GB2463033A

GB2463033A - Method of operating a cryogenic refrigerator with multiple refrigeration stages

Info

Publication number: GB2463033A
Application number: GB0815650A
Authority: GB
Inventors: Philip Alan Charles Walton; Trevor Bryan Husband; Kevin Paul Wastie
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-08-28
Filing date: 2008-08-28
Publication date: 2010-03-03
Also published as: GB0815650D0

Abstract

A method of operating a cryogenic refrigerator 30 with multiple stages 32, 34, wherein each stage is supplied, through corresponding flow metering valves 44, 46, 54, 56, with a relatively high-pressure cryogen gas supply line 48 and a relatively low-pressure gas return line 50, for connection, in use, to a compressor 70 of gas. The method comprises the steps of determining a target temperature range for at least one of the stages 32, 34; and adjusting a flow metering valve 44; 46; 54; 56 in at least one of the supply and return lines for each stage. The proportion of cooling power delivered to the respective stages is thereby varied such that the temperature of the corresponding stage falls within the target temperature range. In an alternative embodiment, the cooling power ratio between the two stages may be used as the control parameter.

Description

METHOD OF OPERATING A CRYOGENIC REFRIGERATOR

WITH MULTIPLE REFRIGERATION STAGES

The present invention relates to methods for operating a refrigerator with multiple refrigeration stages. In particular, but not exclusively, it relates to the control of a multi-stage refrigerator used to cool a cryogen vessel and a thermal radiation shield within a cryostat.

The present invention will be particularly descrthed with reference to cryogenic pulse tube refrigerators (PTR5). The invention may, however, be employed in the control of other types of refrigerators.

A particularly useful application of the present invention lies in the control of a two-stage refrigerator for cooling the cryogen vessel and a thermal radiation shield in a cryo stat containing a superconducting magnet for an MRI imaging 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 (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum 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 cooling the shield(s). 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. In this example, the refrigerator 17 is 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 4-1 OK.

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.

Fig. 2 shows a typical present arrangement of a magnet (not visible) within a cryostat 10, with a mechanical refrigerator 17 providing cooling to the interior of the cryostat. The refrigerator 17 is placed in a helium circuit including high pressure supply line 48, low pressure return line 50 and helium compressor 70.

A magnetic resonance imaging system will comprise further equipment (not illustrated), such as gradient and field coils, shim coils and a patient table. One or more system electronics cabinet(s) 72 house(s) a magnet supervisory system 74 and other control and measurement equipment 76 which control operation of the magnet, and such further equipment, over communications lines 78. The magnet supervisory system 74 receives data input from appropriate system sensors attached to various components of the MRI system. Helium compressor 70 is typically an electromechanical device. It is conventionally mechanically enclosed within the system electronics cabinet(s) 72 but the helium compressor is conventionally a standalone device.

Conventionally, the refrigerator 17 operates to provide a constant cooling power to its first and second stages, regardless of the current or planned thermal load on each stage. For example, the second stage is typically always cooling to a cryogen recondensing temperature, regardless of whether such recondensation is currently needed. The refrigerator usually runs constantly at full power, and is tuned to give optimum all-round performance. Cooling power is in effect wasted as full-power refrigeration is typically not constantly required.

According to the present invention, cooling efficiency, and power consumption, of a refrigerator with multiple refrigeration stages is improved according to the methods and apparatus recited 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 thereof, given by way of examples only, with reference to the accompanying drawings, wherein: Fig. 1 shows a conventional arrangement of a cryo stat; Fig. 2 shows a conventional arrangement of a magnet within a cryostat, and associated equipment; and Fig. 3 shows a cross-section of a multiple-stage refrigerator adapted for use according to the present invention.

