GB2513351A

GB2513351A - Refrigerator Mounting Assembly for Cryogenic Refrigerator

Info

Publication number: GB2513351A
Application number: GB201307355A
Authority: GB
Inventors: Michael Simpkins
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2013-04-24
Filing date: 2013-04-24
Publication date: 2014-10-29
Anticipated expiration: 2033-04-24
Also published as: GB201307355D0; GB2513351B

Abstract

A mounting assembly for a two-stage cryogenic refrigerator (17) comprising a sock (15) having first and second stages corresponding to first and second stages of the refrigerator. In use, the first stage of the refrigerator is in thermal contact with the first stage of the sock and the second stage of the refrigerator is in thermal contact with the second stage of the sock. Fasteners (40) are provided, mechanically clamping the second stage of the refrigerator into contact with the second stage of the sock.

Description

REFRTGERATOR MOUNTING ASSEMBLY FOR CRYOGENIC REFRIGERATOR

The present invention relates to improved arrangements for mounting of a cryogenic refrigerator in a cryostat. In particular, the present invention provides such arrangements for use in cryostats which form part of an MRI system.

Fig. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVO) 14. One or more thermal radiation shields 16 are provided in the vacjum space between the cryogen vessel 12 and the outer vacuum chamber 14.

In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liguid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in Fig. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-lOOK. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.

A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.

The present invention is particularly concerned with mounting arrangements for cryogenic refrigerator 17 and its interface with refrigerator sock 15.

A conventional arrangement for locating a cryogenic refrigerator 17 uses an evacuated sidesock 15. In this arrangement, a two-stage refrigerator is generally used. A first stage 30 of the refrigerator is generally pressed into contact with a first stage of the sidesock. That first stage of the sidesock is generally in thermal contact with thermal radiation shield 16. At a lower, closed, end of the sidesock, a second stage 32 is provided. When in position, the second stage of the refrigerator may be pressed into contact with the second stage of the sidesock. The second stage of the sidesock is typically thermally linked to a heat exchanger which is exposed to gaseous cryogen in the cryogen vessel. In some arrangements, the heat exchanger is exposed directly to the interior of the cryogen vessel. In other arrangements, the heat exchanger is positioned within a small recondensing chamber, which is linked to the main cryogen vessel by one or more passageways.

In such arrangements, it is important to have a suitable mechanical pressure on both first and second stages of the refrigerator, which must be maintained when in use at cryognic temperatures.

In alternative arrangements, the second stage of the refrigerator may be directly exposed to cryogen gas from the cryogen vessel. In such arrangements, cryogen gas recondenses of the second stage of the refrigerator and return to the liquid cryogen in the cryogen vessel.

Refrigerator sock 15 may have a flexible connection of some sort built in, in an attempt to ensure effective mechanical connection despite variations in component sizes due to build tolerances.

Although troublesome, it is important to achieve acceptable thermal contact between the first 30 and second 32 stages of the refrigerator and the corresponding stages of the refrigerator sock 15. The first and second stages of the refrigerator are more clearly visible in Fig. 2. In case of insufficient thermal contact, effective cooling will not be provided to the thermal radiation shield and the heat exchanger, and it may not be possible to maintain the required temperature within the cryogen vessel. For example, if the mating faces of the stages of the refrigerator and the stages of the refrigerator sock are not accurately formed due to assembly tolerances, then the thermal contact surface area, and therefore recondensing performance, will be reduced.

The present invention addresses the problems described above and provides apparatus as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments, in conjunction with the accompanying drawings, wherein: Fig. 1 schematically illustrates a conventional cryogenically-cooled superconducting magnet assembly, which may be modified according to the present invention; Fig. 2 illustrates a conmercially-available cryogenic refrigerator which may be used in an arrangement 0± the present invention; Figs. 3A and 3B show the refrigerator of Fig. 2 modified according to certain features of the present invention; Fig. 4 shows a sock for accommodating a cryogenic refrigerator, according to certain features of the present invention; Fig. 5 shows a similar view to that of Fig. 4, but in which certain features are shown transparent; Fig. 6 shows an axial cross-section through a sock as illustrated in Figs. 4, 5; Fig. 7 shows a view of the refrigerator of Figs. IA, 3B assembled into a sock as shown in Fig. 5; and Fig. 8 shows an axial cross-section through the assembly of Fig. 7.

The present arrangement provides an improved refrigerator sock and improved interface arrangements to ensure effective thermal contact between stages of a two-stage cryogenic refrigerator and corresponding stages of a refrigerator sock.

In an example of the present invention, the refrigerator is mounted in an evacuated refrigerator sock, but the thermal contact surfaces are pressed together by bolts or similar mechanical fasteners. Other similar fixing means may be used in other embodiments, but the important point is that one or more fastener is used which allows a controlled clamping force to be provided between the stages of the refrigerator and the stages of the sock. The controlled clamping force will, if necessary, provide some deformation of one or more stage of the refrigerator and/or one or more stage of the refrigerator sock, thereby to provide an increased contact area between refrigerator and sock. This is beneficial because effective thermal contact may be provided even though some parts of the refrigerator and/or sock may be of inaccurate construction, within allowed manufacturing tolerances.

