Cryostat with active neck tube cooling by a second cryogen

Abstract

A cryostat arrangement has an outer jacket, a first tank with a first cryogen, and a second tank with a second liquid cryogen which boils at a higher temperature than the first cryogen. The first tank comprises a neck tube, whose hot upper end is connected to the outer jacket at ambient temperature and whose cold lower end is connected to the first tank at a cryogenic temperature. The arrangement uses a riser pipe protruding into the second tank through which the second liquid cryogen can flow out of the second tank and into a first heat exchanger in thermal contact with the neck tube. An outflow line is provided through which second cryogen evaporating from the first heat exchanger can flow out and into an optional second heat exchanger. It is thus possible to greatly reduce heat input from the neck tube into the first tank.

Claims

1. A cryostat arrangement for storage of a first cryogen, the cryostat arrangement having an outer jacket and a first tank installed within the outer jacket with the first cryogen in addition to a second tank holding a second liquid cryogen, wherein the first cryogen boils at a lower temperature than the second cryogen and wherein the first tank comprises a neck tube whose hot upper end is connected to the outer jacket at ambient temperature and whose cold lower end is connected to the first tank at cryogenic temperature, the arrangement comprising: a riser pipe protruding into the second tank such that the second liquid cryogen can flow out of the second tank through the riser pipe, a lower end of the riser pipe being located in the second liquid cryogen in the second tank, and a first heat exchanger into which the riser pipe opens directly or indirectly with the riser pipe's upper end, the first heat exchanger having an outflow line such that evaporating second cryogen from the first heat exchanger can flow out through the outflow line, the first heat exchanger being located outside of the neck tube in direct thermal contact therewith so as to provide local cooling via the second cryogen from the riser pipe.

2. The cryostat arrangement according to claim 1, wherein a level of the second liquid cryogen in the riser pipe is above a level in the second tank because of a pressure difference between the outflow line and a gas volume above a liquid surface in the second tank, and the first heat exchanger is fed with second liquid cryogen from the riser pipe.

3. The cryostat arrangement according to claim 1, further comprising an exhaust line through which evaporating second cryogen vents from the second tank, wherein the exhaust line has a flow resistance device and a pressure difference between the outflow line and a gas volume above the liquid surface in the second tank can be controlled by the flow resistance device.

4. The cryostat arrangement according to claim 3, wherein the flow resistance device comprises a control valve.

5. The cryostat arrangement according to claim 1 wherein the outflow line has a pump, and wherein a pressure difference between the outflow line and a gas volume above the liquid surface in the second tank can be controlled by the pump.

6. The cryostat arrangement according to claim 5, wherein the pump comprises a control valve.

7. The cryostat arrangement according to claim 1 further comprising a second heat exchanger arranged in thermal contact with the neck tube above the first heat exchanger, the second heat exchanger providing additional local cooling using evaporating second cryogen from the first heat exchanger.

8. The cryostat arrangement according to claim 7 further comprising a temperature sensor arranged on the neck tube adjacent to the second heat exchanger.

9. The cryostat arrangement according to claim 1 wherein a distributor tank is arranged in thermally conducting contact with the second tank in the outer jacket above the second tank, and wherein second liquid cryogen from the riser pipe is fed into the distributor tank and second liquid cryogen can be conveyed out of the distributor tank into the first heat exchanger.

10. The cryostat arrangement according to claim 9, wherein the distributor tank has the form of a ring.

11. The cryostat arrangement according to claim 1 wherein the outflow line is connected directly or indirectly to a branch piece and the branch piece is connected directly or via a flow resistance device to an exhaust line, and wherein the exhaust line is connected to the second tank.

12. The cryostat arrangement according to claim 1 further comprising at least one of a flow meter located in the outflow line for determining a flow rate of second cryogen flowing out through the outflow line and a flow meter located in an exhaust line through which evaporating second cryogen vents from the second tank for determining a flow rate of the second cryogen outgassing through the exhaust line.

13. The cryostat arrangement according to claim 1 further comprising a temperature sensor arranged on the neck tube adjacent to the first heat exchanger.

14. The cryostat arrangement according to claim 1 further comprising a pressure sensor located in the second tank.

15. The cryostat arrangement according to claim 14 wherein the pressure sensor is located near the lower end of the riser pipe.

