JP2001304790A - Heat exchanger - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small and light-weight heat exchanger used for exchanging heat between two different fluids which react with each other when contacted, wherein inside and outside fluids are less risked to be mixed with each other even when a heating tube is broken, particularly a heat exchanger suitable for an evaporator of a liquid metal cooling fast breeder. SOLUTION: A heating tube 6 has a dual structure of an inner tube 21 and an outer tube 22 which are sufficiently distanced from each other so as not to be broken at once by one cause. An intermediate heating medium is filled in a gap 24 made between the inner tube and the outer tube so as to be naturally convected. Thus heat is exchanged with fluid on the outside of the tube through natural convection.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat exchanger for exchanging heat between two different fluids that react upon contact, and more particularly to a steam generator for a liquid metal cooled fast breeder reactor.

[0002]

2. Description of the Related Art When a single heat transfer tube, such as that usually used in a steam generator of a liquid metal-cooled fast breeder reactor using sodium, is used, when the heat transfer tube is damaged, sodium and water come into contact immediately. It is dangerous because sodium reaction occurs. For this reason, there is a demand for a highly reliable heat exchanger that does not easily damage the heat transfer tube and does not immediately lead to a serious accident even if the heat transfer tube is damaged.

For this reason, conventionally, as a steam generator for a liquid metal-cooled fast breeder reactor, a double heat transfer tube such as an intimate double tube divided into an inner tube and an outer tube and made independent, and a braided double tube. Is used. However, in these double-pipe heat exchangers, the pipe wall is divided into two layers to reduce the risk of breakage at once, but the heat transfer performance of the heat transfer pipe is reduced. For this reason, a method of ensuring heat transfer performance by interposing a heat transfer medium in the gap between the inner and outer pipes or interposing a porous metal is also used.

For example, Japanese Patent Application Laid-Open No. 54-016762 discloses that a heat transfer tube is doubled, helium gas is filled as a heat medium between the inner and outer tubes, and explosion bonding is performed at both ends to seal heat exchange. A vessel is disclosed. However, the contacted double-pipe heat exchanger uses the heat conduction of the heat medium to assist the heat transfer between the inner and outer pipes, and the heat transfer efficiency decreases in inverse proportion to the gap width. The gap between the tubes is reduced to several microns to several hundred microns. Therefore, when a large force is applied, it is inevitable that the inner tube and the outer tube are simultaneously damaged, and the sodium water reaction cannot be avoided effectively.

Japanese Patent Application Laid-Open No. Hei 6-323777 discloses that a porous metal is disposed in a gap between an inner pipe and an outer pipe of a double pipe, and when a pipe wall is torn, an external fluid penetrating into the gap is heated. There is disclosed a braided double-pipe heat exchanger that detects leakage by detecting a sensor provided at an end of the exchanger. The disclosed heat exchanger has a metallurgy between the porous metal and the inner and outer tubes by winding a porous metal made of braided wire on the inner tube, inserting the outer metal into the outer tube, subjecting the inner tube to integral drawing, and performing heat treatment. It is the one that has been combined. It is stated that a helical double heat transfer tube having high thermal conductivity can be obtained by performing heat treatment again after forming into a helical shape.

In this heat exchanger, when the inner tube or the outer tube is damaged, a coolant or steam infiltrates between the inner tube and the outer tube and reaches the end of the heat exchanger through the porous metal to reach the sensor. Can detect leaks. As described above, the disclosed heat exchanger does not use a heat transfer fluid such as liquid metal because it is preferable that the infiltrated coolant and the like flow smoothly in the gap between the inner and outer tubes, and mainly uses porous metal. Heat transfer between the inner tube and the outer tube through heat conduction in the braided wire
Heat transfer efficiency is not always high.

Further, in order to more reliably avoid contact between sodium and water, for example, separation as shown in a sectional view of FIG. 6 and a partially enlarged transverse sectional view of an effective heat transfer portion in FIG. There is a type steam generator. The heat exchanger of the separation type arranges a heat transfer body between two fluid pipes and exchanges heat by heat conduction. Drill a hole in a metal block such as aluminum which has good heat transfer performance. Then, a sodium pipe and a water / steam pipe are inserted so as to be adjacent to each other, and a liquid metal having excellent thermal conductivity, such as bismuth, tin, or indium, is filled between the metal block and the pipe as an intermediate heat medium. The heat of sodium propagates through the wall of the pipe, is transmitted to the aluminum block through liquid metal such as bismuth, and is further conducted in aluminum, and is transmitted into the water / steam pipe through the liquid metal, and removes the water in the pipe. It is sent as steam to a generator.

