CRYOSYSTEM INCLUDING A CIRCULATOR

Description

[0001] The present invention relates to a circulator ensuring the circulation of a refrigerant fluid in a circuit of a cryogenic installation of the cryogenic loop type as well as such a cryogenic installation comprising this circulator.

[0002] Known in the state of the art are cryogenic installations for cooling an element having a thermal load comprising a cryogenic machine of the type " pulsed gas tube » known in English as « pulse tube » and a circuit connecting the cryogenic machine and said element, and in which a refrigerant fluid such as helium or nitrogen circulates.

[0003] In these installations, such a refrigerant fluid is set in motion by a circulator whose operation is controlled in particular from measurements of the flow rate of the fluid requiring the use of a thermal flow meter arranged in the circuit of these installations. Such a circulator comprises a main compartment in which is arranged an electric motor provided with a motor shaft supported by a bearing device of this motor comprising bearings, said motor shaft having one end on which is mounted a centrifugal wheel. In this circulator, electric cables provided for supplying the motor are connected to the latter through a sealed passage thus isolating such a passage from the main compartment in which the motor, the bearing device and the centrifugal wheel are immersed in the refrigerant fluid.

[0004] However, one of the disadvantages of these cryogenic installations is linked to the fact that they are not suitable for applications for which the refrigerant must be maintained at so-called "cryogenic" temperatures. bass » which are lower than 10K. Indeed, at such temperatures, the circulator presents numerous malfunctions and also generates significant thermal losses in these cryogenic installations.

[0005] Indeed, the electric motor of such a circulator is configured to operate at an operating temperature which corresponds to the ambient temperature. As soon as this operating temperature approaches low cryogenic temperatures, the motor is then subject to operating anomalies which can result, for example, in a blockage of the rotational movement of the motor shaft and therefore of the centrifugal wheel resulting from solidification occurring at such temperatures of a lubricant used in the bearings of the bearing device of this motor, and clearances which are too small (thermal contractions).

[0006] Furthermore, when the circulator operates at such low cryogenic temperatures, internal natural convection phenomena originating from the circulation of the fluid in the main compartment of this circulator generate very significant heat exchanges between the area of the compartment where the motor is located at a temperature on average 250K higher than the temperature prevailing in the area of this compartment where the centrifugal wheel is located. These heat exchanges have the consequence of generating considerable heat losses in these cryogenic installations which then make the use of such a circulator incompatible with the operation of these installations at such temperatures.

[0007] Document US 6,213,736 B discloses a circulator according to the preamble of claim 1.

[0008] One of the aims of the invention is therefore to remedy the drawbacks mentioned above and to improve the cryogenic installations known from the state of the art. In particular, the invention proposes a cryogenic installation defined to operate optimally in a wide range of cryogenic temperatures, in particular at low cryogenic temperatures less than or equal to 30K or even less and for example of the order of 5K.

[0009] For this purpose, the invention defined by claim 1 relates to a circulator ensuring circulation of a refrigerant fluid in a circuit of a cryogenic installation from and to an element having a thermal load, the circulator comprising a drive module and a pumping module provided with a centrifugal wheel, said drive module being magnetically coupled to said pumping module in order to drive said centrifugal wheel with a rotary movement.

[0010] In other embodiments: the pumping module comprises a drive shaft comprising a magnetic rotor and said centrifugal wheel at a first end; the pumping module comprises a bearing device supporting a drive shaft of the centrifugal wheel comprising first and second bearings, in particular cryogenic bearings; the pumping module comprises a casing defining a sealed enclosure in which a drive shaft of the centrifugal wheel is included; the casing comprises a casing of the magnetic rotor and a volute; a magnetic rotor is arranged between first and second bearings of a bearing device while being mounted on a drive shaft of the centrifugal wheel;According to the invention, the drive module comprises a magnetic stator, in particular comprising at least two electromagnets arranged around a circumference of all or part of a casing of a magnetic rotor of an envelope of the pumping module so as to cooperate with said magnetic rotor; the magnetic stator and the magnetic rotor mounted on the drive shaft together form an electric motor having operating characteristics substantially similar to those of a rotating motor, and the circulator is a mini-circulator.