16. The cryostat arrangement according to claim 1 further comprising a filling level sensor located in the second tank.

17. The cryostat arrangement according to claim 1 wherein the cryostat arrangement is used for cooling a superconducting magnet assembly as part of a magnetic resonance apparatus.

18. A method for operating a cryostat arrangement according to claim 1, the method comprising adjusting a pressure difference between the outflow line and a gas volume above a liquid surface in the second tank so that there is a flow of the second cryogen through the heat exchanger.

19. The method according to claim 18, further comprising the following steps: i) detecting at least one parameter selected from a) a flow rate on the outflow line or on an exhaust line through which evaporating second cryogen vents from the second tank, using a flow meter, b) a temperature on the neck tube, using a temperature sensor, c) a pressure difference between a pressure of the second liquid cryogen and a pressure in the outflow line, using a pressure sensor and d) a filling level in the second tank, using a filling level sensor, ii) comparing a value of the detected at least one parameter with a predetermined value of that parameter, and iii) adjusting a pressure difference between the outflow line and the gas volume above the liquid surface in the second tank a) as a function of a filling level in the second tank or b) so that the at least one parameter detected in step (i) is substantially equal to the predetermined value of that parameter.

20. The method according to claim 18 wherein the pressure difference between the outflow line and the gas volume above the liquid surface in the second tank is adjusted by means of a pump or a flow resistance device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated in the drawing and is explained in greater detail on the basis of exemplary embodiments, in which:

(2) FIG. 1a shows a schematic vertical sectional view of a first embodiment of the cryostat arrangement according to the invention;

(3) FIG. 1b shows a schematic diagram of the temperature curve along the neck tube axis of a cryostat according to the invention;

(4) FIG. 2 shows a schematic vertical sectional view of a second embodiment of the cryostat arrangement according to the invention with a cooling loop as the first heat exchanger and a valve at the outlet of the exhaust line out of the second tank;

(5) FIG. 3a shows a schematic vertical sectional view of a third embodiment of the cryostat arrangement according to the invention with a second heat exchanger on the neck tube and a branch piece between the outflow line out of the heat exchangers and the exhaust line from the second tank;

(6) FIG. 3b shows an embodiment like that in FIG. 3a, but with a flow meter in the outflow line out of the heat exchangers;

(7) FIG. 3c shows an embodiment like that in FIG. 3a, but with a temperature sensor on the neck tube;

(8) FIG. 3d shows an embodiment like that in FIG. 3a, but with a pressure sensor and/or filling level sensor in the second tank;

(9) FIG. 4 shows a schematic vertical sectional view of another embodiment of the cryostat arrangement according to the invention with a distributor tank for the second cryogen;

(10) FIG. 5 shows a schematic vertical sectional view of an embodiment of the cryostat arrangement according to the invention, with a horizontal room temperature tube;

(11) FIG. 6a shows a schematic vertical sectional view of a cryostat arrangement according to the state of the art, with a horizontal room temperature tube, and

(12) FIG. 6b shows a schematic vertical sectional view of a cryostat arrangement according to the state of the art with a vertical room temperature tube.

DETAILED DESCRIPTION

(13) FIGS. 1a and 2 through 5 of the drawing each show in a schematic diagram preferred embodiments of the cryostat arrangement according to the invention for storage of a first cryogen, in particular for cooling an arrangement of superconducting magnets 20, with an outer jacket 1, 11 and a first tank 2, 12 installed therein and containing the first cryogen, as well as a second tank 16, 16 with a second liquid cryogen, wherein the first cryogen boils at a lower temperature than the second cryogen, and wherein the first tank 2, 12 comprises a neck tube 4 whose hot upper end 5 is connected to the outer jacket 1, 11 at ambient temperature and whose cold lower end 6 is connected to the first tank 2, 12 at cryogenic temperature.

(14) The cryostat arrangement according to the invention is characterized in that a riser pipe 3, 3, 13 protruding into the second tank 16, 16 is provided, through which the second cryogen can flow out of the second tank 16, 16; the lower end of the riser pipe 3, 3, 13 ends in the second liquid cryogen in the second tank 16, 16; a first heat exchanger 7, into which the riser pipe 3, 3, 13 opens directly or indirectly at its upper end; an outflow line 17 connected directly or indirectly to the first heat exchanger 7 is provided, and the second cryogen evaporating out of the first heat exchanger 7 can flow out through this outflow line; and the first heat exchanger 7 is in thermal contact with the neck tube 4, and the neck tube 4 provides local cooling by means of the second cryogen from the riser pipe 3, 3, 13.

