Radio frequency -- vs. super steam vacuum

A discussion of these two kilns for drying large timbers. February 19, 2002

We are curious about radio frequency kilns vs. super steam vacuum kilns for drying large timbers over 8" x 8" in dimension. Located in British Columbia, we need to dry primarily Douglas fir and western red cedar timbers.

Forum Responses
From contributor M:
Hot water, platten vac kilns are better than both steam or RF vac kilns. If I had to choose between steam or RF, I would take the steam. RF vac kilns cost too much to operate and dry uneven. Replacement parts are very costly as well.

We dried some oak lumber in the Vacu-Therm kilns a few years ago and had very good results. They use vacuum alternating with atmospheric pressure cycles.

Gene Wengert, forum technical advisor

From contributor D:
You'd want some heavy duty heating plates if you planned to drop 8" timbers on them. You would need a special technique if you hoped to heat 8" timbers from two surfaces in a vacuum.

RF/V costs a bundle to setup, operate and maintain.

Vacutherm's exchange systems are really slow. They were (are?) also operated by timers. If you got the timers set correctly, you might get those timbers dry in three or four months. That would end up costing about as much as RF!

From contributor R:
We dry 8" x 8" and greater Douglas fir, hemlock, spruce, etc. on a steady basis with RFV technology. Our technology is quite a bit newer than what others in this forum are familiar with, when it comes to RFV drying of timbers. We have 2 commercial installations (one eastern U.S., other in Canada) with two more up and running in each country starting the 2nd quarter 2002. Drying times for these timbers are around 4 - 5 days.

From contributor D:
New oscillators may be much more efficient than the old triodes but the system is still expensive, isn't it? It still requires operators with special training, right or wrong? Is it economically feasible for many more than a few operations?

From contributor R:
We don't run oscillators or triodes in our RFV kilns, which is one reason why this technology is now a commercial success. Our sales have been made on the basis that the customer hasdemonstrated paybacks with HeatWave's technology, including no need to have a skilled person running the kiln.

Our latest sale is a 300 kW amplifier system to a very large commodity softwood lumber producer drying 2" studs, and there's plenty of those types in North America.

From contributor D:
How about maintenance? If your "amplifier" or control system broke down, could anybody other than HeatWave get it running again? Your "amplifier" has to oscillate or you don't have dielectric heating, right?

From contributor R:
Maintenance costs have been part of payback analyses done by all our customers. It has been determined by them, and guaranteed by HeatWave, to be less than conventional kiln maintenance costs.

The amplifier technology used in our equipment is similar to what's found in local TV and radio broadcast equipment. So when HeatWave's 1 year all-inclusive warranty expires, one can use a local RF technician if need be, to service the equipment. It's all laid out in the service manual.

RF amplifiers are based on 50 ohm technology, which is completely different when compared to RF oscillators in terms of how the RF is generated. RF amplifiers are not intended to oscillate.

From contributor D:
Just how long of a period are you talking about? Cheaper than a conventional kiln? So normal plant maintenance has no hope of fixing a system? If you don't change polarity, how do you heat dielectrically?

I guess that you were trying to say is that you do change polarity but not with an oscillator that I am familiar with. Is that right?

From contributor M:
We have been talking about upgrading our old RF kilns. They havent been in operation for about 8 months now. The costs to operate them are way too great. I want to "convert" our 3 RF kilns into hot water vac kilns. My boss doesn't seem interested.

From contributor R:
Changing the look and operation of those old girls is problematic. I'm concerned no amount of surgery might make them work the way they should. Not only is the RF generated improperly (using RF oscillators that cannot be controlled properly, damages wood, runs inefficiently, causes fires, hard to service, etc.), but the electrode design is improper, which causes those "big blue flashes" at the end of the drying schedule (which often has been dealt with by monkeying around with the vacuum pressure). Also, there is non-uniform heating/drying associated with that electrode design. I'm also not a fan of liquid ring pumps, which this older technology uses, because we have found that they don't operate at the vacuum levels where we see significant moisture uniformity improvements.

