Abstract:
A detector, in particular an IR detector in the seeker head of a guided missile, is cooled with expanding gas generated by depressurizing a pressurized fluid. A mixture which forms a positive azeotrope, including argon or nitrogen as a main component and at least one alkane as a secondary component, is expanded as the fluid. The composition is preferably in the region of a eutectic mixture in order to avoid a component freezing adjacent to the expansion nozzle. This makes it possible to considerably extend the life of the cooler in comparison to that when using pure cooling gases such as nitrogen or argon, and to cool the detector down more quickly, subject to the same constraints.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority, under 35 U.S.C. §119, of German patent applications DE 10 2007 004 999.6, filed Feb. 1, 2007 and DE 20 2007 008 674.1, filed Jun. 21, 2007; the prior applications are herewith incorporated by reference in their entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method for cooling a detector, in particular an IR detector in the seeker head of a guided missile. The expanding gas after a pressurized fluid has been expanded is used to cool the detector. 
         [0004]    A method such as this uses the so-called Joule-Thomson effect (also referred to as the Joule-Kelvin effect) for cooling, making use of the expansion of a real gas, which is not the same as that of an ideal gas. When a real gas is expanded below its inversion temperature, then it is cooled down because of the positive Joule-Thomson coefficient—that is to say by means of certain interactions between the gas molecules. This gas which has been cooled down and expanded in turn further cools down the inlet high-pressure gas, by means of a counter-flow heat exchanger, until the expanding high-pressure gas exists in the vapor area of the reverse-flow heat exchanger below the boiling or dewline—that is to say in the wet vapor region—partially as a liquid, condensed phase and partially as a gaseous phase. This cooling down of the gas to the boiling point of the respective real gas with expansion to about 1 bar below its inversion temperature is made technical use of in a versatile manner in order to produce low temperatures or to cool down temperature-sensitive equipment, in particular such as detectors. In particular, optical detectors in the infrared range, so-called IR detectors, must be cooled down to temperatures of below 100 K in order to achieve a good signal-to-noise ratio. 
         [0005]    In the case of expansion or Joule-Thomson coolers based on the Joule-Thomson effect, a suitable pressurization working gas (high-pressure cooling gas) is expanded by means of a restrictor or nozzle, and the emerging gas which has been cooled down by virtue of isoenthalpy expansion is used to reduce the temperature of the gas inlet and of a detector that is disposed adjacent to the expansion nozzle. In this case, the expanding and partially liquefying gas is aimed directly against the detector rear wall to be cooled, or against a thermally highly conductive intermediate wall to which the emerging gas can be applied and on whose front face the detector is arranged. In order to reach the boiling point of the cooling gas and to achieve good cooling performance at this respective boiling point of the respective gas, it is known for the working gas which is supplied on the high-pressure side to the expansion nozzle—also referred to as a restrictor—to be cooled down by flowing in the opposite direction to the inlet working gas, which is being expanded, through an appropriate reverse-flow heat exchanger before finally emerging into the surrounding area. 
         [0006]    Particularly when the installation conditions are confined and the energy resources are low, it is impossible to operate a Joule-Thomson cooler in a closed cooling circuit, for example on the basis of the Linde process, with the expanded working gas being compressed with heat being emitted, and being supplied in a pressurized form to the expansion nozzle once again, in order to cool down the detector. This is because compressors require physical space and a large amount of energy for operation, whose dissipated heat must also be dissipated. So-called open Joule-Thomson coolers are known for confined operational conditions such as these, in which the pressurized working gas is emitted to the surrounding area from a high-pressure gas container after it has been expanded, after the required detector cooling and after flowing back in the reverse-flow heat exchanger. Open Joule-Thomson coolers such as these are used, for example, to cool IR detectors in the seeker heads of guided missiles where neither physical space nor energy are available to allow a compressor system to be used for a closed Joule-Thomson cooler. A two-stage, open Joule-Thomson cooler for cooling an IR detector in the seeker head of a guided missile is described, for example, in European patent specification EP 0 432 583 B1 and U.S. Pat. No. 5,150,579. 
         [0007]    Since, for obvious reasons, the amount of working gas which can be made available from a high-pressure vessel in a missile is restricted, and some of the carrier aircraft cannot supply high-pressure working gas from the aircraft or launcher, also referred to as the launch row, only a limited supply of working gas can be carried in a pressure bottle, either for rapid cooling of an IR detector in the missile itself, for long-lasting detector cooling, for greater cooling power levels or for combinations thereof. 
