CN110691953A

CN110691953A - Heat exchanger for a molten salt steam generator in a concentrated solar power plant

Info

Publication number: CN110691953A
Application number: CN201880034199.6A
Authority: CN
Inventors: 阿尔弗雷德·德蒂尔; 斯特凡·维南德; 伊夫·莱克卢; 里德哈·哈尔扎拉赫; 艾尔多·阿葛涅蒂; 托马斯·鲍蒂尔; 克里斯托弗·德太耶
Original assignee: Cockerill Maintenance and Ingenierie SA
Current assignee: John Cockerill SA
Priority date: 2017-05-24
Filing date: 2018-05-15
Publication date: 2020-01-14
Anticipated expiration: 2038-05-15
Also published as: ZA201907258B; EP3406998A1; WO2018215239A1; AU2018274073A1; MX2019013991A; US20200141568A1; KR20200010318A; IL270461B; EP3406998B1; PE20200088A1; CL2019003253A1; CN110691953B; ES2844382T3

Abstract

Hairpin heat exchanger (1) in which a bundle of parallel U-shaped bent tubes (2) protrudes from the exchanger and is connected via bent tubes (11) to a first header (9) outside one end of an inner shell (3) and an outer shell (4) at a first straight section, respectively, and to a second header (10) outside one end of the inner shell (3) and the outer shell (4) at a second straight section, the first header distributing a first fluid to the bundle of straight tubes (2), the second header collecting the first fluid in the form of liquid, steam or a liquid/steam mixture from the bundle of straight tubes (2).

Description

Heat exchanger for a molten salt steam generator in a concentrated solar power plant

Technical Field

The present invention relates to the field of heat exchangers, in particular heat exchangers, such as evaporators, superheaters, reheaters and economizers, intended for use in heat-conducting fluid steam generators, such as the Molten Salt Steam Generator (MSSG) of a concentrated solar power plant (CSP).

Prior Art

As is known, CSP tower installations typically include one or more solar receivers located at the apex of a central tower. These solar receivers are heated by concentrated incident solar rays and they produce a hot fluid that will be further used to produce high pressure steam that can drive turbines and generate electricity.

More specifically, the CSP tower apparatus has the major components, namely, at least the heliostat solar field, a solar receiver mounted on top of the tower, a steam generator, a steam turbine, and a storage system. In molten salt technology, molten salt is typically heated to 565 ℃ in a solar receiver and stored in a hot storage tank. When power generation is required, hot salt flows from the hot tank to a Molten Salt Steam Generator (MSSG), steam is generated, and the steam is then injected into a steam turbine.

Fig. 1 schematically illustrates components of a typical so-called heat exchanger train for an MSSG. Hot molten salt enters the evaporator 102 from an inlet 100 through a reheater 101 and a superheater 104. Thereafter, the hot salt flows from the outlet of the evaporator 102 to the economizer 103 and then to the outlet 105.

By "shell and tube" heat exchangers is meant in the prior art a type of heat exchanger design suitable for high pressure applications. This type of heat exchanger consists of a large pressure vessel called a "shell" inside which a set of tubes called a "bundle" is located. The first fluid flows through the tubes and the second fluid flows over the tubes within the housing, the first and second fluids having different temperatures in order to transfer heat from the second fluid to the first fluid and vice versa.

There are many variations on the shell and tube design. For example, fig. 2 schematically shows a straight tube type heat exchanger (two-tube side). The end of each tube 21 is connected to a tank or plenum 29 through an aperture provided in a divider plate called a "tubesheet" 27. The tube 21 may be straight as shown in fig. 2, or may be bent into a "U" shape (U-shaped tube).

To provide improved heat exchange between the two fluids, the flow path of the second fluid is generally defined by intermediate baffles 28 forming respective channels, such that the flow of the second fluid changes its direction as it flows from one channel to the next. The baffles are typically in the form of part-circular segments or annular rings and discs mounted perpendicular to the longitudinal axis of the housing 22 to provide a zigzag flow of the second fluid.

