A heat pipe is a thermal management component that can transport large quantities of heat with a very small difference in temperature between the hotter and colder interfaces.
Inside a heat pipe, at the hot interface, a fluid turns to vapor which flows driven by a pressure difference to the cold end of the heat pipe. There the vapor condenses on the cold interface. The liquid falls or is moved by capillary action back to the hot interface to evaporate again and repeat the cycle. The heat transport along the heat pipe therefore is realized by evaporation, vapor flow and condensation, which enables a high heat transport capacity with comparatively low temperature differences.
A typical heat pipe consists of a sealed hollow tube, made out of thermal conductive metals like copper or aluminum. The pipe contains a relatively small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with vapor phase of the working fluid, all other gases being excluded. Inert gases like air are removed by vacuum during the production process of the heat pipe, as they would disturb the evaporation and condensation process.
On the inside of the heat pipe a wick structure is covering the internal walls. The geometry of this wick e.g. the size of the pores and the ratio of pores and solid material produces a capillary force on the liquid phase of the working fluid. This wick structure is typically a sintered metal powder or a series of grooves parallel to the tube axis, but it may in principle be any material capable of exerting capillary pressure on the condensed liquid to drive it back to the heated end. The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end.
Heat pipes contain no moving parts and typically require no maintenance, so they can offer an excellent life time combined with excellent heat transfer performance.
Container materials and working fluids are selected based on the aimed operating temperatures and heat loads. Heat Pipe solution are offered from 2 to 4°K (liquid helium) up to 2000°K (Indium). But the vast majority of heat pipes are offered between
-40°C and +200°C, using ammonia, alcohol, ethanol or water, or mixtures of these working fluids.
The advantage of heat pipes is their excellent heat transport capacity. They are actually a vastly better heat conductor than an equivalent cross-section of all solid heat transfer materials like aluminum, copper or even silver.