesn't seem like it would force any moisture out. But, using a vacuum pump might pull moisture out. I don't know about this because I've never tried it, so take it for what it's worth - speculation only.

 

[Dario]

Gerry,

Again, water cannot be compressed...air can. Vacuum will only work on compressed matter.

You may force out the water if the air entrapped inside doesn't have any other way out and it pushes the water out as it exits/expands. If it finds another way, then it won't bother moving the water.

Imagine your pumps and jacks...if there is a leak...it won't work since air will find the easier way (through a leak).

Hope I am making sense.

 

[pssherman]

Pete,
A vacuum will work to remove the moisture in the wood. By lowering the pressure to below the vapor pressure of water, liquid water will boil/evaporate inside the wood creating vapor. The vapor takes up a lot of room so most of it is forced out of the wood. The evaporation cools the wood so it will need to have heat applied to it if is large or very wet. This is just a simple explanation of vacuum drying which has been around for quite a while now. To learn more on this topic do a Google on "vacuum kiln".

Hope this helps.

Paul in AR

 

[pete00]

thanks for the info all

paul

just came back from google.....

how can seperating two simple things like wood and water be so complicated[B)]

my wife seperated me from my money easy enough,
mabey i should ask her how to do this..[}][}]

oh god... she's gonna know i said that.[8]

 

[Dario]

Originally posted by pssherman
<br />Pete,
A vacuum will work to remove the moisture in the wood. By lowering the pressure to below the vapor pressure of water, liquid water will boil/evaporate inside the wood creating vapor. The vapor takes up a lot of room so most of it is forced out of the wood. The evaporation cools the wood so it will need to have heat applied to it if is large or very wet. This is just a simple explanation of vacuum drying which has been around for quite a while now. To learn more on this topic do a Google on "vacuum kiln".

Hope this helps.

Paul in AR

Paul,

That is a cool system but totally different from what Pete asked since you need to add heat to it.

I am bit confused too...are you saying that by lowering the pressure the water will boil and you will only need a heat source if the wood is big or very wet? This is new to me. [B)]

 

[leehljp]

Dario,

We used to discuss the possibility of "boiling" water until it freezes in our physics classes -&gt; 40+ years ago. "Boiling" has as much to do with the external pressure as with the temp. Most people think that boiling has to do with temperature only. Decrease the pressure and you will lower the evaporation and boiling point/temp.

What I said is not necessarily the technical aspect of it, but it is the practical aspect.

 

[Dario]

Thanks Hank.

I do remember a bit about pressure/temperature relationship...but didn't quite thought that water will actually boil by just lowering the pressure.

That probably explains my marginal grades in Physics [][}]

 

[pssherman]

Perhaps further explanation is in order. Remember I am an engineering professor.

If the pressure in the container is lower than the vapor pressure of water at the given temperature, evaporation will occur. However, this evaporation will cause the pressure in the container to rise AND the temperature of the wood to decrease. The lower temperature also results in a lower vapor pressure. Eventually the pressure will reach the vapor pressure of the cooler temperature. To continue this process, the wood will need to be heated and the vacumm lowered again. In a commercial application, the kiln is stuffed to the max and the wood is very wet. This means that the moisture needs to be continually removed with a vacuum pump and the wood heated to keep the vapor pressure high enough. Temperatures generally do not exceed about 120-140 degrees.

For Pete's application of a few blanks at a time, a canning jar would have a volume many times greater than the blanks. Since the blanks probably are not 'green' removing the moisture would not cool the blanks a great deal and the pressure in the jar would not rise as much. So, you could but a few blanks in the jar and pull a vacuum which would remove some of the moisture and repeat periodically until the blank is dry enough.

There is an article on the article page that describes stabilization with a vacuum by Lee Biggers(?). In this case the vacumm removes the air so the polyurethane can more easily get into the wood.

Hope this clears thing up a little.

Paul in AR

BTW, the minimum vacuum required for various temperatures is:
140 F needs 24 in Hg
120 F needs 26.5 in Hg
100 F needs 28 in Hg
80 F needs 29 in Hg
70 F needs 29.2 in Hg
60 F needs 29.4 in Hg
and a perfect vacumm is about 29.92 in Hg

 

[DWK5150]

Intersting info here I think just a good general about drying wood

http://www.ca.uky.edu/agc/pubs/for/for55/for55.htm

 

[pete00]

Originally posted by pssherman
<br />Perhaps further explanation is in order. Remember I am an engineering professor.

