Vacuum kiln dried wood

[jim mills]

Are there any studies out that show the differences between conventionally kiln dried wood vs. vacuum kiln dried wood? I hear claims that the vacuum process results in less end checking, and obviously speeds up the drying process, but that's about all I've heard. I'm curious if there are claims of the wood being more stable.

[Jim Becker]

A good vacuum kiln can do a very good job even on very thick material. 'Tuber Matt Cremona has been taking his slabs and workbench kit materials to a large vacuum kiln in his area for some time now. The drying seems to be very even and extraordinarily fast at the same time, considering some of that material is like 8" thick. AFAIK, he's toying with actually buying a unit someday to bring that in-house. Considering the level of investment that requires, it says to me that the results he's getting are pretty good. And he makes a chunk of his living with those slabs and heavy work bench kits.

[Maurice Mcmurry]

I learned about the vacuum kiln from John Griffin at Old Standard Wood. He talks a little about it in one of his YouTube's. I am just putting the finishing touches on a small one that fits in the wood cooker (trying to reverse engineer the roasted wood that Warmouth and others are using)


https://youtu.be/H64dtff8x5s

[Jon Grider]

Just askin', too time starved to research my own question, but does vacuum drying kill any of the bugs and critters?

[Maurice Mcmurry]

Great question! I am not sure but would expect so. "OK Google"...

A funny rabbit hole, now I know a little about bed bugs, roaches and vacuum cleaners.

John says he goes "down to almost outer space". That is going to kill bugs.

"almost all insects can survive without oxygen for many hours."

"Water boils at 212°F at sea level. However, if we pull a 500-micron vacuum on a sealed refrigeration system, the water inside of that system will boil at -12°F."

[Richard Coers]

The advantage of a vacuum kiln is that water will boil at 90 degrees F in a hard vacuum. The trouble is that radiant heat does not work in a vacuum. You need heating blankets, or heating manifolds with warm water between every layer of wood. Then taking condensation out of the kiln is not easy either. Another issue with vacuum kilns is that wood density changes in a board (like crotch figure) takes longer to dry in the vacuum than the rest of the board. If a proper conditioning cycle is run in a traditional kiln, I don't think there is an advantage for stress between one to the other.

[Maurice Mcmurry]

When I first met Mr. Griffin he has using a WoodMizer vacuum kiln and having problems with it. His vacuum pump then was a pump and dump water venturi powered by his well. His newer homemade one is interesting.

[Robert Hayward]

The advantage of a vacuum kiln is that water will boil at 90 degrees F in a hard vacuum. The trouble is that radiant heat does not work in a vacuum. You need heating blankets, or heating manifolds with warm water between every layer of wood.
A hardwood supplier a few miles from me has a pretty big vacuum kiln. I have watched his crew load carts with green wood to be rolled into the kiln. There were no blankets or anything else. Just the lumber stacked on the carts ready to go into the kiln. HIs kiln is a commercially made one about 7'X7'X30', I am guessing. The name is on the kiln but I cannot remember it.

[Richard Coers]

A hardwood supplier a few miles from me has a pretty big vacuum kiln. I have watched his crew load carts with green wood to be rolled into the kiln. There were no blankets or anything else. Just the lumber stacked on the carts ready to go into the kiln. HIs kiln is a commercially made one about 7'X7'X30', I am guessing. The name is on the kiln but I cannot remember it. There has to be a temperature differential somewhere. Vacu-Therm uses a alternating cycle of vacuum and atmospheric conditions to draw the water from the core. https://www.vacutherm.com/vacuum-kiln-products/vacupress/vacupress-product-lines/

[Greg Quenneville]

I read recently about high conventional kiln temperatures causing pitch to set hard. Surely that has to be a heat conversion process? Maybe not a factor in many wood species, but perhaps something to consider.

Water may well boil at very low temperatures in a vacuum, but you can still hold it in your hand, and it is more of a gee whiz phenomenon than anything else. (Speaking from experience of a couple of high altitude decompression chamber rides almost fifty years ago)

[Maurice Mcmurry]

I read recently about high conventional kiln temperatures causing pitch to set hard. Surely that has to be a heat conversion process? Maybe not a factor in many wood species, but perhaps something to consider.

Mr. Griffins processes are very species specific. His Spruce is highly prized partly because of his process. He gets the target moisture content without ever exposing the wood to any warmth (quite the contrary). I am working with Maple and Black Locust on a very small scale.

[Bill Space]

The trouble is that radiant heat does not work in a vacuum.

Radiant heat seems to work pretty good across millions of miles of vacuum between the sun and the earth. :)

[Sebastien La Madeleine]

Radiant heat seems to work pretty good across millions of miles of vacuum between the sun and the earth. :)

Right!

Molecules and atoms of the medium can absorb, reflect, or transmit the radiation energy. If the medium is a vacuum, since there are no molecules or atoms, the radiation energy is not attenuated and, therefore, fully transmitted. Therefore, radiation heat transfer is more efficient in a vacuum.

I think what was meant was that you can't heat vacuum like you can air to reach every board. And since radiant heat is going to be absorbed by the first object in line with the source, he meant to say that you have to distribute the source of heat for it to be effective.

One other way that is used is microwave. Inside of the vacuum, they emit microwaves to inject the energy directly in the water molecules inside the wood. This allow the traversal of pretty much all material inside of the kiln.

The trifecta would be a dehumidified vacuum microwaved kiln. This will dry wood extremely fast with very little change in material (no cracking/splitting or warping). They are expensive though to both buy and run.

[Maurice Mcmurry]

I have had some success with the regular microwave, but did not do it scientifically.