In the separate steam generator, the sodium pipe and the water / steam pipe are positionally independent, and the
The possibility of two pipes being damaged at the same time is reduced. However, in the case of a separate steam generator, it is necessary to dispose an aluminum block in the heat exchange area, and the amount of intermediate heat medium to be filled in the heat exchanger is enormous, and the structure is complicated and large. There was an economic problem.

[0009]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a small and lightweight heat exchanger which reduces the risk of mixing of internal and external fluids even when the heat transfer tube is broken. In particular, it is an object of the present invention to provide a heat exchanger suitable for a steam generator of a liquid metal-cooled fast breeder reactor.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, a heat exchanger according to the present invention used for exchanging heat between two different fluids which react upon contact with each other includes a heat transfer tube through which a first fluid passes. The inner pipe and the outer pipe have a double structure, and the gap between the inner pipe and the outer pipe is filled with the intermediate heat medium so as to be capable of natural convection, and heat exchange with the second fluid outside the pipe is performed through the natural convection. Features. In the heat exchanger of the present invention, since natural convection heat transfer is used in addition to heat conduction of the heat medium, the heat transfer coefficient in the gap between the inner and outer pipes gradually decreases almost in proportion to the gap width to the power of 0.22. . Therefore, even if the inner pipe and the outer pipe are sufficiently isolated so as not to be damaged at one time due to one cause, the heat transfer efficiency does not significantly decrease.

It is known that in natural convection heat transfer, the heat transfer efficiency is improved from 1.2 times to 3.1 times as compared with that obtained by only heat conduction. A highly efficient steam generator can be obtained. In the heat exchanger of the present invention, compared with the amount of the liquid metal such as aluminum block and bismuth as the intermediate heat medium required in the conventional separation type heat exchanger, the intermediate heat medium requires much less, and the structure is also small. Simple and compact.

The intermediate heat medium may be sealed in the gap between the inner and outer tubes. If the intermediate heat medium is present only in the portion corresponding to the heat exchange portion in the inner and outer pipe gaps, the amount of the intermediate heat medium used can be reduced without lowering the heat exchange efficiency, which is economical. Further, it is preferable that the inner and outer pipe gaps be defined so that the Rayleigh number is 1000 or more. When the Rayleigh number is less than 1000, the intermediate heat medium present in the gap between the inner and outer pipes is stationary and convection does not occur.
In the above, the intermediate heat medium naturally convects and transfers heat. In particular, when the upper and lower surfaces are fixed walls, the Rayleigh number is 1
It is said that the medium does not flow below about 700.

Further, the gap between the inner and outer pipes in the double-pipe heat exchanger of the present invention is preferably 1 mm or more. When the gap is 1 mm or more, the intermediate heat medium causes natural convection, and the possibility that the inner tube and the outer tube are simultaneously damaged is significantly reduced. The heat exchanger of the present invention may be of a horizontal type in which the heat transfer tubes are oriented substantially horizontally. Compared with the vertical type heat exchanger, the horizontal type heat exchanger is easier to naturally convection, and the weight of the intermediate heat medium and the weight of the steel material can be reduced.

The heat exchanger of the present invention can be applied to heat exchange between water and sodium metal or potassium metal often used in a steam generator of a liquid metal-cooled fast breeder reactor. In this case, the intermediate heat medium is preferably any one of lead, bismuth, tin, and indium, or a mixture thereof. Particularly, lead or bismuth is preferable from the viewpoint of cost and toxicity of the medium. Further, it is preferable that the static pressure of the intermediate heat medium be lower than the static pressure of the second fluid. In this way, even if the outer tube is damaged, the scale of the accident can be prevented from increasing because the intermediate heat medium does not enter the system of the second medium such as sodium.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments with reference to the drawings. In this embodiment, the heat exchanger of the present invention is applied to a steam generator of a liquid metal-cooled fast breeder reactor. FIG. 1 is a cross-sectional view of the present embodiment, FIG. 2 is a schematic cross-sectional view showing a state where the heat transfer tube according to the first embodiment of the present invention is cut along a plane along an axis, and FIG. FIG. 4 is a cross-sectional view of the heat transfer tube cut along a plane perpendicular to the axis, FIG. 4 is a schematic cross-sectional view of the heat transfer tube according to the second embodiment of the present invention cut along a plane along the axis, and FIG. FIG.

