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Thread: Vacuum Kiln questions

  1. #1
    Join Date
    Dec 2015
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    Milwaukie, OR
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    Vacuum Kiln questions

    Looking for some feedback other's experience with vacuum kilns? I've been researching and it looks like the only domestic manufacturer is I-Dry out of Vermont. Anybody using their kiln(s)? If so is it worth the $50K + price tag? If not I-Dry what other models are out there? Also can someone explain to a simpleton like myself what the differences are in various models ie. radio frequency vs. microwaves?

    David M.
    Milwaukie Hardwoods, LLC

  2. #2
    Join Date
    Oct 2007
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    New Hill, NC
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    2,339
    David, I've been looking at vacuum kilns for over ten years, but have not gotten to the point of making the investment. Here is what I've learned.

    Broadly speaking, vacuum kilns fall into two categories. Deep vacuum and intermediate vacuum.

    Deep vacuum kilns dry down to 95% vacuum or more. Because heat does not convect in a full vacuum, some method of introducing heat is necessary in order to vaporize the water and dry the lumber. Options included hollow metal plates in-between the layers of lumber. A hot water / antifreeze mixture is pumped through the plates in order to introduce heat into the lumber for drying. Another option (not used much) was the use of heating blankets in-between the lumber layers.

    Still another option is the use of radio frequency to heat the lumber. Typically there are anode and cathode plates placed in-between the stacked lumber and radio frequency (microwave band) is introduced into the kiln, causing the water molecules to heat up. RF and Microwave kilns are the same thing.

    Intermediate vacuum kilns such as the iDry system use heat convection to vaporize the water. Lumber is stickered in the kiln chamber and fans circulate warmed air through it to vaporize the water. They were developed by Vacutherm back in the 1980's before Vacutherm transitioned into deep vacuum kilns. Recently the son of Vacutherm's founder reintroduced the technology as the iDry system.

    There are several manufacturers of deep vacuum kilns in North America. PC Specialties is one (vacdry.com). Vacutherm also has their Vacupress line which uses hot water plates to heat. RF Kiln Tech in Canada is a third, and there are a couple more that I'm forgetting the names of. There are some Chinese manufacturers of vacuum kilns but quite frankly their products have a reputation of failure after a couple of years. Supposedly they are improving, but to me investing in one has more risk than I'm comfortable with.

    Hands down, the most affordable and proven vacuum kiln currently on the market is the iDry system, which are available in the $50K range on up. Deep vacuum kilns cost much more - most start around 2X to 3X the price of the iDry.

    There are compromises with each technology. The hot water plate system seems to be the most consistent and prevalent technology. There are conflicting reports regarding the consistency of results obtained from RF Vacuum kilns. iDry has brought affordable vacuum drying to many small and mid size operators.

    If you are planning to dry in volume with kiln charges of same species and initial moisture content, I'd look into a hot water system. If you're looking at smaller capacities, then the iDry is worth looking at.

  3. #3
    Join Date
    Dec 2015
    Location
    Milwaukie, OR
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    Thanks for the feedback Scott. It sounds like you've done your homework. $50k is a stretch and to justify it I'd have to see some immediate return on the investment. When I go to the iDry website it looks like my neighborhood, the Pacific NW, has more of their kilns then any other region in the country. We recently were hired on a rush job to dry and process some large cedar slabs and I took some to a local guy with the smallest iDry, ($50K) He was pretty open about his experience and of late has been charging .85 cents per bf per week of drying. Apparently a flat fee per bf wasn't making him enough to pay for the electricity and loan on the kiln. I was surprised that the iDry manufacturers recommend housing the kiln in a room, space heated to 60-70F. Apparently he wasn't doing that and was paying out the nose for electricity to get his kiln up to temp.

    As far as I can tell the advantages of vacuum vs. DH include, 1) being able to dry wood immediately after milling, 2) more consistent drying of thicker material and slabs, 3) less warp and cracking then DH, 4) ability to consistently get to 140F temperature to insure the bugs are cooked. (I can get my DH kiln up to 130F but haven't been able to get above that).

    On another note: I'm redoing my website and want to include some resources ie. glossary, FAQ and an article on basic wood drying. Would it be okay to use your "Sticky" on basics and credit you for it?

    David M.

  4. #4
    A funny story about "drying" a slab in a vacuum kiln:

    Some time ago we were doing a job in Fat City including a bar. The contractor was to supply the material for the reclaimed Doug Fir bar top. We took delivery of a couple of 4" x 30" x 96" or so monsters, real backbreakers, said to have been salvaged from some old West Coast shipwreck and dried in an RF/vacuum kiln. On inspection we saw some charring on one surface, and a closer look showed that the slab had actually caught fire in the kiln and someone had spliced in an almost convincing Dutchman. That piece read <6% mc at the burnt end and 30% at the other. The material went away and was replaced with some resawn material from a NYC warehouse. The original supplier got credit for chutzpah but nothing else.

  5. #5
    Join Date
    Oct 2007
    Location
    New Hill, NC
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    2,339
    Quote Originally Posted by David Mealey View Post
    Thanks for the feedback Scott. It sounds like you've done your homework. $50k is a stretch and to justify it I'd have to see some immediate return on the investment. When I go to the iDry website it looks like my neighborhood, the Pacific NW, has more of their kilns then any other region in the country. We recently were hired on a rush job to dry and process some large cedar slabs and I took some to a local guy with the smallest iDry, ($50K) He was pretty open about his experience and of late has been charging .85 cents per bf per week of drying. Apparently a flat fee per bf wasn't making him enough to pay for the electricity and loan on the kiln. I was surprised that the iDry manufacturers recommend housing the kiln in a room, space heated to 60-70F. Apparently he wasn't doing that and was paying out the nose for electricity to get his kiln up to temp.

    As far as I can tell the advantages of vacuum vs. DH include, 1) being able to dry wood immediately after milling, 2) more consistent drying of thicker material and slabs, 3) less warp and cracking then DH, 4) ability to consistently get to 140F temperature to insure the bugs are cooked. (I can get my DH kiln up to 130F but haven't been able to get above that).

    On another note: I'm redoing my website and want to include some resources ie. glossary, FAQ and an article on basic wood drying. Would it be okay to use your "Sticky" on basics and credit you for it?

    David M.
    David, you have my permission to use my “sticky”. Please reference by website at WPSawmill.com. There is additional information about quartersawing on my site.

    The biggest advantage of vacuum over DH drying is time. Vacuum is 3X faster. You can go directly into a DH kiln after milling; I’ve done it for years with great results. My Nyle (with auxiliary heaters) routinely goes to 160F, but either a very well insulated kiln chamber and/or auxilliary heat source is required.

    Best of success to you.
    Scott

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