Variable-diameter U-shaped tubular evaporator for fused salt heat storage system

Abstract

The invention discloses a variable-pipe-diameter U-shaped tubular evaporator suitable for a high-temperature molten salt energy storage system. The variable-pipe-diameter U-shaped tubular evaporator comprises a tube box, a pass partition plate, a shell, a variable-pipe-diameter U-shaped heat exchange tube bundle, a tube plate, a baffle plate, a tube pass undersaturated water inlet, a tube pass saturated steam outlet, a shell pass high-temperature molten salt inlet and a shell pass low-temperature molten salt outlet. The problems that a traditional constant-pipe-diameter U-shaped pipe evaporator is large in flow resistance, concentrated in thermal stress, low in heat exchange efficiency and the like in a large system are solved. According to the evaporator, the design of the U-shaped heat exchange tube bundle gradually changing along the way is adopted, the flow speed and pressure distribution of fluid in the tube are optimized through a variable-tube-diameter structure, the risks of local overheating and uneven evaporation are reduced, and the flow resistance is effectively reduced. And due to the variable pipe diameter design, the thermal stress concentration phenomenon is reduced, so that the reliability and the service life of the equipment are improved. Through the design, the heat transfer uniformity and the heat transfer efficiency of the evaporator are remarkably improved, and efficient and stable evaporation heat exchange equipment is provided for a large fused salt heat storage system.

Description

Technical Field

[0001] The present invention relates to the technical field of high-temperature thermal energy storage and conversion equipment, and in particular to a variable-diameter U-shaped tube evaporator for a molten salt heat storage system. Background Art

[0002] As a highly efficient thermal energy storage and transmission technology, the molten salt heat storage system has been widely used in the fields of solar thermal power plants, industrial waste heat recovery, and molten salt heat storage coupled with thermal power units for peak and frequency regulation. The system stores thermal energy in the form of high-temperature molten salt, and releases the thermal energy through a heat exchanger when needed for power generation, steam heating or other industrial purposes. Molten salt is an ideal heat storage medium due to its high heat capacity, high temperature stability and good thermal conductivity. How to reliably and efficiently transfer heat from high-temperature molten salt to other working fluids (such as water or other working fluids) to fully utilize the heat is one of the key issues facing the development of molten salt heat storage systems.

[0003] The U-tube evaporator is one of the commonly used types of heat exchangers in molten salt heat storage systems. It transfers the heat of the high-temperature molten salt (shell side) to the water through the U-shaped heat exchange tube bundle, evaporates the under-enthalpy water in the tube into saturated steam, and transports it to the high-temperature molten salt superheater to produce qualified high-temperature steam. Conventional U-tube evaporators are widely used due to their compact structure, high heat transfer efficiency and low manufacturing cost. With the rapid development of solar thermal power stations and large-scale molten salt heat storage coupled thermal power unit technology, if the traditional U-tube evaporator design is still used in large-scale molten salt heat storage systems, there will be technical bottlenecks such as large flow resistance and large thermal stress, and at the same time face the challenge of urgently improving heat exchange efficiency.

[0004] The traditional U-tube evaporator adopts a constant tube diameter design. As the heat is released, the viscosity of the molten salt increases rapidly along the process, resulting in a rapid increase in the flow resistance along the molten salt side. This characteristic leads to a significant increase in the molten salt pump head, which in turn affects the flow stability of the molten salt on the shell side of the heat exchanger. In the traditional U-tube evaporator, the molten salt flow rate is not uniform at different positions, especially in the curved part of the U-tube, which is prone to flow separation and eddy currents, resulting in reduced local heat transfer efficiency.

[0005] In large-scale molten salt heat storage systems, as the capacity and volume of the U-tube evaporator increase, the temperature difference between the inlet and outlet of the molten salt side (shell side) becomes large, resulting in increased heat transfer unevenness and increased thermal stress of the U-tube.

[0006] To solve these problems, existing methods have proposed to improve the heat transfer effect by adding spoilers, but this method will increase the flow resistance; special coatings are used to improve corrosion resistance, but the high cost and complex coating process limit its application. Therefore, the traditional constant pipe diameter design is difficult to fundamentally solve the problems of flow resistance and thermal stress concentration. Summary of the invention

[0007] Technical problem to be solved by the present invention: The purpose of the present invention is to solve the problems of low heat exchange efficiency, large flow resistance, uneven evaporation and concentrated thermal stress of the heat exchange tube in the existing U-tube evaporator, and provide a variable diameter U-tube evaporator for a large molten salt heat storage system. The heat exchange tubes adopt different diameters at different positions to optimize the flow state of the fluid in the U-tube.