(15) The first heat exchanger 7 is preferably arranged above the center of the axial extent of the neck tube 4.

(16) According to the invention, it is thus proposed that the coupling is not realised by a connecting piece whose function is based on thermal conduction in the solid material but instead it is proposed that at least one (or more) heat exchangers be installed at the coupling points on the neck tube 4. These heat exchangers usually have liquid nitrogen flowing through them. The heat exchangers are supplied from the two tanks 16, 16 through the tubular riser pipe 3, 3, 13, which extends as far as the bottom of the second tank 16, 16.

(17) The pressure in the second tank 16, 16 can be adjusted by regulating the outflow rate through a control valve taking into account the atmospheric pressure, so that the desired flow rate and/or the desired height of the liquid level in the riser pipe is obtained for the neck tube cooling. Alternatively, the pressure difference can be established by means of a pump, which is arranged in the outflow line.

(18) In addition, the neck tube section between the heat exchanger and the attachment to the outer jacket may be precooled with the evaporated nitrogen, which further reduces the heat load on the coupling point. However, the reduction in heat load via the exhaust cooling is quite low, so it is necessary to balance between the added complexity and the resulting thermodynamic efficiency.

(19) The temperature gradient in the neck tube segment amounts to approximately 300K over a length of 70 cm, for example, ranging from ambient temperature to the temperature of liquid helium. The neck tube cooling should ideally be mounted as close to the outer jacket as possible such that the temperature gradient from the neck tube cooling to the helium is minimized. However, at the same time, icing of the cryostat on the outside should be prevented. Hence, a good compromise should be sought between effective neck tube cooling to reduce the temperature gradient and prevention of icing.

(20) FIG. 1b shows the curve of the temperature T along the neck tube 4 diagramed schematically. The temperature transition at the heat exchanger 7 is crucial for the flattening of the temperature gradient toward the inside. Therefore, less heat enters the first cryogen (usually helium) at the lower boiling point.

(21) The cryostat arrangement according to the invention is preferably configured in such a way that the level of the second liquid cryogen in the riser pipe 3, 3, 13 is higher than the level in the second tank 16, 16 because of the pressure difference between the outflow line 17 and the gas volume over the liquid surface in the second tank 16, 16, in particular being at the level of the first heat exchanger 7, and so that the first heat exchanger 7 is charged with the second liquid cryogen from the riser pipe 3, 3, 13.

(22) With the cryostat arrangement according to the invention, the outer jacket 1, the first tank 2; 12 (helium tank), the second tank 16; 16 (nitrogen tank) and the neck tube 4 usually delimit an evacuated space. In the embodiments of the cryostat arrangement according to the invention in FIGS. 1a through 5 as well as in the state of the art as illustrated in FIGS. 6a and 6b, the first tank 2; 12 is surrounded by at least one radiation shield 8, which is connected in a thermally conducting manner to a contact surface 9 beneath the first heat exchanger 7 on the outside circumference of the neck tube 4.

(23) An exhaust line 14, which is shown in all the embodiments of the invention depicted in the drawing, through which the evaporating second cryogen vents out of the second tank 16, 16, hasas shown in the embodiments according to FIGS. 2 through 5a flow resistance device 15 on the atmosphere end, which can be designed in particular as a dosing valve, preferably as a control valve, with which the pressure in the nitrogen tank 16, 16 can be controlled. Due to the flow resistance device 15 a pressure difference between the outflow line 17 and the gas volume over the liquid surface in the second tank 16, 16 can be implemented in the exhaust line 14.

(24) In embodiments that are not shown separately in the drawings, the outflow line 17 may have a pump on the atmosphere end, in particular a pump having a dosing valve or having a control valve, preferably a controllable pump so that a pressure difference between the outflow line 17 and the gas line over the liquid surface in the second tank 16, 16 can be implemented by the pump in the outflow line 17.

(25) As shown in FIGS. 1b through 5, the first heat exchanger 7 may have at least one pipe loop, preferably a plurality of pipe loops wound around the outside circumference of the neck tube 4, lying in close contact with the outside circumference of the neck tube 4 to provide good thermal contact. In embodiments that are not shown separately in the drawings, the first heat exchanger 7 may however, also have a tube segment arranged radially around the outside circumference of the neck tube 4, with second liquid cryogen, mostly nitrogen, flowing through the tube segment out of the riser pipe 3, 3, 13.

