FR2754593A1 - Cryogenic refrigeration by Joule-Thomson expansion - Google Patents

Abstract

The cryogenic cooling unit has set a single stage compressor (2) and its drive motor (3) integrated in a single assembly with a cold probe extending from the side. The probe (1) terminates in an expansion orifice (10). A low pressure buffer chamber in the compressor is in permanent communication with the gas circuit returning from the expansion orifice (10). A high pressure buffer chamber is in permanent communication with the gas supply circuit. The compressor has two valves ( 31, 32) which, during the displacement of the compressor piston (16), are alternately open while the other is closed to sequentially draw in the gas and transfer it from the low pressure chamber to the high pressure chamber.

Description

COOLING METHOD AND DEVICE
CRYOGENIC OF COMPONENTS BY RELAXATION
FROM JOULE-THOMSON
The present invention relates to techniques which make it possible to cool components at cryogenic temperature, such as electronic components, or in particular optoelectronic components, by exploiting the expansion of a compressed gas, in a working gas circuit located in the immediate vicinity of the component. to cool.

With regard to the prior art to the present invention, reference will usefully be made to Damien's article
Feger entitled "Cooling of optoelectronic detectors" published in "Engineering techniques, treated
Electronics "pages E4070-1 to 11. There are described in their principles various technological configurations of coolers in connection with the thermodynamic work cycle that they implement.

 Among these configurations, the invention is particularly interested in those which are based on an isenthalpic expansion of Joule-Thomson that the compressed gas undergoes at the outlet of a heat exchanger in which the compressed gas and the expanded gas circulate against the current , both at constant pressure, to the nearest pressure drop. This exchanger is advantageously constituted, as described by the article already cited, by a fin tube wound around a mandrel. Ended by a nozzle defining the expansion orifice, it constitutes what is commonly called the cold finger of the cooler, as opposed to the hot zone on the side of the compressor. I1 is intended to be introduced inside a cryostatic envelope at the bottom of which is the cold zone, in heat exchange contact with the component to be cooled.

 To date, Joule-Thomson cryogenic expansion cooling systems have only been able to find a limited number of applications, despite the great refrigeration capacity that they are capable of developing when they use a high pressure liquefied gas. , such as nitrogen, air or argon. Indeed, the expansion orifice is very small (diameter of a few tens of microns), to the point of causing draconian requirements in the purity of the working gas to avoid the risks of clogging.

Also, it is only possible to operate in a closed circuit by passing the expanded gas through a series of compression stages separated by filters, which results in a construction which is too complex to be able to be suitable for the numerous situations of industrial practice which require safety. , miniaturization and longevity simultaneously.

To resolve these difficulties while enjoying the advantages of the Joule detent cycle
Thomson compared to Stirling cycle or pulsed gas tube coolers, the present invention provides a method and a device for cooling components which are more especially intended for components which, in order to function properly, must be maintained at a temperature within range from 120 K to 200 K. It then allows a miniaturized production ensuring high efficiency, in particular in the absorption of thermal loads of the order of watt for an ambient temperature of 20 C. It advantageously takes the form of a compact monobloc assembly , integrating the pressure generating means with the cold finger. As regards its applications, it is particularly well suited to the case of infrared or CMOS components whose operation is improved at low temperature.

More specifically, the subject of the invention is a device for cryogenic cooling of components by Joule-Thomson expansion, of the type with a closed circuit of thermodynamic working gas, comprising a pressure generator continuously supplying an expansion orifice located in the immediate vicinity of the component to be cooled and an intermediate heat exchanger between the upstream circuit of compressed gas coming from the pressure generator and the downstream circuit of expanded gas in return from said orifice,
characterized
- in that said pressure generator comprises a single-stage compression compressor by a piston (16) movable in a fixed jacket (17) under the control of an associated motor (3), and is integrated into a monobloc assembly with a cold finger (1) terminated by said expansion orifice,
- in that in said compressor (2) there is provided on the one hand a low pressure buffer volume in permanent communication with the downstream gas circuit in return for said expansion orifice, on the other hand a high pressure buffer volume in permanent communication with the upstream gas circuit supplying said orifice continuously,
- And in that said compressor is equipped with two valves (31, 32) which, during the movements of said piston (16), are alternately one operative in the open position while the other is closed, for sequentially sucking and delivering gas from said low pressure buffer volume to said high pressure buffer volume.