[0008] The technical solution of the present invention is: a variable diameter U-tube evaporator for a molten salt heat storage system, comprising a shell, a tube sheet, a tube box, a partition plate, a U-shaped heat exchange tube, a baffle and a drain port, wherein the U-shaped heat exchange tube is fixed on the tube sheet, the tube sheet is connected to the tube box, and the tube box is fixedly connected to the shell through a flange, and is characterized in that the U-shaped heat exchange tubes all adopt a variable diameter structure, and the diameter of the U-shaped heat exchange tubes is large at the inlet section and small at the outlet section.

[0009] Furthermore, a partition plate is provided inside the pipe box, and the partition plate divides the inner cavity of the pipe box into a pipe-side undersaturated water inlet chamber and a pipe-side saturated steam outlet chamber.

[0010] Furthermore, the shell is a vertically arranged cylindrical structure, and a number of baffles are evenly distributed on the inner side of the shell along the vertical direction to guide the flow of the molten salt and enhance the heat exchange effect.

[0011] Furthermore, the baffles are arranged between the shell-side high-temperature molten salt inlet and the shell-side low-temperature molten salt outlet, and the side surface of each baffle is staggered at an angle of 90° to the inner wall of the shell.

[0012] Furthermore, a tube-side undersaturated water inlet is provided on the side wall of the tube-side undersaturated water inlet chamber for introducing undersaturated water; and a saturated steam outlet is provided on the side wall of the tube-side saturated steam outlet chamber for discharging saturated steam.

[0013] Furthermore, a shell-side high-temperature molten salt inlet is provided on the lower side of the shell for introducing high-temperature molten salt; a shell-side low-temperature molten salt outlet is provided on the upper side of the shell for discharging low-temperature molten salt.

[0014] Furthermore, a shell side drain port is provided at the bottom of the shell, and the drain port is equipped with a high temperature resistant and corrosion resistant valve to achieve rapid emptying of the shell side medium.

[0015] Furthermore, the shell and the variable diameter U-shaped heat exchange tube are made of high temperature resistant and high strength materials, and the inner surfaces thereof are treated with anti-corrosion to resist the corrosion of high temperature molten salt and water vapor.

[0016] Furthermore, the tube diameter conversion formula of the U-shaped heat exchange tube is:

[0017]

[0018] Where D(x) is the diameter of the heat exchange tube at distance x, D in is the inlet diameter of the heat exchange tube, D out is the outlet diameter of the heat exchange tube, L is the total length of the heat exchange tube, and x is the distance along the length of the tube, from 0 to L.

[0019] Furthermore, the outlet diameter of the U-shaped heat exchange tube (7) is 0.5-0.8 times the inlet diameter.

[0020] Beneficial effect: Compared with the prior art, the beneficial effect of the present invention is that the present invention provides a variable diameter U-tube evaporator suitable for high-temperature molten salt energy storage systems, and adopts a U-shaped heat exchange tube bundle design with a gradually changing tube diameter along the way. The variable tube diameter structure optimizes the fluid flow rate and pressure distribution in the heat exchange tube, reduces the risk of local overheating and uneven evaporation, reduces thermal stress concentration, and significantly improves the reliability and service life of the equipment. This design effectively improves the uniformity and efficiency of heat transfer, reduces flow resistance, provides efficient and stable evaporation heat exchange equipment for large-scale molten salt heat storage systems, and provides strong technical support for the performance improvement of key equipment in such systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic cross-sectional structure diagram of a variable diameter U-tube evaporator used in a molten salt heat storage system in the present invention.

[0022] Figure 2 It is a schematic structural diagram of the shell and tube sheet of the variable tube diameter U-shaped tube evaporator used in the molten salt heat storage system in the present invention.