The amount of cooling power delivered by each stage of a multiple stage refrigerator is typically fixed, or is difficult to control. For example, manual adjustment of metering valves may adjust the cooling power available to each stage. The present invention proposes a method and apparatus for automatically detecting the thermal environment of the refrigerator stages, and adjusting the relative cooling power supplied by each cooling stage according to the detected thermal environment. In further embodiments, methods are provided in which the relative cooling power supplied by each cooling stage is adjusted according to planned variations in the thermal environment.

Fig. 3 shows a cross-section of a multiple-stage refrigerator adapted for use according to the present invention. The drawing shows a two-stage pulse tube refrigerator (PTR) 30, but the invention is not limited to two-stage refrigerators, or to pulse tube refrigerators. The invention may be applied to any multi-stage refrigerator.

The refrigerator 30 of Fig. 3 comprises a warm end 31, a first refrigeration stage 32 and a second refrigeration stage 34. In use, the first refrigeration stage 32 would be thermally linked to the thermal radiation shield 16 (Fig. 1), while the second refrigeration stage 34 would be arranged to cool the cryogen vessel, possibly by recondensation of cryogen vapour. A first pulse tube 36 is connected between the warm end 31 and the first refrigeration stage 32. A second pulse tube 38 is connected between the warm end 31 and the second refrigeration stage 34. A first regenerator tube 40 is connected between the warm end 31 and the first refrigeration stage 32. A second regenerator tube 42 is connected between the first refrigeration stage 32 and the second refrigeration stage 34.

A rotary valve 43, conventional in itself, controls timing for access to the pulse tubes 36, 38 for a relatively high pressure helium supply line 48 and relatively low pressure helium return line 50. Typically, in a rotary valve, a pattern of channels in a rotary element line up with inlet and outlet ports at appropriate moments to provide the required timing control. Operation of the pulse tube refrigerator is conventional in the manner well known to those skilled in the art.

According to the present invention, flow metering valves 44, 46, 54 and 56 control the rate at which gas can flow through the rotary valve 43 from and to the respective supply and return lines 48, 50. The flow metering valves 44, 46, 54, 56 are controlled by a control circuit 62, to control the gas flow through the refrigerator. The cooling power supplied by the first and second cooling stages is accordingly controlled. As separate flow metering valves are provided for each refrigeration stage, the relative refrigeration power delivered by each stage may be controlled.

A defined quantity of gas is delivered by the compressor 70 to the PTR, and the division of this gas between the stages is adjusted by the flow metering valves 44, 46, 54, 56. The adjustment of the flow metering valves influences the relative cooling power available at each refrigeration stage.

For example, if more gas is directed to the second stage, then the second stage would receive a larger proportion of the available cooling power, and the cooling power at the first stage would reduce.

According to an aspect of an embodiment of the present invention, first refrigeration stage 32 is provided with a first temperature sensor 64, and second refrigeration stage 34 is provided with a second temperature sensor 66. Temperature sensors 64, 66 may be resistance wires, or semiconductor components or any known temperature sensor capable of operating at temperatures as low as 4K.

According to an embodiment of the present invention, control circuit 62 receives temperature data from temperature sensors 64, 66, indicating current temperatures of the first and second refrigerating stages, respectively. The control circuit compares the received data against target values, and adjusts the flow metering valves 44, 46, 54, 56 in order to adjust the relative cooling power delivered to the first and second refrigerating stages 32, 34, to bring their temperatures closer to predefined, or supplied, target temperatures known to the control circuit.

The control circuit is set up to attempt to maintain the temperatures of the cooling stages within a preferred tolerance range around the target temperature.

The flow metering valves may include a stepper motor or other drive device arranged to control the valve element such that data sent to the stepper motor causes the valve to open more, or to close somewhat.

In some embodiments, the target temperatures may be fixed and stored within the control circuit 62. When, for whatever reason, the temperature of one of the refrigeration stages 32, 34 varies from its target temperature, the opening of the flow metering valves will be adjusted to bring the temperatures of the refrigeration stages back to their target values.