 According to a preferred embodiment of the invention, the intermediate exchanger between the high pressure and low pressure circuits is mounted in a cryostatic envelope which is also integral with the assembly and at the bottom of which is the component to be cooled. On the other hand, but there in a manner in itself conventional, said exchanger is preferably constituted by a fin tube surrounded around a cylindrical mandrel mounted in the axis of the cryostatic envelope.

According to other characteristics of the invention, advantageously used in their operating combinations
- The low pressure volume is provided in an internal space of a casing enclosing the compressor and at least partially in said piston, the latter being hollow, while the high pressure volume is advantageously provided, also at the compressor , in a cylinder head fixedly mounted on the same casing
- Each of the compressor valves has a flexible diaphragm valve that is applied in the closed position on an annular seat
- These valves are preferably arranged in series, one on an end face of the piston, the other on a fixed plate internal to the compressor
- The cold finger has a nozzle with a conical annular geometry to guide the high pressure gas leaving a fin tube of the intermediate exchanger, wound around an axial cylindrical tube of the cold finger, up to the expansion orifice located at the end of it
- This nozzle is preferably made by a male part closing the end of a cylindrical mandrel mounted in the axis of the cold finger and a female part formed by a nozzle secured to a tube which limits an annular space for gas circulation low pressure around the heat exchange tube.

 The invention also results in a process for cooling components which essentially consists in implementing a thermodynamic isenthalpic expansion cycle of Joule-Thomson in a device as defined above, previously filled with an appropriate working gas. The latter is therefore subjected in a closed circuit on the one hand to a compression step towards a buffer volume of high pressure gas, on the other hand to an expansion step carried out at the end of a cold finger in the immediate vicinity of the component to be cool, passing through an intermediate stage of heat exchange between the compressed gas and the expanded gas, which is received in a buffer volume at low pressure.

 The working gas advantageously consists of a mixture of nitrogen, or preferably methane, with lower saturated hydrocarbons, such as methane, ethane, propane, in a proportion of between 15% and 70% by weight by weight total of the mixture.

 The compression step, which takes place in a single stage as already indicated with respect to the device of the invention, is advantageously carried out, by the dimensioning of the downstream and upstream circuits, mainly in their buffer volumes provided in the compressor, under a compression ratio of between 2 and 7, preferably between 3 and 5, up to a high pressure between 25 bars and 40 bars, preferably of the order of 30 bars, as determined by the quantity of gas introduced into the device before it is sealed.

 Such conditions imply the existence of a back pressure in the cold zone where the expansion orifice opens, and therefore an increase in the boiling temperature of the mixture compared to that which it would theoretically present under normal conditions. pressure (about 1 bar). However, they have proved to be advantageous for obtaining temperatures in the aforementioned range with good efficiency, both during the period of cooling the component to be cooled and during its maintenance at subsequent cold temperature.

 In addition, for cooling powers of the order of 500 mW to 1 watt (more generally of the order of 50 mW to 2 watts), which are more than sufficient for a number of applications, the method according to the invention can be implemented by using devices as defined above, which remain simple, inexpensive and compact, and which nevertheless allow long-term operating safety without fear of static leakage or contamination of the gas of job.