[0023] In the figure: 1—shell, 2—tube sheet; 3—tube box; 4—pass partition; 5—undersaturated water inlet for tube side; 6—saturated steam outlet for tube side; 7—variable diameter U-shaped heat exchange tube bundle; 8—baffle; 9—shell side high temperature molten salt inlet; 10—shell side low temperature molten salt outlet; 11—drain outlet; 1-1—high temperature molten salt; 1-2—low temperature molten salt; 2-1—undersaturated water; 2-2—saturated steam DETAILED DESCRIPTION

[0024] In order to deepen the understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. The embodiments are only used to explain the present invention and do not constitute a limitation on the protection scope of the present invention.

[0025] like Figure 1-2As shown, the variable diameter U-shaped tube evaporator for the molten salt heat storage system of the present invention comprises a shell 1, a tube sheet 2, a tube box 3, a partition plate 4, a variable diameter U-shaped heat exchange tube 7, a baffle 8 and a drain port 11. Each U-shaped heat exchange tube 7 adopts a variable diameter structure to optimize the fluid flow rate and heat transfer efficiency. The variable diameter U-shaped heat exchange tube is fixed on the tube sheet 2, the tube sheet 2 is connected to the tube box 3, and the tube box 3 is fixedly connected to the shell 1 through a flange.

[0026] Inside the pipe box 3, a partition plate 4 is vertically arranged in the center to divide the inner cavity of the pipe box 3 into two independent cavities: a tube-side undersaturated water inlet chamber and a tube-side saturated steam outlet chamber.

[0027] The tube sheet 2 is connected to the shell 1. The shell 1 is a vertically arranged cylindrical structure for accommodating internal heat exchange components and facilitating the discharge of molten salt.

[0028] On the inner side of the shell 1, a number of baffles 8 are evenly distributed along the vertical direction. The baffles are fixedly connected to the variable diameter U-shaped heat exchange tubes to guide the flow of the molten salt and enhance the heat exchange effect.

[0029] The baffle 8 is arranged between the shell-side high-temperature molten salt inlet 9 and the shell-side low-temperature molten salt outlet 10, and the side of each baffle 8 is staggered at a 90° angle with the inner wall of the shell 1. A tube-side undersaturated water inlet 5 is provided on the side wall of the tube-side undersaturated water inlet cavity for introducing undersaturated water; and a saturated steam outlet 6 is provided on the side wall of the tube-side saturated steam outlet cavity for discharging saturated steam after heat exchange. A shell-side high-temperature molten salt inlet 9 is provided on the lower side of the shell 1 for introducing high-temperature molten salt; a shell-side low-temperature molten salt outlet 10 is provided on the upper side of the shell 1 for discharging low-temperature molten salt after heat exchange. A shell-side drain outlet 11 is also provided at the bottom of the shell 1 to facilitate the discharge of molten salt by gravity. The high-temperature molten salt 1-1 flow inlet and the undersaturated water 2-1 flow inlet of the variable-diameter U-shaped tube evaporator are located on the same side, and the low-temperature molten salt 1-2 flow outlet and the saturated steam 2-2 flow outlet are located on the same side, which optimizes the system's pipeline layout and overall operating efficiency.

[0030] The variable diameter U-shaped heat exchange tube bundle is the core heat exchange component of the variable diameter U-shaped tube evaporator, which is composed of multiple U-shaped heat exchange tubes 7, each of which adopts a variable diameter structure. The purpose of the variable diameter design is to adjust the flow rate of the fluid at different positions to optimize the heat exchange efficiency and reduce the flow resistance. Both ends of the variable diameter U-shaped heat exchange tube are fixed on the tube sheet 2 and connected to the tube box 3. Undersaturated water flows into the variable diameter U-shaped heat exchange tube 7 and absorbs the heat of the molten salt to generate steam during the heat exchange process.

[0031] In order to enhance the heat exchange effect of the shell-side molten salt, a number of shell-side medium baffles 8 are evenly arranged in the vertical direction inside the shell 1. The shape, number and spacing of the baffles are optimized according to the flow velocity and flow rate of the shell-side medium. The baffles are fixedly connected to the variable diameter U-shaped tube bundle to guide the flow path of the shell-side molten salt so that it forms turbulence in the shell. This turbulent state helps to increase the contact area and time between the shell-side medium and the outer surface of the heat exchange tube, significantly improving the heat exchange efficiency.

[0032] The baffles are staggered at 90° to the inner wall of the shell to ensure that the medium changes direction frequently during the flow process, thereby achieving more uniform heat exchange.

