WO2005116514A1 - Reduction des fuites de liquide cryogene lors du transport de cryostats - Google Patents

Reduction des fuites de liquide cryogene lors du transport de cryostats Download PDF

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Publication number
WO2005116514A1
WO2005116514A1 PCT/EP2005/005152 EP2005005152W WO2005116514A1 WO 2005116514 A1 WO2005116514 A1 WO 2005116514A1 EP 2005005152 W EP2005005152 W EP 2005005152W WO 2005116514 A1 WO2005116514 A1 WO 2005116514A1
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WO
WIPO (PCT)
Prior art keywords
thermal
refrigerator
passageway
cryogen
contact
Prior art date
Application number
PCT/EP2005/005152
Other languages
English (en)
Inventor
Timothy John Hughes
Stephen Paul Trowell
Keith White
Original Assignee
Siemens Magnet Technology Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GBGB0411605.9A external-priority patent/GB0411605D0/en
Application filed by Siemens Magnet Technology Ltd filed Critical Siemens Magnet Technology Ltd
Priority to US11/597,654 priority Critical patent/US8950194B2/en
Publication of WO2005116514A1 publication Critical patent/WO2005116514A1/fr

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/04—Cooling

Definitions

  • Superconducting magnet systems are used for medical diagnosis, for example in magnetic resonance imaging systems.
  • a requirement of an MRI magnet is that it produces a stable, homogeneous, magnetic field.
  • a superconducting magnet system which operates at very low temperature. The temperature is typically maintained by cooling the superconductor by immersion in a low temperature cryogenic fluid such as liquid helium.
  • the superconducting magnet system typically comprises a. set of . superconductive windings for producing a magnetic field, in a cryogenic fluid vessel which contains the superconductor windings, immersed in a cryogenic fluid to keep the windings at a superconducting temperature.
  • the cryogenic fluid vessel is typically surrounded by one or more thermal shields, and a vacuum jacket completely enclosing the shield(s) and the cryogenic vessel. . . .
  • An access neck typically passes through the vacuum jacket from the exterior, into the cryogenic vessel. Such access neck is used for filling the cryogenic vessel with cryogenic liquids and for passing services into the cryogenic vessel to ensure correct operation of the magnet system.
  • Cryogenic fluids and particularly helium, are expensive and it is desirable that the magnet system should be designed and operated in a manner to reduce to a minimum the amount of cryogenic liquid consumed.
  • Cryogenic liquid may be lost due to boil-off, caused by thermal leaks into the cryogenic vessel.
  • the vacuum jacket reduces the amount of heat leaking to the cryogenic vessel by conduction and convection.
  • the thermal shields reduce the amount of heat leaking to the cryogenic vessel by radiation. In order to further reduce the heat load - the heat leaking to the cryogenic fluid vessel, and thus the loss of liquid - it is common practice to use a refrigerator to cool 5 the thermal shields to a low temperature.
  • the refrigerator cooling the one or more shields and/or the cryogen vessel is inactive, and is incapable of diverting the heat load from the cryogen vessel.
  • the refrigerator itself provides a ⁇ low thermal resistance path for ambient heat to reach the cryogenic vessel. This in turn means a relatively high level of boil-off during transportation, leading to loss of cryogen liquid.
  • the boiled off cryogen is typically vented to the atmosphere in such circumstances. It is desirable to reduce the loss of cryogen to the minimum possible, both since cryogens are costly and in order to prolong the time available for delivery: the time during which the system can remain with the refrigerator inoperable but still contain some cryogen liquid.
  • the gas boiled off from the cryogen liquid leaves the cryogen vessel solely through the access neck. It is well known that the cold gas from boiling cryogenic liquids can be employed to reduce heat input to cryogen vessels, by using the cooling power of the gas to cool the access neck of the cryogen vessel and to provide cooling to thermal shields by heat exchange with the cold exhausting gas.
  • the refrigerator of the superconductive magnet system When the refrigerator of the superconductive magnet system is turned off for transportation, ambient heat is conducted along the passive refrigerator to reach the thermal shield(s) and/or the cryogen vessel.
  • the refrigerator is typically removably connected to the thermal shield(s) and cryogenic vessel by a refrigerator interface. It has been demonstrated that removing the refrigerator from the refrigerator interface can noticeably reduce the heat load onto the internal parts of the system, and therefore reduce the loss of cryogenic liquid.
  • the benefits of this solution are outweighed by its disadvantages;: the refrigerator must be replaced when putting the sMBI system into operation, and it is desired to keep this latter operation. as simple • as possible.
  • Replacing the: refrigerator may involve difficult . and . skilled • . operations. It is- also required to permit operation of the refrigerator as. soon ; as possible after the magnet system arrives at site, and even before the system has been fully set up, to prevent further loss of cryogen.