The steam generator of this embodiment is of a horizontal type as shown in FIG. 1, and molten sodium, which is a coolant of a liquid metal-cooled fast breeder reactor, is supplied from an inlet nozzle 1 to a shell side, and liquid sodium is supplied from an outlet nozzle 2 The water returns to the metal-cooled fast breeder reactor, and water is supplied from the supply nozzle 3 to the tube side to be converted into steam and sent from the outlet nozzle 4 to a generator or the like. One of a number of heat transfer tubes 6 arranged in the shell 5 of the steam generator is connected to a feed water inlet tube plate 7 and a steam outlet tube plate 8.

FIGS. 2 and 3 are sectional views of the heat transfer tube 6 alone. The heat transfer tube 6 has a double structure of an inner tube 21 and an outer tube 22 as shown in the figure. The inner tube 21 is connected to the feed water inlet tube plate 7 and the steam outlet tube plate 8, and the outer tube 22 is made of sodium. It is connected to an inlet tubesheet 9 and a sodium outlet tubesheet 10. The gap 24 formed between the inner pipe 21 and the outer pipe 22 is formed between the plenum 12 formed between the feed water inlet pipe sheet 7 and the sodium outlet pipe sheet 10 and the steam outlet pipe sheet 8 and the sodium inlet pipe sheet 9. It leads to a plenum 13 formed therebetween. The gap 24 between the inner and outer tubes is maintained by an appropriate gap spacer 23, and the low melting point metal supplied from the respective plenums 12 and 13 is filled as an intermediate heat medium.

Although not shown, the plenums 12 and 13 are preferably provided with a core inside to reduce the free space, in order to save an intermediate heat medium filled therein. The melting point of the intermediate heat medium is lower than the operating temperature, and even if the wall of the heat transfer tube outer tube 22 breaks and mixes with sodium, or if the wall of the inner tube 21 breaks and mixes with water or steam, it is extreme. Those that do not cause a chemical reaction are selected. In addition, it is preferable that the material does not cause environmental pollution even in the event of leakage, and it is preferable to use, for example, lead or bismuth in consideration of economic efficiency.

The gap 24 between the inner tube 21 and the outer tube 22 is
At least 1 m, so that the molten metal generates natural convection
m or more. Although the inner tube 21 and the outer tube 22 are less likely to be damaged at the same time as the inner / outer tube gap 24 is larger, if it is too large, the filling amount of the molten metal becomes large, which is not economical. Also, the heat transfer efficiency in convective heat transfer is considered to be degraded in inverse proportion to the 0.22 power of the gap width, though it is slower than in the case of using heat conduction, and the gap width should not be extremely large. Is preferred.

The spacers 23 are provided at appropriate intervals on the outer surface of the inner tube 21 in order to maintain a gap width between the inner tube 21 and the outer tube 22. The spacer 23 can be formed, for example, by winding a plate having a thickness slightly smaller than the gap width of about 0.6 mm when a gap of 1 mm is provided, or by welding a coil, a cylinder, a notched cylinder, or the like. it can.

Since the heat transfer tubes 6 are long, appropriate heat treatment tubes 6 are bent in the axial direction of the steam generator so as to prevent the heat transfer tubes 6 from contacting each other due to bending in the shell 5 and preventing the liquid sodium from drifting. Heat transfer tube spacers 11 are provided at intervals, and are supported so as to maintain a predetermined heat transfer tube interval and position. The heat transfer tube 6 has a curved tube portion 16 formed in one of the plenums 12 and 13 so as to absorb thermal expansion.

The shell 5 is provided with a shell bellows (not shown) at a substantially central position so that a difference in thermal expansion between the steam generator and the support structure can be absorbed. The plenums 12 and 13 are provided with a device for detecting sodium and water leaked into the gap 24 between the inner and outer pipes. In addition, a cover gas pressure gauge, a rupture plate rupture detection system, and a pressure release device are provided in case of an accident.

FIG. 4 is a sectional view showing another example of the heat transfer tube 6 used in the steam generator of the present embodiment. The cross section in the direction perpendicular to the axis is the same as the heat transfer tube shown in FIG. In the double-tube heat transfer tube 6 used in the present embodiment, the heat exchange between sodium and water is performed by the heat exchange portion sandwiched between the sodium inlet tube sheet 9 and the sodium outlet tube sheet 10. Therefore, it is sufficient that the intermediate heat medium is present in the heat exchange portion in the gap 24 between the inner and outer tubes.