A prior art alternative to the above design shown in fig. 3 is a horizontal hairpin heat exchanger. The hairpin heat exchanger 1 has two housings 22 containing straight portions of U-shaped tubes. The head of the hairpin contains a 180 ° U-shaped bend of the tube. The advantages of this hairpin design are:

no joint expansion system is required, since thermal expansion is naturally governed by the hairpin design;

drainage and venting of the exchanger are easier due to the horizontal position of the straight pipes and the exchanger.

Different concepts of steam generators are known. Sandia (Sandia) reports 93-7084 "research on thermal storage and steam generator problems, becker corporation" reports a combination of these different concepts, listing the advantages and disadvantages of existing steam generators.

In order to increase the efficiency of heat transfer in heat exchangers, it has been known since the 1920 s that baffles mounted in a casing can have a specific shape intended to guide the fluid in a spiral path. Moreover, at the same shell side pressure drop, with the continuous spiral baffle, the heat Transfer rate was increased by about 10% compared to the conventional segmented baffle (j.heat Transfer (2007), vol.129(10), 1425-1431). This mode allows reducing the leakage flow occurring in the segmented baffle and further greatly increases the heat Transfer coefficient (j. heat Transfer (2010), vol.132(10), 101801). Furthermore, flow stratification and stagnant zones (according to calculations) are avoided, which allows for complete evacuation and reduced fouling sensitivity (lower fouling resistance and lower heat transfer area).

Document WO 2009/148822 discloses baffles mounted in a housing to direct fluid into a helical flow pattern at different helix angles as the baffles approach the inlet and outlet respectively. Documents US2,384,714, US2,693,942, US 3,400,758, US 4,493,368 and WO 2005/019758 each disclose each different type of baffle, but the same purpose is to provide a spiral flow pattern of the fluid. Document US 1,782,409 discloses a continuous spiral baffle.

These current solutions are not satisfactory, for example, in terms of thermal gradient flexibility, efficiency (pressure drop, heat transfer coefficient), drainage, natural circulation, etc., and the newly designed steam generator and/or its single heat exchanger should meet technical requirements, for example:

improved thermal efficiency by reducing internal leakage and bypass flow;

-improved pressure drop by reducing local water flow obstruction;

-improved uplift capacity;

-improved reliability;

improvement of fouling behaviour etc

Furthermore, the material and manufacturing costs of forced circulation evaporators are higher than those of natural circulation evaporators due to the capital costs of recirculation pumps.

Object of the Invention

The present invention aims to overcome the drawbacks of the prior art heat exchangers intended for steam generators.

In particular, the present invention aims to obtain a reduced-size evaporator exhibiting high flexibility in terms of thermal gradients and improved efficiency due to lower pressure drop, lower internal leakage (bypass), improved heat transfer coefficient, reduced fouling tendency, easy-to-discharge molten salts, natural circulation (i.e. without circulation pump), long service life, and competitive costs, thanks to an optimized hydrodynamic salt flow.

Another object of the invention is to avoid the use of thicker components, such as the current tube sheets, necessary in the shell-and-tube classical heat exchangers, which would lead to the drawback of high pressure zones adjacent to low pressure zones.

Disclosure of Invention

A first aspect of the invention relates to a hairpin heat exchanger having a first straight segment, a second straight segment and a curved segment connecting the first and second straight segments, each straight segment comprising a portion of a cylindrical first shell or cylindrical inner shell located inside said cylindrical second shell, both forming an inter-shell space enclosing a bundle of parallel U-bends each having a first straight portion and a second straight portion respectively located in said first and second straight segments of the exchanger and a 180 ° bend located in said curved segment of the exchanger, wherein, in use, a first fluid to be heated and evaporated is flowing, and a second shell or cylindrical outer shell respectively provided with an inlet at one end and an outlet for a second fluid at the other end, the second fluid is a hot heat transfer fluid and thus, in use, flows in an inter-shell space and is cooled by heat exchange with the first fluid flowing in the straight tubes, the inter-shell space also enclosing some baffles to direct the second fluid, wherein the bundle of parallel U-bends protrudes from the exchanger and is connected via bends outside the end of the inner shell and the end of the outer shell at the first straight section, respectively, to a first header distributing the first fluid to the bundle of straight tubes, and outside the end of the inner shell and the end of the outer shell at the second straight section, to a second header collecting the first fluid in the form of liquid, vapour or a liquid/vapour mixture from the bundle of straight tubes.