 In accordance with the invention, therefore, use is made of suitable gas compositions, which are subjected to a Joule-Thomson thermodynamic expansion work cycle under conditions of lower high pressure and higher cold temperature than any what had been planned so far. This is how we were able to produce a compact cooling device, integrating in a single miniature assembly a pressure generator with single-stage compressor and a cold finger that does not need to regulate the flow rate of the orifice. expansion to ensure with good efficiency the very different cold production capacities which are necessary on the one hand during the cooling, on the other hand during the simple maintenance at cold temperature of the component.

 In this regard, it is advantageous, in accordance with the invention, to equip the device with means for regulating the rate of movement of the piston of the compressor, by acting on the drive speed by its pilot motor as a function of the temperature. of the component, detected locally by appropriate means. The flow of circulating gas is thus controlled at the nominal temperature desired for proper operation of the component. At its simplest, the control law is proportional.

The invention will now be more fully described in the context of preferred characteristics and their advantages, with reference to the figures of the accompanying drawings which illustrate them and in which
- Figure 1 shows a general cross-sectional view of the cooling device according to the invention;
- Figure 2 illustrates a detail of this device concerning the expansion nozzle provided at the end of the cold finger
- Figure 3 shows the device of Figure 1 in a variant with linear control rather than rotary control.

 The device illustrated in the figures serves to cool an electronic component 5, such as an infrared sensor or a CMOS component, to cryogenic temperature.

Built in dimensions adapted to the requirements of miniaturization, it is presented as a monobloc assembly, with autonomous functioning, which integrates the cooler proper, or cold finger 1, with a pressure generator comprising a compressor 2 and its piloting motor 3.

 The casings which enclose their functional organs are fixed together and closed in a sealed manner, in particular by welding, wherever it is necessary to contain the working gas. The various fixed parts internal to the device are also assembled, preferably by laser welding or by gluing.

 In a conventional manner in itself, the cold finger 1 is introduced into a cryostatic envelope constituted by a double-wall vacuum cryostat 4, at the bottom of which is mounted the component 5 to be cooled. In the particular case illustrated, this cryostat is shown as sharing its internal wall with the external wall 42 limiting the cold finger proper, but it could also be two independent walls, having complementary diameters so that the cold finger s '' introduce into the cryostat with no more clearance than that necessary for its installation.

 As explained above, the device according to the invention is designed to provide the refrigerating power necessary for the cooling of component 5 by the use of a thermodynamic isenthalpic expansion cycle of Joule-Thomson, applied to a working gas. circulating in a closed circuit between the compressor 2 and the expansion orifice 10 of a nozzle 6, which terminates the cold finger 1 in the cold zone, located at the bottom of the cryostat 1, in the immediate vicinity of the component 5.

 Between the two, both on the outward and return journey, the working gas passes through a heat exchanger 7, which extends along the cold finger and which provides a heat exchange between the high pressure compressed gas supplying the expansion port 10 and the expanded gas at low pressure returning to the compressor 2. In practice, this exchanger consists of a fin tube 11 (Fig. 2). The high pressure gas flows inside the tube, while the low pressure gas flows outside, around the fin, in a closed annular space 12 containing the exchange tube 11.

 In order to best ensure the conditions of a Joule-Thomson thermodynamic expansion cycle, it is sought on the one hand that the expansion orifice 10 situated at the end of the cold finger 1 is supplied by a continuous flow of gas at high pressure, on the other hand that in the intermediate exchanger 7, the pressure remains substantially constant both on the high pressure side and on the low pressure side.

The illustrated design meets these requirements, even though, unlike known coolers of this type, there is no large reserve of permanent liquefied gas (the dimensions of which would be prohibitive for long-term autonomy) and the compressor is at a single stage of compression.

 Among the characteristics recommended for this purpose, it can be seen from FIG. 1 that the production of the pressure generator is partly inspired by that of a pressure oscillator for a Stirling cycle refrigerator. But, essentially, it differs from it by the fact that the compressor 2 is equipped with two valves, which open alternately, one in suction, the other in discharge, to sequentially pass the gas from one low pressure buffer volume 8 to a high pressure gas buffer volume 27.

