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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.
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
Many different types of heat pipes have been developed over the years. Manufacturers are now able to make heat pipes in any geometry and specifically tailored to the needs of the consumer. Several types of heat pipes that may be relevant to the House_n project include heat pipes with thermal diodes or thermal switches (including variable conductance heat pipes) and flat plate heat pipes. Flat heat pipes are just that; the orientation of the wick structure is designed so that the liquid is more evenly distributed to the top and the bottom of the plate.
The wick structure in a flat plate is a sintered metal; it is a metal powder that has been molded and heated until the metal has fused, creating a structurally stable metal with small pores within. Flat heat pipes produce a surface that has a relatively uniform temperature distribution and large surface area. These would be useful in the case where one needs to radiate heat uniformly instead of from a point source. The use of flat plates as wall components could be one possible application for heat pipe technology in the house.
Thermal switches in a heat pipe serve to prevent the pipe from working in certain cases. This can be accomplished by introducing a blockage, made possible in a variety of different ways. Methods would include freezing the fluid, placing a magnetically operated vane within the pipe which would block the vapor flow, or using a physical displacement block (which controls the amount of fluid in the reservoir and in the heat pipe by blocking the fluid from being transported by the wick).
Another possible way to stop or control the heat transfer within the pipe would be by limiting the acting surface of the condenser by using an inert gas (this is the principle also behind variable conductance heat pipes). Thermal diodes allow the heat pipe to only work in one direction. In one example of a heat diode, if the location of the condenser and evaporator switch, the liquid becomes trapped in a reservoir whose wicks are not connected to the rest of the pipe. This makes it so that the liquid will not be able to travel down the length of the heat pipe until the condenser and evaporator switch again to heat the liquid to the gaseous phase so it can flow down the pipe once more.
Another example of a thermal diode is when there is excess liquid in a reservoir within the heat pipe. When the evaporator and condenser are switched, the liquid in the reservoir becomes a vapor and condenses on the condenser. This large amount of fluid prevents any vapor from condensing at the other end of the heat pipe and therefore will only allow heat transfer in one direction.
-Heat recovery (heating the incoming air and reabsorbing the heat from
the outgoing air) -Primary method of heating/cooling in building facades
-Incorporation into water heating systems
-Combining the use of heat pipes with trombe walls to heat an area
Andrews, J, Akbarzadeh, A, Sauciue, I.: Heat Pipe Technology,
Dunn, P.D., Reay, D.A.: Heat Pipes, Pergammon, 1994.