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

Robert Darlington rdarlington at gmail.com
Sun Dec 27 00:04:46 UTC 2009


My comments are in-line, below....

On Fri, Dec 25, 2009 at 4:38 PM, Joe Gwinn <joegwinn at comcast.net> wrote:

> At 12:45 AM +0000 12/25/09, time-nuts-request at febo.com wrote:
>
>>
>> Date: Thu, 24 Dec 2009 17:14:38 -0700
>> From: Robert Darlington <rdarlington at gmail.com>
>> Subject: Re: [time-nuts] Cheap Rubidium (heatpipe cooling for)
>> To: Discussion of precise time and frequency measurement
>>        <time-nuts at febo.com>
>>
>>
>> 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.
>>
>
> 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.

>
>
>  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.



>
>  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.  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.

-Bob


>
>
>  -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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