CN216084481U - Cooling device for a cryogenic container - Google Patents

Abstract

The present disclosure relates to a cooling device for a cryogenic container. The device includes: a cryocooler having at least a first cooling stage and a second cooling stage; a package having at least first and second corresponding stages in thermal contact with at least first and second cooling stages, respectively, of a cryocooler, the package including a substantially cylindrical upper wall extending between the first stage of the package and an opening of the package. Embodiments of the present disclosure may provide a more reliable cooling device for a cryogenic vessel.

Description

Cooling device for a cryogenic container

Technical Field

The present disclosure relates to a cooling arrangement for a cryogenic vessel, such as a cooling arrangement for cooling a superconducting magnet to an operating temperature. In particular, the present disclosure relates to cooling by a multi-stage cryocooler housed within a recess formed in an outer wall of a cryostat. The outer wall is commonly referred to as an "outer vacuum container" (OVC), and the recess is commonly referred to as a "kit (sock)". The multi-stage cryocooler may be referred to as a "cold head" (CH).

Background

Fig. 1 shows a conventional arrangement for a cryogen vessel, such as an arrangement for cooling a superconducting magnet to an operating temperature. A portion of the outer wall of the OVC is schematically indicated at 10. This portion (very close to the mounting location of CH 11) may be referred to as the sleeve flange. A cavity (sleeve) 12 extends inwardly into the OVC from the sleeve flange. In this example, the CH has two cooling stages. The first cooling stage 14 cools to a first cryogenic temperature and the second stage 16 cools to a second lower cryogenic temperature. In the case where CH cools the superconducting magnet, the second cryogenic temperature is sufficiently cold that the superconducting magnet can be cooled to a temperature at which it superconducts.

The kit has a plurality of stages corresponding to the plurality of stages of the CH.

The Cold Head (CH)11 used in cryogenic systems is typically mounted in a package 12, which package 12 isolates the CH from the vacuum environment. Due to thermal insulation requirements, the walls of the package are typically designed to be as thin as possible to minimize the thermal conduction load from room temperature at the package flange 10 through the material of the walls of the package to the first stage 22. Bolts 18 are used to attach the cooling head 11 to the kit flange 10. Of course, the bolt 18 may be replaced with other equivalent securing means. The elastomeric seal 20 is typically located between the body of the cooling head 11 and the sleeve flange 10. As the bolts 18 are tightened, mechanical pressure is applied from the first cooling stage 14 of CH 11 to the first stage 22 of the package 12. The sleeve 12 is supported by the sleeve flange 10, but is not retained at the first stage 22. A thermal radiation shield 30 is attached to the first stage 22. The second stage 24 of the cartridge 12 and the second stage 16 of CH 11 define a condensation chamber and no mechanical contact is required between the second stage 16 of CH 11 and the second stage 24 of the cartridge 12.

SUMMERY OF THE UTILITY MODEL

The known solutions can still be improved in terms of the reliability of the heat conduction and the mechanical contact of the cooling device. Therefore, there is a need for a cooling device for a cryogenic vessel that at least partially addresses the above-mentioned problems.

According to a first aspect of the present disclosure, a cooling device for a cryogenic vessel is provided. The cooling device includes: a cryocooler having at least a first cooling stage and a second cooling stage; a kit having corresponding at least first and second stages in thermal contact with at least first and second cooling stages of a cryocooler, respectively; the sleeve comprises a substantially cylindrical upper wall extending between the first stage of the sleeve and the opening of the sleeve, characterized in that the substantially cylindrical upper wall of the sleeve comprises at least one ridge.

In one embodiment, the substantially cylindrical upper wall of the cartridge further comprises at least one recess.

In one embodiment, the substantially cylindrical upper wall of the sleeve comprises a plurality of alternating recesses and protrusions to form a bellows structure, the device further comprising a plurality of reinforcing rods extending parallel to the cylindrical upper wall of the sleeve between the opening of the sleeve and the first stage of the sleeve.