[0033] The shell 1 and the variable diameter U-shaped heat exchange tube 7 are made of high temperature resistant and high strength materials, and the inside is treated with anti-corrosion to resist the corrosion of high temperature molten salt and water vapor.

[0034] The shell side drain port 11 at the bottom of the shell 1 is an important component of the variable diameter U-tube evaporator during shutdown or material change. The drain port is designed with a high temperature and corrosion resistant valve to achieve rapid emptying of the shell side medium.

[0035] Before starting the equipment, first confirm whether the connection status of all pipes and valves is normal to ensure that there is no leakage. Before starting, the variable diameter U-shaped tube evaporator should be preheated to prevent the molten salt from solidifying and blocking the heat exchanger due to cold start. After preheating, start introducing the working medium: molten salt as a heat source enters the variable diameter U-shaped tube evaporator shell 1 through the shell side high temperature molten salt inlet 9; undersaturated water as a heating medium enters the variable diameter U-shaped heat exchange tube 7 through the tube side inlet 5.

[0036] The undersaturated water enters the undersaturated water inlet chamber of the tube side from the undersaturated water inlet of the tube side, and is then evenly distributed to each variable diameter U-shaped heat exchange tube, flows through the bending section of the variable diameter U-shaped heat exchange tube and the tube side superheated steam outlet, and experiences sub-subcooled boiling, nucleate boiling and saturated steam states respectively, and finally passes through the tube side saturated steam outlet chamber of the tube box, and flows into the downstream superheater through the tube side superheated steam outlet.

[0037] The external working medium of the U-shaped heat exchange tube bundle is molten salt. The high-temperature molten salt flows from the high-temperature molten salt inlet of the shell side through the shell side and each baffle, and finally becomes low-temperature molten salt and flows out from the low-temperature molten salt outlet of the shell side.

[0038] The corresponding data is obtained through simulation calculation. When the molten salt enters the variable diameter U-tube evaporator from the shell inlet 9 at a flow rate of 600kg / h, the molten salt flows upward along the channel in the shell and gradually fills the shell 1. The inlet and outlet temperatures of the molten salt are 365℃ and 290℃. After heat exchange and cooling in the heat exchanger, it leaves the shell outlet 10. At the same time, a complex flow path is formed through the action of the baffle 8, which increases the turbulence of the flow and improves the heat exchange efficiency.

[0039] At the same time, 0.3Mpa undersaturated water enters from the tube side medium inlet 5 at a flow rate of 40kg / h, and is guided into the variable diameter U-shaped heat exchange tube 7 through the partition plate 4 in the tube box 3. The water exchanges heat with the molten salt in the heat exchange tube, is heated from the initial temperature of 25℃ to the saturated evaporation temperature of 133.53℃, and is finally discharged from the tube side medium outlet 6 as saturated steam.

[0040] In the heat exchange process, the variable diameter U-shaped heat exchange tube 7 is responsible for transferring heat. Through the efficient heat transfer design, the heat of the molten salt is quickly transferred to the water in the tube. According to the data, the surface heat transfer coefficient of the shell side is 608.42W / (m 2 ·K), and the surface heat transfer coefficient of the tube side is 1993.4W / (m 2 ·K), the higher heat transfer coefficient proves that the equipment is operating under better heat transfer performance. The total heat transfer coefficient of the variable diameter U-tube evaporator is 409.48W / (m 2 K), the heat transfer area is 0.27m 2 , the thermal power reaches 18.753KW, ensuring efficient heat exchange between molten salt and water.

[0041] During operation, by controlling the flow and flow rate of molten salt and water, the heat flow balance between the two can be adjusted to ensure that the temperature and pressure of the output steam meet the requirements. By adjusting the valves of the shell-side high-temperature molten salt inlet 9 and the tube-side undersaturated water inlet 5, precise control of the fluid flow can be achieved.

[0042] The maintenance and cleaning of the molten salt and water circulation system is a vital task. After completing an operation cycle, the flow of molten salt and water will gradually slow down and eventually stop. Through the design of the shell side drain port 11, the medium can be effectively discharged and the accumulation of residues can be reduced. In addition, in order to ensure the long-term and efficient operation of the equipment, regular inspection and maintenance are essential, especially in high temperature and high pressure working environments, the cleaning and sealing inspection of the heat exchanger is particularly important. During the equipment maintenance process, the shell 1 can be disassembled to conduct a comprehensive inspection and cleaning of the interior.

