How fast is to fast drying wood?

[Kelvin]

I'm looking at turning my aux heater on to only one of my solar kilns and cranking the temps to max.  I see that Nyhle kilns say they can dry 18% to 6% in 7 days so i imagine if i got the temps really up and and tried baking the lumber i could push it as hard as possible.  See any possible problems?  Gene said something like you can't over do 18%MC wood.  Thanks
KElvin

[Den Socling]

A vacuum kiln can dry from 18% to 6% in one day so there is no such thing as too fast. The danger is moisture gradients and the resulting stress. If a kiln charge is truly 18%, with no pockets or cores at 25+, then you need heat for diffusion and you don't have any gradient that the extra heat will exacerbate.

[woodhaven]

Is 18% a good rule of thumb? At what mc can you safely go up to 150 or 160 degrees without problems?

[etat]

you don't have any gradient that the extra heat will exacerbate.

Hey Den, I profess to being a dummy.  Will you please explain this again, please.  

[Den Socling]

In green wood, you have "free" water and "bound" water. The free water is regular, liquid water that can move through the wood. The bound water is chemically bound in the cell walls.

The free water adds to the volume of the cells. When the free water is gone, the cells shrink. If the cells on the outside shrink much more than the cells in the core, stress is created.

The moisture content which represents the free water gone and only bound water left is around 28%. Again, if all the free water is gone, the cells have all shrunk to their new size and you aren't going to create any new stress that leads to cracks or warp.

But, you may make 18% the rule to make certain that nothing in the kiln charge is above 28%.

[etat]

Thank you Den.  I actually understand now.  Start out slow so the cells shrinking don't cause uneven pressure in the wood as you start drying out the water.  Is there an average mc of say fresh cut oak.  How do you dry your wood down to 18 percent before putting it in your vacum kiln?  

[Den Socling]

The MC of freshly cut Red oak averages around 70%. It goes into vacuum kilns at 70% where it takes from 5 (6/4) to 14 (14/4) days to dry.

The average MC can be as low as 42% for Ash to 100% for soft maple. That's one reason you are careful when mixing species.

[Fla._Deadheader]

Sounds like vac drying is really quick. Is there a way to ballpark how many inches of vacuum you pull, and for how long a period of time, during the drying cycle, and not have it become high tech in explaning it

Just some basics would be Ok. When we get a place to set up a kiln, I will get back with MC's of REALLY wet wood, and see what you can advise.

[Den Socling]

I always speak of reduced pressure instead of vacuum. It is easier to get everything clear.

The atmosphere's pressure can push a column of mercury to a height of about 760 millimeters. Millimeters of mercury is a mouthful so somebody came up with the term 'Torr'. One millimeter of mercury is one Torr.

In our vac kilns, we reduce the atmospheric pressure from 760 to 50 Torr. (That's about 28" of mercury if you want to talk vacuum.) Water boils at about 100'F at this pressure.

We pull the pressure down as soon as the load is preheated and keep it down until it's dry.

[Fla._Deadheader]

  Thanks Den. I guess that means the compressor runs as long as the wood is being dried?

[shopteacher]

Den, what size vac pumps are used to pull that much vac in a bldg. that size?  When I did refrigeration work I used a 2 stage pump to dry out the system and could pull about 29" hg.  I also had a sensor connected to tell me when the system was to a certain micron level of moisture, a little impractical for a kiln though.  I imagine kiln comstruction goes to great lengths to seal it airtight, right. Would it be practical to build a small vac kiln from scratch?  I know woodmizer had small vac kiln and maybe still do.  Are there any major or consistant problem encountered with that type of kiln?  Does the kiln also use a refrigeration system with the vaccum?

[Den Socling]

Well, this leading off topic but the vacuum pumps that I use are the liquid-ring type. They create vacuum by slinging water outward. There's no compressor.

the chamber used for a vacuum kiln is relatively small. They are filled completely (normally) with wood and, yes, no pinholes are allowed in the welds.

Unfortunately, it is not practical to build vac kilns under 1000 bf. And a 1000 bf kiln is barely practical. I just had a guy from Canada here today. He came in with the idea of me helping him build a 1200 bf kiln. He left 5 hours later with the idea that he was going to buy a 4000 bf kiln that we build.

[shopteacher]

Den, I see your in about middle of the state. I live a little SW of Pittsburgh.  I'd be interested in maybe making a trip up your way come spring.  I've been to Grizzly, in Williamsport a couple times, how far are you from there?

[Den Socling]

Shopteacher,
You go by us on the way to Grizzly. Isn't that some place to shop? you should visit Hermance while you're in the area, too.
Den