CLOSED CYCLE CRYOGEN RECIRCULATION SYSTEM AND METHOD

Abstract

There is provided refrigeration system (1) and method for remote cooling of a thermal load having a first portion (27) and a second portion (25). The system comprises a cold source (4) having a first cooling stage (5) and a second cooling stage (6), the temperature of the first cooling stage being higher than the temperature of the second cooling stage. The system also comprises a cryogen circuit for circulation of a cryogen flow in a closed cycle, the closed cycle being thermally coupled to the cold source. The system further comprises a compressor (7) for compressing and circulating the cryogen flow in the cryogen circuit. The cryogen circuit comprises a first conduit for thermally connecting the first cooling stage of the cold source to the first portion of the thermal load so as to cool said first portion towards the temperature of the first cooling stage, and a second conduit for thermally connecting the second cooling stage of the cold source to the second portion of the thermal load so as to cool said second portion to wards the temperature of the second cooling stage. The cryogen flow in the system is a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow.

Description

RELATED APPLICATIONS

The present application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/EP2015/079195, filed 10 Dec. 2015, which claims priority to European Patent Application No. 14197216.6, filed 10 Dec. 2014. The above referenced applications are hereby incorporated by reference into the present application in their entirety.

FIELD OF THE TECHNOLOGY

The present disclosure relates generally to the field of cryogenic cooling systems, and more particularly, to small-scale and low cooling capacity cryogenic cooling systems which are suitable for cooling objects which are located remotely, such as, but not limited to, superconducting magnets.

BACKGROUND

Large scale and large capacity cryogenic cooling systems, such as helium refrigerators and liquefiers are well known in the art.

For small-scale low-capacity cryogenic cooling systems, with cooling power in the Watt-range at temperatures below 5K, cryocoolers are commonly used. They traditionally consist of a compressor, a cold head and wherein the cold head contains one, two or maximum three cooling stages, with more or less predetermined temperature ranges. Due to their design, systems based on cryocoolers are traditionally closed cycle systems which are called “cryogen free” as they provide only cold surfaces for the cooling of thermal loads.

Such cryocoolers are typically used for direct contact cooling of laboratory-sized objects to be cooled down and maintained at the desired temperature, such as samples, superconductors, etc. Other applications include re-condensation of vaporized fluids such as helium at 4.5K, hydrogen at 20K or nitrogen at 70K.

This results in a significant temperature gradient within the object to be cooled, which means that the cryocooler needs to be at a significantly lower temperature than the desired load temperature resulting in a lower efficiency of the system and lower cooling power.

To avoid temperature gradients within the object to be cooled, bath cooling is a well-known solution and this is still commonly performed in cryogenic laboratories in universities and research centres. This solution was also used for first-generation MRI magnets. Such systems are open-cycle, and therefore require a storage dewar which has to be refilled or changed on a regular basis, and thus requires additional handling of a hazardous cryogenic fluid. In cases where the required performance is achievable within the range of cryocoolers, cryocoolers are well adapted to re-condensate the vaporized liquid.

However, where bath cooling is not possible or where remote cooling is required, such as when the cryocooler cannot be located in sufficient proximity to the load for any reason, state of the art cryocoolers are not suitable for such cooling purposes.

There is therefore a need for a compact refrigeration system that is capable of providing low-capacity refrigeration for remote cooling of a load. The inventors of the present invention have addressed the above shortcomings of conventional low-capacity cryogenic refrigerators, as will be explained below.

SUMMARY OF INVENTION

According to one aspect of the present invention, there is provided a refrigeration system for remote cooling of a thermal load having a first portion and a second portion, the system comprising: a cold source having a first cooling stage and a second cooling stage, the temperature of the first cooling stage being higher than the temperature of the second cooling stage; a cryogen circuit for circulation of a cryogen flow in a closed cycle, the closed cycle being thermally coupled to the cold source; and a compressor for compressing and circulating the cryogen flow in the cryogen circuit, wherein the cryogen circuit comprises a first conduit for thermally connecting the first cooling stage of the cold source to the first portion of the thermal load so as to cool said first portion towards the temperature of the first cooling stage, and a second conduit for thermally connecting the second cooling stage of the cold source to the second portion of the thermal load so as to cool said second portion towards the temperature of the second cooling stage, and wherein the cryogen flow is a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow. It is especially interesting to operate the system of the present invention with a two phase cryogen flow, as the refrigeration using the latent heat of the fluid is advantageous, because as the liquid evaporates the fluid does not fluctuate in temperature, thus allowing a uniform temperature distribution of the fluid at the thermal load. Because of this uniform temperature distribution, the conventional solution to the problem as identified in the Background, of operating the cold source at a lower temperature than is desired for the load is no longer required, resulting in a higher cold source performance. Some advantages of the refrigeration with two phase flow include the reduced mass flow in comparison to a single phase flow, and also that the two phase fluid may be returned from the thermal load at the same temperature (thus reducing hotspots).

