A heat pipe is a “hermetic two-phase heat-transfer element" that moves heat from its evaporator to its condenser through a continuous vaporization–condensation cycle of a working fluid inside a vacuum-sealed envelope. Because latent heat transport requires only a tiny temperature difference, the device is often referred to as a “thermal superconductor." Its basic construction consists of a metallic shell, a porous wick lining the inner wall, and a precisely metered charge of working fluid. When the tube is tilted or mounted vertically, the wick can be omitted, yielding a simpler and cheaper gravity-assisted heat pipe (thermosiphon).

Working fluids are selected according to the intended temperature range: ammonia, acetone, or R134a for low temperatures (-60 °C to +60 °C); water for the medium range (50 °C to 250 °C); and naphthalene, sodium, or potassium for high temperatures (250 °C to 1200 °C). The envelope material must be chemically compatible with the fluid; the copper-water pair is the most stable, whereas carbon-steel-water combinations require corrosion inhibitors. Heat transport is limited by five characteristic constraints—capillary, sonic, entrainment, boiling, and condenser limits—that must be checked during design.

Heat pipes offer excellent temperature uniformity, high power density, passive operation, and flexible geometry. They are now widely used in spacecraft thermal control, CPU/GPU vapor chambers, LED cooling, flue-gas waste-heat recovery, ground-freezing “thermal piles" along the Qinghai–Tibet Railway, and battery thermal management in electric vehicles. Advances in micro/nano fabrication and 3-D printing continue to push heat-pipe technology toward even higher power densities and more demanding operating conditions.