Cross heat exchanger, method of manufacturing a cross heat exchanger and cryocooler

Abstract

The invention relates to an alternating heat exchanger for cryogenic coolers, which utilizes helium as working gas and heat storage medium, having at least one cell with a cell wall enclosing a cavity having a plurality of partial cavities. The partial cavities are connected to one another by at least one connecting channel and are completely enclosed by the cell wall, with the exception of the at least one connecting channel leading to the other partial cavities. The cavity of the at least one cell is filled with helium as heat storage material. In addition, the alternating heat exchanger has a flow channel for the working gas helium, which is constructed between the individual partial cavities, and a pressure equalization hole in the form of a capillary, which penetrates the cell wall and constitutes a permanently open connection between the working gas helium outside the cavity and the heat storage material helium inside the cavity. The partial cavities have support elements inside them, which support the cell walls delimiting a partial cavity with respect to one another.

Description

Technical Field

[0001] This disclosure relates to an AC heat exchanger using helium as the working gas for a cryogenic cooler, a method for manufacturing such an AC heat exchanger, and a cryogenic cooler equipped with such an AC heat exchanger. Background Technology

[0002] Helium is commonly used as the working gas in cryogenic coolers. Helium possesses a high heat capacity in the temperature range of 2K to 20K, which is comparable to that of rare earth compounds in this range. Therefore, the use of helium as a material for heat exchangers has been proposed. Helium-filled, enclosed hollow bodies made of glass or metal are known as heat exchanger structures, as disclosed in US 2012 / 0304668 A1, DE 10319510 A1, DE 102005007627 A1, CN104197591 A, DE 19924184 A1, and US 4359872 A. However, this basic idea has not yet been translated into any finished product. Furthermore, the helium-filled spheres lead to wear, which shortens the lifespan of the cryogenic cooler. A fundamental problem with these known enclosed hollow bodies containing helium lies in the complex helium filling process under overpressure. Due to the overpressure, the wall thickness of the hollow body had to be increased, which resulted in a decrease in heat transfer resistance.

[0003] The paper "Heat Capacity Characterization of a 4K Regenerator with Non-Rare Earth Material" presented at Cryocoolers 19, International Cryocooler Conference, Inc., Boulder, CO. 2016, proposes a structure with an absorbent material capable of absorbing helium for use as an AC heat exchanger in cryogenic coolers. This AC heat exchanger has a complex and cumbersome structure, and there is a risk that some of the absorbent material will be carried away by the working gas flow. Because the absorbent particles are carried away, the lifespan of cryogenic coolers with such AC heat exchangers will be drastically shortened.

[0004] JP HO7318181 discloses an AC heat exchanger in which a hollow body filled with helium is sealed by thermal contraction. CN 104 197 591 A discloses an AC heat exchanger having rectangular hexahedral units filled with helium as the heat storage material. These units are sealed after filling, therefore there are no pressure balancing holes. JP S62-233688A discloses an AC heat exchanger in which a metal is used as the heat storage material for heat storage; helium is not used as the heat storage material.

[0005] An AC heat exchanger is disclosed in JP2011190953A, comprising small tubes open on both sides, these tubes containing helium as a heat storage material. Therefore, the helium-filled tubes have pressure balancing holes, allowing pressure balance to be achieved between the tubes and the working helium gas during operation of the cooler or AC heat exchanger. A disadvantage of this AC heat exchanger is that adjacent units filled with helium as the heat transfer material overlap, and the overlapping sections of the unit walls do not contribute to heat exchange. Therefore, the functionality of this known AC heat exchanger is limited.

[0006] An AC heat exchanger designed to use helium as both the working gas and the heat storage material is disclosed in WO 2018 / 104410 A1. This known AC heat exchanger includes a cavity with multiple interconnected tubular cavities. Flow channels for the working gas helium are formed between the cavities. A pressure balancing orifice, in the form of a capillary, penetrates the unit wall, creating a continuously open connection between the working gas helium outside the cavity and the heat storage material helium inside the cavity. The thinner the unit wall, the better the heat transfer between the working gas helium and the heat storage material helium through the unit wall. However, a certain thickness of the unit wall is necessary to prevent it from cracking or breaking during pressure fluctuations during AC heat exchanger operation. Summary of the Invention

[0007] Therefore, based on WO 2018 / 104410 A1, the purpose of this disclosure is to describe an AC heat exchanger that uses helium as the working gas and heat storage material, which can achieve more efficient heat transfer through the unit walls than WO 2018 / 104410 A1.

[0008] This objective is achieved through the features of the invention.

