EP0947787B1 - Device for a thermal connection in a cryogenic machine - Google Patents

Description

The subject of the present invention is a device comprising a cold finger and a device for thermal bond between the end of the cold finger and a load which should be brought to cryogenic temperature during use.

The invention finds a particularly application important, although not exclusive, when the machine refrigerator works using the Stirling cycle. However, it can also be used when this machine uses another closed cycle or even a cycle open, for example the Joule Thomson cycle.

The above machines provide cold to the end, generally consisting of a cover thick, with a cold finger whose base is directly or indirectly in contact with an environment high temperature. To reduce conduction losses, a very thin-walled tube made of a material having low thermal conductivity, such as steel stainless or titanium. The tube, being thin, has a very low mechanical strength and very low stiffness. Any force exerted on its end can consequently deforming the cold finger, which has consequences particularly serious when this finger contains a moving element, which is the case with cycle machines Stirling.

We have therefore sought to make devices thermal bonding which both have low thermal resistance and apply only low forces on the end of the cold finger. We notably carried out thermal bonding devices consisting of a braid of copper wires having mass and stiffness also weak as possible. However, this solution is not fully satisfactory. A braid of mass and stiffness low to high thermal resistance. To assemble the braid on the cover of the cold finger, you must access directly to that finger and to the charge, which is hardly compatible with the realization of a efficient thermal insulation. The fragility of the finger cold makes assembly delicate. So that the braid has the flexibility required it must have a length and a volume important.

The disadvantage of using a thermal braid additional when the same load is cooled by two machines, this to ensure redundancy. If one machine is stopped, for example due to a fault, the parasitic thermal leak by the cold finger of this machine, which remains in thermal connection with the load, adds to the power required by the load.

Also known (US-A-4 802 345) a device for thermal bond between a cold finger and a load, consisting of a narrow clearance in which there are gases at least one of which is noncondensable at the temperature of operation. Close play is essential and makes difficult decoupling.

Document US-A-4,178,775 describes a cryostat for a infrared detector cooled by a refrigerating machine open cycle. Blotting paper traps liquefied gas at near the infrared detector. This blotter doesn't play a pumping role, but only reserve.

The invention aims in particular to provide a device for thermal link for cryogenic machine responding better than those previously known to the requirements of the practical, in particular by reducing the thermal gradient between the end of the cold finger and the load, avoiding a mechanical connection between the cold finger and the load and allowing a realization of low mass and low volume with fewer assembly constraints.

To this end, the invention notably proposes a device thermal bonding according to claim 1.

The deformable wall can in particular be constituted by a thin-walled revolution bellows, connecting a base cold finger and the spray plate. He will be in generally preferable effect to avoid direct fixation bellows on the cold finger, the thickness of which is very weak, generally around a tenth of a mm.

The condensation and vaporization interval will be usually 1 to 10 mm. The pumping element by capillarity interposed between the end of the finger and the plate reduces the entrainment of drops forming towards the outside by the gases. This pumping element can have various constitutions. It can be made up of a pellet of wicking porous material, occupying the interval between the tip of the cold finger and the plate. This tablet may in particular be made of felt of silica, or fiberglass, or even synthetic material, with pores of a few tens of microns diameter. The circulation of the liquid from the periphery can also be facilitated by grooves engraved in the end.

The plate can be extended by a surrounding shirt the end part of the cold finger to avoid training liquid droplets outside the range by gas from vaporization.

Means of thermal insulation, generally made up of a Dewar vase, will be provided around enclosure and load to reduce losses thermal. However, such isolation is no longer necessary when the device is intended to operate in space, where there is a deep vacuum.

• la figure 1 est une vue en coupe d'un dispositif ;
• la figure 2 montre une variante.

The above characteristics as well as others will appear better on reading the following description of a particular embodiment, given by way of non-limiting example. The description refers to the accompanying drawings. In the drawings:

• Figure 1 is a sectional view of a device;
• Figure 2 shows a variant.

