Vacuum kiln dried wood

[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_ba...rinciples.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.

[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.