Whoa?
A radio frequency vacuum kiln dried live edge slab?

What the Heck is a Radio Frequency Vacuum Kiln?

We get a lot of questions about how a Radio Frequency Vacuum Kiln works to dry live edge slab lumber. The easiest way to explain it is that it’s very similar to a microwave oven. We load a stack of wood slabs into the kiln and apply pressure to the stack. There are no plates between the staff and we apply electrical charge to those plates. We will explain more of those details further in this article.

 

Let it Air Dry!

The first step after cutting is the air drying process. We may allow our slabs to dry up to six months or one year depending on our workload. When using a properly built and performing Radio Frequency Vacuum Kiln, a long air drying process is not required. This just happens to be the cycle of our workload based on the number of slabs we produce. Depending on our production, we can transfer our air drying inventory to kiln drying slabs at any time.

 

The vacuum kiln that you see in this image is about a 20-foot-long chamber that can fit a timber cart about 5 meters long. You can see here on the timber cart … It’s always a jigsaw puzzle. We usually have two boules of Indonesian Suar wood or South American black walnut that are about 4-5m long. We try to use the total length that we have available in the kiln.

All of our lumber comes from sustainable forest, legal harvested Indonesian sources. The supply was originally planted with the intention of being harvested and ten trees are always planted for one that is taken. We then saw-mill it at our factory. For the three-inch walnut, we like to air-dry it for four to six months to give it optimal color. At this thickness, once it goes in the vacuum kiln it’s going to have a little bit better moisture content. Less moisture is almost always better.

 

Wait…So What’s a Vacuum Kiln?

A lot of people will say, “Well, what’s a vacuum kiln?” There are a few different kinds of them and we have several types of each. The one we are specifically talking about today is a radio frequency vacuum kiln. Basically, what that means is that there’s a chamber that all the wood goes into, the door’s close, and then we initiate a vacuum pump. The pump will suck out the majority of the air in the chamber. By doing that, we’re removing the atmosphere pressure, and we’re able to vaporize water at a much lower temperature. Out in the standard atmosphere, we’re about 101 kPa. Inside the kiln, once we turn the vacuum on, we put it down to about 7 to 10 kPa. This would boil water or vaporize water at around to 42 degrees. As the wood dries out, we’ll raise the vacuum up a little bit to get to higher temperatures, but in general, you’ll have lower temperatures in a vacuum kiln drying.

The other advantage is that it holds the material very, very flat. Inside the kiln, there is a hydraulic press that’s going to push down on the wood. As the wood dries, it will shrink and the hydraulic press will actually adjust and apply more pressure. They would stack that is about 4 feet tall we can see 4 to 5 inches of height loss. That is basically moisture leaving the wood and the slab becoming more dense.

 

Electrically Charged Metal Plates

In the vacuum kiln, we have electrically charged metal plates. This is actually going to transfer the radio frequency from a plate to the other plate, and the wood is in between. What we’re trying to achieve with that is that the electricity that’s flowing from plate to plate is actually shaking the water molecule at about 6,800 times per second. That creates friction, and that creates heat. We need that heat energy to warm the live edge wood slabs up to a certain temperature to be able to boil that water away. This process is very similar to what a microwave does to your food when you cook it.

On our kiln cart, we’re going to have a negative sheet, which is the whole body of the kiln, and then we’re going to load on about 3 to 4 thick slabs of wood, and then we’re going to have another metal sheet, another layer of live edge slabs and so on. Once we put that sheet on, this is going to be the positive sheet. Basically, it’s not connected to the negative sheet, but it is flowing electricity between the negative and the positive, shaking that water molecule, which is then generating heat.

 

 

The example above shows the vacuum kiln with two positively charged plates and three grounded plates for a total of five plates. In theory, you could use any number of plates, but this seems to be an efficient setup that yields high productivity and is still fairly easy to manage.

The side effect of this electrically charged heat is that the wood is actually drying from the inside out. When we start drying, we’ll actually have the outside of the wood will be almost like in a steam bath, a really nice environment for the wood, not necessarily for us because there’s going to be no oxygen in there, and it wouldn’t be pretty if we hopped in the chamber. But for the wood, the lack of oxygen and the hydraulic press all is a whole system to keep the wood flat during drying, and restrict defects and prevent the slabs from checking or splitting.
 

