In article ,
"Leo Lichtman" wrote:

What troubles me is that the window fogs up, and water collects on the
bottom of the chamber. If the vacuum is causing the water to evaporate, why
doesn't it evaporate from the areas outside the wood? If the system is
working, it must be some other mechanism than vacuum drying.

Certainly operating it at a minimal vacuum ("15", probably 15 inches of
mercury, or about 1/2 atmosphere - a very low vacuum indeed) offers some
room for doubt about its method of operation. The standing water (there
is mention of removing it) would indicate that the water in the wood is
not boiling off, so the only means I can guess at for the expulsion of
this water from the wood would be small gas bubbles in the pores driving
(free) water out the pores. That would leave the bound water behind.
Fogging is pretty much a given, as the thing will be operating at 100%
humidity for whatever pressure and temperature it's running at. On the
other hand, at 1/2 an atmosphere, heat (mentioned in another reply) will
convect quite well, there being very little vacuum insulation effect at
that high a pressure.

With a better vacuum heat is not really needed, as water will boil at
and below room temperature, and if the wood self-cools (due to boiling
of water in it) to below freezing, water will sublime out of the ice -
freeze drying (intentional freeze drying starts with freezing the object
to be dried). I believe that some of the commercial kilns (or at least a
research project test kiln of commercial size I've read about) have a
more effective means (than contact blankets) of providing heat in vacuum
to improve cycle times, however - microwaves.

Most of the degrade issues of any drying process remain on the table
with vacuum drying, as I see it; this may be one reason for choosing
such a low level of vacuum, as extreme speed of drying can aggravate
degrade. Also, heat can improve some degrade issues by allowing the wood
to bend plastically - so if it's restrained flat and heated sufficiently
(not very applicable to bowls, I'm afraid) it can relieve stresses that
might tend to twist or cup it, and set flat. Also, sufficient heat to
kill bugs is a good thing.

--
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