An outer-baffled shell-and-tube heat exchanger

Abstract

The application discloses an outer-bend flow tube shell heat exchanger, which comprises a shell, two tube plates and a head fixedly connected with the shell, a plurality of heat exchange tubes, a shell side inlet, a shell side outlet, a tube side inlet and a tube side outlet; characterized in that a plurality of partitions are fixedly arranged in the shell to form a plurality of independent heat exchange chambers, and the heat exchange tubes all pass through the partitions; two adjacent heat exchange chambers are connected through a flow guide pipe fixedly connected to the outer side wall of the shell, and the flow guide pipes are fixedly connected to the outer side walls of the opposite sides of the shell in sequence and at intervals; the application can effectively avoid the formation of flow dead zones on the shell side and increase the effective heat exchange area, thereby improving the heat exchange efficiency, effectively reducing the risk of temperature overrun, being easy to manufacture, being low in cost, being high in shell volume utilization, improving the overall economy of the equipment and being easy to manufacture large-scale heat exchangers.

Description

Technical Field

[0001] This invention relates to a shell-and-tube heat exchanger, and more particularly to an externally baffled shell-and-tube heat exchanger. Background Technology

[0002] A heat exchanger, also known as a heat exchanger, is a key piece of equipment widely used in chemical, petroleum, power, food, and many other industrial production processes. Its basic function is to facilitate heat transfer between fluids at different temperatures, effectively transferring some of the heat from the hot fluid to the cold fluid to achieve process objectives such as heating, cooling, evaporation, or condensation. Heat exchangers play an indispensable role in improving production efficiency, optimizing energy utilization, and ensuring product quality.

[0003] A shell-and-tube heat exchanger is an indirect heat exchanger that uses the wall surface of a tube bundle enclosed in a shell as the heat transfer surface. Shell-and-tube heat exchangers have become the most widely used type of heat exchanger due to their significant advantages, such as simple structure, reliable operation, low cost, wide flow cross-section, and ease of scale removal. Their structure mainly consists of components such as the shell, tube sheet, end caps, heat exchange tubes, and baffles (or baffle rods). The baffles or baffle rods create flow obstructions within the shell, which are crucial factors affecting heat transfer. In the actual operation of shell-and-tube heat exchangers, a common problem is the existence of flow and heat transfer dead zones on the shell side. Heat transfer dead zones refer to areas where heat transfer efficiency is significantly reduced due to poor fluid flow or structural design defects. These areas not only affect the overall performance of the heat exchanger but may also accelerate scaling, further reducing heat transfer efficiency and increasing maintenance costs.

[0004] Although different forms of baffles or baffle rods are widely used in heat exchangers, each form may cause dead zone problems to varying degrees.

[0005] Single-bow baffle: The dead zone created by this type of baffle is usually located on the leeward side of the baffle, because a low-speed zone is formed behind the baffle when the fluid goes around it, causing the fluid to stagnate;

[0006] Double-bow baffles: Compared to a single-bow design, double-bow baffles guide fluid flow in both directions, reducing (but not eliminating) the dead zone on the leeward side. However, dead zones may still appear in certain areas between the two baffles, especially when the baffle spacing is large.

[0007] Disc-shaped baffle: Disc-shaped baffles guide the fluid circumferentially and have a smaller leeward dead zone than bow-shaped baffles. However, due to the disc-shaped baffle structure, a dead zone will appear in the central region of this design, because the flow velocity in the central part is lower when the fluid flows around the disc surface, which easily forms a stagnant zone.

[0008] Spiral baffles: This is a new type of baffle design that causes fluid to flow in a spiral pattern within the heat exchanger. In principle, this avoids the short-circuiting and dead-zone problems inherent in traditional baffles, and ensures uniform fluid distribution within the tube bundle, improving heat transfer efficiency and reducing local temperature gradients. It offers advantages such as uniform flow, low pressure drop, and low vibration and noise. However, in practical engineering, non-continuous spiral baffles still pose a dead-zone risk at the leeward side of the discontinuous structure's junction. Continuous spiral baffles, with their central helix angle reaching 90 degrees, cannot be perforated on the spiral surface, preventing the baffle from covering the shell-side center region. This results in a short circuit in the center, affecting heat transfer and relatively reducing the effective heat transfer area. Furthermore, the diameter of the ineffective center region increases with the baffle pitch. On the other hand, the complex three-dimensional structure of spiral baffles makes their manufacturing process more difficult than traditional baffles, especially in the production of large-scale industrial equipment where high precision is required, leading to manufacturing difficulties and increased costs.

