Patent Application
20100005814
This application claims Paris Convention priority of DE 10 2008 031 486.2 filed Jul. 3, 2008 the complete disclosure of which is hereby incorporated by reference.
The invention relates to a method for cooling a cryostat configuration during transport, wherein the cryostat configuration comprises a superconducting magnet coil in a helium tank containing liquid helium, which is surrounded by at least one radiation shield, wherein the cooling inside the cryostat configuration in stationary operation is performed entirely without liquid nitrogen using a refrigerator. The invention also relates to a cryostat configuration with an outer shield within which a helium tank for liquid helium is installed, which contains a superconducting magnet coil and with at least one radiation shield surrounding the helium tank, wherein the cryostat configuration is cooled by a refrigerator entirely without liquid nitrogen during stationary operation.
Such a configuration is known, for example, from U.S. Pat. No. 7,141,90 B2.
Superconducting magnets consist of windings made of superconducting wire that are cooled down to temperatures of 4.2 Kelvin using liquid helium. The principal task of the cryostat configuration is to maintain the superconducting magnet at the specified operating temperature with liquid helium while consuming as little coolant as possible.
The main components of known cryostat configurations are the helium tank, which contains the superconducting magnet coil and liquid helium, the outer jacket, which acts as an outer vacuum vessel, and a neck tube, which connects the helium tank to the outer shield. The helium tank is surrounded by a vacuum space, which is delimited by the helium tank itself, the neck tube, and the outer jacket.
To reduce the heat input into the helium tank and thus into the superconducting magnet coil, it was previously common practice to surround the helium tank not only with radiation shields but also with a nitrogen tank containing liquid nitrogen, which was permanently installed in the cryostat. However, such configurations require frequent maintenance as the nitrogen tank usually has to be refilled every two weeks due to evaporation of the liquid nitrogen.
EP 0 468 425 A2 describes a cryostat configuration with a nitrogen tank that surrounds the helium tank, which is cooled by means of a refrigerator. It is thereby possible to reduce the nitrogen consumption and, therefore, the number of maintenance interventions in this way. However, all cryostat configurations with a nitrogen tank have the disadvantage that the resulting cryostat configuration is very large due to incorporation of the nitrogen tank and therefore occupies a large amount of space in the laboratory.
Modern cryostats of superconducting magnets therefore use a refrigerator to directly cool the radiation shield. Depending on the type of construction, the refrigerator is also able to prevent evaporation of helium by recondensing the helium gas in the helium tank. Such a cryostat configuration is known, for example, from U.S. Pat. No. 7,140,190 B2 and www.bruker-biospin.com/biospec, www.bruker-biospin.com/mri_usr_technology. Such cryostat configurations, which use refrigerators for the direct cooling of radiation shields, can be operated fully nitrogen free. This permits more compact cryostat configurations to be implemented and maintenance costs to be saved because maintenance interventions, such as replenishing the nitrogen and vacuum testing of the nitrogen vessel, are no longer necessary.
Transport of such cryostat configurations to their destination is preferably performed in the cold state because, in many installation locations, no service personnel with expert knowledge are available, and the cooling time of a superconducting magnet configuration disposed in a cryostat configuration as described above can take up to three weeks, during which time specialist personnel must be available on site.
However, the transport of cryostat configurations in the cold state is a time-critical and expensive undertaking. Transport by airfreight, although less problematic, is, however, very expensive. Transport by sea is considerably cheaper but very time consuming. Since the refrigerator consumes a large amount of electrical energy, it is almost logistically impossible to run it while the cryostat configuration is being transported: The compressor of the cold head requires up to 15 kW of electrical power, a large part of which is converted to heat and usually has to be dissipated by water-cooling. For this, a water circuit has to be connected to a water cooler during transport, which consumes roughly the same amount of electrical power as the compressor. The entire system must be monitored by a control unit, which also requires electrical energy. Furthermore, the large quantity of waste heat must be allowed to escape from the transport system which will otherwise heat up. All these constraints would make operating a refrigerator during transport extremely complicated and therefore very expensive.
