Straight tube structure and baffle optimization method for molten salt electric heater

Abstract

The application discloses a straight pipe structure and baffle optimization method for a molten salt electric heater, the straight pipe structure comprising a cylindrical shell, the two sides of the cylindrical shell being respectively provided with a molten salt inlet and a molten salt outlet which are communicated with the inner cavity of the cylindrical shell, the two ends of the cylindrical shell being respectively provided with an inlet pipe plate and an outlet pipe, the outer side of the inlet pipe plate being provided with a straight pipe type electric heating pipe bundle and a junction box connected with the straight pipe type electric heating pipe bundle, the two ends of the straight pipe type electric heating pipe bundle being respectively sealed and fixed on the inlet pipe plate and the outlet pipe plate, the inner cavity of the cylindrical shell being alternately provided with baffles in the length direction thereof, the baffles being provided with pipe holes through which the straight pipe type electric heating pipe bundle passes, the baffles located at the upper part of the inner cavity of the cylindrical shell comprising at least one truncated baffle, the end of the truncated baffle close to the inner wall of the cylindrical shell being cut off by a predetermined length and forming a transverse flow guide notch, and the temperature field uniformity can be greatly improved and the risk of over-temperature can be eliminated without increasing the flow resistance.

Description

Technical Field

[0001] This invention relates to the field of molten salt energy storage technology, and in particular to a straight tube structure and baffle optimization method for molten salt electric heaters. Background Technology

[0002] Molten salt electric heaters are key equipment in the thermoelectric decoupling process of thermal storage systems. The principle of molten salt electric heaters is the same as that of traditional tubular electric heaters, significantly improving the electro-thermal conversion efficiency and reducing energy waste. Existing shell-and-tube molten salt electric heaters often achieve good heat transfer through baffles, but this structure generates significant flow resistance, increasing power consumption. Furthermore, a large flow dead zone appears in the shell side, making it easy for the wall temperature of the heating tubes to exceed the upper limit of the molten salt's operating temperature due to insufficient heat transfer, resulting in uneven heating and localized overheating of the molten salt. Currently, the main industrial solution to the localized overheating problem in molten salt electric heaters is multi-stage segmented heating, but this method suffers from problems such as large equipment steps, large footprint, and high equipment investment costs.

[0003] The high-temperature characteristics and changes in the physicochemical properties of molten salt during use pose significant challenges to electric heaters. Taking solar salt as an example, when the molten salt temperature exceeds 580℃, it decomposes rapidly, producing nitrite and oxygen. This not only alters the physicochemical properties of the molten salt and destroys its thermal storage performance, but also severely affects the service life of the electric heater, adversely impacting the safety of the thermal storage system.

[0004] To enhance heat transfer, alternating upper and lower arc-shaped baffles are often used in the shell side. However, at high Reynolds numbers, the flow dead zone formed between the baffle ends and the shell becomes particularly prominent, easily leading to localized molten salt overheating and decomposition, threatening system safety. Existing technologies include creating small holes in the baffles to guide the jet flow and dissipate the dead zone (e.g., CN118031420A), but for large-scale equipment, the perturbation range of the small holes is limited. A more direct and efficient structure is needed to improve the uniformity of the shell-side flow field in large equipment. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a straight-tube structure and baffle optimization method for a molten salt electric heater. This straight-tube structure for the molten salt electric heater, by setting a truncated baffle and forming a stable transverse flow channel, can effectively carry out the retained high-temperature molten salt and mix it with the mainstream. Without significantly increasing the flow resistance, it can greatly improve the uniformity of the temperature field and eliminate the risk of overheating.

[0006] A straight-tube structure for a molten salt electric heater includes a cylindrical shell. A molten salt inlet and a molten salt outlet communicating with the inner cavity are respectively provided on both sides of the cylindrical shell. An inlet tube sheet and an outlet pipe are respectively installed at both ends of the cylindrical shell. A straight-tube electric heating tube bundle and a junction box connecting the straight-tube electric heating tube bundle are installed on the outer side of the inlet tube sheet. The two ends of the straight-tube electric heating tube bundle are respectively sealed and fixed to the inlet tube sheet and the outlet tube sheet. Baffles are alternately arranged vertically along the length of the inner cavity of the cylindrical shell. The baffles have tube holes through which the straight-tube electric heating tube bundle passes. The baffle located above the inner cavity of the cylindrical shell includes at least one truncated baffle. The end of the truncated baffle near the inner wall of the cylindrical shell is cut off by a predetermined length to form a transverse flow guiding notch.

