A molten salt heat exchange tube with an inlet structure and a molten salt heat exchange device comprising the same

Abstract

The application belongs to the technical field of energy storage, and particularly relates to a molten salt heat exchange pipe with an inlet structure and a molten salt heat exchange device comprising the same. The molten salt heat exchange pipe with the inlet structure comprises a pipe body, which is used for molten salt flow and heat exchange; one end of the pipe body is provided with an inlet structure, the inlet structure is a gradually enlarged diameter structure relative to the inner diameter of the pipe body, and a funnel-shaped inlet is formed; and a connecting structure is arranged between the two sides of the inlet structure. Without relying on additional energy input, the application improves the flow and heat exchange environment of sodium nitrate molten salt in the initial stage of entering the heat exchange pipe, and reduces the viscosity and crystallization tendency at the inlet. The molten salt heat exchange device improves the stability, uniformity and overall heat exchange efficiency of the molten salt heating process by arranging the enlarged diameter inlet structure at the inlet of the heat exchange pipe and cooperating with the heat conduction connecting structure, and simultaneously introducing the shell diameter change, pipe bundle partition design and asymmetric pipe box volume configuration at the device level.

Description

Technical Field

[0001] This invention belongs to the field of energy storage technology, specifically relating to a molten salt heat exchange tube with an inlet structure and a molten salt heat exchange device including the same. Background Technology

[0002] With the increasing proportion of renewable energy generation, the demand for large-scale, long-term energy storage technologies in energy systems is becoming increasingly urgent. Molten salt energy storage, as a technology that stores and releases energy in the form of sensible heat, has advantages such as large storage capacity, wide operating temperature range, high technological maturity, and suitability for coupling with systems such as solar thermal power generation, industrial waste heat recovery, and thermal power peak shaving. It has become one of the important development directions in the field of high-temperature energy storage. By heating molten salt and storing thermal energy during the storage phase, and then releasing the heat for power generation or heating during the release phase, molten salt energy storage systems can effectively improve energy utilization efficiency and enhance the flexibility of the power system, which is of great significance for building a new energy system.

[0003] In existing molten salt energy storage systems, nitrate-based molten salts are widely used as heat transfer and energy storage media due to their moderate melting point, high specific heat capacity, good chemical stability, and relatively low cost. Among them, sodium nitrate and its mixed molten salt with potassium nitrate are one of the most common molten salt systems in engineering applications. Sodium nitrate exhibits good thermal stability and heat transfer performance under high-temperature liquid conditions, making it suitable for long-term cyclic use in the medium- and high-temperature range. Therefore, it is widely used in molten salt energy storage and molten salt heat transfer systems.

[0004] Sodium nitrate has a high melting point. During system startup or low-load operation, the molten salt tends to be at a low temperature close to its melting point, at which point its viscosity increases significantly and its fluidity deteriorates markedly. Furthermore, when the temperature in a localized area is insufficient or the heat distribution is uneven, sodium nitrate is prone to crystallization, leading to pipe blockage, localized heat exchange failure, or even equipment damage. Therefore, ensuring that the molten salt heats up rapidly and uniformly upon entering the heat exchange unit, while avoiding the formation of a low-temperature, high-viscosity zone and the risk of crystallization in the inlet area, is a key technical challenge in the design of molten salt heat exchange systems.

[0005] Existing heat exchangers for heating molten salts mostly employ a shell-and-tube heat exchange structure, where the molten salt flows inside the heat exchange tubes and the heating medium flows in the shell side, with heat transfer achieved through the tube walls. While this type of heat exchanger is relatively mature in conventional fluid heat exchange scenarios, it still has shortcomings when directly applied to heating sodium nitrate molten salt. For example, traditional heat exchange tubes typically use a straight tube structure with a constant diameter. When the molten salt enters the heat exchange tube inlet, the flow path changes abruptly, and the velocity distribution becomes uneven, easily forming a low-velocity stagnation zone near the inlet. Simultaneously, the inlet section is usually located at the lowest temperature in the system, where heat exchange conditions are relatively weak. This makes it difficult for the molten salt to obtain sufficient heat in this region in a timely manner, further exacerbating viscosity increases and the tendency for crystallization.

[0006] In summary, sodium nitrate molten salt energy storage systems face problems such as low inlet temperature, high viscosity, and easy crystallization under heating conditions. These problems deteriorate the safety, stability, and heat exchange efficiency of the molten salt heating process. Summary of the Invention

[0007] To address the above problems, the present invention provides a molten salt heat exchange tube with an inlet structure, comprising a tube body for molten salt flow and heat exchange; an inlet structure at one end of the tube body, the inlet structure being an expansion structure that gradually increases in diameter relative to the inner diameter of the tube body, forming a funnel-shaped inlet; and a connecting structure between the two sides of the inlet structure.

