A 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems

Abstract

The present invention discloses a 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems, which is characterized in that it includes a housing, and an inner core cylinder and an outer core cylinder are arranged inside the housing; the inner and outer core cylinders are connected and fixed to the inner wall of the housing through connecting rods; wherein, a first finned copper tube is wound on the outer side of the inner core cylinder, and a second finned copper tube is wound on the outer side of the outer core cylinder; both ends of the inner core cylinder are conical closed structures protruding outward; both ends of the housing are conical opening structures protruding outward, and one conical opening structure at one end of the housing serves as the heat flow inlet of the heat exchanger, and the other end serves as the heat flow outlet of the heat exchanger; the gaps between the first finned copper tube, the second finned copper tube and the housing are the flow channels of the heat flow of the heat exchanger; one end on the same side of the first finned copper tube and the second finned copper tube is used to connect to the cold flow inlet of the heat exchanger, and the other end is used to connect to the cold flow outlet of the heat exchanger; wherein, the flow directions of the cold flow and the heat flow of the heat exchanger are opposite.

Description

Technical Field

[0001] The present invention relates to a 2K cryogenic heat exchanger applied to a large flow rate condition (10 g / s) for improving the liquid production efficiency of a 200W@2K cryogenic system, and in particular to a cryogenic heat exchanger adopting a double-layer finned tube structure form. Background Art

[0002] The 2K superfluid helium cryogenic system is responsible for providing a cryogenic environment of about 2K for high-performance superconducting devices, ensuring that the superconducting devices can operate under extreme conditions such as strong magnetic fields and strong currents. Generally speaking, the more powerful the superconducting device is, the greater the cooling capacity of the required cryogenic system is, and the greater the generated liquid helium flow rate is. The 2K cryogenic heat exchanger is one of the key devices in the 2K superfluid helium cryogenic system. It is located before the last-stage Joule-Thomson (J-T) throttle valve in the refrigeration cycle and can use the 2K return gas to subcool the saturated liquid helium before throttling to improve the liquid output rate of the throttle valve. Its main performance indicators are the heat transfer efficiency and the pressure loss of the 2K return gas. The heat transfer efficiency directly affects the efficiency of the 2K cryogenic system, while the pressure loss of the 2K return gas will affect the performance of the rear-end rotating equipment (pumping pump or cold compressor). In addition, the volume of the heat exchanger is also an important indicator, and an excessively large volume is not conducive to system integration.

[0003] With the continuous expansion of the current 2K cryogenic system load, the requirements for the 2K cryogenic heat exchanger have also increased. For example, 2K heat exchangers with a standard flow rate of 10 g / s are designed in both the superconducting module test system of the second phase of the Dalian Coherent Light Source and the superconducting module project of the second phase of the China Spallation Neutron Source to meet the operation requirements of superconducting devices.

[0004] The heat exchanger structure forms that were applicable to small flow rate (5 g / s) working conditions in the past cannot meet the performance requirements of larger flow rate working conditions (10 g / s). For example, in the literature (Prabhat Kumar Gupta and Roger Rabehl. Numerical modeling of a 2K J-T heat exchanger used in Fermilab Vertical Test Stand VTS-1[J]. Cryogenics, 2014, 62:31-36.), the literature (Ruixiong Han et al. Design optimization, construction and testing of 2K Joule-Thomson heat exchanger for a superfluid helium cryogenic system[J]. Applied Thermal Engineering, 2020, 180) and the literature (Ashish Kumar, Hirotaka Nakai, Kota Nakanishi, Design optimization of the 2K heat exchanger for the superfluid helium cryogenic systems at KEK, Cryogenics, Volume 111, 2020, 103173, ISSN 0011-2275, https: / / doi.org / 10.1016 / j.cryogenics.2020.103173.) about the finned tube-wound 2K heat exchanger, the designed flow rate is 5 g / s for all. When they work at a flow rate of 10 g / s, the heat transfer efficiency and pressure loss will both show a cliff-like drop, unable to meet the usage requirements. Summary of the Invention

[0005] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems. The present invention re-performs the structural design and calculation verification to realize a 2K cryogenic heat exchanger that can work under the working condition of 10 g / s flow rate, and it is necessary to take into account the heat transfer efficiency, pressure drop and volume.