         [0008]    The volumes of high-pressure bottles with a maximum volume of up to 500 cm 3  carried in conventional air-to-air missiles and which are restricted for physical space reasons are disadvantageously suitable only for a restricted cooler running time for cooling an IR detector. In the stated conditions and depending on the ambient temperature, cooler running times of merely between 1.5 and 3 hours can currently be achieved using air, nitrogen or argon as the working gas, which make it possible to reduce the temperature of the IR cooler to below the 100 K boiling point. New types of operational scenarios for modern combat jets are now, however, based on flying times which may be from 6 to 8 hours, during which the IR detector in the missile must be kept at a cryogenic operating temperature of below 100 K, for the missile to be ready for operation at all times. 
         [0009]    In other applications with short cooler running times, detector cooling-down times of less than 1.5 to 2 seconds must be achieved by means of the Joule-Thomson cooler from the restricted high-pressure bottle volumes of a few cubic centimetres, but at high gas pressures. This is the situation, for example, with portable surface-to-air missiles against enemy combat aircraft (so-called “Manpads”—Manportable Air Defense Systems—and with marine missile defense systems (against so-called Seaskimmers—missiles approaching at low level) and enemy combat aircraft. In certain circumstances—for example when carrying out improvements to missiles—a combination of a considerably shorter cooling-down time of the IR detector with a longer cooler running time and greater cooling loads to be transported away from the IR detector must be achieved from these small bottle volumes at the same time. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    It is accordingly an object of the invention to provide a method of cooling a detector, in particular an IR detector in the seeker head of a guided missile as described above, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows for a cooler running time that is as long as possible to be achieved with a restricted supply volume of working gas. 
         [0011]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method of cooling a detector, the method which comprises: 
         [0012]    providing a pressurized fluid, the pressurized fluid being a mixture of argon or nitrogen as a main component and at least one alkane as a secondary component, the mixture forming a positive azeotrope; and 
         [0013]    expanding the pressurized fluid and using an expanding gas to cool the detector. 
         [0014]    In other words, for a method for cooling a detector, in particular an IR detector in the seeker head of a guided missile, in which the expanding gas is used as a cooling gas and a cooling liquid to dissipate heat from the detector after a pressurized fluid has been expanded, in particular above the critical point, this object is achieved according to the invention in that a gas mixture which forms a positive azeotrope, in which the boiling point of the mixture is below that of the pure components, including argon or nitrogen as a main component and at least one alkane as a secondary component, is expanded as the fluid. 
         [0015]    The invention is in this case based on the consideration that the cooling gases that have been used until now with boiling points below 90 K, such as nitrogen, argon, oxygen or air, do not have sufficient cooling capacity for the desired running time extension. In consequence, even when using a high-pressure bottle in which the working gas has a pressure of more than 300 bar applied to it, it is not possible to achieve the desired running time extension by a factor of at least 2 to 3. Their cooling capability—that is to say the integral Joule-Thomson coefficient—is simply too low. However, for further possible working gases such as neon, helium or hydrogen with boiling points below 100 K, the inversion temperatures are below room temperature, of about 25° C. Expansion of working gases such as these in the given cooler conditions and without a previous initial cooling of the high-pressure gas below its respective inversion temperature therefore does not lead to the desired cooling down, but to an increase in the gas temperature. Furthermore, even when using initial cooling for these gases, the cooling capability or the integral Joule-Thomson effect is too low. 
         [0016]    Furthermore, the invention is based on the consideration that certain gases, inter alia those with a relatively high molar mass, such as the alkanes, admittedly have considerably greater cooling capacities in terms of the Joule-Thomson effect, but their boiling points are all above 100 K. It is therefore impossible to use pure alkanes as a cooling gas for cooling the IR detector to temperatures of at least 100 K. 
