"Bill Rubenstein" wrote in message
. com...
Fred:

Interesting.... but several thoughts come to mind...

The heat in the box around the vacuum chamber may heat the walls of the
chamber. Heat can move via convection or conduction. The problem is,
that with no (or very little) air in the chamber, convection cannot take
place. So, conduction is the only way of transferring the heat to the
wood. Since the wood is minimally in contact with the chamber, I doubt
that the heat can have any effect at all.

Vacuum kilns are a proven technology. But, they rely on heating blankets
in contact with each and every piece of wood. Also, if the wood is thick,
the surface is going to dry much faster than the core leading to all sorts
of problems.

When the pressure is lowered, the boiling point of water is lowered --
that 212F is at a standard pressure only. Vacuum kilns work by raising
the temperature and lowering the pressure so as to change the water in the
wood to vapor which passes through the cell walls more easily than water.

I tend to think that Fine Woodworking is right.

Well, I'm going with the cheeseheads at FPL. It's relative humidity and
Raoult's law all the way. Raising the temperature of the air in the chamber
effectively lowers the relative humidity, pulling a vacuum allows escape of
water to air with application of less energy in the form of heat.
Combination is a quick way to get rid of the unbound water, that's for sure.

For the individual who said that cold air is a low humidity environment -
half correct. Low _absolute_ humidity, high _relative_ humidity. With
Raoult's law, the vapor pressure over the wood will dictate how much water
can leave and at what rate. At saturation - the dewpoint - none can leave.
That's why you have to continue to remove the humid air whether in a
standard kiln, vacuum kiln, or slowly through a barrier like a paper bag,
anchorseal or news paper.