Vacuum Kiln Principles

Vacuum kilns cause evaporation at low temperatures, but they still require heat and that heat is tricky to supply because of the vacuum. Here's an explanation. July 29, 2011

I was reading up on some small vacuum kiln projects, and I found an article where someone said they were using a Wood-Mizer vacuum kiln, and drawing about 200 too, which is about 8 inches of vacuum (so the article said). My understanding is these commercial vacuum kilns also heat the lumber.

Seems to me that the goal here is to convert the water within the wood to steam, and thus vacate the wood. To do this, we introduce a vacuum which lowers the atmospheric pressure around the wood, and we know the lower the atmospheric pressure, the lower the temperature at which water boils.

So I did some more digging, and I see that if I can draw 28 inches of vacuum, water will boil at 100 degrees F. If I can draw more than that, I won't need to heat my wood (having to heat the wood just adds more complexity to the puzzle).

I built me a very small chamber that draws about 29.5 inches of vacuum. If my shop is heated to 70 degrees, water will boil in 29.18 inches of vacuum... So it seems my shop is warm enough, and I'm drawing enough vacuum, that the wood in my chamber should expel its water as steam, without needing to mess with heating blankets and such.

Does it sound like I'm on the right track? My desire is to draw enough vacuum that I won't need to heat the wood.

Forum Responses
(Sawing and Drying Forum)
From contributor K:
Sorry, physics kills you here. As you pump out some water vapor, the evaporation will cool the wood. You can actually bring the wood down to freezing temperatures and below by this process. The colder wood (and the colder water contained in that wood) evaporates water much more slowly. You have to add heat. But this is difficult, since the wood is in a (partial) vacuum. Ambient heat (the temperature outside the vacuum kiln) goes in very slowly because of the vacuum (like a thermos). You need to put radiant heat, like toaster wires or microwave radiation, inside the kiln. There is no way to avoid needing heat inside a vacuum kiln.

From the original questioner:
Might a couple of large light bulbs work?

From Professor Gene Wengert, forum technical advisor:
Without air to carry the heat to the wood, how will heat transfer to the wood occur, especially if you have a stack of wood? Also, even in a vacuum, you still need 1000 BTU per pound of water evaporated, and that is a serious amount of energy.

Note that not all species are porous enough to allow vapor to flow easily through the wood. If you have porous and non-porous in the same load or even in the same piece, it would seem that final MCs could vary quite a bit.

If you want to avoid blankets, etc., perhaps you can use alternating vacuum and the atmospheric pressure with hot air supplied when at atmospheric.

From contributor S:
Is this why transmitted energy sources have been investigated to support moving the energy to the wood to compensate for the evaporation energy loss? Something like induction or RF heating?

From the original questioner:
"Without air to carry the heat to the wood, how will heat transfer to the wood..."
How does the sun do it?

"it would seem that final MCs could vary quite a bit."
Are you saying that it's possible to over dry the wood?

"with hot air supplied when at atmospheric"
Interesting concept, but the hot air may cause the wood to dry unevenly, thus cracking the ends.

From contributor T:
Think of the sun as an ongoing nuclear explosion. That's not exactly accurate, but close enough for illustration. This nuclear fusion emits electromagnetic radiation. Radiant energy does not need a medium to pass through, as does convection or conduction.

There's a lot more to it than that, but I won't try to elaborate because I have forgotten most of it. But there you have your solution to vacuum kilns. After the vacuum is pulled down to as near 30 inches of mercury as you can get, set off an extremely small thermonuclear device inside the chamber. It will evaporate all of the free and bound water. If it's too large it will evaporate all of the wood, the chamber, the operator and the surrounding suburbs, so use caution when calculating the yield.

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.

From contributor K:
You are definitely on the right trail using (incandescent) light bulbs. They radiate almost all their energy in infrared, which is exactly what is needed. You would have to illuminate the wood uniformly in order to get even drying, and the amount of heat you put in to dry the wood will be about the same as with an electrically heated kiln. One problem - bulbs are not made to run in a vacuum, so if they are inside the vessel, they may break. You could certainly try it.

Another problem is that while drying wood, you pump out not just water, but a mixture of other volatile and corrosive chemicals that can damage your vacuum pump depending on how it is constructed. Keep that in mind.

From the original questioner:
My plan is to use a venturi valve to draw the vacuum, thus no corrosion to a vacuum pump.

From Professor Gene Wengert, forum technical advisor:

Why use a light bulb? Why not use a strip heater or simple electrical heater? 100 watts = 341 btu/hr. Light bulbs are for light and not for heat generation. Also, 1000 btu heaters are very inexpensive. Also, 500 bf of green lumber needs about 3 million btu for drying, including heat loss from the building, etc. But remember that without air, you cannot transfer heat from a bulb or heater. That is why vacuum chambers have used heated plates or blankets in intimate contact with the wood, or used radio frequency.

Regarding Vacu-Therm that uses alternating vacuum and atmospheric, it works well. The key is the cycle time and the amount of heating.

In any system, the rate of drying can be controlled (must be controlled) to control quality. It is this factor that makes commercial vacuum kilns work well or poorly. What is the safe rate? How much lumber is ruined before that number is determined?

From contributor T:
About 3 years ago I stumbled across the site of a wood turners club, and read an article of one of their members who had built a very small vacuum kiln.

He used a 12" irrigation pipe and glued a cap on one end and fabricated a removable cap for the other end with a gasket, etc. I can't remember the length of the pipe but I want to say in the 18" range. Inside he installed a column of small shelves that took up about a third or so of the length of the pipe, maybe a half. The other half contained a small cooling fan such as found in a microwave oven and for heat he used an incandescent bulb.

He was drying very small blocks of wood - I can't remember what he was making, but it was a niche product that he was having good success selling and this is why he wanted a way to dry his blanks quickly. In the article he had given all of the necessary info including some pretty good pictures of his kiln.

He raved about how effective it was. He struck me as someone who had only a very basic understanding of the principles involved, and maybe that's why he had no reservations in going through all the trouble to build it. Sometimes we know too much. He addressed the issue of heat transfer in the vacuum and said that he was aware that although his wood was in a near-vacuum, there was still some air in there and that his fan would move what little there was and transfer what little heat was present across the wood.

This is in response to the question, which stated "very small." I guess his unit would qualify as very small. I don't remember how long he had been using it but I want to say a year. Assuming his unit works as he claims it did, the variables involved are many. I don't remember how far down he said he pulls the vacuum. The blanks were not as small as pen blanks, but they weren't as big as bowl blanks either.

From contributor D:
Use Gene's suggestion of discontinuous vacuum. It's very simple. Stick a temperature probe (RTD) in a block of wood and you can make the cycle times nearly perfect.

From contributor R:
I've wondered about using the resistance grids they sell for bedding in mortar, for heating a ceramic bathroom floor. Add layers of that between the wood? They must get above body temp to feel warm. Just thinking?

From contributor D:
The resistance grid might give you the same problem that Wood-Mizer had with their vacuum kiln. In areas where the wood is relatively dry, the wood can get very warm. In areas of relatively high moisture content, the wood is cooled by evaporation. This all leads to problems in controlling temperature and inconsistency in moisture content.

From Professor Gene Wengert, forum technical advisor:
Adding to contributor D's comments, I ran a Wood-Mizer vacuum kiln for a year and we did not have a fire, but the wood was quite charred looking a few times in a few spots. Note that this uneven heating also occurred in the rf/vacuum kilns that I saw in SC. In fact, in all kilns that I have seen or worked with (except Vacuum-Therm and contributor D's kiln), uneven final MC has been a problem.