HEAT EXCHANGER USING HEAT PIPES

Abstract: Heat pipe type heat exchanger which comprises: a container (20) for containing a first heating medium therein or causing the same to flow therethrough; at least one heat pipe (25) extending liquid-tight with a substantially horizontal axis through the container; and a tube (27) extending gas-tight and axially through the heat pipe for causing a second heating medium to flow therethrough, whereby the heat exchange is effected between the first and second heating mediums through the walls of the heat pipe and the tube. In case a plurality of heat pipes are pro­ vided, the individual tubes (27) extending axially through the corresponding heat pipes are connected at their ends to each other by means of bends (33) such that they make a zigzag or meandering piping.

Description: The present invention relates to a heat exchanger for exchanging heat between fluids at higher and lower temperatures through heat pipes and, more particularly, to a heat exchanger which is effective in case the heat exchange is accomplished between the cooling medium of liquid metal and water of a nuclear reactor. As is well known in the relevant art, the heat pipes transfer heat as the latent heat of a working fluid by sealing up closed tubes with a condensable fluid as the working fluid, after the tubes have been evacuated, and by circulating the working fluid within the closed tubes through evaporations and condensations. Since the heat pipes have excellent thermal conductivity, therefore, an efficient heat exchange can be performed if the heat pipes are used in a heat exchanger for the heat exchange between two kinds of fluids to be refrained from any contact and mixing. Fig. 4 is a schematic diagram showing one example of the heat exchanger of the prior art. This heat exchanger is constructed by inserting a plurality of heat pipes 3 into and arranging them across higher- and lower-temperature chambers 1 and 2 isolated from each other. If a hotter fluid 4 is supplied to the higher-temperature chamber 1 whereas a colder fluid 5 is supplied to the lower-temperature chamber 2, the working fluid in the heat pipes 3 evaporates at the higher-temperature ends of the heat pipes 3 so that its resultant steam flows to the lower-temperature ends of the heat pipes 3, until the working fluid radiates its heat and condenses. Thus, the heat is exchanged between the hotter and colder fluids 4 and 5. Since the passages for these hotter and colder fluids 4 and 5 are thus isolated from each other, the heat exchanger shown in Fig. 4 is effective for the heat exchange between such substances, e.g., liquid sodium and water as will produce an intense reaction. Since, however, these endothermic and exothermic portions for the heat pipes are isolated, the heat exchanger of Fig. 4 is defective in its large size. Since, moreover, the heat pipes 3 are made of tubes which are thinned to reduce their total thermal resistance and to have an excellent thermal conductivity, the heat exchanger shown in Fig. 4 has found it difficult to weld the heat pipes 3 in a sealed state to the chambers 1 and 2, respectively, and had had their welded portions positioned in the opposed walls of the individual chambers 1 and 2 so that it has been accompanied by a problem that the heat pipes 3 nd the individual chambers 1 and 2 have been remarkably difficult to weld or seal up. In case, on the other hand, the fluid passages can be freely set, the prior art may have used a shell tube type heat exchanger which can be small-sized. Fig. 5 is a schematic diagram showing one example of the shell tube type heat exchanger. This heat exchanger is constructed such that a meandering tube 11 for the colder fluid 5 is arranged in a closed shell 10 for the hotter fluid 4 so that the heat exchange may be effected between the hotter and colder fluids 4 and 5 through the wall of the meandering tube 11. This shell tube type heat exchanger of the prior art shown in Fig. 5 can be small-sized without any reduction in the heat transfer area. Since, however, what exists between the hotter and colder fluids 4 and 5 is the wall of the meandering tube 11, the hotter and colder fluids 4 and 5 will directly contact or mix with each other even if the meandering tube 11 turns slightly defective with pin holes or the like. This makes it impossible to use the shell tube type heat exchanger of Fig. 5 for the heat exchange between the intensely reactive substances such as the sodium and water which are used as the cooling mediums of the nuclear reactor. Another heat exchanger using the heat pipes for exchanging heat between the primary and secondary cooling mediums of the nuclear reactor, i.e., the sodium and water is disclosed in the US-A-4 560 533. As shown in Figs. 6 and 7, a heat pipe 13 using mercury as a working fluid 12 has its inside partitioned into a plurality of compartments by baffle plates having fluid vents 14. The heat pipe 13 thus constructed is dipped upright in sodium 16 used as a cooling medium of a nuclear reactor, and a U-shaped