[0011] The invention also relates to a cryogenic installation for cooling an element having a thermal load comprising such a circulator ensuring circulation of a refrigerant fluid in a circuit of this cryogenic installation from and to said element.

[0012] Advantageously, the cryogenic installation comprises at least one pulsed gas tube type cryogenic machine and a compression and control unit.

[0013] Advantageously also, the cryogenic installation comprises a cryostat provided with an enclosure maintained under an isolating vacuum.

[0014] Specifically, the cryostat includes the circulator, the cryogenic machine and the circuit.

[0015] A circulator according to the invention is defined by claim 1.

[0016] Embodiments of this circulator are defined by claims 2 to 6.

[0017] A cryogenic installation according to one aspect of the invention is defined by claim 7.

[0018] Embodiments of this installation are defined by claims 8 to 11.

[0019] Other advantages and characteristics of the invention will appear more clearly on reading the description of a preferred embodiment which follows, with reference to the figures, produced as an indicative and non-limiting example: there figure 1 represents a cryogenic installation of the cryogenic loop type for cooling an element receiving a thermal load, according to an embodiment of the invention, and the Figures 2A And 2B represent schematic views of drive and pump modules of the first and second variants of a circulator, among which only the circulator of the second variant illustrated by the Figure 2B is in accordance with the invention.

[0020] In the following description, like reference numerals designate like parts or parts having similar functions.

[0021] An embodiment of the cryogenic installation 1 of the cryogenic loop type, for cooling an element 3 having a thermal load according to the invention is described below with reference to the figure 1 . Such a cryogenic installation 1 is configured to operate at all cryogenic temperatures, for example below 30K or 25K, in particular at cryogenic temperatures known as " bass » which are less than 5K or 10K. In this cryogenic installation 1, element 3 otherwise called " user » , may be a device or object to be cooled that generates or receives this thermal load.

[0022] The cryogenic installation 1 mainly comprises the element 3 presenting the thermal load, a circulator 2, a circuit 6 in which a refrigerant fluid 4 circulates and a cryogenic machine 5.

[0023] This cryogenic installation 1 also comprises a compression and control unit 7 connected to the cryogenic machine 5. This cryogenic installation 1 may comprise a cryostat (not shown) in which the following components of this installation 1 are located: the circulator 2, the cryogenic machine 5 and part of the circuit 6. It will be noted that these components are of course connected to this circuit 6. Such a cryostat comprises an enclosure maintained under a dynamic vacuum also called " isolation vacuum» to minimize heat losses by gas convection and conduction. It should be noted that this cryostat isolation vacuum surrounds the entire circulator 2, thus contributing to the fact that the circulator 2 is maintained at a substantially constant temperature corresponding to a low cryogenic temperature, for example, of 5K or even less. It should be noted that the quality of the required isolation vacuum corresponds to the area commonly called "secondary vacuum", with a residual pressure level typically between 10 -7< and 10 -4< mbar.

[0024] In the cryostat, these components are mechanically fixed by parts such as fixing flanges which are made of a very low thermal conductive material to limit thermal leaks by thermal conduction.

[0025] In the cryogenic installation 1, the circuit 6 provides a thermal link between the cryogenic machine 5 and the element 3 presenting the thermal load. Such a circuit 6 thus contributes to conducting this thermal load towards the cryogenic machine 5 in order to evacuate it. More precisely, when the element 3 generates this thermal load in the fluid 4, the latter then causes an increase in the temperature of the refrigerant fluid 4 in relation to its specific heat and its mass flow rate. This increase in temperature of the refrigerant fluid 4 may however remain moderate depending on the importance of the mass flow rate of this fluid 4. Subsequently, when the refrigerant fluid 4 carrying the thermal load passes the level of the cryogenic machine 5, it cools down by depositing this thermal load and starts again for a new cycle.In this context, it will be noted that such a circuit 6 is a closed heat-transfer loop which carries the refrigerant fluid 4 between the cryogenic machine 5 and the element 3 presenting this thermal load.

[0026] In this circuit 6 of the cryogenic installation 1, the refrigerant fluid 4 may be a pressurized fluid of gaseous or liquid type which is maintained at a temperature below 5K. In the present embodiment, this refrigerant fluid 4 is a liquid such as supercritical helium at a pressure of approximately 20 bars.

