Why does drying wood in a vacuum kiln produce a better product than traditional methods?
It all comes down to time and quality.
Waiting for wood to dry outdoors is a long process! In open air, wood will need an entire year per inch to dry.
In the Kiln (or VacDry), Cedar can be dried in just a few days, Pecan will take a couple of weeks and a 3” Walnut slab will require 4 weeks.
It is important that wood be dry prior to using it for any projects because over time, as “wet” (or raw or whatever you call it) wood dries, it can warp, twist, or move, creating a real disaster! We are happy to dry our wood for your project or we can dry wood that you bring.
Why does David's Vacuum Kilns Work So Well?
When the first vacuum kilns were built many decades ago, it was thought that the vacuum was just going to pull water out of the wood. Nothing could have been further from the truth.
Den, the president of PCS VacDry started looking at drying wood in vacuum chambers about 35 years ago. What got him interested was the fact that once in awhile a load of wood turned out well while most of the time the results were terrible.
There was a lot information learned over the years but a little bit of science needs to be understood if you want to know why VacDry kilns work as that are intended.
1. There is a unit of pressure called Torr. The atmosphere above you has weight and it is pushing down on you with the pressure of 760 Torr.
2. Matter, like water, is made up of molecules. The molecules are always in motion. The motion increases as the temperature increases.
3. Water molecules in motion bounce off one another and the result is an outward push called vapor pressure.
One more thing. When the vapor pressure of water equals the surrounding pressure, the water boils. This means that it turns to vapor within the liquid and not just from the surface. That’s why you see bubbles in a pot of boiling water.
When water is heated to 212’F, the vapor pressure is 760 Torr. Since the pressure around it is also 760 Torr, the water boils. Also, if you add more heat, the water boils faster but it doesn’t get any hotter. Note: there is some difference with your elevation. The weight of air is 760 Torr at sea level and less in Colorado.
Here is where things get interesting. If you reduce the surrounding pressure, the vapor pressure doesn’t need to be so high to make water boil. In the VacDry vacuum chamber, the pressure is so low that water boils at 104’F. That means the wood is only 104’.
The VacDry control system can precisely control temperature so we know what the vapor pressure of the water will be. In addition, the VacDry control system can precisely control the negative pressure inside the chamber. With these under control, we can easily control the drying rate of wood. It the wood is porous, we can add a lot of heat and boil out the water quickly. If the wood is dense and it is difficult to remove water, we add heat more slowly.
There is an added benefit to drying wood this way. When wood is dried in a conventional kiln, warm dry air is blow over the surface of the wood, lifting water molecules off the surface and carrying them away. In order for water to move from the inside of the wood to the outside surface, this process must happen very slowly. Otherwise the wood's surface can shrink and crack.
With the VacDry kiln, water boils from under the surface and you can now dry much more quickly. How quickly? If we look at Red Oak that is 3” thick, it will take more than a year to dry using conventional methods of air drying and in a conventional kiln. In a VacDry kiln, the wood is dry in 11 days. That’s about 35 times faster! And the quality is much better with the VacDry kiln. Air drying is a destroyer but you can’t put 3” wet oak into a conventional kiln.
Another factor that makes VacDry kilns able to dry difficult-to-dry species is humidity. Since the VacDry is boiling water with the pressure difference described above, it doesn’t matter if the atmosphere in the vacuum chamber is very humid. It can be full of water vapor which is protecting the surface of the wood from drying, shrinking and cracking even while water is still being pulled out of the wood.
The final point that we will mention here is the pressure control inside the vacuum chamber. In early vacuum kilns, the inventors would simply start a vacuum pump and they expected water to come out. That does not work. The reason is that when the water in the wood changes from a liquid to vapor, the vapor carries away heat. The water left behind is colder so its vapor pressure is lower. If the pump continues to run, the water in the wood will stop evaporating. The early inventors got around this problem by installing timers to start and stop the vacuum pump.
This is where an opportunity came up for Den. He was hired as a consultant by a company that had eight Italian vacuum kilns. The kilns were supposed to be identical but statistics were kept and it was clear that some kilns consistently dried faster and with higher quality than others. After looking very closely at components and controls, he discovered that the timers were not all set the same. He took the settings from the good kilns and put them in the less-than-good kilns. Sure enough the good kilns became bad and the bad kilns became good.
This made Den wonder what was going on and how could this be used? Timers could not be used because sometimes they would have an optimal on/off period and other times the setting would not be as good. This led to controlling the chamber pressure set point (vacuum) with a “dead band”. A dead band is the highest and the lowest levels that a control system allows a process value to be. If there is a lot of water boiling, the pressure goes up quickly and the “off” period is short. If the wood is nearly dry and little water is being boiled out, the pressure goes up slowly and the “off” period is longer. It is all automatic and no timers are needed.