In other embodiments, the target temperatures are supplied dynamically to the control circuit, for example by an external controller 68, for example magnet supervisory system 74 which acts to exert overall control of an MRI imaging system as a whole. Such arrangements enable the relative cooling power delivered to the respective refrigeration stages 32, 34 to be adjusted based on current imaging system activity. In yet other embodiments, target temperatures may be adjusted in advance of changes to the imaging system activity, so that the relative cooling power to the respective stages may be adjusted in advance of system activity, and the refrigerator will operate to maintain appropriate temperatures of the first and second refrigeration stages 32, 34 during the system activity.

In other embodiments of the present invention, temperature sensors 64, 66 may not be provided at the refrigeration stages. Temperature sensors may instead, or additionally, be provided at the refrigerated thermal radiation shield and at the cryogen vessel or cryogen recondenser; or at any suitable position within the cryo stat.

In alternative embodiments, open-loop control may be exercised. In such arrangements, the target temperature is replaced by a cooling ratio, determining the relative proportion of cooling power which is to be applied to each of the refrigeration stages. Like the target temperatures discussed above, the cooling ratio may be stored in the control circuit 62, or may be dynamically supplied to the control circuit. Such arrangements enable the relative cooling power delivered to the respective refrigeration stages 32, 34 to be adjusted based on current imaging system activity. In yet other embodiments, the cooling ratio may be supplied in advance of changes to the imaging system activity, so that the relative cooling power to the respective stages may be adjusted in advance of imaging system activity, and the refrigerator will operate to maintain temperatures of the first and second refrigeration stages during the imaging system activity.

It has been found that adjusting the proportion of cooling power provided to each stage influences the temperature gradient of the refrigerator and may make it less efficient. The control circuit 62 should be arranged to limit the variation in cooling power distribution so that overall refrigerator efficiency does not become intolerably low.

Some likely situations in which the present invention would be useful will now be discussed.

During pre-cool, where a warm magnet within a cryogen vessel is being cooled before final filling with liquid cryogen, all of the available cooling power could be provided to the first cooling stage 32 to cool the radiation shield 16 to a temperature in the region of 80-lOOK. The refrigerator is typically more efficient at cooling to 80-lOOK than it is for cooling to 4-10K.

The cooling power diverted from the 4-10K second stage 34 will accordingly be more efficiently used in cooling the shields to 80-lOOK. The shield 16 will accordingly reach operational temperature more quickly than when the refrigerator 30 is conventionally used, supplying cooling power at both 80-lOOK and 4-10K.

During an imaging sequence of an MRI imaging system, an increased heat load into the cryogen vessel may be expected, for example from resistive gradient coils which become active during imaging but which do not generate heat when the imaging system is inactive. During imaging, then, it is useful for more cooling power to be directed to the second cooling stage 34, to keep the cryostat cold, than to the first stage 32, as the thermal radiation shield 16 may be allowed to warm by a few degrees during the imaging process. In this way, recondensing margin -the ability to recondense all of the boiled off cryogen back into liquid form -may be maintained despite an increased heat input from the gradient coils.

At inactive times, when the magnet is at field, cooled to its operating temperature but no imaging is taking place, the refrigerator 30 should act to maintain constant temperatures of the refrigeration stages. The control circuit may usefully monitor the temperatures of the first 32 and second 34 refrigeration stages, in embodiments provided with temperature sensors 64, 66. Alternatively, the refrigerator may revert to operation with stored default cooling power distrthution between the refrigeration stages.

The preceding description has concentrated on the use of the present invention in adapting the opening of flow metering valves 44, 46, 54, 56 to change the cooling power delivered to each refrigeration stage 32, 34 in order to change the temperature or cooling power delivered to each refrigeration stage 32, 34. However, the present invention also finds application in maintaining a constant temperature or cooling power for the refrigeration stages over time.