 These two buffer volumes are located inside the casing 13 of the compressor, which is pierced with channels 14 and 15 for communication with the cold finger 1, respectively on the side of the high pressure circuit supplying the cold finger and on the side of the downstream circuit to low pressure. The compression of the gas is obtained by periodic displacement of a piston 16, movable in a jacket 17 with cylindrical internal section, fixedly mounted in the casing 13.

 In the embodiment of Figure 1, the piston 16, movable vertically in the arrangement shown, is driven by rotary type control. The motor 3 is an electric motor, the electric winding of the stator 18 and the rotor 19 having been shown in particular. The shaft 20, integral with this rotor, is mounted on bearings between the bottom of the casing 23 enclosing the clean elements of the motor and a flange 24 sealed against the casing 13 of the compressor. The internal space of the engine is thus isolated from that of compressor 2.

 Beyond the flange 24, the shaft 20 carries an eccentric 21 whose eccentric part is mounted by means of a bearing in the lower end of a rod rod 22, which controls the piston 16. At its other end, the rod 22 is in practice connected, also by bearing, to a transverse axis 25 integral with the piston 16. Indeed, the latter is hollow, in order to provide an internal space participating in the definition of the volume at low pressure 8 and of thus reducing the overall size.

 On the other hand, in accordance with Figure 1, the invention provides, according to its preferred embodiments, that the two suction and discharge valves are arranged in series rather than in parallel. Both consist of valves with deformable diaphragm, as is known to be produced by means of annular sectors suitable for joining in a sealed manner in the closed position bearing on a cooperating seat and for deviating when they are pushed out of said seat.

 A first valve 31 thus formed is fixed by screws in its center on the outer face (upwards in FIG. 1) of a plate 26, mounted across the casing 13 and sealed thereon, facing the orifices gas passage through said plate. When the piston rises, it opens in a chamber 27 internal to the cylinder head 28 of the compressor. It closes when the piston descends, under the effect of the pressure difference between the two faces of the plate 26.

 The second valve 32, active in opening alternately with the previous one, is fixed in a similar manner, but on the piston 16 itself. It automatically closes associated gas passages which pass through the end transverse wall 29 of the piston when the latter rises. It opens when the piston descends, the first valve 31 being closed. It then places the internal space 8 of the piston in communication with an intermediate compression chamber delimited by the two valves 31 and 32.

 In each case, it can be seen from the figure that the gas passage channels, annularly distributed, emerge in an annular recess in the plate 26, or respectively in the wall 29, which is hollowed out between the central point of attachment of the deformable membrane and the perimeter forming its proper seat.

 The buffer volume 8 of the low pressure circuit, partly formed inside the hollow piston 16, extends into the internal space of the compressor housing 13.

Compared to this solution, a membrane device would have the advantage of isolating the low-pressure volume from any contamination by the mechanical movement transmission members. But the solution adopted is superior by creating a sufficient volume of tranquilization of the gas for a minimum overall size.

In addition, it solves any problem of recovering leaks between piston and liner during compression.

 The channel of the low pressure downstream gas circuit 15 opens directly into the internal space of the compressor, which it puts into communication with a distribution chamber 33, formed at the foot of the cold finger 4, at the outlet of the annular space 12 .

 On the upstream side at high pressure, the channel 14, drilled through the compressor casing 13, places the chamber 27 in communication with a chamber 34 which, inside the compressor casing 13, completes it to form the buffer volume at high pressure. From there, the working gas is forced into the fin tube 11 of the exchanger 7.

 At the end of the cold finger 1 is the nozzle 6 which guides the high pressure gas emanating from the heat exchange tube 11 towards the expansion orifice 10. The precise embodiment of the nozzle appears better in FIG. 2, where we see that it forms an annular space with conical geometry 35, formed between a male part 36 and a female part 37. The exchange tube 11 opens directly in this annular space.

