Vacuum, Dehumidification, and Microwaves in One Kiln?

Here's a long, highly speculative, but informative discussion of the concept of using microwaves, vacuum, and dehumidification for drying wood in a prototype kiln. April 20, 2011

What is the reason of buying a dehumidification system and not a vacuum system when the vac does the job in a fraction of the time? I need to dry 30mbf hardwood lumber monthly.

Forum Responses
(Sawing and Drying Forum)
From contributor C:
The initial equipment cost can be higher for vacuum but, if you are drying thick or difficult species, dehumidification can be impractical.

From Gene Wengert, forum technical advisor:
I agree with Contributor C and would add that energy cost, stacking cost, maintenance, and operating labor must also be considered, in addition to equipment cost. It is usually best when looking at costs to look at the cost per MBF. Further, the cost will change if you consider air drying prior to kiln drying. Perhaps the biggest cost factor of all is quality loss (called degrade) or, stated positively, the lack of quality loss. This is oftentimes the greatest benefit of vacuum drying. Drying time is a factor but not overwhelming.

From contributor W:
There are a number of factors to consider such as species, thickness, initial and final moisture content, cost of energy, labor costs, whether you buy or saw yourself, etc. In my somewhat biased opinion, between the two choices, DH will be the best for most any species up to 8/4 and you need to look at vacuum for anything thicker. I have seen very successful DH kilns drying 50,000 BF per charge of 16/4 red oak. Some places have both but unless you are dealing in a very specific product, the first kiln you buy will probably work out best as DH. Put a sample monthly production list together listing, species, thickness, initial and final moisture contents and volume of each. Ask the various manufacturers to give you their estimates of costs.

From contributor C:
To contributor W: Are you suggesting that A DH kiln can dry 16/4 red oak? Even if it was air dried for a year, this would be a challenge in a DH kiln. If you can dry 16/4 red oak from green, I'm in the wrong business.

From contributor W:
We had one customer who had two kilns that each held 50 MBF of 16/4 red oak and he turned it over twice a year for many years. It had to go in green because I doubt you could successfully air dry 16/4 oak. 200,000 BF of 16/4 red oak commanded quite a price. Because it dried so slowly, they did it with a pretty small DH system. We have another customer who has about 30 DH kilns and dries nothing but 8/4 and thicker oak.

From contributor C:
I wonder what the drying cost and degrade would be with a 26 week schedule. You're talking about 26 weeks and a vacuum kiln can do it in three weeks. We are not comparing apples to apples.

From contributor X:
My experience drying even 12/4 RO in my DH kiln was not good. I don't put any species in it thicker than 6/4 (except for walnut and mesquite, etc.) and then some of them such as RO and most of the interlocking grain species I air dry first.

The ideal situation for me is a small vacuum kiln for my higher premium burls and figured woods, with a DH kiln for the furniture grade (which I have) and a solar for the building grade lumber (which I have had 90% complete for several seasons). The big hurdle in the vacuum kiln arena has been heating the wood. I have recently decided to build a radar vacuum kiln, because I have all the background to do this. I was a radar tech in USCG and worked for my dad's HVAC company when I got out of the service.

I realize there's more to it than injecting about 5000 watts of microwave energy into a vacuum but I am going to take a shot at it anyway. I have already built a radar transmitter using a 3-bank magnetron cluster (untested as yet except for a couple of two minute test runs) I just need to figure out what kind of membrane I need at the end of the waveguides so that it will allow the signals to pass undistorted into the chamber, but that will also not implode when they are exposed to 29+ inches of mercury. I guess I'm sort of asking for ideas without coming right out and asking so as not to hijack the OP question. I'm hoping some sort of thin phenolic might withstand the pressure without absorbing any of the signal.

I am making a point for the OP though, which is that every type of kiln has its place, in addition to the ones mentioned also steam and perhaps condensation kilns even, if you happen to have a big cooling tower and heat exchanger lying around with free or cheap wood scraps. I feel certain that all the posters here would also agree that most if not all kiln types have their place, just not all would probably agree exactly how each type of kiln system is optimized, or in what application it is best utilized. I love these kinds of threads - this is where I get to learn something new. We are supposed to do that every day.

From contributor C:
Isn't there a penetration problem with high frequency? Also, isn't there a problem with wet pockets with any form of dielectric heating?