According to a second aspect of the present disclosure, a cooling device for a cryogenic container is provided. The cooling device includes: a cryocooler having at least a first cooling stage and a second cooling stage; a kit having corresponding at least first and second stages in thermal contact with at least first and second cooling stages of a cryocooler, respectively; the package comprises a substantially cylindrical upper wall extending between the first stage of the package and the opening of the package, characterized in that the thermal contact between the first stage of the cryocooler and the first stage of the package is provided by a contact ring with flexible fingers, the contact ring being located between the first stage of the cryocooler and the first stage of the package to form a thermal contact between the first stage of the package and the first stage of the cryocooler.

Embodiments of the present disclosure may make a cooling device for a cryogenic vessel more reliable.

Drawings

The above and further objects, features and advantages of the present disclosure will become more apparent from the following description of certain embodiments, given by way of example only, taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a conventional arrangement of a cooling head mounted within a kit of external vacuum vessels;

2(a) to 2(d) represent respective example embodiments of the present disclosure; and is

FIG. 3 illustrates example components that may be employed in embodiments such as that shown in FIG. 2 (d).

Detailed Description

In this arrangement, the upper wall 26 of the sleeve 12 is the stressed member. The mechanical force applied by the bolts 18 to provide intimate mechanical contact between the first cooling stage 14 of CH 11 and the first stage 22 of the sleeve 12 is fully loaded onto the upper wall 26 of the sleeve 12, which sleeve 12 is a thin walled tube. To withstand such axial tension, the material of the upper wall 26 must be at least thick enough so that it does not significantly deform upon application of pressure from the bolt 18. However, using a thicker material means that the upper wall 26 may have a higher thermal conductivity than the preferred thermal conductivity. The material of the upper wall 26 must be impermeable to any gases that may be present in the cartridge, such as helium. In this way, these gases do not diffuse from within the sleeve into the vacuum insulation provided by the OVC. Although a load bearing material of low thermal conductivity may be used to fabricate the upper wall 26, such material may be permeable to helium or other gases found in the kit 12.

Thus, the present disclosure provides an improved package, and in particular an improved upper wall 26 of package 12, which enables mechanical pressure to be provided between the first cooling stage 14 of the CH and the first stage 22 of the package, and effective thermal contact to be provided between the first cooling stage 14 of the CH 11 and the first stage 22 of the package 12, by tightening the bolts 18 between the cooling head 11 and the package flange 10, without the need for a thick upper wall 26 of excessive thermal conductivity. The accessories (including bolts 18, sleeve flange 10, elastomeric seal 20 in this example) provide a limited range of movement between sleeve flange 10 and CH 11 at room temperature.

In a cooling apparatus for a cryogenic vessel, such as a superconducting magnet used for cooling an MRI system, a zero-boil off apparatus is generally employed. For example, in a helium-based cryogenic cooling device, a two-stage cold head CH 11 is typically used. The first cooling stage 14 of the cooling header CH 11 is typically arranged to cool cryogenic system structures such as heat shields 30, suspension devices (not shown in the drawings). The first cooling stage 14 of CH 11 also removes heat from the second stage 16 of CH. The second stage 16 of CH typically cools a refrigerant such as helium below its boiling point and condenses it into a liquid.

As shown in fig. 1, CH 11 is housed in a set 12, the set 12 including an upper thin walled tube 26, a first stage 22, a lower thin walled tube 32, and a second stage 24 at the bottom of the set. The thermal radiation shield 30 is thermally connected to the first stage 22 of the package where the thermal radiation shield 30 is cooled by the first cooling stage 14 of CH 11. In helium-based systems, the first cooling stage 14 may be cooled to a temperature of about 30-40K during operation. Thus, a good thermal contact is required between the first cooling stage 14 of the CH and the first stage 22 of the package, which can be provided by applying pressure when tightening the bolts 18. The force applied by tightening the bolts 18 is carried by the tension in the upper wall tube 26. A refrigerant, such as helium, typically directly contacts the second stage 16 of CH. Therefore, the package 12 must prevent such refrigerant from escaping into the vacuum region provided by the OVC as an insulating means. Thus, in an environment where the exterior is a vacuum, the sleeve 12 must withstand relatively high internal air pressures.

