A heat pipe is a closed evaporator-condenser system. The sealed shell is a hollow tube lined with a capillary structure or wick. A working fluid at a specified vapor pressure saturates the wick capillaries in a state of equilibrium between liquid and vapor.
Liquid in the wick evaporates when the heat pipe starts absorbing heat. The vapor fills the hollow area of the heat pipe, called the vapor space, and diffuses heat evenly across the heat pipe. Heat distribution across the heat pipe happens quickly and determines the high thermal conductivity of the heat pipe.
When a spot along the heat pipe drops below the evaporation temperature, the vapor contacts the cooler wick and shell and releases its latent heat into the casing. The vapor no longer has enough energy to maintain a gaseous form and condenses back into liquid then seeps into the wick structure. Capillary action within the wick returns the condensate to the evaporator region and completes the operating cycle.