[0009] The interior of the partial cavity is filled with helium as the heat storage medium during the initial operation of the AC heat exchanger via capillary tubes in the unit walls. Because a similar pressure ratio exists within the cavity or partial cavity during operation, the unit walls can be designed to be relatively thin. However, a certain thickness of the unit walls is necessary to prevent them from cracking or breaking during pressure fluctuations during AC heat exchanger operation. Since the partial cavity has supporting elements within it, the unit walls can be designed to be even thinner, as these thin unit walls are supported on these supporting elements. Due to the thinner unit walls, heat transfer through the unit walls is improved. The ratio of the cavity volume to the capillary opening area or outflow resistance is chosen such that the pressure within the cavity or partial cavity remains almost constant or at least only slightly varies within the operating frequency range of the cooler (approximately 1 to 60 Hz). This operating principle is similar to that of a capacitor at high frequencies – if the capacitance is sufficiently high and the voltage change is small, the capacitor is almost unaffected by voltage variations. In typical applications, the pressure within the unit always fluctuates around the average pressure of the cooling system, typically around 16 bar. Therefore, stable pressure is important because when the cavity pressure fluctuates between, for example, 8 and 24 bar in each cycle without contributing to cooling, the volume of one or more cavities will make the largest contribution to the "dead-point volume." The opening area or outflow resistance of the pressure balancing orifice is selected such that, before the AC heat exchanger starts operating and during the start-up phase, helium enters one or more cavities due to the existing pressure ratio. Because of the high outflow resistance of the pressure balancing orifice, the aforementioned "capacitor effect" occurs as the pressure fluctuates in the region of the AC heat exchanger with the cooler's operating frequency. During the start-up phase, the temperature of the working gas helium and the helium in the AC heat exchanger cavities decreases. Therefore, the volume of helium decreases, and helium simultaneously flows further into the AC heat exchanger cavities through the pressure balancing orifice. That is, during the start-up phase, helium must be replenished until the operating temperature and pressure are achieved.

[0010] The unit is penetrated by a flow channel defined by the unit wall. This results in an increased heat exchange surface, thereby improving heat transfer between the helium gas inside the cavity and the working gas outside. The flow channel is preferably constructed as an elongated opening. The elongated open flow channels for the working gas are preferably straight and parallel to each other, firstly to minimize flow resistance, and secondly to ensure that the tubular cavities between them are constructed identically. Due to the straightness and parallelism, equidistant flow channels are formed in a simple manner between the two flow channels.

[0011] As an alternative, the flow channels are arranged parallel to each other between the partial cavities.

[0012] Pressure balancing holes can also be provided by the looseness that occurs during the manufacturing of the unit.

[0013] To improve heat exchange between the working gas helium and the heat-storing helium located in the hollow body, the surface of the flow channel is provided with a turbulent structure.

[0014] In 3D printing, rectangular or circular cavities can be manufactured as a single unit or in two separate steps from two components. The openings within these cavities that are blown out of material after 3D printing can then be sealed. Because these openings have a very small cross-sectional area, welding methods are suitable for this purpose.

[0015] Preferably, the support element is provided with a narrow, blind-hole-shaped opening through which the working gas, helium, can enter. This allows it to absorb thermal stress generated during 3D printing in an accordion-like manner, preventing cracks from forming in the material.

[0016] The heat exchangers according to this disclosure are particularly suitable for, in particular, Stirling coolers, Gifford-McMahon coolers, or pulse tube coolers.

[0017] The entire AC heat exchanger preferably has a thickness of 5 mm to 100 mm along the flow direction of the working gas. Attached Figure Description

[0018] Preferred embodiments of the present disclosure are described below with reference to the accompanying drawings.

[0019] The attached diagram shows:

[0020] Figure 1 This is a perspective sectional view of the first embodiment of the AC heat exchanger, and

[0021] Figure 2 This is a perspective sectional view of the second embodiment.

[0022] Wherein: 1-AC heat exchanger; 2-unit; 4-unit wall; 4-1-planar unit wall; 4-2-strip unit wall; 6-i-partial cavity; 8-capillary tube; 10-flow channel for working gas; 12-surround connecting channel; 14-support element. Detailed Implementation

