EP0258093B1 - Joule-thomson cooler and cryostat provided with this cooler - Google Patents

Description

The present invention relates to Joule-Thomson coolers and cryostats intended to very quickly obtain low temperatures between 80 and 130K. These coolers are used for example to cool infrared detectors. It relates more particularly to a Joule-Thomson cooler of the type described in the preamble of claim 1.

The most common technology in this field consists in using a double-walled Dewar as cryostat, the inner wall of which forms a cylindrical well on the bottom of which is fixed the element to be cooled. The cooler proper comprises a cylindrical core on which is wound helically a finned capillary tube provided at its end with expansion means, which can be constituted by a simple drilling or by a more complex flow control device.

Although these devices have been continuously improved, they are difficult and expensive to produce industrially, on the one hand because of the complex configuration of the Dewar, which has a well and a double wall, and on the other hand because of the high mechanical precision necessary to ensure a sealed introduction without distortion of the cooler into the cylindrical well of the cryostat.

GB-A-1 168 997 has proposed a cooler of the aforementioned type, which reduces these difficulties. However, it is more of a principle than an industrially feasible device.

The object of the present invention is to provide a technique making it possible to produce such devices industrially in a simple and economical manner and with improved performance.

To this end, the subject of the invention is a Joule-Thomson cooler of the aforementioned type, characterized by the content of the characterizing part of claim 1.

It should be noted that FR-A-2 477 406 describes the use of a porous mass around a smooth tube of a Joule-Thomson cooler.

Other characteristics of the cooler according to the invention are described in subclaims 2 to 7.

• un refroidisseur Joule-Thomson tel que défini ci-dessus ; et
• une enceinte sous vide entourant ledit emplacement central et reliée à joint étanche à l'enveloppe, laquelle débouche à l'extérieur de cette enceinte.

The invention also relates to a cryostat comprising:

• a Joule-Thomson cooler as defined above; and
• a vacuum enclosure surrounding said central location and connected to a tight seal with the envelope, which opens to the outside of this enclosure.

• la figure 1 représente en plan les pièces qui constituent un refroidisseur suivant l'invention ;
• la figure 2 est une demi-vue partielle en coupe axiale d'un cryostat comprenant le refroidisseur de la figure 1 ;
• la figure 3 est une vue en élévation, à plus petite échelle, de ce cryostat ;
• les figures 4, 6 et 7 illustrent en coupe axiale trois phases de la fabrication d'une variante du refroidisseur de la figure 1 ;
• la figure 5 est une vue prise suivant la flèche V de la figure 4 ; et
• la figure 8 est une vue en coupe axiale d'une variante du cryostat de la figure 2.

Some examples of implementation of the invention will now be described with reference to the accompanying drawings, in which:

• Figure 1 shows in plan the parts which constitute a cooler according to the invention;
• Figure 2 is a partial half-view in axial section of a cryostat comprising the cooler of Figure 1;
• Figure 3 is an elevational view, on a smaller scale, of this cryostat;
• Figures 4, 6 and 7 illustrate in axial section three phases of the manufacture of a variant of the cooler of Figure 1;
• Figure 5 is a view taken along arrow V of Figure 4; and
• FIG. 8 is a view in axial section of a variant of the cryostat of FIG. 2.

• un tube capillaire 2 en acier inoxydable enroulé en une spirale plane à spires jointives ; l'extrémité extérieure de ce tube est pourvue d'un filtre 3, tandis que son extrémité intérieure est obturée et présente un orifice de détente calibré 4 orienté axialement (figure 2) ;
• deux plaquettes planes 5 constituées d'une matière poreuse à porosités ouvertes qui peut être une mousse, un feutre ou un fritté thermiquement conducteur, en pratique métallique, et notamment une mousse de nickel compactée ayant une porosité volumique de 35 % ; chacune de ces plaquettes à la forme d'un disque dont le diamètre est nettement supérieur au diamètre extérieur de la spirale 2 et qui présente un trou central 6 ayant un diamètre du même ordre que la spire intérieure de cette spirale ;
• deux plaques planes 7 ayant une bonne résistance aux contraintes mécaniques et thermiques, notamment en acier inoxydable. Ces plaques 7 ont le même diamètre que les disques 5.