[0006] The main features of the present invention are as follows: The high-pressure tube side inlet is high-temperature saturated liquid helium (3.0 barA, about 4.5 K), and the low-pressure shell side inlet is negative-pressure 2K saturated helium vapor below atmospheric pressure (3100 PaA, about 2K). Under normal circumstances, the flow rates on both sides are consistent, and it can operate under the condition of a maximum standard mass flow rate of 10 g / s. The main heat exchange components include a double-layer core cylinder, double-layer finned tubes with different specifications, and an outer gas channel. The inner and outer double-layer finned tubes are connected by a tee pipe outside the shell. Through detailed design calculations, the present invention balances the flow resistance and heat exchange capacity of the inner and outer layers, and has the advantages of simple structure, good heat exchange effect, and strong adaptability to non-standard working conditions.

[0007] A 2K negative-pressure double-layer finned tube heat exchanger for cryogenic systems, characterized in that it comprises a housing, an inner core cylinder is arranged inside the housing, and an outer core cylinder is arranged between the inner core cylinder and the housing; the inner core cylinder and the outer core cylinder are fixedly connected to the inner wall of the housing through connecting rods; wherein,

[0008] The outer side of the inner core cylinder is wound with a first finned copper tube, and the outer side of the outer core cylinder is wound with a second finned copper tube;

[0009] Both ends of the inner core cylinder are conical closed structures protruding outward; both ends of the housing are conical opening structures protruding outward. One conical opening structure of the housing is used as the inlet of the heat flow of the heat exchanger, and the other conical opening structure is used as the outlet of the heat flow of the heat exchanger; the gaps between the first finned copper tube, the second finned copper tube and the housing are the flow channels of the heat flow of the heat exchanger;

[0010] One end on the same side of the first finned copper tube and the second finned copper tube is used to connect to the inlet of the cold flow of the heat exchanger, and the other end is used to connect to the outlet of the cold flow of the heat exchanger; wherein, the flow rate difference between the cold flow and the heat flow of the heat exchanger is less than the set error, and the flow directions are opposite.

[0011] Furthermore, the inner diameters and lengths of the first finned copper tube and the second finned copper tube are the same. The fin density of the second finned copper tube is higher than that of the first finned copper tube, and the fin height of the second finned copper tube is lower than that of the first finned copper tube.

[0012] Further, the inner core tube has a length of 615 mm, an outer diameter of 114 mm, and an inner diameter of 102 mm; the outer core tube has a length of 573 mm, an outer diameter of 168.4 mm, and an inner diameter of 156.4 mm; the first finned copper tube has a length of 1200 mm, an inner diameter of 8 mm, a fin height of 6 mm, a fin thickness of 1 mm, a fin density of 328 / m, and 29 winding turns; the second finned copper tube has a length of 1200 mm, an inner diameter of 8 mm, a fin height of 5 mm, a fin thickness of 1 mm, a fin density of 475 / m, and 22 winding turns.

[0013] Further, the first finned copper tube and the second finned copper tube are wound around the outer sides of the inner core tube and the outer core tube by an integrally extruded forming method.

[0014] Further, one ends on the same side of the first finned copper tube and the second finned copper tube are respectively connected to the two ports of the first tee, and the other port of the first tee serves as the inlet of the cold flow of the heat exchanger; the other ends on the same side of the first finned copper tube and the second finned copper tube are respectively connected to the two ports of the second tee, and the other port of the second tee serves as the outlet of the cold flow of the heat exchanger.

[0015] Further, the cold flow of the heat exchanger is saturated liquid helium, and the hot flow of the heat exchanger is saturated helium vapor.