[0009] Baffles: Baffles are typically designed to guide the fluid through small-scale local velocity fluctuations (turbulence, coherent structures) within the tube bundle, thereby improving heat transfer efficiency. Although the dead zone of a baffle is significantly smaller than that of a traditional baffle plate, the change in fluid path as the fluid flows through the baffle creates low-velocity dead zones upstream or downstream of the baffle. Furthermore, baffles have limited effect on fluid disturbance, resulting in relatively lower heat transfer efficiency compared to baffle plates. This leads to increased overall equipment size and cost, and the possibility of failing to meet design requirements under extreme operating conditions.

[0010] The aforementioned different forms of baffle structures have their own advantages and disadvantages in heat exchangers, but all suffer from dead zone problems to varying degrees. Although optimizing the shape, spacing, and flow guiding structure of the baffles can reduce some of the dead zone, it cannot completely solve this problem. While existing technologies can barely meet the established functional requirements, their performance often involves certain compromises. Facing increasingly complex and changing working environments and emerging application demands, a typical technical challenge in heat exchanger applications is the use of HITEC molten salt (KNO3-NaNO2-NaNO3 (53%-40%-7%)) as a medium for energy storage and peak shaving heat exchange equipment. HITEC molten salt is a low-melting-point molten salt widely used in industrial fields. Due to its excellent energy storage characteristics, it has recently been used as a heat storage and heat transfer medium for energy storage and peak shaving in thermal power plants. Energy storage and peak shaving involves extracting steam as a heat source to heat the molten salt medium and store it when there is surplus steam for power generation. During peak power generation periods, the heat stored in the molten salt is released to provide heating and generate electricity. Energy storage and peak shaving can significantly improve the economics of power plants and further meet the needs of grid load fluctuations and external heating. HITEC molten salt has low freezing and decomposition temperatures, thus its operating temperature is strictly limited. The crystallization temperature of HITEC molten salt is 142℃, and its decomposition temperature is 454℃. Because the NaNO2 in its composition is easily oxidized, its freezing point will further increase. Therefore, in engineering, the typical operating temperature of this molten salt medium is 190℃-450℃. Exceeding 450℃ carries the risk of chemical decomposition, generating nitrogen oxides and nitrogen gas. When using HITEC molten salt as a heat exchange medium, precise temperature control of the molten salt medium is crucial for system safety. In coal-fired power plant energy storage and peak shaving projects, the steam parameters of mainstream supercritical units are higher than 550℃, while the steam parameters of newly built ultra-supercritical units are higher than 600℃. Considering heat exchanger design, equipment safety, and other factors, low-temperature, low-pressure molten salt media should ideally be used in the shell side of the heat exchanger to reduce equipment costs and ensure equipment safety. However, in existing heat exchangers, molten salt tends to form flow dead zones when it flows through the shell side, resulting in low local flow velocity or recirculation. This reduces the local heat exchange performance outside the tubes, causing the heat exchange tube wall temperature to be closer to the steam temperature inside the tubes. The local temperature of the heat exchange tube surface in contact with the molten salt medium exceeds the limit, and the molten salt decomposes. This shows limitations in terms of heat exchange efficiency, durability, and safety. Summary of the Invention

[0011] The purpose of this invention is to address the aforementioned shortcomings of the prior art by providing an externally baffled shell-and-tube heat exchanger that effectively avoids the formation of flow dead zones on the shell side and increases the effective heat exchange area, thereby improving heat exchange efficiency, effectively reducing the risk of temperature exceeding limits, and is easy to manufacture, has low cost, high shell volume utilization, improves the overall economy of the equipment, and is easy to manufacture large heat exchangers.

[0012] To achieve the above objectives, the present invention provides an externally baffled shell-and-tube heat exchanger, comprising a shell, two tube sheets and end caps fixedly connected to the shell, several heat exchange tubes, a shell-side inlet, a shell-side outlet, a tube-side inlet, and a tube-side outlet; characterized in that: several baffles are fixedly disposed inside the shell to form several independent heat exchange chambers, and the heat exchange tubes all pass through each baffle; two adjacent heat exchange chambers are connected by a drain pipe fixedly connected to the outer side wall of the shell, and each drain pipe is sequentially and spaced apart on the outer side wall of opposite sides of the shell.

