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

[Robert Darlington]

On Thu, Dec 24, 2009 at 1:32 PM, Bob Camp <lists at cq.nu> wrote:

> Hi
>
> A heat pipe might work if the fluid had a sufficiently low boiling point.


The working fluid in a heat pipe will boil at every temperature above its
melting point.  I tend to use water because it's cheap, but have made them
with 3M "engineered fluids", Fluorinert, and denatured alcohol.  I've found
that ordinary solder works just fine.  A trick to make these things easy to
build is to use a ball valve at the top (I'm assuming there is a top and
we're going with gravity return because it's simple).  I've got a few that
are still under vacuum for several years now in this configuration.  My
giant heat pipe of doom is a 10 foot stick of 1/2" copper with a ball valve
at one end and an end cap at the other.  There is perhaps 100ml water in
there total, and no air.  You can either boil the liquid until it builds up
a nice head of steam, or go the easy way and pull a vacuum with a pump and
just close the valve.  These things are incredible.  If you pack snow around
the end of this thing, the other end that is ten feet away gets cold almost
immediately.  They want to stay isothermal and the heat transfer is at the
speed of sound through the working fluid.  Delays are introduced because
you're dealing with a thermal mass of copper pipe that needs to change
temperature along with the working fluid so it's not quite instant, but
still about 10,000 times faster heat transfer than copper by itself.  They
are certainly handy for getting heat out of confined spaces.

-Bob



> The rubidium isn't terribly tolerant of high temperatures, and I'm going to
> pick up some heat rise as I put it inside some baffles / shields. You need
> to find something that fits a fairly narrow window.
>
> I suspect that a recirculating water loop is a more practical approach to
> carry away the heat. It's got a pump to move the water, but the rest of it
> is fairly simple.
>
> Bob
>
>
> On Dec 24, 2009, at 2:49 PM, Joe Gwinn wrote:
>
> > 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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