[0016] Further, the cold flow of the heat exchanger is saturated liquid helium at an absolute pressure of 3.0 bar and 4.5 K, and the hot flow of the heat exchanger is saturated helium vapor at an absolute pressure of 3100 Pa and 2.0 K.

[0017] The advantages of the present invention are as follows:

[0018] The main feature of the present invention is to design a double-layer Hampson heat exchanger for a 2K cryogenic system with a large flow rate (10 g / s), and optimize the design by using detailed flow and heat transfer formulas. Finally, by changing the fin size, the resistances on both the tube side and the shell side of the inner and outer layers are made basically equal, and the heat transfer coefficients on both the inner and outer sides are made basically equal. It solves the problem that the flow distribution of such heat exchangers is usually not uniform enough, improves the overall heat transfer efficiency, and has the characteristics of simple structure and convenient maintenance. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of a single-layer finned tube structure.

[0020] Figure 2 It is a schematic diagram of a double-layer wound tube structure.

[0021] Description of the reference numerals: 1 - finned copper tube, 2 - inner core tube, 3 - outer core tube, 4 - outer shell, 5 - fixing short rod, 6 - tee. Detailed implementation mode

[0022] The present invention will be further described in detail below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0023] Since previous literature results and test results have shown that the tube-fin structure (also known as the Hampson structure) performs well under cryogenic conditions of 2K. At the same time, a single-layer structure can meet the performance requirements under a flow rate condition of 5 g / s, and has the advantages of compact volume, simple processing, and easy maintenance. However, if it is desired to apply to a larger flow rate condition, the overall volume required for a single-layer structure will be too large, and it will also cause a large resistance loss. Therefore, a double-layer structure is considered for use under a 10 g / s condition. For such a design, the biggest difficulty lies in the flow rate distribution. If the flow rates on the tube side and the shell side cannot be well balanced, the heat transfer efficiency will be greatly affected (Reference: Pacio J C, Dorao C A. A review on heat exchanger thermal hydraulic models for cryogenic applications[J]. Cryogenics, 2011, 51(7): 366 - 379.).

[0024] The basic structural form of the 2K cryogenic double-layer finned tube heat exchanger involved in the present invention is as Figure 2 shown.

[0025] Two layers of finned copper tubes formed by integral extrusion are tightly wound around two layers of 304 stainless steel core cylinders, and are connected by three-way pipes at both ends. The two layers of 304 stainless steel core cylinders and the outermost stainless steel shell are connected by spot welding with stainless steel short rods. The working medium at the tube side inlet is saturated liquid helium with an absolute pressure of 3.0 bar and 4.5K. The gap between the finned copper tube and the shell is the shell side flow channel, and the working medium at the shell side inlet is saturated helium vapor with an absolute pressure of 3100 Pa and 2.0K. The flow directions on the tube side and the shell side are opposite, and the whole heat exchanger presents a "cross-counterflow" form.

[0026] Obviously, it is hoped that the flow rates of the inner and outer layers on both the tube side and the shell side should be as consistent as possible to ensure the highest possible heat transfer efficiency. Otherwise, on the one hand, it may deviate from the design condition and cause the heat transfer efficiency to decrease. On the other hand, a radial temperature difference will also be generated between the inner layer and the outer layer, resulting in unbeneficial radial heat transfer, thereby reducing the overall heat transfer efficiency.

[0027] The flow rate is inversely proportional to the flow resistance generated by the flow structure. The greater the resistance, the smaller the flow rate. Therefore, in the present invention, finned tubes of different sizes are designed for the inner and outer layers to balance the resistance of the two layers.

[0028] The calculation adopts the distributed parameter method to model and design the heat exchanger. The heat exchanger is divided into several small heat exchange units. Heat transfer and fluid calculations are performed on each small heat exchange unit to obtain the heat transfer coefficient and Fanning friction factor. Finally, the heat transfer coefficient and friction factor of the heat exchanger as a whole are obtained by combining the micro-element calculation results, which are used for heat exchanger design.