While a refrigerator, such as a PTR 30, may initially be set up to deliver the required amount of cooling to each stage 32, 34, the cooling performance of the PTR may deteriorate over time, for example due to gas contamination within the PTR refrigerator. As discussed above, the timing of ingress and egress of helium gas into the PTR tubes is controlled by rotary valve 43, while the flow rate of the gas is controlled by the flow metering valves 44, 46, 54, 56 according to the present invention. It is found that the initial fine adjustment of the flow metering valves may be affected by an accumulation of debris created by wear to the face of the rotary valve 43. This accumulation of particles may change the flow characteristics of the flow metering valves. According to the methods of the present invention, by monitoring the temperatures of the cooling stages 32, 34, and adjusting the flow metering valves to maintain the target temperatures within a defined tolerance band, the continued cooling to the required temperature and the required cooling power can be maintained despite the change in flow characteristics of the metering valves.

The present invention may allow the required cooling to be provided without constantly operating the refrigerator at full power. By reducing -10-the power of the refrigerator as a whole, wear on the rotary valve may be reduced.

In the embodiments described above, control of both the relatively high pressure inlet flow metering valves 44 and 54, and the relatively low pressure return flow metering valves 46 and 56 are controlled. It has been found that the relatively high pressure inlet flow metering valves 44 and 54 provide a relatively coarse control of the distribution of cooling power between the stages of the refrigerator, while control of the relatively low pressure return flow metering valves 46 and 56 provides a finer control. In certain embodiments, it may be found sufficient to control either only the relatively high pressure inlet flow metering valves; or to control only the relatively low pressure return flow metering valves. In other embodiments, it may be found appropriate to adjust the relatively high pressure inlet flow metering valves 44, 54 only if the temperature of the cooling stages has deviated from the target value by more than a certain amount, with control of the relatively low pressure inlet flow metering valves 46, 56 being used if the temperature of the cooling stages has deviated from the target value by less than the certain amount.

While the present invention has been described with reference to a limited number of specific examples, many variations and modifications will be apparent to those skilled in the art, and form part of the present invention as defined by the appended claims. For example, the control circuit 62 may be located near the warm end of the refrigerator, or the control circuit may be remotely located and connected to receive the necessary data and to perform its function. Similarly, although the present invention has been described with particular reference to the use of helium as the cryogen employed by the refrigerator, the present invention applies to the -11 -use of any suitable cryogen. Further, the function of the control circuit may be performed by a computer or a magnet supervisory system, remotely located and connected to receive the necessary data and to perform the function of the described control circuit. Selection of the proportion of cooling power to be delivered to each cooling stage, or the target temperature for each cooling stage, may be performed remotely, and may be made in advance of corresponding operation of the imaging system. Alternatively, such selection may be made in response to the commencement of, or request for commencement of, corresponding operation of the imaging system.

Claims

-12 -CLAIMS: 1. A method of operating a cryogenic refrigerator (30) with multiple stages (32, 34), wherein each stage is supplied, through corresponding flow metering valves (44, 46, 54, 56), with a relatively high-pressure gas supply line (48) and a relatively low-pressure gas return line (50), for connection, in use, to a compressor (70) of gas, said method comprising the steps of: -determining a target temperature range for at least one of the stages (32, 34); and -adjusting a flow metering valve (44; 46; 54; 56) in at least one of the supply and return lines for each stage, whereby the proportion of cooling power delivered to the respective stages is varied such that the temperature of the corresponding stage falls within the target temperature range.
2. A method according to claim 1 wherein the method further comprises: -measuring the temperature of at least the corresponding stage; and -adjusting the flow metering valves in accordance with the measured temperature(s).
3. A method according to claim 1 or claim 2 wherein the target temperature range is determined in accordance with present temperatures of the stages.
4. A method according to claim 1 or claim 2 wherein the target temperature range is determined in accordance with predicted temperature changes of the stages.