cooling water tube 17 is inserted downward into that heat pipe 13. As a result, the working fluid 12 evaporates on the inner wall face of the heat pipe 13 and comes into contact with the outer circumference of the cooling water tube 17 to give its latent heat to the water in the cooling water tube 17 so that the heat is exchanged between the sodium 16 and the water. The heat exchanger shown in Figs. 6 and 7 can be small-sized, because the cooling water tube 17 is disposed in the heat pipe 13, and can avoid the contact and mixing between the sodium 16 and the water. Since, however, the inner wall face of the heat pipe 13 in its entirety acts as the evaporator for the working fluid 12, the baffle plates 15 are indispensable for distributing the working fluid 12 vertically all over the inner wall face of the heat pipe 13 so that the heat exchanger is troubled by the more complex structure, the worse productivity and the higher production cost. Incidentally, there is also disclosed in the prior art, as in Japanese Patent KOKAI No. 61 - 235688, a heat regenerator which uses heat pipes arranged in horizontal positions. In this heat regenerator, an outer tube having its two ends sealed up is mounted on the outer circumference of an intermediate portion of an inner tube, and the sealed chamber defined by the outer circumference of the inner tube and the inner circumference of the outer tube is sealed up with a working fluid, thus constructing each of the thermal diode type heat pipes. These heat pipes are arranged in the horizontal positions and in multiple stages within a regenerative substance, and the individual inner tubes are connected to one another. As a result, in case a heating medium is introduced into the inside of the inner tubes, a heat transfer is established in a higher-temperature layer of the regenerative substance than the heating medium from the regenerative substance to the heating medium by the actions of the heat pipes. In a lower-temperature layer of the regenerative substance than the heating medium, on the other hand, the heat pipes remain inactive, because they are of the thermal diode type, so that no heat exchange is caused between the regenerative substance and the heating medium. This raises no disturbance in the temperature layers formed in the regenerative substance so that the regenerative substance can be prevented from becoming cold, namely, efficient regenerations can be ensured. According to Japanese Patent KOKAI NO. 61 -235688, however, the apparatus disclosed has its heat pipes arranged in the horizontal positions which match the temperature layers formed by the regenerative substance, and accordingly the inner tubes protruding from the heat pipes are also dipped in the regenerative substance. As a result, defects such as pin holes, if any, in the inner tubes will invite a danger that the heating medium flowing in the inner tubes directly contacts and mixes with the regenerative substance. This makes it impossible to convert the apparatus into the heat exchanger to be used for the heat exchange between the metallic sodium and water which will intensely react if they contact. From US-A-4 566 527 the use of heat pipes through which a medium flows for heating surfaces, for example a bridge deck, is known. The inner tube of the heat pipe is coated with wick material and the interior of the heat pipe is partially filled with a heat transfer medium vaporizing in the wick material of the inner tube through which a hot fluid flows and condensing again in tubes provided in the surface and connected to the heat pipe, thus transferring the heat to the surface to be heated. From US-A-3 677 329 it is known to use heat pipes for process furnaces or ovens, with the interior of the inner tube of the heat pipe being used as process zone. To obtain a process temperature as constant as possible the inner tube is coated with a wick material in contact with a working fluid which is provided in the interior of the heat pipe. The working fluid vaporizes due to the heat from the exterior tube and condenses at the inner tube. In this manner the process zone is controlled so as to maintain the condensation tempera-ture of the working fluid exactly. From FR-A-602 833 a heat exchanger is known in which heat pipes connected in staggered manner extend through a container filled with a hotter medium to be cooled. The cooling medium flows through the inner tube of the heat pipe. The interior of the heat pipe is partially filled with a working fluid. The heat pipes are arranged horizontally such that the working fluid is in contact with the inner wall of the outer tube and vaporizes at the latter. The vaporized working fluid condenses at the cooler inner tubes and thus transfers the heat to the latter. It is, therefore, an object of the present invention to provide a heat pipe type heat exchanger which can ensure an efficient heat exchange without any contact and mixing of higher- and lower-temperature fluids and which is so simple in structure that it can be small-sized. In accordance