From the original questioner:
Thanks for all the backup. By the sounds of it the best system for me to go with is the DH. All my lumber is under 8/4, mostly 4/4, all for furniture use (oak, ash, poplar). So for 30-40 mbf monthly is it better to get one big kiln or two or three small ones instead?

From contributor W:
It depends. Are you doing several species and thicknesses? Do you saw or buy lumber? How do you sell your lumber - truckload or small quantity? It is almost always better to have several small chambers than one big one unless you are filling trucks or containers or you always do the same product.

From contributor C:
Yep. DH is for you. Whether it's one, two, or three kilns, it would depend on which was your bread winner. I believe you could mix a little ash with oak but you might not want to slow down poplar that was loaded with oak.

From contributor X:
I'm under the impression that most RF/V kilns use radio waves which among other things generally carry less energy than microwaves, but I don't know why designers choose radio waves over microwaves (if they do). Maybe the lower frequencies of radio waves tend to have less of the drawbacks you mention. I already successfully employ a microwave oven to dry very small blocks of wood like for bottle stoppers and even up to blanks for small bowls etc. in some species so I know the idea itself is sound.

I don't plan to make this large scale. I have plenty of items lying around to use as a chamber, the largest being a 750 gallon propane tank. If I use it I'll seal the inside with an aluminum-based sealer, but I will paint a small piece of metal with it and nuke it to make sure the sealer does not absorb any degree of energy. If it does I'll just keep testing sealers until I find one where the molecules don't get excited by 2.45GHz with a wavelength of nearly 5".

I briefly considered tying the three high voltage circuits together and operating them with a single control circuit, but I quickly decided that three independent circuits would give me a great deal of flexibility. I'm just using residential microwave oven components. I haven't decided from which direction and at what locations to introduce the waves into the chamber, because I don't understand how the waves interact inside the chamber and with the wood yet. I don't know squat about any of that so it's all a crap shoot at this point. I also thought about mounting the magnetrons inside the chamber. I wouldn't need to mess with waveguides that way; I'd just let the waves bounce around until they found some cellulose to bite into. That won't work though because they need constant airflow over the cooling fins, and I'd have to make some elaborate modifications to cool them inside a vacuum chamber.

So the biggest hurdle I have to even getting to the 'arc-n-spark' stage is trying to decide where (location/direction), and how (the mechanical integrity of the waveguide/cavity resonator intrusion needed to introduce the waves into the chamber). I also briefly though about tying all three waveguides together and introducing them into the chamber via one common waveguide. That would minimize the possibility of a leak, but I have no idea if the waves would conflict with each other. I could probably find that out fairly easily though, and waveguides are easy to build.

I think I'll have to end up having to use what's called a cavity resonator. If so, here's where an engineer earns his gravy in my book. The resonators must have the proper dimensions and shape or the frequencies will go bananas once they get there. I think there's probably software that will do this by now, but I know an EE that could do this for me I think.

From contributor K:
To contributor X: Why would you need to isolate the waveguide from the vacuum chamber? Why not build the waveguides vacuum tight? The magnetron is a vacuum tube.

From Gene Wengert, forum technical advisor:
When vacuum drying, the boiling point of water is lowered, perhaps to 100 F. This allows for rapid removal of water, especially the free water without the negative of high heat. Drying is by mass flow and not by diffusion for the most part - technically speaking. Further, the shrinkage, if the proper settings are used (and this is why you need to hire Contributor C and use his equipment) does not occur substantially during the removal of the free water. The more or less even movement of water throughout the entire piece allows for fairly even shrinkage, which avoids the steep moisture and shrinkage gradients that cause many defects in oak.

The use of microwave or radio frequency to provide the energy needed for drying has been tried for many decades. As Contributor C alludes to, the problem is getting the energy to heat evenly, as higher MC areas plus mineral areas will absorb more energy and heat more than drier areas. In one load of 12/4 red oak (white oak does not dry well at all) in a vacuum/rf dryer resulted in 40% of the piece with honeycomb and also a wide final MC spread. Every piece that had defects also finished the drying cycle at a high MC. I do believe that rf energy and drying, especially for hardwoods, are not compatible due to the variations in the electrical properties of wood. (The equipment is good, but the wood's variation is the issue.) One large operation in PA using this vac/rf equipment finally had to go with air drying prior to kiln drying to get reasonable (but not cost effective) final MC results.