Consequently, and in contrast to the refrigeration systems using the sensible heat of the fluid, using the latent heat requires a lower mass flow at a given cooling capacity.

Notwithstanding the above, the system of the present invention can also operate in single phase conditions using the latent heat of the cryogen.

The system of the present invention is the combination of a cold source and a cryogen circulation system; it provides a low capacity (for example, 1 or 2 Watts at 4.5 K) refrigeration system with a cryogen mass flow at low temperatures in a closed cycle at mass flow rates in the order of magnitude of <0.1 g/second for two phase flow or up to 1 g/second for single phase gas or supercritical helium flow. The closed nature of the cryogen cycle allows for non-stop refrigeration, i.e. the system does not need to be stopped in order to change or re-fill the cryogen. Further, the system advantageously does not suffer from the drawbacks of contact cooling, for example, temperatures gradients across the load, punctual cooling or elaborated thermal anchoring.

The refrigeration system may preferably comprise at least one heat exchanger, which is used to thermalize the cryogen to at least one of the cold source stages and to exchange the enthalpy respectively between the go and return streams of the cryogen.

The compressor of the refrigeration system may be a circulation pump at room temperature. The advantage of using warm pumps with respect to any other solution is that the compression work can be directly extracted to the ambient environment by any common means, such as air or water heat exchangers. This is especially important when high compression ratios are required, for example, for a Joule-Thomson expansion step.

In addition, the system of the present invention can provide one or two (or in some cases more) cryogen flows at different temperatures at the same time. In other words, the cryogen flows in a single circuit but can be taken at two different stages of the refrigeration process simultaneously. The same cryogen can therefore be used at two different temperature levels.

Some applications of the system of the present invention include low capacity cryogenic refrigeration where contact cooling with a common cold source is not suitable, where re-filling the cryogen source is not an option (for example, in a dewar), where re-condensation of the used cryogen must be integrated into the system, where use of an immersion cryostat is not suitable, and where the available space next to the thermal load presents some constrains (for example available space or radioactive environment). Other uses may include recuperative cycles, multistage cascade cooling, helium condensation, helium liquefaction and remote cooling.

Preferably, the first portion of the thermal load could be a thermal shield, and said thermal shield may be actively cooled. The cryogen at the higher temperature (e.g. 60 K) can be used to cool, for example, said active thermal shield. Preferably, the second portion of the thermal load could be a superconducting magnet. The cryogen at the lower temperature (e.g. 4.2 K) could be used to cool, for example, said superconducting magnet. When operating with two phase helium, the configuration of the refrigeration system of the present invention would provide a uniform temperature profile at the second portion of the thermal load. This is particularly advantageous in the case where the second portion of the thermal load is a superconducting magnet coil and it is important that a uniform temperature profile with a low temperature gradient is achieved on the coil.

Each of the first conduit and the second conduit may preferably be located within a transfer line, and preferably said transfer line has low thermal losses. An advantage of the remote set-up of the “active” parts of the system (the cold source and the compressor) away from the “passive” parts of the system (the thermal load e.g. the superconducting magnet) is that no harmful vibrations are transferred to the thermal load. In addition, said “active” parts are spatially separated from the thermal load. This is advantageous in case the load is in an environment with special constraints (for example radioactivity or limited space availability).

Preferably, the first and second cooling stages of the refrigeration system and the first and second conduits of the closed cycle cryogen circuit may all connected in series. With respect to prior art systems using parallel cryogen circuits, the arrangement of the system of the present invention avoids the need for elaborate and expensive cryogenic control systems and equipment.

The cold source of the refrigeration system may preferably be, but is not limited to, a cryocooler. Any cold source providing a continuous cooling performance at the desired temperature would be suitable. However, cryocoolers exhibit advantages which make them particularly adapted to the present invention: they are commercially available, easy to use and being cryogen-free, cryocoolers obviate the need for a cryogen flow. This therefore also reduces the complexity of the system, and additionally limits the need for controls and instrumentation. In its simplest form, the system could be implemented completely without controls (as per FIG. 1 and FIG. 2). Such a system is very robust and offers a turn-key solution to users which therefore also do not need to be particularly trained in cryogenics. In more sophisticated implementations (e.g. FIG. 3), a limited set of controls and instrumentation can be used, increasing the versatility and flexibility of the system, which would be particularly suited for the cooldown of larger cold masses.

