boiling point of fluid used in closed heatpipes are not really important, as one can easily change the pressure inside the heatpipe and drasticly change the bp temp. Freezing point, of course, gotta be 0'C or lower.

I've read some articles online regarding the fluid used in heatpipes. Mostly used water under very low pressure allow vapourization to occur at lower temperature. I also did read somewhere that some manufacturer used low molecular weight, non corrosive hydrocarbons, with pressure lower than atmospheric. Some just used solid copper heatpipes.

Just my 2cents of knowledge to share.

Currently studying chemical engineering, year 2, university of nottingham.