However, if the refrigerator is not run, the radiation shield continues to heat up until the dissipating helium cools down the radiation shield in accordance with the energy that is incident upon the radiation shield, and an equilibrium temperature is established. This is usually around 100-150 Kelvin. The thermal losses are then considerable and can result in evaporation rates of 5 liters of LHe (liquid helium)/hour. The residence time (i.e. the time until no more helium is to be found in the magnet) sinks to well below one month and makes transport to remote regions or by ship practically impossible without the magnet becoming dry and therefore heating up during transport, that is, there is no more liquid helium in the helium tank. In such cases, only expensive airfreight transport is possible.
The object of this invention is therefore to suggest a method by which the consumption of liquid helium can be greatly reduced during transport and the possible transport time of a cooled superconducting magnet configuration therefore increased.
This object is inventively solved by switching off the refrigerator during transport and instead conveying liquid nitrogen from an external nitrogen vessel via a supply tube from the nitrogen vessel to the cryostat configuration and bringing it into thermal contact with the radiation shield by means of a thermal contact element in the cryostat configuration.
In the inventive method, the radiation shield is therefore not cooled by means of the refrigerator during transport but by means of liquid nitrogen, which is, however, not stored in the cryostat configuration itself but outside in one or more external nitrogen vessels. This makes it possible to maintain the radiation shield at temperatures of approx. 80 K. Neither electrical energy nor a water supply is required during transport. Instead of liquid nitrogen, other liquefied gases with a temperature above that of liquid helium can be used, for example, air, or an inert gas. By this method, the evaporation rate of the helium in the helium tank can be reduced by a factor of 1.5-12, preferably of 2.5-12, without generating waste heat, without the need to introduce elaborate provisions for running a refrigerator or having to dispense with the compact design of the cryostat configuration cooled without nitrogen in stationary operation.
With the inventive method, it is therefore possible to implement a low-cost and simple method of transport cooling without dispensing with the advantages of a nitrogen-free-cooled cryostat configuration in stationary operation (fewer maintenance interventions and nevertheless a compact cryostat configuration).
The supply of liquid nitrogen to the contact element is preferably activated when transport starts and deactivated again at the end of transport. Therefore, the radiation shield is only cooled with nitrogen during transport. Consequently, the nitrogen vessels are only required during transport and can be removed from the cryostat configuration after transport and returned to the supplier. The nitrogen vessels are dimensioned such that they can be returned together with the usual installation kit.
In a preferred variant of the inventive procedure, the refrigerator is removed from the cryostat configuration before the supply of liquid nitrogen to the contact element is activated and installed again after the liquid nitrogen supply has been deactivated.
The inventive cryostat configuration for performing the procedure described above is characterized in that a thermal contact element is provided of which at least one stationary section is in permanent thermal contact with the radiation shield, and in that a supply tube is provided for introducing the liquid nitrogen from a nitrogen vessel disposed outside the cryostat configuration to the contact element.
In a highly preferred embodiment, the nitrogen vessel is removed from the cryostat configuration before stationary operation starts. The inventive cryostat configuration therefore does not differ in its dimensions during stationary operation from cryostat configurations of prior art. What is more, the nitrogen vessels can thereby be easily refilled and possibly used for other tasks.
The stationary section of the contact element can either be a solid body integrated in the cryostat configuration or a hollow body that makes contact with the radiation shield and through which nitrogen can be introduced via the supply tube.
In a special embodiment, at least one section of the supply tube for supplying liquid nitrogen is permanently installed in the cryostat configuration. The permanently installed section of the supply tube can be connected to an external section of the supply tube that leads to the nitrogen vessel for transporting the cryostat configuration.
It is particularly advantageous, for example, if the supply tube for introducing liquid nitrogen can be removed from the cryostat configuration before stationary operation starts. The supply tube required for cooling during transport therefore occupies no space within the cryostat configuration during stationary operation.
In a further variation of this embodiment, a supply tube for introducing liquid nitrogen is detachably connected to the stationary section of the contact element. Such an embodiment applies, for example, when the stationary section surrounds a hollow space and the supply tube that can be removed from the cryostat configuration is connected directly to the hollow space of the stationary section in order to introduce the nitrogen for transport cooling.
Alternatively, in a highly preferred further embodiment of the inventive cryostat configuration, the supply tube for introducing liquid nitrogen is connected to a movable section of the contact element and can be removed from the cryostat configuration together with the latter before stationary operation starts. The moving section of the contact element, like the stationary section, can be a hollow space filled with nitrogen or a solid body that is in thermal contact with the nitrogen from the external nitrogen vessel.