Preferably, the cold source may be contained within a cryostat or vacuum vessel for insulation purposes. Said cryostat or vacuum vessel is separate and independent from a cryostat of the thermal load(s). Preferably, the cryostat or vacuum vessel may comprise an actively cooled thermal shield to reduce undesired thermal radiation to the colder components.

Preferably, the system may further comprise a Joule-Thompson (JT) expansion step, which may be a JT pipe. Due to the JT-expansion, the final cryogen temperature at the thermal load is lower than after thermalizing at the second stage of the cold source. Therefore, the cold source can operate at a higher temperature than the final cryogen temperature. Since the cold source performance increases when working at a higher temperature, the overall cooling capacity would therefore increase which may lead to an overall higher cooling capacity of the cold source.

Preferably, depending on the construction of the system, it may act as a refrigerator or as a liquefier. For example, gas helium (GHe) may be liquefied to liquid helium (LHe). However, the main design objective of the present invention is to achieve a refrigeration system rather than a liquefaction system.

According to another aspect of the present invention, there is provided a method for cooling remotely cooling a thermal load using a refrigeration system, the method comprising: selecting the temperature of a first cooling stage of a cold source of the system to be higher than the temperature of a second stage of cooling of the cold source; circulating a cryogen flow in a closed cycle around the cryogen circuit of the refrigeration system, the closed cycle being thermally coupled to the cold source, the cryogen circuit comprising a first conduit for thermally connecting the first cooling stage of the cold source to a first portion of the thermal load so as to cool said first portion towards the temperature of the first cooling stage, and a second conduit for thermally connecting the second cooling stage of the cold source to a second portion of the thermal load so as to cool said second portion towards the temperature of the second cooling stage; and compressing the flow in the cryogen circuit using a compressor of the refrigeration system, wherein the cryogen flow is a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow.

BRIEF DESCRIPTIONS OF DRAWINGS

Certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a refrigeration system and a load to be cooled according to the present invention;

FIG. 2 is a schematic diagram of a refrigeration system according to the present invention; and

FIG. 3 is a schematic diagram of another refrigeration system according to the present invention.

DETAILED DESCRIPTION OF DRAWINGS

Reference will now be made to FIG. 1, which is a schematic diagram of a system according to the present invention. FIG. 1 shows a refrigeration system (1) and a thermal load (25, 27) to be cooled, each being installed separately in two separate cryostats (2) The cold source (4), which in this case may be a cryocooler, cools the cryogen to the required temperature, and then the cryogen is delivered to the thermal shield (25) and to the load (27) through a pipe system (26, 28). A low-loss transfer line (24) is also shown.

FIG. 2 illustrates the refrigeration system (1) in more detail. This is one of the many possible configurations, which can vary according to the cooling requirements. This particular configuration could be considered as the baseline of more complex configurations potentially resulting in higher performances. An example of those more complex configurations is presented in FIG. 3. The refrigeration system (1) in FIG. 2 consists of a pre-cooling refrigerator and a cryogen circuit. The two systems are thermally coupled to each other. Most of the components are inside a vacuum chamber or cryostat (2) and, where necessary, some of them are also thermally protected by an actively cooled thermal shield against undesired thermal radiation (3).

The pre-cooling refrigerator consists in this case of a commercial 2-Stage (5, 6) cryocooler (4) (GM-type, Stirling type, Pulse Tube type, or other) or any other suitable refrigeration system. The first cooling stage (5) has a temperature that is higher than the temperature of the second cooling stage (6).

There is also shown in FIG. 2 a compressor (7) (or gas pump) at room temperature, a pipe system (8, 9) for the cryogen and various heat exchangers (10, 11, 12, 13, 15, 16, 17).

The cryogen circuit starts with a compressor (7) at room temperature, which is used to circulate the cryogen. The cryogen circulates through a pipe system, which consists of the feed pipeline (8) and the return pipeline (9). The feed pipeline (8) starts in the flow direction at the compressor (7), goes through different heat exchangers (10, 11, 12, 15, 16, 17) and ends at the feed through connection (23) to the thermal load. Correspondingly, the return pipeline (9) starts at the feed through connection (23) from the thermal load, goes through different heat exchangers (13, 12, 11) and ends at the compressor (7).

