Generically HEAT PIPE is a Heat Engine, which is designed for heat transfer by the implementation of an evaporation-condensation cycle. The cycle work is spent on the maintenance of the internal circulation of the heat-carrying agent Bologa M (1991). The distinguishing feature of the Heat Pipe is its quasi-passive nature: i.e. the absence of a mechanical pump or other type of motive moving part for the maintenance of fluid circulation.

Consequently, heat pipes can be classified according to the physical phenomenon which is responsible for fluid circulation:

• Capillarity

• Two-phase loops with capillary pump (see Section 317)
• Capillary driven heat pipes Faghri (1995) of the classical type. This type of heat pipe was the first to be applied in engineering practice. Traditionally, the general term "heat pipe" is used for denote this particular type of heat transfer device. In the Section 311, Section 312, Section 313, Section 314, Section 315 and Section 316 the main physical and technical aspects of classical heat pipes will be discussed.

• Gravity