[0013] This invention utilizes several baffles fixed inside the shell and several drainage pipes installed outside the shell. The medium inside the shell flows sequentially through each heat exchange chamber, transferring the dead zone of the original baffle plate to the outside of the shell. This creates a through-flow region inside the shell, avoiding the problem of dead zones on the leeward side of the traditional baffle plate flow region. Furthermore, all tube bundles inside the shell are placed in the through-flow region, which not only increases the effective heat exchange area and improves the shell volume utilization rate, but also improves the external convective heat transfer efficiency. This makes the surface temperature of the heat exchange tubes closer to the shell-side medium, effectively reducing the risk of temperature exceeding limits. Moreover, the drainage pipes are easy to manufacture and have low cost, improving the overall economy of the equipment. The manufacturing difficulty is independent of the equipment size, making it easy to manufacture large heat exchangers.

[0014] As a further improvement of the present invention, the axes of the shell-side inlet, shell-side outlet and each drain pipe are in the same plane on both sides of the shell; this can ensure that the medium forms pure flow through the shell, further improving the heat exchange efficiency.

[0015] As a further improvement of the present invention, the inner diameter of the drain pipe is 20% larger than the distance between the two partitions of the heat exchange chamber to which it is connected; so as to ensure that the medium is more evenly distributed along the axial direction of the equipment after entering the heat exchange chamber through the drain pipe, and the surface temperature of the heat exchange pipe is closer to that of the shell-side medium, effectively reducing the risk of temperature exceeding the limit.

[0016] As a further improvement of the present invention, the axial distance between the axis of the shell-side inlet, the shell-side outlet, and the inlet and outlet of each drain pipe and the axial distance between the axis of the inlet and outlet of each drain pipe and the axial distance between the two sides of the partition is less than 70% of the distance between the two sides of the partition; to prevent each inlet and outlet from being relatively offset in the axial direction of their respective heat exchange chambers, and to ensure that the medium is relatively evenly distributed in the axial direction after entering the heat exchange chamber.

[0017] As a further improvement of the present invention, the maximum distance between the baffles on both sides of each heat exchange chamber is no more than 50% of the minimum distance; when the distance between the baffles on both sides of each heat exchange chamber is adjusted independently according to the thermal performance requirements, it can prevent the medium pressure fluctuation in each heat exchange chamber from being too large, ensure the uniformity of heat exchange, and prevent the temperature from exceeding the limit.

[0018] In summary, this invention can effectively avoid the formation of flow dead zones on the shell side and increase the effective heat exchange area, thereby improving heat exchange efficiency, effectively reducing the risk of temperature exceeding limits, and is easy to manufacture, has low cost, high shell volume utilization, improves the overall economy of the equipment, and is easy to manufacture large heat exchangers. Attached Figure Description

[0019] Figure 1 This is a front view of Embodiment 1 of the present invention.

[0020] Figure 2 This is a partial view of a drainage tube with a different structure used in Example 1.

[0021] Figure 3 This is a partial view of a drainage tube with a different structure used in Example 1.

[0022] Figure 4 This is a front view of Embodiment 2 of the present invention.

[0023] Figure 5 This is a front view of Embodiment 3 of the present invention.

[0024] Figure 6 for Figure 5 Top view. Detailed Implementation

[0025] The invention will be further described below with reference to the accompanying drawings.