The device shown schematically on the Figure 1 comprises a thin tube 10, one end of which is attached to a base 12 belonging to the cryogenic machine and the other end of which is closed by a cover 14, which will generally be thicker than the cylindrical wall of the tube. In general this cover will be attached. he can however be in one piece with the rest of the tube. In general, the side wall of the tube is made up of a material with low thermal transmittance, for example stainless steel, titanium or alloy with titanium base. In the case of a machine intended for provide a cooling power of 1W to 90K, in an atmosphere at a maximum temperature of 300K, cold finger can for example have a diameter of 12 mm, a thickness 0.1 mm and a length of about 60 mm.

The device shown in Figure 1 is intended for cool a charge contained in a vacuum cryostat. This cryostat has an outer casing 16, for example glass with a silver inner side to be reflective. This outer envelope 16 is fixed on the base 12 by means not shown and the sealing between the atmosphere and a volume 30 which will be defined later is ensured by an O-ring 18. An annular zone 19 of the envelope intended for the fixing and the connection waterproof can be thickened to increase its rigidity.

The thermal bonding device includes a plate 20 slightly larger in diameter than the cover 14, having a face opposite that of the cover. This plate can be made of metal with high conductivity thermal. It is intended to be rigidly connected to the load to be cooled (not shown). The plate can be also attached to a partition 24 which can be viewed as the internal envelope of the cryostat. This envelope is fixed mechanically to the outer casing 16 at locations not shown in the figure. A flexible wall, shown in the form of a metal bellows 26, connects the bottom of the envelope 24, carried by the plate 20, to the zone annular 19 for reinforcing the outer casing 16. It thus separates a vacuum space 28 from an internal volume 30 surrounding the cold finger 10. Due to the flexibility of the bellows, parts 20 and 24, mechanically linked to the load of use, remain free compared to relative movements that can have in relation to them the parts 18 and 16 and therefore the end of the cold finger 14.

The internal volume 30 is occupied by gas chosen in depending on the temperature to which the plate 20. One can in particular use nitrogen, oxygen, air or argon. This latter gas has the advantage of being a neutral gas and to have a saturation curve slightly above that of nitrogen, resulting in pressure lower when the temperature of volume 30 is that of the environment on earth, for a quantity of liquid predetermined at 90K in enclosure 30. Often we will provide a ballast tank 32 connected to volume 30, so that limit the pressure of the gas contained in volume 30 when the temperature is that of the environment.

We will generally give the interval 22 a thickness nominal between 1 and 10 mm. This interval is occupied by a porous organ forming a wick of circulation of liquid by capillarity. The thickness of the interval may also be chosen based on the accuracy of positioning that can be expected during assembly and risks of displacement during operation, by example following accelerations or vibrations.

To prevent drops from forming on the cover 14 are dragged to a warmer part of the finger cold, the plate 20 is advantageously extended by a shirt 34 surrounding the end portion of the cold finger. So that the gas only liquefies against the cover 14, opposite the plate 20, the end part of the wall side of the cold finger can be isolated by a sleeve 36 made of thermal insulating material, over a length of the order of the centimeter. This sleeve may in particular be made of material expanded with closed porosity.

The operation of the device is then as follows, when the assembly shown in the figure is initially at room temperature. Volume 30 is completely filled with gas. When the refrigeration machine works, the gas temperature gradually decreases. Finally she reaches, at the end of the cold finger, the liquefaction temperature. Drops of liquefied gas get form and accumulate against the cover 14 and get bigger, gradually invading the porous organ. If the plate 20 is then at a temperature higher than the boiling point of liquid at prevailing pressure in volume 30, liquid vaporizes on contact with the plate by absorbing heat. Steam recondense on cover 14 and the cycle continues until the temperature of the plate 20 reaches that of the tip of the cold finger. Interval 22 can then fill completely with liquid which will vaporize again if the heat transfer by conduction of the insufficient liquid to keep plate 20 below of the boiling point. Interval 22 can play the role of the condenser of a heat pipe using the same gas as that present in volume 30 and distributing the cold in the plate 20 and if necessary the wall 24.

