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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 hotter and colder interfaces.

Inside a heat pipe, at the hot interface a fluid turns to vapour and the gas naturally flows and condenses on the cold interface. The liquid falls or is moved by capillary action back to the hot interface to evaporate again and repeat the cycle.

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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 exerting capillary pressure on the condensed liquid to drive it back to the heated end. 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. This type of heat pipe is known as a thermosiphon or a Perkins Tube, after Jacob Perkins.

Heat pipes contain no moving parts and typically require no maintenance, though non-condensing gases that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials of construction, may eventually reduce the heat transfer effectiveness. This is particularly acute when the working fluid's vapour pressure is low.

The materials and coolant chosen in design depend on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications (2-4K) to mercury (523-923K) & sodium (873-1473K) and even indium (2000-3000K) for extremely high temperature conditions. However, the vast majority of heat pipes for low temperature applications use some combination of ammonia (213-373K), alcohol (methanol (283-403K) or ethanol (273-403K)) or water (303-473K) 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 230 MW/m^2 have been recorded (nearly 4 times the heat flux at the surface of the sun).^[1]

A level of control over the total pressure in the heat pipe can be obtained by controlling the amount of working fluid. Water, for instance, expands 1600 times when it vaporizes at 1 atmosphere. If 1/1600 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 is, say, 5 atmospheres, one could use a quantity of water equal to 5/1600 of the total volume.

Active control of heat flux can be effected by adding a variable volume liquid reservoir to the evaporator section. Variable conductance heat pipes employ a large reservoir of inert immiscible gas attached to the condensing section. Varying the gas reservoir pressure changes the volume of gas charged to the condenser which in turn limits the area available for vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodated with a single design.

A modified heat pipe with a reservoir having no capillary connection to the heat pipe wick at the evaporator end can also be used as a thermal diode. This heat pipe will transfer heat in one direction, acting as an insulator in the other.

Flat heat pipes

Thin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes. These components are a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system.

Compared to a one-dimensional tubular heat pipe, the width of a two-dimensional heat pipe allows an adequate cross section for heat flow even with a very thin device. These thin planar heat pipes are finding their way into “height sensitive” applications, such as notebook computers, and surface mount circuit board cores. Companies such as Novel Concepts can produce flat heat pipes as thin as 0.5 mm (thinner than a credit card).

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 condenses, releases its latent heat, and warms 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 molecules in a gas is approximately the speed of sound and in the absence of non condensing gases, this is the upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is dependent on the rate of condensation at the cold end.

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.

When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat pipe until the resulting vapour has purged the non condensing gases from the pipe and then seals the end.

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

The main reason for the effectiveness of heat pipes is due to the evaporation and condensation of the working fluid, which requires/releases far more energy than simple temperature change. Using water as an example, the energy needed to evaporate one gram of water is equivalent to the amount of energy needed to raise the temperature of that same gram of water by 540 °C. Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts.

Origins

The general principle of heat pipes using gravity (commonly classified as two phase thermosiphons) dates back to the steam age. The modern concept for a capillary driven heat pipe was first suggested by R.S. Gaugler of General Motors in 1942 who patented the idea.^[2] The benefits of employing capillary action were independently developed and first demonstrated by George Grover at Los Alamos National Laboratory in 1963 and subsequently published in the Journal of Applied Physics in 1964.^[3] Grover noted in his notebook:^[4]

"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."

Between 1964 and 1966, RCA was the first corporation to undertake research and development of heat pipes for commercial applications (though their work was mostly funded by the US government). During the late 1960s NASA played a large role in heat pipe development by funding a significant amount of research on their applications and reliability in space flight following from Grover's suggestion. NASA’s attraction to heat pipe cooling systems was understandable given their low weight, high heat flux, and zero power draw. Their primary interest however was based on the fact that the system wouldn’t be adversely affected by operating in a zero gravity environment. The first application of heat pipes in the space program was in thermal equilibration of satellite transponders. As satellites orbit one side is exposed to the direct radiation of the sun while the opposite side is completely dark and exposed to the deep cold of outerspace. This causes severe discrepancies in the temperature (and thus reliability and accuracy) of the transponders. The heat pipe cooling system designed for this purpose managed the high heat fluxes and demonstrated flawless operation with and without the influence of gravity. The developed cooling system was the first description and usage of variable conductance heat pipes to actively regulate heat flow and/or evaporator temperature.

Publications in 1967 and 1968 by Feldman, Eastman, & Katzoff first discussed applications of heat pipes to areas outside of government concern and that did not fall under the high temperature classification such as; air conditioning, engine cooling, and electronics cooling. These papers also made the first mentions of flexible, arterial, and flat plate heat pipes. 1969 publications introduced the concepts of the rotational heat pipe with its applications to turbine blade cooling and the first discussions of heat pipe applications to cryogenic processes.

Starting in the 1980s Sony began incorporated heat pipes into the cooling schemes for some of it’s commercial electronic products instead of the more traditional finned heat sink with and without forced convection. The initial applications were to tuners & amplifiers but they soon spread to other high heat flux electronics applications. During the late 1990s increasingly hot microcomputer CPUs spurred a threefold increase in the number of U.S. heat pipe patent applications. As heat pipes transferred from a specialized industrial heat transfer component to a consumer commodity most development and production moved from the U.S. to Asia. Modern CPU heat pipes are typically made from copper and use water as the working fluid.

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 spacecraft 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 oriented towards the Sun.

Heat pipes are used to dissipate heat on the Trans-Alaska Pipeline System. Heat produced by friction and turbulence in the moving oil would conduct down the pipe's support legs and melt the permafrost which anchors them. 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.

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 effectively 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 around 1/80th of the original conductivity.

In addition, below a certain temperature, the working fluid will not vaporize at all, and the thermal conductivity will be reduced to that of the solid metal casing. One of the key criteria for the selection of a working fluid is the desired operational temperature range of the application. The lower temperature limit typically occurs a few degrees above the freezing point of the working fluid.

References

^ Jim Danneskiold, Los Alamos-developed heat pipes ease space flight. Los Alamos News Release, April 26, 2000.
^ Gaugler, Richard (1944), , Dayton, Ohio: U.S. Patent Office, pp. 4, 2350348
^ 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

Heat Pipe Science and Technology, Amir Faghri, Taylor and Francis 1995.