[0023] Figure 1 and Figure 2Two embodiments of the present disclosure are shown, each being a cylindrical AC heat exchanger 2 having a circular cross-section, with only half of the AC heat exchanger 2 shown in each embodiment. The AC heat exchanger 2 includes a unit 2 having a unit wall 4 surrounding a cavity with a partial cavity 6-i. This unit wall 4 is penetrated by a pressure balancing orifice, in the form of a capillary tube 8. The unit 2 has an annular cross-section and is arranged in a tubular flow channel for the working gas helium. The interior of the cavity is filled with helium as a heat storage material during operation. The partial cavities 6-i form a planar structure parallel to the longitudinal axis of the unit 2. Elongated, open, parallel flow channels 10 for the helium as the working gas are constructed between the planar partial cavities 6-i. The partial cavities 6-i are interconnected in the edge region of the cylindrical AC heat exchanger 2 by a connecting channel 12 and together with the partial cavities 6-i form the cavity. The planar and parallel partial cavities 6-i extend along the entire height or length of the cylindrical unit 2, and are formed by two planar unit walls 4-1 arranged at a distance from each other, sealed at the edge region by strip unit walls 4-2. Elongated open flow channels 10 that completely penetrate the unit 2 are arranged between the various partial cavities 6-i.

[0024] Supporting elements 14 are provided inside the partial cavity 6-i, which support the planar unit walls 4-1 to each other. Figure 1 In the first embodiment shown, the support element 14 is constructed as a small rectangular hexahedron distributed inside the partial cavity 6-i. The support element 14 can also be configured as a cylinder, a circle, or a sphere.

[0025] exist Figure 2 In the second embodiment shown, the support element is strip-shaped, extending from the strip unit wall 4-2 to form a tortuous channel. The strip support element 14 is provided with an elongated opening (not shown) for the entry of the working gas, helium. This allows the thermal stress generated during 3D printing to be absorbed in an accordion-like manner, preventing cracks from forming in the material.

Claims

1. An AC heat exchanger (1) for a cryogenic cooler, utilizing helium as the working gas and heat storage medium, having: At least one unit (2) having a unit wall (4) surrounding a cavity having multiple partial cavities (6-i). in, The partial cavities (6-i) are interconnected by at least one connecting channel (12), and are entirely surrounded by unit walls (4) except for the at least one connecting channel (12) leading to the other partial cavities (6-i). The cavity of at least one unit (2) is filled with helium as a heat storage material. A flow channel (10) for the working gas helium, which is constructed between the respective partial cavities (6-i), and The pressure balance hole is in the form of a capillary (8), which penetrates the unit wall (4) and forms a continuously open connection between the working gas helium outside the cavity and the heat storage material helium inside the cavity. Its features are: Each of the aforementioned partial cavities (6-i) has a support element (14) inside it, which supports the unit walls (4) defining the partial cavity (6-i) to each other, and The support element (14) is constructed in the shape of a strip and extends from the unit wall into the partial cavity (6-i).

2. The AC heat exchanger (1) according to claim 1, characterized in that: The cavity is constructed into a tubular shape by means of the arrangement and shape of the support elements and the shape of the unit wall.

3. The AC heat exchanger (1) according to claim 1, characterized in that: The partial cavity (6-i) is constructed to bend by the arrangement and shape of the support element (14) and the shape of the unit wall.

4. The AC heat exchanger (1) according to claim 1, characterized in that: The strip support element (14) is provided with a blind hole-shaped elongated opening, through which the working gas helium can enter.

5. The AC heat exchanger (1) according to claim 1, characterized in that: At least 50% of the unit wall surrounding the partial cavity (6-i) is constructed to be planar.

6. The AC heat exchanger (1) according to any one of claims 2 to 5, characterized in that: The partial cavity (6-i) has a rectangular cross-section.

7. The AC heat exchanger (1) according to any one of claims 2 to 5, characterized in that: The partial cavity (6-i) has a circular cross-section.

8. The AC heat exchanger (1) according to claim 5, characterized in that: The flow channel (10) has a rectangular cross-section between the partial cavities (6-i).

9. The AC heat exchanger (1) according to claim 1, characterized in that: The at least one unit (2) is constructed as a disk with a circular cross-section, and the connecting channel (12) connecting the partial cavity (6-i) is provided in the edge region of the disk-shaped unit (2).

10. The AC heat exchanger (1) according to claim 1, characterized in that: Due to the lack of tightness during the manufacturing of the heat exchanger (1), a capillary tube (8) is formed.

11. The AC heat exchanger (1) according to claim 1 or 2, characterized in that: The outer side of the unit wall has a turbulent structure in the flow channel (10) for the working gas helium.

12. A method for manufacturing an AC heat exchanger (1) according to claim 1, characterized in that: The AC heat exchanger (1) was manufactured by 3D printing.

13. The method according to claim 12, characterized in that: The partial cavity (6-i) has openings after 3D printing, and these openings are subsequently closed.

14. A cryogenic cooler in the form of a Stirling cooler, a Gifford-McMahon cooler, or a pulse tube cooler, having at least one alternating current heat exchanger, characterized in that, The AC heat exchanger is the AC heat exchanger (1) according to any one of claims 1 to 11.

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