Figs. 2-8 show refrigerator 17 and refrigerator sock 15 with their axis A-A approximately horizontal. In embodiments of the present invention, axis A-A will typically be approximately vertical, as shown in Fig. 1, but is shown approximately horizontal in the drawings for ease of representation. The sidesock can be at any angle although the

S

fridge works better vertical, either "upright" as shown in Fig. 1 or inverted.

Fig. 2 shows a two-stage cryogenic refrigerator 17, as commercially available, to which the present invention may be applied. The refrigerator has a first stage 30 and a second stage 32. An OVC flange 34 is provided to attach the refrigerator to the CVC 14, and which is used to provide a vacuum seal for the refrigerator sock 15. In operation, the first stage 30 is cooled to a temperature of about 50-80K, and the second stage is cooled to a temperature of about 4K, to provide recondensation of helium. The inner workings of the cryogenic refrigerator 17 are not the subject of the present invention.

Figs. 3A and 33 show a cryogenic refrigerator 17 similar to that shown in Fig. 2, modified according to an aspect of the present invention, from two viewpoints. A bracing piece 36 is shown attached to the second stage 32. A lower surface 44 of the second stage protrudes beyond the bracing piece 36. The bracing piece 36 is shown formed of more than one piece, assembled together around the second stage by fasteners 45, and mechanically attached to the second stage by further fasteners 42. Three protrusions 48 are shown, being parts of the bracing piece which extend radially away from the second stage 32. More or fewer than three may be provided, but three is the presently preferred number. Each of the protrusions carries a captive fastener 40. The captive fastener may be a bolt with recessed hexagonal head, although equivalent fastenings may be used. The purpose of the bracing piece and the fasteners will be explained below.

Fig. 4 shows an example of a refrigerator sock 15 according to an aspect of the present invention. First stage 61 is shown.

When installed within a cryostat, first stage 61 will be in thermal contact with the thermal radiation shield 16. A heat exchanger 70 is provided at the closed end of the sock, but is not visible in Fig. 4, as a recondensing chamber 50 is positioned around the heat exchanger. Cryogen feed and return pipes 52 are shown. In use, these would provide access between the cryogen vessel 12 and the recondensing chamber 50.

A bellows arrangement 54 is provided in a wall 56 of a lower section of the sock 15. A wall 58 of an upper part of the sock does not require a bellows section, since variation in build tolerance may be accommodated between the CVC and first stage by an 0-ring seal (not illustrated) at the interface between the OVC and the refrigerator flange 34. Mechanical tie rods brace first stage 61 of the sock against second stage retaining structure 63. As shown, the tie rods are simple rods 60 with threaded ends, and nuts 62 or similar fasteners bear against the first stage 61 of the sock and the second stage retaining structure 63, providing tension in the tie rods. In the illustrated embodiment, four tie rods 60 are shown, although more or fewer could be used.An upper interface piece 64 is shown. In use, interface piece 64 will typically be welded into a corresponding hole in OVC 14, to seal the interior of the sock from the interior of the OVC, and provide a mounting point for OVC flange 34.

Fig. 5 shows a similar view of the refrigerator sock 15, this time with the walls 58, 56 of The sock shown transparent. In this drawing, it is shown that the first stage 61 of the sock is provided with a cut-out 66 of suitable shape and size to allow the bracing piece 36 attached to refrigerator 17 to pass through. Second stage 68 is visible, along with heat exchanger 70 which is thermally linked to second stage 68.

End piece 72 is shown, closing the end of the sock, and braced against first stage 61 by retaining structure 63 and tie rods 60. End piece 72 contains tapped holes or recesses 74 to accommodate fasteners 40, as will be explained below. Item 64 is welded to the OVC, and will need to have a central hole which is large enough hole for bracing piece 36 and first stage interface piece 38 to pass through.

Fig. 6 shows a cross-section through the structure of Fig. 5, taken in a plane containing axis A-A. The detailed structure of the lower end of the sock, described above, is more clearly illustrated in this drawing.

Fig. 7 shows a view, similar to the view in Fig. 5, where the walls 56, 58 of the sock are shown transparent. Fig. 8 shows a similar view, in cross-section, taken in a plane containing axis A-A. The refrigerator 17 is shown in place. Protrusions 48 of the bracing piece 36 are mechanically attached to the end piece 72 by fasteners 40 which may be recessed-hex headed M8 or Mb bolts, for example. As mentioned above, second stage 32 of the refrigerator protrudes beyond the bracing piece 36.

Tension in fastener 40 causes end surface 44 of second stage 32 of the refrigerator to press onto an exposed surface of the second stage 68 of the refrigerator sock. This places the second stage of the refrigerator in effective thermal contact with the second stage 68 of the sock, and the heat exchanger 70. By appropriate selection of the axial length of the wall 56 of the lower part of the sock, and force required to deform bellows 54, one can ensure that, at the same time that effective thermal contact is provided between the second stage 32 of the refrigerator 17 and the second stage 68 of the sock, a suitable pressure is provided between the first stage 30 of the refrigerator, first stage interface piece 38 and the first stage 61 of the sock.