A good DH operation can, when lumber prices are good, make a gross profit of about $25 per day per MBF. Subtract the operating costs from this value. It is possible to operate a DH kiln with 12/4/ and maybe even 16/4 green red oak, especially if there is a technique to avoid mold growth in this closed environment (moth balls worked in one experiment), and get good quality (no bacterially infected oak; no white oak). The potential profit when drying 4/4 would be 180 days x $25 or $4500. If the selling price of the KD material is perhaps $6000 per MBF or even more. It might be possible to break even with thick oak green from the saw. In fact, I did see one government contract that needed thick oak for a special project and the asking price was even higher, as it is a rare item and they were in a hurry. Remember, you can dry lumber all day, but you only make a profit when the KD lumber is sold.

From contributor X:
Gene, I can't punch holes in your assessment, your logic is bullet proof as applied to a conventional lumber drying operation. But I failed to emphasize why I hope my intended use of a MW/V kiln can succeed. I'm not going to be drying lumber, I only want to dry small to medium sized turning blocks and short craft boards i.e. luthier sets etc. and blanks along the lines of pens, bottle stoppers, up to medium bowl and vase sizes. Perhaps larger if that is successful.

I realize moisture can still get trapped in one area of a 8" cubed burl block more so than an area even an inch away, but my thinking is that the frequency of that happening and the degree to which it happens, will be much less than with the large scale operations where only lumber dimensions are being dried.

Another thing - I went to the back of the pasture yesterday evening and looked at that 750 gallon tank - it's huge. I forgot how big that thing is. Perhaps you were seeing it in your mind's eye clearer than me, and naturally thinking I'd be filling it up with lumber and trying to do something which you are already aware is not really doable.

I got this hair-brained idea based on the success I've had in using just my shop MW to dry single to several small blocks at a time. That is very small scale success granted, but I have had acceptable results doing this, especially when I've given the blocks the alcohol bath treatment prior to, but even when not. All of this in an ambient environment - no vacuum.

All I'm aiming to do is take this small scale and up it about three or four times in capacity, and instead of the relatively inefficient water-displacing alcohol bath, replace that with a vacuum environment.

Knowing how I intend to apply it, does this change your opinion about the possibility of it being successful? You're used to applying your expertise to things on a big scale and I realize that, but if I can employ this technology in the way I hope, I would consider it highly beneficial to my small operation. If you still feel confident it cannot be as successful as I envision (or even close) then I have to ask myself if I want to proceed with this. Not trying to put any pressure on you or anything, but I'm glad I'm not in your shoes when you get questions like this.

Contributor K - it isn't necessary to isolate the waveguides from the chamber, in fact they have to be hard connected to the chamber in order to allow the signal to pass into the chamber. I wouldn't want to build waveguides that are also vacuum chambers themselves. Magnetrons have a small vacuum cavity that houses the cathode, but this does not have any relevance to, or offer any benefit that I can see in the relationship between the waveguide and chamber. Building vacuum-tight waveguides would be very difficult because they are just loosely (relatively speaking) connected to the magnetron mechanically. This is why I think a cavity resonator would be the easiest approach.

From contributor G:
About ten years ago, I did basically what you want to do Contributor X. I made a steel box 2'x2'x5' out of 12gauge, reinforced it with 2"x2" angle iron. The top was removable to put lumber in it. I used the guts from about a 1kw microwave. To get the signals into the chamber, I cut a hole about 2"x3" (whatever size was coming out of the microwave), put 1/4" plexiglass over the hole for the microwaves to go into chamber. Just inside the chamber where the entry hole was I put a small metal fan (it was the one removed from the original microwave when I dismantled it) to disperse the microwaves (this was a common technique in the 80's).

On the bottom of the box, welded a 1/2" pipe flange to connect to 3 pieces of 4" PVC pipe that was sealed with a ball valve on bottom to drain liquid out. Never did have any liquid come out as it all went through the vacuum pump.

The problems I had were:

1. Finding an airtight seal that the microwaves would not affect. I finally used a closed cell foam strip covered with the very thin metal tape used for ductwork.

2. Because the liquid ended up going through the vacuum pump, it literally would eat the metal. I did not bother to invest in the proper pump which I should have.