In some cases, it would be beneficial to use a gas mixture in volume 30 so that the thermal bond can operate in a wider temperature range : for example, we will take a mixture of argon, methane, carbon dioxide and ammonia to cover an area ranging from ambience to - 180 ° C. So whatever the temperature of the working load, at least one of these gases will be in its boiling range, while the others will be in gaseous, liquid or solid form and will only intervene by conduction in the transfer thermal. This possibility can be interesting for applications operating at variable temperatures or to facilitate the transient for cooling the system, allowing the initiation of the thermal link to temperatures higher than the nominal temperature use.

The thermal gradient between the cover and the plate is very low, the boiling flow is usually 1 to 10 W / cm ² , even under micro-gravity. No force is exerted by the load on the end of the cold finger, since there is no mechanical connection between the plate and the cold finger, the porous material having no appreciable stiffness. The nominal difference between the cover and the plate can be chosen to a value sufficient to compensate for any manufacturing tolerance and any relative displacement. Because the tolerances are high, the cold finger can be easily integrated into a system. The plate 20 constitutes only a small excess length, usually less than 10 mm.

In the case of a system with a load provided with two machines to create redundancy, thermal leaks generated by a faulty machine are very small, because stopping this machine will cause the cold finger, vaporization of the liquid and reduction of heat transfers which will only be done in mode conductive through steam, between the cover and the plate.

As indicated above, means can be designed to pump liquid to the center of the cover. We can in particular provide means using capillary forces, such as furrows radials bringing the liquefied gas from the periphery of the cover towards its center.

When the device is intended to operate only in space, therefore under vacuum, the cryostat can be omitted and in this case the bellows 26 simply connects a annular plate tightly fixed to the base 12 (or the base itself) at a bottom extending the plate 20.

In Figure 2, where the organs corresponding to those of FIG. 1 are designated by the same reference number, the pumping element 40 constitutes the condenser of a heat pipe 42 for cooling a charge located at distance. For this, the porous material 40 does not occupy only the area facing the cold finger 14. It extends into a conduit 42. The porous material does not introduce any mechanical coupling, due to its texture. The liquid-gas interface 44 is likely to be move through the porous material, depending on the thermal power dissipated in the load. Grooves internal gas return to the condenser part can be arranged inside the duct 42.

Claims (9)

1. Device comprising a cold finger and thermal link device for use between an end, at cryogenic temperature, of the cold finger and a load, comprising a plate (20) confronting said end, for connection with the load, mechanically separate from the end and defining with said end a condensation and vaporization gap occupied by a capillary pumping element and comprising also a deformable wall connected to the plate and defining an enclosure accommodating said gap and surrounding at least the end of the cold finger and the portion of the cold finger which is close to the end, said enclosure being occupied by at least one gas having a condensation temperature selected responsive to the cryogenic temperature to be given to the load.
2. Device according to claim 1, characterized in that said pumping element is a pellet of wick-forming porous material occupying the whole gap comprised between the end of the cold finger and the plate (20).
3. Device according to claim 1 or 2, characterized in that the plate (20) is extended by a jacket surrounding the end portion of the cold finger for hindering shift of drops of liquid out of said gap due to egress of said gas upon vaporization thereof.
4. Device according to claim 3, characterized by a thermally insulating sleeve (36) surrounding the end portion of the cold finger.
5. Device according to any one of claims 1 to 4, characterized in that the deformable wall comprises a flexible bellows having a rotational symmetry, and thin wall connecting a base of said cold finger and said plate.
6. Device according to any one of Gaims 1 to 5, characterized in that thermally insulating means are positioned around the enclosure and the load.
7. Device according to any one of claims 1 to 6, characterized in that said enclosure is occupied by a mixture of a plurality of gases having different boiling temperatures.
8. Device according to any one of claims 1 to 7, characterized in that the gap (22) is sized to constitute a condenser of a heat pipe.
9. System comprising a load and two cryogenic machines, characterized in that each of said machines comprises a cold finger and a link device connected to the load according to any one of claims 1-8.

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