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

[Joe Gwinn]

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