So in summary, can the 70's generation RFV kilns you're stuck with get modified to incorporate technology that make these things work properly? Yes. How much would it cost? We're talking pretty well everything has to get changed except the chamber.

From contributor D:
Now that is interesting. You want to go lower than 28 torr?

Liquid-rings go low enough if there is a little supporting equipment backing them up. And they will run for decades with no maintenance.

From the original questioner:
We still do not have any definitive answers, but some confirmation of what we already know... that possibly, the new RFV technology (4th generation) is the only system that can effectively dry the large timbers with minimal degrade. We would still like to hear from someone experienced with the SSV technology from Europe, if possible.

As suppliers of large-dimensional timbers, we are currently offering to make arrangements for RFV drying, but are considering SSV as a less expensive drying option.
We have found that the outrageous capital cost of the Heatwave RFV kiln has prevented us from purchasing ourselves and has forced us to consider other alternatives.

Maybe over time, as more people begin to manufacture the RFV kilns, it will bring the costs down so companies like ours can purchase this technology.

From contributor D:
Vacuum kilns are expensive to set up. RF adds a whole lot more. Don't hold your breath waiting for price to come down. RF/V will never be applicable to anything but timbers and there isn't enough market for dry timbers.

From contributor M:
A few years ago, I had a chance to play around with a little test kiln from Mouldrup. The thing would hold around 500ft. I had great results with thin oak, but anything over a 2x2 square would not dry without defects.

My RF/V kilns are some of the first ever built. We used to use them to pre-dry thick RO squares. These old junkers work great if an even moisture content isn't a problem. We would dry down to an average MC of 20% and finish the squares in our Hot Water Vac Kilns. (The best way to dry thick hard wood quickly.) I believe that the cost of RF drying is too great to be profitable.

I operate 8 Hot water vacuum kilns that hold an average of 7500ft. I can dry 4.5x4.5 aoft maple squares in around 10 days. These squares have an average BMC of 75%. Green from the saw to 6% in 10 days! I think you should be able to dry 8x8 Douglas fir in a hot water vac kiln.

From contributor R:
I heard stories about the circa 1970's kiln being used to "predry" the oak, and then it's finished in a more conventional type of kiln. The kiln HeatWave has recently sold to a hemlock commodity stud mill does the exact opposite. This RFV kiln (which will be by far the world's largest) is being used to redry hemlock "wets" that come out of a conventional kiln.

I'm a firm believer of utilizing kilns where the technology best fits. So when it comes to commodity lumber, certainly conventional kilns easily remove water out of the wood, especially above fiber saturation point. However, the experience with hemlock drying is that a portion of the total population is very difficult to dry, and some of the hemlock continues to come out as wet lumber - even when the average MC of the total population is dried to a low MC.

RFV drying is very efficient at these low MC's. To redry hemlock "wets", drying times are in the order of 5 - 10 hours (removal of 10% to 20% average MC). For red oak, I would expect those drying times to be the same. So lumber that's being redried that goes through a HeatWave kiln has relatively short drying schedules, which translates to the strong economics to pay for the equipment. It all gets down to how fast and efficiently the kiln can process wood and at what quality this wood comes out.

We've now had similar, positive results with redrying black spruce, alpine fir and plywood veneer (Douglas fir, hemlock, alpine fir). The appealing aspect of not having to sticker your wood or veneer when redrying provides additional savings in labor. Being able to apply full RF power at very low MC's and at vacuum pressures that are below 28 torr, is an engineering challenge that I'm pleased to say we've been able to accomplish.

From contributor D:
Contributor M's RF/V predried oak was loaded into hot water vac kilns because they're very efficient at low MC's. How could it be economically feasible to load predried softwood into an expensive RF kiln?

If you feel that you need to go under 28 torr, you must have discovered some new gas laws. Tell us about them.

From contributor M:
I should add that we did find a way to get our 3x3 RO squares to an even 6% in our RF/V kilns but again, the cost of operation was too great. This also caused some damage to some of the components. It has been awhile but I think the average run time was 158 hours. 7 days is pretty fast for 3" RO.

