WO2015092697A1

WO2015092697A1 - Cooling loop for superconducting magnets

Info

Publication number: WO2015092697A1
Application number: PCT/IB2014/066997
Authority: WO
Inventors: Joshua Kent HILDERBRAND
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-12-20
Filing date: 2014-12-17
Publication date: 2015-06-25

Abstract

A system including a cryogenic refrigeration apparatus including a compressor having a fluid suction intake and a fluid discharge. The refrigeration apparatus also includes a cryocooler comprising an expander with a refrigeration stage. The system also includes a cooling loop connected at a first end to the fluid discharge of the compressor of the refrigeration apparatus and at a second end to the fluid suction intake of the compressor of the refrigeration apparatus. A pressure differential between the fluid discharge and the fluid suction intake causes a working fluid to flow through the cooling loop from the fluid discharge to the fluid suction intake past the refrigeration stage of the expander to decrease a temperature of the fluid and further past an object being cooled (e.g. a superconducting magnet of an MRI system) to remove heat from the object being cooled.

Description

Cooling Loop For Superconducting Magnets

Background

[0001] Superconducting magnets, as often used in magnetic resonance imaging ("MRI") devices, need to have their coils kept at extremely low temperatures for superconductivity to persist. The present industry standard approach to accomplish such cooling is to submerse the coils, which are most commonly made of a niobium-titanium alloy, in a liquid helium bath. However, because of the high expenditures involved in this approach, methods for accomplishing the requisite cooling without liquid helium submersion are desirable.

Summary Of The Invention

[0002] A system includes a refrigeration apparatus including a compressor having a fluid suction intake and a fluid discharge The refrigeration apparatus also includes an expander providing refrigeration. The system also includes a cooling loop

connected at a first end to the fluid discharge of the

compressor of the refrigeration apparatus and at a second end to the fluid suction intake of the compressor of the refrigeration apparatus. A pressure differential between the fluid discharge and the fluid suction intake causes a working fluid to flow through the cooling loop from the fluid discharge to the fluid suction intake past the expander to decrease a temperature of the fluid and further past an object being cooled to remove heat from the object being cooled.

[0003] A method for cooling an object, including cooling an expander of a refrigeration apparatus. The refrigeration apparatus also includes a compressor having a fluid suction intake and a fluid discharge. The method also includes forcing a fluid through a cooling loop having a first end coupled to the fluid discharge of the compressor of the refrigeration apparatus and a second end coupled to the fluid suction intake of the compressor of the refrigeration apparatus. A pressure

differential between the fluid discharge and the fluid suction intake causes the fluid to flow through the cooling loop through an expander heat exchanger coupled to the expander to decrease a temperature of the fluid and further through a load heat exchanger coupled to the object being cooled to remove heat from the object being cooled.

Brief Description Of The Drawings

[0004] Figure 1 shows a schematic view of a cooling loop for a superconducting magnet according to an exemplary embodiment.

[0005] Figure 2 shows an exemplary method of operation for the exemplary cooling loop of Figure 1.

Detailed Description

[0006] The exemplary embodiments may be further understood with reference to the following description and the related appended drawings, wherein like elements are provided with the same reference numerals. Specifically, the exemplary

embodiments relate to a forced cooling loop for a

superconducting magnet.

[0007] Superconducting magnets are used, among other

applications, in magnetic resonance imaging ("MRI") machines that are commonly found in hospitals and other types of medical facilities. A superconducting magnet, in which electric current flows through coils of superconducting wire having substantially no electrical resistance to generate a magnetic field, may typically be expected to have a service life on the order of ten years. To achieve this extended service life, the coils must be kept at an extremely low temperature in order for the resistance therein to be low enough for superconducting state to occur and for the magnetic field to persist without a need for the current to be supplemented from external sources. For coils made from a niobium-titanium alloy, as is most common, this temperature is approximately 4 degrees Kelvin (referred to herein as "4 K") .

[0008] The industry standard approach to maintaining this low temperature is to submerse the coils in a bath of liquid helium. 4 K refrigeration may be provided by consuming liquid helium or by using a 4 K cooler to cool the liquid helium bath. The volume of liquid helium required by either approach is large, and liquid helium prices are high and increasing. Therefore, an alternative method for providing the requisite 4 K cooling to the coils without using a liquid helium bath is desirable.

