Heat Pipes: Moving Heat With Phase Change, Not Conduction

A pipe that moves heat, not just conducts it

Solid copper conducts heat well, but it is still conduction: heat diffuses down a temperature gradient at a rate set by the material's thermal conductivity, a few hundred W/(m·K) for copper. A heat pipe is a sealed, evacuated tube partly filled with a working fluid and lined with a wick, and it moves heat by phase change instead — carrying latent heat as vapour rather than diffusing sensible heat through a solid. The result is an effective axial thermal conductivity of tens of thousands of W/(m·K), one to two orders of magnitude above solid copper, in a device with no moving parts and no external power.

The cycle: evaporate, travel, condense, return

At the evaporator end, heat vaporises the working fluid soaked into the wick; the vapour's latent heat of vaporisation absorbs a large amount of energy for a small temperature rise. That vapour is now at slightly higher pressure than the cooler end, so it flows almost freely down the pipe's open core to the condenser, where it gives up its latent heat to whatever is cooling that end and condenses back to liquid. The liquid then has to get back to the evaporator to close the loop, and gravity cannot be relied on if the pipe is horizontal or the evaporator is above the condenser — so the wick does the work instead, by capillary action.

Capillary pumping: the wick as the pump

The wick — sintered metal powder, grooves, or a fine mesh — is a porous structure with small effective pore radius r. Surface tension across the liquid-vapour meniscus in those pores generates a capillary pressure that pulls liquid back toward the evaporator with no external pump:

ΔP_capillary ≈ 2σ·cos(θ) / r_eff

σ    = surface tension of the working fluid
θ    = wetting (contact) angle, ≈0 for a well-wetted wick
r_eff = effective pore radius of the wick structure

Operating requirement:  ΔP_capillary  ≥  ΔP_liquid + ΔP_vapour + ΔP_gravity

That inequality is the heat pipe's entire operating envelope. If the capillary pressure the wick can generate is exceeded by the sum of the viscous liquid-flow loss, the vapour-flow loss and any adverse gravity head, the wick dries out at the evaporator and the pipe hits its capillary limit — its most common failure mode. Finer wicks generate more capillary pressure but also more viscous resistance to the liquid flow, so wick design is a direct trade-off.

Other operating limits

Capillary limit is usually the binding one, but three others can dominate depending on temperature and geometry. The sonic limit occurs at very low vapour density (near the fluid's freezing point) when vapour velocity approaches the local speed of sound and the vapour flow chokes, capping throughput regardless of the temperature gradient. The entrainment limit occurs when fast vapour flow shears droplets off the liquid film in the wick against the direction the liquid needs to travel. The boiling limit occurs at high heat flux when nucleate boiling inside the wick creates vapour bubbles that block liquid resupply to the evaporator surface — the porous-media analogue of critical heat flux in pool boiling.

Choosing a working fluid

The fluid must be liquid across the whole operating range and have a high latent heat, high surface tension and low viscosity — favouring vapour transport and capillary pumping while minimising internal losses. Water is the workhorse near room temperature and electronics-cooling ranges (0–200°C) because its combination of latent heat and surface tension is hard to beat; ammonia and methanol serve colder ranges; liquid sodium and other alkali metals extend heat pipes to hundreds or over a thousand degrees for aerospace and nuclear applications.

Frequently asked questions

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