[0043] Experimental conditions

[0044] Inlet pipe diameter: Din = 100mm

[0045] Total length of U-shaped heat exchange tube: L = 1m, length of U-shaped bending part is Lbend = 0.2m

[0046] Outlet pipe diameter: including three ratios within the optimal range and two ratios outside the optimal range

[0047] Case 1: Dout = 0.4·Din = 40mm;

[0048] Case 2: Dout = 0.5·Din = 50 mm;

[0049] Case 3: Dout = 0.65·Din = 65mm;

[0050] Case 4: Dout = 0.8·Din = 80 mm;

[0051] Case 5: Dout = 0.9·Din = 90 mm;

[0052] Experimental data

[0053] Taking into account factors such as heat transfer efficiency, pressure drop, turbulence and bubble breakage, the following experimental data are obtained:

[0054]

[0055]

[0056] Comparative Analysis

[0057] Heat transfer efficiency: In the best range (cases 2, 3, and 4), the heat transfer efficiency of these three ratios is high (390-430 W / m 2 ·K). The outlet diameter in the range of 0.5-0.8 times can effectively increase the flow rate, enhance turbulence and promote bubble breakage, thus achieving efficient heat exchange.

[0058] Out of range (cases 1, 5): Case 1, where the outlet pipe diameter is less than 0.5 times, has the highest heat exchange efficiency (435 W / m 2 ·K), but the pressure drop increases significantly, which may cause system instability. Case 5 where the outlet pipe diameter is greater than 0.8 times the heat transfer efficiency decreases significantly (360W / m 2 ·K), indicating that the excessively large pipe diameter reduces fluid disturbance and is not conducive to efficient heat exchange.

[0059] Pressure drop: within the optimal range: The pressure drops of cases 2, 3 and 4 are 2500Pa, 1800Pa and 1200Pa respectively, which are within a reasonable range and will not cause excessive operating pressure of the system.

[0060] Out of range: Case 1 has the highest pressure drop (3100Pa), which significantly increases the system energy consumption and pump requirements. Case 5 has the lowest pressure drop (900Pa), but at the same time the heat transfer efficiency is low, indicating that an excessively large outlet diameter will weaken turbulence and reduce the heat transfer effect.

[0061] Turbulence disturbance and bubble breakage rate: within the optimal range: In cases 2, 3 and 4, the turbulence disturbance and bubble breakage rate are moderate, which helps to achieve efficient heat exchange between water and the tube wall in the final section and ensure smooth flow of the fluid.

[0062] Out of range: Although the turbulence and bubble breakage rate of case 1 are high, excessive disturbance will increase the pressure drop and make the system unstable. The turbulence and bubble breakage rate of case 5 are low, which is not conducive to efficient heat exchange at the end of evaporation.

[0063] Thermal stress concentration and steam quality: Within the optimal range: Cases 2 and 3 have higher outlet steam quality and lower thermal stress concentration risk, which are suitable for practical applications. Although the steam quality of Case 4 is slightly reduced, it can still reach a higher saturated steam level.

[0064] Out of range: The smaller outlet diameter of Case 1 increases the flow velocity and disturbance, but creates a higher risk of thermal stress concentration at the bend. Although the thermal stress distribution of Case 5 is balanced, the larger outlet diameter is not conducive to the full evaporation of water, and the steam quality is low.

[0065] Summary: By comparison, it can be concluded that the optimal choice is for the outlet diameter to be 0.5-0.8 times the inlet diameter. This ratio can provide efficient heat exchange while maintaining appropriate pressure drop and turbulent disturbance, ensuring that the bubbles are fully broken and evaporated into saturated steam, avoiding thermal stress concentration.

[0066] The U-shaped heat exchange tube bundle of the present invention is composed of a plurality of U-shaped heat exchange tubes with gradually changing tube diameters along the way, wherein the tube diameter of the inlet section is large and the tube diameter of the outlet section is small; the larger inlet tube diameter helps to reduce the flow velocity at the inlet of undersaturated water, reduces the resistance loss of the inlet undersaturated water, helps to evenly distribute the water flow, reduces the thermal stress of the evaporation starting section and the risk of uneven thermal stress of different U-shaped heat exchange tubes.

