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 100 kW/(m⋅K) for long heat pipes, in comparison with approximately 0.4 kW/(m⋅K) 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:
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.
Grover's suggestion was taken up by NASA, which led heat-pipe development in the 1960s, particularly regarding applications to aid 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 (and thus reduces reliability and accuracy) 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
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