According to a preferred embodiment of the invention, the hairpin heat exchanger further comprises one or a suitable combination of the following features:

-the hairpin heat exchanger is horizontal and the flow of the second fluid is co-current or counter-current with respect to the flow of the first fluid therein;

-the first and second headers are straight and cylindrical, or spherical;

-the first fluid is a fluid comprising feedwater or supercritical carbon dioxide;

-the second fluid is a molten salt or a mixture of molten salts, diathermic oil or liquid sodium;

-the baffles are in the form of continuous helical baffles;

the baffle is assembled, preferably welded or bolted, to the cylindrical inner casing;

-providing tube sheets (tube sheets) between the first header, the second header and the hairpin sections of the exchanger comprising a cylindrical inner shell and a cylindrical outer shell, respectively;

the tube sheet is oval and provided with passages to allow the U-bends to pass sealingly through the tube sheet;

the tube plate is designed with a thickness suitable to withstand low pressures;

-sealing means are provided between the outer casing and the baffle;

the hairpin exchanger is equipped with a distribution jacket for uniformly feeding the second fluid from the heat-transfer fluid inlet to the heat exchanger;

the distribution jacket has a plurality of openings distributed over 360 ° on its inner surface, said openings preferably feeding the second fluid to the first turn of the helical baffle.

A second aspect of the invention relates to the use of a hairpin heat exchanger as described above as an evaporator.

A third aspect of the invention relates to the use of a hairpin heat exchanger as described above as a superheater.

A fourth aspect of the invention relates to the use of a hairpin heat exchanger as described above as a reheater or economizer.

A fifth aspect of the invention relates to the use of an evaporator, superheater, reheater and economizer as described above, forming at least one heat exchanger train in a Molten Salt Steam Generator (MSSG). Advantageously, the superheater, reheater and/or economizer are operated in countercurrent, while the evaporator is operated in cocurrent.

It is also within the scope of the invention for the molten salt steam generator to be a single-pass or forced circulation steam generator.

Drawings

Figure 1 diagrammatically shows components of a typical heat exchanger train for a molten salt steam generator.

Figure 2 schematically shows an embodiment of a "shell and tube" straight tube heat exchanger according to the prior art.

Fig. 3 shows a perspective view of a prior art horizontal hairpin generator.

Fig. 4A and 4B show a plan view and a front view, respectively, of a preferred embodiment of a heat exchanger according to the present invention.

Fig. 5 is a longitudinal sectional view of a heat exchanger according to the embodiment of fig. 4.

Fig. 6A and 6B show views corresponding to fig. 4, respectively, but with a support system of a heat exchanger.

Fig. 7 is a detailed longitudinal section of an exchanger according to the invention, centred on an oval tube plate.

Fig. 8A and 8B show a perspective view and a cross-sectional view, respectively, of the oval tubesheet described above.

Detailed Description

The present invention relates to a new design for a horizontal hairpin heat exchanger 1 as shown in fig. 4 to 8.

The heat exchanger has a reciprocating flow between two fluids. A first fluid, typically a mixture of water and steam, circulates through a first bundle of parallel horizontal straight tube sections 2 in a first straight portion of the hair clip, and then through a second bundle of parallel horizontal straight tube sections 2 in a second straight portion of the hair clip. The first bundle of tubes 2 is connected to the second bundle of tubes 2 by a 180 ° bend in the head of the hairpin, forming a U-shaped bend.

Supercritical carbon dioxide is another example of a first fluid useful in the present invention.

According to an alternative embodiment, a first bundle of straight tube sections may discharge fluid through a thick (thicker) tube housing into a valve cap, into which a second bundle of straight tube sections also terminates. Thus, according to this particular embodiment, these tubes do not have U-bend sections.