  • the present invention accordingly addresses the problem of cryogen loss from an inoperative superconductive magnet system, in particular the problem that the inoperative refrigerator presents a heat load on the magnet system which results in loss of cryogenic liquid.
  • part of the boil-off gas is directed from the cryogen vessel through the refrigerator interface and past the refrigerator to cool the refrigerator.
  • Some of the heat conducted along the refrigerator into the system is intercepted and removed by that part of the boil-off gas.
  • the heat load onto the cryogenic vessel is thereby reduced, which in turn reduces the boil-off of cryogen from the cryogenic vessel.
  • This part of the boil-off gas is then vented from the system along with the remainder of the boil-off gas, for example to leave the cryogenic liquid vessel via the access neck.
  • FIG. 1 shows a cross-section of a superconducting magnet system for use in ., an MRI system, adapted according to an embodiment of the present invention
  • Fig. 2 shows a cross section of part of the superconducting magnet system of Fig. 1 in more detail
  • Fig. 3 shows certain details of the embodiment of the invention shown in Fig. 2;
  • Fig. 4 shows a view corresponding to that of Fig. 3, according to another embodiment of the invention.
  • Fig. 5 shows an embodiment of the present invention adapted for shield cooling.
  • FIG. 1 shows a cross-section of a superconducting magnet system 3 for use in an MRI system, adapted according to an embodiment of the present invention.
  • a two-stage cryogenic refrigerator 1 is removably connected by an interf ce sock (also known as an interface sleeve) 2, such that its first stage cools the shield 20 and its second stage cools the cryogenic vessel 5.
  • the refrigerator is preferably arranged as a recondensing refrigerator.
  • a heat exchanger cooled by the second stage of the refrigerator is exposed to the interior of the cryogenic vessel 5, for example by a tube 4.
  • the refrigerator is, in operation, thereby enabled to reduce the consumption of cryogenic liquid by recondensation of boiled off cryogen back into its liquid state.
  • Superconductive magnet coils are provided in cryogenic vessel 5.
  • the interface sock is a chamber extending from the exterior of the cryostat 3 to be in thermal connection with . the cryogen vessel 5.
  • the. interior - of -the ⁇ yogen: vessel may be exposed" to . the . interior of the sock.
  • the sock is preferably composed of a thin wall of a , material of relatively low thermal conductivity, such as certain grades of . stainless steel.
  • the coils are immersed in a cryogenic liquid 5a.
  • a thermal. , shield 20 is provided around the cryogenic vessel.
  • a vacuum jacket 22 encloses the cryogenic vessel and the shield in a vacuum.
  • a central bore 24 is provided, to accommodate a patient for examination.
  • An access neck 7 is provided to allow access to the cryogenic vessel 5.
  • a pipe 6 provides a gas conduit from the top of the interface sock 2 to the top of the access neck 7.
  • Boil-off gas from the cryogen 5a may flow from the cryogen vessel 5 through tube 4, through interface sock 2 and along pipe 6 to the access neck 7.
  • the advantage provided by the presence of the pipe 6 is that, during transportation, a proportion of the boil-off gas from the boiling cryogen passes up through the interface sock 2, past the refrigerator 1. This cools the refrigerator 1 and reduces the ambient heat being conducted into the superconductive magnet system by the inoperative refrigerator 1.
  • the pipe 6 is closed by one or more valves when the superconductive magnet system is in operation.
  • FIG. 5 Figure 2 shows a more detailed view of the refrigerator interface sock 2 and the pipe 6.
  • boil-off of the cryogen 5a will occur, and boil-off gas will be produced at a temperature slightly above the boiling point of the cryogen.
  • Liquid helium is currently used in many superconductive magnet 10 systems. In such a system, the boil-off ' gas will have a temperature in the range of 4K.
  • the refrigerator 1 will be exposed to an ambient temperature of
  • Boil-off gases generated in cryogenic vessel 5 may leave the vessel either by the access neck 7, or, according to an aspect of the present invention, through the tube 4, through the interface sock 2 past the refrigerator, and then through pipe 6. These two paths preferably meet just upstream of an
  • Pipe 6 is preferably fitted with a valve 12 which is open during transportation but may be closed during normal operation of the magnet system when the refrigerator is operational.
  • pipe 6 may be fitted with a means 13 to regulate the flow of gas past the refrigerator, conveniently realized by use of a suitably sized orifice.
  • the orifice may be of fixed size, or may be adjustable.
  • FIG. 3 shows further detail of the refrigerator in the interface sock and in particular the thermal connection 15, 30 of the first stage of the refrigerator to the shield 20.
  • thermal connection between the first stage heat exchanger 28 and its contact flange 15, and the thermal contact 30 linked to shield 20 is achieved by using a pressed taper, although other means known in the art may alternatively be employed.