[0027] In this cryogenic installation 1, the circuit 6 for circulating the refrigerant fluid 4 is connected to the cryogenic machine 5. This cryogenic machine 5 otherwise called " cryocooler » , is preferably of the pulsed gas tube type known in English as " pulse tube » .Such a machine is used to produce cryogenic temperatures below 120K. This cryogenic machine 5 includes an exchange zone otherwise called " cold end » which is intended to evacuate the thermal load carried by the refrigerant fluid 4 coming from the element 3 having generated such a load. In this embodiment, this cryogenic machine 5 is capable of delivering a low cooling power which is between 1 W and 10 W approximately, depending on the desired temperature level. The temperature delivered by the cryogenic machine 5 takes on an equilibrium which is a function of the thermal load to be evacuated. The circulator 2 adapts to the temperature / thermal load pair of the user 3.

[0028] As we have seen previously, the refrigerant 4 is circulated in the circuit 6 by the circulator 2. This circulator 2 visible on the figure 1 is preferably a mini-circulator 2 otherwise called “ mini cold circulator » or even « microcirculator". This circulator 2 is defined to create a pressure difference in the fluid 4 necessary to overcome the circulation pressure losses of the latter in the circuit 6. This circulator 2 is a small device of generally cylindrical shape. It may have a length L of between 70 and 100 mm, and preferably 70 mm, as well as a substantially circular section with a diameter of the order of 50 to 60 mm, and preferably 50 mm. Such a circulator 2 is particularly suitable for delivering hydraulic power of between 1 and 100 mW, and preferably 10 mW. The circuit 6 will be sized to reduce this hydraulic power while maintaining the capacity to transport the thermal load. Indeed, this hydraulic power corresponds to the friction power of the fluid 5 on the walls, which degrades into heat, which must be evacuated in addition to the thermal load of the user 3, and other losses such as eddy currents.

[0029] In the cryostat, this circulator 2 is preferably arranged as close as possible to the circuit 6 of the cryogenic machine 5 and in particular at the level of the exchange zone of this machine 5. This circulator 2 mainly comprises a drive module 8a, 8b and a pumping module 9a, 9b provided with a centrifugal wheel 10 structurally defined to operate at cryogenic temperatures, as well as electrical cables 11 supplying the drive module 8a, 8b. In this circulator 2, the drive module 8a, 8b is magnetically coupled 51 to the pumping module 9a, 9b in order to drive the centrifugal wheel 10 with a rotary movement.The magnetic coupling 51 constitutes a component of the circulator 2 which comprises components 18, 28, 29, 38, 39 of this circulator 2 which are described below and which participate in the transmission of a movement without mechanical contact, without energy input and without wear through a casing 12 of the pumping module 9a, 9b of this circulator 2.

[0030] This pumping module 9a, 9b therefore comprises this casing 12 which comprises a sealed enclosure 13 in which is included a drive shaft 14 of the centrifugal wheel 10. Such a centrifugal wheel 10 is capable of carrying out a rotary movement capable of causing a circulation of the refrigerant fluid 4 in the circuit 6 and in particular in this enclosure 13. During this circulation, the refrigerant fluid 4 is then sucked into this enclosure 13 at the level of an inlet opening 15a defined in the casing 12 and evacuated through an outlet opening 15b of this enclosure 13. These openings 15a, 15b are connected to the circuit 6 respectively by inlet and outlet pipes 16a, 16b of the circulator 2. It will be noted that the inlet opening 15a has an axis a1 which is coincident with central axes a2, a3 of the centrifugal wheel 10 and of the shaft training 14.Regarding the outlet opening 15b, it comprises an axis a4 which is preferably in a plane perpendicular to the axis a1 of the inlet opening 15a. In this casing 12, the drive shaft 14 of the centrifugal wheel 10 is provided with two ends 17a, 17b, one of which, the first end 17a, comprises this centrifugal wheel 10 fixed by gluing, fitting or even screwing. This drive shaft 14 also comprises a magnetic rotor 18 provided with at least one magnetic element 19, in particular a single cylindrical permanent magnet, in particular a large permanent magnet, or at least two permanent magnets of opposite polarities, for example two small permanent magnets. The magnetic element 19 preferably has a magnetization which can be parameterized according to the torque required to drive the centrifugal wheel 10 in a rotational movement.In the present embodiment, this magnetization is partial because the required torque is low due to the fact that the necessary hydraulic power is low and the mechanical power to be exerted on the drive shaft 14 is also low. It will be noted that if the chosen magnetization is total instead of partial, the magnetic volume of the magnets is limited accordingly.