The compressor (7) compresses and moves the cryogen gas through pipe runs. The first heat exchanger (10) re-cools the cryogen gas back from room temperature. Subsequently a group of heat exchangers (11-13) transport the cryogen heat from the feed pipeline (8) to the return pipeline (9). The cryogen is additionally cooled in the feed pipeline (8) by the cryocooler (4) at the heat exchangers on the first (15, 16) and second stage (17). After the cryogen is being pre-cooled in the feed pipeline (8), it flows towards the feed through connection (23) to the thermal load.

The cryogen can be taken at two different stages of the refrigeration process simultaneously, allowing its use at least at two different temperatures, or in case of need, at intermediate temperatures at temperature levels between ambient and first stage, or temperatures between first and second stage. Additional pipes and connections may be needed for this functionality. The following description focuses on the two stage configuration, however, other configurations comprising more stages are also conceivable. The cryogen at a higher temperature will be delivered over a pipe system to the thermal shield (25) of the load, while the colder cryogen will go to the load (27). In both cases, the cryogen will return to the refrigeration system (1) after being used in order to provide a closed cycle system.

Depending on the configuration of the refrigeration system (1) and the chosen cryogen, the system can be set up to provide a mass flow of a cryogen as any one of a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow. Therefore, the sensible heat (the cryogen is gaseous or supercritical and changes its temperature at the load to be cooled) or the latent heat (the liquid cryogen evaporates and therefore it does not change his temperature) of the cryogen can be used for refrigeration. After the cryogen refrigerates the load it returns to the compressor (7) through the return pipeline (9), thus closing the cycle.

FIG. 3 illustrates the refrigeration system (1) in a more complex configuration and potentially with a higher performance than in FIG. 2. As mentioned before, the many possible configurations of the system can vary according to the cooling necessities. However, some basic principles presented in FIG. 2 remain the same in FIG. 3. In this particular case, the refrigeration system (1) consists of a pre-cooling refrigerator, a cryogen circuit comprising a Joule Thompson (JT) expansion step and a transfer line (24). Most of the components are inside a vacuum chamber or cryostat (2) and some of them are also thermally protected by an actively cooled thermal shield (3) against undesired thermal radiation.

The pre-cooling refrigerator consists in this case of a commercial 2-Stage (5, 6) cryocooler (4) or any other suitable refrigeration system (see above). The JT refrigerator includes a compressor (7) at room temperature, a pipe system (8, 9) for the cryogen, various heat exchangers (10-17) and a Joule Thompson (JT) expansion device (18). In addition, in order to reduce flow impedances and to accelerate cool down, several bypass valves (19-22) can be installed in case of necessity.

The JT refrigerator starts with a compressor (7) at room temperature, which is used to compress and circulate the cryogen. The cryogen circulates through a pipe system, which consists of the feed pipeline (8) and the return pipeline (9). The feed pipeline (8) starts in the flow direction at the compressor (7), goes through various heat exchangers and valves (10-12, 15, 23, 16-18) and ends at the feed through connection (23) to the load. Correspondingly, the return pipeline (9) starts at the feed through connection (24) from the load, goes through different heat exchangers (14-11) and ends at the compressor (7).

For remote cooling, a transfer system is required. This may comprise an optimized transfer line (24) which delivers the cryogen to the thermal loads. The optimization consists of minimizing the losses on all lines, and in particular in the cold circuit. This allows a more efficient integration into the overall system.

After the compression of the cryogen gas, the first heat exchanger (10) cools the cryogen back from room temperature. Afterwards a group of heat exchangers (11-14) transport the cryogen heat from the feed pipeline (8) to the return pipeline (9). The cryogen is additionally cooled in the feed pipeline (8) by the cryocooler (4) at the heat exchangers on the first (15, 16) and second stage (17). After the cryogen is being pre-cooled in the feed pipeline (8), it flows through the JT expansion device (18).

The cryogen can be taken at two different stages of the refrigeration process simultaneously, allowing his use at two different temperatures. The cryogen at a higher temperature will be delivered over a pipe system to the loads thermal shield (25), while the colder cryogen will go to the load (27). In both cases, the cryogen will return to the refrigeration system (1) after being used, in order to provide a closed cycle system.

Depending on the configuration of the refrigeration system (1) and the chosen cryogen, the system can be set up to provide a mass flow of either sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas. Therefore, the sensible heat (the cryogen is gaseous and changes his temperature) and/or the latent heat (the liquid cryogen evaporates and therefore it does not change his temperature) of the cryogen can be used for refrigeration. After the cryogen refrigerates the load it returns to the compressor (7) through the return pipeline (9), closing the cycle.