[0008] This invention features a funnel-shaped expansion structure at the inlet end of the tube, gradually increasing in diameter relative to the inner diameter of the tube. This allows the sodium nitrate molten salt to smoothly transition from a larger flow cross-section to a normal diameter channel upon entering the heat exchange tube, avoiding the jetting effect and local backflow caused by abrupt changes in the flow path at a conventional constant-diameter inlet. This reduces the low-velocity stagnation zone at the inlet. For sodium nitrate molten salt, which has high viscosity and poor fluidity at low temperatures, this smoothly transitioning expansion inlet helps to form a more uniform and stable velocity distribution, enabling the molten salt to establish a continuous and controllable flow state at the inlet section, reducing the risk of crystallization caused by local stagnation. Simultaneously, the expansion structure increases the heated area of ​​the molten salt in the inlet region and extends its effective residence path at the inlet, allowing the molten salt, still in a low-temperature, high-viscosity state, to obtain more sufficient heat exchange conditions in the initial stage of entering the heat exchange tube, promoting a rapid temperature increase. Furthermore, the connecting structure between the two sides of the inlet structure, without blocking the flow path, shapes and constrains the flow field at the inlet, helping to suppress flow deviation and eddy development in the inlet section, making the molten salt flow more stable and uniform. By comprehensively regulating the inlet flow state as described above, this invention improves the flow and heat exchange environment of sodium nitrate molten salt in the initial stage of entering the heat exchange tube without relying on additional energy input, reduces the viscosity and crystallization tendency at the inlet, and enhances the safety, stability and heat exchange efficiency of the molten salt heating process.

[0009] Furthermore, the connecting structure is made of a thermally conductive material with a higher thermal conductivity than the tube body. This allows the inlet structure to actively introduce and distribute heat while controlling the flow state, further enhancing the thermal environment of the molten salt inlet region. Since sodium nitrate molten salt often exhibits a significant axial temperature gradient in the initial heating stage, and the inlet section is located at the low-temperature end of the overall heat exchange system, relying solely on convective heat transfer is insufficient to compensate for the insufficient heat in the inlet region. By using a thermally conductive material for the connecting structure, a stable solid thermal conduction channel is formed between the two sides of the inlet structure. This allows heat accumulated in the heated high-temperature areas or on the tube wall to be conducted along the connecting structure to the vicinity of the inlet. Thus, even under conditions of low molten salt flow rate and limited convective heat transfer, preheating of the inlet region can still be achieved. This design, which compensates for heat in the inlet section through solid thermal conduction, helps reduce the radial and axial temperature difference at the inlet, improves local temperature uniformity, and thereby reduces the risk of a sudden increase in viscosity and crystallization of sodium nitrate molten salt at the inlet due to insufficient temperature. Through the synergistic effect of the thermally conductive connection structure and the expanded inlet structure, this invention creates a more stable and continuous heat-fluid coupling environment in the inlet section, providing a strong guarantee for the smooth entry of molten salt into the heat exchange tube and the rapid establishment of effective heat exchange conditions.

[0010] Furthermore, the axis of the inlet structure is coaxial with the axis of the tube body, and the expansion angle of the inlet structure is a gradual expansion angle. This is beneficial for establishing a stable and controllable axisymmetric flow state in the initial stage of molten salt entering the heat exchange tube. The coaxial structure can avoid the molten salt from generating biased flow and asymmetric velocity distribution at the inlet, allowing the fluid to flow uniformly into the normal diameter channel along the tube body axis, thereby reducing local shear stress concentration and flow instability. The gradual expansion angle allows the molten salt to experience a smooth flow transition in the expansion inlet section, avoiding flow separation and the formation of backflow vortices caused by excessively rapid expansion. For sodium nitrate molten salt with high viscosity and poor fluidity at low temperatures, this coaxial gradual expansion structure can suppress the formation of low-velocity stagnation zones and dead zones in the inlet section, allowing the molten salt to form a continuous and stable mainstream channel when entering the heat exchange tube, promoting the uniform development of the velocity and temperature fields, thereby improving the heat exchange conditions in the inlet section, reducing the risk of crystallization, and improving the stability and heat exchange efficiency of the molten salt heating process.

[0011] Furthermore, by setting the ratio of the inlet structure length to the length of the normal diameter section in the tube to between 1:6 and 1:8, the inlet section provides sufficient space for flow shaping and pre-heat exchange without significantly increasing the overall tube length. On the one hand, this length ratio ensures a smooth transition of the molten salt before it enters the normal diameter tube section, preventing abrupt velocity changes and flow separation caused by an excessively short inlet section. On the other hand, it also prevents the inlet structure from becoming too long, thus avoiding the formation of a low-velocity stagnation zone or occupying the effective heat exchange length. This achieves a reasonable balance between flow stability and heat exchange efficiency, further reducing the viscosity and crystallization risk at the molten salt inlet.

[0012] Furthermore, setting the cross-section of the connecting structure to be rectangular, with the long side of the rectangle arranged along the axial direction of the tube, increases the effective stress-bearing area of ​​the connecting structure in the axial direction, improving its structural strength and wear resistance under long-term scouring conditions of high-temperature molten salt, and mitigating the wear on the edges of the connecting structure caused by tiny solid particles that may be entrained in the sodium nitrate molten salt. On the other hand, this orientation results in a relatively small projected size of the connecting structure in the radial direction, thus stabilizing and shaping the inlet flow field without significantly obstructing the main molten salt channel, which helps reduce the generation of local turbulence and irregular eddies. In addition, the continuous axial extension of the rectangular cross-section structure helps form a relatively stable flow boundary in the inlet region, guiding and constraining the molten salt flow, thereby further improving the flow uniformity and heat transfer conditions in the inlet section.

[0013] Furthermore, by configuring multiple connecting structures and arranging adjacent connecting structures perpendicular to each other in cross-section, a multi-directional synergistic flow shaping and support effect is achieved within the inlet structure. On one hand, multiple connecting structures can circumferentially separate and constrain the inlet channel, preventing overall flow deviation or unilateral concentration of molten salt in the inlet section and promoting a balanced velocity distribution in all directions. On the other hand, the perpendicular arrangement allows the connecting structures to function simultaneously in different radial directions, helping to suppress eddies and secondary flows developing in a single direction within the inlet section, enhancing the axisymmetry and stability of the flow field. In addition, this cross-arrangement improves the overall stiffness of the inlet structure, making the connecting structures less prone to deformation under long-term scouring conditions of high-temperature molten salt, thereby ensuring flow stability while further enhancing the reliability and consistency of the heat transfer process in the inlet region.

[0014] On the other hand, the present invention provides a molten salt heat exchange device for heating molten salt. The molten salt heat exchange device includes a shell, a heat exchange tube bundle disposed within the shell, a first tube box and a second tube box respectively disposed at both ends of the shell, and end caps for sealing the first tube box and the second tube box. The heat exchange tube bundle includes multiple heat exchange tubes for molten salt flow, and both ends of each heat exchange tube are connected to the first tube box and the second tube box respectively through a first tube sheet and a second tube sheet. The first tube box is provided with a molten salt inlet, and the second tube box is provided with a molten salt outlet. The shell is provided with a heating medium inlet and a heating medium outlet. In particular, at least one heat exchange tube is the aforementioned molten salt heat exchange tube with an inlet structure, and the inlet structure is located near the end of the molten salt inlet.

[0015] Furthermore, by setting a larger diameter at the molten salt inlet end and a smaller diameter at the molten salt outlet end, the shell diameter can be optimized to match the heat load and heat exchange requirements at different axial positions during molten salt heating. Since the molten salt has a lower temperature and higher viscosity at the inlet end, the requirements for heat exchange capacity and thermal stability are higher. Increasing the shell diameter at the inlet side expands the shell-side flow cross-sectional area and local heat capacity, reducing the flow velocity and residence time of the heating medium in this region, thereby enhancing the heat exchange intensity in the inlet area and improving the preheating conditions of the molten salt. At the molten salt outlet end, the molten salt temperature has significantly increased and its fluidity has enhanced, reducing the urgent need for heat exchange. A correspondingly smaller shell diameter can increase the flow velocity and scouring capacity of the heating medium, preventing redundant heat accumulation in the high-temperature zone and improving overall heat exchange efficiency. In summary, this shell diameter design, which varies from large to small along the molten salt flow direction, achieves a distribution of shell-side heat exchange capacity and molten salt heat demand over the entire heat exchange length, improving the stability and energy utilization efficiency of the molten salt heat exchanger under heating conditions.