Paul in AR

So professor(feel like im back in school, ready to flunk a test)

Is this correct
If i have a container big enough to hold a 3 by 6 inch piece of wet wood, and apply a vacumm to it it may dry the wood if its not too wet

If its too wet if i apply heat to it in the range you mentioned. It will dry the wood.

Does the heat need to be in the inside of the container with the wood, or can the container be heated(like sticking in it an oven.

thanks in advance for answereing.

boy my wife is gonna like this one...

 

[Dario]

Thanks Paul!

Now, I have another idea to play with []

"BTW, the minimum vacuum required for various temperatures is:
140 F needs 24 in Hg
120 F needs 26.5 in Hg
100 F needs 28 in Hg
80 F needs 29 in Hg
70 F needs 29.2 in Hg
60 F needs 29.4 in Hg
and a perfect vacumm is about 29.92 in Hg"

He-he-he...I know Texas heat will be of some benefit. A closed container wrapped in black plastic will easily hit 140 degrees here during summer. [}]

 

[pssherman]

Originally posted by pete00
<br />

Originally posted by pssherman
<br />Perhaps further explanation is in order. Remember I am an engineering professor.

Paul in AR

So professor(feel like im back in school, ready to flunk a test)

Is this correct
If i have a container big enough to hold a 3 by 6 inch piece of wet wood, and apply a vacumm to it it may dry the wood if its not too wet

If its too wet if i apply heat to it in the range you mentioned. It will dry the wood.

Does the heat need to be in the inside of the container with the wood, or can the container be heated(like sticking in it an oven.

thanks in advance for answereing.

boy my wife is gonna like this one...

Note that I have now added the required vacuum levels for various temperatures in my previous post. The wood must be kept at these temperatures.
Since there is virtually no air in the container, heating by convection will not work. Direct contact and thermal radiation (infrared) are the only ways to heat the boards in the container. This is why commercial kilns are so very expensive and problematic.

Another alternative for drying small pieces of wood is to use a microwave oven. This topic has been discussed on this forum recently.

I use an old freezer (nonworking) with a light bulb continously on to control moisture content and to reduce moisture in "wet" blanks that I get from ebay. The inside of the freezer is about 20 F warmer than outside (my unheated shop), which reduces the relative humidity. The lower relative humidity corresponds to a lower moisture content in the wood.( see the link in DWK's post) This method is slower than using a vacuum but is easier to accomplish.

Paul in AR

 

[Johnathan]

Wow, that was a good lesson Professor. Too much for my brain to handle, I'm just a simple minded musician!Sounds like it would work, if anyone has success with the wood drying let me know.

 

[pete00]

thanks paul..

back to the drawing board, i have articles on most of the drying methods. I must admit i get lost in some of the explanations of why the process works.

really appriciate all the comments and advice from all.
thanks....pete

 

[pssherman]

He-he-he...I know Texas heat will be of some benefit. A closed container wrapped in black plastic will easily hit 140 degrees here during summer.

Dario
For this to work you would need to use a clear container and wrap the wood tightly in black plastic.[]
However, if you are considering doing lots of wood (like 100 bf or more) you could use this concept for a solar kiln. Lots of homeowners/small time sawmills use these to dry lumber. They are easy to make and relatively inexpensive as far as kilns go.[]

Paul in AR

 

[pssherman]

Originally posted by pete00
<br />thanks paul..

back to the drawing board, i have articles on most of the drying methods. I must admit i get lost in some of the explanations of why the process works.

really appriciate all the comments and advice from all.
thanks....pete

Last year I read many articles on the various methods of drying wood, probably some of the same ones you found. My engineering background (30 years) helped me to be able understand the how and the why of these methods. If you have any questions send me an email and I will try to explain what they are saying.

Paul in AR

 

[leehljp]

Another way to get the moisture out of wood is to MICROWAVE it. Very effective, but should be done carefully! I Don't recommend anyone stick a piece of "wet" wood in the microwave on high for 3 minutes!

Thirty seconds and wait a few minutes; 30 seconds again and wait about 10 to 15 minutes; 30 seconds again and wait until it is room temp. Repeat again after room temp has been reached.

