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

[Joe Gwinn]

At 12:00 PM +0000 12/27/09, time-nuts-request at febo.com wrote:
>
>Date: Sat, 26 Dec 2009 17:04:46 -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>
>
>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.

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