[0016] Furthermore, by designing the heat exchange tubes located in the outer region of the heat exchange tube bundle with a larger diameter expansion inlet structure, the expansion ratio or expansion section length of the outer tubes is greater than that of the heat exchange tubes in the central region of the bundle. This helps to compensate for the non-uniformity of heat exchange and thermal environment at different locations within the tube bundle. Because the outer heat exchange tubes are closer to the inner wall of the shell, their inlet regions are more prone to forming relatively low-temperature and unstable flow zones due to factors such as shell heat dissipation, shell-side flow bypass, and weakened local heat exchange conditions. By implementing a stronger diameter expansion design at the inlet of the outer heat exchange tubes, the molten salt velocity gradient can be further reduced and the flow shaping effect enhanced in the inlet section. This allows high-viscosity molten salt to establish a stable and continuous flow state at the inlet of the outer tubes, thereby reducing the difference in salt inlet between each heat exchange tube. In summary, this differentiated inlet structure design helps improve the consistency of the overall inlet operating conditions of the tube bundle, suppresses the generation of local cold spots and crystallization risks, and thus improves the heat exchange uniformity, operational stability, and system reliability of the entire molten salt heat exchanger.

[0017] Furthermore, setting the volume of the first tube box to be larger than that of the second tube box facilitates the formation of a stable inlet buffer and flow equalization space before the molten salt enters the heat exchanger tube bundle. Since the first tube box is located on the molten salt inlet side, where the molten salt has a lower temperature and higher viscosity, the larger tube box volume reduces velocity fluctuations at the inlet, allowing the molten salt to diffuse sufficiently and achieve uniform distribution before entering multiple heat exchanger tubes. This avoids low-velocity stagnation zones at the inlet of individual heat exchanger tubes due to insufficient flow. Simultaneously, the increased volume of the first tube box also acts as a thermal buffer, reducing transient temperature and pressure fluctuations under inlet conditions and lowering the risk of molten salt crystallization and blockage at the inlet side. In contrast, the second tube box is located on the molten salt outlet side, where the molten salt has a higher temperature and better fluidity, resulting in a relatively lower demand for buffering and flow equalization. Therefore, designing the first tube box volume to be larger than that of the second tube box helps improve the operational stability and overall heat exchange reliability of the molten salt heat exchanger under heating conditions.

[0018] The beneficial effects of this invention are: (1) By setting an enlarged inlet structure at the inlet end of the heat exchange tube with a gradually increasing diameter relative to the inner diameter of the tube, the molten salt can achieve a smooth transition from a large cross section to a small cross section when entering the heat exchange tube, avoiding the jet effect, backflow and low-speed stagnation zone caused by the sudden change of flow channel at the traditional constant diameter inlet. This structure can establish a more stable and uniform inlet flow state when the molten salt is in a low temperature and high viscosity state, reduce the risk of local stagnation and crystallization, and improve the flow and heat exchange conditions of the molten salt in the initial stage of entering the heat exchange tube.

[0019] (2) The present invention sets the connection structure between the two sides of the inlet structure as a heat-conducting material, so that the inlet area has an additional heat transfer path on the basis of flow control, which can conduct heat from the heated area or the tube wall in the heat exchange tube to the inlet section, thereby enhancing the preheating effect of the inlet area. This design is particularly effective under the condition of low molten salt flow rate and limited convective heat transfer capacity, which helps to reduce the temperature gradient at the inlet, reduce the possibility of sudden increase in molten salt viscosity and crystal precipitation, and improve the stability of the heating process.

[0020] (3) The present invention sets the cross-section of the connecting structure as rectangular, and the long side of the rectangle is arranged along the axis of the tube body. This is beneficial to improve the wear resistance while ensuring the structural strength, and to reduce the local wear of the connecting structure caused by long-term scouring of high-temperature molten salt. In addition, the orientation of the cross-section makes the flow obstruction effect of the connecting structure in the radial direction smaller, which can guide and stabilize the flow field in the inlet section without significantly increasing the flow resistance, and further improve the flow uniformity and heat transfer conditions in the molten salt inlet area.

[0021] (4) The present invention has a shell with a larger diameter at the molten salt inlet and a shell with a smaller diameter at the molten salt outlet. This is beneficial to match the shell-side heat transfer capacity with the heat demand of the molten salt along the molten salt flow direction. Increasing the shell diameter at the inlet can reduce the flow rate of the heating medium and increase the local heat capacity, thereby strengthening the preheating conditions of the low-temperature molten salt; decreasing the shell diameter at the outlet helps to improve the scouring capacity of the heating medium, avoid heat transfer redundancy in the high-temperature area, and improve the overall heat transfer efficiency and operational stability.

[0022] (5) The present invention sets the volume of the first tube box to be larger than that of the second tube box, so that the molten salt forms a larger buffer and flow equalization space before entering the heat exchange tube bundle, which helps to reduce the flow velocity fluctuation at the inlet and improve the salt inlet uniformity among multiple heat exchange tubes; this structure can reduce the possibility of low-velocity stagnation zones at the inlet of individual heat exchange tubes and buffer the temperature and pressure fluctuations on the inlet side, thereby reducing the risk of molten salt crystallization and blockage, and improving the operational reliability of the molten salt heat exchange device under heating conditions. Based on the above beneficial effects, this invention has good application prospects in the field of energy storage technology. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a molten salt heat exchanger tube with an inlet structure.