         [0017]    Finally, the invention recognizes the fact that the characteristic of a high cooling capacity with regard to the Joule-Thomson effect and a desired low boiling point taking into account the gas-mixture thermodynamics can be achieved only in a gas mixture having a plurality of components. Specific real gas mixtures which form a so-called positive azeotrope, that is to say they have mixture boiling points below the boiling temperatures of the individual pure components in the T,x diagram, exist only in a few specific cases in particular because of specific interactions between the individual components. In this case, inter alia, a vapor pressure curve plotted over the concentration ratio of the components on a p,x diagram with a specific composition also has a local maximum. In this context, x represents the concentration, T the temperature and p the pressure. Furthermore, in the case of binary mixtures composed of two components, the boiling curve and vapor-pressure curve touch one another in the phase diagram. In this case, the gas mixture behaves like a pure gas. A mixture of this composition is referred to as an azeotrope or as an azeotropic mixture: on boiling or condensation, azeotropic mixtures behave like a pure substance. When mixtures are composed to form positive azeotropic mixtures, in which the vapor-pressure curves have a maximum, all these positive-azeotropic mixtures, with these very specific compositions, have boiling points which are well below those of the individual pure gas components. For example, if the condensed phase of the expanded cooling gas vaporizes in a vapor area or expansion area of a cooler by heat absorption from the detector side, then this azeotropic mixture boils like a pure gas at this reduced mixture boiling point, which remains constant, without any change to the mixture composition. In the case of mixtures composed of more than two gas components, depending on the number of components, an azeotropic point with a specific concentration no longer occurs, in accordance with the Gibb&#39;s phase rule, but a two-dimensional or multiple-dimensional “field/region” with specific concentration areas which can be determined precisely for the individual gas components. Since azeotropic mixtures are pressure-dependent, the respective azeotropic mixture must be determined for the pressure ranges that occur in the vapor area, around 1 to 3 bar (0.1-0.3 MPa). 
         [0018]    Extensive experiments have now shown that a mixture comprising argon or nitrogen (possibly also air) as a main component and at least one alkane as a secondary component, with the alkane or alkanes being chosen such that the mixture forms a positive azeotrope (with a boiling point of below 100 K), behaves like a pure gas with one of the main components in comparison to the higher boiling-point components of the alkanes in the region with the low boiling points, in only specific concentration ratios which comprise the tightly limited azeotropic composition, for cooling of an open Joule-Thomson cooler. Under the conditions which then occur, a dynamic equilibrium comprising a liquid phase and a gas phase of the azeotropic mixture at the temperature that corresponds to the boiling point of this liquid/gas phase is created in the expansion area (vapor area) downstream from the expansion nozzle. Furthermore, the gas mixture comprising main and secondary components is chosen such that this results not only in a positive-azeotropic gas mixture but that the respective mixture boiling point is as far as possible below 100 K. 
         [0019]    A mixture composed of gas and liquid for cooling the detector is formed after the expansion process in the vapor area (wet vapor region). The essential feature in this case is that a relatively large amount of liquid and a relatively small amount of gas are produced over wide pressure ranges (from 3 to 50 MPa) since the amount of liquid determines the cooling capability of the cooler arrangement mainly by means of the vaporization enthalpy of this azeotropic mixture. During the cooling of the detector, the liquid phase which is produced in the vapor area is changed continuously to the gas phase at a constant boiling point. The cooler performance is in this case governed mainly by the mixture vaporization enthalpy, and less by the pure convective gas cooling. On the high-pressure side, new gas/liquid mixture flows continuously via the expansion nozzle into the vapor area, thus maintaining the cooling process. As long as a liquid phase is present, the cooling system remains at the constantly low azeotropic mixture boiling point, in this case at temperatures below the required 100 K. Since the azeotropic mixture in the vapor area and composed of gas and liquid in both phases has the same concentration and behaves like a pure gas, the azeotropic mixture composition in the two phases does not change over time either, and the mixture boiling point therefore also remains below 100 K, even though the individual alkanes have boiling points of well over 100 K. The expanded gas, like the gas which is created in the vaporizing amount of liquid as well, flows through the reverse-flow heat exchanger out of the vapor area into the surrounding area, in order to initially cool the high-pressure inlet. The gas phase from the vapor area is finally dissipated to the exterior as consumed gas mixture. At the same time, these azeotropic mixtures with the main components of nitrogen or argon make use of the greater cooling capacity of an alkane as a secondary component in comparison to nitrogen or argon, by virtue of the higher Joule-Thomson coefficients, that is to say cooling capacities. Furthermore, the higher molar mass of most of the alkanes that are used here is also used such that an even longer running time of the Joule-Thomson cooler can be achieved overall, with the same volumetric amount of pressurized fluid. 
