History

The development of the heat pipe originally started with Angier March Perkins who worked initially with the concept of the working fluid only in one phase (he took out a patent in 1839 on the hermetic tube boiler which works on this principle). Jacob Perkins (descendant of Angier March) patented the Perkins Tube in 1936 and they became widespread for use in locomotive boilers and baking ovens. The Perkins Tube was a system in which a long and twisted tube passed over an evaporator and a condenser, which caused the water within the tube to operate in two phases. Although these early designs for heat transfer systems relied on gravity to return the liquid to the evaporator (later called a thermosyphon), the Perkins Tube was the jumping off point for the development of the modern heat pipe. The concept of the modern heat pipe, which relied on a wicking system to transport the liquid against gravity and up to the condenser, was put forward by R.S. Gaugler of the General Motors Corporation. According to his patent in 1944, Gaugler described how his heat pipe would be applied to refrigeration systems. Heat pipe research became popular after that and many industries and labs including Los Alamos, RCA, the Joint Nuclear Research Centre in Italy, began to apply heat pipe technology their fields. By 1969, there was a vast amount of interest on the part of NASA, Hughes, the European Space Agency, and other aircraft companies in regulating the temperature of a spacecraft and how that could be done with the help of heat pipes. There has been extensive research done to date regarding specific heat transfer characteristics, in addition to the analysis of various material properties and geometries.

How They Work

A metal cylinder is sealed with a fluid within it creating a closed system. One end of the tube is heated and the other is cooled. The heat source (the evaporator) causes the fluid to boil and turn to vapor (this is absorbing energy as heat). This also creates a pressure difference that causes the vapor to flow towards the cooler end of the tube. Once the vapor reaches the cold end of the tube (the condenser), the fluid changes phase again from vapor back to a liquid (releasing the energy as heat). This liquid returns to the hot (evaporator) end by means of a wick so that the liquid can repeat the process. This process is capable of transporting heat from a hot region to a colder region. It requires no addition of external energy and can be manufactured to have any geometry or property desired.

Components of a Heat Pipe

A heat pipe has three different components: the casing, the working fluid, and the wick. The casing can be made out of a variety of different materials, depending on the specifications and the working fluid (some combinations are not compatible, for material compatibility see to Appendix A). Most heat pipes currently used have copper, stainless steel, or aluminum casings. The wick is often a woven wire mesh that is composed of very small pores. Stainless steel is easiest to work with but copper is also used. Aluminum on the other hand, is difficult to weave and therefore in using this material it is difficult to achieve a small pore size (for material pore sizes, see Appendix B). The pore size is important because the wick operates under the principle of capillary action. Capillary action describes how fluid in a very small tube will be forced up through this tiny opening causing the fluid to rise. This fluid transport against gravity is passive and can be attributed to the atmospheric pressure pushing the through the small pores, and the surface tension felt between the molecules of the fluid itself (thereby ensuring a continuos stream of fluid moving up the wick). The wick is usually located against the inside walls of the heat pipe and can have various geometries as seen here.

To determine the appropriate working fluid, there are several considerations to be weighed. The vaporization and condensation point of the fluid, in addition to its operating temperature and its latent heat of vaporization. The primary concern should be whether or not the fluid's operating temperature is suitable for the design needs. Secondly, is it's have a high latent heat of vaporization (will this fluid absorb a lot of energy to change phase from liquid to gas, and will it thereby release a lot of energy when it changes back to a liquid). The higher the latent heat of vaporization, the more efficient the liquid (so one would require less liquid to dump the same amount of energy). The problem with using many of these working fluids is that some are flammable and some may be toxic which poses quite a problem in many applications.

Current Applications

Heat pipes have become widely used to cool the CPU's of computers due to the fact that they can be manufactured at such a small scale. They act as heat sinks for the processors and other components of computers that generate substantial heat. Heat pipes are also ideal to use in laptops because the materials in a heat pipe are not only very efficient and take up minimal space, but they are also extremely lightweight.

Heat pipes have already had a significant impact on climate control systems in buildings by increasing the effectiveness and efficiency of dehumidifying systems. This has become quite useful for many buildings not only in humid climates but also buildings that contain moisture-sensitive products (libraries, museums, and supermarkets). In the dehumidification process, it is necessary to cool the air entering the dehumidifier to roughly 55 degrees Fahrenheit. Most current methods require energy to pre-cool the air, but using a heat pipe to perform the same task would take not energy at all. Cooling the air before dehumidifying it is important because the colder the air, the greater the amount of water that can condense out of it. Once the air has been dehumidified at a relatively low temperature, the other end of the heat pipe warms the air back to room temperature (once again, using no energy). The Sony Corporation has been incorporating heat pipes as heat sinks in its tuner-amplifier products. Other applications of heat pipes include using them to cool buried high voltage cables and de-icing roads.

Specific Types of Heat Pipes