Brainstorming About Vacuum Kilns

Inquiring minds discuss why vacuum kilns are so expensive, how they work, and the possibilities of building your own or even improving on the concept. July 30, 2007

Are there plans available showing how to build a small vacuum kiln? I would like to use a vacuum kiln to dry 8/4 to 20/4 hardwoods up to possibly 8' in length. Size of the kiln would be in the 100 to 500 bf range.

I called today and got sticker shock at the price of a commercial dryer for 500'. I have thought about building a vacuum kiln and I can't see why a chamber couldn't be lined with hot water or glycol tubing so that heat can be added or subtracted. I thought about using, for the physical housing or box, stainless steel, epoxied coated steel, possibly something even like lexan or delrin with some kind of rubber or other moveable membrane. There are a number of vacuum techniques that are available for the refrigeration and hvac industry - why not one of these? I know that the process involves reducing the atmospheric pressure so that water moves or boils at lower temperatures and pressures.

Surely there is a way if one has the mechanical skills. Even if I can get a vacuum kiln built, I still will need a go/by checklist of what to do with temperatures, heat, etc.

Forum Responses
(Sawing and Drying Forum)
From contributor N:
I am the last one to poo-poo anybody's ideas, especially if it sounds like such a fun project as that. But you might find you have almost as much (or more) money in a setup like you are talking about, and you may have to tinker with it for a year getting it dialed in, as if you just bought one (maybe used?). If you are looking in the 100-500 bft range, build a little d/h kiln. Less that $300 and you are in business, with hardware store bought parts, and a heck of a lot less work designing and building it.

From contributor D:
Just a couple of points. You can only transfer heat by having the tubes in contact with the wood and because wood is a decent insulator, the tubes need to be very close together. Also, the vacuum pumps used in refrigeration won't move enough water or last more than a few days in this kind of application. Anyway, is it impossible? Of course not. Is it practical? Not really. Drying quality for many species and thicknesses is poor. 4/4 oak generally does not do well in a vacuum kiln, for example. Operating costs are usually quite high.

From the original questioner:

I already own two small commercial dry kilns and they work great for 4/4 material, but it is the length of time involved (3 months, 6 months, 9 months for thicker stock, and I have done some of this) that got the wheels rolling. I recently have been laminating 4/4 stock in a vacuum bag to get the thickness that I might need. Thus the question.

One of the commercial companies asked $55,000 for a 500 bf kiln. Surely I can build one for far less than that. Even at $15,000 it would be a bargain.

I would further like to know if there has been research done on a hyperbaric chamber which would probably be the opposite of a vacuum? (Liquids don't compress for all practical terms, but possibly something could be used that would replace the water at high atmospheric pressure, or possibly the wood itself would force the water out.) Just a thought.

From Professor Gene Wengert, forum technical advisor:
I agree that pipes on the wall will not work. You must have intimate contact with the wood. You also need a very good pump.

Building the chamber is only the first step. Learning the best settings is the second, and much more complex, step. I have seen a lot of wood ruined in a vacuum kiln, especially thicker stock, because the controls were not appropriate and/or operator did not know how to run the unit. One might say that it is a very steep learning curve, and quite a bit of wood will be lost (= $$$). The commercial units are expensive because of the equipment and control costs.

For the small quantities you need, it would be less expensive to contract out your specialty drying. Get in touch with someone who can tell you the names of vacuum companies that could do your drying.

From contributor S:
It is my understanding that vacuum kilns are the state of the art cure-all for speed and stability for drying specialty woods. I think I understand the concept and some of the physics involved as far as achieving the vacuum - heat transfer to the wood and such. But is it safe to actually remove the water that fast from the wood? I mean, I could put wood in a hot oven and remove moisture fast, but how would a vacuum kiln not allow wood stress and degrade? Aside from the claims, what is the science of it?

The chamber I don't see as being all that tough to build. It would, I understand, have to withstand the forces applied by outside pressure and be openable and resealable, maintain the vacuum and such, but these are relatively static calculation and design issues.

Stainless, by the way, would be a great material if one had cheap access and the ability to work with it on hand. And then again, why build a chamber - would a bag work?

Heat transfer to the wood (to the moisture, I am guessing, would be more correct) I understand that heat is not conducted very well by the vacuum and that the source would need to be in contact with the wood. Going on memory, I thought that in a vacuum kiln, electric blankets of sorts are used. Could these blankets not be fluid filled with water or a saline or a wax and be circulated by a pump? What kinds of temps are we talking? Could a heat pump be used with tubing grids or radiator plates? Also, if the heat is not being lost into the vacuum, shouldn't the heating of the wood be more efficient? Could waste heat from another process not be utilized?

Is it all a pipe dream? Why couldn't a resourceful guy build his own vacuum kiln? Are there any vacuum kiln sellers that have an open explanation of drying rates and settings or charts and how to achieve these?

It would be great to be able to make a small stack of wood alternated with some flat radiator plates in between layers, hook the tubing up to a tea kettle, wrap and seal the stack, start the vacuum pump, turn the fire on under the kettle, and a week later have a load of dry wood. Nope, sounds too easy - I better stick with trying to hit the lottery!