The force applied by the bolt 18 and the internal gas pressure create significant mechanical stress in the upper thin walled tube 26. By way of example, the upper thin-walled tube 26 may be substantially cylindrical. The axial force applied by the bolt 18 will cause tension in the upper thin walled tube 26 in the axial direction, while the gas pressure within the sleeve will cause tension in the upper thin walled tube 26 in the radial direction. This stress will change and may increase as the CH cools in operation due to the different thermal contraction of CH 11 and sleeve 12.

In some arrangements, a piston seal O-ring arrangement may be used. Such an arrangement may provide tolerances to give the required contact force between the first stage of the refrigerator and the first stage of the kit. In some other arrangements, other piston seals may be used, and other sealing arrangements are possible, such as what is known as a face seal.

In some cases, it has been found that the mechanical stress in the upper thin-walled tube 26 may exceed the yield limit of the upper thin-walled tube in the event of an overpressure of refrigerant gas in the kit, or over-tightening of the bolts 18, and then plastic deformation may occur to reduce the contact force between the first stage 22 of the kit and the first cooling stage 14 of the CH. Thus, the required thermal contact may become insufficient. Fatigue of the material of the upper wall tube may cause leaks in the sleeve, allowing refrigerant gas to escape from the sleeve into the vacuum within the OVC, thereby destroying the insulating effect provided by the OVC. It has been found that the vacuum tight joints at the respective ends of the upper thin walled tube 26 are susceptible to failure from excessive loads. Such joints may include, for example, welds or vacuum brazes. Of course, other types of joints may be used.

By way of respective embodiments of the present disclosure, an improved kit device is provided in which the force exerted by an attachment device, such as a bolt 18, is limited.

Fig. 2(a) - (d) show schematic diagrams of concepts according to respective embodiments of the present disclosure. In each case, the present disclosure seeks to eliminate excessive tension-induced plastic deformation in the upper thin-walled tube 26 and ensure good thermal contact between the CH first cooling stage 14 and the first stage 22 of the package. Features that are the same as those shown in figure 1 have corresponding reference numerals.

Referring first to the embodiment of fig. 2(a), instead of the upper thin-walled tube 26 of fig. 1, an upper thin-walled tube 36 is provided. The upper thin walled tube 36 is substantially cylindrical and rotationally symmetric about a longitudinal axis, as in this example CH 11.

In particular, in the embodiment of fig. 2(a), the upper thin-walled tube is provided with a single bulge 38. A ridge (bump) 38 is provided on the upper thin-walled tube, the ridge 38 having a shape with a widened central portion and narrower end portions at upper and lower ends thereof. When CH 11 is assembled by tightening the bolt 18, tension in the upper thin-walled tube 36 may cause the ridge 38 to expand axially by elastic deformation, and this elastic expansion of the ridge limits the stress in the material of the upper thin-walled tube 36. This expansion will provide sufficient elastic deformation to limit the stress in the upper thin-walled tube 26 and avoid plastic deformation thereof.

As the bolts 18 are tightened, a limit is reached where the coolant header CH 11 can no longer be driven closer to the sleeve flange 10. At this time, it can be said that the cooling head CH 11 "bottoms out" with respect to the sleeve flange 10 so that no additional force can be applied to the upper thin-walled tube 36. This will limit the maximum force that must be supported by the upper thin walled tube 36. The flexibility in the upper thin walled tube 36 limits the maximum mechanical load in the sleeve 12.

A second embodiment is shown in fig. 2 (b). In this embodiment, the upper thin-walled tube 36 with the raised portion 38 of fig. 2(a) is replaced with a different upper thin-walled tube 46. The upper thin-walled tube 46 is substantially rotationally symmetric about the longitudinal axis, as is CH 11 in this example. Instead of a single bulge being provided in the upper thin-walled tube 46 of the embodiment of fig. 2(a), the upper thin-walled tube 46 is provided with a plurality of windings 48, 49. In a manner similar to the embodiment of fig. 2(a), the coils 48, 49 allow for elastic deformation of the upper thin-walled tube 46 to accommodate any excessive tension applied by the bolt 18 and prevent plastic deformation of the upper thin-walled tube 46. More windings 48, 49 may also reduce the thermal conductivity between the room temperature end of CH 11 and the first cooling stage 14 of CH 11 because the thermal path provided by the upper thin walled tube 46 is substantially longer.