[0029] The free flow area obtained by the fin arrangement (A fc )for

[0030] A fc =πD e [(d f -d o )(1-nt)]

[0031] The outer surface area of the finned tube (A s )for

[0032]

[0033] The circumference of the outer tube along the axial direction (s o )

[0034]

[0035] The circumference of the inner surface of the tube along the axial direction (s i )

[0036]

[0037] Shell side hydraulic diameter (D h )for

[0038]

[0039] Based on the free flow area on the shell side (A fc The Reynolds number Re on the shell side of the block is

[0040]

[0041] The flow rate per unit area G is often used to calculate the heat transfer coefficient, which is calculated by the following formula:

[0042]

[0043] In the above geometric formula, d i d o d f and are the diameters of the inner tube, outer tube and fin tube respectively, t is the fin thickness, n is the number of fins per unit length, D e is the average diameter of the coil, see Figure 1 shown.

[0044] The overall heat transfer coefficient U is calculated by the following formula. Assume the fin efficiency is unified.

[0045]

[0046] Here, h i and h o are the heat transfer coefficients on the tube side and the shell side respectively, and are obtained from the following two formulas respectively.

[0047]

[0048]

[0049] where Pr and Re are the Prandtl number and the Reynolds number respectively, and k is the thermal conductivity of the working fluid.

[0050] There are various definitions for the efficiency of a heat exchanger. The commonly used calculation method is to calculate using the enthalpy difference. The heat transfer efficiency formula calculated using the enthalpy difference is as follows.

[0051]

[0052] where is the enthalpy of the hot fluid at the cold fluid inlet temperature. Similarly, if the cold fluid is the fluid with the minimum heat capacity, the effectiveness of the heat exchanger is defined by the following formula.

[0053]

[0054] where is the enthalpy of the cold fluid at the hot fluid outlet temperature. H in the above two formulas h and H c are the enthalpy values of the hot fluid and the cold fluid at each point, and have a one-to-one correspondence with Th and Tc, and can be obtained by querying the software.

[0055] The pressure drop calculation is an important part in the design of a heat exchanger. The pressure drop per unit length of a Hampson-type heat exchanger can be calculated according to the following formula.

[0056]

[0057] This formula is a general formula. Whether it is the shell side or the tube side, it can be calculated through this formula. Here, ρ is the density of the working fluid, f is the Fanning friction coefficient of the working fluid, G is the mass flow rate per unit area, and the friction coefficients on the tube side and the shell side can be calculated respectively according to the following two formulas.

[0058] f h = 0.184Re -0.2 (1 + 3.5(d i / D e ))

[0059]

[0060] The physical properties of the flowing working fluid helium are obtained using the Hepak software.

[0061] Design calculations are carried out according to the above formula. First, ensure that the inner diameters and lengths of the inner and outer finned tubes are the same, so as to ensure the same tube-side resistance. However, since the core cylinder of the outer layer is larger in diameter than the inner layer, the number of turns wound on the outer layer is less than that on the inner layer, which means that the heat transfer path of the outer layer is shorter than that of the inner layer. Therefore, it is necessary to increase the pressure drop per unit length of the outer layer.

[0062] Adjust the shell-side resistance and heat transfer efficiency of the inner and outer layers by adjusting the density and height of the fins on the winding tube, that is, increase the fin density of the outer layer while reducing the fin height of the outer layer, so that the calculated resistance and heat transfer efficiency on both sides are basically equal. The final design results are shown in the following table.

[0063] Inner layer Outer layer Outer diameter of core tube 114mm 168.4mm Inner diameter of core tube 102mm 156.4mm Length of core tube 615mm 573mm Number of turns of winding tube 29 22 Length of winding tube 1200mm 1200mm Inner diameter of winding tube 8mm 8mm Height of fin 6mm 5mm Thickness of fin 1mm 1mm Density of fin 328 / m 475 / m

[0064] Calculated according to this structure, the heat transfer efficiency at a flow rate of 10 g / s is 91%, and the flow resistance is 78 Pa.

