🌡️ Heat Pipe

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.