 The female part 37, which has the orifice 6 at its center, is formed by a ruby tip which, after having been metallized on its external surface, is brazed inside a cylindrical tube 38 constituting the body of the finger cold around which the exchange tube 11 is wound.

The male part 36 is produced by the closed end of an axial needle or mandrel 39, which is in the form of a hollow tube made of a material with low thermal conductivity substantially equivalent to that of the tube 38.

 Although this does not appear in the figures, the male part of the nozzle is threaded on the surface. This further improves safety against the risks of gas stagnation and the formation of polluting deposits.

 At the rear of the nozzle 6, the hollow tubular needle 39 extends over the entire length of the body tube 38, in which it is mounted to slide without play. The sliding capacity is eliminated in the device once constructed, by welding or brazing the two tubular elements one on the other. It is used beforehand to adjust the flow rate of gas which is released through the expansion orifice 10 in operation.

 We see in Figure 1 that there is provided for this purpose, a tip 40, which allows to move the mandrel 39 in the tube 38 by a suitable tool of translation, combined with a temporary pneumatic supply device and means also provisional gas flow measurement. The available cooling capacity will depend on this setting, in conjunction with other parameters that can be set separately.

 By the female part 37 of the nozzle, the end of the cold finger 1 ends in the immediate vicinity of a bottom 41 closing the external tubular wall 42 which completes the body of the cooler with the internal tube 38, leaving space between them annular 12 in which the heat exchanger is housed, namely the fin tube 7. It is on the rear face of this bottom 41 that component 5 to be cooled is fixed, generally by gluing.

 Referring now to the alternative embodiment illustrated in FIG. 3, we will not recall any more the elements which remain unchanged compared to FIGS. 1 and 2, in particular with regard to the cold finger 1 and the control motor 3. The compressor 2 is also designed as above in its functional relationships with the cold finger.

 The differences are mainly observed in the structure of the piston of the compressor, which here bears the reference 51, and in the mechanical means of transmission of movement driving the piston from the motor shaft 20.

 The piloting mode remains of the rotary type, but the crankshaft effect actuates a movement transmission mechanism by movable ball bearing in a groove formed transversely outside the piston.

 More specifically, the offset crankshaft axis of the eccentric 52 rises at the end of the shaft 20 is integral with the internal cage 53 of a ball bearing whose external cage 55 is trapped in a rectilinear groove 56 hollowed out through the piston 51 perpendicular to the plane of the figure. This arrangement ensures the reciprocating movement of the piston while immobilizing it in rotation. Of course, the piston 51 is configured in the same place to facilitate the circulation of gas in return from the cold finger to the buffer volume 57 formed inside the piston itself, and thus avoid detrimental pressure drops.

 Thanks to the provisions adopted, in particular by the capacity of the buffer volumes which act as reservoirs, it is ensured that the gas in circulation has an upstream pressure and a downstream pressure (relative to the expansion orifice) which remain practically constant in operation . This contributes to enabling the cooling device which has just been described to implement a true Joule-Thomson cycle.

 Returning now to FIG. 1, there can be seen schematically illustrated the regulation means which advantageously replace the flow adjustment which was previously carried out by acting on a variable passage section of the expansion orifice. According to the invention it is on the periodic rhythm of the compression that one acts.

 To this end, there is associated with the device a regulation system 61, which acts on the supply of the motor 3 to vary the speed of rotation of the motor shaft 20, and therefore the rate of movement of the piston 16. The regulation is controlled by the temperature of component 5, detected by conventional means in themselves on this component. A comparison is made continuously against a fixed value corresponding to the nominal temperature desired for proper operation of the component.

 For a simply proportional control law, the high pressure applied to the upstream gas circuit supplying the detent orifice varies progressively during cooling, but then settles at a substantially constant value during permanent operation which maintains the component at the desired cryogenic temperature.

