At 12:00 PM +0000 12/27/09, [email protected] wrote:
Date: Sat, 26 Dec 2009 17:04:46 -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]>
My comments are in-line, below....
On Fri, Dec 25, 2009 at 4:38 PM, Joe Gwinn <[email protected]> wrote:
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.
$1000 a gallon! Or $5 a drum when you buy it at a salvage auction.
That explains why low-end heatpipes use alcohol or acetone.
Actually, one ought to be able to use the freon intended for
automobile air conditioners, for a whole lot less money, even new.
>> 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.
>
Mine have been running for a few years with no sign of needing to be pumped
down again. They just work.
> 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?
>
Nope, just whatever was on the shelf at the local hardware store.
Stainless ball with brass valve body. Teflon bearing surface.
Ahh. A quarter-turn ball valve, used as a cutoff. The term "ball
valve" isn't quite precise in plumbing parlance.
These are very good, but still they are not hermetic, and will over
decades (if not a few years) lose their working fluid. I bet that
while water will be contained, freon will diffuse right through the
teflon seal of the ball valve. So, there's the tradeoff.
>> 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?
>
Education, fun, and then later a demonstration piece. It's fun to rapidly
move the thing along its axis, upward and then stop. The slug of water
moves up and then slams back down to the bottom and sounds like a steel ball
in the pipe. It makes a satisfying clang sound. A couple of years back
when I did a demo, people were convinced I had a metal part in there that
was loose. I opened the valve and out came a 100ml water and nothing
else. Too cool, and you can make them at home for next to nothing.
This definitely sounds like a good physics demo for school use.
Before
I started using vacuum pumps to pump them down, I'd use a blowtorch to boil
the water and use the valve to throttle the steam coming out. Once the
steam is coming out really fast you basically just quickly close the valve
and remove from the heat source. That's it! For smaller diameter pipes I
use other methods and other working fluids because heating tends to just
eject the sometimes very expensive fluid.
What sizes, what fluids, what purposes?
Joe
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