From contributor D:
Contributor M, when I had your kilns at VELDA, we dried everything to 6%. MC checks would show everything between 5 and 7% but we would do 100% MC checks before the molders. We would still find a wet piece here and there. We were drying 6/4 and 8/4 RO squares in about 60 hours.

The problem in a vacuum kiln is that the rate is dependent on the ability of the wood to conduct water vapor from the interior to the surface. When the path is open, then drying is fast; when the path is restricted, drying is slow. The permeability of wood varies greatly, even within the same piece, although not as much in FAS, knot-free, all sap- or all heartwood. But who has the luxury to dry this wood? Even so, the problem is that some pieces over-dry while others are still wet, even in the same piece of wood.

I have looked at 6 different vacuum kilns (different manufacturers) and this MC variation has been in all, although VacuTherm did use equalization to minimize this affect. Remember that the affect is due to characteristics in the wood, not the equipment shortcomings. This variation is easily fixed by equalization, but that means more time. Further, I have seen fires in several rf-vac kilns, such as those used at Velda, where the wood overdried and then overheated. I have seen one manufacturer that claimed the variation was eliminated, but in truth it was not. Another person sold their vacuum kilns (used for several years), but refused to sell the technology that they had developed over the time they had them.

RF-VAC has been used to dry softwoods and hardwoods, green to 6% MC, green to 20% MC (followed by normal kiln drying--this worked well), and also 25% MC to 6% MC. The results have been mixed, with the manufacturers often not understanding the quality requirements for kiln-dried wood. Often third party tests are with very small chambers and just a few pieces of wood or are with wood that does not have a narrow final MC requirement (such as SD=0.5). Sometimes third party tests are financially involved with the manufacturer and bad test results would mean loss of funding.

I have seen 12/4 red oak FAS dried in RF-Vacuum in South Carolina (kilns were also supplied to VA and PA, which I observed) and the amount of clear wood that ended up with honeycomb was quite high.

The best results that I ever observed were for squares. Also, the hybrid kilns, such as VacuTherm using a mix of conventional and vacuum, were much better than those that strictly followed vacuum drying.

Gene Wengert, forum technical advisor

From contributor R:
Certainly permeability affects how quickly wood dries in a vacuum kiln, and that's a variable wood property with some species having a very large range in permeability. However, RF heating is unique in being a mode of heat transfer that's volumetric (all other modes of heat transfer - conductive, convective, radiative and microwave) is from the outside surface to the core. This fundamental difference in heat transfer becomes beneficial for RF in drying large cross sections.

In addition, dielectric heating (which microwave and RF are both subsets of) are affected by the dielectric property known as the loss factor. The higher the MC, the higher the loss factor. This results in a "selective heating" where pieces with higher MC's are absorbing more RF energy than pieces at a lower MC. Therefore dielectric heating is fundamentally different than other modes of heat transfer because of this "selective" heating. This results in drying uniformity that is better than other forms of vacuum drying technologies when the average MC is low.

I'd like to suggest that the technology HeatWave has introduced into the marketplace has now dealt with what were previously thought to be RFV "drying" issues but were in fact related to improperly designed equipment (drying issues such as fires, internal honeycomb, non-uniform heating, non-uniform drying, etc.) or to improper RFV kiln schedules, and not to the fundamental heat transfer advantages this drying technology uniquely possesses. Whether HeatWave's technology will be successful commercially will be up to the marketplace and purely a function of paybacks to the customer.

The paper I presented was on the results of 4 different sawmill companies that investigated RFV redrying. All 4 of these mills were hemlock operations. Since this paper was written, the results have been repeated with black spruce, alpine fir, and with Douglas fir and spruce/pine veneer, with similarly strong paybacks. All of these softwoods are well known for their variability in wood permability.

The first 300 kW RF power amplifier RFV kiln installation is going into Washington State, operation scheduled for July 2002. Extensive analysis for over a year by the customer on all aspects of the commercial drying economics - drying times, lumber quality, energy costs, maintenance costs, environmental costs, etc., were evaluated before this significant investment was undertaken.

If the wood is impermeable and it is wet, the water will be heated, converted to steam, the pressure will increase, and then blow the wood apart. If the wood is permeable enough, the temperature will stay at 212 F until the liquid water is gone. Permeability is indeed controlling even in RF drying, unless the wood is permeable enough.