Directly cooling the magnet coils may require the use of a continuous flow refrigeration system (or "cooler") such as a Brayton or Joule-Thompson ("JT") cooler, thermal links between a Gifford-McMahon ("GM") cooler and the coils, or a combination of the two .

[0009] Providing refrigeration directly to magnet coils using industry-standard GM coolers typically requires either the use of thermally conductive links or gas circulation loops to thermally connect the 4 K stage of the cooler expander (in the case of a GM cooler, the expander may alternatively be referred to as a "coldhead") to the magnet coils. To maintain the coil temperature within its requisite parameters, this connection should be highly efficient, capable of transporting the entire heat load with only fractions of a degree of temperature

difference. Depending on the context, conductive links that might be suitable for this purpose include solid conductors

(e.g., copper wire or bar), two-phase devices such as heat pipes, or convective loops. Although these approaches all provide thermal links to the expander, each has disadvantages.

[0010] To provide the requisite heat transportation, a solid conductor preferably has a large material cross-section. Given the long lengths of conductor that are typically required, the resulting conductor is heavy, complicated to properly integrate, and expensive. Further, the heavy solid conductors generate significant mechanical loads due to shock and vibrations from transportation and imaging. These mechanical loads must be reacted at the connection points, which may lead to mechanical failures in the expander and other sensitive components.

[0011] Heat pipes may deliver improved thermal performance as compared to solid conductors, with significantly reduced mass. However, heat pipe working fluids are only suitable for narrow operating temperatures; therefore, multiple heat pipes with different working fluids connected in parallel are required to provide refrigeration over a large enough temperature range to support cooling the system down from higher temperatures. Even using multiple working fluids, the entire temperature range experienced during a cool down is not adequately covered in order to permit efficient heat pipe operation. [0012] Convective cooling loops may be used over the entire temperature range of interest, without requiring the large mass associated with a solid conductor. However, the implementation of a convective cooling loop typically requires the use of complex heat exchanger arrangements. Further, the cooling loops themselves may include numerous joints and long tubing runs, making practical implementation difficult. Additionally, convective cooling loops typically require large temperature differentials to produce substantial density differences coupled with a significant height difference to generate enough static head to produce flow rates substantial enough to carry the required heat loads. This may require the expander to operate at a substantially lower temperature than the device being cooled, leading to increased cool-down and recovery times.

[0013] Fluid circulators may provide forced cooling and are capable of overcoming the disadvantages of solid conductors, heat pipes, and convective cooling loops noted above. Forced cooling is effective over large temperature ranges, is

lightweight, requires fewer tubing joints and less overall tubing, and may closely couple the temperatures of the expander (e.g., the coldhead of a GM cooler) and the refrigerated item (e.g., the coils of a superconducting magnet) in order to shorten cool-down and recovery times.

[0014] The exemplary fluid circulation system 100 illustrated in Figure 1 may provide for fluid circulation to cool the coils of a superconducting magnet using a compressor that already exists as part of a refrigeration system to provide for fluid circulation. It will be apparent that, though the exemplary embodiment is described with specific reference to the coils of a superconducting magnet, as is commonly use in an MRI device, the broader principles may be equally applicable to a mechanism for cooling any other object to an extremely low temperature. It will also be apparent to those of skill in the art that the specific arrangement of components shown in Figure 1 is only exemplary, and that various other layouts providing for the flow of working fluid that will be described hereinafter may also be possible without departing from the broader concepts illustrated by the exemplary embodiment.

[0015] The fluid circulation system 100 (or, for brevity, "system 100") includes a compressor 110. The compressor 110 may be part of a refrigeration system (indicated in Figure 1 by a bounding box having a dashed border) that lowers the temperature of an expander 120 in order to cool the coils of a

superconducting magnet. In the exemplary embodiment, the refrigeration system is a GM cooler and the expander is the coldhead thereof, but those of skill in the art that other types of refrigeration systems, such as a Brayton cooler, a JT cooler, etc., may be used in the alternative without departing from the broader principles described herein. Further, in the exemplary embodiment, the refrigeration system may be a multi-stage GM cooler, and the expander 120 may be a multi-stage expander having a first cooling stage 122 at a temperature T_i22 of 40.0 K and a second cooling stage 124 at a temperature T_i24 of 3.996 K.