(26) FIGS. 3a through 5 show embodiments of the cryostat arrangement according to the invention, in which a second heat exchanger 10, which is in close contact with the outside circumference of the neck tube 4 to provide thermal coupling and provides additional local cooling for the neck tube 4 by means of the second cryogen evaporating out of the first heat exchanger 7, is arranged above the first heat exchanger. In these embodiments, the outflow line 17 is also connected directly or indirectly to a branch piece 18, which is in turn connected to an exhaust line 14 either directly or by way of a flow resistance device 15 and this exhaust line is in turn connected to the second tank 16, 16, so that the cryogen flow rate through the heat exchanger can be adjusted through the control of the pressure in the second tank 16, 16.

(27) In the embodiment according to FIG. 3b, a flow meter 19 for determining the flow rate of the second cryogen flowing out through the outflow line 17 is arranged in the outflow line 17. Alternatively, or additionally, a flow meter for determining the flow rate of the second cryogen outgassing through the exhaust line 14 may also be arranged in an exhaust line 14. In both cases control of the pressure in the second tank 16, 16 can thus be achieved.

(28) In the embodiment according to FIG. 3c, a temperature sensor 21 is arranged on the neck tube 4 in the region of the first heat exchanger 7 and/or of the second heat exchanger 10. Pressure control of the cryogen pressure in the second tank 16, 16 is also possible with the measured temperature as a manipulated variable.

(29) In the embodiment according to FIG. 3d, a pressure sensor 22 is provided in the second tank 16, 16, preferably near the bottom, in particular near the lower end of the riser pipe 3, 3, 13 which is designed as an intake section. Alternatively, or additionally, a filling level sensor 22 may also be provided in the second tank 16, 16.

(30) FIG. 4 shows an embodiment of the cryostat arrangement according to the invention in which a distributor tank 16, preferably designed in the form of a ring, is arranged in the outer jacket 1 above the second tank 16, 16, so that it is in thermally conducting contact with the second tank 16, 16, and the second liquid cryogen from the riser pipe 3 can be fed into the second tank, on the one hand, and the second liquid cryogen can be forwarded from this tank to the first heat exchanger 7, on the other hand.

(31) In the embodiment shown here, the distributor tank 16 is arranged on an upper cover lid of the radiation shield 8. This distributor tank 16 causes a drop in temperature of the shield cover and therefore also causes a drop in temperature of the 80K shield in the bore. In addition, the control of the liquid level in the riser pipe, which is described in greater detail below, allows an increase of the amount of the second cryogen stored in the system if the distributor tank 16 has a sufficiently large volume.

(32) FIG. 5 shows an embodiment of the invention in which a room temperature tube 23 is provided, passing horizontally through the first tank 12. The second tank 16 may be arranged in the form of a ring around the helium tank 12 (the axis of symmetry of the second tank is also horizontal). The lower end of the riser pipe 13 protrudes into the lower region of the second tank.

(33) FIGS. 6a and 6b illustrate cryostat arrangements that are known from the state of the art, such as those discussed in detail above.

(34) FIG. 6a shows a cryostat arrangement with a horizontal room temperature tube 23 which is comparable to the embodiment of the invention illustrated in FIG. 5except, of course, for the modifications according to the invention.

(35) Finally, FIG. 6b shows a cryostat arrangement with a vertical room temperature tube, such as that described in detail in DE 10 2004 034 729 B4, which was cited in the introduction above.

LIST OF REFERENCE NUMERALS

(36) 1, 11 Outer jacket 2, 12 Tank for first cryogen (helium tank) 3, 3, 13 Riser pipe 4 Neck tube 5 Hot upper end of the neck tube 6 Cold lower end of the neck tube 7 First heat exchanger 8 Radiation shield 9 Contact surface 10 Second heat exchanger 11 Outer jacket 12 Tank for first cryogen (helium tank) 13 Riser pipe 14 Exhaust line 15 Flow resistance device (control valve) 16, 16 Tank for second cryogen (nitrogen tank) 16 Distributor tank 17 Outflow line 18 Branch piece 19 Flow meter 20 Magnet arrangement 21 Temperature sensor 22 Pressure sensor 22 Liquid Level Sensor 23 Room temperature tube