Indeed, the dry spots do not receive as much energy, but any energy they receive is used for heating and not evaporation. So again, they will over-heat.

Gene Wengert, forum technical advisor

From contributor D:
Gene, overheating doesn't start fires in RF kilns. Arcing starts fires in RF kilns. Blankets in Wood-Mizers can overheat and start fires but, of course, you can't start fires with water heated vac kilns.

Wood temperature is limited by chamber pressure as long as water is being vaporized. You can overheat and honeycomb if you shell dry first. Shell dried wood coming out of a vac kiln is a very miserable experience!

RF is scary because it can honeycomb wood that looks perfect on the outside. This is one cause of the bad image that vacuum dried wood has obtained. (Another is crazy claims made by manufacturers.) I always assumed that this type of honeycomb was caused by drying too fast. But I could dry red oak at rates up to 1.5% per hour without honeycomb.

P.S. Dry wood isn't heated dielectrically so it doesn't continue to get hotter. It's the water molecules that are caused to change polarity with the electrode that get heated. I used to think that RF was great because of this "selective" heating.

Put a piece of wood in your microwave oven in the kitchen (and keep a fire extinguisher handy and also a rapid way to put the piece of wood into a bucket of water and some tongs). Sure, water heats, but the wood also has molecules affected by the dielectric field. You can smoke oven-dry wood in a microwave by using high power in a matter of a few minutes. Dry wood is heated dielectrically.

Gene Wengert, forum technical advisor

From contributor D:
There is a HUGE difference between the frequency of a microwave and an RF kiln. You are comparing a lighthouse beacon to a flashlight.

From contributor R:
Sounds like contributor D has the scars on his forehead from using RF oscillators over the years. He's 100% correct that there's a huge difference between microwave and RF heating. Both are forms of dielectric heating but it stops there when trying to compare the two. Microwaves can probably be best characterized as heating with electromagnetic waves whereas RF would be characterized as heating with electromagnetic fields - all to do with the operating wavelengths.

In addition, home or industrial microwaves put power densities into the product in the order of 5000 kW/m3 whereas RFV drying power densities are in the order of 4 kW/m3 or less. Obviously you're going to blow up your wood if the power densities are too high. RF amplifiers have absolute control of RF power output, whereas this is very difficult to do with oscillators.

Contributor D is also correct that the fires are associated with high voltage arcing, coupled with the unique tendency of RF oscillators wanting to continue to oscillate and supply RF power directly into the "disturbances" that are occurring between the two plates. So if there's a problem, an oscillator continues on its merry way and puts more energy into that problem area. Not a good thing and something which has poisoned the well and hardened negative opinions over the years on the technology.

For RF amplifiers, should there ever be a problem that occurs between the applicator (or any other component of the RF output circuit), amplifiers detect it as reflected power, and the RF power is either reduced or shut off completely. No more carbonization events with amplifier technology.

So if RF/Vacuum drying issues are now commercially addressed as it relates to:

- elimination of fires
- absolute control of RF power in order to eliminate honeycoming
- the ability to run at very low vacuum pressures with no high voltage arcing
- the RF able to operate with energy efficiency at the full range of load impedances (from green right down to 2% MC)

The question is: are there any opportunities out there for this technology, in spite of its high capital cost? Redrying wood is certainly one of those opportunities and there's likely other ones as well. I also agree with Den that for thick cross-sections, the market is limited but the premiums could pay for the equipment (although it's probably on a case by case basis).

I also don't know how permeable wood has to be, as it relates to the commercial opportunities with this technology. It seems based on you easterner's experiences that red oak has a high enough permeability where the circa 70's RFV technology that was used made some commercial sense. We find in softwoods that the permeability is high enough for many commercial applications. We have also had good success with birch and aspen, which is permeable enough to dry 16/4 cross sections in the order of 5 - 7 days. I'm sure this won't be the case for something like white oak but I've got limited experience with it. There's a lot left to learn here. Conventional drying has about a 40 year jump in research on kiln schedule development.