The inventive cryostat configuration is particularly advantageous if a neck tube is provided to contain a cold head of the refrigerator, wherein one end of the neck tube is connected to the outer jacket and its other end is connected to the helium tank, wherein the contact element is disposed inside the neck tube and the stationary section of the contact element makes thermally conductive contact with a cold stage of the cold head during stationary operation and with the liquid nitrogen transported via the supply tube during transport operation. The thermal contact between the radiation shield and the nitrogen is therefore established in the neck tube, which is usually used to accommodate the cold head. In this case, no additional modifications have to be made to the magnet (cooling tubes etc.), which would result in additional losses during normal operation.
In a further advantageous embodiment of the inventive cryostat configuration, the stationary section of the contact element comprises at least two contact surfaces between which a gas gap is located, via which heat is dissipated from the radiation shield. As far as possible, the two contact surfaces make form-fit contact, wherein the gas gap is preferably filled with helium gas. This results in very good thermal contact without the need to apply mechanical force (e.g. using screws).
One advantageous embodiment comprises at least one fixing facility for fixing at least one nitrogen vessel, which is disposed on the exterior of the outer shield. The nitrogen tank can then be detachably connected to the exterior of the outer shield.
The refrigerator is preferably either a pulse tube cooler or a Gifford-McMahon cooler.
The cold head of the refrigerator preferably comprises two cold stages that can be operated at different cryogenic temperatures. The first cold stage can be used in stationary operation of the cryostat configuration to cool the radiation shield, while helium from the helium tank can be reliquefied with the second (colder) cold stage.
The advantages of the invention are particularly beneficial if the superconducting magnet coil is part of an MR magnet, in particular, part of an NMR, MRI, or FTMS configuration.
The invention also relates to a transport vessel for transporting an inventive cryostat configuration with an installed nitrogen vessel with liquid nitrogen and a supply tube that leads from the nitrogen vessel to a connection device for connection to a cryostat configuration.
To simplify and accelerate loading and transport of the transport vessel, it is advantageous if the transport vessel has standard dimensions, in particular according to ISO 668. The inventive cryostat configuration can be conveyed by means of such a transport vessel with a wide range of different means of transport (ocean-going and inland waterway vessels, railroad, truck) and quickly transshipped.
Further advantages of the invention can be derived from the description and the drawing. The characteristics stated above and below can be used individually or any number of them may be used in any combination. The embodiments shown and described are not intended as an exhaustive list but are examples used to describe the invention.
a a detail view of the inventive cryostat configuration in
b a detail view of the inventive cryostat configuration in
The superconducting magnet coil 2 to be cooled is located in helium tank 8 containing liquid helium 9, which is surrounded by a radiation shield 10. A vacuum space is disposed between the radiation shield 10 and the helium tank 8, and between the radiation shield 10 and outer jacket 3. To reduce the liquid helium losses, a heat-conducting connection is established on the inventive cryostat configuration 1 between the liquid nitrogen 6 emanating from the external nitrogen vessel 4 and the radiation shield 10. The radiation shield 10 is in permanent thermal contact with a stationary section 11a of a contact element via a flexible coupling element 11c, wherein the stationary section 11a can itself be connected to a non-stationary, movable section 11b of the contact element with thermal conductance. The movable section 11b is disposed here as a hollow space and mechanically connected with the supply tube 7 in such a way that the liquid nitrogen 6 can make its way from the external nitrogen vessel 4 into the hollow space of the movable section 11b of the contact element.
Heat transfer from the radiation shield 10 to the liquid nitrogen 6 is implemented by the thermal contacting of the movable section 11b and the stationary section 11a. A discharge valve 17 is disposed in the nitrogen vessel 4 to avoid overpressure in the nitrogen vessel 4 resulting from the introduction of heat to the nitrogen. Instead or in addition, a valve to diffuse the nitrogen gas can also be disposed in the supply tube 7.
In the embodiment illustrated in
a and 2b illustrate a contact surface 13 constructed from a material with high heat conductance, e.g. copper plate, to improve the heat conductance between the movable section 11b and the stationary section 11a, and which is part of the movable section 11b (not shown in
As
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 031 486.2 | Jul 2008 | DE | national |
