Vacuum Kilns: Energy Use and Drying Times

A discussion of the performance you can expect in the real world from a vacuum wood-drying kiln. June 13, 2014

We dry birdseye and curly maple in four DH kilns. I am interested in purchasing a vacuum kiln for faster turnaround and better color. Has anyone had success with VacuTherm or other manufacturers?

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
I can tell you that the maple dried in a VacuTherm kiln was whiter than conventional drying. Of course, the whiteness in conventional drying ranges all over the map. But I also saw some maple that was dried in another vacuum kiln that was dried too slowly and was not really white. Of course, log handling (storage time in warm weather) is an issue to consider too as it seems to affect color. I wonder if the economics of faster turnaround will justify the higher drying cost with 4/4. I would think that thicker stock will easily support a vacuum kiln.

From the original questioner:
Gene - thanks for your prompt response. I spoke with someone from Woodmizer last week. I asked if there was a "best practices" publication for hardwood saw and planing mills.

From Gene Wengert, forum technical advisor:
For hardwood lumber drying, the most recent and thorough publication is Drying Hardwood Lumber, which is available through the US Forest Service, Princeton, WV.

From contributor W:
I have visited one company in Quebec that has purchased several Vacutherm systems and dries nothing but maple. I think they only dry thick maple in the vacuum units. I may be wrong about that but that is what they were doing when I was there. They must be satisfied because they keep re-ordering and expanding the vacuum operation.

From Carl Hagstrom, Systems Administrator at WOODWEB
We recently retrieved over two hundred Forest Product Laboratory articles, and cataloged them within WOODWEB's Knowledge Base, including the Drying Hardwood Lumber publication Gene refers to. You can easily download (and print) the publication. Just click on the link below, and follow the download instruction.

Drying Hardwood Lumber

From contributor G:
I have a spreadsheet that I use to calculate things like energy use when someone wants to know how much it is going to cost to dry a certain wood in our vacuum kilns. I don't imagine that the numbers will be directly applicable to a competitors kiln design, but it might give you a ballpark idea. This is using hard maple 4/4 from 70% to 7%, our VacDry 2.5, a fuel oil hot water heater and a three day schedule with a four hour turnaround per load (assuming no timesaving features):

- VadDry 2.5, 230,000 board feet per year, 2017 per load
- 457 gallons water removed
- $56 to vaporize 457 gallons ($1.79 per gallon, 85% efficiency)
- $46 to run the pumps, cooling water and control systems (3 days, 117kW a day at $0.13per kW/hr)
- Cost to dry per thousand board feet: $50.57

Please keep in mind these are only the numbers I can control. Everyone has different overhead figures which you can add onto that number. The schedule is also very conservative at three days, considering we are currently providing a five to six day schedule for green baseball bat billets. We might eventually recommend a more aggressive 48 or 36 hour schedule for 4/4 hard maple.

From contributor M:
I thought I'd drop in with a few questions/comments myself here. I've been running a Wood-Mizer VK2000 for about four years now, drying everything from 3/4 maple drawer stock to 9x14x25' white pine timbers. I'm somewhat puzzled by the dry times those of you with vacuum kilns are stating. When I first got the kiln and followed the schedules provided we could dry lumber in 72-96 hours but the degrade was brutal. After talking to Wood-Mizer and other people running VK2000's, we slowed things way down and have had pretty good results, but our time in kiln is six-eight days. An average load will pull out 350 plus gals at about 33/gals every 12 hours, slowing down at the end of each stage. I realize all kilns are different, but the idea of pulling 450 gals out in 48 hours sounds like a disaster waiting to happen based on how my kiln runs. I've found running heavy heat cycle's (heat duty on VK's) case hardens the lumber and basically stops the drying process while warping and twisting the daylights out of it, as well as pushing the RH up so high that the vacuum can't get down to where it should be (1.5 +/- in/mg). I guess my question is, is anybody out there running a VK2000 and noticed the same thing, or am I missing the boat somewhere? I know I haven't provided all the parameter's, but will fill in as necessary.

From contributor D:
The problem with your Wood-Mizer is the tiny vacuum pump. It will work somewhat with a low heat duty because you are evaporating much less water and the vacuum pump can keep up. I wasn't going to do any more upgrades but I did a VK2000 a couple weeks ago for a company that is drying HM baseball bat billets. I visited them today and they said they dried their second load in six days.

From contributor A:
I have to disagree with Contributor D on the vacuum pump size on a Wood-Mizer Vk 2000. The only thing the pump has to do is reach the absolute pressure desired and make up for leaks. The steam pressure produced in the process is a wash because when it condenses it creates vacuum to match. The only limit to the pump that I have noticed is the temperature of the water in the pump. No matter what size pump you have you cannot pull absolute pressure below the boiling point of the liquid in the ring of the pump. I agree that 3/4 lumber can be dried faster than six-eight days safely, but don't just go turn up the speed till we get some of this figured out for you.

