Working Fluid in Heat Pipes

The more fuid in there, the more heat transfer. How is your pipe?

Heat pipes operate on the principle that the Latent heat of evaporation of a fluid is much greater than the heat required to elevate the temperature of the fluid by, let us say, a degree.

Therefore, The fluid used and the environment pressure must be worked out such that when the pipe end is heated, the fluid evaporates at the expected working temperature, then migrates to the area where that heat is collected (cooling fluid or air). There, the gas is cooled and it will condensate and flow back to the area where the heat need to be removed... repeat cycle.

For Water, the Latent heat of evaporation is ~ 640 kCal/kg of water evaporated at the evaporation temperature (which depends on the pressure prevailing in the heat pipe). this is against the 1 kCal/kg/degreeC when heating the water when below the evaporating temperature...

This explains why the heat pipe is so efficient to transfer heat from hot source to cold source.

From here on you should be able to work out the qty of fluid required for your model/type/fluid used heat pipe...

Condense not condensate. L.H. of vaporization of water is 542 kCal/kG not 640.