This is what I do but it is not scientific. Some might have better and more precise recommendations than mine.

I have a vaccum chamber but not a pressure chamber. I am "planning" on getting one of those HF pressure pots in the next couple of months. It should work well with microwaved "dry" blanks.

<b>WARNING:</b> I decided to microwave a couple of fresh corn cobs after I had eaten the corn. (I wanted to attempt a corn cob pen.) Works great and shrinks them considerably - to a point and then it will BURN them BLACK! DAMHIKT! [)]

 

[Dario]

I've been using the microwave drying method for years...so much so that LOML, gave me the microwave and it is now in my "shop" (garage).

Ofcourse that is only because she already got a new one. []

 

[tnilmerl]

How to boil water until it freezes. This was (and still is) the trick question I as my kids science teachers. Ability to correct answer (or not) demonstrates a fundamental understanding of the principle of energy transfer and pressure equilization. The reason the freezes is that energy is required co convert the moisture to a vapor as the system attempts to equilize the pressure. The energy is usually drawn from the environment, and heat is most readily accessible and convertible energy source. Heat is pulled from the water as it transitions from a liguid to a gas. If you can pump the air out fast enough and create a vacuum, then the water freezes. Haven't you ever seen a sci-fi space movie when someones spacesuit is torn? Same thing.

For small pen blanks, a vacuum pot is easy to assemble and void the air from it.

I haven't tried it, but I wonder if anyone has tried to use a veneer vacuum press system to dry out larger samples?

 

[pssherman]

Originally posted by tnilmerl
<br />I haven't tried it, but I wonder if anyone has tried to use a veneer vacuum press system to dry out larger samples?

This might actually have enough surface contact, for 1 or 2 layers of wood, to transfer the heat from the outside. You would probably need to have the vacuum pump running continuously though.

Dario, this might work with your Texas sunshine.

Paul in AR

 

[rtparso]

Ok Prof you did real good accept for one principle that limits the rate that the water can be removed. Much of the water is not in a free state. It is either in inclusions (cells) or bonded (at a molecular level) to other materials in the wood. The bonded water will for the most part will not be removed with drying. That leaves the water in the free state or the included water. Under a vacuum the water in the free state is removed from the ends of the capillaries but due to the capillary action the rate that the water can be removed is limited. Not that the vacuum does not accelerate the process, it is gut limited. The water in the cells has to pass through the cell walls and the speed of drying is limited to the rate that the water can pass through the cell walls. If you freeze the wood the ice crystals will rupture the cells (or some of the cells) allowing the vacuum to remove more water quicker. So vacuum will accelerate the drying rate and so will heat. Jeff Knight built a vacuum kiln last year and I learned a lot about the process. One of the problems is once you get a vacuum how do you get the heat to the wood. If you are doing pen blanks in a glass jar you could use a heat lamp since infrared heat will pass through a vacuum.

 

[tnilmerl]

and this is why I love this group so much! People are able to articulate a lucid and valid response to opinions postulated from half remembered science and experiences from 30 years ago.

Back on the veneer vacuum system. I think the entire system would be valid. The critical component would be the pump itself. The ones I've seen are relatively low-power and not designed to draw that many pounds of pressure. Some have a simple design and cannot be run for an exteneded period without risk of burning out the motor. Higher grade pumps have a shutoff system to prevent burnout.

The addition of heat lamps is a good idea. I was thinking a small room heater would do the trick.

I missed the original intent of the question. Was it to accelerate drying? On green wood, what about using a stablizing agent such as pentacryl? Water in the wood is required for effective absorbtion of the agent. I believe there is a molecular reaction between the internal water molecules and the stablizing agent. Explains why the wood cannot be dry to use the agent. Also recommended for green wood.

I have seen people use pentacryl and a vacuum pump system which causes all the air evacuate the wood. Causes the submerged blank to foam like crazy, then it settles down and is absorbed at a higher capacity than at normal air pressure.


Blah, blah, blah, how I ramble.....

 

[low_48]

We went through several ideas like these on the Forestry Forum. You can also put the wet wood in a vacuum veneering bag and then submerge that in hot water. This would let you do a single large slab if you wanted. Of course the bag must be perfect, and the hot water circulated to keep it warm. Another caution, all the moisture and acid from the wood will come through the pump.