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. In order to achieve the required stability, it is common to use 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 the thermal shields to a low temperature. It is also known to use such a refrigerator to directly refrigerate the cryogen vessel, thereby reducing the cryogen fluid consumption. It is also known to use a two-stage refrigerator, in which a first stage is used to cool the thermal shield(s), and the second stage is used to cool the cryogenic vessel.
It is desirable that such superconducting magnet systems should be transported from the manufacturing site to the operationals site containing the cryogen liquid, so that they can be made operational as quickly as possible. During transportation, 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. Indeed, 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.
In prior configurations, 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.
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. However, the benefits of this solution are outweighed by its disadvantages the refrigerator must be replaced when putting the MRI 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.
The present invention therefore provides methods and apparatus as defined in the appended claims.
According to an aspect of the present invention, in order to minimize the loss of cryogen during transportation of superconductive magnet systems, or indeed at any time that the refrigerator is turned off, 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.
The above, and further, objects, characteristics and advantages of the present invention will be described with reference to a number of specific embodiments, given by way of examples only, in conjunction with the accompanying drawings, wherein:
Superconductive magnet coils (not shown) 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. In some embodiments, the interior of the cryogen, 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.
According to an embodiment of the present invention, 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. Preferably, the pipe 6 is closed by one or more valves when the superconductive magnet system is in operation.
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 exhaust valve 26 (
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. In addition, 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.
As mentioned above, the boil-off gas which flows through lower part 8 of sock 2, past, the refrigerator's second stage must traverse the thermal connection 15, 30 which thermally links the refrigerator first stage to the thermal shield 20.
The boil-off gas may pass through the thermal connection via channels providing a passageway past or through the contact flange 15. In one embodiment illustrated in
In alternative embodiments, 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 contact flange 15.
As the boil-off gas flows past the refrigerator, initially at a temperature of about 4K, the refrigerator is cooled. The heat removed by the boil-off gas heats the gas as it passes upwards through the sock. Although the boil-off gas has been heated, it remains at a very low temperature. The boil-off gas will accordingly be very effective to cool the refrigerator along its entire length, and to cool the shield 20 by cooling the thermal interface 30 during its passage through or past the contact flange 15 and/or the thermal interface 30.
In addition to cooling of the shield 20 via the thermal link 19, and as illustrated in
This configuration maximises 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.
According to an aspect of the present invention, a restrictor orifice 13 may be placed on the pipe 6. This may be a fixed orifice or an adjustable orifice. By limiting the rate of gas flow in the tube 6, 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. By suitably controlling the dimensions of the channel through the thermal connection and the orifice 13, the cooling of the different parts of the refrigerator 1 by escaping boil-off gas may be controlled. 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. For this latter purpose, 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 for monitoring the amount of cryogen remaining in the cryogen vessel.
The presence of 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.
In an alternative embodiment, shown in
In tests, it has been found that the 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.
While the present invention has been described with reference to a limited number of embodiments, given by way of examples only, the invention may be modified in numerous ways, which will be apparent to those skilled in the relevant art. For example, while the above example has described an MRI magnet system which is fitted with a very low temperature refrigerator for recondensation of cryogen gas so that in normal operation there would be no loss of cryogen, the present invention may be applied to more effectively remove the heat conducted by an inoperative refrigerator to thermal shield(s) used on a magnet system where only the shield(s) is/are refrigerated so as to reduce but not eliminate cryogen loss during normal operation.
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.
Number | Date | Country | Kind |
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0411605.9 | May 2004 | GB | national |
0423637.8 | Oct 2004 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2005/005152 | 5/12/2005 | WO | 00 | 9/11/2007 |
Publishing Document | Publishing Date | Country | Kind |
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WO2005/116514 | 12/8/2005 | WO | A |
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Number | Date | Country | |
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20080155995 A1 | Jul 2008 | US |