with the invention, this object is solved by the features as claimed in claim 1 or claim 2. According to the heat exchanger of the present invention, therefore, the heat exchange between the first and second heating mediums can be established in the container, and the area for the heat exchange is enlarged so that the heat exchanger can be accordingly small-sized. In the present invention, therefore, either the first or second heating medium may be metallic sodium whereas the other may be water. Even in this case, the heat pipes separate the metallic sodium from the water so that these two mediums can be prevented in advance from directly contacting and intensely reacting. The invention will become apparent from the following description taken with reference to the accompanying drawings, in which: Fig. 1 is a schematic section showing a heat pipe type heat exchanger according to the present invention; Figs. 2 and 3 show transverse sections of two heat pipes; Fig. 4 is a schematic view showing one example of the heat pipe type heat exchanger according to the prior art; Fig. 5 is similar to Fig. 6 but shows one example of the shell tube type heat exchanger according to the prior art; Fig. 6 is a schematic view showing another example of the heat pipe type heat exchanger according to the prior art for the heat exchange between sodium and water; and Fig. 7 is an enlarged transverse section taken along line IX - IX of Fig. 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Fig. 1, a container or shell 20 is formed in its opposed walls with an inlet 22 and an outlet 23 so that a colder fluid (e.g., water) 26 to have its heat exchanged may flow therein in one direction. The shell 20 is equipped with a plurality of double pipes 24 which extend horizontally through the right and left walls of the shell 20. As better seen from Figs. 2 and 3, each double pipe 24 is constructed of: an outer tube 25 having its two ends closed; and an inner tube 27 which extends gas-tight and coaxially through the outer tube 25 while sealing the same so as to provide a passage for a hotter fluid (e.g., liquid sodium) 21. The inside of the outer tube 25, namely, the chamber having an annular section between the outer tube 25 and the inner tube 27 is sealed up with a predetermined condensable fluid as its working fluid 28 after it has been evacuated. As the working fluid 28, incidentally, there can be used a variety of fluids in accordance with a target temperature and the kind of fluid to be heat-exchanged. In case the hotter fluid is sodium whereas the colder fluid is water, for example, mercury can be employed as the working fluid. The double pipes 24 thus constructed are arranged in such generally horizontal positions within the shell 20 as to extend through the right and left walls of the shell 20 and are fixed liquid-tight in those walls by the use of means for welding them from the outside. Of these double pipes 24, the pipe 24 positioned at the side of the inlet 22 has its inner tube 27 providing a outlet 31 for the hotter fluid at its one end, whereas the pipe 24 positioned at the side of the outlet 23 has its inner tube 27 providing an inlet 32 for the hotter fluid at its one end. Every adjacent pipes 24 have their inner tubes 27 connected at the ends to each other by connecting pipes 33 such as return bends. As a result, the double pipes 24 are formed as a whole into one zigzag or meandering piping. By the use of the heat exchanger thus constructed, a heat exchange is accomplished between the higher-temperature fluid 21 and the lower-temperature fluid 26. For this operation, the colder fluid 26 is introduced into the shell 20 from the inlet 22 to the outlet 23, and the hotter fluid 21 is introduced into the meandering piping from the inlet 32 to the outlet 31. Since, in this instance, the double pipes 24 are arranged in the horizontal positions, the working fluid 28 in the heat pipes 30 is accumulated in the bottom of the outer tubes 25 by its own weight. Each inner tube 27 is offset downward with respect to the corresponding outer tube 25, as shown in Fig. 2, so that it may be partially dipped in the working fluid 28 in the liquid phase. In an alternative, as shown in Fig. 3, each inner tube 27 may be covered on its outer circumference with an annular wick 29′ and equipped with radial wick 29″ which extends radially in an upright position from the outer face of the inner tube 27 and the inner face of the outer tube 25 so that the working fluid in the liquid phase may be supplied to the outer circumference of the inner tube 27 acting as the evaporator by those circular and radial wicks 29′ and 29″. As is now apparent from the description thus far made, according to the present invention, tubes are extended axially through heat pipes which are arranged in horizontal positions, and the outer circumferences of the heat pipes and the inner circumferences of the tubes are used as endothermic portions and exothermic portions so that the heat exchanger of the present invention can have its total structure