According to the invention, the bundle of tubes 2 in each straight section is located between a cylindrical inner housing 3 and a cylindrical outer housing 4, as shown in fig. 5.

The inner space 5 delimited by the two

housings

3,4 allows to keep a heat source, preferably a second fluid, within the annular flow path. The second fluid is a heat transfer fluid, such as molten salt that has been heated by a solar receiver at the apex of the CSP tower apparatus. By bringing the flow of heat transfer fluid into contact with these bundled tubes 2, the heat transfer fluid transfers heat to the parallel flowing first fluid flowing through the tubes 2. The first fluid and the second fluid may be co-current or counter-current without departing from the scope of the invention. Similarly, the heat source or second fluid may be any heat transfer fluid, such as water, thermal oil, liquid sodium, fluidized bed, and the like.

As shown in fig. 6, the cylindrical outer shell 4 or the distribution jacket connected thereto is provided at one end with an inlet nozzle 6, respectively an outlet nozzle 6, through which the heat-conducting fluid is led to enter, respectively leave, the heat exchanger 1. Similarly, an outlet nozzle 7, an inlet nozzle 7 are provided at the other end of the cylindrical outer casing 4, respectively, to discharge the cooled heat transfer fluid, introducing the hot fluid, respectively.

Advantageously, as mentioned above, the heat transfer fluid is uniformly distributed on the shell at 360 ° (inlet, circulation, fluid temperature) thanks to a distribution jacket (see below) located at the inlet nozzle of the heat exchanger.

In order to increase the heat exchange efficiency, as in fig. 6, in the straight part of the hairpin exchanger there is provided a space 5 with an enclosed continuous helical baffle 8, which allows to direct the flow of the heat transfer fluid. The heat transfer fluid then flows helically in a heat exchanger (e.g., an evaporator operating in natural circulation) between the inner and outer housings according to an annular flow path. The continuous spiral baffle configuration ensures that the second fluid flows slowly without any abrupt direction changes or dead zones, as in a heat exchanger with vertical flow baffles. In this way, the heat transfer rate is greatly increased and the pressure drop is greatly reduced compared to heat exchangers having conventional segmented baffles (see above).

According to one embodiment, the cylindrical inner housing 3 and the baffle 8 may be welded or bolted. In addition, sealing means may be provided between the outer casing 4 and the baffle 8 to avoid parasitic flows.

At each outer end of the straight portions of the hairpin exchanger, as shown in fig. 7, an annular bundle of parallel straight tubes 2 is connected to at least one cylindrical linear header 9, 10 via a suitable elbow 11 located in a position external to the inner and

outer housings

3, 4. The header axis is orthogonal to the hairpin heat exchanger axis.

More specifically, as shown in fig. 4 to 6, at the first end of the exchanger, the bundle of straight tubes 2 is connected to at least a first cylindrical linear header 9 or entry header 9, which supplies the straight tubes 2 with a first fluid, while at the second end of the exchanger, the first fluid running inside the bundle of tubes 2 is collected from the bundle of tubes 2 by at least a second cylindrical linear header 10 or exit header 10. When there are a large number of tubes 2 in the bundle, more than one entry header 9 or exit header 10 may be required.

Furthermore, as shown in fig. 7, the bundles of straight tubes 2 are connected to the inlet header 9 or to the outlet header 10 in the region outside the inner shell 3, the outer shell 4 of the hairpin exchanger by means of suitable bent tubes 11. Thus, the use of tube sheets and/or high pressure bulb collectors, valve covers and headers as in the prior art so-called "shell and tube" heat exchangers is avoided in the present invention because they are simply replaced with cylindrical headers that are moved outside the hairpin heat exchanger.

In shell-and-tube configurations, the first fluid (typically water) is typically at high pressure within a quasi-spherical vessel or plenum. On the other side of the tubesheet, the salt flowing around the tube bundle is kept at a low pressure, requiring a very thick tubesheet to withstand the pressure differential. The configuration of the invention provides an extension tube connected to a standard header (cylindrical, spherical, etc.) at the end of the exchanger, in which the high-pressure fluid circulates. This allows the thickness of the tube sheet to be reduced, with limited, if any, pressure. More specifically, in the rectangular cross-section of fig. 7, it can be seen that the pressure drop supported by the tube sheet 16 is controlled by the difference in the external (air) pressure 12 and the internal heat transfer fluid pressure 13.