[0024] Figure 2 This is a schematic diagram of an entrance structure.

[0025] Figure 3 This is a schematic diagram of a molten salt heat exchanger.

[0026] In the figure: 1. Molten salt heat exchange tube with inlet structure; 11. Tube body; 12. Inlet structure; 13. Connection structure; 2. Shell; 3. First tube sheet; 4. Second tube sheet; 5. First tube box; 6. Second tube box; 7. Molten salt inlet; 8. Molten salt outlet; 9. Heating medium inlet; 10. Heating medium outlet. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0028] This embodiment provides a molten salt heat exchange tube 1 with an inlet structure, which is suitable for a heat exchange unit for heating molten salt in a sodium nitrate molten salt energy storage system.

[0029] like Figure 1 and Figure 2As shown, the molten salt heat exchanger tube 1 with an inlet structure includes a tube body 11, which is a hollow straight tube structure. The interior of the tube body 11 forms a flow channel for molten salt flow and heat exchange. The tube body 11 is made of a high-temperature resistant and nitrate-corrosion-resistant metallic material, such as 316L stainless steel, Inconel alloy, or heat-resistant low-alloy steel. The outer diameter of the tube body is 25-50 mm, the wall thickness is 2-4 mm, and the length of the normal diameter portion of the tube body 11 is 1500-3000 mm.

[0030] An inlet structure 12 is provided at one end of the tube body 11. The inlet structure 12 is an expanding structure that gradually increases in diameter relative to the inner diameter of the tube body 11, and is funnel-shaped in general. The inlet structure 12 is coaxially arranged with the axis of the tube body 11, so that the molten salt can smoothly enter the heat exchange tube along the axial direction of the tube body. The expansion half-angle of the inlet structure 12 is a gradual expansion angle, preferably 5°-10°, to avoid flow separation or backflow in the inlet section. The ratio of the length of the inlet structure 12 to the length of the normal diameter portion of the tube body 11 is controlled within the range of 1:6 to 1:8. For example, when the length of the normal diameter portion of the tube body is 2400 mm, the length of the inlet structure is 300-400 mm.

[0031] A connecting structure 13 is provided between the two sides of the inlet structure 12. The connecting structure 13 spans across the opposite sides of the inner wall of the inlet structure to connect the structural areas on both sides of the inlet structure. The connecting structure 13 is made of a thermally conductive material, such as copper, copper alloy, or high thermal conductivity stainless steel, whose thermal conductivity is superior to that of the tube body 11 substrate, thereby enhancing the heat transfer capacity of the inlet area. The connecting structure 13 is integrally formed with the inlet structure 12 or is fixedly connected by welding. The location of the connecting structure 13 does not block the inlet flow channel, so as to ensure that the molten salt can flow smoothly into the interior of the tube body 11.

[0032] In this embodiment, when sodium nitrate molten salt enters the heat exchange tube from the inlet structure 12, it first forms a stable and uniform flow state in the expanded diameter inlet section, and with the assistance of the connecting structure 13, the thermal environment of the inlet area is improved. Then, it enters the normal diameter section of the tube body 11 to complete the further heat exchange process. This reduces the viscosity of the molten salt at the inlet and the risk of crystallization, and improves the stability and reliability of the molten salt heating process.

[0033] Example 2 Based on this embodiment 1, such as Figure 2 As shown, the cross-section of the connecting structure 13 is rectangular, and the longer side of the rectangular cross-section is set along the axial direction of the tube 11. Here, "the longer side is set along the axial direction" means that the longer side of the rectangular cross-section is parallel to the axis of the tube 11 in spatial orientation.

[0034] The rectangular cross-section has a length of 6-20mm and a width of 3-8mm, which enables the connection structure to have sufficient structural strength and wear resistance under long-term scouring conditions of high-temperature molten salt, while maintaining a small flow resistance dimension in the radial direction to avoid significant obstruction to the main flow channel of molten salt.

[0035] Furthermore, in this embodiment, the connecting structure 13 is provided in multiple ways. Adjacent connecting structures 13 are arranged perpendicularly to each other on the cross section of the entrance structure, specifically four, which are evenly distributed around the circumference of the entrance structure 12. Two adjacent connecting structures 13 extend in mutually perpendicular radial directions on the cross section, thereby forming a cross-support and multi-directional flow constraint structure inside the entrance structure.