         [0020]    In this case, the expression of fluid in the pressure vessels means the aggregate state of the pressurized gas mixture above the critical point on a T,s diagram, at which no separation of liquid from gas is evident, that is to say there is no meniscus. In this case, T represents the temperature and s the entropy. The individual gas components nitrogen, argon and the various alkanes have a maximum in their cooling capacity at one specific pressure, that is to say Joule-Thomson coefficient, which is generally in the range from 200 to 400 bar (20-40 MPa) at room temperatures. However, the individual gas components exist in the mixture only below their partial pressure, which is below the total pressure, corresponding to the gas-mixture composition. The gas mixtures in the high-pressure bottle must thus be at a higher total pressure in order to optimize the cooling process in order to ensure that the individual gas components, at their partial pressure, approach the region of optimum cooling performance as much as possible. This may necessitate pressures of up to 500 bar and possibly even of 800 bar in the pressure vessels. Higher pressures also at the same time mean greater available amounts of gas in the gas container and therefore additionally a longer cooler running time, as well. 
         [0021]    The use of alkanes also offers the advantage that, as a result of the higher Joule-Thomson coefficients, the cooling process collapses only at considerably low residual pressures in a high-pressure supply, thus resulting in longer cooler running times in comparison to nitrogen and argon, since the gases that remain in the high-pressure bottle can be used down to lower pressures for cooling purposes. 
         [0022]    The use of alkanes furthermore offers the advantage that a large number of organic impurities which originate, for example, from gas production, from compression or from a pressure bottle or from pipeline systems, are dissolved in the gas and therefore cannot be precipitated adjacent to the expansion restrictor, and therefore cannot end the cooling process by blocking the restrictor. 
         [0023]    In one advantageous refinement of the invention, the boiling point of the azeotrope should be below 100 K, in particular below 90 K. Depending on the chosen mixture, it is sufficient in this case for the vapor pressure curve of the selected gas mixture on the p,x diagram to have a mixture-specific local maximum. The boiling point of the mixture on the T,x diagram then has a minimum and is below that of the pure gases that are involved. In the case of mixtures comprising a plurality of gases, the mixture boiling point—for the mixtures chosen here—should not be significantly higher than that of the main component nitrogen or argon, that is to say between 85 and 100 K. 
         [0024]    In order to prevent a component in the mixture from freezing during expansion of the gas mixture, which results in a temperature reduction, and which to this extent can lead to undesirable accumulation on the expansion nozzle, it is advantageous for the sought mixture to be chosen from main and secondary components such that the required azeotropic gas mixture also has a composition in the vicinity of the eutectic composition. For this purpose, it is absolutely essential for the various gas components to be soluble in one another in the liquid, condensed phase. In order to ensure that the solid phase is not left, which could therefore lead to blocking of the gas flow, during the expansion adjacent to the nozzle, the individual gas components must be soluble in one another in the condensed liquid state; this means: component a is soluble in component b in a dual mixture, component c is soluble in at least one of the components a and b in a mixture of three items, component d is soluble in at least one of the components a, b and c, etc. It is therefore necessary not only to determine a specific mixture composition for a positive azeotrope but, at the same time, the mixture composition must also be designed so as to create a eutectic in order that the boiling point of the fluid that is completely in solution assumes a lower freezing point temperature than the associated boiling point of the mixture. 
         [0025]    A mixture with a eutectic composition of its components is, specifically, characterized in that the melting point of the mixture is lower than the melting points of the pure components. This is also an important precondition for use of azeotropic mixtures in Joule-Thomson coolers, since the freezing and melting points of the individual alkanes used here are higher than the boiling point of the azeotropic gas mixture with these alkanes. In order to prevent accumulation on the expansion nozzle as a result of individual gas components freezing in this case, the proposed azeotropic gas mixture must preferably also have a eutectic composition: in the case of the gas mixtures proposed here, the mixture melting point must preferably remain below the mixture boiling point. No individual solid aggregate state may occur in the mixture liquid phase, that is to say no individual components are deposited adjacent to the expansion nozzle. Mixed phases, in which one of the components is in the liquid phase and the other component is in the solid aggregate state, do not exist for a mixture with a eutectic composition. If the azeotrope thus has a composition in the vicinity of the eutectic composition, then this prevents individual components from freezing, for example the secondary components in the mixture, significantly reducing the proportion of the component which freezes out. 
         [0026]    In particular, a melting point of below 90 K, or even better below 85 K is advantageous for a eutectic composition of the sought mixture. 
         [0027]    At the moment, IR detectors are cooled using gas mixtures which in combination with one another achieve two or all three of the following effects at very high gas pressures and with limited available high-pressure gas containers, by virtue of the higher Joule-Thomson coefficients associated with them: 
         [0028]    i. allow longer running times from the limited volume, or 
         [0029]    ii. allow shorter detector cooling-down times, or 
         [0030]    iii. allow a greater cooling load. 