According to one embodiment of the invention shown in fig. 7 and 8, the tube plate is preferably used in the form of an oval tube plate 16 or the like, which has apertures or channels 17 for the parallel tubes 2. The welding of the tubes 2 only to the oval tube sheet 16 is intended to ensure fluid tightness. For the reasons described above, these oval tubesheets 16 advantageously have a lower thickness than prior art flat tubesheets.

Today, customers often need to increase the speed of ascent and stoppage. Thicker vessel walls or headers are not suitable for accepting higher temperature gradients and are more prone to fatigue, thereby shortening the useful life of the heat exchanger. In this case, the present invention extends the life of the heat exchanger components.

Fig. 7 also shows a detailed view of an embodiment of the entry/exit distribution jacket 30 from the fluid inlet/outlet 6, 7 into the hairpin heat exchanger. The uniform distribution of the second fluid at the entry/exit in the heat exchanger is ensured by a series of distribution openings 31, which are located over the inside of the distribution jacket 30 by 360 °, preferably in the first turn 32 of the helical baffle 8.

The present invention is flexible and is intended to be applied to a range of heat exchanger designs used in MSSG technology, such as reheaters, superheaters, preheaters and evaporator plants, where all common parts are made according to the common heat exchanger design of the present invention.

As described above, the reduced temperature hot molten salt, for example, first flows in parallel through a reheater and superheater for recombination and into an evaporator, and then into a preheater/economizer in series.

In the current embodiment, the hot molten salt enters the system at high temperature, e.g., 563 ℃ and certainly below 565 ℃, which is the degradation temperature of commonly used molten salts. However, within the scope of the present invention, the thermally conductive fluid may withstand temperatures up to 700 ℃. All metal parts are advantageously made of stainless steel or precious metals, which can withstand temperatures as high as 600 ℃ or higher.

The temperature at which the cold salt leaves the preheater is typically in the range of 290 ℃ to 300 ℃ or above the minimum temperature, i.e. the freezing temperature of the heat transfer fluid (as low as 240 ℃ for molten salts such as sodium derivatives). Alternatively, in this case, any heat-conducting fluid, such as for example a heat-conducting oil, may be used instead of the molten salt, for example in the operating temperature range of 80 ℃ (condensation and/or crystallization temperature) to 380 ℃ (example degradation temperature).

The high pressure water flows in the tubes or pipes and not at the shell side, which results in a smaller thickness of the tube sheet and header/shell and therefore a higher thermal gradient capability.

Although the design of the exchanger according to the invention is optimized for natural circulation operation, it can also be used in single-pass or forced circulation steam generators.

REFERENCE SIGNS LIST

1 hairpin heat exchanger

2 straight pipe (segment)

3 cylindrical inner housing

4 cylindrical outer casing

5 space between the casings

6 heat transfer fluid inlet

7 heat transfer fluid outlet

8 spiral baffle

9 inlet straight collecting pipe

10 outlet straight header

11 pipe crimping (segment)

12 Low pressure first fluid (air)

13 Low pressure second fluid (molten salt)

14U-shaped elbow

15 high pressure fluid (water/steam)

16 oval tube plate

17 pipe channel

18 front cover

19 rear cover

20 support body

21 straight pipe

22 casing

23 shell side fluid intake

24 tube side fluid entry

25 tube side fluid outflow

26 shell side fluid outflow

27 tube sheet

28 baffle

29 tank or plenum or valve cover

30 distribution jacket

31 to the distribution opening of the first helical turn (or pitch) of the baffle