[0026] Example 1

[0027] like Figure 1 As shown, this embodiment of an external baffle shell-and-tube heat exchanger includes a shell 1, two tube sheets 2 and end caps 3 fixedly connected to both ends of the shell, several heat exchange tubes 4, a shell-side inlet 5, a shell-side outlet 6, a tube-side inlet 7, and a tube-side outlet 8. Several baffles 9 are fixedly installed inside the shell 1, forming several independent heat exchange chambers. The heat exchange tubes 4 all pass through each baffle 9. The baffles 9 on both sides of all heat exchange chambers are equally spaced. Adjacent heat exchange chambers are connected by a drain pipe 10 welded to the outer wall of the shell. The drain pipe 10 is 1 An 80-degree bend is provided, and each drain pipe 10 is sequentially and intermittently fixed to the outer side walls of opposite sides of the shell 1; the axes of the shell-side inlet 5, the shell-side outlet 6, and each drain pipe 10 are in the same plane on both sides of the shell 1; the inner diameter of each drain pipe 10 is greater than 20% of the distance between the two partitions 9 of each heat exchange chamber it is connected to; the axial distance between the axis of the shell-side inlet 5, the shell-side outlet 6, and the inlet and outlet of each drain pipe 19 and the axial distance between the axis of the inlet and outlet of the shell-side inlet 5, the shell-side outlet 6, and the axial distance between the axis of the inlet and outlet of the drain pipe 19 and the axial distance between the axis of the two partitions 9 on both sides is less than 70% of the distance between the two partitions 9 (or the distance between the partition 9 and the tube sheet 2).

[0028] This embodiment uses a straight-tube BEM-type heat exchanger for heating molten salt with high-temperature steam. The molten salt flows through the shell side, and the high-temperature steam flows through the tube side, employing a pure counter-current heat exchange method. The steam inlet temperature is 550°C, and the molten salt outlet temperature is 390°C. By fixing several baffles 9 inside the shell 1 and installing several guide pipes 10 outside the shell, the medium inside the shell flows through each heat exchange chamber sequentially, transferring the dead zone of the original baffle plate to the outside of the shell 1. In addition, the axes of the shell-side inlet 5, the shell-side outlet 6, and each guide pipe 10 are in the same plane on both sides of the shell 1, forming a pure through-flow region inside the shell 1. This avoids the dead zone of the traditional baffle plate. The problem of dead zones on the leeward side is addressed. On the other hand, all heat exchange tubes 4 inside the shell 1 are through-flow zones, and all heat exchange tubes are effectively utilized. The flow velocity and heat transfer performance of the through-flow zone are much greater than those of the baffle zone. This not only increases the effective heat exchange area and the shell volume utilization rate, but also improves the external convective heat transfer efficiency and makes the surface temperature of the heat exchange tube 4 closer to the shell-side medium. Even if the steam inlet temperature is significantly higher than the molten salt limit temperature of 450°C, the risk of temperature exceeding the limit can be effectively reduced. Furthermore, the guide tube 10 is easy to manufacture and has a low cost, which improves the overall economy of the equipment. Moreover, the manufacturing difficulty is independent of the equipment size, making it easy to manufacture large heat exchangers.

[0029] The larger inner diameter of the inlet pipe and the prevention of large axial offset of each inlet and outlet in their respective heat exchange chambers ensure that the medium is more evenly distributed axially after entering the heat exchange chamber through the inlet pipe 10. The surface temperature of the heat exchange tube 4 is closer to that of the shell-side medium, further reducing the risk of temperature exceeding the limit. As proven by use, the average wall temperature of the heat exchange tube 4 is below 420 degrees Celsius, and the local maximum temperature is 449 degrees Celsius. At the same time, the temperature of the high-temperature steam has dropped below the molten salt limit temperature when it leaves the flow zone, so there is no risk of excessive temperature in the subsequent heat exchange process.

[0030] This embodiment is not limited to the described implementation method, such as... Figure 2 , Figure 3 As shown, the drainage pipe can also adopt the structure of two straight pipes clamping double 90-degree elbows 12 or two straight pipes clamping headers 13; the spacing of the baffles 9 on both sides of each heat exchange chamber can be adjusted according to the thermal performance requirements, but the maximum spacing of the baffles 9 on both sides of each heat exchange chamber is not greater than 50% of the minimum spacing, so as to prevent the medium pressure fluctuation in each heat exchange chamber from being too large, to ensure the uniformity of heat exchange, and to prevent the temperature from exceeding the limit.