[0036] Through the above settings, the rectangular cross section and the multiple interconnecting structures 13 arranged perpendicularly to each other improve the overall rigidity and durability of the inlet structure 12 under high temperature and high viscosity molten salt conditions while ensuring the smooth flow of the inlet channel. They also play a stabilizing and shaping role in the flow field of the inlet section, further improving the flow uniformity and heat exchange conditions of the molten salt in the initial stage of entering the heat exchange tube.

[0037] Example 3 Based on Example 2, the surface of the connecting structure 13 is provided with several protrusions. These protrusions are distributed along the length and / or circumferential direction of the connecting structure 13, and are used to disturb the near-wall flow of molten salt in the inlet region, thereby weakening the stable boundary layer formed by the low-temperature, high-viscosity molten salt on the surface of the connecting structure and reducing the occurrence of local low-velocity stagnation. By providing the protrusions, the flow activity and heat transfer effect in the inlet region can be enhanced without significantly increasing the overall flow resistance, improving the temperature uniformity of the inlet section and reducing the risk of molten salt crystallization. The shape and size of the protrusions can be rationally designed according to actual working conditions to avoid the formation of attachment initiation points for molten salt crystallization while improving flow and heat transfer conditions.

[0038] Example 4 This embodiment provides a molten salt heat exchange device for heating molten salt, which is suitable for energy storage or start-up heating conditions in sodium nitrate molten salt energy storage systems.

[0039] like Figure 3 As shown, the molten salt heat exchanger includes a shell 2, a heat exchange tube bundle disposed within the shell 2, a first tube box 5 and a second tube box 6 respectively disposed at both ends of the shell 2, and end caps for sealing the first tube box 5 and the second tube box 6. The shell 2 is a vertically arranged cylindrical shell, and its material is selected from high-temperature resistant and nitrate corrosion resistant carbon steel, low alloy heat-resistant steel or stainless steel. The inner diameter of the shell is 300-800mm, and the shell length is 2-5m, which is used to form the shell-side space for the flow of the heating medium.

[0040] The heat exchange tube bundle is disposed inside the shell 2 and consists of multiple heat exchange tubes 1, which are arranged parallel to each other along the axial direction of the shell 2. The heat exchange tubes are made of the same high-temperature and corrosion-resistant metal material as in Examples 1-3, such as stainless steel 316L or heat-resistant alloy steel, with an outer diameter of 20-40 mm and a wall thickness of 2-4 mm. The number of heat exchange tubes can be set to 20-100 depending on the heat exchange capacity requirements. Both ends of each heat exchange tube are fixed and sealed by the first tube sheet 3 and the second tube sheet 4, respectively, so that the heat exchange tube bundle and the shell 2 form a reliable tube-side and shell-side isolation structure.

[0041] The first tube box 5 is located at the lower end of the shell 2, and the second tube box 6 is located at the upper end of the shell 2. The first tube box 5 has a molten salt inlet 7, and the second tube box 6 has a molten salt outlet 8. The volume of the first tube box 5 is 1.2-2.0 times that of the second tube box 6, so as to form a stable buffer and flow equalization space before the molten salt enters the heat exchange tube bundle, reduce the flow rate fluctuation of the low temperature and high viscosity molten salt at the inlet, and improve the uniformity of salt inlet among multiple heat exchange tubes. The second tube box 6 is mainly used to collect the heated molten salt, and its relatively small volume is sufficient to meet the outflow requirements.

[0042] The shell 2 has a heating medium inlet 9 at the top and a heating medium outlet 10 at the bottom. The heating medium can be saturated steam, superheated steam, or high-temperature heat transfer oil. The heating medium flows from top to bottom through the shell side and forms countercurrent heat exchange with the molten salt flowing from bottom to top in the heat exchange tube, so as to improve the overall heat exchange efficiency and improve the preheating conditions on the molten salt inlet side.

[0043] In this embodiment, at least one heat exchanger tube in the heat exchanger tube bundle adopts a molten salt heat exchanger tube 1 with an inlet structure as in Embodiments 1-3. Its inlet structure 12 is located near the first tube box 5, i.e., the molten salt inlet 7, allowing the molten salt to establish a stable flow state through the enlarged inlet structure in the initial stage of entering the heat exchanger tube. With the assistance of the connecting structure 13, the flow and heat exchange environment of the inlet section are improved. The remaining heat exchanger tubes can be conventional equal-diameter heat exchanger tubes, or, depending on system requirements, some or all of them can be molten salt heat exchanger tubes 1 with inlet structures.

