A dodge occurs to me - a homebrew heat pipe:
<http://en.wikipedia.org/wiki/Heat_pipe>.
Make the cold plate of copper, to which is soldered a meandering
piece of copper tubing, which tubing is also soldered to a copper
radiator plate that is above the coldplate, forming a closed loop
with a fill tube attached by a T. Braze all tubing connections, as
for freon refrigeration systems. (Soft solder is too porous to work
for the joints, but is OK for attaching tubes to plates.)
Insulate the two tubes running between coldplate and radiator plate
from one another.
Put enough working fluid into the system to fill the tubing that is
soldered to the coldplate, but no more. Warm the system up so the
vapor drives all the air out, pinch the fill tube off and fold it
back, and braze the end shut. (It's not critical to get absolutely
all the air out.)
Making the radiator plate be above the coldplate (the boiler)
implements what amounts to an oldtime two-pipe water vapor heating
plant. Vapor goes up one pipe, condensed fluid returns via the
other. I lived in a house with such a system. The difference
between a vapor plant and a steam plant is pressure: the vapor plant
runs below atmospheric pressure, while the steam plant runs at or
slightly above.
Make sure that things are arranged so the returning fluid does not
pool anywhere but in the coldplate, or the heat pipe will bang like
an old steam heating system.
There is a brazing filler metal intended for copper-to-copper joints
that is widely used for freon systems:
<http://www.uniweld.com/catalog/alloys/silver_brazing_alloys/phos_copper.htm>.
The zero silver phos stuff is adequate, cheap and widely available.
While copper-to-copper needs no flux, copper-to-brass does, so also
get the flux. Plumbing supply houses and welding equipment stores
are likely sources. You will also need a torch or pair of torches
able to raise the tubing joints to an orange heat in a reasonable
length of time.
Depending on the chosen working fluid, the cold plate temperature
will not rise above the boiling point of the fluid unless the system
is too small (in radiator heat removal capacity) to easily handle the
10 or 20 thermal watts that are passing through.
What fluid to use? Anything common and thermally stable that does
not attack copper. Alcohol (methyl or ethyl) and water are common
choices, as are the various freons. I bet acetone would also work.
Anyway, one controls the coldplate temperature by a combination of
choice of working fluid and internal pressure.
I have seen commercially made heat pipes for cooling Intel CPUs
advertised, but I don't know that these units can be adapted.
Anyway, a heat pipe system will stabilize the coldplate temperature
fairly accurately despite variations in thermal load, has no moving
or electrical parts, and may be sufficient by itself. If not
sufficient, it can be used as the outer stage in a two-stage ovening
scheme.
Joe Gwinn
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