Vac. kiln

[69bronco]

Ran across this pic.

  Says it's a load of green ash,ad walnut, sinkers and cookies. Thought it was interesting.  Comments..Den

[Den Socling]

That's Paul in Canada. He bought a Chinese radio frequency vacuum kiln. A big mistake IMHO. The ones I ran had oscillators that were a big problem. Paul thought his new RF/V didn't have what I had. He sent me a picture and it is a spitting image of what I had. Another guy in NY had one and he said it was good for about 6 months then everything was corroding. Paul said the Chinese had to send two engineers to set it up and show him how to run it. I've had a fair amount of interesting email with Paul.

[69bronco]

How does that work with no stickers? Are there some sort of heat pads between layers? Don't need to reveal any trade secrets, just curious 

[Den Socling]

There is a dielectric field created between electrodes. The energy oscillates. Switching direction of positive and negative a couple million times a second. Water is bipolar which means that one side of the molecule is positive and the other is negative. As a result, the water molecules flip back and forth with the oscillation and the friction causes heat.
It seems like a great idea. 30 or so years ago, I was a great fan of RF/V but there are a few bad problems. The strength of the dielectric field is not even through out the kiln charge. The result is uneven MC. The kiln charge is a capacitive part of the circuit and it changes making adjustments necessary. Capacitance decreases as MC comes down. Oscillators are inefficient making drying cost high. Very high voltage makes life dangerous and difficult. I used to impress people by taking my knife and dragging down the crack of the cabinet door. It would leave a shower of sparks from the RF. I had my operators wash everything with solvent once a month. One high voltage transformer was $14,000 and the oscillator tube itself was, I believe, $12,000. Before this company hired me, they were blowing up transformers and tubes at least once a month.
The Chinese are offering this junk dirt cheap and some people are falling for it.

[serg]

Hi! I agree with Den. I believe this method of warm-up yesterday.

[Den Socling]

I wondered for a minute about what "warm-up yesterday" could mean. I think he was saying that it is old heating technology.

[GeneWengert-WoodDoc]

I have seen three attempts at rf vacuum drying.  The first was tanoak baseball bats in California. The next was in SC a few decades later, and they had several kilns in WVa and PA.  And the third was decades later recently.  The problem with all three is that the dielectric behavior of wood is not merely an effect of the water in wood, but also the wood itself.  I even saw some dry regions of one piece of lumber begin smoking, as energy went into heating the wood even though this piece was dry.  The SC process worked somewhat ok for thinner material, like 4/4 red oak, if the wood was dried to about 25% MC first, but the overall price is too high compared to conventional.

There was one rf kiln - non-vacuum - in Memphis, TN that dried persimmon golf club heads.  It ran for years.

[Den Socling]

I called the fire department one night when we opened a door and found 5000 BF of Red Oak on fire. Our usual cause of fires was arcing on top of the load. I solved that by laying sheets of aluminum on top of the load.

[Den Socling]

So long to this post. At the request of Paul's business associate.

[69bronco]

Good for you!

[Den Socling]

I deleted this post. The guy deserved it but it was just too mean for me to do.

[longtime lurker]

Imitation is the sincerest form of flattery and all that...

Sadly, Chinese imitation is mostly void of royalties for patent holders.

I seen a Chinese RF ( non vac) kiln advertised not long back for cheap cheap. Its hard for any manufacturer to compete with them on price for sure: I am unsure why they haven't picked their quality up across the board because I've seen well made Chinese stuff and its okay, still cheaper then the Korean stuff and similar quality.

[Carlos Sheng]

I called the fire department one night when we opened a door and found 5000 BF of Red Oak on fire. Our usual cause of fires was arcing on top of the load. I solved that by laying sheets of aluminum on top of the load.

I'm little confused,why the wood on fire, the wood is under vacuum and wet condition

[GeneWengert-WoodDoc]

The fire does not occur when the kiln is first started, but rather when the wood is fairly dry.  What happens is that several pieces become quite dry ahead of all the rest of the wood in the kiln.  The rf field is not uniform, so these pieces are in the high energy input region.  Once dry, the energy input heats these wood pieces above 212 F.  This heat results in rapid oxidation, which we would call charring.  The technical term is pyrolysis.  The gasses released are highly flammable.  As there is no oxygen, the wood does not flame and gas does not ignite, but the wood does smoke and char, which is reason enough to call the fire department.  The smoke is highly flammable. When the door is opened, the incoming oxygen in the air results in aggressive flaming.  I saw several rf vacuum kilns smoke as well as one non-rf vacuum kiln smoke some pieces.