The heat transfer tube 6 shown in FIG. 4 has a structure in which a bellows 26 is provided between the inner tube 21 and the outer tube 22 at a position outside the heat exchange section to seal the gap between the inner and outer tubes. An intermediate heat medium is sealed in the sealed interior 24, and argon gas is press-fitted into the plenums 12 and 13 through argon gas supply nozzles 14 and 15, respectively, and a static pressure of the argon gas is applied from the outside 25 of the bellows 26. The static pressure applied to the intermediate heat medium is made smaller than the pressure of sodium so that the intermediate heat medium does not enter the sodium system even when an outer tube breakage accident occurs.

Further, the plenums 12, 1 at both ends of the heat exchanger are provided.
3 is provided with a leak detector (not shown) to remove sodium or steam mixed in the intermediate heat medium into plenums 12 and 13.
And the detection by each detector to detect the occurrence of an accident. When the intermediate heat medium is pressurized to 2 atm or more, a steam explosion at the time of an accident can be prevented. The intermediate heat medium performs natural convection due to a temperature difference in the inner and outer pipe gaps 24, and transfers heat of sodium to water to be vaporized.
Since the heat transfer tube 6 needs to hold the intermediate heat medium only in the heat exchange section, it is possible to greatly reduce the total amount of the heat medium while maintaining the heat exchange capacity.

There is no difference between the heat transfer tube of FIG. 2 and the heat transfer tube of FIG. 4 regarding the heat transfer effect of the heat exchanger. In the heat exchanger of the present invention, the heat transfer tube has a double structure of an inner tube and an outer tube, and a natural convection is possible in a gap between the inner and outer tubes using a liquid metal that does not react with a heat exchange substance as an intermediate heat medium. And heat exchange takes place between the substance in the tube and the substance in the shell via natural convection. The amount of heat transfer in the heat exchanger of the present invention greatly increases due to natural convection as compared with the case where heat transfer is performed only by heat conduction of the heat medium.

FIG. 5 is a diagram illustrating the contribution of natural convection. The circle positions in the figure plot the Rayleigh number Ra calculated based on the use conditions of the heat exchanger on the horizontal axis and the Nusselt number Nu at that time on the vertical axis. The Rayleigh number Ra is a variable used as an index for natural convection generated when a stationary fluid between horizontally placed parallel plates is heated from below. The Rayleigh number Ra is the distance l between the upper and lower plates, the acceleration g of gravity, the kinematic viscosity ν, the temperature conductivity α,
Nu = l ³ gγΔT / (να) using the body expansion coefficient γ and the temperature difference ΔT between the lower surface and the upper surface.

When the upper and lower surfaces are solid walls, when the Rayleigh number is 1708 or less, the fluid does not produce convection. Although the heat transfer tube of the present embodiment is not a horizontal parallel plate, as can be seen from the figure, the Nusselt number Nu increases from around 1000 with the Rayleigh number Ra, and eventually exceeds 3.0.
Since the Nusselt number Nu represents the ratio of the amount of heat transmitted through the surface of an object placed in a fluid to the amount of heat transmitted only by heat conduction, the heat exchanger of the present invention has a higher heat transfer than a conventional heat exchanger. Even if the heat area is small, the same heat transfer effect can be obtained.

As described above, the heat exchanger of the present invention fills the space between the inner and outer tubes with the intermediate heat medium flowing therethrough and utilizes heat transfer by natural convection of the heat medium. The heat transfer efficiency is remarkably improved as compared with the conventional double tube heat exchanger containing an intermediate heat medium. The intermediate heat medium only needs to be filled in the gap between the inner tube and the outer tube. Therefore, a liquid metal reservoir is formed at the bottom of the heat exchanger, and the liquid metal is filled between the hole of the aluminum block and the tube wall. Compared with the conventional separation tube type heat exchanger, the amount of the intermediate heat medium used can be greatly reduced. Also, the amount of steel used in the entire heat exchanger can be saved. For example, in order to process an exchange heat amount corresponding to 1000 MWt / unit, a conventional separation tube type steam generator requires a steel material weight of 1030 t and an intermediate heat medium amount (aluminum block and liquid metal) of 1020 t. On the other hand, in the steam generator of the present embodiment, the steel weight 64
0 t and the amount of the intermediate heat medium (liquid metal only) 700 t.

