Chinese-built radio frequency vacuum (RF/V) wood dry kilns

by Dennis Socling

Let me state up front that I have never operated a Chinese-built RF/V kiln, but I used to be responsible for the operation of eight 5000 boardfoot American-built PowerDry RF/V kilns in the early 1980s.  As the president of PCS VacDry and having to fight against decades of 'bad press' that poorly engineered vacuum kilns have generated, I have an interest in seeing this outdated and compromised RF/V design get the notoriety I believe it deserves.

I have come across two companies in North America who have recently purchased Chinese-built radio frequency vacuum kilns because of the Chinese claims that they are great kilns, combined with a ridiculously low price. After a discussion with one of our potential customers in Australia and discovering they had decided to buy the Chinese RF/V kiln, I decided it was time to set the record straight: substantial compromises were made to reach a price point


Carbon Steel Chambers

To begin, it is an extremely bad idea to use a carbon steel vacuum chamber. One company that I have talked with who has one of these Chinese kilns says that the carbon steel is protected from moisture and water vapor with a layer of fiberglass. What will likely happen over time is this: the fiberglass will slowly develop cracks and pinholes, and water will eventually get behind it.  Every time vacuum is pulled on the chamber, the water between the fiberglass and carbon steel will vaporize.  When water vaporizes, it increases in volume by 1700x. This pushes yet more fiberglass loose, fracturing the bond between the carbon steel and the fiberglass. 

Further, the carbon steel is going to rust in contact to water and water vapor and further weaken the bond...and the operator won't even know this is happening, much less be able to do anything about it!  At some point, the rusted carbon steel is going to weaken to the point where it can implode or "suddenly pressurize", and the kiln will be worth nothing but the price of scrap metal.  A discussion with a different owner of a fiberglass / carbon steel chamber RF/V kiln revealed the hidden problem...their kiln chamber was OK for about 6 months before corrosion took over.

A long time ago I was a consultant for a company that had carbon steel vacuum chambers that were lined with thin stainless steel. One night an operator was pulling vacuum and the carbon steel chamber suffered a sudden collapse to unseen corrosion. Once the carbon steel vacuum chamber gave way, air pressure on the stainless steel lining wrapped it tight around the kiln charge. The Red Oak newels that were in the kiln had to be taken out by hand, one at a time. The kiln cart never made it out of the pile of scrap that was left over.


Radio Frequency Drying as a heat source

The Chinese are also using a copy of very old technology that was rejected by the US market more than 30 years ago: what I see is a straight copy of the equipment I operated 35 years ago.  Here is a link to a new Chinese radio frequency generator that appears to be a direct clone of ancient (as in, the patent was granted in 1976) PowerDry technology that failed in the US market more than 35 years ago:

The tube that is on the right hand side of the photo (inside the yellow housing) cost us $12,000 35 years ago! They are oscillators used to broadcast radio signals. Underneath the oscillator is the transformer enclosure. One is high current and the other is high voltage. If I remember correctly, the high voltage transformer was around 12,000 volts. This had to be rectified from AC to DC current and it took electronic know how to keep functioning rectifiers. The average man in the street couldn’t do it.

The discs with the blue centers are capacitors. They would commonly blow apart on us. The copper windings are inductors. Arcs would jump across them. The flat metal that looks like copper is beryllium. We used to buy the stuff in rolls to replace what burned.  I had my operators wash everything once a month with solvent to keep moisture-trapping dust off the components in an effort to prevent arcing.

Heat (energy) loss is inherently high with RF, and this drives up ongoing drying costs.  Unlike PCS VacDry kilns where the heat is effectively contained within the kiln charge and entirely used to vaporize water and dry the wood, the longer the RF/V kiln runs the higher the drying cost will be due to those substantial ongoing heat losses.  The potential customer in Australia that had thought about buying a Chinese kiln ended up calculating that our higher price would be recovered after a short period of operation because of PCS VacDry’s much lower operating costs.

Since the water in the wood is part of the electrical circuit, the moisture content of the wood has a direct impact on what is called the 'dielectric field'. Since not all of the wood dries at the same rate, the dielectric field typically becomes uneven throughout the kiln charge. This can lead to uneven moisture content: one area can end up wet while another can be over-dry. We had a real problem with this when trying unsuccessfully to operate the PowerDry kilns. And as our kilns got older, the electrodes distorted and the problem just got worse.

The unequal dielectric field also caused us arcs between the wood and the electrodes that would catch the wood on fire. The fires were mostly contained by the vacuum but if you pulled out a smoldering load, it could ignite. I had to call the local fire department one night to soak down a load of Red Oak at the end of the drying schedule.

Another severe problem with radio frequency drying is the drying defect referred to as “honeycomb” due to stresses from the typically uneven drying. For those who are not familiar, honeycomb looks like a tear or crack inside the wood. From the outside, the wood can look perfect. The problem becomes a nightmare after you ship a load of honeycombed wood to a customer.


Operator Controls

Looking inside at the controller wiring of a 'modern' RF/V kiln, you would see a nightmare. Extensive relay logic and hundreds of connectors, each a potential point of failure. You would need a dedicated technician who was intimately familiar with the system to keep basic electrical issues like a single bad relay contact from shutting down the entire drying process.

Reportedly, two Chinese engineers spent two weeks putting the kiln together and teaching the operator how to run it. Unlike our "set and forget" kilns, with RF the wood is part of the electrical circuit.  If you look at those meters in the picture linked, you see that somebody made black marks on the plastic: as the moisture content of the wood changes you have to frequently readjust the oscillator to try to keep the needles near those marks.  And when you dial in one, that change will impact the others:

Additional reading:

I think RF/V has one appropriate use case. The biggest pieces of softwood that I can dry in a reasonable time frame are around 18”. If you need to dry timbers bigger than that, radio frequency is one way to get heat deep inside of a piece of wood.

PCS VacDry customers typically set up their own kilns. They don’t need on-site training. We've got permanent stainless steel chambers.  With our carefully engineered heating system we can ensure effortlessly uniform temperatures throughout the kiln charge, load after load with no intervention by the kiln operator.  And they get free daily checks of their kiln's operation through the Internet by PCS VacDry.  PCS VacDry vacuum kilns are designed to operate for decades.

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