From contributor M:
You can build a hybrid version of the vacuum kiln known as a discontinuous vacuum kiln. What this does is alternate between a vacuum and atmospheric pressure to allow heat transfer. Finding or building a container that will withstand the vacuum seems to be the biggest technical challenge.

As far as contributor D's comments about drying quality being poor in a vacuum kiln, I disagree with this. I have had many species of 4/4, 6/4 and 8/4 material dried in a vacuum kiln with excellent results. In all fairness to contributor D, he sells DH kiln controllers. I can not speak to his experience with vacuum kilns. Those that have one of his DH controllers speak highly of them.

Den from PC Specialty checks in here every so often. He builds and sells vacuum kilns. If you have not spoken with Den or his son Garret, you need to. They are a wealth of knowledge in terms of vacuum kiln construction.

Like you, I run a conventional kiln and would love to have a vacuum kiln for drying thicker stock. I am not sure you can dry 20/4 material; I believe that 16/4 is the thickest material that can be dried. Here again, Den is the best person to speak on what thickness can or can not be dried in a vacuum kiln. I picked up my kiln controller and sensors from PC Specialty; it is a great setup. I jokingly tell my friends that my dog could dry wood with my setup... but of course being a trained kiln operator does not hurt.

From contributor C:
For the price range you seem comfortable with, why not just buy a used vacuum kiln? There are usually several Wood-Mizer Vk-1000, or better, VK-2000 available online. Besides the capacity difference, 1000/bdft vs. 2000/bdft, the two machines are designed differently as well. I have a 2000, and have talked to a few people with 1000's. I would recommend the 2000.

I've seen VK-1000's for $5000-$9000, and average price of a VK-2000 seems to be around $13,000.

A lot of people have had bad experiences with Wood-Mizers, and as an owner of one myself, I can sympathize. It's taken me 7 years to get comfortable with running it, and I still learn something to tweak my schedules almost every run. It's my opinion, and only my opinion, that the problems created improperly vacuum drying are no worse than those created by improperly drying conventionally or DH, it only appears that way because it happens in days instead of 6 weeks.

I have successfully dried 6X14 24' long timbers, although they were pine, as well as typical 4/4-12/4 lumber. Red oak can be a challenge, though the batches I had the most problems with were diseased, as I learned from one of the Doc's posts awhile ago.

I agree with almost everyone here that vacuum kilns are tough to learn, and one of the big secrets, at least to me when I got mine, is that the manual should start out "Once upon a time..." The schedules Wood-Mizer printed are ridiculous, and the explanation of how the unit works is lacking at best. Once you learn the actual mechanics of what needs to happen, and what the numbers you are seeing really mean, you can figure out what needs to happen per load pretty easily.

The units are not turn on and walk away; they require attention and thought. Nor are they cheap to run, being purely electric. The heating blankets are delicate, and don't last more than 2-3 years if you are lucky, and there are other little parts that can make you pull your hair out. Yes, there are some engineering/design faults in the unit, but nothing that can't be solved. You should see some of the nonsense on my mid-end CNC router! However, it can be profitable and satisfying to properly dry a load of lumber as fast as is possible with a vacuum.

There are some modifications that need to be to be made on the machines, but they are simple and cheap. For what you are looking to do, I think it might be a realistic solution to your situation. I think you'll find it's not as bad as some will lead you to believe; if you ask questions and draw on the catastrophes that have befallen others before you, perhaps you can keep your own catastrophes a little smaller.

From the original questioner:
I thought why not use a vacuum bag (PVC or other the kind that we use to do vacuum glue work) and place the bag in some liquid (water, glycol, etc.), totally immersed for heat exchange and draw a vacuum on the bag and maintain a heat source on the liquid and maintain a vacuum. I can't see why a bag with lumber (granted a relatively small amount if the liquid must touch every member, but still maybe in the 100 bf range) that is immersed in a heated liquid with a constant vacuum wouldn't do the same thing that a commercial kiln would.

From contributor B:
While it seems to me that contributor C may have the most practical approach with the used WM VK 2000, I would like to add a bit to the idea of building one's own vac kiln. First, the bag chamber would still need a sturdy frame to hold the bag off the charge... more material science probably involved than one might expect. Second, I recently went to a very large metal salvage yard. It is amazing the already fabricated stainless steel that are available at $1.50 - $3.50/pound. Steel is moving fast in the junkyards, so you have to show up when it is available, but if you get there when a factory plant is being disassembled, you can be amazed at the bargains you might find.

From contributor E:
You guys have laid out a lot of work for me [PC Specialties]. I have wanted to design a low cost vacuum kiln for years. I never have been able to do it. I have helped people dry one or a few-shot items in a plastic bag. Someday, I'll put the procedure on the internet. I have been trying to help people do their own at the Forestry Forum. Not like our kilns, but something doable. The last guy was working on a cross between Italian discontinuous and resurrected Russian steam-vac. He hasn't posted lately.

If you have the proper vac kiln, you can definitely knock people's socks off. I dried some heavy, flat sawn red oak flitches including the pith. When somebody said the wood would be too stressed to be used for flooring, I cut some. Seen in the photo.