         [0031]    The fluid preferably has an initial pressure of more than 100 bar, in particular of more than 300 bar, in particular of more than 500 bar, and in particular preferably more than 800 bar from a compressed-gas container of limited availability applied to it, in order that the partial pressures of the individual gas components are in the optimum pressure range for their respective cooling capacity. The maximum initial pressure to be provided is therefore governed by the individual gases, their molar components in the mixture and thus their various partial pressures. The total pressure should be chosen as a function of the gas composition such that the specific partial pressures of the individual gases come as close as possible to the maximum specific Joule-Thomson coefficient. This necessarily leads to quite high initial pressures for the gas mixture. The amount of stored fluid is increased by appropriately high compression, with a positive effect on the running time of the Joule-Thomson cooler. A pressure of up to more than 500 bar (in some cases even of up to 800 bar) can be applied in a compact form to the fluid by use of a high-pressure gas bottle. Pressure bottles with a maximum filling pressure of 350 bar are available without problems as standard equipment, and pressure bottles of up 800 bar are even available for trials purposes. 
         [0032]    In one advantageous refinement, a mixture comprising 30 to 70% by volume of nitrogen and 20 to 80% by volume of methane is used as a fluid. Even this simple mixture results in operating temperatures of an IR detector to be cooled of below 100 K and extends its running time by a factor of 2, in comparison to the use of pure nitrogen. Ethane is preferably added as a further secondary component to this fluid, making up a proportion of 10 to 40% by volume. The other components, that is to say nitrogen and methane, in this case make up proportions of 20 to 40% by volume and 10 to 40% by volume, respectively. Once again, the boiling point remains below 100 K here, although the running time extension is in this case more than a factor of three. 
         [0033]    A mixture comprising 30 to 70% by volume of nitrogen, 15 to 35% by volume of ethane and 15 to 35% by volume of propane has been found to be another suitable mixture for use as the fluid. In order to further reduce the achievable low temperature and greater cooling capacity, a proportion of 10 to 30% by volume of methane can be added to this mixture, as a further component. The other components, that is to say nitrogen, ethane and propane, in this case make up proportions of 20 to 70% by volume, 10 to 25% by volume and 10 to 20% by volume, respectively. In all situations which lead to a positive-azeotropic and eutectic mixture, the boiling point of the gas mixture is below 100 K, no solid phase occurs, and the running time is extended by a factor of 3 to 4 in comparison to that of nitrogen, depending on the ambient temperature. 
         [0034]    All the gas mixtures mentioned here for running time extension in Joule-Thomson coolers additionally provide significantly shorter cooling-down times, greater cooling loads and combinations thereof, as a result of the greater cooling capacity, and this may be a critical factor for certain missile types. 
         [0035]    In one alternative refinement, a mixture comprising 45 to 60% by volume of argon and 35 to 50% by volume of methane is used as the fluid. The azeotrope in this mixture, which has a composition of 56% by volume of argon and 44% by volume of methane, admittedly has a slightly higher boiling point of about 96 K than argon (argon has a boiling point of 87.3 K), but the boiling point has been reduced sufficiently in comparison to methane such that a wet vapor mixture of the azeotropic composition occurs in the expansion area, with a boiling point of below 100 K. This results in a fluid which can be used, and which makes it possible to reduce the temperature of a detector to the desired operating point of below 100 K. 
         [0036]    Further details relating to fluids based on nitrogen, with alkanes being used as secondary components, can be found in particular in GB 1 3336 892. The compositions described there are, however, specified for use in a cooling circuit with low compression pressures, in particular for closed circuits with a maximum pressure of 30 bar. It is not possible to predict their characteristics when used in non-equilibrium conditions in an open Joule-Thomson cooler. 