 

Increased Production with a Vacuum Kiln

Speed is the third and most awesome side effect of the vacuum kiln, in that we can take this three-inch wood, which is generally about 30% moisture content now, probably a little higher because that’s right on the outside, probably more in the 40, 45 range. We’re able to dry this in about eight to 10 days through this vacuum kiln with virtually no degrade in the wood, which is pretty awesome. It’s an exciting tool to have. We load the vacuum kiln and we unload the vacuum kiln every 8-10 days and have it running close to 365 days a year.

Our team will spend a half day unloading and reloading the vacuum kiln with beautiful live edge slabs. It’s a lot quicker unloading, usually, than loading it, because when you do load it it’s a bit of a jigsaw puzzle, and you want really good contact, wood to wood surface. We usually have about four feet of total height. We usually also have about four feet of width, and then the cart itself is 16 and a half feet long. We always try and maximize the amount of live edge slab that goes in the kiln, because the amount of electricity it uses is going to be the same. So the cost to dry the wood, it just makes sense to put as much in as you can. So we maximize our yield and extend that cost savings to our customers.

We always inspect the wires as we roll the cart in. Make sure nothing’s catching or bumping the wires out of the way. Two plates are going to be a positive signal, and the top, middle, and bottom is a negative signal. So we don’t want those plates interacting in a way would cause any shorts.

Once the whole timber cart has been rolled in, what controls the drying process is the temperature of the internal moisture the wood. We like to drill a hole in one of the thickest slabs so we can place a thermometer in it. This way we can gauge and monitor the temperature throughout the slab drying process. We always make sure the thermometer cable is nowhere near plate cables. This is another way to avoid getting zapped or causing a short.

We have a hydraulic plate that pushes and holds the wood down the entire time as it dries. This holds the wood very flat. Then as the wood shrinks, it’s going to continuously add pressure, and keep the wood as flat as possible throughout the entire process.

One of the last things to do is shut and seal the door. The door on our radio frequency kiln is double insulated and it is very heavy.

 

High Tech Wood Controller

We use a computerized control module to control the drying process. It is essentially the brains of the operation. It controls a 20,000-watt microwave. This also controls the vacuum pressure, as well as the temperatures. It has a touchscreen that we can touch, and it even lights up. The load that we just took out of the kiln had 152 hours of heating time, so probably that was in there for six days of heating time. But actually, the load was in there for eight days. That just shows you, it does shut on and off as it reaches its max temperature. It drained about 700 liters of water out of the last load.

We can set the min and max temperature based on the live edge wood slab thickness. When we’re doing three-inch-thick walnut, we set the max temperature at about 40 degrees Celsius, and I’ll have a low of about 38 degrees. This machine is going to heat up to 40 degrees Celsius. Once it hits that, it’ll shut off, and then cool down until 38, and then turn back on and heat to 40, and it’s going to repeat that. Throughout the eight to 10-day cycle, we raise the temperatures along the way.

There’s also a vacuum pressure, which, we’re in an atmosphere of 101 kPa. I’m going to be pulling it down initially to 9 kPa, and then it’ll rise to 12. Again, the vacuum pump will kick on and pull it back down to 9, and it’ll cycle just like that.

We get a really cool cycle of vacuum atmospheric pressure and heat. The humidity fluctuates along with temperature and pressure.

 

Press the Start Button

When we hit the start button, a fan turns on along with a high-frequency generator. Then it’s going to build up some electrical charge. We have to wait for a few minutes until we can process to the next step, but we’re going to see here that the voltage turns on. Then we wait a few minutes for things to get going before turning on the vacuum.  The vacuum pump uses an air compressor to generate negative pressure and reduce the atmospheric pressure as mentioned above.

 

Slabs are Drying and Almost Ready for Production

That’s it! The wood slabs are loaded and the machines are all fired up! In about eight to 10 days, we’ll have a nice, dry load of live edge wood slabs that will be ready for making some of the world’s most beautiful furniture.

We hope this article has been informative especially if you were interested in learning how live edge wood slabs our dried in a radio frequency or microwave oven kiln.