At 12:45 AM +0000 12/25/09, [email protected] wrote:
Date: Thu, 24 Dec 2009 17:14:38 -0700
From: Robert Darlington <[email protected]>
Subject: Re: [time-nuts] Cheap Rubidium (heatpipe cooling for)
To: Discussion of precise time and frequency measurement
<[email protected]>
On Thu, Dec 24, 2009 at 1:32 PM, Bob Camp <[email protected]> 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.
Well, I've been thinking about this, and I used the term "heat pipe"
too loosely. Both the one- and two-pipe systems mentioned here have
no wicks, and so technically are two-phase thermosyphons, which
depend on gravity to circulate vapor and condensate. A true heat
pipe has a wick, and will work in zero gravity.
One gets significant heat transfer by phase change so long as the
vapor pressure in the heat input end is high enough to generate
enough vapor to carry the thermal power flow, and this makes the pipe
isothermal. However the temperature (although constant along the
pipe) varies with the thermal power flow (in thermal watts) being
carried.
What I'm looking for is related but different: A device where the
heat transfer capacity varies sharply with temperature, so that there
is a range of heat transfer rates over which the input-end
temperature will be substantially constant. This is why I envision
the fluid boiling (versus evaporating), which is actually out of the
operating regime of a true heat pipe.
I tend to use water because it's cheap, but have made them
with 3M "engineered fluids", Fluorinert, and denatured alcohol.
Fluorinert. I think that's what the expensive commercial CPU-cooling
heatpipes use.
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.
I wouldn't have thought that an ordinary ball valve would be tight
enough, allowing the water to escape and the air enter, slowly,
although I suppose one can replace the water if it comes to that.
But I think people want to build this exactly once, so I followed
refrigeration practice. A properly made hermetically sealed
refrigeration system keeps its working fluid essentially forever. I
suppose one can use a refrigeration fill valve, say from an
automobile air conditioning system, but these all leak to some degree.
Is the ball valve anything special?
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.
What is the purpose of the heatpipe of doom? Education?
-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.)
[snip] [This is really a kind of "thermosyphon", as discussed above.]
> >
> 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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