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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
