[time-nuts] Variable Conductance Heat Pipes for temperature control

[Bob Camp]

Hi

After a quick read, I still don't see much on just how good the thermal control can get. Of course I could have missed that page ...

Bob


On Jan 23, 2010, at 1:33 PM, Joe Gwinn wrote:

> Back in December 2009 I proposed (in "Re: [time-nuts] Cheap Rubidium (heatpipe cooling for)") use of heat pipes to stabilize the temperature of a Rubidium oscillator within an insulated box.   Turns out I was anticipated by ~40 years.
> 
> While standard heat pipes offer near isothermality , they don't offer temperature constancy as the heat flow and/or condenser temperature vary.  However, there are hints in the literature that near constant temperature can be achieved, complete with pictures of a Thermacore International product used to control the temperature of a Rb oscillator used in a cell phone tower or the like.
> 
> General description from Thermacore: <http://www.thermacore.com/products/variable-conductance-heat-pipe.aspx> 
> 
> So, I did some digging.  It turns out that the technology was worked out circa 1970, under the rubric Variable Conductance Heat Pipes (VHCPs), which are described in all standard books on heat pipes, but usually only in passing, unclearly.  However, the older books tended to be better on the subject.
> 
> The basic principle of a VHCP is simple.  One builds a standard heatpipe, but in addition to the usual working fluid (that carries heat by evaporation and condensation) one puts some non-condensible gas inside as well.  A classic example would be a heatpipe made of copper and containing water plus nitrogen.
> 
> In operation, the flow of water vapor from evaporator to condenser sweeps the nitrogen into the condenser, which becomes partially blocked by the resulting nitrogen-gas plug.  If the heat flow increases, the pressure inside the pipe increases, compressing the nitrogen plug, unblocking more of the condenser area, thus reducing the temperature change at the evaporator.
> 
> Anyway, the modern textbook on heatpipes is "Heat Pipes - Theory, Design, and Application" fifth edition, David Reay and Peter Kew, Butterworth-Heinemann (Elsevier) 2006.  (The Thermacore product for Rb oscillators mentioned above is shown in Figure 7.2 on page 277.)
> 
> The references in Reay and Kew lead one to "Heat pipe theory and practice - a sourcebook", S.W. Chi, Hemisphere Publishing (McGraw-Hill), 1976.  (This has an entire chapter on "controlled heat pipes", including a nice description of how to design a VHCP, all based on work done at TRW for the space program.
> 
> The key reference in Chi is to the report written by TRW, "Theory and design of variable conductance heat pipes", B.D. Marcus, NASA Contractor Report CR-2018, April 1972.  A pdf scan of this 252-page report is available for free download from NASA'a Technical Report Server: <http://ntrs.nasa.gov/search.jsp?R=190521&id=9&as=false&or=false&qs=Ns%3DPublicationYear%257c0%26N%3D4294915469>
> 
> Copper-water-nitrogen VCHPs seem simple enough for fabrication in a home shop, although long-term retention of nitrogen (or any such gas) will require hard brazed or welded hermetic seams and seals (versus soft solder and ball valves).
> 
> Joe Gwinn
> 
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