Vacuum Kilns - Pros and Cons

      Testimony from operators who have used vacuum kilns. May 11, 2005

Question
I represent a hardwood export company based in Southern Africa. We export logs and wet-off-saw lumber. We are considering exporting KD lumber and would like to hear from either manufacturers or users on the pros and cons of a vacuum kiln. I believe that it's possible to dry approximately 6 cubic meters in 6 days. I don't want to confuse this with board feetů Is that possible?

Forum Responses
(Sawing and Drying Forum)
About a year ago we seriously inquired about buying a vacuum kiln for drying heavy oak. Being very skeptical about vacuum kilns, the kiln company said they would dry some heavy oak for us as proof that their kiln was what we needed. Needless to say, the final MC, after drying, varied greatly and we had approximately 15% degrade due to checking. Vacuum kilns may have their place for certain drying operations, but not for ours at this time.



I too have been interested in vacuum kilns for a long time. The thought of drying 4/4 hardwood lumber in three days and 8/4 in six days is very exciting. I have tried to research this subject and haven't learned a whole lot, except claims from the companies that sell them.

For example, I've heard that white oak won't dry in a vacuum kiln, but red oak will. I know of one very reputable company that sold them for a while and scrapped the idea because they simply had too many problems. Uneven drying is another issue that I have caught rumors about. I also would be interested to hear how much testing has been done on African and South American species. If there are problems among the North American species, then I can only assume that problems could and will exist in other species located throughout the world.

If vacuum kiln drying could be or has been perfected, I would be one of the first to invest. However, I would like to hear this information from an expert such as Dr. Wengert, rather than just the companies that are selling them. I would like to read some independent research or government studies. One other source of information would be people who actually own and operate vacuum kilns.



From Gene Wengert, forum technical advisor:
I did some controlled experiments in a Vacu-Therm kiln. The results were very good indeed. It is the only vacuum kiln I have seen over the past 25 years that was able to provide uniform final MCs with an average of 7% MC with very high quality. At the time, the cost was high, but I am told that the cost is lower now. (Vacu-Therm was so certain that their kiln was excellent, that they allowed me to run a test load and then compare the results to a conventionally dried test load. They were able to compete very well in quality.)

In addition to my experience, I do study the world literature on vacuum drying (and other new drying systems) to see what others are finding. Recently I heard about a new drying operation in Hawaii; the report was not good, but I would like to visit it and see for myself. I wonder if the IRS would approve?

Because the supply of kiln dried hardwoods is so large in the USA, it is difficult to benefit from being able to go from tree to finished lumber in a week or so. In other words, you cannot charge extra to cover any extra costs involved in fast processing. If you dry lumber and then it sits around for a month waiting to be sold, the idea of fast drying is no longer a benefit. This would be a question for you in Africa to answer.

In short, any drying system, not just vacuum, must compete cost-wise with conventional systems... steam or DH or hot water. The cost must include energy, capital, degrade, drying time and so on.

Further, the definitions of "dry" and "quality" are key. What is the target final MC and the spread of final MC tolerated? What about quality, including color and drying stress? In other words, with any system, it is possible to dry the lumber, but will it be dried with the required quality and to the required MC level? Answering these questions is where a researcher and practitioner, like myself and others, can be useful, as they can view a drying system from an outside perspective.

I have seen people discuss their particular drying system and compare it to other systems, but they had never run the other systems (or the old system they compared their new system to was 25 years old), so their view was somewhat slanted. It is my advice to compare a new and different system to the modern conventional systems, not old conventional systems.

I have seen people with a new drying system dry lumber but without the final lumber quality that most buyers would require. I have seen several people with special new drying systems spend a great deal of time trying to get them to work well. Certainly one advantage of conventional drying systems is that you can spend most of your time drying lumber rather than doing engineering or design work for the manufacturer. With conventional systems, you also can find a wealth of technical information from the past literature and from other operators.



I've been using a WM vacuum kiln for 17 years. In the last three months I bought four more kilns. They are wonderful machines. The problem with the kilns is, as a rule, people don't like to follow the manufacturer's directions.

Two of the kilns I bought from a mill/drying operation. The first kiln they bought used, and dried for 2-3 years with it, then they bought a brand new one from WM. On the second load they dried with the new one, they opened it at the end of the drying cycle and the lumber caught fire. Was this the result of poor design or a faulty kiln? No. They just didn't follow the directions and loaded air-dried lumber with green lumber. The controlling thermister in each blanket section will dry the whole section by reference to that thermister board. If you place the thermister on a green board and include dry (air dried) boards in that section, guess what? The dryer boards come to dry quickly and the green board thermister keeps calling for heat. Result: at best, over-dried boards, at worst, totally fried, overheated lumber ready to burst into flames the minute you open the kiln and add oxygen.

I can go on and on about how people misuse these kilns. If you load same species, same size, all sawn at about the same time, you will get excellent results. You can dry air-dried lumber, also - just don't expect the few green pieces you want to include at the last minute to dry.

I saw, dry, and process hard maple for the billiards industry. We also manufacture squares for baseball bats. The vacuum kiln is way faster than conventional kilns but it is up to the kiln operator to use his skill and knowledge and not get in too much of a hurry. These kilns have controls on them but some of the vacuum kiln operators don't have self control and think they know everything. When they get poor results they blame the equipment, not themselves.

We get excellent results because 1. we have learned how to operate a vacuum kiln, 2. we have learned how to dry lumber. In very frank conversations with WM I have learned that the people who operate the vacuum kiln were either very successful or it was a disaster, because people either follow the directions or they don't.

I know of one company that has added cooling towers and other modifications to the WM vacuum kiln and I think they are unnecessary. I'm no expert, but I can tell you we run our kilns 7 days a week year round and I would not be in business this long if not for the WM kilns.

I don't work for WM and I have never been paid for any PR or recommendations I've given. They are straight shooters and service what they sell. I'm on my third WM mill. My second mill was a 40 electric I bought new for $17,000. I used it for 9 years and sold it for $11,000. I now use a 40 super hyd electric and it's a wonderful machine. I have one secret I'll share... I let them ( RESHARP) do my bands. Can't beat the service - we can't sharpen them as good.



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  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Air Drying Lumber

  • KnowledgeBase: Primary Processing: General

  • KnowledgeBase: Primary Processing: Kiln Operation


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