-13 -
5. A method of operating a cryogenic refrigerator (30) with multiple stages (32, 34), wherein each stage is supplied, through corresponding flow metering valves, with a relatively high-pressure gas supply line (48) and a relatively low-pressure gas return line (50), for connection, in use, to a helium compressor (70), comprising the steps of: -determining a target cooling power ratio to be delivered to the respective stages (32, 34); and -adjusting a flow metering valve (44; 46; 54; 56) in at least one of the supply and return lines for each stage, whereby the proportion of cooling power delivered to the respective stages is varied such that the target cooling power ratio is delivered to the respective stages.
6. A method according to any preceding claim wherein cryogen gas is supplied through the supply line and withdrawn through the return line at a defined, constant rate, and the division of this gas between the stages is adjusted by the flow metering valves (44, 46, 54, 56) to influence the relative cooling power available at each refrigeration stage.
7. A method according to any preceding claim wherein a rotary valve (43) controls timing for access to the refrigerator stages for the supply line (48) and the return line (50), while the flow metering valves (44, 46, 54, 56) control the rate at which gas can flow through the rotary valve (43) from and to the respective supply and return lines (48, 50).
8. A method according to any preceding claim wherein the relatively high pressure inlet flow metering valves (44, 54) are adjusted if the temperature of the refrigerator stages has deviated from the target value -14 -by more than a certain amount, with control of the relatively low pressure inlet flow metering valves (46, 56) being used if the temperature of the refrigerator stages has deviated from the target value by less than the certain amount.
9. A cryogenic refrigerator (30) with multiple stages (32, 34), wherein each stage is connectable, through corresponding flow metering valves (44, 46, 54, 56), with a relatively high-pressure gas supply line (48) and a relatively low-pressure gas return line (50), for connection, in use, to a compressor (70) of gas, each flow metering valve being connected for control by a control circuit (62), said control circuit being provided with a target temperature range for at least one of the stages (32, 34); -the control circuit being arranged to adjust a flow metering valve (44; 46; 54; 56) in at least one of the supply and return lines for each stage, whereby the proportion of cooling power delivered to the respective stages may be varied such that the temperature of the corresponding stage(s) falls within the target temperature range(s).
10. A cryogenic refrigerator according to claim 9 wherein the control circuit (62) is connected (64, 66) to receive temperature data indicating the current temperature of at least the corresponding stage(s); and the control circuit is arranged to adjust the flow metering valves in accordance with the received temperature data.
11. A cryogenic refrigerator according to claim 9 or claim 10 wherein the target temperature range is stored within the control circuit (62).
12. A cryogenic refrigerator according to claim 11 wherein a first refrigeration stage (32) is provided with a first temperature sensor (64), and a second refrigeration stage (34) is provided with a second temperature sensor (66), the first and second temperature sensors being arranged to supply the temperature data to the control circuit.
13. A cryogenic refrigerator according to claim 9 or claim 10 wherein the target temperature range is supplied to the control circuit (62) from an external controller (74).
14. A cryogenic refrigerator (30) with multiple stages (32, 34), wherein each stage is connectable, through corresponding flow metering valves (44, 46, 54, 56), with a relatively high-pressure gas supply line (48) and a relatively low-pressure gas return line (50), for connection, in use, to a compressor (70) of gas, each flow metering valve being connected for control by a control circuit (62), said control circuit being provided with a target range of cooling power ratio to be delivered to the respective stages (32, 34); the control circuit being arranged to adjust a flow metering valve (44; 46; 54; 56) in at least one of the supply and return lines for each stage, whereby the proportion of cooling power delivered to the respective stages may be varied such that the cooling power ratio delivered to the respective stages falls within the target range.
15. A cryogenic refrigerator according to any of claims 9-14, further comprising a rotary valve (43) arranged to control timing of access to the refrigerator stages by the supply line (48) and the return line (50). -16-
16. A cryogenic refrigerator according to any of claims 9-15, wherein the flow metering valves are adjusted by a drive device arranged to control a valve element such that data sent to the drive device by the control circuit causes the valve to open more, or to close somewhat.
17. A cryogenic refrigerator according to claim 16, wherein the drive device comprises a stepper motor.