How Does a Heat Pipe Work? Heat Pipes Explained

A heat pipe is a heat transfer mechanism that can transport large quantities of heat with a very small difference in temperature between the hot and cold interfaces.

Heat pipes are extensively used in many modern computer systems, where increased power requirements and subsequent increases in heat emission have resulted in greater demands on cooling systems. Heat pipes are typically used to move heat away from components such as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment.

On the internal side of the tube's side-walls a wick structure exerts a capillary force on the liquid phase of the working fluid. This is typically a sintered metal powder (sintering is a method for making objects from powder, by heating the material until its particles adhere to each other) or a series of grooves etched in the tube's inner surface.  The basic idea of the wick is to soak up the coolant.

Heat pipes contain no moving parts and require no maintenance and are completely noiseless.  In theory, it is possible that gasses may diffuse through the pipe's walls over time, thus reducing this effeciveness. 

The vast majority of heat pipes uses either ammonia or water as working fluid. Extreme applications may call for different materials, such as liquid helium (for low temperature applications) or mercury (for extreme high temperature applications).

The advantage of heat pipes is their great efficiency in transferring heat. They are actually a better heat conductor than an mass of solid copper.

Mechanism
Heat pipes use evaporation and condensation to move heat quickly from one place to another. A typical heat pipe is a sealed tube containing a liquid and a wick. The wick extends from one end of the tube to the other and is made of a material that attracts the liquid--the liquid "wets" the wick. The liquid is called the "working fluid" and is chosen so that it tends to be a liquid the temperature of the colder end of the pipe and tends to be a gas at the temperature of the hotter end of the pipe. Air is removed from the pipe so the only gas it contains is the gaseous form of the working fluid.

The pipe functions by evaporating the liquid working fluid into gas at its hotter end and allowing that gaseous working fluid to condense back into a liquid at its colder end. Since it takes thermal energy to convert a liquid to a gas, heat is absorbed at the hotter end. And because a gas gives up thermal energy when it converts from a gas to a liquid, heat is released at the colder end.

The vapor pressure over the hot liquid working fluid at the hot end of the pipe is higher than the vapour pressure over fluid at the cooler end of the pipe (where it condenses), and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour releases its latent heat, warming the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapour impede the gas flow, and reduce the effectiveness of the heat pipe, where vapor pressures are low.

The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action.  Most heatpipes used in heatsinks today have wicks, and are effective in vertical or horizontal orientation.

In summary: inside a heat pipe, "hot" vapor flows in one direction, condenses to the liquid phase, which flows back in the other direction to evaporate again and close the cycle.

Limitations
When heated above a certain temperature, all of the working fluid in the heat pipe will vaporize and the condensation process will cease to occur; in such conditions, the heat pipe's thermal conductivity is reduced to the heat conduction properties of its solid metal casing alone. As most heat pipes are constructed of copper, an overheated heatpipe will generally continue to conduct heat at only around 1/80th of their original conductivity.