[0065] Although specific embodiments of the present invention are disclosed for illustrative purposes, which are intended to help understand the content of the present invention and implement it accordingly, those skilled in the art can understand that: without departing from the spirit and scope of the present invention and the appended claims, various substitutions, changes, and modifications are possible. Therefore, the present invention should not be limited to the content disclosed in the best embodiments, and the scope of protection required by the present invention is defined by the scope of the claims.

Claims

1. A 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems, characterized in that, It includes a housing, an inner core cylinder is arranged inside the housing, and an outer core cylinder is arranged between the inner core cylinder and the housing; the inner core cylinder and the outer core cylinder are fixedly connected to the inner wall of the housing through connecting rods; wherein, The outer side of the inner core cylinder is wound with a first finned copper tube, and the outer side of the outer core cylinder is wound with a second finned copper tube; Both ends of the inner core cylinder are tapered closed structures protruding outwards; both ends of the housing are tapered opening structures protruding outwards. One tapered opening structure at one end of the housing serves as the inlet of the heat flow of the heat exchanger, and the tapered opening structure at the other end serves as the outlet of the heat flow of the heat exchanger; the gap between the first finned copper tube, the second finned copper tube and the housing is the flow path of the heat flow of the heat exchanger; The same-side ends of the first finned copper tube and the second finned copper tube are used to connect to the inlet of the cold flow of the heat exchanger, and the other ends are used to connect to the outlet of the cold flow of the heat exchanger; wherein, the flow rate difference between the cold flow of the heat exchanger and the heat flow of the heat exchanger is less than the set error, and the flow directions are opposite.

2. The 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems according to claim 1, characterized in that, The inner diameters and lengths of the first finned copper tube and the second finned copper tube are the same. The fin density of the second finned copper tube is higher than that of the first finned copper tube, and the fin height of the second finned copper tube is lower than that of the first finned copper tube.

3. The 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems according to claim 2, characterized in that, The length of the inner core cylinder is 615 mm, the outer diameter is 114 mm, and the inner diameter is 102 mm; the length of the outer core cylinder is 573 mm, the outer diameter is 168.4 mm, and the inner diameter is 156.4 mm; the length of the first finned copper tube is 1200 mm, the inner diameter is 8 mm, the fin height is 6 mm, the fin thickness is 1 mm, the fin density is 328 / m, and the number of winding turns is 29; the length of the second finned copper tube is 1200 mm, the inner diameter is 8 mm, the fin height is 5 mm, the fin thickness is 1 mm, the fin density is 475 / m, and the number of winding turns is 22.

4. The 2K negative pressure double-layer finned tube heat exchanger for cryogenic system according to claim 1, wherein, The first finned copper tube and the second finned copper tube are wound on the outer sides of the inner core cylinder and the outer core cylinder by an integrally extruded molding method.

5. A 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems according to claim 1, characterized in that, The same-side ends of the first finned copper tube and the second finned copper tube are respectively connected to the two ports of a first three-way joint, and the other port of the first three-way joint serves as the inlet of the cold flow of the heat exchanger; the same-side other ends of the first finned copper tube and the second finned copper tube are respectively connected to the two ports of a second three-way joint, and the other port of the second three-way joint serves as the outlet of the cold flow of the heat exchanger.

6. A 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems according to any one of claims 1 to 5, characterized in that, The cold flow of the heat exchanger is saturated liquid helium, and the heat flow of the heat exchanger is saturated helium vapor.

7. A 2K negative pressure double-layer finned tube heat exchanger for cryogenic systems according to claim 6, characterized in that, The cold flow of the heat exchanger is saturated liquid helium with an absolute pressure of 3.0 bar and 4.5 K, and the heat flow of the heat exchanger is saturated helium vapor with an absolute pressure of 3100 Pa and 2.0 K.

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