[0007] As a preferred embodiment of the above technical solution, the cut-off length of the cut-off baffle is 10-100mm.

[0008] As a preferred embodiment of the above technical solution, the perforations are uniformly arrayed on the baffle plate.

[0009] As a preferred embodiment of the above technical solution, the cylindrical shell is supported and positioned by two sets of saddles arranged in parallel.

[0010] As a preferred embodiment of the above technical solution, the two ends of the cylindrical shell are respectively connected to the inlet tube sheet and the outlet tube sheet via mating flanges. The molten salt inlet is located on the side of the shell near the end of the inlet tube sheet, and the molten salt outlet is located on the side of the shell near the end of the outlet tube sheet.

[0011] An optimization method for baffles in any of the above-mentioned straight-tube structures, the specific optimization process is as follows:

[0012] S1, determine the arrangement spacing and quantity of the baffles;

[0013] S2, determine the number of cut-off baffles 8;

[0014] S3, determine the curve of the length of the baffle to be cut off and the proportion of the overheated region;

[0015] S4, select the length of the baffle that needs to be cut off, where the overheated region in the curve has the smallest proportion.

[0016] As a preferred embodiment of the above technical solution, in step S3, an optimization model of the curve corresponding to the lowest point of the curve is established. The input of the optimization model is the electric heating power and the molten salt inlet flow rate. The output of the optimization model is the highest temperature and the optimal point is selected.

[0017] As a preferred embodiment of the above technical solution, in step S4, the proportion of the overheated region is the ratio of the volume of molten salt with a temperature higher than 580°C in the molten salt electric heater to the total volume of molten salt in the molten salt electric heater.

[0018] The beneficial effects of this invention are as follows:

[0019] A straight-tube electric heating tube bundle is used, with its two ends fixed to both ends of the shell via inlet and outlet tube sheets, respectively. The structure is stable and facilitates individual tube maintenance. A predetermined length is cut off at the end of one or more baffles near the upper part of the shell-side flow channel, thereby forming a stable transverse flow guide channel at that location.

[0020] Molten salt flows in an "S" shape within the shell side, guided by baffles. When passing through the truncated baffles, a portion of the fluid no longer completely bypasses the baffle ends but flows directly laterally through the transverse guide channels. This transverse fluid acts like a "fluid scraper," directly penetrating and sweeping across the traditional dead zone behind the downstream baffle, severely disrupting the vortex structure there and effectively carrying out the retained high-temperature molten salt to mix with the mainstream. Combined with the straight-tube layout, the heat load distribution along the heating tubes can be more easily optimized through zoned power control, matching the flow field optimized by the truncated baffles, achieving end-to-end temperature uniformity control from "structural guidance" to "power matching." This design is intuitive to modify and easy to manufacture, making it particularly suitable for large-scale equipment. It significantly improves temperature field uniformity and eliminates the risk of overheating without substantially increasing flow resistance. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of the electric heater in the embodiment.

[0022] Figure 2 This is a schematic diagram of the cut-off baffle plate in the embodiment.

[0023] Figure 3 This is a three-dimensional surface plot showing the change in the proportion of the overheated region with the number of baffles and the cut-off length.

[0024] The attached figures are labeled as follows: 1-cylindrical shell, 2-molten salt inlet, 3-molten salt outlet, 4-inlet tube sheet, 5-junction box, 6-straight tube electric heating tube bundle, 7-baffle plate, 8-cut-off baffle plate, 9-lateral guide notch, 10-saddle. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0026] like Figure 1 , Figure 2 The diagram shows a straight-tube structure for a molten salt electric heater, comprising a cylindrical shell 1. Molten salt inlets 2 and molten salt outlets 3, respectively, are provided on both sides of the cylindrical shell 1, communicating with its inner cavity. An inlet tube sheet 4 and an outlet pipe are respectively installed at both ends of the cylindrical shell 1. A straight-tube electric heating tube bundle 6 and a junction box 5 connecting the straight-tube electric heating tube bundle 6 are installed on the outer side of the inlet tube sheet 4. The two ends of the straight-tube electric heating tube bundle 6 are respectively sealed and fixed to the inlet tube sheet 4 and the outlet tube sheet. Baffles 7 are alternately arranged vertically along the length of the inner cavity of the cylindrical shell 1. The baffles 7 have holes through which the straight-tube electric heating tube bundle 6 passes. The baffle 7 located above the inner cavity of the cylindrical shell 1 includes at least one truncated baffle 8. The end of the truncated baffle 8 near the inner wall of the cylindrical shell 1 is cut off by a predetermined length to form a transverse flow guiding notch 9.

[0027] In this embodiment, the cut-off length of the truncated baffle 8 is 10-100mm. Specifically, a flow dead zone is easily formed behind the top baffle 7 of the molten salt electric heater. The cut-off section of the upper truncated baffle 8 can form a jet by a certain length, which shortens the residence time of the high-temperature molten salt in the molten salt electric heater. At the same time, the fluid of the jet has a entrapment effect on the surrounding fluid, reducing or eliminating the flow dead zone behind the baffle 7.

[0028] In this embodiment, the tube holes are uniformly arrayed on the baffle plate 7.

[0029] In this embodiment, the cylindrical shell 1 is supported and positioned by two sets of saddles 10 arranged in parallel.

[0030] In this embodiment, the two ends of the cylindrical shell 1 are respectively connected to the inlet tube sheet 4 and the outlet tube sheet via mating flanges. The molten salt inlet is located on the side of the shell near the end of the inlet tube sheet 4, and the molten salt outlet 3 is located on the side of the shell near the end of the outlet tube sheet.

[0031] An optimization method for baffles in any of the above-mentioned straight-tube structures, the specific optimization process is as follows:

[0032] S1, determine the arrangement spacing and quantity of baffles 7;

[0033] S2, determine the number of cut-off baffles 8;

[0034] S3, determine the curve of the length of the baffle 7 to be cut off and the proportion of the overheated region;

[0035] S4, select the length of the baffle 7 that needs to be cut off, where the overheated region has the smallest proportion in the curve. The three-dimensional surface plot showing the change in the overheated region proportion with the number of baffles and the cut-off length is shown below. Figure 3 As shown.

[0036] In this embodiment, in step S3, an optimization model of the curve corresponding to the lowest point of the curve is established. The input of the optimization model is the electric heating power and the flow rate of the molten salt inlet 2. The output of the optimization model is the highest temperature and the optimal point is selected.

[0037] In this embodiment, in step S4, the proportion of the overheated region is the ratio of the volume of molten salt with a temperature higher than 580°C in the molten salt electric heater to the total volume of molten salt in the molten salt electric heater.

[0038] During use, molten salt fluid enters the electric heater shell from molten salt inlet 2. Part of the molten salt fluid passes through the transverse guide notch 9 and generates a jet. After leaving the transverse guide notch 9, the molten salt fluid quickly forms turbulence, which shortens the residence time of the high-temperature molten salt at the top in the molten salt electric heater. At the same time, the jet fluid has a suction effect on the surrounding fluid, reducing or eliminating the stagnant dead zone on the back of the baffle 7. In addition, the transverse guide notch 9 increases the transverse flow area of ​​the shell-side fluid, reduces the flow resistance of the molten salt, reduces scale and corrosion, and improves the overall heat transfer performance of the molten salt to a certain extent. Another part of the fluid flows around the cut-off baffle 8 and the molten salt fluid flows out through the molten salt outlet 3.

[0039] The following description is based on specific embodiments.

[0040] Example 1

[0041] Electric heater power: 20kW / m2, molten salt temperature rises from 290℃ to 560℃, molten salt inlet flow rate 2 is 24.44kg / s.

[0042] Before the upper end of the baffle 7 is cut off, high-temperature molten salt accumulates at the top of the molten salt electric heater, and the molten salt forms a "heat transfer dead zone" due to stagnation at the back of the baffle 7, further hindering the flow and heat transfer of the molten salt. The cut-off portion at the upper end of the baffle 8 forms a transverse flow guide notch 9. Molten salt flows through this notch, forming a jet, which shortens the residence time of the high-temperature molten salt at the top in the molten salt electric heater. Simultaneously, the jet fluid has a vortex effect on the surrounding fluid, reducing or eliminating the stagnation dead zone at the back of the baffle 7. Furthermore, the presence of the transverse flow guide notch 9 increases the transverse flow velocity of the molten salt, which reduces the flow resistance of the molten salt and improves the overall heat transfer performance of the molten salt to a certain extent.