[0044] In this molten salt heat exchanger, low-temperature sodium nitrate molten salt enters the first tube box 5 from the molten salt inlet 7. After buffering and flow equalization, it enters the heat exchange tube bundle. In the heat exchange tube 1 with the inlet structure, the flow shaping and preliminary heating of the inlet section are completed first. Then, it fully exchanges heat with the shell-side heating medium in the normal diameter tube section. Finally, it is collected by the second tube box 6 and discharged from the molten salt outlet 8, thereby achieving stable and efficient heating of the molten salt.

[0045] Preferably, an ultrasonic transducer is installed on the outer side of the end cap on one side of the molten salt inlet 7, and the ultrasonic transducer is fixedly connected to the outer surface of the end cap. Since the end cap has a continuous rigid connection with the tube box, tube sheet, and heat exchange tube inlet section, the high-frequency vibration generated by the ultrasonic transducer can be transmitted to the tube box and heat exchange tube inlet area through the end cap structure, causing the heat exchange tubes near the inlet structure 12 to generate micro-vibrations with small amplitude and controlled frequency. This micro-vibration can periodically disturb the flow state of the molten salt in the near-wall region of the inlet section, weakening the stability of the thermal boundary layer, improving the flow activity and temperature uniformity of the low-temperature, high-viscosity molten salt at the inlet, thereby enhancing the heat exchange effect of the inlet section and reducing the risk of crystallization and adhesion. Arranging the ultrasonic transducer on the outer side of the end cap allows for non-contact vibration excitation of the inlet area without damaging the sealing structure of the heat exchange device or exposing the transducer directly to the high-temperature molten salt environment, thus balancing engineering reliability and operational safety.

[0046] Example 5 Based on Example 4, the shell 2 is configured as a variable diameter structure along the molten salt flow direction, that is, the shell section with a larger diameter at the end near the molten salt inlet 7 and the shell section with a smaller diameter at the end near the molten salt outlet 8.

[0047] Specifically, the shell 2 includes a first shell section and a second shell section connected sequentially along the axial direction. The first shell section is located at the position corresponding to the molten salt inlet 7, and its inner diameter is 500-800 mm. The second shell section is located at the position corresponding to the molten salt outlet 8, and its inner diameter is 300-600 mm. The first shell section and the second shell section can be connected by a conical transition section or a stepped transition section to avoid significant separation or sudden changes in local pressure drop in the flow within the shell side.

[0048] By setting a larger shell diameter at one end of the molten salt inlet 7, a larger flow cross-sectional area and heat capacity are formed in the shell side in this region, thereby reducing the flow velocity of the heating medium and extending its residence time in the inlet region, which is beneficial to enhancing the preheating conditions of the low-temperature molten salt. Conversely, reducing the shell diameter at the molten salt outlet 8 can increase the flow velocity and scouring capacity of the heating medium in the high-temperature region, avoiding redundant heat exchange capacity. This achieves a match between the shell-side heat exchange capacity along the molten salt flow direction and the molten salt heat demand, improving the overall heat exchange efficiency and operational stability of the molten salt heat exchanger under heating conditions.

[0049] Example 6 Based on Example 5, the heat exchange tube bundle is divided into a peripheral region and a central region according to its spatial location, wherein the heat exchange tube located in the peripheral region is configured with an enlarged inlet structure 12.

[0050] Specifically, the outer region of the tube bundle refers to one or more turns of heat exchange tubes adjacent to the inner wall of the shell, accounting for 20%-40% of the total number of heat exchange tubes; the central region of the tube bundle comprises the remaining heat exchange tubes located within the aforementioned outer region. For heat exchange tubes located in the outer region, the expansion ratio of their inlet structure 12 is set to 1.3-1.6, or the length of their expansion section is 300-450 mm; while for heat exchange tubes located in the central region, the expansion ratio of their inlet structure is set to 1.1-1.3, or the length of their expansion section is 200-300 mm. The aforementioned expansion ratio is the ratio of the maximum inner diameter of the inlet structure 12 to the normal inner diameter of the tube body 11.

[0051] By incorporating a larger diameter inlet structure into the heat exchange tubes in the outer region of the tube bundle, the flow shaping capability and the inlet transition path at the molten salt inlet are enhanced. This helps to compensate for potential issues such as weaker heat exchange conditions and uneven temperature distribution in the outer heat exchange tubes due to their proximity to the inner wall of the shell, thereby reducing the risk of low-velocity stagnation zones and cold spots forming at the inlet of the outer heat exchange tubes. This differentiated inlet structure design improves the consistency of salt inlet among multiple heat exchange tubes, enhances the uniformity of flow and heat exchange conditions in the entire heat exchange tube bundle inlet section, and further improves the operational stability and overall heat exchange performance of the molten salt heat exchanger under heating conditions.

