[time-nuts] Cheap Rubidium (heatpipe cooling for)
Joe Gwinn
joegwinn at comcast.net
Sun Dec 27 14:33:36 UTC 2009
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
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