[time-nuts] Cheap Rubidium (heatpipe cooling for)

Joe Gwinn joegwinn at comcast.net
Thu Dec 24 19:49:38 UTC 2009


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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