[0052] Example 7 Based on Example 6, the volume of the first tube box 5 is set to be greater than the volume of the second tube box 6 in order to enhance the buffering and flow equalization capacity of the molten salt inlet side.

[0053] Specifically, the first tube box 5 is located at the end where the molten salt inlet 7 is located, and its effective internal volume is 0.05-0.20 cubic meters; the second tube box 6 is located at the end where the molten salt outlet 8 is located, and its effective internal volume is 0.03-0.12 cubic meters. Preferably, the volume of the first tube box 5 is 1.2-2.0 times the volume of the second tube box 6. The axial height and radial dimensions of the first tube box 5 are correspondingly increased to form a larger inlet cavity space, allowing the low-temperature, high-viscosity molten salt to fully diffuse and achieve uniform distribution before entering the heat exchange tube bundle.

[0054] In summary, this invention addresses the key engineering challenges faced by molten salt in heating systems, such as low inlet temperature, high viscosity, and easy crystallization. It proposes a systematic improvement scheme from two levels: the heat exchanger tube inlet structure 12 and the overall device structure. By incorporating an expanded-diameter inlet structure 12 and a connecting structure 13 at the heat exchanger tube inlet, the flow state and thermal environment of the molten salt inlet are synergistically optimized. Simultaneously, at the device level, variations in shell diameter, differentiated tube bundle design, and asymmetrical tube box volume configuration are introduced, enabling the low-temperature molten salt to establish stable and uniform flow and heat exchange conditions from the initial stage of entering the heat exchange unit. This invention can improve the operational stability and heat exchange efficiency of molten salts such as sodium nitrate during the heating process, reduce the risk of crystallization and blockage, and is applicable to fields such as molten salt energy storage, solar thermal power generation, industrial waste heat recovery, and high-temperature heat transfer systems. It is of great significance for improving the safety, reliability, and engineering application value of high-temperature energy storage systems.

[0055] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A molten salt heat exchange device for heating molten salt, comprising a shell, a heat exchange tube bundle disposed within the shell, a first tube box and a second tube box respectively disposed at both ends of the shell, and end caps for sealing the first tube box and the second tube box; the heat exchange tube bundle includes multiple heat exchange tubes for molten salt flow, and both ends of each heat exchange tube are connected to the first tube box and the second tube box respectively through a first tube sheet and a second tube sheet; the first tube box has a molten salt inlet, and the second tube box has a molten salt outlet; the shell is provided with a heating medium inlet and a heating medium outlet; characterized in that: At least one heat exchange tube is a molten salt heat exchange tube with an inlet structure. The molten salt heat exchange tube with an inlet structure includes a tube body for molten salt flow and heat exchange. One end of the tube body is provided with an inlet structure, which is an expansion structure that gradually increases in diameter relative to the inner diameter of the tube body, forming a funnel-shaped inlet. A connecting structure is provided between the two sides of the inlet structure, and the inlet structure is close to the molten salt inlet end. The heat exchange tubes located in the outer region of the heat exchange tube bundle have larger expansion inlet structures. The expansion ratio of the expansion inlet structure of the heat exchange tubes in the outer region is greater than that of the expansion inlet structure of the heat exchange tubes in the center region of the tube bundle, or the expansion section length of the expansion inlet structure of the heat exchange tubes in the outer region is greater than that of the expansion section length of the expansion inlet structure of the heat exchange tubes in the center region of the tube bundle.

2. The molten salt heat exchanger as described in claim 1, characterized in that: The thermal conductivity of the connecting structure is higher than that of the tube body.

3. The molten salt heat exchanger as described in claim 1, characterized in that: The axis of the inlet structure is coaxial with the axis of the pipe body, and the expansion angle of the inlet structure is a gradual expansion angle.

4. The molten salt heat exchanger as described in claim 1, characterized in that: The ratio of the length of the inlet structure to the length of the normal diameter portion of the tube is 1:6 to 1:

8.

5. The molten salt heat exchanger as described in claim 1, characterized in that: The cross-section of the connecting structure is rectangular, and the long side of the rectangle is along the axial direction of the tube body.

6. The molten salt heat exchanger as described in claim 5, characterized in that: The connection structure consists of multiple structures, with adjacent connection structures arranged perpendicular to each other in cross-section.

7. The molten salt heat exchanger as described in claim 1, characterized in that: The shell has a larger diameter at the molten salt inlet end and a smaller diameter at the molten salt outlet end.

8. The molten salt heat exchanger as described in claim 1, characterized in that: The volume of the first pipe box is greater than the volume of the second pipe box.

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