What are Heat Pipes?
A heat pipe itself is a simple heat transfer device which is used to convey heat from one location to another using an evaporation-condensation cycle. A collection or bundle of heat pipes within a common housing or casement used to transfer heat to one medium to another is referred to as a Heat Pipe Heat Exchanger (HPHX). Heat pipes can be referred to as “superconductors” of heat due to their rapid transfer ability with minimal heat loss. The entire process only makes use of three major components including the container (the pipe), the working fluid inside the pipe and the thermosyphon effect. Before studying the details of how a heat pipe functions, it might be best to understand the history of heat pipes and the progress of heat pipe technology.
History of Heat Pipe Technology
The history of the heat pipe can be traced back to Angier March “AM” Perkins |1| who in 1831 took out a patent on a “hermetic boiler tube”. He dabbled with the idea of a working fluid, but only in the sense of a single phase device at a high pressure. In 1936, a later descendant, Jacob Perkins |2|, was granted a patent on what was then referred to as the Perkins Tube in which a long, twisted tube filled with water was passed over an evaporator and then a condenser. This advancement utilized a case in which the encapsulated water functioned in two phases; liquid turning to steam. These early designs relied on gravity so that the condensed water would travel back to the evaporator, but nonetheless they were the precursor to the present heat pipe technology.
It was in 1942, F.W. Gay |3| devised what is today known as the current gas-to-gas heat pipe heat exchanger utilizing a partition plate seperating the bottom evaporating end of the heat pipe from the top side condensing end. Included in the patent were the addition of inlet air ducts and the use of external fins to improve on the tube-to-gas heat transfer process. Also included was the proposal of utilizing different working fluids such as mercury and methanol that would then accommodate various exhaust gas temperatures.
In 1944, the concept of the modern heat pipe was introduced by R. S. Gaugler |4| of the General Motors Corporation who patented a lightweight heat transfer device originally destined for a refrigeration process. However, the process never made it past the patent stage as an alternative solution was found.
It wasn’t until the interest of the U.S. space program that the idea of the use of heat pipes was resurrected. The resurgence of heat pipes was put forth as a suggestion by L. Trefethen |5| in 1962 followed by a patent application for satellite heat control by Theodore Wyatt |6| in 1963. It was not until Grover and his co-workers |7| of the Los Alamos Scientific Laboratory rediscovered the concept in late 1963 and built prototypes with the impetus being provided by heat pipe technology. Grover also coined the name “heat pipe” and stated, “Within certain limitations on the manner of use, a heat pipe may be regarded as a synergistic engineering structure which is equivalent to a material having a thermal conductivity greatly exceeding that of any known metal”.
Grover built his first heat pipes using water as the working fluid and then promptly followed those tests with a liquid sodium heat pipe for an operational range of 1100°K or 1500°F. With those successes, both high temperature and ambient temperature procedures were soon explored by many technicians and scientists in the heat transfer field. As an example, the concept of a Variable Conductance or Temperature Controlled Heat Pipe was first described by Hall of RCA in a patent application dated October 1964. However, the effect of a noncondensing gas was shown in Grover’s original publication, its significance for achieving variable conductance was not immediately recognized. In subsequent years, the theory and technology of Variable Conductance Heat Pipes was greatly advanced, most notably by Bienert and Brennan at Dynatherm |8| and Marcus at TRW |9|
In 1966, the first cryogenic heat pipe was developed by Haskin of the Air Force Flight Dynamic Laboratory at Wright – Patterson Air Force Base where today still, heat pipes continue to be tested and utilized for jet engine applications.
April 5, 1967, showed the first “zero g” demonstration of a heat pipe conducted by a group of engineers at the Los Alamos Scientific Laboratory. The first successful flight experiment overcame the reluctance that many spacecraft designers had for using this new heat pipe technology to solve temperature control problems with orbiting spacecraft. From those experiments, most of today’s spacecraft vehicles now rely on heat pipes to either control the temperature of individual components or of the entire vehicle structure. Some past examples were the ARS – E, OAO, ATS F&G spacecraft and the Sky Lab.
Minus the NASA advancements, the development for commercial applications of heat pipes progressed at a much slower pace. In 1968, RCA developed a heat pipe heat sink for transistors used in aircraft transmitters. This represented the first commercial application of heat pipes.
It wasn’t until the early 1970’s, with all of the previous development that commercial energy recovery applications became possible with temperatures that would be in the 175°-750°F range. Still, because of the high cost and time to manually produce heat pipes, they were only within reach of the more well-heeled companies such as IBM, RCA and the likes.
Today, significant advancements in manufacturing processes, computerized modeling and expanded research have shown that heat pipe technology can solve many of the critical problems and issues in heat transfer and temperature control affordably and effectively.
More details about heat pipe technology history can be found at this link: http://en.wikipedia.org/wiki/Heat_pipe
|1| Perkins, Angier March (“A.M.”), “hermetic heating tubes” a.k.a. “Perkins System”, 1831 UK Patent 6146. |Back|
|2| Perkins, Jacob, “Perkins Tube”, April 1936 UK Patent 7059 |Back|
|3| Gay, Frazier W. (“FW”), “Heat transfer means”, August 1942, United States Patent 1725906 |Back|
|4| Gaugler, R. S., “Heat Transfer Device”, June 1944, U. S. Patent 2,350,348. |Back|
|5| Trefethen, L., “On the Surface Tension Pumping of Liquids or a Possible Role of the Candlewick in Space Exploration”, G. E. Tech. Info., Ser. No. 615 D114, Feb. 1962. |Back|
|6| Wyatt, Theodore, “Satellite temperature stabilization system”, Oct 1964, U.S. Patent US3152774 A |Back|
|7| Grover, G. M., Cotter, T. P. and Erikson, G. F., “Structures of Very High Thermal Conductivity”, J. Appl. Phys., 35, 1990 (1964) |Back|
|8| Bienert, W. B., Brennan, P. J., “Transient Performance of Electrical Feedback Controlled Variable – Conductance Heat Pipes”, ASME Paper 71 – Av – 27, SAE/ASME/AIAA Life Support and Environmental Control Conference, San Francisco, California, July 12 – 14, 1971. |Back|
|9| Marcus, B. D., “Theory and Design of Variable Conductance Heat Pipes”, Reports no. 1and 2, TRW 13111 – 6027 – RO – 00, Contract NAS 2 – 5503, April 1971. |Back|