FR3052245B1 - CRYOGENIC DEVICE WITH COMPACT EXCHANGER - Google Patents

Abstract

This cold generation device implements the "Joule-Thomson" relaxation principle. It comprises a heat exchanger in which circulates a countercurrent fluid under high pressure and under low pressure. The heat exchanger consists of the stack of pellets (5) made of porous material, and in particular sintered, constituting a cylindrical mandrel, at the periphery and in contact with which is wound a capillary (10) in which the high fluid circulates. pressure, the low pressure fluid flowing against the current inside the porous mandrel thus formed.

Description

CRYOGENIC DEVICE WITH COMPACT EXCHANGER

FIELD OF THE INVENTION The invention lies in the general field of cold machines, and more particularly in cold generating devices intended to allow the operation of certain types of detectors, and more particularly infra-red detectors of the type cooled, also called quantum infra-red detectors.

It is more particularly devices of the type in question implementing as a source of cold, the principle of relaxation called "Joule-Thomson".

PRIOR STATE OF THE TECHNIQUE

In the particular context of the infra-red detectors, it is desired for obvious reasons of space, to limit the volume of the cryogenic source. In fact, miniature cryogenic machines frequently use the "Joule-Thomson" relaxation principle, thus making it possible to have a large cryogenic power, and therefore a rapid cooling, in particular of infra-red detectors or electronic compounds. requiring for 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 that occurs between the high pressure fluid and the low pressure fluid before the expansion of the fluid intervenes. The efficiency of heat exchange is therefore essential. For this purpose, the devices of the prior art implement a Hampson type countercurrent 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 occurs at the periphery of the sleeve, at which the low-pressure fluid flows counter-currently.

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. While the heat exchange surface is increased, the presence of the fins, because of their thickness, increases the spacing between two consecutive turns, and therefore reduces the number of turns of the capillary for a length. given the chuck, 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. One then comes up against the problem of congestion of said exchanger, and therefore the cold machine.

It has also been proposed to reduce the axial conduction in the exchanger, inherent to the implementation of the chuck, and a source of loss and efficiency. The invention is aimed at a device of the type in question which makes 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 cooling time of the installation. without altering the size of the existing devices or conversely, at constant TMF, to reduce the size of such devices.

SUMMARY OF THE INVENTION To this end, the invention proposes a cold generation device implementing the "Joule-Thomson" expansion principle, comprising an exchanger in which a fluid under high pressure circulates in countercurrent. and under low pressure.

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 in which the high pressure fluid circulates, the low fluid pressure circulating against the current inside the porous mandrel thus formed.

In other words, the invention basically consists in replacing the mandrel and fins of the prior art with a stack of sintered and porous material, favoring the heat 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 in which circulates the high pressure fluid, and consequently allows to optimize the compactness of the cold generating device.

Advantageously, there is interposed between each of the pellets made of sintered material, a porous thermal insulating fabric, typically made of glass fibers. The interposition of such glass fiber elements, thus non-heat conducting, reduces the axial conduction and corollary optimizes the operation of the cold generator device.

Advantageously, the pellets are made from sintered silver or sintered copper.

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

According to an advantageous characteristic of the invention, the turns of the capillary are not in contact with each other. For this purpose, an insulating wire, typically made of glass fiber acting as a spacer, is coiled with the said capillary.

BRIEF DESCRIPTION OF THE FIGURES

The manner of carrying out the invention and the advantages which result therefrom will emerge more clearly from the description which follows, given by way of indication and not by way of limitation, in support of the appended figures. FIG. 1 is a diagram illustrating the "Joule-Thomson" expansion principle implemented at the level of the cold generating device, FIG. 2 is a schematic representation of the device of the invention, FIG. 3 is a view similar to FIG. FIG. 2 illustrating the respective circuit of the high pressure and low pressure fluid; Figure 4 is a schematic representation of a cryostat; FIG. 5 is a schematic representation in partial sagittal section of

One of the cryostat parts of Figure 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 therefore shows the operating diagram of a device implementing the "Joule-Thomson" trigger. This diagram shows the source of high pressure fluid HP, this fluid can be a typically argon gas, nitrogen or air, 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 chamber (4).

FIG. 2 shows the heart of the exchanger according to the invention. This is constituted by the stack of pellets (5), made of porous material, and in particular sintered silver base. Silver is indeed a very good thermal conductor and is also easy to sinter. One could also consider using copper instead of 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 pellets (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 an advantageous characteristic of the invention, these pellets (5) are separated from each other by a spacer (9), made of a non-conductive porous material, typically consisting 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. The assembly thus constituted by the pellets and the spacers constitutes a cylindrical mandrel, in contact with which is wound a capillary (10) in 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 outer diameter of 0.5 millimeters and an inner diameter of 0.3 millimeters.

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

The realization of such a device can be performed in the following manner.

In the first place, the pellets (5) are produced using a mold shaped according to 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 melting temperature of the silver, in order to obtain a simple sintering without causing the melting of the powder.

After completion of the pellets, their stacking is carried out by intercalating the thermal insulating elements (9), the latter having an external diameter less than or equal to that of the pellets (5), so that they can not come into contact with the capillary (10).

These pellets and the spacers are threaded onto the holding rods (6), for example threaded, and locked by means of the nuts (8). Lin mandrin is de facto constituted.

The capillary, for example made of cupro-nickel alloy undergoes treatment consisting of a silver deposit, for example by electrolysis, if the pellets are made of silver sintered. This deposit is intended to promote subsequent contact with the pellets (5), especially when proceeding to the fixing of said capillary by welding or solder. Thus, after winding the capillary (10) around the mandrel, the assembly is placed in an oven to generate the soldering phenomenon.

Alternatively, it may be envisaged to consolidate the assembly thus constituted by a thermal conductive binder, for example consisting of a type of "solgel" type of glue film loaded with metal powder, whitewashed in the capillary / pellet 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. For this purpose, it should be recalled that the device of the invention is intended to be integrated in a cylindrical well of a cryostat, as shown in FIG. 4. Such a cryostat (11) is traditionally maintained under empty. It receives within the enclosure that defines an infrared detector (12), positioned vertically above a window (13) transparent to the radiation to be detected. Finally, it comprises two wells (14), within 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.

FIG. 5 shows 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) is positioned in an insulating material, for example made of glass fibers or polyester fibers such as sold under the trademark terylene®, coming to bear between two consecutive turns of the capillary (10), that is to say in the interval between said turns, and against the inner wall of the cylindrical well (14). The wire (15) is thus wound along the mandrel and then fixed at its two ends, typically by gluing.

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

The device according to the invention makes it possible to considerably increase the heat exchange area in comparison with the devices of the prior art, of the type comprising a finned capillary, typically 1000 times with constant bulk. It is therefore easily conceivable 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 maintaining the same performance as the devices of the machine. prior art. These results are particularly significant in the context of cooled infrared detectors.

Claims (8)

1. Apparatus for generating refrigeration using the "Joule-Thomson" expansion principle, comprising a heat exchanger (1) in which a fluid under high pressure and under low pressure flows in countercurrent, characterized by: κ in that the heat exchanger consists of the stack of pellets (5) made of porous material, and in particular sintered, constituting a cylindrical mandrel, at the periphery and in contact with which is wound a capillary (10) within which circulates the high-pressure fluid, the low-pressure fluid circulating countercurrently within the porous mandrel thus formed; 81 and in that between each of the pellets (5) is interposed a porous thermal insulating element (9). 2. Device for generating cold according to claim 1, characterized in that the porous heat insulating element (9) consists of a fabric, in particular made of glass fibers. Cooling device according to one of claims 1 and 2, characterized in that the pellets (5) are cylindrical in that the thermal insulating spacer elements (9) are circular in shape, and in that that the diameter of the intermediate elements (9) is less than or equal to the outer diameter of the pellets (5). 4. Cooling device according to one of claims 1 to 3, characterized in that the pellets (5) are made of sintered silver or copper. 5. Cold generating device according to one of claims 1 to 4, characterized in that the capillary (10) is made of metal, in particular copper, stainless steel, or cupronickel alloy. 6. Device for generating cold according to claim 5, characterized in that the capillary (10) receives a silver deposit, depending on the nature of the constituent material of the pellets (5), before its winding on the mandrel consisting of stacking pellets (5). 7. Device for generating cold according to one of claims 1 to 6, characterized in that the turns defined by · the winding of the capillary (10) around the mandrel formed by the stack of pellets (5) are not in contact with each other. 8. Device for generating cold according to claim 7, characterized in that a thermal insulating wire (15) is wound on the intervals between the turns, said wire being in particular made of glass fibers.