32 first helical turn of baffle

Molten salt inlet of 100 MSSG

101 MSSG reheater

Evaporator of 102 MSSG

103 MSSG energy saver

104 MSSG superheater

105 MSSG molten salt outlet

Claims

1. A hairpin heat exchanger (1) having a first straight section, a second straight section and a curved section connecting the first and second straight sections, each straight section comprising a portion of a cylindrical first shell or cylindrical inner shell (3) and a portion of a cylindrical second shell or cylindrical outer shell (4), the cylindrical first shell (3) being located inside the cylindrical second shell (4), both forming an inter-shell space (5) enclosing a bundle of parallel U-bends (2) each having a first straight portion and a second straight portion respectively in the first and second straight sections of the exchanger and a 180 DEG bend in the curved section of the exchanger, in use a first fluid to be heated and evaporated flowing therein, the cylindrical outer shell (4) being provided with an inlet (6) respectively at one end, and at the other end an outlet (7) for a second fluid, which is a hot heat-conducting fluid, whereby in use the second fluid flows in the inter-housing space (5) and is cooled by heat exchange with the first fluid flowing in the straight tubes (2), the inter-housing space (5) further enclosing a plurality of baffles (8) to guide the second fluid, wherein the bundle of parallel U-shaped bent tubes (2) protrudes from the exchanger and is connected via a bent tube (11) to a first header (9) outside an end of the inner housing (3) and an end of the outer housing (4) at the first straight section and to a second header (10) outside an end of the inner housing (3) and an end of the outer housing (4) at the second straight section, respectively, the first header distributing the first fluid to the bundle of straight tubes (2), the second header collects the first fluid in the form of liquid, vapour or a liquid/vapour mixture from the bundle of straight tubes (2).

2. The hairpin heat exchanger (1) according to claim 1, wherein the hairpin heat exchanger is horizontal and the flow of the second fluid is co-current or counter-current with respect to the flow of the first fluid.

3. The hairpin heat exchanger (1) according to claim 1, wherein the first and second headers (9, 10) are straight and cylindrical, or spherical.

4. The hairpin heat exchanger (1) according to claim 1, wherein the first fluid is a fluid comprising feedwater or supercritical carbon dioxide.

5. The hairpin heat exchanger (1) according to claim 1, wherein the second fluid is a molten salt or a mixture of molten salts, diathermic oil or liquid sodium.

6. The hairpin heat exchanger (1) according to claim 1, wherein the baffle (8) is in the form of a continuous spiral baffle.

7. The hairpin heat exchanger (1) according to claim 1, wherein the baffle (8) is assembled, preferably welded or bolted, to the cylindrical inner housing (3).

8. The hairpin heat exchanger according to claim 1, wherein a tube sheet (16) is provided between the first header (9), the second header (10) and the hairpin section of the exchanger containing the cylindrical inner and outer shells (3, 4), respectively.

9. Hairpin heat exchanger according to claim 8, wherein the tube sheet (16) is oval and provided with a passage (17) to allow the U-bend (2) to pass sealingly through the tube sheet (16).

10. Hairpin heat exchanger according to claim 8, wherein the tube sheet (16) is designed with a thickness suitable for withstanding low pressures.

11. Hairpin heat exchanger according to claim 1, wherein sealing means are provided between the outer shell (4) and the baffle (8).

12. The hairpin heat exchanger according to claim 1, wherein the hairpin heat exchanger is equipped with a distribution jacket (30) for uniformly feeding the second fluid from the heat transfer fluid inlet (6, 7) to the heat exchanger.

13. The hairpin heat exchanger according to claim 12, wherein the distribution jacket (30) has a plurality of openings (31) distributed over its inner surface by 360 °, the openings (31) preferably feeding the second fluid to a first turn (32) of the helical baffle (8).

14. Use of the heat exchanger according to claim 1 as an evaporator.

15. Use of the heat exchanger according to claim 1 as a superheater.

16. Use of the heat exchanger of claim 1 as a reheater or economizer.

17. Use of the evaporator according to claim 14, use of the superheater according to claim 15, and use of the reheater and/or economizer according to claim 16, forming at least one heat exchanger train in a Molten Salt Steam Generator (MSSG).

18. The use according to claim 17, wherein the superheater, the reheater and/or the economizer are operated counter-currently, while the evaporator is operated co-currently.

19. The use according to claim 17, wherein the molten salt steam generator is a single pass or forced circulation steam generator.