Claims (12)

 1. Device for cooling components by Joule-Thomson expansion, of the closed circuit type of thermodynamic working gas, comprising a pressure generator (2, 3) continuously supplying an expansion orifice (10) located in the immediate vicinity of the component (5) to be cooled and an intermediate heat exchanger (7) between the upstream compressed gas circuit from the pressure generator (3) and the downstream expanded gas circuit in return for said orifice,  characterized  - in that said pressure generator comprises a single-stage compression compressor by a piston (16) movable in a fixed jacket (17) under the control of an associated motor (3), and is integrated into a monobloc assembly with a cold finger (1) terminated by said expansion orifice,  - in that in said compressor (2) there is provided on the one hand a low pressure buffer volume (8) in permanent communication with the downstream gas circuit returning from said expansion orifice (10), on the other hand a volume high pressure buffer (27) in permanent communication with the upstream gas circuit supplying said orifice,  - And in that said compressor is equipped with two valves (31, 32) which, during the movements of said piston (16), are alternately one operative in the open position while the other is closed, for sequentially aspirating and discharging gas from said low pressure buffer volume (8) to said high pressure buffer volume (27).  2. Device according to claim 1, charac terized in that said low pressure volume (8) is formed in an internal space of a casing (13) enclosing the compressor (2) and at least partly in said piston (16 ), the latter being hollow.  3. Device according to claim 1 or 2, characterized in that said high pressure volume (27) is formed in a fixed cylinder head (28) of the compressor (2).  4. Device according to any one of claims 1 to 3, characterized in that each of said valves (31, 32) comprises a flexible membrane valve applying in the closed position on an annular seat.  5. Device according to any one of claims 1 to 4, characterized in that said valves (31, 32) are arranged in series, one on an end face (29) of said piston (16), the other on a fixed plate (26) of the compressor.  6. Device according to any one of claims 1 to 4, characterized in that the cold finger (1) comprises a nozzle (6) with conical annular geometry for guiding the high pressure gas leaving a finned tube (11) of said exchanger, wound around an axial cylindrical tube (38, 39), up to the expansion orifice (10).  7. Device according to claim 6, charac terized in that said nozzle comprises a male part closing the end of a cylindrical mandrel (39) mounted in the axis of the cold finger and a female part constituted by a nozzle (37) integral with a tube (38) of the cold finger which limits an annular space (12) for circulation of the low pressure gas around said fin tube (11).  8. A method of cooling components by isenthalpic Joule-Thomson expansion of a thermodynamic working gas used in a device according to any one of claims 1 to 8, said working gas being subjected thereto, repeatedly and in a closed circuit, on the one hand at a stage of compression towards a buffer volume of high pressure gas, on the other hand at an expansion stage carried out at the end of a cold finger in the immediate vicinity of the component to be cooled, passing by an intermediate stage of heat exchange between the compressed gas and the expanded gas, carried out along said cold finger inside a cryostatic envelope,  characterized in that said compression step takes place in a single stage, under a compression ratio of between 2 and 7 (preferably between 3 and 5), up to a high pressure of between 25 bars and 40 bars (preferably from '' order of 30 bars).  9. Method according to claim 8, characterized in that the working gas consists of a mixture of nitrogen or methane with lower saturated hydrocarbons, such as methane, ethane, propane, in a proportion between 15 % and 70% by weight of the total weight of the mixture.  10. Method according to claim 8 or 9, characterized in that, to ensure that the component is cooled before it is kept at cryogenic temperature, the compression rate is adjusted at a speed greater than that useful for generating the cooling power required by normal running.  11. Process according to claim 8, 9 or 10, characterized in that the flow of gas at high pressure supplying the expansion orifice is permanently regulated by acting on the speed of control of the movements of the piston (16) of the compressor depending on the temperature of the component (5).  12. Device according to any one of claims 1 to 7, characterized in that it comprises means (61) for regulating the piloting speed of said piston (16) by action on said motor (3), with slaving to the temperature of said component (5).