  • the thermal connection may employ indium metal to improve the thermal contact, between contact flange 15 and thermal contact 30.
  • Fig. 3 illustrates certain alternative arrangements providing a path for the boil-off gas through the thermal connection.
  • the boil-off gas may pass through the thermal connection via channels providing a passageway past or through the contact flange 15.
  • a channel 14 is cut into an outer contact face of the contact flange.
  • a channel 16a is cut into an inner face of the contact flange with a connected radial channel 16 cut into the upper surface of the contact flange.
  • the channel 14 is shown in position in the right hand side, with the channels 16, 16a in position being shown on the left hand side.
  • an oblique hole 17 may be drilled or otherwise formed through the contact flange 15, to provide a passageway for gas to flow between lower sock portion 8 and upper sock portion 9.
  • the thermal shield 1 20 which' is thermally connected to the first cooling 'stage: ' -, of the refrigerator by thermal link 19, which may be of any suitable known •type; such as flexible copper braiding. . , : , - , .. • ⁇ ; • . . :. ' ⁇ ,5 ' ⁇ ⁇ - . ⁇ y
  • passageways such as those shown at 14, 16, 16a, 17 may alternatively, or additionally, be provided in the thermal contact 30 rather than only in the ontact flange 15, , . . . . 0
  • the boil-off gas flows past the refrigerator, initially at a temperature of about 4K, the refrigerator is cooled.
  • cryogen boil-off gas may be used to cool the shield 20 directly; in much the same way as it is used to cool the refrigerator.
  • Cold gas may be taken from helium vessel 5 via pipe 31, which is preferably of low thermal conductivity, and passed through a tube 32 which is in close thermal contact with the shield.
  • the tube would exit from the vacuum jacket 22 via pipe 33, which is preferably of low thermal conductivity, into the venting system via pipe 34.
  • Gas flow may be controlled by use of valves and orifices, in the same manner as described below for refrigerator cooling. By this means, the gas flow may be balanced to optimize th cooling.performance for the system. ,- .. ., . .
  • This configuration niaximises the use of the. gas enthalpy to cool the shield, and may be used to.. minimize the cryogen losses during transport of the. system. Liquid cryogens may also be passed through this. heat exchanger, tube to reduce the time required for initial cool-down of the system from, room temperature.
  • Refrigerator 1 may be of any known type, such as a Gifford-McMahon or pulse tube refrigerator.
  • the upper parts of the refrigerator in particular, may contain relatively delicate mechanical parts. There is a risk that the flow of boil-off gas past the refrigerator, as provided by the present invention, may damage certain parts of the refrigerator by cooling them to a temperature far below their normal operating temperature. In certain embodiments of the present invention, therefore, steps must be taken to ensure that the refrigerator is not excessively cooled by the boil-off gas to such an extent that damage to the refrigerator may be caused.
  • a restrictor orifice 13 may be placed on the pipe 6. This may be a fixed orifice or an adjustable orifice.
  • the mass flow of boil-off gas past the refrigerator may be controlled, and so the refrigerating effect of the boil- off gas on the various parts of the refrigerator may be controlled.
  • the passageway such as 14; 16, 16a; 17 through the thermal connection 15, 30 also acts as a gas flow rate regulation.
  • the orifice 13 may also be suitably sized to limit the gas flow through pipe 6 to balance the flow through pipe 6 with the flow of boil-off gas., through the access neck 7.
  • the gas flow in tube 6 and. in the access neck 7 may. be measured, to ensure appropriate, cooling ..of the .. refrigerator.
  • the gas flows may also be measured for other purposes, such as.i . for monitoring the amount of cryogen remaining in the cryogen vessel..
  • orifice 13 has also been found beneficial in preventing a convection flow of boil-off gas, which might otherwise flow in- a path through sock 2, pipe 10 and access neck 7 back into the cryogenic vessel, or vice versa.
  • connecting pipe 10 may extend into the upper part of the sock.
  • This pipe may be thermally insulated 10a.
  • Such an embodiment would have the advantage that the boil-off gas does not flow past the upper parts of the refrigerator, and the cooling effect on the more sensitive parts of the refrigerator may in this way be limited.
  • cryogen loss from a cryogenic magnet system adapted according to the present invention is reduced to approximately 50% of the loss form the same system which has not been modified according to the present invention.
  • the present invention may be applied to more effectively
  • the present invention may also be applied to the reduction of cryogen loss , from any cryogenic vessel provided with a refrigerator which, when inoperative, provides a thermal load onto the cryogen vessel.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Containers, Films, And Cooling For Superconductive Devices (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)

