Heat pipe


A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces.
At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid, releasing the latent heat. The liquid then returns to the hot interface through capillary action, centrifugal force, or gravity, and the cycle repeats.
Due to the very high heat-transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors. The effective thermal conductivity varies with heat-pipe length and can approach for long heat pipes, in comparison with approximately for copper.
Modern CPU heat pipes are typically made of copper and use water as the working fluid. They are common in many consumer electronics like desktops, laptops, tablets, and high-end smartphones.

History

The general principle of heat pipes using gravity, commonly classified as two-phase thermosiphons, dates back to the steam age. Angier March Perkins and his son Loftus Perkins created the Perkins Tube, which achieved widespread use in locomotive boilers and working ovens. Capillary-based heat pipes were first suggested by R. S. Gaugler of General Motors in 1942, who patented the idea, but did not develop it.
George Grover independently developed capillary-based heat pipes at Los Alamos National Laboratory in 1963; his patent of that year was the first to use the term "heat pipe", and he is often referred to as "the inventor of the heat pipe". He noted in his notebook:
Grover's suggestion was taken up by NASA, which led heat-pipe development in the 1960s, particularly regarding applications to and reliability in space flight. This was understandable given the low weight, high heat flux, and zero power draw of heat pipes, and that they would not be adversely affected by a zero gravity environment.
The first space application was the 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 outer space. This causes severe temperature discrepancies of the transponders. The heat pipe designed for this purpose managed the high heat fluxes and demonstrated flawless operation with and without the influence of gravity. That cooling system was the first to use variable-conductance heat pipes to actively regulate heat flow or evaporator temperature.
NASA tested heat pipes designed for extreme conditions, with some using liquid sodium as the working fluid. Other forms of heat pipes cool communication satellites. Publications in 1967 and 1968 by Feldman, Eastman, and Katzoff first discussed applications of heat pipes for wider uses such as in air conditioning, engine cooling, and electronics cooling. These papers were the first to mention flexible, arterial, and flat-plate heat pipes. Publications in 1969 introduced the concept of the rotational heat pipe with its applications to turbine-blade cooling and contained the first discussions of heat-pipe applications to cryogenic processes.
Starting in the 1980s, Sony began incorporating heat pipes into its commercial electronic products in place of both forced-convection and passive-finned heat sinks. Initially they were used in receivers and amplifiers, soon spreading to other high-heat-flux electronics applications.
During the late 1990s, increasingly high-heat-flux microcomputer CPUs spurred a threefold increase in the number of U.S. heat-pipe patent applications. As heat pipes evolved from a specialized industrial heat-transfer component to a consumer commodity, most development and production moved from the U.S. to Asia.
CPU heat pipes are typically made of copper and use water as the working fluid.

Structure, design, and construction

A typical heat pipe consists of an envelope, a wick, and a working fluid. The envelope is made of a material that is compatible with the working fluid such as copper for water heat pipes, or aluminum for ammonia heat pipes. Typically, a vacuum pump removes the air from the pipe, which is partially filled with a working fluid and then sealed. The working-fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range.
The operating temperature of a given heat pipe system is critically important. Below the operating temperature, the liquid is too cold and cannot vaporize into a gas. Above the operating temperature, all the liquid has turned to gas, and the environmental temperature is too high for the gas to condense. Thermal conduction is still possible through the walls, but at a greatly reduced rate of thermal transfer. In addition, for a given heat input, a minimum working-fluid temperature must be attained, while at the other end, any additional increase in the heat-transfer coefficient from the initial design tends to inhibit the heat-pipe action. This can be counterintuitive, in the sense that if a heat-pipe system is aided by a fan, then the heat-pipe operation may potentially be severely reduced. The operating temperature and the maximum heat-transport capacity—limited by its capillary or other structure used to return the fluid to the hot area—are closely related.
Working fluids are chosen according to the required operating temperatures, with examples ranging from liquid helium for extremely low-temperature applications to mercury, sodium, and even indium for extremely high temperatures. The vast majority of heat pipes for room-temperature applications use ammonia, alcohol, ethanol ), or water. Copper/water heat pipes have a copper envelope, use water as the working fluid, and typically operate from. Water heat pipes are sometimes partially filled with water, heated until the water boils and displaces the air, and then sealed while hot.
The heat pipe must contain saturated liquid and its vapor. The saturated liquid vaporizes and travels to the condenser, where it is cooled and condensed.
The liquid returns to the evaporator via the wick, which exerts capillary action on the liquid. Wick structures include sintered metal powder, screen, and grooved wicks, which have a series of grooves parallel to the pipe axis. When the condenser is located above the evaporator in a gravitational field, gravity can return the liquid. In this case, the pipe is a thermosiphon. Rotating heat pipes use centrifugal forces to return liquid from the condenser to the evaporator.
Heat pipes contain no moving parts and typically require no maintenance, though non-condensable gases that diffuse through the pipe's walls, that result from breakdown of the working fluid, or that exist as original impurities in the material, may eventually reduce the pipe's effectiveness.
The heat pipe's advantage over many other heat-dissipation mechanisms is its efficiency in transferring heat. A pipe one inch in diameter and two feet long can transfer at with only drop from end to end. Some heat pipes have demonstrated a heat flux of more than, about four times that of the Sun's surface.
Some envelope/working-fluid pairs that appear to be compatible are not. For example, water in an aluminum envelope develops significant amounts of non-condensable gas within hours or days. This issue is primarily due to the oxidation and corrosion of aluminum in the presence of water, which releases non-condensable hydrogen gas.
In an endurance test, pipes are operated for long intervals and monitored for problems such as non-condensable gas generation, material transport, and corrosion.
The most commonly used envelope/wick/fluid combinations include:
Other combinations include stainless-steel envelopes with nitrogen, oxygen, neon, hydrogen, or helium working fluids at temperatures below 100 K, copper/methanol for electronics cooling when the heat pipe must operate below the water range, aluminum/ethane heat pipes for spacecraft thermal control in environments when ammonia can freeze, and refractory-metal envelope / lithium fluid for applications above.
Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size, and coolant all affect the optimal temperature. Outside of its design heat range, thermal conductivity is reduced to the heat-conduction properties of its envelope. For copper, that is around 1/80 of the design flux. This is because below the range, the working fluid never vaporizes, and above the range, it never condenses.
Few manufacturers can make a traditional heat pipe smaller than 3 mm in diameter due to material limitations. Researchers have shown that heat pipes containing graphene can improve cooling performance in electronics.

Types

In addition to standard, constant-conductance heat pipes, other types include:
  • Vapor chambers, which are used for heat-flux transformation, and surface isothermalization;
  • Variable-conductance heat pipes, which use a non-condensable gas to change the heat pipe's effective thermal conductivity as power or the heat-sink conditions change;
  • Pressure-controlled heat pipes, a type of VCHP where the reservoir volume or the NCG mass can be changed, to increase precision;
  • Diode heat pipes, which have a high thermal conductivity in the forward direction, and a low thermal conductivity in the reverse direction;
  • Thermosiphons, which return the liquid to the evaporator by gravitational/accelerational forces; and
  • Rotating heat pipes, which return the liquid to the evaporator by centrifugal forces.