[0067] The curved portion of the U-shaped heat exchange tube adopts a constant tube diameter design, which can adjust the flow rate and pressure distribution of water flow and bubbles, increase turbulent disturbance at the curved portion, increase bubble breakage, and reduce thermal stress concentration at the curved portion.

[0068] The outlet diameter of the U-shaped heat exchange tube is 0.5-0.8 times the inlet diameter. The smaller outlet diameter of the U-shaped heat exchange tube causes a series of polymerization and fragmentation processes between water and generated bubbles, increases the flow rate and disturbance in the tube, improves the heat exchange efficiency between the working medium and the tube wall, and ensures that the water at the end of evaporation can quickly absorb heat and completely evaporate into saturated steam.

[0069] The present invention is not limited to the specific structures and methods described in the above embodiments. Any equivalent replacement or improvement based on the concept of the present invention should be within the protection scope of the present invention.

Claims

1. A variable diameter U-shaped tube evaporator for a molten salt heat storage system, comprising a shell (1), a tube sheet (2), a tube box (3), a partition plate (4), a U-shaped heat exchange tube (7), a baffle (8) and a drain port (11), wherein the U-shaped heat exchange tube (7) is fixed on the tube sheet (2), the tube sheet (2) is connected to the tube box (3), and the tube box (3) is fixedly connected to the shell (1) through a flange, characterized in that: The U-shaped heat exchange tubes (7) all adopt a variable diameter structure, and the diameter of the U-shaped heat exchange tubes (7) is large at the inlet section and small at the outlet section.

2. The variable diameter U-shaped tube evaporator according to claim 1, characterized in that: A partition plate (4) is provided inside the pipe box (3), and the partition plate (4) divides the inner cavity of the pipe box (3) into a pipe-side undersaturated water inlet chamber and a pipe-side saturated steam outlet chamber.

3. The variable diameter U-shaped tube evaporator according to claim 1, characterized in that: The shell (1) is a vertically arranged cylindrical structure, and a plurality of baffles (8) are evenly distributed along the vertical direction on the inner side of the shell (1) to guide the flow of molten salt and enhance the heat exchange effect.

4. The variable diameter U-shaped tube evaporator according to claim 3 is characterized in that: The baffles (8) are arranged between the shell-side high-temperature molten salt inlet (9) and the shell-side low-temperature molten salt outlet (10), and the side surface of each baffle (8) is staggered at an angle of 90° to the inner wall of the shell (1).

5. The variable diameter U-shaped tube evaporator according to claim 1, characterized in that: A tube-side undersaturated water inlet (5) is provided on the side wall of the tube-side undersaturated water inlet chamber for introducing undersaturated water; a saturated steam outlet (6) is provided on the side wall of the tube-side saturated steam outlet chamber for discharging saturated steam.

6. The variable diameter U-shaped tube evaporator according to claim 1, characterized in that: A shell-side high-temperature molten salt inlet (9) is provided on the lower side of the shell (1) for introducing high-temperature molten salt; and a shell-side low-temperature molten salt outlet (10) is provided on the upper side of the shell (1) for discharging low-temperature molten salt.

7. The variable diameter U-shaped tube evaporator according to claim 4, characterized in that: A shell side drain port (11) is provided at the bottom of the shell (1), and the drain port is equipped with a high temperature resistant and corrosion resistant valve to achieve rapid draining of the shell side medium.

8. The variable diameter U-shaped tube evaporator according to claim 1, characterized in that: The shell (1) and the variable diameter U-shaped heat exchange tube (7) are both made of high temperature resistant and high strength materials, and their inner surfaces are subjected to anti-corrosion treatment to resist the corrosion of high temperature molten salt and water vapor.

9. The variable diameter U-tube evaporator according to any one of claims 1 to 8, characterized in that: The tube diameter conversion formula of the U-shaped heat exchange tube (7) is: Where D(x) is the diameter of the heat exchange tube at distance x, D in is the inlet diameter of the heat exchange tube, D out is the outlet diameter of the heat exchange tube, L is the total length of the heat exchange tube, and x is the distance along the length of the tube, from 0 to L.

10. The variable diameter U-tube evaporator according to claim 9, characterized in that: The outlet diameter of the U-shaped heat exchange tube (7) is 0.5-0.8 times the inlet diameter.

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