[0031] This invention controls the flow velocity by adjusting the baffle spacing, thereby controlling the shell-side heat transfer performance and pressure loss. The baffle spacing design method is consistent with that of the single-baffle type. For the thermal calculation of the external baffle scheme, a single-baffle type with a 20% notch is used as a reference. For shell-side Reynolds numbers (indicating the liquid flow state: laminar or turbulent) below 4000, the shell-side heat transfer coefficient is increased by +35%; for shell-side Reynolds numbers between 4000 and 8000, the increase is +20%; and for shell-side Reynolds numbers above 8000, the increase is +30%. Experimental verification shows that the average shell-side heat transfer coefficient (W / m²K) of the external baffle scheme is 20%-40% higher than that of single-baffle / double-baffle / disc annular baffles, and 30%-80% higher, respectively. Compared to baffle rods, although baffle rods provide more uniform flow than traditional baffles, external baffle systems still offer better heat transfer performance and more uniform flow. The shell-side average heat transfer coefficient (W / m^2K) of external baffle systems is 100%-180% higher than that of baffle rods. This is because baffle rods provide limited disturbance, resulting in significant differences in heat transfer performance between the external baffle system and the flow-through region of traditional bow-shaped baffles. Compared to spiral baffles, depending on the configuration of the spiral baffles, the shell-side average heat transfer coefficient of the external baffle system is significantly higher. The heat transfer efficiency (W / m^2K) is 5%-160% higher than that of spiral baffles (spiral angle 5-30 degrees). This is because the external baffle design is a full through-flow system, which has a stronger heat transfer effect than the oblique through-flow of spiral baffles. On the other hand, because the central region of the spiral baffle has a large swirl angle, drilling cannot be completed, and heat exchange tubes or baffles are usually not installed in this region, resulting in a significant volume waste. For the same shell diameter, the external baffle design has a larger effective heat transfer area than the spiral baffle, resulting in higher shell volume utilization, improved overall equipment economy, and easier manufacturing of large heat exchangers.

[0032] Example 2

[0033] like Figure 4 As shown, this embodiment is an external baffle BFU type shell and tube heat exchanger 16, whose heat exchange tubes 17 are U-shaped, and the two tube sheets and end caps are connected to the same end of the shell and isolated by baffles 18; similarly, several baffles 9 are fixed inside the shell to form several independent heat exchange chambers, and two adjacent heat exchange chambers are connected by a drain pipe 10 welded to the outer wall of the shell.

[0034] Example 3

[0035] like Figure 5 , Figure 6 As shown, this embodiment is an externally folded hairpin heat exchanger 19. The shell and heat exchange tubes are both U-shaped. According to the working conditions, several baffles 9 are fixed in the high-temperature section of the shell at the steam inlet. The two adjacent heat exchange chambers are connected by a drain pipe 10 fixed to the outer wall of the shell.

[0036] The above embodiments have been used to illustrate the invention, but it should be understood that the above embodiments are only for illustrative purposes and are not intended to limit the invention to the scope of the described embodiments. The purpose of the invention is to address the above-mentioned shortcomings of the prior art by providing an externally baffled shell-and-tube heat exchanger that can effectively avoid the formation of flow dead zones on the shell side and increase the effective heat exchange area, thereby improving heat exchange efficiency, effectively reducing the risk of temperature exceeding limits, and is easy to manufacture, has low cost, high shell volume utilization, improves the overall economy of the equipment, and is easy to manufacture large heat exchangers.

Claims

1. An outer-baffled shell-and-tube heat exchanger comprising a shell, two tube sheets and a head fixed to the shell, a number of heat exchange tubes, a shell-side inlet, a shell-side outlet, a tube-side inlet, a tube-side outlet; characterized in that: The shell is provided with several partitions to form several independent heat exchange chambers, and the heat exchange tubes all pass through the partitions; two adjacent heat exchange chambers are connected through a flow guide pipe fixed to the outer side wall of the shell, and the flow guide pipes are fixed to the outer side walls of the opposite sides of the shell in sequence and at intervals.

2. An externally folded tube shell heat exchanger as claimed in claim 1, wherein: The axes of the shell side inlet, the shell side outlet and the flow guide pipes are in the same plane at the two sides of the shell.

3. An outer-baffled shell-and-tube heat exchanger according to claim 1 or 2, characterized in that: The inner diameter of the flow guide pipe is greater than 20% of the distance between the two partitions of the heat exchange chamber connected by the flow guide pipe.

4. An externally folded tube and shell heat exchanger as claimed in claim 3, wherein: The axial distance between the axes of the shell side inlet, the shell side outlet and the inlets and outlets of the flow guide pipes and the axial distance from the two side partitions is less than 70% of the distance between the two side partitions.

5. An externally folded tube and shell heat exchanger as claimed in claim 4, wherein: The maximum distance between the two side partitions in each heat exchange chamber is not greater than 50% of the minimum distance.

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