UK How a heat pipe moves heat
A heat pipe moves heat by boiling and condensing a liquid inside a sealed tube. The fluid absorbs a large amount of heat as it evaporates at the hot end and gives it up as it condenses at the cold end, so a slim copper pipe can carry heat away from a processor far more effectively than a solid copper bar of the same size. In practice its effective conductivity is many times that of solid copper.
That is why heat pipes sit inside most fanless PCs. They carry heat from a small processor die to a large finned case that can pass it to the surrounding air, with no fan and nothing to wear out.
Inside a heat pipe
A heat pipe has three parts: a sealed copper envelope, a wick lining its inner wall and a small charge of working fluid. Air is pumped out before the pipe is sealed, so the pressure inside is low and water boils at well below its usual boiling point. The fluid is chosen to suit the operating temperature, and water is the usual choice for electronics.
Heat pipes for computers are often flattened and bent to fit between the processor and the fins. A vapour chamber works on the same principle in a flat, wide form, spreading heat in two directions rather than carrying it along a line.
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Evaporator. The hot end, clamped to the processor through a copper block. The fluid boils here and absorbs heat.
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Vapour flow. The vapour moves along the hollow core towards the cooler end, carrying that heat with it.
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Condenser. The cool end, bonded to fins or the chassis wall. The vapour condenses and releases its heat.
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Wick. A sintered powder, grooved or mesh lining that draws the liquid back to the hot end by capillary action.
Wick types compared
The wick decides how well a heat pipe returns liquid, and so how it copes with orientation and heavy loads. Many pipes combine two types.
| Wick type | How it is made | Working against gravity | Typical use |
|---|---|---|---|
| Sintered powder | Copper powder fused to the inner wall | Best: strong capillary pull | Computers mounted in any orientation, high heat loads |
| Grooved | Fine channels cut along the inner wall | Poor: relies on gravity | Horizontal or gravity-assisted layouts |
| Mesh | Layers of woven copper screen | Moderate | Thin, flattened pipes and general use |
| Composite | A mix, such as grooves with a sintered layer | Good | Designs that need both capacity and flexibility |
The choice is made by the chassis designer, but it explains why a manufacturer may specify a mounting direction for a fanless computer, and why two systems with similar fins can perform differently.
Orientation and limits
A heat pipe works best when the hot end sits below the cold end, because gravity helps the condensed liquid run back. Working against gravity, the wick has to lift the liquid on its own and the pipe carries less heat; sintered powder wicks cope with this best. Tight bends and heavy flattening also reduce how much heat a pipe can move.
Every heat pipe has a maximum heat load. Beyond it the wick cannot return liquid fast enough, the hot end dries out and its temperature climbs, and the processor responds with thermal throttling. Water-filled pipes also stop moving heat while the water is frozen and typically resume once the processor warms them. For a fanless computer, this is why the manufacturer’s mounting orientation and temperature ratings deserve attention.
Heat pipes in fanless computers
In the fanless systems we specify, heat pipes usually carry heat from the processor to the fins on the case, sometimes alongside a direct conduction path to a cold plate. When we match a processor to a chassis we check its TDP against the chassis rating, the hottest temperature at the installation and the way the computer will be mounted.
Heat pipes are not the only part of the path. The thermal pad or paste between the processor and the copper block, the area of fins on the case, and the air around the computer all set how much heat can escape. A pipe that moves heat well cannot help if the fins are enclosed in a sealed cabinet with no air movement.
Our guide to how a fanless PC works covers the full heat path, and our comparison of active vs passive cooling explains when a heat-pipe design suits an installation and when fans are the better fit.
Quick answers on heat pipes
Heat pipes: common questions
What liquid is inside a heat pipe?
Usually water. Water suits the temperatures inside electronics. Other fluids, such as ammonia or methanol, are used for much colder conditions, and liquid metals for very high temperatures.
Do heat pipes wear out?
Not in normal use. With no moving parts they last for the life of the computer. A pipe can fail if it is punctured, crushed or slowly loses its vacuum.
Does heat pipe orientation matter?
Yes. Pipes work best with the hot end lower than the cold end. Sintered wicks reduce the effect, but mounting rules in a datasheet should be followed.
Is a heat pipe better than solid copper?
Yes, for moving heat a distance. It carries heat many times better than a copper bar of the same size, so it can link a small processor to a large finned case.
What is the difference between a heat pipe and a vapour chamber?
Shape. Both use evaporation and condensation. A heat pipe carries heat along a line; a vapour chamber is flat and spreads heat across an area.
We build fanless computers with the processor, chassis and mounting matched to the site temperature as standard, so the heat path is sized for the work the system has to do.