[Richard Coers]

Radiant heat seems to work pretty good across millions of miles of vacuum between the sun and the earth. :)
For some reason, someone always brings up the sun and earth when a discussion of vacuum kilns starts. I'll let a professor rebut your comment; From Professor Gene Wengert, forum technical advisor:
The sun heats due to emitted radiation, but in outer space it only heats the surface of what the energy hits. Without air, there would not be heating within the stack.You will find over-drying and under-drying from piece to piece and within an individual piece. Contributor D's dryer addresses this issue, but others I have seen and used did not except for Vacuum-Therm.
The oscillation between vacuum and atmospheric must be done carefully indeed. Humidity at atmospheric is important.
https://www.woodweb.com/knowledge_base/Vacuum_Kiln_Principles.html

[jim mills]

Lot's of interesting comments on the subject. It's clear why there is no definitive answer to the question. I've asked a couple vacuum driers, and they dont have answers. My conventional kiln drying sawyer doesn't know either. It appears that perhaps vacuum drying "freezes" the wood in it's current state, resulting in less movement & checking in the drying process. The question that remains is how the vacuum dried wood will react long term with exposure to environmental cycles of warm/cold, and humid/dry conditions. My sawyer doesn't paint his log ends, and claims the end checking is part of the stress relief process of drying wood.

[jim mills]

I visited Old Standard Wood several years go, and got the nickle tour. It was my first exposure to vacuum drying. John's comments were that the process simply speeds up the process.

[Thomas Wilson]

My unscientific guess is that if bugs are dried to 7% moisture they would be dead. A jar (with ventilated lid) full of various critters in the vacuum kiln would give experimental confirmation. Science can be fun. I have a wood pile that could be used for specimen gathering.

[Maurice Mcmurry]

I thought about putting a bug in a big syringe, closing it most of the way, capping the end then opening it. I better go make sawdust instead.

[Bill Space]

For some reason, someone always brings up the sun and earth when a discussion of vacuum kilns starts. I'll let a professor rebut your comment; From Professor Gene Wengert, forum technical advisor:
The sun heats due to emitted radiation, but in outer space it only heats the surface of what the energy hits. Without air, there would not be heating within the stack.You will find over-drying and under-drying from piece to piece and within an individual piece. Contributor D's dryer addresses this issue, but others I have seen and used did not except for Vacuum-Therm.
The oscillation between vacuum and atmospheric must be done carefully indeed. Humidity at atmospheric is important.
https://www.woodweb.com/knowledge_base/Vacuum_Kiln_Principles.html


Not trying to start a debate, but this professor agrees that radiant heating does work in a vacuum.

And he does raise an interesting point. Without a medium to move heat through convection (and conduction) radiant heating would not work well with a snack of lumber, as the pieces within the pile would essentially shielded by the outer layer and not heated.

But for a slab this would be different, since the surface of the slab would be heated, and the slab itself would heat to the center by conduction, similar to the way stacks of boards are heated in a normal kiln.

This may be the reason the person referred to initially may be finding the vacuum kiln very useful for drying slabs, which might not be the same with stacks of boards.

[Scott T Smith]

Vacuum kilns can provide extremely high quality drying, but like with all kilns the skill of the operator plays a role.

They can be broadly divided into two different categories - deep vacuum and shallow vacuum.

Shallow vacuum kilns such as the iDry are able to use atmosphere to convey heat into the stickered lumber, because they do not pull a deep enough vacuum to remove all of the atmosphere from the chamber.

Deep vacuum kilns require some type of heat source that is either touching the lumber (heat exchanger plates placed in-between each layer of wood), heat blankets, or at the top end of the technology scale microwaves (RF).

Shallow vacuum kilns have a heat exchanger present in the kiln and air is recirculated across the heat exchanger and through the stickered lumber.

Heat exchangers in "most" deep vacuum kilns consist of tubular plates that are between the layers of lumber. They recirculated an antifreeze solution that is heated from an external heat source.

Almost all baseball bat blanks are dried in vacuum kilns. Many vacuum kilns still run a sterilization cycle at the end of the kiln run in order to heat sterilize any pests.

[jim mills]

If water boils at such a low temperature under a vacuum, why is heat a necessary part of the process? The sterilization process I get.

[Richard Coers]

If water boils at such a low temperature under a vacuum, why is heat a necessary part of the process? The sterilization process I get.
It is very rare for it to hover around 90 degrees F where I live. Most people want to dry lumber year round.

[Scott T Smith]

If water boils at such a low temperature under a vacuum, why is heat a necessary part of the process? The sterilization process I get.

The vacuum process results in a significant cooling effect due to the water evaporation. This impedes moisture transfer in the lumber.

[Maurice Mcmurry]

I subscribed to Legacy Lumber's YouTube Chanel. This video shows that Sebastian describes , it's very interesting.


https://youtu.be/sNPXnpMKPjk

[Kevin Jenness]

As Scott Smith said, operator skill plays a large part in the results with any kiln. Some time ago a client delivered huge chunk of reclaimed Doug Fir for a bar, something like 6" x 48" x 12'. The story was it had been recovered from a shipwreck and "dried" in a RF vacuum kiln. It was ungodly heavy and further inspection showed that it registered 5% mc at one end and 30% at the other, and had scorch marks around the massive dutchman that had been let in to make up for the effect of the kiln-induced fire. That piece went back to the supplier.

I am hoping to get a tour of the Idry factory over in Barre soon. A friend of my son is employed there as an engineer and they are doing a sample load of 16/4 white oak and hickory for him.