[0031] This casing 12 of the pumping module 9a, 9b comprises a volute 20 and a casing 21. This casing 21 otherwise called " bell » or even « sealing bell", is defined to house the magnetic rotor 18 mounted on the drive shaft 14 and constitutes a part of the casing 12 having the general shape of a cylindrical tube. This casing 21 has characteristics linked to a low generation of eddy currents when exposed to a variable magnetic field. It is preferably of low thickness as described below. In addition, it has properties of resistance to internal pressure, in particular at pressures below 50 bars. In addition, this casing 21 is made of a metallic material having low electrical conductivity properties; it may, for example, be made of stainless steel or even a titanium alloy retaining low electrical conductivity over the entire range of working temperatures, including at 5K. It will be noted that this metallic material has a non-laminated structure, incompatible with the required sealing.

[0032] The volute 20 comprises the inlet and outlet openings 15a, 15b of the enclosure 13 of this casing 12. This volute 20 defines a part of the enclosure 13 of the casing 12 in which the centrifugal wheel 10 located on the first end 17a of the drive shaft 14 is arranged. As regards the casing 21, it also defines a part of the enclosure 13 of this casing 12 in which the magnetic rotor 18 mounted on the drive shaft 14 is located.

[0033] The pumping module 9a, 9b also includes a bearing device 22 also called “ pivoting» supporting the drive shaft 14 of the centrifugal wheel 10 and which is located in the casing 12 partly in the volute 20 and in the casing 21. This bearing device 22 is in particular defined to absorb the forces resulting from the rotation of this drive shaft 14 provided with the centrifugal wheel 10. This bearing device 22 comprises two bearings 23a, 23b or groups of bearings subsequently called first and second bearings 23a, 23b. These bearings 23a, 23b which are precision bearings, can be of any known nature suitable for operating at cryogenic temperatures in particular below 5K and supporting an axial load and a radial load, they can be for example cold bearings also called cryogenic bearings. These bearings 23a, 23b make it possible to support, guide and center the drive shaft 14 in the casing 12 of the pumping module 9a, 9b.

[0034] In the circulator 2, the casing 12 and in particular the housing 21 delimits the enclosure 13 of this circulator 2. The drive module 8a, 8b of this circulator 2 is arranged in this part.

[0035] In reference to the Figures 1, 2A And 2B , this circulator 2 can be produced according to two variants in which the drive and pumping modules 8a, 8b, 9a, 9b are different. Only the variant shown in Figure 2B is part of the invention.

[0036] In the first variant, the drive and pump modules 8a, 9a are visible on the Figure 2A, the casing 12 may result from a sealed assembly of the volute 20 with the casing 21 or may be a single-piece part. The drive shaft 14 of the pumping module 9a arranged in this casing 12 extends from the top 24 of this casing 21 towards the inlet opening 15a while being supported by the bearing device 22 comprising the first and second bearings 23a, 23b. In this variant, these first and second bearings 23a, 23b are preferably cold bearings. On this drive shaft 14, the first bearing 23a is mounted on the second end 17b of the drive shaft 14. On this drive shaft 14, these first and second bearings 23a, 23b are arranged in such a way that a gap is defined between them for the arrangement of the magnetic rotor 18 on the drive shaft 14. In other words, the magnetic rotor 18 is in this variant, arranged between these two bearings 23a, 23b.In this configuration, it will be noted that the magnetic rotor 18 and all or part of the bearing device 22 are arranged in the casing 21 of the envelope 12.