The fasteners 40 must be tightened after the refrigerator 17 has been placed in the sock 15. Access must be provided for a tool to reach the heads of fasteners 40 once the refrigerator is in place. Typically, the heads of fasteners 40 are about 400mm below the surface of the OVC.

As shown in Figs. 3A, 3E, 4, 7, 8 access holes 74 are provided in the first stage interface piece 38 and interface piece 64 to allow tool, such as a long allen key, to reach the heads of fasteners 40 to tighten them. Similarly, as shown in Fig. 7, the cut-out 66 in the first stage 61 of the sock 15 is aligned with the fasteners 40. These are also aligned with the fasteners 40. Accordingly, once the refrigerator 17 is located in the sock 15, a tool such as a long allen key or screwdriver, as appropriate for the type of fastener 40 selected, is passed through access holes 76, 74 and cut-out 66 to reach fasteners 40. Fasteners 40 are then tightened to a predefined torgue, which is sufficient to ensure an effective contact surface area between end surface 44 of second refrigerator stage 32 and the adjacent surface of the second stage 68 of the sock.

Preferably, the length of the lower wall 56 of the sock, including bellows 54, is such that the tightening of the fasteners 40 causes some compression of the bellows 54.

Alternatively, or in addition, the relative thermal expansion coefficients of the components cause some compression of bellows 54 as the refrigerator cools to its operational temperature. The compression of the bellows 54 ensures that an appropriate interface pressure is provided between the first stage 30 of the refrigerator and the first stage 61 of the sock. Such interface pressure remains within a tolerable range even though the precise axial separation between first and second stages of the refrigerator and first and second stages of the sock may vary due to build tolerances. Later on, a vacuum is pumped in the sock, the bellows will relax due to loss of internal atmospheric pressure as discussed in further detail below.

The fasteners 40 are accessed through upper interface piece 64. Preferably, the fasteners are captive, and in addition to providing clamping force, they can be used as jacking screws for removal of the refrigerator.

Another feature of this design is the tie rods 60 which span the first 61 and second 68 stages of the sock 15. When the refrigerator 17 is fitted, the sock 17 has atmospheric pressure internally and vacuum externally, on the surface exposed to the interior of the CVC. Atmospheric pressure acting on the base of the sock 15 will tend to extend the bellows. Under these conditions the tie bars restrain the base to prevent over-extension of the bellows 54. When the refrigerator 17 is fitted and a vacuum is drawn within the sock 15. The bellows are slightly compressed, causing the tie bars to become inactive therefore preventing the tie bars acting as a heat transfer path during operation of the refrigerator 17.

In preferred embodiments of the present invention, a conformal layer of indium or a thermally conductive grease suitable for use at a temperature of about 4K may be provided between first stage 61 of the sock and the first stage 30 of the refrigerator. This conformal layer assists with ensuring an effective thermal contact between the first stage of the refrigerator and the first stage of the sock. Similarly, a conformal layer of indium or a thermally conductive grease suitable for use at a temperature of about 4K may be placed between the second stage of the refrigerator and the second stage of the sock. A piston-type 0-ring seal may be provided at the OVC to enable build toThrances to be taken up at the first stage.

Claims

CLAIMS1. A mounting assembly for a two-stage cryogenic refrigerator (17) comprising a sock (15) having first (61) and second (68) stages corresponding to first (30) and second (32) stages of the refrigerator (17) wherein, in use, the first stage of the refrigerator is in themal contact with the first stage of the sock and the second stage of the refrigerator is in thermal contact with the second stage of the sock, characterised in that fasteners (40) are provided, mechanically clamping the second stage of the refrigerator into contact with the second stage of the sock.
2. A mounting assembly according to claim 1, wherein the sock comprises a lower wall (56) extending between the first stage and the second stage, and an upper wall (58) extending away from the first stage in a direction opposite to the lower wall, wherein the lower wall comprises a bellows portion (54)
3. A mounting assembly according to claim 1 or claim 2, wherein fasteners (40) comprise bolts or similar mechanical fasteners.
4. A mounting assembly according to any preceding claim, wherein a bracing piece (36) is provided, attached to the second stage (32) of the refrigerator, the fasteners (40) acting on the bracing piece to press the second stage of the refrigerator into contact with the second stage of the sock.
5. A mounting assembly according to claim 4, wherein a lower surface (44) of the second stage of the refrigerator protrudes beyond the bracing piece (36) and is in contact with the second stage (68) of the sock.
6. A mounting assembly according to any preceding claim, wherein mechanical tie rods (60) brace first stage (61) of the sock against a second stage retaining structure (63) to mechanically restrain the second stage of the sock against the first stage of the sock.
7. A mounting assembly according to any preceding claim, wherein the sock (15) is provided with an upper interface piece (64)
8. A mounting assembly according to claim 2 wherein the length of the lower wall (56) of the sock, including bellows (54), is such that tightening of the fasteners (40) causes some compression of the bellows (54)
9. A mounting assembly according to claim 2 wherein the relative thermal contraction of the components causes some compression of bellows (54) as the refrigerator cools to its operational temperature.