The cryostat represented in FIGS. 2 and 3 comprises a Joule-Thomson cooler 1 made up of the parts represented in FIG. 1, namely:

• a stainless steel capillary tube 2 wound in a flat spiral with contiguous turns; the outer end of this tube is provided with a filter 3, while its inner end is closed and has a calibrated expansion orifice 4 oriented axially (FIG. 2);
• two flat plates 5 made of a porous material with open porosities which can be a foam, a felt or a thermally conductive sinter, in metallic practice, and in particular a compacted nickel foam having a volume porosity of 35%; each of these plates in the form of a disc whose diameter is significantly greater than the outer diameter of the spiral 2 and which has a central hole 6 having a diameter of the same order as the inner turn of this spiral;
• two flat plates 7 having good stress resistance mechanical and thermal, in particular stainless steel. These plates 7 have the same diameter as the discs 5.

To make the cooler 1 from the above parts, they are superimposed in the order shown in Figure 1, namely a plate 7, a disc 5, the spiral 2, the other disc 5 and the other plate 7, this coaxially, and they are fixed to each other by gluing or stirring.

This cooler is then integrated in the general shape of a flat disc in a cryostat as shown in FIGS. 2 and 3. An element 8 to be cooled, for example an infrared detector, is fixed in the center of a two plates 7. On has coaxially on either side of the cooler 1 two tubular sections 9 each ending in an outer flange 10 provided with a circular groove 11, which receives an annular seal 12. The peripheral region of the cooler is pinched between the two flanges 10, the tightening being ensured by two annular flanges 13 which bear on the flanges 10 and which are crossed by bolts 14. This tightening causes on the one hand a crushing of the seals 12 on the plates 7, which allows d '' to obtain a vacuum seal, and on the other hand a slight approximation of the peripheral regions of the plates 7 and the discs 5, the latter, which are relatively flexible, coming into contact mutual tact in the middle plane of the cooler, although this valve in contact is not essential. The cooler 1 then defines, between the plates 7, a planar spiral 2 embedded in the porous mass formed by the two discs 5, this mass leaving a central chamber 15 free in which opens the expansion orifice 4, which is oriented towards element 8 to be cooled.

As can be seen in FIG. 3, the cryostat is completed by two tubular sections 16 respectively extending the two sections 9 and placed in communication by a side tube 17. One of the sections 16 is hermetically sealed, while the other can connect via a line 18 to a vacuum pump (not shown).

In operation, after having established a secondary vacuum (less than 10⁻³mb) in the enclosure constituted by the elements 9, 16 and 17, the external end of the tube 2 is connected to a source 19 of a gas under very high pressure, for example argon under 700 bars. Gas high pressure, purified by the filter 3, circulates in the spiral 2, is relaxed and therefore cooled as the orifice 4 passes, and enters the central chamber 15. From there, it circulates radially outward through the porous discs 5, and exits freely to the atmosphere through the periphery of the latter. The low pressure gas countercurrently cools the high pressure gas contained in the spiral 2, essentially by means of the discs 5, so that the temperature rapidly decreases in the chamber 15, until liquid forms therein. at the temperature that corresponds to the pressure there. This pressure is defined by the pressure drop of the low pressure circuit through the discs 5. It should be noted that the passage section offered to the low pressure gas increases as the expansion ends and the gas heats up. , which is very favorable with regard to the pressure drops of the low pressure circuit.

To improve the heat exchange between the low pressure and high pressure gases, it is preferable to fix the spiral 2 to the discs 5 by brazing. In addition, to reduce the radial heat input by conduction, it is preferable to fix the plates 7 to the discs 5 by gluing.

