Advantages of vacuum drying

What does this leading-edge technology have to offer? April 9, 2003

Question
We are considering purchasing a vacuum kiln (hot water platen type). We only deal in high grade lumber. Our goal is to provide premium kiln dried lumber and components to cabinet suppliers. Being a small manufacturer, is this type of kiln the best option for our business? The drying speeds of these kilns are very fast as opposed to conventional kilns. They are also far superior for drying thick stock. I would like to further understand the downside (if any) of using a vacuum kiln. I'm not concerned about the capital investment. I'm only interested in providing the best quality material with the most reliable control. Will vacuum do this for me or should I further consider DH?

Forum Responses
(From WOODWEB's Sawing and Drying Forum)
Drying thick stock such as maple is a good application for vacuum. It is unlikely that quality of 4/4 material will be as good as with a conventional or DH kiln. If quality is your concern, for 4/4 to 8/4, DH or conventional is the best way to go. If speed is your main concern or you are drying very thick material, vacuum is a good choice. Evenness of moisture content is harder to achieve with vacuum. Operating costs and capital costs are higher with vacuum. There is a good reason that 99% of the kilns sold in the US each year are conventional and DH kilns.


From the original questioner:
I can understand why 99% of the kilns in the US are conventional or DH, considering the volume capacities for large manufactures and the small capital expense for the hobbyist. I spoke with a lumber company that dried 69,000 bdft in November with a vacuum kiln and reported only 18 bdft rejected due to high moisture content. I also spoke with a conventional kiln manufacturing rep and he stated that vacuum drying was the future of drying lumber. The guys that are investing in vacuum kilns seem to be expanding. I can't see why they would be buying more vacuum kilns if they were getting bad results. Speed seems to be a major advantage for vk. A typical order for red oak could be dried, machined, and shipped in less time than it takes to dry it in a conventional kiln. From what I understand, low grade lumber is tough to dry in a vacuum with consistent results because of the knots and density of wood surrounding them. Is there anything I'm missing as to the downside of vacuum drying?

It all comes down to $$$.

I would visit a kiln drying the same lumber that you are interested in before investing in "new" technology. If you cannot visit, could you send a load of your lumber to one of their kilns and have them dry it for you? Stay with it so you can record drying times, etc. Check energy consumption.

The rejection of wood for high MC depends on the customer. Many customers do not even check the MC.

A well-run conventional drying operation can have very little degrade if run properly and if the wood is good to begin with. A vacuum kiln cannot do better for any products that I have seen, except for thick red oak.

The person who is selling you the kiln… if you ask them for the exact procedure for drying your species and thicknesses, can they tell you and write it down? Will they give you a performance guarantee? Is this just a guess, or do they have experience with this being dried in a vacuum? I have seen a lot of lumber ruined while an operator is trying to figure out how to dry in a vacuum.

Vacuum drying patents exist that are over 120 years old, I am told. Vacuum has been around for years, but has not been economically viable for standard lumber products. I have yet to see a vacuum kiln dry to uniform final MCs, except for VacuTherm.

Gene Wengert, forum technical advisor


From contributor D:
At $0.08/kwh, it cost $14 per day to run our 4000 BFt vac kiln. You can easily dry 8/4 SM squares in 5 days, so that works out to $17.50 per 1000, not counting the heat. A leisurely 7 days for lumber will cost you $24.50/1000.

As far as heat is concerned, you don't need to heat a structure, you don't lose heat through walls and you don't dump heat with vents. It takes less than 20,000 btu's per hour per 1000 BFt to dry red oak at 1/2% per hour.

Gene, tell us about your experience with VacuTherm and their uniform MC.


I sent some 5/4 red oak to Vacu-Therm and had them dry it. The MC was extremely uniform (SD under 0.6). They use an equalization process to account for the variation in wood-moisture movement variations, initial MC variations, heartwood/sapwood, etc. The only negative we noted was that the color difference between sapwood and heartwood was much higher than with conventional wood - they did not discolor the wood!

Cost, of course, is always a concern, but one would have to check that out for a specific situation. With thick lumber, conventional costs would be extremely high, so vacuum will win in most cases. For 4/4, I doubt that vacuum will win, especially with a wood waste boiler that is also disposing of wood waste. (Stop burning and you would have to pay to have the wood hauled away.)

Is the electric energy cost you gave above just for the vacuum pump? With 1000 BF of red oak, you need to evaporate about 3000 pounds of water, which will require 3 million BTUs or about 870 kWh per MBF.

Gene Wengert, forum technical advisor


From contributor D:
Gene, that's the cost of vacuum and heating water pumps. Cost to run the controller is negligible, as is the cooling water pump.

We have no problem with heat from wood waste boilers. Steam works great and helps get RH up in cold chambers. And, we have a customer in New York who uses an outdoor furnace. Since his vac kilns don't need a lot of heat, they rarely drop in temperature when the fire goes low.

You're slightly high on btu's but close enough. Divide 3 million by 64 (% change in MC) and divide that by 2 (1/2% per hour). What do you get?


From contributor M:
There are downsides to any type of dry kiln. What species do you want to dry? How thick is it?

Vac kilns are great for drying thick wood and yes, there are a few down sides. If you don't care about the cost of buying a vac kiln, the cost of operating one should be no problem, either. If you need to dry thick hardwood fast, go for it.


