What is a heat pipe?
A heat pipe is a sealed tube that moves heat far better than a solid bar of the same metal. Inside, a small amount of working fluid boils at the hot end and condenses at the cold end, carrying the heat along in the fluid itself instead of conducting it through the wall.
Because no pump or fan is needed, a heat pipe is completely passive. You find them in laptop coolers, satellite panels, solar collectors and anywhere heat has to be carried from a small hot spot to a wide cooling surface.
Why tune the parameters?
Three settings decide how a heat pipe behaves: how much heat is poured in, how strongly the cold end is cooled, and how warm the surroundings are. Together they set both end temperatures and the small difference between them.
Slide them far enough and you reach the pipe's limit. Above a certain heat load the wick can no longer return liquid quickly enough, the evaporator runs dry and the temperature jumps — which is exactly what the status readout is watching for.
1. Heat enters at the evaporator
Whatever needs cooling is attached to the evaporator end. Its heat passes through the wall into the working fluid, so the metal never has to carry the heat the whole length of the pipe.
2. The fluid boils into vapour
The absorbed heat boils the liquid, turning it into vapour that fills the central channel. Boiling is the reason a heat pipe can swallow so much heat in such a small spot.
3. Vapour flows to the condenser
Boiling raises the pressure slightly at the hot end, and condensing lowers it at the cold end. That tiny pressure difference is enough to sweep the vapour along the open core.
4. Vapour condenses and releases heat
At the cold end the vapour gives up its heat and turns back into liquid. The fins spread that released heat over a large area so the surrounding air can carry it away.
5. The wick returns the liquid
The condensed liquid soaks into the wick, whose fine pores pull it back to the evaporator by capillary action. With the liquid home, the loop is closed and the pipe can run continuously.
Does a heat pipe need a pump?
No. The vapour is pushed along by the pressure difference set up by boiling, and the liquid returns by capillary action in the wick. With no pump or fan inside, a heat pipe is a passive device that simply needs to be sealed and filled once.
What is the working fluid inside?
It depends on the temperature range. Water works well around 30 to 200 °C, ammonia is used for colder duties, and liquid metals such as sodium are used for very high-temperature applications.
Why does it stop working above a certain heat load?
Every heat pipe has a limit. Push too much heat in and the wick cannot return liquid fast enough, so the evaporator dries out and the temperature climbs sharply. That is the failure the status readout warns about.
Why is the temperature almost the same at both ends?
Because the fluid itself carries the heat, the pipe behaves like a conductor thousands of times better than solid copper. The temperature drop from end to end stays small even when the load is large.
What does the wick do?
The wick lines the inside of the pipe and provides the fine pores that pull condensed liquid back to the evaporator. Without it, the liquid would have no way to return and the cycle would stall.
Can a heat pipe work against gravity?
Yes, thanks to the wick. A wick-based heat pipe can lift liquid upwards as well as along a level pipe, though it performs best when the evaporator sits below the condenser and gravity helps the return.