How are VacDry vacuum kilns more energy efficient, and save me money per kiln charge?

Conventional kilns skim water molecules off the surface of wood by moving warm and dry air across the wood. Drying must be done very slowly to allow time for water molecules to migrate to the surface. It is very easy to over-dry the surface and cause case-hardening and cracks.  In a conventional kiln, the longer your kiln schedule, the longer those circulation fans are running and the more heat and steam that ends up vented to the great outdoors.

Other vacuum kiln manufacturers have believed incorrectly that the lowered boiling point of water would make drying fast and wood spare the wood of any drying degrade. To a small degree, this has been true.

In a VacDry vacuum kiln, only two small energy efficient water pumps run continuously.  The main drying force of the vacuum pump only runs when needed.  Just as significant, vacuum is a great insulator and the wood is in direct physical contact with the heat source: practically every BTU that enters the kiln chamber is used to directly evaporate water, which tremendously increases efficiency.  Convective losses are practically nonexistent, conductive losses are very minimal, and the naturally minor radiant losses are kept in check with a small amount of insulation.


VacDry Kilns are the only that use a property of water called vapor pressure. When water’s vapor pressure equals ambient (surrounding) pressure, water boils. An example is water at sea level. Pressure is measured by a column of Mercury which is measured in “Torr”. At sea level, the atmospheric pressure is 760 Torr. If you heat water to 212’F, the vapor pressure is 760 Torr and the water boils. The water changes from a liquid to gas. In addition, the water can’t be heated above 212’F. A hotter burner will increase boiling but the water stays at 212’.

VacDry uses precise control of the vapor pressure of the water in the wood. This is combined with precise control of pressure in the vacuum chamber. The result is perfectly controlled drying. We can chose the fastest rate that the wood will give up water.

Since VacDry is using a pressure gradient instead of a humidity gradient, VacDry can boil away water from beneath the surface of the wood. This eliminates the moisture gradient that results in surface and end checks with conventional methods.  The result is beautiful and stress free wood.

Why would degrade (damage from drying) be much lower with VacDry Kilns?


First of all, you put your wood into your VacDry vacuum kiln as soon as you cut it. All air drying causes damage. Even if you don’t see open cracks, they may have started within hours depending upon species. If you are intentionally air drying, degrade can be major. You need to coat the ends with sealer and you still need to cut longer than needed so that you can trim off end checks. You may also shoot yourself in the foot by cutting thicker than needed so that you can plane off surface checks and warp. None of this is needed in a VacDry Kiln.

Secondly, as described earlier, the VacDry drying process does not create the moisture gradient between shell and core that is a result of drying conventionally. With our process, water boils inside the wood while the wood is a gentle 100’F. And our patented conditioning process further enhances the elimination of any moisture content gradient. With straight grain species, you will find 7% from shell to core and from end to end.  When drying species with twisted grain, the gradient will still be minimal in most cases. The most tightly twisted figured wood can take more drying time.

Finally, conditions inside a VacDry Kiln are not the result of an operator’s inspection and interpretation of samples. Drying conditions are not determined by what the operator feels is right. And conditions do not vary throughout the kiln. In your VacDry Kiln, the control system manages the drying conditions and the conditions are exactly the same for every piece of wood in the kiln. Furthermore, our 35 years of research has taught us what conditions will give us the desired final moisture content. A kiln charge might be dry in 8 days with a +/-1.5% variance. If you let it in the kiln for 9 days, you would expect =+/-1%. If you let it sit 10 days, it would still be at the target final MC but now +/-0.5%. That’s the way VacDry vacuum kilns work.

VacDry vacuum kilns are designed to allow for precise control of the factors that set the drying rate of the wood.  With precise control of the heat, the humidity, and the chamber pressure, schedules do not set conditions that create excessive moisture gradients between the core and the shell of the wood.  This means fewer end checks, fewer surface checks, and less degrade around existing defects such as knots and inclusions.  After installing our vacuum kilns, bat billet manufacturers who used to cut 40" billets realized they were needlessly throwing away good wood, and ended up with 37" billet lengths due to effectively zero introduced degrade from the drying process!

 

How effective are VacDry vacuum kilns for just-in-time manufacturing?

Begin with the fact that a huge log-yard is no longer necessary. VacDry Kilns can get wood out the door so fast that you don’t need to make sure the logs are already in inventory. In many cases, the wood can be loaded, dried and unloaded in less than 10 days.

VacDry Kilns do best with wood that is freshly cut. You want to avoid the exposure to air that results in the beginning of drying degrade.

Now you don’t need to know how many weeks are required for air-drying before your wood can even go into a kiln. What would air-drying conditions be? You would need to forecast relative humidity and wind speed. What a pain in the behind!

Then, if you are drying in conventional kilns, you have to add weeks or months drying from a moisture content that you can’t be sure of.

VacDry Kilns skip all of this. Saw, dry, ship and that’s it – two weeks or less!  For the first time, the forest products industry can take advantage of just-in-time manufacturing.   You can take orders you weren't able to respond to in time, and quickly get the needed dried wood in inventory.  You can out compete your competitors when your drying times are reduced by 60 to 80%!

Why is wood dried in a VacDry Kiln so bright, colorful and beautiful?

Wood will come out of a VacDry Kiln looking exactly as it did the day it was sawn. The reason is that it was dried in a vacuum chamber with approximately 1/14th of the air pressure that you breathe. The result is that water boils and the wood dries around 100’F (38’C). This prevents the caramelizing of sugars: caramelizing is the temperature dependent oxidation of sugar. Caramelized sugar turns brown. Our low drying temperature along with low levels of oxygen results in wood where caramelizing is not seen. It looks as it did when it came off the saw.

Wood which has been exposed to air for a short time will turn brown outside but after it comes out of the VacDry Kiln and goes to machining, the original color is exposed.

Some companies expose species kiln Koa and walnut to air for months to purposely darken it.

In my opinion, this vacuum dried walnut couldn’t be more beautiful.

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