Heat pipe

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The heat sink (aluminium) with heat pipe (copper)

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.

1 Construction
2 Mechanism
3 Origins
4 Applications
5 Limitations
6 Notes
7 References
8 See also
9 External links

[edit] 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.

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 or a series of grooves parallel to the tube axis, but it may in principle be any material capable of soaking up the coolant. If the heat pipe has a continual slope with the heated end down, no inner lining is needed. The working fluid simply flows back down the pipe.

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. However, the vast majority of heat pipes uses either ammonia or water as working fluid.

The advantage of heat pipes is their great efficiency in transferring heat. They are actually a vastly better heat conductor than an equivalent cross-section of solid copper. Heat flows of more than 230MW/m^2 have been recorded (6 times the heat flow from the surface of the sun).^[1]

A level of control over the total pressure in the heat pipe can be obtained by controling the amount of working fluid. Water, for instance, expands1600 times when it vaporizes at 1 atmosphere. If one sixteen hundredth of the volume of a heat pipe is filled with water, when all the fluid is just vaporized, the pressure will be one atmosphere. If the safe working pressure of the pipe in question (including the X2 engineering safety factor) is, say, 5 atmospheres, one could use a quantity of water equal to 5 sixteen hundredths of the total volume.

[edit] Mechanism

Diagram showing components and mechanism for a heat pipe containing a wick

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 (caused by contamination for instance) in the vapour impede the gas flow, and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low. The velocity of vibrating molecules in a gas is approximately the speed of sound and in the absence of non condensing gases, this is the velocity with which they travel in the heat pipe.

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.

An interesting property of heat pipes is the temperature over which they are effective. On first glance, you would suspect that a water charged heat pipe would only start to work when the hot end reached 100 degrees and the water boiled resulting in the mass transfer which is the secret of a heat pipe. However, the boiling point of water is dependant on the pressure under which it is held. In an evacuated pipe, water will boil right down to 0 degrees. Heat transfer will start therefore, when the hot end is warmer than the cold end. Similarily, a heat pipe with water as a working fluid can work well above 100 degrees C.

In summary: inside a heat pipe, "hot" vapor flows in one direction, condenses to the liquid phase, and flows back in the other direction to evaporate again and close the cycle. One reason for the effectiveness of heat pipes is the amount of heat that an evaporating fluid absorbs and then returns when it condenses. For water for instance, to evaporate one gram of water takes as much heat as would be needed to raise that same gram of water by 80 degrees C.

[edit] 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^[2] in 1964.

Grover noted in his notebook: (from ^[3])

"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."

[edit] 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 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.

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.

Heat pipes are used to dissipate heat on the Alaska Oil Pipeline. Heat from the friction of the oil against the wall of the pipe and from the turbulence of the oil would conduct down the legs of the pipe to melt the permafrost into which they penetrate. Heat pipes with radiators at the top are used on each leg to keep them cold so they won't melt the permafrost and let the pipeline collapse.

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 °C and are being used in Antarctica to heat water.

[edit] 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's thermal conductivity is reduced to the heat conduction properties of its solid metal casing alone. As most heat pipes are constructed of copper (a metal with high heat conductivity); an overheated heatpipe will generally continue to conduct heat at only around 1/80th of their original conductivity.

[edit] Notes

↑ Grover, G.M., T. P. Cotter, and G. F. Erickson (1964). "Structures of Very High Thermal Conductance". Journal of Applied Physics 35 (6): 1990-1991..
↑ Heat Pipe research at LANL