The elastic deformation of the thin walled tube coils 48, 49 absorbs the displacement of the first cooling stage 14 of CH 11 when the CH is bolted, while ensuring that there is sufficient force to achieve good thermal contact between the first cooling stage 14 of CH 11 and the first stage 22 of the package. The embodiment of fig. 2(b) differs from the embodiment of fig. 2(a) in the number of windings 48, 49 provided in the upper thin-walled tube of the set 12. Embodiments having multiple convolutions 48, 49 such as shown in fig. 2(b) may make the upper wall thin-walled tube 46 more flexible (flex) than the upper wall thin-walled tube 36 of fig. 2 (a).

In the embodiment of fig. 2(a) and 2(b), the upper thin walled tube is provided with a single or several windings 38, 48. The shape change of the coil due to the axial force exerted by the bolt 18 limits the stress in the upper thin walled tube and avoids direct elongation of the straight tube (as in conventional devices). Direct elongation of the straight tube in conventional devices may result in permanent plastic deformation of the upper thin-walled tube. The elastic deformation of the coils of fig. 2(a) and 2(b) may maintain sufficient force to ensure good thermal contact between the first cooling stage 14 of CH 11 and the first stage 22 of the bundle 12.

It has been found that permanent plastic deformation of the upper thin-walled tube 46 is beneficial in the first assembly. This will allow the shape of the upper thin walled tube 46 to be changed to compensate for manufacturing tolerances of the kit. Plastic deformation of the upper thin wall tube 46 on the first assembly onto the cooling tip CH 11 will improve the fit between the upper thin wall tube 46 and the cooling tip CH 11. Alternatively, rather than such plastic deformation with the cooling tip CH 11 on the first component, such permanent plastic deformation may be intentionally performed as part of the part fabrication. This plastic deformation step may also improve the elastic properties by introducing favorable residual stresses in the winding.

However, it has been found that excessive convolutions 48, 49 can cause the thin walled tube to become excessively flexible. In such embodiments, the overly flexible thin-walled tube may not be able to maintain sufficient force between the first stage 22 of the set 12 and the first cooling stage 14 of CH 11.

The embodiment of fig. 2(c) addresses this problem. The embodiment of fig. 2(c) provides an upper thin-walled tube 56 in place of the upper thin-walled tube 26 of fig. 1. The upper thin walled tube 56 is rotationally symmetric about the longitudinal axis, as is CH 11 in this example.

The upper thin walled tube 56 has a plurality of convolutions so that it can be considered a bellows. The bellows is significantly flexible in the axial direction and therefore may not be able to hold sufficient force against the first stage 22 of the package to make effective thermal contact with the first cooling stage 14 of CH. The upper thin walled tube 56 provides a long thermal conduction path from room temperature at the OVC 10 to the first stage 22 of the kit. Thus, the thermal load between the two components is reduced by the bellows in the upper thin-walled tube 56. However, in this example, the bellows are too flexible to maintain sufficient force to ensure good thermal contact between the first stage 22 of the package and the first cooling stage 14 of the refrigerator.