When the entire system is switched on, the cryogen starts circulating and the cryocooler (4) starts cooling down from room temperature. During this period of time, the bypass-valves can be opened (19-22) in case of necessity. Consequently, the cryogen can bypass some components (13, 14, 18), reducing the overall pressure losses. Once the cool down process is finished, the bypass-valves (19-22) can be closed again.

KEY

• • 1. Refrigeration system
  • 2. Cryostat or vacuum chamber
  • 3. Thermal Shield of the Cryogen Recirculation System
  • 4. Cryocooler
  • 5. First stage of cryocooler
  • 6. Second stage of cryocooler
  • 7. Compressor
  • 8. Feed pipeline
  • 9. Return pipeline
  • 10. Heat exchanger 1 (HX1)
  • 11. Heat exchanger 2 (HX2)
  • 12. Heat exchanger 3 (HX3)
  • 13. Heat exchanger 4 (HX4)
  • 14. Heat exchanger 5 (HX5)
  • 15. Heat exchanger at the first stage before going to the thermal shield of the load (HX1stA)
  • 16. Heat exchanger at the first stage after going to the thermal shield of the load (HX1stB)
  • 17. Heat exchanger at the 2nd Stage (HX2nd)
  • 18. JT expansion device
  • 19. Bypass-valve for Heat Exchanger 4 (HX4) at the feed side
  • 20. Bypass-valve for Heat Exchanger 5 (HX5) at the feed side
  • 21. Bypass valve for JT expansion device
  • 22. Bypass-Valve for Heat Exchanger 4 and 5 (HX4 and HX5) at the return side
  • 23. Feed through connection for the transfer lines
  • 24. Transfer line
  • 25. Thermal Shield of the load
  • 26. Pipe system for the thermal shield of the load
  • 27. Load
  • 28. Load pipe system
  • 29. Thermal shield of the transfer line

Claims

1. A refrigeration system (1) for remote cooling of a thermal load having a first portion (27) and a second portion (25), the system comprising:

a cold source (4) having a first cooling stage (5) and a second cooling stage (6), the temperature of the first cooling stage being higher than the temperature of the second cooling stage;

a cryogen circuit for circulation of a cryogen flow in a closed cycle, the closed cycle being thermally coupled to the cold source; and

a compressor (7) for compressing and circulating the cryogen flow in the cryogen circuit,

wherein the cryogen circuit comprises a first conduit for thermally connecting the first cooling stage of the cold source to the first portion of the thermal load so as to cool said first portion towards the temperature of the first cooling stage, and a second conduit for thermally connecting the second cooling stage of the cold source to the second portion of the thermal load so as to cool said second portion towards the temperature of the second cooling stage, and

wherein the cryogen flow is a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow.

2. A refrigeration system (1) according to claim 1, wherein the first portion (27) of the thermal load is a thermal shield.

3. A refrigeration system (1) according to claim 1, wherein the second portion (25) of the thermal load is a superconducting magnet.

4. A refrigeration system (1) according to claim 1, wherein the system further comprises a transfer line (24) in which one or both of the conduits are located, and wherein the transfer line has low thermal loss.

5. A refrigeration system (1) according to claim 1, wherein the first (5) and second (6) cooling stages and the first and second conduits of the closed cycle cryogen circuit are all connected in series.

6. A refrigeration system (1) according to claim 1, wherein the cold source is a cryocooler.

7. A refrigeration system (1) according to claim 1, wherein the cold source is contained in a cryostat that is separate and independent from a cryostat of the thermal load.

8. A refrigeration system (1) according to claim 7, wherein the cryostat comprises an actively cooled thermal shield (3).

9. A refrigeration system (1) according to claim 1, wherein the system further comprises at least one heat exchanger.

10. A refrigeration system (1) according to claim 1, wherein the system comprises means for performing a Joule-Thompson expansion step (18).

11. A refrigeration system (1) according to claim 1, wherein the system can also act as a liquefier.

12. A method for cooling remotely a thermal load using a refrigeration system (1), the method comprising:

selecting the temperature of a first cooling stage (5) of a cold source (4) of the system to be higher than the temperature of a second cooling stage (6);

circulating a cryogen flow in a closed cycle around the cryogen circuit of the refrigeration system, the closed cycle being thermally coupled to the cold source, the cryogen circuit comprising a first conduit for thermally connecting the first cooling stage of the cold source to a first portion (27) of the thermal load so as to cool said first portion towards the temperature of the first cooling stage, and a second conduit for thermally connecting the second cooling stage of the cold source to a second portion (25) of the thermal load so as to cool said second portion towards the temperature of the second cooling stage; and

compressing the flow in the cryogen circuit using a compressor (7) of the refrigeration system,

wherein the cryogen flow is a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow.