EP3465030B1 - Cryogenic device with compact exchanger - Google Patents

Description

FIELD OF THE INVENTION

The invention lies in the general field of cold machines, and more particularly in cold generation devices intended to allow the operation of certain types of detectors, and more particularly infrared detectors of the cooled type, also called detectors. quantum infrared.

It relates more particularly to devices of the type in question using as a cold source, the so-called "Joule-Thomson" expansion principle.

PRIOR STATE OF THE ART

In the particular context of infrared detectors, it is desired, for an obvious reason of space, to limit the volume of the cryogenic source. In fact, miniature cryogenic machines frequently use the “Joule-Thomson” expansion principle, thus making it possible to have significant cryogenic power, and therefore rapid cooling, in particular of infrared detectors or electronic compounds. requiring their operation to operate at particularly low temperatures.

It is known that the performance of such cryogenic machines depends on the efficiency of the heat exchange which occurs between the high pressure fluid and the low pressure fluid before the expansion of the fluid occurs. The efficiency of the heat exchange is therefore essential.

To this end, the devices of the prior art use a Hampson type counter-current exchanger, in which the high pressure fluid circulates in a capillary surrounding a cylindrical sleeve or mandrel, closed by an insulating foam. The heat exchange takes place at the periphery of the sleeve, at the level of which the low pressure fluid circulates against the current. The document FR 2 477 406 A1 discloses a cold generating device according to the preamble of claim 1.

In order to optimize this heat exchange, it has been proposed to increase the exchange surface between the high pressure fluid and the low pressure fluid, by providing the capillary with radial fins. If certainly, the heat exchange surface is increased, on the other hand, the presence of the fins, due to their thickness, increases the spacing between two consecutive turns, and therefore decreases the number of turns of the capillary for a given length of the mandrel, neutralizing at least partially the desired optimization of the exchange.

For the same purpose, it has also been proposed to increase the length of the exchanger, and more particularly the length of the capillary. We then come up against the problem of the size of said exchanger, and therefore of the cold machine.

It has also been proposed to reduce the axial conduction in the exchanger, inherent in the implementation of the mandrel, and source of loss and efficiency.

The invention relates to a device of the type in question making it possible at the same time to increase the efficiency of such a device, in particular by reducing the TMF, that is to say the time for the installation to cool down. , without altering the size of existing devices or on the contrary, at constant TMF, reducing the size of such devices.

STATEMENT OF THE INVENTION

To this end, the invention proposes a device for generating cold implementing the principle of "Joule-Thomson" expansion, comprising a heat exchanger within which a fluid under high pressure and under low pressure circulates.

According to the invention, the heat exchanger consists of the stack of pellets made of porous material, and in particular sintered, constituting a cylindrical mandrel, in contact with which is wound a capillary within which circulates the high pressure fluid, the low fluid pressure circulating against the current inside the porous mandrel thus formed.

In addition, there is interposed between each of the pellets made of sintered material, a porous thermal insulating fabric, typically made of glass fibers.

In other words, the invention basically consists in replacing the mandrel and the fins of the prior art by a stack of sintered and porous material, promoting the thermal exchange of the low pressure fluid with the high pressure fluid circulating in the capillary. peripheral in contact with said material.

This optimization of the exchange results from the nature of the material constituting the mandrel, and also makes it possible to dispense with the fins optimizing the heat exchange of the prior art, and consequently, makes it possible to optimize the concentration of turns of the capillary within which the high pressure fluid circulates, and consequently optimizes the compactness of the cold generating device.

In addition, due to the intercalation between the pellets of sintered material of thermally insulating grids, typically made of glass fibers, therefore non-conductive of heat, the axial conduction is reduced and consequently the operation of the cold generating device is optimized. .

Advantageously, the pellets are made based on silver sinter or copper sinter.

The capillary is made of metal, typically copper, stainless steel, or even a cupronickel alloy.

• ▪ isoler thermiquement deux spires consécutives du capillaire ;
• ▪ isoler thermiquement lesdites spires du tube extérne ou puits dans lequel le dispositif de l'invention est susceptible d'être introduit ;
• ▪ assurer une étanchéité du dispositif de l'invention avec un tel tube externe ou puits, contraignant le fluide basse pression à passer au travers des pastilles en matériau fritté, induisant une optimisation du rendement.

According to an advantageous characteristic of the invention, the turns of the capillary are not in contact with one another. To this end, a thermally insulating wire, typically made of fiberglass and acting as a spacer, is wound simultaneously with said capillary. Such a wire performs various functions:

• ▪ thermally insulate two consecutive turns from the capillary;
• ▪ thermally isolate said turns from the external tube or well into which the device of the invention is capable of being introduced;
• ▪ sealing the device of the invention with such an external tube or well, forcing the low pressure fluid to pass through the pellets made of sintered material, inducing an optimization of the yield.

BRIEF DESCRIPTION OF THE FIGURES

• la figure 1 est un schéma illustrant le principe de la détente « Joule-Thomson » mise en œuvre au niveau du dispositif générateur de froid.
• la figure 2 est une représentation schématique du dispositif de l'invention.
• la figure 3 est une vue analogue à la figure 2 illustrant le circuit respectif du fluide haute pression et basse pression ;
• la figure 4 est une représentation schématique d'un cryostat ;
• la figure 5 est une représentation schématique en section sagittale partielle de l'une des partie du cryostat de la figure 4.

The manner of carrying out the invention and the advantages which ensue therefrom will emerge more clearly from the description which follows, given by way of indication and without limitation in support of the appended figures.

• the figure 1 is a diagram illustrating the principle of "Joule-Thomson" expansion implemented in the cold generating device.
• the figure 2 is a schematic representation of the device of the invention.
• the figure 3 is a view analogous to the figure 2 illustrating the respective circuit of the high pressure and low pressure fluid;
• the figure 4 is a schematic representation of a cryostat;
• the figure 5 is a schematic representation in partial sagittal section of one of the parts of the cryostat of the figure 4 .

DETAILED DESCRIPTION OF THE INVENTION

We therefore represented in relation to the figure 1 the operating diagram of a device implementing the “Joule-Thomson” expansion. This diagram shows the source of HP high pressure fluid, this fluid can be a typically argon, nitrogen or air gas, and the return of said fluid after expansion.

The double coil (1) shows the countercurrent heat exchanger between the high pressure fluid emanating from the high pressure source HP, and the low pressure fluid, after expansion at the evaporator (2), an expansion valve (3) being mounted before the evaporator. The assembly is integrated within a vacuum enclosure (4).

We have represented within the figure 2 the heart of the exchanger according to the invention. This consists of the stack of pellets (5), made of porous material, and in particular of silver-based sintered material. Silver is indeed a very good thermal conductor and moreover proves easy to sinter. We could also consider using copper to replace silver.

Typically, the porosity of these pellets is close to 100 nanometers. In other words, the orifices generated by the sintering of the pellets have a typical diameter of 100 nanometers.

These pads (5), of generally cylindrical shape, are for example assembled to each other by means of fixing rods (6), emanating from the high pressure connector (7), and provided with nuts (8) at their lower base. . Alternatively, the pellets can be glued together.

According to the invention, these pellets (5) are separated from each other by an interlayer or grid (9), made of a non-conductive porous material, typically made of a woven fiberglass. These spacers have a typical thickness of 0.3 millimeters. The implementation of such spacers tends to oppose any axial thermal conduction, optimizing the heat exchange surface between the two flows, respectively low pressure and high pressure.

The assembly thus formed by the pellets and the spacers constitutes a cylindrical mandrel, in contact with which a capillary (10) is wound, within which the high pressure fluid circulates. This capillary is for example made of copper, stainless steel or a cupro-nickel alloy. It typically has an external diameter of 0.5 millimeter and an internal diameter of 0.3 millimeter.

Due to the porous nature of the pellets (5), the low pressure fluid passes through them and cools them. In turn, having regard to the fact of their good thermal conductivity character, the pellets cool the high pressure fluid which circulates in the capillary. In fact, good thermal contact is necessary between the capillary and the pellets.

The production of such a device can be carried out as follows.

First, the pellets (5) are produced using a mold shaped as a function of the desired shape of said pellets. The silver powder is poured into the mold, and the temperature of the mold is raised to a temperature below the silver melting temperature, in order to obtain a simple sintering without causing the powder to melt.

After the pellets have been produced, they are stacked by inserting the thermal insulating elements (9), the latter having an external diameter less than or equal to that of the pellets (5), so that they cannot come into contact with the capillary (10).

These pads and the spacers are threaded onto the retaining rods (6), for example threaded, and locked by means of the nuts (8). A mandrel is therefore de facto constituted.

The capillary, for example made of cupro-nickel alloy undergoes a treatment consisting of a deposit of silver, for example by electrolysis, if the pellets are made of silver sinter. The purpose of this deposit is to promote subsequent contact with the pellets (5), in particular when the capillary is fixed by welding or by soldering. Thus, after winding the capillary (10) around the mandrel, the assembly is placed in an oven to generate the phenomenon of brazing.

Alternatively, it can be envisaged to consolidate the assembly thus constituted by a thermal conductive binder, for example consisting of a film of “solgel” type adhesive loaded with metal powder, brushed in the capillary / tablet area.

According to an advantageous characteristic of the invention, it is sought to avoid any contact between the consecutive turns of the capillary, in order to avoid any thermal bridge between them.

To this end, it should be recalled that the device of the invention is intended to be integrated into a cylindrical well of a cryostat, as illustrated within the figure 4 . Such a cryostat (11) is traditionally maintained under vacuum. It receives within the enclosure that it defines an infrared detector (12), positioned directly above a window (13) transparent to the radiation to be detected. Finally, it comprises two wells (14), into which are inserted in each of them a device according to the invention, in order to generate the cold necessary for the operation of said detector.

We have represented within the figure 5 , a schematic view in partial sagittal section of one of the wells (14) provided with the device of the invention.

Thus, in order to force the low pressure fluid, and in particular the low pressure gas, to pass through the porous pellets (5), a wire (15) made of insulating material, for example made of glass fibers or polyester fibers such as as marketed under the registered trademark terylene®, coming to bear between two consecutive turns of the capillary (10), that is to say in the interval separating said turns, and against the internal wall (16) of the cylindrical well (14 ). The wire (15) is thus wound along the mandrel, then fixed at its two ends, typically by gluing.

Thus, with the introduction of this wire (15), it overcomes any thermal bridge between the turns on the one hand, and between the turns and the well (14).

The consecutive turns of the capillary (10) are therefore thermally insulated from each other. In addition, the turns of the capillary (10) are thermally insulated from the well (14).

Finally, the presence of the wire (15) provides a seal for the device with respect to said well, forcing the low pressure fluid to pass through the pellets (5), and therefore contributing to optimizing the yield of the device of the invention.

In the particular case of the implementation of the device of the invention with an infrared detector, the operating temperature of the latter is typically between 77K and 250K.

The pressure of the high pressure fluid is typically between a few tens to a few hundred bars.

The device according to the invention makes it possible to considerably increase the heat exchange surface in comparison with the devices of the prior art, of the type comprising a capillary with fins, typically 1000 times with constant congestion. It is therefore easily understood that the efficiency of such a cold machine is itself increased, or that the size of such a cold machine can be significantly reduced, while retaining the same performance as the devices of the invention. prior art. These results are particularly appreciable in the context of cooled infrared detectors.

Claims (8)

1. A cold generation device implementing the "Joule-Thomson" expansion principle, comprising a heat exchanger (1) having a fluid under high pressure and under low pressure circulating in counterflow therein, wherein the heat exchanger is formed of a stack of pellets (5) made of a porous material, and particularly a sintered material, forming a cylindrical mandrel, having a capillary (10) wound at the periphery thereof and in contact therewith, the capillary having the high-pressure fluid circulating therethrough, the low-pressure fluid circulating in counterflow inside of the porous mandrel thus formed, characterized in that between each of the pellets (5) is interposed a thermally-insulating porous element (9).
2. The cold generation device of claim 1, characterized in that the porous thermally-insulating element (9) is formed of a fabric, particularly made of fiber glass.
3. The cold generation device of any of claims 1 and 2, characterized in that the pellets (5) have a cylindrical shape, in that the thermally-insulating intercalary elements (9) have a circular shape, and in that the diameter of the intercalary elements (9) is smaller than or equal to the external diameter of the pellets (5).
4. The cold generation device of any of claims 1 to 3, characterized in that the pellets (5) are made up of sintered silver or copper.
5. The cold generation device of any of claims 1 to 4, characterized in that the capillary (10) is made of metal, particularly of copper, of stainless steel, or of cupronickel alloy.
6. The cold generation device of claim 5, characterized in that the capillary (10) receives a silver deposit, which is a function of the nature of the material forming the pellets (5), before its winding on the mandrel formed by the stack of pellets (5).
7. The cold generation device of any of claims 1 to 6, characterized in that the spirals defined by the winding of the capillary (10) around the mandrel formed by the stack of pellets (5) are not in contact with one another.
8. The cold generation device of claim 7, characterized in that a thermally-insulating yarn (15) is wound on the intervals separating the spirals, said yarn being particularly made up of fiber glass.

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