Heat pipe

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.

Construction

A typical heat pipe consists of a sealed hollow tube. A thermoconductive metal such as copper or aluminium is used to make the tube. The pipe contains a relatively small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with vapour phase of the working fluid, all other gases being excluded.

Internally, a wick structure overcomes gravitational forces (or because of their absence in the case of space applications). This is typically a metal powder sintered onto the tube's side-walls, but may in principle be any material capable of soaking up the coolant.

Heat pipes contain no moving parts and typically require no maintenance, though non-condensing gases that diffuse through the pipe's walls may eventually reduce the effectiveness, particularly when the working fluid's vapour pressure is low.

The materials and coolant chosen depends on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications to mercury for high temperature conditions.

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

Mechanism

Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature (hence they tend to equalise the temperature within the pipe).

When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe.

The vapour pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapour pressure over condensing working fluid at the cooler end of the pipe, 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 in the vapour impede the gas flow, and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour presures are low.

The condensed working fluid then flows back to the hot end of the pipe, either by force of gravity in the case of vertically oriented heat pipes, or through capillary action in the case of heat pipes containing wicks, or heat pipes that are orientated horizontally relative to gravity.

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.

Origins

While the general principle of heat pipes using gravity dates back to the steam age, the benefits of employing capillary action were first noted by George Grover at Los Alamos National Laboratory in 1963 and subsequently published in the Journal of Applied Physics in 1964.

Grover noted in his notebook: (from )

"Heat transfer via capillary movement of fluids. The "pumping" action of surface tension forces may be sufficient to move liquids from a cold temperature zone to a high temperature zone (with subsequent return in vapor form using as the driving force, the difference in vapor pressure at the two temperatures) to be of interest in transferring heat from the hot to the cold zone. Such a closed system, requiring no external pumps, may be of particular interest in space reactors in moving heat from the reactor core to a radiating system. In the absence of gravity, the forces must only be such as to overcome the capillary and the drag of the returning vapor through its channels."

Applications

Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are found. Heat pipes have since been used extensively in space craft as a means for managing internal temperature conditions.

Heat pipes are extensively used in many modern computer systems, where increased power requirements and subsequent increases in heat emission have resulted 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.

Heat pipes are also being widely used in solar thermal water heating applications in combination with evacuated tube solar collector arrays. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and orientated towards the Sun.

In solar thermal water heating applications, an evacuated tube collector can deliver up to 40% more efficiency compared to more traditional "flat plate" solar water heaters. Evacuated tube collectors eliminate the need for anti-freeze additives to be added as the vacuum helps prevent heat loss - these types of solar thermal water heaters are frost protected down to more than -35 degrees C and are being used in Antarctica to heat water.

Limitations

Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size and coolant all have an effect on the optimal temperatures in which heat pipes work.

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 becomes completely ineffective.

References

Heat Pipe research at LANL

External links

HVAC | Computer hardware cooling | Heat

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