[0037] In this variant, the drive module 8a comprises an electric motor 25 connected to a source of electrical energy via the electric cables 11. This motor 25 can be chosen from electric motors of the state of the art such as alternating current or direct current brushed motors or even brushless direct current motors also called " brushless motor » or engine « step by step». This motor 25 comprises a motor shaft 26 provided with a free end comprising a support element 27 for at least two magnetic elements 28 of opposite polarities, in particular at least two permanent magnets. This motor shaft 26 comprises a central axis a5 which coincides with the axis a3 of the drive shaft 14. This support element 27 is defined to surround the circumference of all or part of the casing 21. This support element 27 may be circular or tubular in shape; it may be, for example, a cap, a dome, a blind tube or even a ring. Each magnetic element 28 is arranged on a lateral portion 29 of this support element 27 opposite a peripheral wall of the casing 21, so that central axes a7, a8 of each magnetic element 28 and of the magnetic rotor 18 coincide.In other words, in such an arrangement, each magnetic element 28 and the magnetic rotor 18 are aligned with each other along these central axes a7, a8. In this context, it will be noted that the casing 21 has a thin thickness of between 300 and 600 µm, and which is preferably 300 µm, so that the generation of eddy currents is minimized. The air gap present between the magnetic rotor 18 and each magnetic element 28 of the support element 27 must be sufficiently small, for example of the order of a few millimeters at most, in order to ensure the magnetic coupling 51 between the drive and pumping modules 8a, 9a.

[0038] Thus in this configuration, the drive and pumping modules 8a, 9a are magnetically coupled 51 to each other by means of the support element 27 provided with at least two magnetic elements 28 of opposite polarities connected to the motor 25 by the motor shaft 26, and the magnetic rotor 18 mounted on the drive shaft 14 of the centrifugal wheel 10. Indeed, each magnetic element 28 of the support element 27, being set into rotation by the motor 25 of the drive module 8a, causes the centrifugal wheel 10 to rotate by being magnetically coupled 51 to the magnetic rotor 18.

[0039] In the second variant, which is part of the invention, the drive and pumping modules 8b, 9b of which are visible on the Figure 2B, the casing 12 is formed by the assembly of the volute 20 and the casing 21. The casing 21 of the magnetic rotor 18 and the volute 20 are assembled with each other so as to define the enclosure 13 of the casing 12. To do this, the volute 20 and the casing 21 each comprise a connection zone with the corresponding fixing flange 30 defined in the cryostat. The connection zone is provided with a groove 31, 32 in which a sealing element 33, 34 of the O-ring type is located so that the sealing element 33, 34 is located between this connection zone and the flange 30. It will be noted that this O-ring can belong to the HELICOFLEX ™ family of seals making it possible to ensure sealing at cryogenic temperatures.

[0040] The connecting area of the volute 20 is mechanically connected to the flange 30 by connecting elements 35 such as screws. The sealing bell 21 made of titanium alloy is welded to the flange 30 or cut from the mass with it.

[0041] The drive shaft 14 of the pumping module 9b arranged in this casing 12, extends from the top 24 of the casing 21 towards the inlet opening 15a while being supported by the bearing device 22 comprising the first and second bearings 23a, 23b. This bearing device 22 is provided with a support component 36 in which these first and second bearings 23a, 23b are located and a preload spring 37 which keeps them axially spaced from each other in this component 36. This support component 36 is mounted partly in the casing 21 so as to be held in a fixed position. This support component 36 is located on the drive shaft 14 between the magnetic rotor 18 and the centrifugal wheel 10. This centrifugal wheel 10 and this magnetic rotor 18 are in this variant respectively mounted on the first and second ends 17a, 17b of the drive shaft 14.In this configuration, it will be noted that the magnetic rotor 18 and all or part of the bearing device 22 are arranged in the casing 21.

[0042] In this variant, the drive module 8b comprises a magnetic stator 38 which may comprise at least two electromagnets 39 making it possible to create a magnetic field rotating around the circumference of the casing 21 so as to cooperate with the magnetic rotor 18. The magnetic stator 38 is arranged at the peripheral wall of this casing 21, so that central axes a7, a9 of this magnetic stator 38 and of the magnetic rotor 18 are coincident. In other words, in such an arrangement, the stator and the magnetic rotor 38, 18 are aligned with each other along these central axes a7, a9. In this context, it will be noted that the casing 21 has a thin thickness of between 300 and 600 µm, and which is preferably 300 µm, so that the generation of eddy currents is minimized.The air gap between the magnetic rotor 18 and the magnetic stator 38 must be sufficiently small, for example of the order of a few millimeters at most, in order to ensure sufficient magnetic coupling.