small-sized. In the heat exchanger of the invention, moreover, the heat pipes intermediate the heat exchange between the first and second fluids. Because of the high heat conductivity of the heat pipes, the efficiency of this heat exchange can be substantially equivalent to that to be effected through a single metal wall. In the heat exchanger of the invention, still moreover, those portions of the tubes for the second heating medium, which are disposed in the container, are covered with the heat pipes so that what occurs is the leakage of the second heating medium into the heat pipes to prevent in advance the second heating medium from directly contacting or mixing with the first one even if the tubes become defective with the pin holes. This similarly applies to the case in which the heat pipes become defective. In this case, too, the first heating medium in the container will leak into the heat pipes at the worst, but the two heating mediums are prevented from contacting or mixing with each other. Such defects can be instantly detected by measuring the pressure in the heat pipes. As a result, the heat exchanger of the present invention can be effectively applied to the heat exchange between the sodium and water which are used as the cooling mediums of a nuclear reactor. Since the heat pipes are arranged generally horizontally, furthermore, the distribution of the working fluid in the heat pipes to the evaporator may be exemplified by the natural flow of the working fluid itself or by the use of the ordinary wick. As a result, the structure of the heat pipes can be simplified. In addition, the heat pipes may be fixed to the container from the outside and sealed up so that the heat exchanger of the present invention can enjoy an excellent productivity.

Claims: A heat pipe type heat exchanger for cooling liquid sodium with water in a nuclear power plant comprising: a container (20) for containing water as a first medium (26) therein or causing the same to flow therethrough; heat pipes (24) extending liquid-tight with a substantially horizontal axis through said container (20) and each of the heat pipes (24) has its two ends protruding to the outside of said container (20) and welded to said container (20) from the outside of the same and connected in a zigzag shape to the ends of another heat pipe (24) by bends wherein said heat pipe (24) is sealed up with mercury as working fluid (28) which will evaporate, when it is heated, and will condense when it loses its heat and an inner tube (31;27) extending gas-tight and axially through said heat pipe (24) for causing sodium as a second medium (21) to flow therethrough, whereby the heat exchange is effected between said first (26) and second mediums (21) through the walls of said heat pipe (24) and said inner tube (31;27), in such a way that the hotter fluid sodium (21) flows through the inner tubes (31;27) of the heat pipes (24) to cause the outer circumferences of the inner tubes (31;27) contacting the working fluid (28) to act as the evaporator and the inner circumferences of the outer tube (25) as the condensor, wherein said inner tube (31;27) is so offset from the axis of said heat pipe (24) that it is partially dipped in the working fluid (28) in a liquid phase. A heat pipe type heat exchanger for cooling liquid sodium with water in a nuclear power plant comprising: a container (20) for containing water as a first medium (26) therein or causing the same to flow therethrough; heat pipes (24) extending liquid-tight with a substantially horizontal axis through said container (20) and each of the heat pipes (24) has its two ends protuding to the outside of said container (20) and welded to said container from the outside of the same and connected in a zigzag shape to the ends of another heat pipe by bends wherein said heat pipe is sealed up with mercury as working fluid (28) which will evaporate, when it is heated, and will condense when it loses its heat and a inner tube (31;27) extending gas-tight and axially through said heat pipe (24) for causing sodium as a second medium (21) to flow therethrough, whereby the heat exchange is effected between said first (26) and second mediums (21) through the walls of said heat pipe (24) and said inner tube (31;27), in such a way that the hotter fluid sodium (21) flows through the inner tubes (31;27) of the heat pipes (24) to cause the outer circumferences of the inner tubes (31;27) contacting the working fluid (28) to act as the evaporator and the inner circumferences of the outer tube (25) as the condensor, wherein said heat pipe (24) includes a porous, cylindrical wick (29) covering the outer circumference of said inner tube (31:27) for establishing a capillary action; and a plate-shaped porous wick (29) to the working fluid (28) in a liquid phase for establishing a capillary action.

Inventors: MOCHIZUKI, MASATAKA, SUGIHARA, SHINICHI, YATSUHASHI, MOTOHARU
Applicants: DORYOKURO KAKUNENRYO KAIHATSU JIGYODAN, FUJIKURA LTD.
Application year: 1987
Main classification: F28D15
IPC codes: F28D15