A heat pipe consists of a vacuum tight envelope, a wick structure and a working fluid. The heat pipe is evacuated and then back-filled with a small quantity of working fluid, just enough to saturate the wick.
The atmosphere inside the heat pipe is set by an equilibrium of liquid and vapour. As heat enters at the evaporator, this equilibrium is upset generating vapour at a slightly higher pressure.
This higher pressure vapour travels to the condenser end where the slightly lower temperatures cause the vapour to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the capillary forces developed in the wick structure.
This continuous cycle transfers large quantities of heat with very low thermal gradients. A heat pipes operation is passive being driven only by the heat that is transferred. This passive operation results in high reliability and long life.