A heat pipe is a passive heat transfer device that exploits the latent heat of vaporisation to move large amounts of thermal energy with minimal temperature difference. A sealed tube contains a small amount of working fluid (water, ammonia, acetone, or sodium, depending on operating temperature). Heat applied at the evaporator end vaporises the fluid; the vapour travels to the cooler condenser end and releases heat as it condenses.
The heat actually transported is Q = min(ΔT/R_total, Q_max): the temperature difference sets the driving conduction, but the wick's capillary limit Q_max caps it. Push ΔT high enough, or make the pipe long enough that Q_max (∝ 1/L) falls below the driving load, and the evaporator dries out — the panel flags DRY-OUT and the transported power stops rising with ΔT.
The condensed liquid returns to the evaporator via a wick structure that uses capillary action, gravity, or centrifugal force. The wick is a porous material (sintered copper powder, mesh, or grooves) lining the tube wall. This closed-loop cycle requires no pump — capillary pressure drives the fluid as long as the heat load remains within the pipe's operating limits.
Heat pipes achieve effective thermal conductivities thousands of times higher than solid copper, making them indispensable in laptop cooling (heatpipe + fan), spacecraft thermal control, nuclear reactor cooling, and permafrost foundations in cold climates. Variable conductance heat pipes (VCHPs) use a non-condensable gas to regulate heat transport automatically.
Heat pipes transport energy as latent heat of vaporisation, which is hundreds of times larger per unit mass than sensible heat. The phase-change cycle carries enormous amounts of energy with tiny temperature differences, giving effective conductivities of 10,000–100,000 W/m·K compared to copper's 400 W/m·K.
The working fluid is chosen based on operating temperature. Water is ideal from 30°C to 200°C; ammonia for cryogenic to room temperature; methanol and acetone for intermediate ranges; and liquid sodium for very high temperatures (500–1100°C) in nuclear or space applications.
Operating limits include the capillary limit (wick cannot return enough liquid), the boiling limit (nucleate boiling disrupts liquid flow), the entrainment limit (high vapour velocity strips liquid from the wick), and the sonic limit (vapour velocity reaches the speed of sound). Exceeding any limit causes dryout and failure.
Yes. Heat pipes are standard in laptop and desktop CPU coolers, where they carry heat from the processor to a remote fin array cooled by a fan. Their flat, flexible designs fit thin laptops and deliver effective cooling without active components beyond the fan itself.
In microgravity, heat pipes rely entirely on capillary action from the wick to return liquid to the evaporator. Spacecraft heat pipes must be designed with a sufficiently fine wick to provide adequate capillary pressure independent of orientation, a key advantage over gravity-fed thermosiphons.