To reduce the cooling time, that is to say the time necessary for the appearance of liquid in the chamber 15, as well as the temperature of this liquid, it is necessary to reduce the heat capacity of the cooler as well as the losses load of the low pressure circuit, without reducing the efficiency of the heat exchange between the low pressure and high pressure gases nor favoring the heat input by conduction between the periphery of the cooler, which is at room temperature, and the zone central cooled.

This complex problem finds a solution in the embodiment, the manufacture of which is illustrated in FIGS. 4 to 7: two strips of metallic fabric, for example of bronze mesh with square mesh of 50 microns of wire diameter and 80 microns of opening, are each wound on a porous core 20, for example in sintered bronze, having a hollow end 21 which is flush with one side of the corresponding strip (FIG. 4). The face 22 of each winding situated on the side of the end 21 is rectified, then (FIG. 6) the two faces 22 are glued or brazed coaxially on the two faces of the spiral 2. The assembly thus obtained is then cut with a saw on either side of the spiral 2 and then rectified in turn to obtain an assembly consisting of the spiral and the two discs 5 with the desired thickness. Finally, the plates 7 are attached to the two faces of this assembly, a few shims being arranged between the peripheral zones of these two plates.

With this construction, each disc 5 consists of a narrow strip of metallic fabric wound in a spiral. It has good thermal conduction in this axial direction, which is that of the fabric, and poor thermal conduction in the radial direction, due to the large number of thermal resistances of contact in series existing between the successive layers of fabric.

There was thus produced a thermally anisotropic low pressure circuit, that is to say having in the axial direction, which is that of the desired heat exchange, a thermal conductivity much greater than its thermal conductivity in the radial direction, which is that of parasitic heat losses.

In fact, the cryostat of Figures 2 and 3 constitutes an experimental device. For a series production, the variant of FIG. 8 would preferably be adopted, which differs therefrom in the following points: the disks 5 and the plates 7 have substantially the same outside diameter as the spiral 2, and the plates 7 have a flange cylindrical device 7A directed opposite the spiral. A washer 23 is fixed by brazing, welding or gluing to each flange 7A, after putting under vacuum, so as to constitute on each side of the cooler a sealed vacuum chamber.

Furthermore, in this variant, as in that of FIGS. 2 and 3, the plates 7 can be made of glass or of a ceramic material and, in this case, directly integrate the element 8 to be cooled, which is generally an electronic circuit, in their central part.

Claims (8)

1. Joule-Thomson cooler, of the type comprising an open casing (7) consisting of two approximately planar plates and having a central location for cooling, and a pipe coil (2) for conveying gas under high pressure, this pipe coil forming a planar spiral with a pressure relief orifice (4) at the centre and being disposed between the two plates of the casing, characterised in that the spiral (2) consists of a smooth capillary tube wound into a spiral with contiguous turns, and in that a porous plate having a central hole (6) is disposed between one face of the spiral and the casing, this central hole forming a pressure relief chamber (15) into which said pressure relief orifice (4) opens.
2. Cooler according to claim 1, characterised in that the pipe coil (2) is spaced apart from the casing (7).
3. Cooler according to claim 1, characterised in that the porous plate (5) consists of a foam, a felt or a metal frit.
4. Cooler according to claim 1, characterised in that the thermal conductivity of the porous plate (5) in the axial direction of the spiral (2) is much greater than its thermal conductivity in the radial direction.
5. Cooler according to claim 4, characterised in that the porous plate (5) consists of a narrow strip of metal fabric wound into a spiral coaxially with said spiral (2).
6. Cooler according to any of claims 1 to 5, characterised in that the different layers of the cooler are fixed to each other by glueing or brazing.
7. Cooler according to any of claims 1 to 6, characterised in that the inner end of the spiral (2) is blocked, the pressure relief orifice (4) being formed laterally in an axial direction, close to this end.
8. Cryostat, characterised in that it comprises:

   - a Joule-Thomson cooler according to any of claims 1 to 7; and

   - a vacuum enclosure surrounding said location and connected with a sealing joint to the casing (7), which opens out on the outside of this enclosure.

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