[0043] This magnetic stator 38 is connected to an electrical energy source via the electrical cables 11. It will be noted that the control of the electrical power supply making it possible to vary the rotary movement of the centrifugal wheel 10 can be carried out with or without a rotor position sensor. In particular, sensorless technologies (" sensorless ") known from the state of the art and which are based on the measurement of electrical parameters and / or parameters varying according to the position of the rotor can be implemented in the present invention. It will be noted that the Hall effect probes conventionally used as position sensors in Brushless motors do not operate at low temperatures.

[0044] In this configuration, it will be noted that in this variant of the circulator 2, the magnetic stator 38 and the magnetic rotor 18 mounted on the drive shaft 14, although separated by the sealing bell 21, together form an electric motor, in particular a motor having operating characteristics which are similar to those of a rotating motor, in particular a synchronous motor of the “brushless” motor type.

[0045] Thus, in this second variant, the drive and pumping modules 8b, 9b are magnetically coupled to each other via the magnetic stator 38 which is capable of generating a rotating field in the direction of the magnetic rotor 18 mounted on the drive shaft 14 in order to cause the centrifugal wheel 10 to rotate.

[0046] In the second variant, the motor rotor is submerged in the fluid. This is made possible by interposing the casing 21 (or sealed bell) between the rotor and the stator. This simplifies the magnetic coupling. Everything happens as if one were replacing an external bell of the magnetic coupler and its rotation motor with a single brushless motor stator.

[0047] Preferably, in the second variant, the casing 21 (or sealed bell) is optimized. In particular, its thickness is reduced as much as possible to withstand internal pressure. The casing is, for example, made of metal, in particular stainless steel or titanium. The material of the casing is chosen to be as resistive as possible over the temperature range, in order to combat the development of eddy currents that generate thermal load. The sealed casing 21 between the rotor and the stator is, for example, made of a material with an electrical resistivity greater than 0.5 µΩ.m -1< at the working temperature, i.e. at a temperature below 30K, or even below 25K. An electrically insulating material will be advantageously used, provided that it provides the required sealing at cryogenic temperature (typically below 30K, or even below 25K), the circulator being surrounded by a secondary vacuum at 10 -6< mbar.

[0048] The sealed casing 21 between the rotor and the stator preferably has a diameter as small as possible (to limit eddy currents), ideally less than or equal to 12 mm.

[0049] The sealed casing 21 between the rotor and the stator has a thickness as small as possible (to limit eddy currents), for example less than or equal to 0.5 mm.

[0050] Preferably, in the second variant, the stator operates under the same vacuum as previously mentioned, namely of the order of 10 -6< mbar. The thermal power of the stator (due to the Joule effect and various magnetic losses) is therefore, for example, evacuated by contact of the stator with the casing 21, itself in thermal contact with the cryogenic fluid. For this, the stator is advantageously mounted just sliding on the casing. Thus, the stator comprises a bore in which the casing 21 is housed. For example, a clearance of less than or equal to 0.05 mm may be provided between the casing and the stator, in particular between the casing and the bore of the stator. This reduced clearance ensures good heat transfer between the rotor and the casing 21. The bore of the stator may be produced by molding a synthetic material, for example an epoxy resin, in particular Stycast ®< .The synthetic material can be loaded with a high thermal conductivity constituent to improve its thermal conductivity.

[0051] Preferably, in the second variant, the rotational speed of the rotor is deliberately greatly reduced to reduce the eddy currents developed in the casing 21. For example, the rotational speed is as low as possible (to limit the eddy currents). In particular, the rotational speed is ideally less than 50 Hz. However, it remains compatible with the production of a sufficient mass flow rate of cryogenic fluid.