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The ability of a good vac kiln to dry very difficult wood makes it obvious what they can do with anything in North America. These pics are an Amboyna burl from SE Asia before and after.

But building your own vac kiln involves structural problems (possibility of implosion), mechanical problems (pumps and heat exchangers), control problems (monitoring all the parameters that need to be controlled), and technique (you could ruin wood with our kilns if you didn't know the settings). But when everything is done right, there is no cheaper way to dry wood.

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The "before" picture.

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I remember contributor M's loads. They gave me more gray hair. I never saw such a mix of species and thickness.

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Sure glad he has his own kiln now, be it conventional.

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I have rebuilt a few Wood-Mizers but won't anymore. I can fix the chamber pressure control problems and assist with heat transfer problems, but the electric blankets are bad. Dry areas of wood overheat. Interestingly enough, I redesigned the VK2000 before Wood-Mizer said they were giving up because of the overheated dry areas. I suggested a design change that the WM engineers wanted to try but we never did.

Finally for tonight, if the control system is built right, you turn a vac kiln on and go home. If you wonder from home what is going on, you connect to the computer that is collecting data 24/7 to make sure the independent control system is doing as advertised.

Click here for higher quality, full size image

From contributor J:
I have a vk1000 that I would sell for 5,000.00, with new blankets and a complete rebuilt control box from Wood-Mizer. Woodmizer does still support most parts. I bought my unit from a guy who had bought it new and used it for years, then retired. I was going to use it for drying thick stock, since we do a lot of rustic wood, but since we bought it I have grown a lot bigger and never used it. I did investigate into a vacuum kiln for a long time and agree that the best solution is a used Wood-Mizer. Learn a lot about vacuum drying, then you can upgrade to a 50,000+ machine if you find it works for you. From my own perspective, I think they are best for the smaller operation; they do need a lot of attention. We have a few 10,000-15,000 ft. kilns and just never got the extra time to mess with it, but if you do master a unit at 5,000-10,000, it will pay for itself fast.

From contributor I:
What about contributor S's vacuum bag idea? Why couldn't you apply a vacuum press idea to a small vacuum drying device? I'm not talking about drying 1000s of bf here, say, just enough for a project or two. All the mechanics of the vacuum chamber are to keep it from collapsing under atmospheric pressure. Why not let the "chamber" collapse against the wood? The stickers would hold the boards apart and vertical stickers could be inserted if necessary. This would certainly minimize warping. Wrap the stacked wood in very heavy poly, install a vacuum fitting and suck away. The poly would be tight against the wood, facilitating heat transfer, plus you could watch the wood as it dries, looking for checks, etc. Any small leaks in the poly would not be a big deal because the additional air would help purge the water vapor from the bag. If direct contact of the wood with the poly was not wanted, the wood could be wrapped in a thick porous fabric witch would allow moisture to escape. I personally think the idea has some merit and deserves some noodling! This would be nice for small batches of wood. Thoughts?

From contributor E:
I thought I mentioned this idea above. But you can't sticker any wood. There is no heat transfer to stickered layers. You can put one piece in a plastic bag and heat that piece by immersing in hot water. But it is a lot more complicated than it sounds. There are mechanical problems and control problems, etc. But, one time a guy called here and asked about drying a thick slab to make a table that had been ordered by Jerry Seinfeld. I had no kiln-time available but I told him about this idea. It worked for him and he sold several more expensive tables using this method. But, more recently, he will send a load of slabs for tables and I dry them in a regular vac kiln.

This chamber is 4' high and 4' wide. It was filled with 10 slabs for tabletops. They were being dried one at a time in plastic bags.

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From contributor J:
I wonder if you could do the vacuum bag idea and stick in between slabs and put into a conventional kiln, only using heat and no compressor. Would ambient heat of the air in the kiln transfer to the lumber? Maybe not the most efficient way, but may work for a few large slabs or small quantity of lumber.

From contributor V:
My idea was to convert a vac kiln and use microwave as the heat source. The goal would be to eliminate the labor intensive blanket and stacking process my kiln requires.

Contributor E, has microwave heating in a vac kiln been investigated?

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor K:
We have been using a WDE (Maspell) dryer for three years now. We are mainly drying 4/4 and 2/4 oak for flooring.

We leave the wood drying in the wind until it drops under 25%, then put it in the dryer, which is really a load, start and don't look behind the machine. It has a controller with settings for various wood species and thicknesses that runs the machine without any user intervention.

Drying times are 2-3 days for 2/4 oak and 5-6 days for 4/4 oak. Drying is perfect. We can put 3-4 m3 of wood stickered planks in it, like normal piles in a conventional kiln. Heat transfer is done by water vapor from the wood and hot water circulating the cylindrical walls of the kiln. The vacuum is as low as 100 mbar. We also dried afrormosia, sapeli, moabi, wenge, afzelia, etc. with excellent results.

The unit cost us, including installation costs, in the order of 75-80K euro. The investment should be paid off in 8-10 years but more important is the flexibility of being able to dry small batches very fast.