In this embodiment of the present disclosure, a stiffener 60 is provided around the upper thin walled tube 56 of the sleeve 12. At least two stiffeners, but preferably at least three stiffeners, are provided distributed around the upper thin walled tube 56 of the sleeve 12 to hold the first stage 22 of the sleeve 12 in a fixed position relative to the OVC sleeve flange 10. More than three reinforcing rods may be provided. As shown in fig. 2(c), the stiffener 60 may be located outside the sleeve, or may be located inside the sleeve, extending between the first stage 22 of the sleeve and the OVC sleeve flange 10. The stiffener 60 holds the first stage 22 of the sleeve 12 in a fixed position relative to the OVC sleeve flange 10. The stiffeners 60 thus constrain the first stage 22 of the sleeve in a fixed position relative to the OVC sleeve flange 10 so that a suitable interfacial force can be established between the first stage 22 of the sleeve 12 and the first cooling stage 14 of CH 11. The reinforcing rods are made of a material having good mechanical elasticity but low thermal conductivity. For example, resin-impregnated carbon fibers or resin-impregnated glass fibers may provide suitable materials. The upper end of the reinforcing rod 60 may be attached to the lower surface of the kit flange 10 by any suitable means, such as bolts or resin bonding. For example, bolts 62 are shown attaching the stiffener 60 to the first stage 22 of the sleeve 12. Of course, other means such as resin bonding or interference fit may be used instead. When the CH is mounted to the kit flange 10, the bolts 18 are tightened and the seal 20 is compressed between the body of the CH 11 and the kit flange 10. The stiffener 60 holds the first stage 22 of the package in a fixed position relative to the package flange 10 and, when the bolts 18 are tightened, interfacial forces are generated between the first stage 22 of the package and the first cooling stage 14 of the CH to establish effective thermal contact. The upper thin walled tube 56 having a bellows configuration does not participate in establishing this interfacial force. At least two reinforcing rods 60 should be provided, but preferably at least three reinforcing rods are provided. The interfacial force is established by the tension in the stiffener 60.

In the embodiment of fig. 2(c), the upper thin-walled tube 56 need not withstand any bolt forces resulting from tightening the bolt 18. The upper thin walled tube 56 also need not support axial pressure loads. Thus, the upper thin-walled tube 56 may be constructed of a thinner material than would otherwise be possible. A plurality of stiffeners 60 are used to fix the relative position of the first stage 22 of the sleeve with respect to the sleeve flange 10 of the OVC. The bolt force generated by tightening of the bolt 18 is transmitted to the reinforcing bar 60. The thin walled tube 56 may then usefully be embodied as a corrugated tube having excellent flexibility and a long heat conduction path.

Fig. 2(d) shows another embodiment of the present disclosure. In the embodiment of fig. 2(d), a contact ring 64 with flexible fingers is provided to make thermal contact between the first stage 22 of the package and the first cooling stage 14 of the CH. In this embodiment, no windings are provided in the upper thin walled tube 66. Of course, a winding may also be provided. In this embodiment, when the bolts 18 are tightened to compress the seal 20 between the body of the CH 11 and the package flange 10, the first stage 22 of the package 12 exceeds the first cooling stage 14 of the CH and is not in direct mechanical or thermal contact therewith. Instead, a contact ring 64 with flexible fingers is provided, the contact ring 64 being located between the first stage 22 of the sleeve 12 and the first cooling stage 14 of the CH. The contact ring 64 should be a flexible thermally conductive material, such as copper.

When CH 11 is inserted into the sleeve 12 and the bolts 18 are tightened to compress the seal 20, the contact ring 64 with the flexible fingers is pressed into thermal contact with the first stage 22 of the sleeve 12 and the first cooling stage 14 of CH 11. The flexibility of the fingers of the contact ring 64 ensures that thermal contact is provided between the first stage 22 of the nest and the first cooling stage 14 of the CH without exerting excessive force on the upper thin walled tube 66.

Another benefit of embodiments such as that shown in fig. 2(d) is that the contact pressure due to the flexible fingers making thermal contact between the first stage 22 of the package and the first cooling stage 14 of the CH will be significantly less affected by the gas pressure in the package than is the case in some other embodiments of the present disclosure. In some other described embodiments, the contact pressure between the first stage of the cooling head CH 11 and the first stage 22 of the package varies as the pressure of the gas in the package varies.

Fig. 3 shows some examples of contact rings that may be used in the embodiment of fig. 2 (d). Other equivalent structures may be provided, but these need to be thermally conductive and flexible so that they are compressed between the first stage 22 of the kit and the first cooling stage 14 of the CH.