From the original questioner:
We run a lot of red oak, poplar, and soft maple. I'm mainly interested in drying red oak, as this is what we use the most. We use 4/4-8/4 but 4/4 is the most common. I'm under the impression that I can dry 4/4 red oak in 5-6 days from green in a vacuum kiln. Is there a particular reason why it's only an advantage for thick lumber? It just seems to me that if it will dry thick material exceptionally well, then why wouldn't it work on 4/4? The difference between green red oak and dried is around .65 per bdft. If I dried 5 loads per month (2500 bdft chamber) then I save 6500.00 per month over what the equivalent amount of kd red oak would cost me. If I dry thicker species, then the cost of green versus the cost of kd is even greater. Is there a problem with drying 4/4 red oak in a vac kiln? Not having to put it on sticks to air dry seems like a major advantage to me.

The ability to dry thick stock will allow me to target specialized, better-paying markets, which will maximize the advantage of being a small manufacturer. The ability to take an order for any profiled dimension, dry the material, machine, and ship it in less time than it takes a large millwork business to dry the material alone would give me an obvious competitive edge. I believe the operating cost is comparable to a conventional kiln but the speed is what impressed me the most.


From contributor M:
One of the problems with drying 4/4 RO in a vac kiln is that you will see some bow near the top of the load. You may want to consider weight on the top of the load. Do you need to condition your lumber? This is not possible in a vac kiln. Red oak dried in a vac kiln will have a lot of stress. 8 to 10 days sounds like a better RO schedule than 5 to 6 days. I think that's pushing it too hard.

From contributor D:
8 to 10 days for 4/4 is absurd. You can dry 8/4 in less time.

Stress is caused by bad schedules and/or the inability of the control system to hold the schedule.


From contributor M:
Maybe I should have said "8 to 10 days in our vac kilns." Our cooling system is way too small. I also like to keep the WB elevated on the red oak to prevent checking. This slows the drying.

From the original questioner:
Gene, did you notice an unusual amount of stress in the 5/4 RO that you sent to VacuTherm? I notice a % of bowed lumber that comes from our present supplier and they use a pre-dryer, conventional kiln drying process and steam for conditioning. I've also sent several loads back due to honeycomb. I do believe it's possible to ruin a load of red oak no matter what process one is using if the operator doesn't know what he's doing. However, with vacuum one has more immediate heat control and environmental humidity control than with conventional. The mere fact that lumber is placed between aluminum plates rather than sticks is bound to give the material more stability during the drying process. I picked up a load of red oak a few days ago and noticed a load of red oak that just came out of their kiln. It was all random length and some of the lumber had very little support on the ends. About 1/3 of the stack was all curled up where the only support was the stacking strips. Do you agree that lumber can be flattened during the drying process due to the softening principle of wood? If so, could this be identified as an advantage of vac over conventional?

From contributor M:
I think a lot of our bow problem in our vac kilns is how the wood is cut. We try to put the longer pieces on the bottom and the shorter pieces on the top. I don't dry much lumber but when I do, we will load the boards first and put the squares on top to help keep the lumber flat. If you are looking to dry just lumber in a vac kiln, you may want to look at some kind of weight for the top of the load. Most honeycomb in a vac kiln comes from drying too slow. I don't think you can "soften" wood in a vac kiln because of the low drying temp. I never go above 66 degrees c.

From contributor D:
You need to dry with vacuum and not heat. If you depend on heat without watching pressure, you skim the surface as in a conventional kiln. This will lead to stress, warp and even honeycomb. You should have a computer to log the chamber pressure. Then you would understand a lot more about vac kilns.

"Drying too slow" causes honeycomb? Drying slowly without keeping the RH up causes shell-drying and that leads to honeycomb. If you're drying something like a 4-1/2" x 9" dimension, you have to go slow because of the uneven shrinkage. But you avoid shell drying by keeping RH up.

You don't need weight to avoid warp, but if you need to resist cup, and you have a rectangular chamber (instead of cylinders, like you have), you can put an inflatable bladder on top of the load and let a little air into it.

Gene, the Vacu-Therms I have seen have had very poor pressure control and little or no RH control. There is no reason to consider "moisture movement variations" or "initial MC variations" or "heartwood/sapwood." I don't believe that Vacu-Therm has any equalization process.


Contributor M, wood can have natural tendencies to bow, cup, twist or crook (side bend). These tendencies exist regardless of the drying system; they are not a result of drying gradients, etc. but are a result of shrinkage within the wood in different amounts.

Contributor D, I know that the Vacu-Therm kilns that Jim Parker used to dry my wood had equalization included. As a result, he achieved excellent uniform final MC.

Anyone, there was a report from Clemson University about the moisture gradients and stresses in rf vacuum drying (Power Dry Kilns). Indeed, they were fairly high. Honeycomb did occur.

In the Wood-Mizer kilns, we were able to create drying stresses and also had a wide range of final MCs.

Gene Wengert, forum technical advisor


From contributor D:
Gene, maybe a visit to a conventional kiln for equalization was included. I just visited Parker's website. There's nothing about equalization and there is no equipment there that is capable of equalizing.

As I said, you certainly can cause stress in vac kilns but Power Dry's junk is not in the same class as today's kilns.

Related Articles