[0052] Preferably, in the second variant, the stator / rotor air gap is deliberately increased and / or the magnetization of the magnets is deliberately reduced to combat eddy currents. The design of the motor here is unusual because the aim is not to obtain the most efficient or effective motor possible for a given quantity of electrical energy. The aim here is to obtain a motor that disturbs the heat transfer loop as little as possible, i.e., that heats the cryogenic fluid as little as possible. For example, the rotor-stator air gap is larger than strictly necessary to accommodate the sealed casing in order to reduce the value of the magnetic field at the casing. This makes it possible to limit eddy currents. For example, the rotor-stator air gap is greater than 3 mm.

[0053] Preferably, in the second variant, for the same purpose, the rotor diameter is limited (to limit eddy currents). For example, the rotor diameter is less than or equal to 6 mm. The limitation of the magnetic volume must remain compatible with the production of sufficient mechanical torque for starting the centrifugal wheel, residual friction in the bearing, as well as the hydraulic resistive torque. Advantageously, the rotor comprises a dipole magnet of small diameter, for example less than 6 mm. The presence of only two poles, relatively distant (for example between 2 and 3 mm) from the sealed casing makes it possible to limit (at a given rotational speed) the production of eddy currents by minimizing the variations in the magnetic field seen by the casing.

[0054] Preferably, in the second variant, the motor is of the "brushless" type with a permanent magnet rotor. Operation at cryogenic temperatures makes motor control based on Hall effect sensors inoperative. Motor control is therefore preferably carried out "in open loop".

[0055] In the various variants, the production of the centrifugal wheel guide bearing is a critical element. In fact, low temperatures prohibit any conventional lubrication. Thus, the bearing is preferably produced by dry bearings mounted in polychlorotrifluoroethylene rings, in particular Kel-F ®< or Neoflon ®< , or Vespel ®< , to maintain a little flexibility with respect to the differential contraction of the various materials when brought to very low temperatures (typically less than 30K).

[0056] Preferably, in the second variant, the pivot assembly is made up of two "cryogenic" type bearings. The clearances in these bearings at room temperature (300K) are significant and are reduced by contraction of the components to achieve an optimal value for good operation at the cryogenic working temperature (typically less than or equal to 30K). The optimal operating clearance minimizes the resistance torque to rotation of the centrifugal wheel and maximizes the service life. These bearings are chosen to be as small as possible, ideally with a diameter of less than 7 mm, in order to limit the friction torque and the engine torque requirement. The bearings are advantageously of the deep groove type and O-shaped mounting. Of course, these bearings could also be X-shaped mounting.

[0057] Preferably, in the second embodiment, the spring 37 exerts an axial preload on the two bearings, in particular an axial preload of 1 N. The stiffness of the spring is chosen so that when brought to a very low temperature (typically less than 30K), the value of this preload does not vary by more than 10% as a result of dimensional variations.

[0058] Preferably, in the second embodiment, the mounting of the rotor is of the "cantilever" type, that is to say that the rotor is mounted cantilevered relative to the two bearings.

[0059] Preferably, in the second embodiment, unlike certain industrial circulators whose motor remains at ambient temperature when the impeller and the volute are at cryogenic temperature, the circulator constitutes an isothermal or substantially isothermal assembly operating at the cryogenic working temperature (typically less than or equal to 30K). The circulator is advantageously installed at the same level, i.e. at the same altitude, as the cold source, in particular at the 5' cold end of the cryogenic machine of the "pulsed gas tube" type. Furthermore, the circulator is advantageously thermally coupled to the cold source. This coupling can be achieved by a copper bar. The circulator is thus permanently maintained at the lowest temperature and the impeller thus always drives the coldest and densest fluid. This avoids loss of priming of the circulator and / or the formation of gas "plugs" at the low points of the circuits.

[0060] In the circulator 2, it will be noted that in the first variant which is not part of the invention, the magnetic coupling 51 comprises said at least two magnetic elements 28 of opposite polarities arranged in the support element 27 and the rotor 18 mounted on the drive shaft 14. Concerning the second variant which is part of the invention, this magnetic coupling 51 comprises the magnetic stator 38 arranged at the peripheral wall of this casing 21 and the rotor 18.