[0016] The compressor 110 includes a suction 112 intaking gas and a discharge 114 expelling gas at, for example, 2275.3 kPa. The compressor 110 may be any type of compressor known in the art and used as part of a refrigeration system. In the

exemplary embodiment operable to cool a load to 4 K, the working fluid is helium; however, in other higher-temperature

embodiments, such as those that might be appropriate for future higher-temperature superconductors, an alternative working fluid, such as neon, argon, nitrogen, etc., may be used. Those of skill in the art will understand that the working fluid may be gas or liquid, and, depending on the specific system

configuration, may also transition between gas and liquid, or vice versa, within the system.

[0017] The discharge 114 may expel fluid at a temperature Tn₄ of 300 K, a pressure P114 of 2275.3 kPa, and a mass flow rate m of 0.380 grams per second. This mass flow rate may remain constant throughout the loop. The discharge 114 may be coupled to a first valve 130; the coupling used here and throughout the system 100 may be circular tubing or any other appropriate shape of flow duct. The purpose of the first valve 130 may be to regulate the flow rate of fluid and the pressure within the forced cooling loop in order to maximize the efficiency of the heat transfer steps that will be described hereinafter. The first valve 130 may therefore be operable to restrict the flow of fluid so that its outflow is at a temperature Ti₃₀ of 300.704 K and a pressure P130 of 1156.1 kPa.

[0018] The first valve 130 may be coupled to a first intake 142 of a first recuperative heat exchanger 140. The fluid arriving at the first intake 142 may be at the temperature Ti₃₀ of 300.701 K and a slightly diminished pressure of P142 of

1154.9 kPa. This decrease in pressure, and similar decreases occurring throughout the fluid circulation system 100, may simply be a natural decrease as fluid flows through the

connections between the various elements of the fluid

circulation system 100 described herein. The fluid arriving at the first intake 142 may be cooled according to the standard function of the heat exchanger 140, and may subsequently be expelled from first exhaust 144 at a temperature Ti₄₄ of 45.193 K and a pressure P144 of 1097.2 kPa. The fluid that is

correspondingly warmed by the heat exchanger 140 to accomplish this cooling will be described hereinafter.

[0019] The exhaust 144 of the first recuperative heat exchanger 140 may be coupled to an intake 152 of a first stage heat exchanger 150. Fluid may arrive at intake 152 of at the temperature Ti₄₄ of 45.193 K and a pressure P152 of 1096.1 kPa. The fluid passing through the first stage heat exchanger 150 may be cooled by heat exchange with the first cooling stage 122 of expander 120. The first cooling stage 122 may provide

refrigeration q_i22 of 10.274 W to the fluid circulating through system 100 at the temperature T122 of 40.0 K. Fluid may leave the first stage heat exchanger 150 through an exhaust 154 at a temperature Ti₅₄ of 40.104 K and a pressure P154 of 1090.6 kPa.

[0020] The exhaust 154 of the first stage heat exchanger 150 may be coupled to a second recuperative heat exchanger 160.

Fluid may arrive at intake 162 of the second recuperative heat exchanger 160 at the temperature T154 of 40.104 K and a pressure Pi62 of 1089.5 kPa. The fluid arriving at the intake 162 may be cooled according to the standard function of the second

recuperative heat exchanger 160, and may subsequently be expelled from exhaust 164 at a temperature Ti64 of 4.855 K and a pressure Pi64 of 983.3 kPa. The fluid that is correspondingly warmed by the second recuperative heat exchanger 160 to

accomplish this cooling will be described hereinafter.

[0021] The exhaust 164 of the second recuperative heat exchanger may be coupled to an intake 172 of a second stage heat exchanger 170. Fluid may arrive at intake 172 of the second stage heat exchanger 170 at the temperature Ί±β4 of 4.855 K and a pressure P172 of 982.3 kPa. The fluid passing through the second stage heat exchanger 170 may be cooled by heat exchange with the second cooling stage 124 of expander 120. The second cooling stage 124 may provide refrigeration q_i24 of 0.996 W to the fluid circulating through system 100 at the temperature T124 of 3.996 K. Fluid may leave the second stage heat exchanger 170 through an exhaust 174 at a temperature Ti₇₄ of 4.016 K and a pressure P174 of 977.4 kPa. As described above, the working fluid may

transition between a gaseous and liquid state as it flows

through the system 100; in the exemplary embodiment, the fluid arriving at the intake 172 may be gaseous helium and the fluid departing at the exhaust 174 may be subcooled liquid helium.

[0022] The exhaust 174 of the second stage heat exchanger 170 may be coupled to a load heat exchanger 180. Fluid may arrive at intake 182 of the load heat exchanger 180 at the temperature T174 of 4.016 K and a pressure P₁₈2 of 976.4 kPa. The load heat exchanger 180 may perform heat exchange at the superconducting coils (or, as noted above, other load) being cooled by the fluid circulation system 100. In the exemplary embodiment, the load heat exchanger 180 may maintain the required temperature at the superconducting coils to ensure that the superconducting state persists. Fluid may leave the load heat exchanger through

exhaust 184 at a temperature Ti84 of 4.543 K and a pressure Pi84 of 927.6 kPa after receiving an input heat load qiso of 0.500 W from the superconducting magnetic coils.

[0023] The exhaust 184 of the load heat exchanger 180 may be coupled to a second intake 166 of the second recuperative heat exchanger 160. Fluid may arrive at the second intake 166 at the temperature Ti84 of 4.543 K and a pressure P166 of 926.6 kPa . Heat may be exchanged, according to the standard function of a heat exchanger, between the fluid flowing from first intake 162 to first exhaust 164 and the fluid arriving at second intake 166. The result of this heat exchange is that the fluid taken in at first intake 162 and expelled at first exhaust 164 is cooled from 40.104 K to 4.855 K, as described above, and the fluid taken in at second intake 166 is heated from its intake

temperature Ti₈₄ of 4.543 K to a resulting temperature of 39.843 K. This warmed fluid leaves the second recuperative heat

exchanger 160 through a second exhaust 168 at a temperature Ti₆8 of 39.843 K and a pressure Pi68 of 836.3 kPa. As described above, the working fluid may transition between a gaseous and liquid state as it flows through the system; in the exemplary

embodiment, the fluid arriving at the intake 166 may be

subcooled liquid helium and the fluid departing at the exhaust 168 may be gaseous helium.

[0024] The second exhaust 168 of the second recuperative heat exchanger 160 may be coupled to a second intake 146 of the first recuperative heat exchanger 140. Fluid may arrive at the second intake 146 at the temperature Ti68 of 39.843 K and a pressure Pi46 of 835.5 kPa. As described above with reference to second

recuperative heat exchanger 160, heat may be exchanged between the fluid flowing from first intake 142 to first exhaust 144 and the fluid arriving at second intake 146. The result of this heat exchange is that the fluid taken in at first intake 142 and expelled at first exhaust 144 is cooled from 300.704 K to 45.193 K, as described above, and the fluid taken in at second intake 146 is heated from its intake temperature Ti₆8 of 39.843 K to a resulting temperature of 295.476 K. This warmed fluid leaves the first recuperative heat exchanger 140 through a second exhaust 148 at a temperature Ti₄₈ of 295.476 K and a pressure Pi48 of 793.7 kPa.

[ 0025 ] The second exhaust 148 of the first heat exchanger 140 may be coupled to a second valve 190. Fluid may arrive at the second valve 190 at the temperature Ti48 of 295.476 K and a pressure P190 of 792.9 kPa. The second valve 190, like the first valve 130, may serve to regulate the flow of fluid through the fluid circulation system 100. The second valve 190 may also be coupled to a suction 112 of the compressor 110. The suction 112 may intake the circulating fluid at a low pressure of 792.9 psi. Fluid may flow out of the second valve 190 and into the suction 112 at a temperature Ti₉₀ of 295.451 K and a pressure Pn₂ of 792.9 kPa . The fluid may then be compressed again by the compressor 110 and re-circulated through the fluid circulation system 100 starting at the discharge 114 as described above.

[ 0026 ] The system 100 has been described with reference to a specific set of components disposed and configured to operate in a manner so as to perform a specific task (i.e., to cool the coils of a superconducting magnet to a desired 4 K temperature) . However, various modifications to the exemplary system 100 may be possible without departing from the broader principles embodied therein. First, as noted, though the cooling system comprising the compressor 110 and the expander 120 has been disclosed to be a GM cooler, other embodiments may comprise a Brayton cooler, a JT cooler, a Stirling cooler, a Pulse Tube cooler, etc.

[ 0027 ] Additionally, though the valves 130 and 190 of the exemplary system 100 are positioned directly adjacent to the discharge 114 and suction 112, respectively, of the compressor 110, the placement within system 100 in other embodiments. In one embodiment, a valve placed at a cold location within the system 100 could serve as an expander for a JT cooler to provide refrigeration in addition to regulating flow. In another

embodiment, differing numbers of valves could be used, including using one valve or even no valves at all to provide for maximum pressure differential within the system 100. Additionally, the valves 130 and 190 could be automated in order to optimize the flow rate through the system 100 for a current status of the load being cooled by the system 100. For example, the flow parameters described above with reference to the exemplary system 100 are appropriate for use to maintain a superconducting magnet in a persist (e.g., superconducting) state, but where a magnet has quenched and recovery is occurring, it may be

desirable for the valves 130 and 190 to be fully open in order to allow fluid to flow through the system 100 at as high a flow rate as possible, in order to expedite recovery from quench.

[0028] Further, the exemplary system 100 provides cooling to a single load at load heat exchanger 180. However, the system may be modified to include a secondary load heat exchanger (e.g., between second exhaust 168 of second recuperative heat exchanger 160 and second intake 146 of first recuperative heat exchanger 140) to provide forced cooling at a second temperature. Such an embodiment may be adapted to provide forced cooling to a

secondary load, such as a thermal shield. Further, the

compressor of the exemplary system could operate cryogenically, with the warmest temperature of the working fluid being roughly 45 K. In such an embodiment, the first recuperative heat

exchanger 140 could be eliminated, further simplifying the system. [0029] Figure 2 illustrates an exemplary method 200 by which the exemplary system 100 may operate. In step 210, the expander 120 is cooled through the operation of the refrigeration apparatus, which, as noted above, comprises the compressor 110 and the expander 120. This may follow the standard operation of a refrigeration apparatus that is known in the art, and it will be apparent to those of skill in the art that the specific details of this step may vary for different types of

refrigeration apparatuses. In step 220, fluid is forced through a cooling portion of a cooling loop extending from the discharge 114 to the fluid suction 112. During the cooling portion of the cooling loop, the fluid is cooled through the use of heat exchangers coupled to the expander 120 (e.g., first stage heat exchanger 150 and second stage heat exchanger 170) or to fluid in a warming portion of the cooling loop (e.g., first

recuperative heat exchanger 140 and second recuperative heat exchanger 160), which will be discussed below.

[0030] In step 230, fluid continues to be forced through the cooling loop to a load portion of the cooling loop. During the load portion of the cooling loop, the fluid is operative to remove heat from the object being cooled by the cooling loop through the use of a heat exchanger (e.g., load heat exchanger 180) . In step 240, the fluid continues to be forced through the cooling loop to a warming portion of the cooling loop. In this portion of the cooling loop, the fluid is warmed to accomplish the corresponding cooling in the cooling portion of the cooling loop through the use of heat exchangers coupled to the cooling portion of the cooling loop (e.g., first recuperative heat exchanger 140 and second recuperative heat exchanger 160) as described above. Following step 240, the fluid returns to the fluid suction 112 the method 200 terminates. However, it will be apparent to those of skill in the art that the method 200 may typically be performed continuously in order to achieve constant cooling for the object being cooled.

[0031] The exemplary embodiment uses the compressor of an existing refrigeration system to provide the driving impetus to propel fluid through various stages of a forced cooling loop, such as heat exchangers and GM coolers, and eventually to an object being cooled by a loop, such as the coils of a

superconducting magnet for an MRI device. By doing so, the exemplary embodiment provides the advantages of a forced cooling loop over prior techniques, such as solid conductors, heat pipes, convective loops, or submersion in liquid helium. The exemplary embodiment further provides these advantages without requiring a separate compressor or other apparatus for driving the fluid through the loop, the resulting increased overall system cost, and the corresponding space and power needed to run such an apparatus. By providing such forced fluid cooling, the

exemplary embodiment may further provide for appropriate cooling to maintain superconducting temperatures in the coils of a superconducting magnet, without requiring large quantities of expensive liquid helium.

[0032] It will be apparent to those skilled in the art that various modifications may be made to the exemplary embodiments, without departing from the spirit or the scope of the invention. Thus, it is intended that the present invention cover

modifications and variations of this invention provided they come within the scope of the appended claims and their

equivalents .

Claims

What is claimed is:

1. A system, comprising:

a refrigeration apparatus comprising a compressor having a fluid suction intake and a fluid discharge, the refrigeration apparatus further comprising an expander providing refrigeration; and

a cooling loop connected at a first end to the fluid

discharge of the compressor of the refrigeration apparatus and at a second end to the fluid suction intake of the compressor of the refrigeration apparatus, wherein a pressure differential between the fluid discharge and the fluid suction intake causes a working fluid to flow through the cooling loop from the fluid discharge to the fluid suction intake past the expander to decrease a temperature of the fluid and further past an object being cooled to remove heat from the object being cooled.

2. The system of claim 1, wherein the object being cooled is a coil of a superconducting magnet of a magnetic resonance imaging device

3. The system of claim 1, wherein the fluid comprises one of helium, neon, argon, nitrogen and air.

4. The system of claim 1, wherein the cooling loop comprises a heat exchanger cooling the fluid flowing from the fluid

discharge of the compressor to the object being cooled and correspondingly warming the fluid flowing from the object being cooled to the fluid suction intake of the compressor.

5. The system of claim 4, wherein the heat exchanger is a recuperative heat exchanger.

6. The system of claim 4, wherein the cooling loop further comprises a further heat exchanger further cooling the fluid flowing from the heat exchanger to the object being cooled and correspondingly warming the fluid flowing from the object being cooled to the heat exchanger.

7. The system of claim 1, wherein the cooling loop comprises a heat exchanger connecting a refrigeration stage of the expander to the cooling loop to decrease the temperature of the fluid.

8. The system of claim 7, wherein the expander is a multistage expander, and wherein the cooling loop further comprises a further heat exchanger connecting a further refrigeration stage of the expander to the cooling loop to further decrease the temperature of the fluid.

9. The system of claim 1, wherein the refrigeration apparatus is one of a Gifford-McMahon cooler, a Joule-Thompson cooler, a Stirling cooler, a Brayton cooler, and a Pulse Tube cooler.

10. The system of claim 1, wherein the temperature of the fluid is substantially equal to 4 Kelvin when the fluid flows past the object being cooled.

11. The system of claim 1, wherein the pressure differential is 200 psi.

12. The system of claim 1, wherein a mass flow rate of the fluid is 0.380 grams per second.

13. The system of claim 1, wherein the cooling loop comprises a valve regulating a mass flow rate of the fluid.

14. The system of claim 1, wherein the cooling loop comprises a load heat exchanger removing heat from the object being cooled and correspondingly warming the fluid.

15. The system of claim 14, wherein the cooling loop further comprises a secondary load heat exchanger removing heat from a further object being cooled and correspondingly warming the fluid.

16. A method for cooling an object, comprising:

cooling an expander of a refrigeration apparatus, the refrigeration apparatus further comprising a compressor having a fluid suction intake and a fluid discharge; and

forcing a fluid through a cooling loop having a first end coupled to the fluid discharge of the compressor of the

refrigeration apparatus and a second end coupled to the fluid suction intake of the compressor of the refrigeration apparatus, wherein a pressure differential between the fluid discharge and the fluid suction intake causes the fluid to flow through the cooling loop through an expander heat exchanger coupled to the expander to decrease a temperature of the fluid and further through a load heat exchanger coupled to the object being cooled to remove heat from the object being cooled.

17. The method of claim 16, wherein the object being cooled is a coil of a superconducting magnet of a magnetic resonance imaging device.

18. The method of claim 16, wherein the fluid comprises one of helium, neon, argon, nitrogen and air.

19. The method of claim 16, wherein the cooling loop comprises a heat exchanger cooling the fluid flowing from the fluid discharge to the object being cooled and correspondingly warming the fluid flowing from the object being cooled to the fluid suction intake .

20. The method of claim 16, wherein the cooling loop comprises a heat exchanger connecting a refrigeration stage of the expander to the cooling loop to decrease the temperature of the fluid.