There a lot of variables with Wood-Mizerís kiln operation. Itís sort of like a chain gang, if the variables are the right distance apart you can go quite fast, if not you have a mess. Going by the vague sketch of symptoms you gave I would say your biggest problem is shell temperature too low in the first part of the run. I added a shell temp control to my kiln and I run it in the fan circuit. I run my shell temp higher at the start of the run and decrease it throughout the run. The harder the wood, the higher the temp of the shell. Donít worry about your vacuum getting a little behind, it will do this when the water is really flowing out fast. It will catch back up toward the end of the run. I like to say there are three speeds to coordinate, and three members of the chain gang. One: Speed of the heat into the wood. Two: Speed of the heat/steam out of the wood. Three: Speed of the heat through the shell or steam to the condenser. The most common problem I have seen is number three getting ahead of the other two in the winter. The next thing I have seen is over-drying in the winter. My first load was a load of cherry that I dried to about 2%, which caused it to twist severely with the factory schedule.

From contributor M:
I couldn't agree more with some of your points. The factory schedules can be used to start a nice fire when you get home, but thatís about it. I am running a temp switch on the cooling fans, as well as a heat exchanger (radiator style) on the reservoir (note on this and all associated parts, stainless steel). I also agree with your description of a proper run, high RH (vacuum press) in the beginning, dropping toward the end of the cycle. We maintain indoor temps of 65-75 F (can be higher in July/August, only so much cross ventilation can do).

Here is what I've noticed/figured out. By raising the shell temp up (say 105 F), we create a nice moist environment (RH) that the charge sits in, which is great, but the shell temp starts to drop and the charge temps go up which I believe is because so much moisture is condensing that itís actually cooling the shell. Another way to illustrate this is that I'm sure you've noticed that the shell temp climbs and the charge temps drop as/immediately after the vac pump has run. I take this as it has removed the over-saturation moisture, allowing more moisture to release from the charge and start condensing on the walls or become part of the general RH.

My understanding is the charge's temps drop as moisture escapes, because the water is taking the energy applied to it (elec/heat) to convert to steam, rather than the wood itself absorbing and storing that energy. The water then floats around as RH, looking for someplace to dump that energy, which it does on the kiln wall (condensing), creating heat, which then has to be scrubbed (cooling fans) in order for other water molecules to be able to do the same thing. As moisture is removed, the wood itself starts to hold energy (heat), indicating that less moisture is present.

So in my experience, turning down the vacuum duty to allow the shell temp to come up works somewhat, but the moisture inside actually fights this, cooling the shell because the vac pump isn't removing the condensed moisture. In my opinion the pump should be functioned by an algorithm of shell temp and vac pressure, not a timed cycle, but thatís a whole other topic. On a typical run, say red oak, we run a VD of 2-3, HD 85+/-, IT 120, IDP 40, FT 140, FDP 15. At the beginning of the run, we'll see vac readings around 2.2-2.7, dropping to 1.6-1.7, and then climbing again in stage 3, trying to end the run at 1.5. The fans are set to click on at 105F (approximate, I don't know which switch you're running, mine leaves a little to be desired for precision).

From contributor D:
To contributor A: Your chamber pressure is a result of water vapor pressure, condensing on the chamber wall and vacuum pump capacity. Condensation doesn't always work. When your vacuum pump is large enough to reduce chamber pressure, it is reducing the boiling point of water. Likewise, if you let the chamber pressure increase, the boiling point increases and the temperature of the wood increases. Having a vacuum system that is capable of keeping the chamber pressure under control is a key to successful vacuum kiln operation. Some wood, like maple and cherry, can take a lot of abuse but try 12/4 red oak and see what you get without tight control.

By keeping temperature of the chamber high, you are keeping relative humidity elevated. This, of course, is good for wet wood. You don't need a humidity gradient to dry wood in a vac kiln. Another way to control the fans is through humidity. I have added humidity sensors and then used a SSR output to control fan speed to maintain the desired RH. You would find that if you could closely control wood temperature. You wouldn't see any more wet or over-dried wood.

From contributor D:
To contributor M: When your vac pump kicks on, it starts to lower chamber pressure. As a result, more water molecules will have the necessary energy to vaporize. When they vaporize, they take heat with them. In a vac kiln with good control, wood temperature stays low as long as there is free water evaporating. Not until the wood is nearly dry will it start to reach heating temperature. The heat is carried by the vapor to the chamber walls or to the seal water if you don't have a vac pump condenser.