Question
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