[0043] Before the upper part of the cut-off baffle 8 was removed, numerous molten salt overheating areas appeared on the back of the three rear baffles 7 above the molten salt electric heater, with the highest temperature reaching 607.85℃. When the temperature exceeds 580℃, the solar salt decomposes and deteriorates rapidly. Therefore, without optimization, the molten salt electric heating system posed a safety risk of overheating. After the baffle 7 was cut off and optimized, the overheating areas on its back were significantly reduced, and the temperature uniformity was greatly improved, eliminating the molten salt overheating phenomenon.

[0044] Example 2

[0045] Electric heater power: 5kW / m2-40kW / m2, molten salt inlet 2 flow rate is 4.07 kg / s-32.59 kg / s, molten salt temperature is increased from 290℃ to 560℃, and the cut-off length of the cut-off baffle 8 is 25mm.

[0046] To further enhance the uniformity of temperature distribution in the molten salt electric heater, the uniformity of molten salt temperature distribution under different system operating parameters was verified. The molten salt inlet 2 temperature was 290℃, the molten salt outlet temperature was designed to be 560℃, the power of the straight tube section of the electric heating rod was 5kW / m2-40kW / m2, and the molten salt inlet 2 flow rate was 4.07 kg / s-32.59 kg / s.

[0047] The uniformity of molten salt distribution is higher under low electric heating power and low flow rate. When the system operating parameters change from electric heater power and molten salt flow rate of 30kW / m2 and 24.44kg / s to 22.5kW / m2 and 18.33kg / s, the maximum temperature of molten salt will drop from 607.85℃ to 579.75℃. At this time, the uniformity of molten salt distribution is greatly improved.

[0048] This invention, by removing a certain length of the upper baffle and optimizing the electric heating power and flow rate, significantly improves the temperature distribution uniformity of molten salt under high power and high flow conditions while ensuring structural reliability, fundamentally suppressing local overheating. Through the innovative combination of a straight tube structure and the truncated baffle 8, supplemented by a multi-parameter optimization method, this invention provides a solution for large-scale, high-power molten salt electric heaters that offers high thermal uniformity, high reliability, and easy maintenance.

[0049] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A straight tube structure for a molten salt electric heater, characterized in that: The device includes a cylindrical shell with a molten salt inlet and an outlet on each side communicating with its inner cavity. An inlet tube sheet and an outlet pipe are installed at each end of the cylindrical shell. A straight-tube electric heating tube bundle and a junction box connecting the straight-tube electric heating tube bundle are installed on the outer side of the inlet tube sheet. The two ends of the straight-tube electric heating tube bundle are sealed and fixed to the inlet tube sheet and the outlet tube sheet, respectively. Baffles are alternately arranged vertically along the length of the inner cavity of the cylindrical shell. The baffles have holes through which the straight-tube electric heating tube bundle passes. The baffle located above the inner cavity of the cylindrical shell includes at least one truncated baffle, the end of which near the inner wall of the cylindrical shell is cut off by a predetermined length to form a transverse flow guide notch.

2. A straight tube structure for a molten salt electric heater according to claim 1, characterized in that: The cut-off length of the cut-off baffle is 10-100mm.

3. A straight tube structure for a molten salt electric heater according to claim 1, characterized in that: The perforations are evenly distributed in an array on the baffle plate.

4. A straight tube structure for a molten salt electric heater according to claim 1, characterized in that: The cylindrical shell is supported and positioned by two sets of parallel saddles.

5. A straight tube structure for a molten salt electric heater according to claim 1, characterized in that: The two ends of the cylindrical shell are connected to the inlet tube sheet and the outlet tube sheet respectively through mating flanges. The molten salt inlet is located on the side of the shell near the inlet tube sheet, and the molten salt outlet is located on the side of the shell near the outlet tube sheet.

6. A method for optimizing a baffle plate in a straight-tube structure as described in any one of claims 1-5, characterized in that: The specific optimization process is as follows: S1, determine the arrangement spacing and quantity of the baffles; S2, determine the number of cut-off baffles 8; S3, determine the curve of the length of the baffle to be cut off and the proportion of the overheated region; S4, select the length of the baffle that needs to be cut off, where the overheated region in the curve has the smallest proportion.

7. The baffle optimization method according to claim 7, characterized in that: In step S3, an optimization model for the curve corresponding to the lowest point of the curve is established. The inputs to the optimization model are the electric heating power and the molten salt inlet flow rate. The output of the optimization model is the highest temperature and the optimal point is selected.

8. The baffle optimization method according to claim 7, characterized in that: In step S4, the overheated region is the ratio of the volume of molten salt with a temperature higher than 580°C in the molten salt electric heater to the total volume of molten salt in the molten salt electric heater.

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