Abstract

Afin de réduire au minimum la fuite de liquide cryogène lors du transport d'un système à aimant supraconducteur, ou lorsque le réfrigérant est éteint, une partie du gaz évaporé est entraînée du réservoir cryogénique à travers l'interface du réfrigérant en passant par le réfrigérant pour refroidir ce dernier. Une partie de la chaleur transmise au réfrigérant dans le système est captée et éliminée par ladite partie de gaz évaporé. Ceci permet de réduire la charge thermique à laquelle est soumis le réservoir cryogénique, et d'atténuer par conséquent l'évaporation du liquide cryogénique du réservoir cryogénique. Cette partie de gaz évaporé est ensuite ventilée du système conjointement avec le reste de gaz évaporé, par exemple, pour quitter le réservoir cryogénique via le col d'accès.
PCT/EP2005/005152 2004-05-25 2005-05-12 Reduction des fuites de liquide cryogene lors du transport de cryostats WO2005116514A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
US11/597,654 US8950194B2 (en) 2004-05-25 2005-05-12 Reduction of cryogen loss during transportation

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GB0411605.9 2004-05-25
GBGB0411605.9A GB0411605D0 (en) 2004-05-25 2004-05-25 Reduction of croygen loss during transportation
GB0423637.8 2004-10-25
GB0423637A GB2414536B (en) 2004-05-25 2004-10-25 Reduction of cryogen loss during transportation of cryostats

Publications (1)

Publication Number Publication Date
WO2005116514A1 true WO2005116514A1 (fr) 2005-12-08

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102009027429B4 (de) * 2008-07-03 2011-09-01 Bruker Biospin Gmbh Verfahren zur Kühlung einer Kryostatenanordnung während des Transports, Kryostatenanordnung mit Transportkühleinheit und Transportcontainer zum Transportieren der Kryostatenanordnung

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5265430A (en) * 1992-06-03 1993-11-30 General Electric Company Actively cooled baffle for superconducting magnet penetration well
US5381666A (en) * 1990-06-08 1995-01-17 Hitachi, Ltd. Cryostat with liquefaction refrigerator
EP0726582A1 (fr) * 1995-02-10 1996-08-14 Oxford Magnet Technology Limited Améliorations relatives à des aimants supraconducteurs
US5586437A (en) * 1995-09-06 1996-12-24 Intermagnetics General Corporation MRI cryostat cooled by open and closed cycle refrigeration systems

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5381666A (en) * 1990-06-08 1995-01-17 Hitachi, Ltd. Cryostat with liquefaction refrigerator
US5265430A (en) * 1992-06-03 1993-11-30 General Electric Company Actively cooled baffle for superconducting magnet penetration well
EP0726582A1 (fr) * 1995-02-10 1996-08-14 Oxford Magnet Technology Limited Améliorations relatives à des aimants supraconducteurs
US5586437A (en) * 1995-09-06 1996-12-24 Intermagnetics General Corporation MRI cryostat cooled by open and closed cycle refrigeration systems

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102009027429B4 (de) * 2008-07-03 2011-09-01 Bruker Biospin Gmbh Verfahren zur Kühlung einer Kryostatenanordnung während des Transports, Kryostatenanordnung mit Transportkühleinheit und Transportcontainer zum Transportieren der Kryostatenanordnung
US8448455B2 (en) 2008-07-03 2013-05-28 Bruker Biospin Gmbh Method for cooling a cryostat configuration during transport and cryostat configuration with transport cooler unit

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