Another method I used for vacuum drying was making sure all lumber was same exact size, putting down black plastic with PVC pipe same size as lumber with holes in pipe facing lumber at each end of lumber and connecting it to vacuum, putting entire package in sun to provide heat. Both methods worked well for small quantities except for the pump problem.

If you attempt it, invest the money in a good vacuum pump that uses the water seals (I do not remember proper terms). Also, be sure the chamber can withstand the pressure when drawing a vacuum. Otherwise it will implode. The metal box I built would make terrible noises and I always had something heavy in between (normally my backhoe) when I would draw a vacuum. It was fun to play with but not practical because I never invested in a good vacuum pump that could withstand the liquids.

From contributor C:
The vacuum pump that you need is called a liquid ring. To keep warm water vapor out of the pump, you need a shell and tube heat exchanger standing on its end. Of course, this all complicates things because you need running water every time the pump starts. And that leads to a vacuum controller. And it goes on and on.

This may sound silly but I have thought about using one of those vacuum bags used for storing garments for small jobs like you are talking about. The bag of wood could be submerged in a tub of water with a strip heater and a thermostat. Every time the bag fills with vapor, pull a vacuum. Water increases in volume 1700x when it changes from liquid to gas so it should be obvious when you need to start the vacuum pump.

From contributor X:
I plan to install the stirrers in the cavity resonators. I wanted to use the rotary types that came in these ovens but couldn't figure an easy way to spin them inside the resonators. I'll use cooling fans also. I just have to decide how to seal the resonators through the wall of the chamber. What did you mean when you referred to a seal that wouldn't be affected by the signal, the door seal or the seal around your plexiglass? And I'm surprised to hear that plexiglass is transparent to MW energy I wouldn't have guessed that.

Contributor C - the simpler any system is the better. I've considered a bag before (I think you even suggested to me) and that should be simple enough to test. I think the reason I'm so bent on the MW/V setup is because I have had good results with the MW - but introducing the vacuum into the equation appears to be much more involved than I had hoped, even on a small scale. I don't know if I'll go forward or not, but if I don't I sure would like to figure out a use for my Frankenwave contraption.

From contributor G:
Contributor X: The problem on the seals was actually (I think) the adhesive used would get hot from the microwaves and release. Also used silicon caulking on the plexiglass to seal it to steel and microwaves would set it on fire. Finally used 2-3" wide aluminum duct tape (the thin high quality stuff the pro's use) folded over with a 1/4" thick closed cell foam strip (used on doors/windows/etc). The aluminum kept the microwaves from getting to anything else, also helped to seal chamber from microwave leakage which would happen without it.

Contributor C - when I was playing with vacuum drying the clothes bags were not available or else I did not know of them. However, I have used them very successfully for vacuum bags when I glue veneer to wood.

From contributor T:
OK after reading the above thread I know that a DH or solar kiln is what would work best for my situation. My question now is will a small DH kiln run equally well loaded at 500 bf as it will at 3000 bf?

From contributor W:
You can dry 500 board feet in a 5000 BF DH kiln equally well but the operating cost will be a little higher. While the compressor only runs based on the amount of water to be removed, and therefore does not cost more, the fans and heat loss from the larger chamber can add to energy consumption. But it is usually in the neighborhood of 1-2 cents per board foot. But most people hate to tie up a big chamber for a small load. The ownership cost (interest, taxes, insurances, maintenance, overhead, etc.) are the same each day whether the kiln is full or empty. That is why a lot of people who dry for others charge by the day and not the board foot.

From contributor I:
In your discussion you are only comparing DH kilns vs vac kilns with heating plate technology. Modern vac kilns do not use heating plates anymore as it limits the kilns in capacity and it requires effort and extra equipment to load. Modern vac kilns use steam as a heating media and are equipped with fans and heating coils just like conventional kilns.

As a company having built vac kilns, DH kilns, and conventional kilns for more than 40 years now we may say that superheated steam vacuum kilns are the best solution for thick lumber. As a matter of fact we combine modern steam vac kilns with heat pumps which is the most efficient way.

From Gene Wengert, forum technical advisor:
Do you have any vacuum kilns that are working in the U.S.? I have seen four different style vacuum kilns and only one used hot plates or blankets. The others used steam, microwaves and alternating vacuum and atmospheric. I did see one of your vacuum kilns about 20 years ago in NW West Virginia, but it did not work satisfactorily and was removed.