Heat pipes and with success are applied long ago to heating of rooms, showing higher efficiency in comparison with traditional radiators of hot-water heating. They are widely applied also to cooling of technical systems which work demands heat extraction.
Operation principle of heat pipe
The patent for the concept of thermal tube of capillary type has been taken out by the representative of the General Motors company in 1942. In two decades the idea has gained basic further development. In 1963 American J. Gruver from laboratory in Los Alamos has visually shown efficiency of such design.
The operation principle of heat pipe is rather simple and clear even to the person far from physics. It is enough to acquire that liquids at evaporation absorb heat energy, and at the time of condensation actively it is given.
The simplest device of this kind represents the tube soldered on both sides in which there is flying liquid. If to heat one side of the device, liquid will begin to evaporate. At the same time steam is condensed on the opposite end of tube. Then liquid spontaneously is returned to heat source. The cycle can repeatedly be repeated.
Work of heat pipe of the most widespread type happens so. Heat which is transmitted through the body to the heat carrier due to heat conductivity is failed to pipe. The liquid moistening the special match which is inside evaporates. Further liquid is condensed in heat extraction zone that leads to flooding of match. The difference of capillary pressures in two different zones of heat pipe leads to emergence of pressure differential. The system turns into the peculiar "capillary pump". Except capillary forces during the work of pipe so-called mass forces can work: electromagnetic, centrifugal, gravitational. Their action is capable both to improve circulation in pipe, and to complicate it.
The heat pipe can work with smooth walls effectively only in one situation – when the source of heat is located near lower pipe end, being under inclination.
Inventors have finished design, having given to tube the chance to function practically in any situation. It was for this purpose sufficient to place so-called match in the device. Any material having the "developed" surface can become it. At the same time the condensed liquid will have opportunity to move on match due to capillary effect at various provisions of tube.
Advantages of heat pipe
The heat pipe has found deserved popularity in various industries of the equipment, including quite specific areas. The device of this kind has very high heat conductivity which is significantly exceeding this characteristic at copper. The heat pipe is capable to transfer heat to long distance at small cross-section while at metal rod the ability to transfer heat energy decreases in proportion to the section relation to length.
Heat transfer speed in tube is very high and is limited only to the speed of evaporation of the heat carrier and rates of its condensation.
The heat pipe is capable to work in any temperature condition. If it is correct to pick up the heat carrier, the tube can be applied at temperature as about +300 degrees Celsius, and at temperatures coming to absolute zero that does such system suitable for use in the space equipment.
The device using the principle of work described above will be durable. Nothing limits the term of operation of heat pipe as neither pipe metal, nor match, nor the heat carrier wear out.
The fault tolerance of thermal tube is amazing, in this simple design there is nothing to break. The tube will work until laws of physics are fair.
Construction features of heat pipe
From the theoretical point of view the heat pipe represents the evaporating and condensation device used for transfer of heat where there is transfer of warmth of steam formation by means of evaporation of liquid in zone of receipt of heat and condensation of vapors in the field of the heat sink. The closed cycle of operation of the heat carrier is usually supported by action of capillary forces.
Parameters of heat pipe can be operated, changing in it pressure. It will give the chance to the heat carrier to make transition from one phase to another in the required temperature condition.
There are certain requirements to constructive parts of heat pipe. For example, the liquid used in tube should not be subject to decomposition, should not enter chemical reaction with material of which the tube and match are made. It is more preferable to do cover of pipe of material with high heat conductivity. The match and tube have to be moistened with the heat carrier well.
Copper, aluminum, steel of different type become body material of heat pipe usually.
The match in heat pipe is saturated with fluid phase of the heat carrier. Not only the mesh, but also sintered porous material structures, as well as special grooves on inside face of tube, perforated screens and so on can become match. The main thing that the structure of match allowed to transfer liquid from condensation zone to the field of heating due to capillary forces.
Now the match is most often made of metal. This constructive part has appearance of grid or becomes weaved out of very fine wire; such technical solution has received the name of metal felt.
As the heat carrier in this system water, acetone, alcohol, acetone, freona can act. In those tubes which should work in the unusual modes of temperatures sodium, mercury, liquid helium or silver can be used. Main requirement to the heat carrier: it has to be pure substance in chemical sense or connection which is capable to act both in liquid, and in steam phase and also to have the moistening effect.
Heat pipes have become widespread not only in heating services of houses, but also in technology schemes of cooling of components of modern computers (processors, video cards). Advantage of heat pipe in this case is defined by the fact that there is opportunity to remove from crystal of the small area at most of heat and to disseminate it on radiator with large number of edges.