Further, in the heat exchanger according to the present invention, since the distance between the inner tube and the outer tube of the double tube type heat transfer tube is sufficiently large, there is a possibility that the inner tube and the outer tube are simultaneously damaged due to the same cause. It is significantly less likely to occur as compared to a double tube heat exchanger. Further, a device in which the static pressure of the intermediate heat medium is smaller than the pressure of sodium has an advantage that the post-treatment becomes easy because the intermediate heat medium does not enter the sodium system even when an outer tube breakage accident occurs. Further, since sodium and vapor mixed in the intermediate heat medium leak into the plenums at both ends of the heat exchanger, detection of the respective detectors can promptly alert the occurrence of an accident.

The heat exchanger of the present invention has a great effect when applied to a steam generator of a liquid metal-cooled fast breeder reactor, but generally causes a chemical reaction when fluids to be exchanged heat are mixed. Needless to say, it can be widely applied to general industrial heat exchangers since it can be used for any object that produces some undesirable result. Although the above embodiment relates to a horizontal heat exchanger, the present invention is based on the fact that the intermediate heat medium transfers heat by natural convection in the gap between the inner and outer tubes. If the gap is made sufficiently large, it goes without saying that the effects of the present invention can be obtained by generating natural convection even in a vertical heat exchanger. In particular, when spacers are provided at appropriate intervals between the inner and outer tubes, local natural convection is likely to occur in a region sandwiched by the spacers, and the present invention is applied to a vertical heat exchanger, whereby a smaller heat transfer is achieved. It is possible to obtain a heat exchanger having a heat area and excellent event liability.

[0032]

As described above, since the heat exchanger of the present invention uses a double tube type heat transfer tube having a sufficiently large space between the inner and outer tubes, an accident occurs in which the inner tube and the outer tube are simultaneously damaged. It is safe because it is difficult. Further, since the natural convection of the intermediate heat medium filled in the gap between the inner and outer pipes is used, the heat transfer efficiency is remarkably improved. Furthermore, since the intermediate heat medium only needs to be filled in the gap between the inner and outer tubes, the amount of use can be greatly reduced,
Also, the amount of steel used in the entire heat exchanger can be saved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a steam generator of a liquid metal-cooled fast breeder reactor using the heat exchanger of the present invention.

FIG. 2 is a cross-sectional view of the heat transfer tube according to the first embodiment of the present invention, taken along a plane along an axis.

FIG. 3 is a cross-sectional view of the heat transfer tube of the present embodiment cut along a plane perpendicular to an axis.

FIG. 4 is a cross-sectional view of a heat transfer tube according to a second embodiment of the present invention, taken along a plane along an axis.

FIG. 5 is a diagram illustrating performance in the present embodiment.

FIG. 6 is a cross-sectional view showing an example of a conventional double tube heat exchanger.

FIG. 7 is a sectional view of a heat exchange part of the heat exchanger of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sodium inlet nozzle 2 Sodium outlet nozzle 3 Feed water inlet nozzle 4 Steam outlet nozzle 5 Shell 6 Heat transfer tube 7 Feed water inlet tube plate 8 Steam outlet tube plate 9 Sodium inlet tube plate 10 Sodium outlet tube plate 11 Heat transfer tube spacer 12,13 Plenum 14 , 15 Argon gas supply nozzle 16 Curved tube 21 Inner tube 22 Outer tube 23 Gap spacer 24 Gap between inner and outer tubes 25 Bellows exterior 26 Bellows

Claims (7)

[Claims] 1. A heat exchanger for exchanging heat between two different fluids which react upon contact with each other, wherein a heat transfer tube through which a first fluid passes has a double structure of an inner tube and an outer tube. A heat exchanger characterized in that the pipe gap is filled with an intermediate heat medium so as to be capable of natural convection, and heat is exchanged with a second fluid outside the heat transfer tube via natural convection of the intermediate heat medium. 2. The heat exchanger according to claim 1, wherein the intermediate heat medium is sealed in the gap between the inner and outer pipes. 3. The gap between the inner and outer pipes having a Rayleigh number of 1000
The heat exchanger according to claim 1 or 2, wherein the intermediate heat medium is configured to transfer heat by natural convection as described above. 4. The heat exchanger according to claim 1, wherein the gap between the inner and outer pipes is 1 mm or more. 5. The heat exchanger according to claim 1, wherein the tube is of a horizontal type oriented substantially horizontally. 6. The method according to claim 1, wherein the first fluid is water, the second fluid is sodium metal or potassium metal, and the intermediate heat medium is any of lead, bismuth, tin, indium or a mixture thereof. The heat exchanger according to any one of claims 1 to 5, wherein: 7. The heat exchanger according to claim 1, wherein a static pressure of the intermediate heat medium is lower than a static pressure of the second fluid.