According to the present disclosure, the risk of failure due to plastic deformation of the upper thin walled tube of the sleeve 12 is reduced or eliminated by increasing the elastic deformation of the upper thin walled tube of the sleeve 12, or by providing the contact ring 64 with flexible fingers or equivalent structure to provide thermal contact between the first stage 22 of the sleeve 12 and the first cooling stage 14 of the CH.

In the embodiment of fig. 2(a) and 2(b), the displacement of CH pushed down by tightening the bolt 18 is compensated by elastic deformation of the upper thin-walled tube or of the flexible fingers or equivalent of the contact ring 64 in the embodiment of fig. 2 (d). In the embodiment of fig. 2(c), the displacement of CH pushed downward by tightening the bolt 18 is compensated by the additional reinforcement bar 60.

Elastic deformation of the upper thin walled tube or thermal contact (such as contact ring 64) means: the tensile stress in thin walled tubes is significantly reduced compared to conventional devices such as that shown in fig. 1. This reduces the risk of failure of the part due to plastic deformation that can be observed in conventional devices such as that shown in figure 1. Because of this reduction in tensile stress, a thinner wall can be used as the upper thin-walled tube. This therefore reduces the thermal conductivity of the upper thin-walled tube. Furthermore, in embodiments employing ridges 38, convolutions 48, 49 or bellows, the thermal path provided by the upper thin wall tube from the OVC sleeve flange 10 to the first stage 22 of the sleeve 12 is elongated due to these features. This further reduces the thermal conductivity of the upper thin walled tube. This reduced thermal conductivity of the upper thin walled tube reduces the conductive thermal load on the first cooling stage 14 to CH 11.

Thus, the present disclosure reduces the risk of failure of the upper thin walled tube due to mechanical overload. Often, such mechanical overload may cause the cartridge to leak. The thermal contact between the first stage 22 of the package 12 and the first cooling stage 14 of the CH may also be improved compared to the arrangement of fig. 1. All of these improvements make cryogenic systems more reliable.

Claims (4)

1. A cooling apparatus for a cryogenic vessel comprising:

-a cryocooler (11) having at least a first cooling stage (14) and a second cooling stage (16);

-a kit (12) having at least a first stage (22) and a second stage (24) respectively, the at least first stage (22) and second stage (24) being in thermal contact with the at least first cooling stage (14) and second cooling stage (16) of the cryocooler, respectively;

the sleeve (12) comprises a substantially cylindrical upper wall (36; 46; 56) extending between the first stage (22) of the sleeve and an opening of the sleeve,

characterized in that said substantially cylindrical upper wall of said set comprises at least one protuberance (38; 48).

2. Cooling device for a cryogenic vessel according to claim 1, whereby the substantially cylindrical upper wall of the kit further comprises at least one recess (49).

3. Cooling device for a cryogenic vessel according to claim 1, whereby the substantially cylindrical upper wall of the kit comprises a plurality of alternating recesses and protrusions to form a bellows structure (56),

the apparatus also includes a plurality of reinforcing rods (60), the plurality of reinforcing rods (60) extending parallel to the cylindrical upper wall of the sleeve between the opening of the sleeve and the first stage (22) of the sleeve.

4. A cooling apparatus for a cryogenic vessel comprising:

-a cryocooler (11) having at least a first cooling stage (14) and a second cooling stage (16);

-a kit (12) having at least a first stage (22) and a second stage (24) respectively, the at least first stage (22) and second stage (24) being in thermal contact with the at least first cooling stage (14) and second cooling stage (16) of the cryocooler, respectively;

the sleeve (12) comprises a substantially cylindrical upper wall (36; 46; 56) extending between the first stage (22) of the sleeve and an opening of the sleeve,

characterized in that the thermal contact between the first cooling stage (14) of the cryocooler and the first stage (22) of the package is provided by a contact ring (64) with flexible fingers, the contact ring (64) being located between the first cooling stage (14) of the cryocooler and the first stage (22) of the package to form thermal contact between the first stage (22) of the package and the first cooling stage (14) of the cryocooler.

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