This is a very interesting topic for me as heatpipes have become the bane of my computing life.
I've made a computer chill box using a striped down ac unit which cools the internal air temp to -30c at idle and -22c at full pc load.
I wanted to use standard air cooling components but found that heatpipes don't work at these temps.....the working fluid is almost invariably distilled water and it of course freezes and so does not move the heat through phase change as designed.
Solid metal conductive heat sinks have no problem for obvious reasons but are not as effective as a heatpipe design.
I tried a zalman 8900 which combines conductive and heat pipe cooling, this enables the conducted heat to thaw the heatpipes but there is a second problem with heat pipes, they are tuned to a certain operational temperature. The pressure in them is reduced to allow the water to boil at lower than 100c....from my experimentation I think they kick in at about 30c......below that temp the water does not boil and so does not transfer heat.
It's not too much of a problem for cpu cooling,I now have a zalman 7500 solid copper star flower cooler.
The problem is with current graphics cards, atm I have 2x gtx460 palit sonic cards which have solid heat sinks. But most modern cards now use heatpipes or vapour chambers which employ the same heat transfer method.
So I'm going to be struggling when I come to up grade.
I've toyed with the idea of changing the cooling fluid, butane liquifies at -1c and isobutane at -11c so would be good candidates....but would be an awkward job.....perhaps easier with a vapour chamber than trying to do all the convoluted heatpipes.
Does anyone here have any ideas?
It would be great if you made a topic with pictures. I've always thought that any below ambient temp. solutions would greatly suffer from potential risk of condensation short-circuit, but I guess an air tight and well insulated unit would work.
Not only does water have a vapor pressure below the boiling point, like several people mentioned earlier. Even below the melting point (0°C) water has a vapor pressure. *mind blown* Although compared to atmospheric pressure (~100 000 Pascal) it may not be enough to transport heat efficiently or in high capacity.
Boiling "point" is a rather meaningless little equilibrium between vapor pressure and atmospheric pressure, it's completely misconstrued as a fundamental material constant at which liquid suddenly explodes into gas. Even though every one of us hangs socks out to dry without heating them to the boiling point, even during the winter in Alaska and Siberia people do this.
My guess is: The problem with cold temperatures is that they effectively block the wick, water will still evaporate but it will collect at the cool end of the pipe as ice, causing the heatpipe wick at the hot end to run dry. Ice crystallization could also break the wick structure.
I guess a custom block water/glycol cooling solution would be the obvious choice. Some retail graphics cards already come with a water-block attached. WC has the advantage of bringing temperatures pretty much down down to ambient.