A heat pipe is filled, under a vacuum, with a 'fraction of a percent by volume' (well under 1% of the volume) by water, ethanol, acetone, sodium, mercury or some other coolant.
Source: Wiki Heat Pipe Page
E.X. If its wat er then you will need to drop the pressure a lot.
If its propane on the other hand, you will need to reaise the pressure til the boiling point is in the positive #'s.
Just a thought , propane may not be a good choice. R-11 although no longer in use today due to its contributing factors in ozone depletion would have made a good choice when used as a refrigerant in a Heat Pipe.
I have provided a link that can let the viewer make their own choice. l