[0061] Furthermore, in these two variants of the circulator 2, the centrifugal wheel 10, by thus performing a rotary movement, causes the refrigerant fluid 4 to circulate in the circuit 6 of the cryogenic installation 1, the fluid 4 is then sucked in through the inlet opening 15a to circulate in the enclosure 13 of the casing 12 and to be evacuated from the latter through the outlet opening 15b towards the cryogenic machine 5. When the refrigerant fluid 4 circulates in the enclosure 13, the drive shaft 14, the magnetic rotor 18 and the bearing device 22 with its bearings 23a, 23b are then immersed in this fluid 4.

[0062] Thus the invention contributes to improving the operation of the cryogenic installation 1 at cryogenic temperatures below 120K, in particular by reducing the thermal losses resulting from the circulator 2. Furthermore, in this invention, the circulator 2 can be configured to operate in a cryogenic installation 1 in which the required hydraulic power and the mechanical power to be exerted on the drive shaft 14 are low.

Claims

1. Circulator (2) for circulating a refrigerant fluid (4) at a temperature equal or less than 30K in a circuit (6) of a cryogenic installation (1) from and to an element (3) having a thermal load, the circulator (2) including a magnetic rotor (18) and a drive module (8a, 8b) and a pumping module (9a, 9b) including a centrifugal wheel (10), said drive module (8a, 8b) having a magnetic coupling (51) to said pumping module (9a, 9b) in order to drive said centrifugal wheel (10) in a rotary movement, the pumping module (9a, 9b) comprising an envelope (12) defining a sealed enclosure (13) containing a drive shaft (14) of the centrifugal wheel (10), the envelope (12) comprising a casing (21) of the magnetic rotor (18) and a volute casing (20), characterized in that the drive module (8b) comprises a magnetic stator (38), in particular a magnetic stator including at least two electromagnets (39), arranged around a circumference of all or part of the casing (21) of the magnetic rotor (18) of the envelope (12) of the pumping module (9b) in such a manner as to cooperate with said magnetic rotor (18).

2. Circulator (2) according to the preceding claim, characterized in that the pumping module (9a, 9b) comprises the drive shaft (14) including the magnetic rotor (18) and said centrifugal wheel (10) at a first end (17a).

3. Circulator (2) according to either one of the preceding claims, characterized in that the pumping module (9a, 9b) comprises a bearing assembly (22) supporting the drive shaft (14) of the centrifugal wheel (10) and comprising first and second bearings having rolling bodies (23a, 23b), in particular cryogenic bearings having rolling bodies.

4. Circulator (2) according to Claim 3, characterized in that the magnetic rotor (18) is arranged outside the two bearings having rolling bodies (23a, 23b) of the bearing assembly (22) and mounted on the drive shaft (14) of the centrifugal wheel (10).

5. Circulator (2) according to any of the preceding claims, characterized in that the magnetic stator (38) and the magnetic rotor (18) mounted on the drive shaft (14) together form an electric motor, in particular a motor having operating characteristics substantially similar to those of a rotary motor.

6. Circulator (2) according to any of the preceding claims, characterized in that the circulator is a minicirculator having a length comprised between 70 mm and 100 mm and the circulator is adapted for delivering an hydraulic power comprised between 1 and 100 mW.

7. Cryogenic installation (1) for cooling an element (3) having a thermal load comprising a circulator (2) according to any one of the preceding claims for circulating a refrigerant fluid (4) in a circuit (6) of that cryogenic installation (1) from and to said element (3).

8. Cryogenic installation (1) according to the preceding claim, characterized in that it comprises at least one cryogenic machine (5) of pulsed gas tube type and a compression and control unit (7).

9. Cryogenic installation (1) according to either one of Claims 7 and 8, characterized in that it comprises a cryostat including an enclosure in which an insulating vacuum is maintained.

10. Cryogenic installation (1) according to the preceding claim, characterized in that the cryostat comprises the circulator (2), the cryogenic machine (5) and the circuit (6).

11. Cryogenic installation (1) according to any one of Claims 7 to 10, characterized in that it comprises a cold source (5'), in particular a cold end (5') of a cryogenic machine of "pulsed gas tube" type, the circulator being mounted at the same level or at the same altitude as the cold source, in particular the centrifugal wheel being mounted at the same level or at the same altitude as the cold end (5') of the cryogenic machine of "pulsed gas tube" type.