A variable-tube-number shell-and-tube heat exchanger for cold-end heat exchange in an SCO2 power cycle

Abstract

The present invention belongs to the field of heat exchange, and specifically discloses a variable-tube-number shell-and-tube heat exchanger for cold-end heat exchange in an SCO2 power cycle, which can improve the problem of mismatch between heat transfer performance and heat exchanger caused by excessive changes in the physical properties of supercritical carbon dioxide. The main structure includes: a shell, a tube sheet head, heat exchange tubes, a baffle plate for dividing the flow path, and a baffle plate. The shell is provided with an inlet and an outlet for the cold-side fluid, the head tube sheet is provided with an inlet and an outlet for the hot-side fluid, and the baffle plate in the tube sheet divides the tube side into several flow paths with different tube numbers. In the present invention, the supercritical carbon dioxide working medium exchanges heat with the shell-side fluid in the tube side with variable tube numbers, and adapts to the change of the physical properties of the fluid in the tube side by changing the tube numbers in the tube side, thereby improving the overall heat transfer performance of the heat exchanger.

Description

Technical Field

[0001] The present invention belongs to the field of heat exchangers, and particularly relates to a variable-tube-number shell-and-tube heat exchanger for cold-end heat exchange in a supercritical carbon dioxide power cycle. Background Art

[0002] Supercritical carbon dioxide (SCO2) has good stability and heat conduction characteristics, and carbon dioxide has a wide source and low cost. The Brayton cycle using supercritical carbon dioxide as the circulating working fluid can greatly reduce the power consumption of the compressor and improve the efficiency of the cycle by utilizing the physical property change characteristics near the critical point. At the same time, the size of the system can be reduced. During the cooling process of SCO2, as the temperature approaches the critical point, its physical properties will change violently, and this change has an important impact on the performance of supercritical carbon dioxide Brayton power generation. The variable-tube-number shell-and-tube heat exchanger is a heat exchanger that changes the number of heat exchange tubes in the tube side on the basis of the traditional shell-and-tube heat exchanger to adapt to the physical property change characteristics of supercritical carbon dioxide in the critical region. Compared with the traditional shell-and-tube heat exchanger, this heat exchanger has a smaller size and better heat exchange performance.

[0003] In the cold-end heat exchanger of the supercritical carbon dioxide power cycle, the physical properties of the SCO2 working fluid change greatly within the pressure and temperature ranges of the heat exchanger operation. The heat transfer coefficient of supercritical carbon dioxide is small in the region far from the critical point and high near the critical point, while the physical properties of the cooling water change little during the whole heat exchange process and the change of the heat transfer coefficient is not large, resulting in the mismatch between the heat transfer coefficient in the tube side and the heat transfer coefficient in the shell side, which affects the overall heat exchange performance of the heat exchanger. Summary of the Invention

[0004] The purpose of the present invention is to provide a variable-tube-number shell-and-tube heat exchanger for cold-end heat exchange in the SCO2 power cycle, so as to improve the problem of mismatch between the heat transfer coefficients on the cold and hot sides caused by the excessive change of the local heat exchange performance of the heat exchanger due to the violent change of the physical properties of the supercritical carbon dioxide working fluid, and thus solve the problem of excessive change of the local heat exchange performance of the heat exchanger caused by the change of the physical properties of the supercritical carbon dioxide working fluid.

[0005] To solve the above problems, the present invention adopts the following technical solutions:

[0006] A variable-tube-number shell-and-tube heat exchanger for the cold-end heat exchange of an SCO2 power cycle, which can improve the problem of mismatch between heat transfer performance and the heat exchanger caused by excessive changes in the physical properties of supercritical carbon dioxide, and improve the overall heat transfer performance of the heat exchanger, including: a shell 1, with the main heat exchange space for the hot fluid and the cold fluid inside, and a cold-side fluid inlet 2 and a cold-side fluid outlet 3 are provided on the shell; a head tube box 4, located on both sides of the shell 1, with a hot-side fluid inlet 6 and a hot-side fluid outlet 5; tube sheets 7, located inside the shell 1 and the head tube box 4, separating the cavities where the cold-side fluid and the hot-side fluid are located, tube holes 8 through which heat exchange tubes pass are provided on the tube sheets, and a baffle plate groove 9 is provided, which divides the heat exchange area into different regions according to the number of tube passes, and the hot-side fluid inlet 6 is communicated with the region with the fewest number of tube passes, and the hot-side fluid outlet 5 is communicated with the region with the most number of tube passes, and the hot fluid enters the corresponding tube pass for heat exchange under the action of the baffle plate groove 9; a baffle 10, arranged radially inside the shell 1 to make the cold-side fluid flow in a curve, enhance the disturbance of the cold-side fluid, and strengthen the cold-side heat transfer; the hot fluid flowing through the heat exchange tubes is a variable-property working medium.

[0007] Further, supercritical carbon dioxide working medium flows in the heat exchange tubes and exchanges heat with the cold-side fluid, and the tube pitch of the heat exchange tubes is equal.

[0008] Further, the tube holes on the tube sheet 7 connected to the heat transfer tubes have the same diameter and are arranged in an overall equilateral triangle pattern; the baffle plate groove 9 is asymmetrically distributed on the tube sheet 7; the baffle plate groove 9 divides the tube layout area into 4 to 6 mutually independent regions A1, A2, A3, A4... with different numbers of tubes, and the number of heat exchange tubes corresponding to each region is n1, n2, n3, n4... respectively.

[0009] Further, the heat exchange tubes are all straight tubes with equal diameters and the same thickness, and according to the change in the physical properties of the heat exchange working medium, the number distribution of the heat exchange tubes in different tube passes, that is, in different regions, changes accordingly.

[0010] Further, n1 < n2 < n3 < n4 <....

[0011] Further, 0.6 < n i / n i+1 < 1, where i = 1, 2, 3, 4....

[0012] Further, pressure sensors and temperature detectors are arranged in each of the regions A1, A2, A3, A4... for detecting pressure and temperature.

[0013] In the present invention, by changing the form that each process of the traditional shell-and-tube heat exchanger has the same tube layout method to the form that the number of tubes per pass changes according to the change in the physical properties of the hot-side fluid, the problem of mismatch between heat transfer performance and the heat exchanger caused by excessive changes in the physical properties of the hot-side fluid is effectively solved, and the overall heat transfer performance of the heat exchanger is improved.

[0014] To more clearly and precisely illustrate the above features and objectives of the present invention, the following provides preferred embodiments in conjunction with the accompanying drawings for detailed description as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1a It is a schematic three-dimensional structure diagram of an embodiment of the present invention.

[0016] Figure 1b It is a schematic diagram of the tube sheets at both ends.

[0017] Figure 2a It is a schematic diagram of the split arrangement of the tube sheets at the fluid inlet and outlet ends.

[0018] Figure 2b It is a schematic diagram of the split arrangement of the tube sheets at the other end.

[0019] Figure 3 It is a graph showing the variation of the heat transfer coefficients in the tube side and shell side along the heat transfer process when a traditional shell-and-tube heat exchanger is used for supercritical carbon dioxide-water heat transfer.

[0020] Figure 4 It is a graph showing the variation of the heat transfer coefficients in the tube side and shell side along the heat transfer process when the present invention is used for supercritical carbon dioxide-water heat transfer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] To make the main features, specific functions, and ultimate objectives of the present invention clearer and easier to understand, the present invention will be further elaborated below in combination with specific examples and corresponding diagrams.

[0022] As Figure 1a and Figure 1b shown, a variable-tube-number shell-and-tube heat exchanger for cold-end heat transfer in an SCO2 power cycle according to the present invention includes: a shell 1 with an internal main heat transfer space for hot and cold fluids, and a cold-side fluid inlet 2 and a cold-side fluid outlet 3 are provided on the shell; a head tube box 4 located on both sides of the shell 1, with a hot-side fluid inlet 6 and a hot-side fluid outlet 5 provided; tube sheets 7 located in the shell 1 and the head tube box 4, separating the cavities where the cold-side and hot-side fluids are located. Tube holes 8 through which heat exchange tubes pass are provided on the tube sheets, and split partition grooves 9 are provided to divide the heat exchange area into different regions according to the number of tube passes. The hot-side fluid inlet 6 is communicated with the region with the fewest number of tube passes, and the hot-side fluid outlet 5 is communicated with the region with the most number of tube passes. The hot fluid enters the corresponding tube passes for heat exchange under the action of the split partition grooves 9; baffle plates 10 are arranged radially in the shell 1 to make the cold-side fluid flow in a curved path, enhance the disturbance of the cold-side fluid, and strengthen the cold-side heat transfer; the hot fluid flowing through the heat exchange tubes is a variable-property working fluid.

[0023] Figure 2a and Figure 1bAs shown in the figure, it is a schematic diagram of the tube sheet split arrangement of the embodiment of the present invention. The number of tube passes of the shell-and-tube heat exchanger in the example of the present invention is 6. For a traditional shell-and-tube heat exchanger, the number of tubes in each process is roughly equal, while in the present invention, the number of heat exchange tubes in each tube pass, that is, the six regions A1, A2, A3, A4, A5, and A6, increases along the tube pass.

[0024] Figure 3 It is a graph showing the change of the heat transfer coefficients on the tube side and the shell side along the heat transfer process when a traditional shell-and-tube heat exchanger is used for supercritical carbon dioxide-water heat transfer. It can be seen from the figure that when a traditional shell-and-tube heat exchanger is applied to supercritical carbon dioxide, due to the drastic change in the physical properties of supercritical carbon dioxide during the entire heat transfer process, there will be a serious mismatch between the heat transfer coefficient on the carbon dioxide side and the heat transfer coefficient on the water side during the entire heat transfer process, resulting in the overall heat transfer coefficient being lower than the heat transfer coefficient on either side, and the overall heat transfer performance of the heat exchanger deteriorates.

[0025] To solve the above problems, the present invention considers improving the mismatch between the heat transfer coefficients on the carbon dioxide side and the water side and enhancing the overall heat transfer performance by redesigning the number of heat exchange tubes in each process of the tube side on the basis of a traditional shell-and-tube heat exchanger.

[0026] According to the embodiment of the present invention, the cold-side fluid enters the tube box space from the cold-side fluid inlet 2 on the shell, and flows out from the cold-side fluid outlet 3 after completing heat transfer in the internal space of the shell; the hot-side fluid enters the heat exchanger from the hot-side fluid inlet 6 on the head tube box 4, flows through six processes with unequal numbers of tubes, exchanges heat with the cold fluid through the tube wall in the shell space, and leaves the heat exchanger from the hot-side fluid outlet 5 after completing heat transfer.

[0027] By changing the heat exchange tubes with equal numbers in each pass of the traditional shell-and-tube heat exchanger to a shell-and-tube heat exchanger with an increasing number of tubes along the process, the embodiment of the present invention can significantly improve the serious mismatch problem between the heat transfer coefficients on the supercritical carbon dioxide side and the water side during the entire heat transfer process, thereby enhancing the overall heat transfer performance of the heat exchanger.

[0028] According to the embodiment of the present invention, the number of heat exchange tubes in each process is n1, n2, n3, n4, n5, and n6 respectively;

[0029] According to the embodiment of the present invention, the relationship between the number of heat exchange tubes in each process is 0.6 < n i / n i+1 < 1, where i = 1, 2, 3, 4, 5;

[0030] Example 1

[0031] Taking the heat exchange between supercritical carbon dioxide and water as an example, the physical properties of supercritical carbon dioxide change drastically during the entire heat exchange process. The temperature of supercritical carbon dioxide is far from the critical state at the inlet section, and the heat transfer coefficient is small. As the heat exchange progresses, its temperature approaches the critical temperature, and the heat transfer coefficient increases rapidly. In contrast, the change in the heat transfer coefficient on the water side during the heat exchange process can be ignored. During the entire heat exchange process, there is a serious mismatch between the heat transfer coefficients on the carbon dioxide side and the water side, resulting in the overall heat transfer coefficient being lower than that of either side, and the overall heat transfer performance of the heat exchanger deteriorates.

[0032] Figure 2 is a schematic diagram of the tube sheet split flow arrangement of the embodiment of the present invention. The number of tube passes of the shell-and-tube heat exchanger in the embodiment of the present invention is 6. The number of heat exchange tubes in each pass changes with the process. Specifically, the number of heat exchange tubes increases along the process.

[0033] In the initial stage of supercritical carbon dioxide heat exchange, the temperature is far from the critical point, and the heat transfer coefficient is small, lower than that of the water side. By reducing the number of heat exchange tubes in the initial process, the heat transfer coefficient of supercritical carbon dioxide is appropriately increased, and the difference in heat transfer coefficients between the cold and hot sides is reduced.

[0034] In the latter stage of heat exchange, the temperature of carbon dioxide decreases and approaches the critical temperature, and the heat transfer coefficient continuously increases, getting closer to and even higher than that of the water side. As the temperature of carbon dioxide continues to decrease, the difference in heat transfer coefficients between the cold and hot sides continues to expand. By increasing the number of heat exchange tubes in the latter process of heat exchange, the heat transfer coefficient of carbon dioxide can be appropriately reduced, and the difference in heat transfer coefficients between the cold and hot sides can be reduced.

[0035] In the embodiment of the present invention, the number of heat exchange tubes in each process gradually increases. The specific relationship between the number of tubes in each pass is: 0.6 < n i / n i+1 < 1.

[0036] By designing a shell-and-tube heat exchanger with an increasing number of tubes along the process, the mismatch problem between the heat transfer coefficients on the carbon dioxide side and the water side in the entire heat exchange process can be significantly improved, thereby enhancing the overall heat transfer performance of the heat exchanger.

Claims

1. A variable-tube-number shell-and-tube heat exchanger for cold-end heat exchange in an SCO2 power cycle, which can improve the problem of mismatch between heat transfer performance and the heat exchanger caused by excessive changes in the physical properties of supercritical carbon dioxide, and improve the overall heat transfer performance of the heat exchanger. It is characterized in that, Comprising: A shell (1) with a main heat exchange space for hot fluid and cold fluid inside. The shell is provided with a cold-side fluid inlet (2) and a cold-side fluid outlet (3); a head tube sheet (4) located on both sides of the shell (1), provided with a hot-side fluid inlet (6) and a hot-side fluid outlet (5); a tube sheet (7) located inside the shell (1) and the head tube sheet (4), separating the cavities where the cold-side fluid and the hot-side fluid are located. The tube sheet is provided with tube holes (8) through which heat exchange tubes pass, and provided with partition plate grooves (9) that divide the heat exchange area into different regions according to the number of tube passes. The hot-side fluid inlet (6) is communicated with the region with the least number of tube passes, and the hot-side fluid outlet (5) is communicated with the region with the most number of tube passes. The hot fluid enters the corresponding tube passes for heat exchange under the action of the partition plate grooves (9); a baffle plate (10) is arranged radially inside the shell (1) to make the cold-side fluid flow in a curved manner, enhance the disturbance of the cold-side fluid, and strengthen the cold-side heat exchange; the hot fluid flowing through the heat exchange tubes is a variable-property working medium; The tube holes on the tube sheet (7) connected to the heat transfer tubes have the same diameter and are arranged in an overall equilateral triangle pattern; the partition plate grooves (9) are asymmetrically distributed on the tube sheet (7); the partition plate grooves (9) divide the tube layout area into 4 to 6 mutually independent regions A1, A2, A3, A4... with different numbers of tubes, and the number of heat exchange tubes corresponding to each region is n1, n2, n3, n4... respectively; The heat exchange tubes are all straight tubes with equal tube diameters and the same thickness. According to the change of the physical properties of the heat exchange working medium, the number distribution of the heat exchange tubes in different tube passes, that is, in different regions, changes accordingly, n1 < n2 < n3 < n4 <....

2. The variable-tube-number shell-and-tube heat exchanger according to claim 1, characterized in that: Supercritical carbon dioxide working medium flows in the heat exchange tubes and exchanges heat with the cold-side fluid, and the tube pitch of the heat exchange tubes is equal.

3. The variable-tube-number shell-and-tube heat exchanger according to claim 1, characterized in that: 0.6 < n i / n i+1 < 1, where i = 1, 2, 3, 4… 4. The variable-tube-number shell-and-tube heat exchanger according to claim 1, characterized in that: A pressure sensor and a temperature detector are arranged in each of the regions A1, A2, A3, A4...

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