         [0037]    The fluids used do not exhibit ideal behavior. Particularly at low temperatures, such as those which can occur during use of a guided missile, significant pressure reductions occur in a pressure bottle. Particularly at low temperatures of down to neg. 45° C. which missiles can be reduced to, this results in noticeable cooling performance losses since the pressure difference for the gas which is expanded in this way is reduced. In one advantageous refinement of the invention, the pressurized fluid is therefore temperature-stabilized. This is achieved, for example, by means of heating mats or integrated heating elements, Peltier elements or by means of existing dissipative heat sources, such as electronics, in which case, by way of example, heat tubes can be used for heat transport. Temperature-stabilizing elements such as these are used in particular when the cooling power losses that occur as a result of the pressure loss cannot be compensated for in situ by the ambient temperature, which is low in any case. If temperature stabilization takes place, then the pressure loss in the real mixture can be compensated for, thus leading to a further increase in the running time of the Joule-Thomson cooler. In general, the Joule-Thomson effect with the gases that are used increases as the temperatures become lower. The pressure drop in the case of a relatively cool environment can be compensated for once again by this effect (partially). However, additional temperature stabilization of the pressure vessel with a relatively cool Joule-Thomson cooler will then additionally contribute to a further running-time extension. 
         [0038]    When the fluid from a pressurized pressure bottle is expanded, then the heating means to be used for temperature stabilization must necessarily be arranged such that they act on the pressure bottle. By way of example, heating mats or the like can surround the pressure bottle. 
         [0039]    When heating elements are used for temperature stabilization, they can lead to a pressure increase as the pressure in the pressure bottle decreases, and they can therefore be used to increase the cooling performance of the Joule-Thomson cooler. This is achieved in particular by the heating elements heating the pressurized fluid above the ambient temperature. A value of about 50° C. has been found to be particularly suitable for this purpose in practice. 
         [0040]    None of the fluid compositions mentioned is toxic; however, in certain mixture ratios, they are explosive when introduced into air containing oxygen. In order to reduce this explosive behavior, it is advantageous to admix heptafluoropropane with a content of between 5 and 15% by volume to the fluid as a further component. In addition, alternatively or in combination, tetrafluoromethane can be admixed to make up a content of between 3 and 20% by volume. Both components are licensed as flame-resistant agents and can be used to replace bromium trifluoromethane, which is no longer permissible for environmental reasons (in accordance with the Montreal Agreement). These two components are therefore used to constrain/suppress burning of the combustible alkanes. Since, in particular, tetrafluoromethane has a quite high cooling capability, this component can also be used to extend the running time of the Joule-Thomson cooler and to decrease any possible explosion risk of the alkane mixture that is used. 
         [0041]    In one preferred refinement, the expanded gas flows in the opposite direction to that before it was expanded, in order to cool the pressurized fluid. This refinement results in the initially mentioned reverse-flow cooling, with the fluid which is supplied to the high-pressure side of the expansion nozzle or restrictor being cooled down by the returning gas flowing in the opposite direction through an appropriate reverse-flow heat exchanger, before finally emerging into the environment. 
         [0042]    In one advantageous development of the method, a further pressurized fluid is expanded, with the expanding gas of the further fluid being used to cool the pressurized fluid before it is expanded. This measure results in a multistage Joule-Thomson cooler with the fluid that is used to cool the detector exchanging heat with the expanding gas and the further fluid cooled down in this way, before emerging from the expansion nozzle. This initial cooling means that the fluid which is used to cool the detector can be cooled down to a very low temperature even before it is expanded, as a result of which the further reduction in its temperature that results from this expansion process in comparison to the inversion temperature results in an improvement in the cooling performance. 
         [0043]    In particular, the fluid which is used for cooling can be precooled by the first expansion stage to such an extent that, at the low temperature which then occurs adjacent to the expansion nozzle or restrictor, the subsequent isoenthalpy expansion results in a specific cooling power which is sufficiently great that the detector is cooled down from room temperature to the required detector temperature of below 100 K within a short time. This two-stage embodiment of the cooler makes it possible in particular to achieve cooling times for cooling down from 295 K to below 100 K of less than two seconds. The latter is now a requirement for guided missiles whose IR detectors must be cooled down to the operating temperature within this time in order, for example, to make it possible to detect targets flying at supersonic speed, sufficiently quickly. 
         [0044]    Since the fluid which is used for cooling is precooled even before it is expanded, there is no longer any need to carry out additional precooling by means of a reverse-flow heat exchanger using the returning expanded gas. In particular, the fluid which has been expanded and has been cooled down to its boiling point can be sprayed onto the rear face of the detector to be cooled, during the expansion process, in the form of a spray coolant. The latter allows a cooler design without any mechanical link to a moving detector. 
         [0045]    However, the temperature of the fluid can be reduced further before its expansion by the expanded gas of the further fluid flowing in the opposite direction to that before it was expanded, in order to cool the pressurized fluid. In this case, the fluid which is used for cooling not only makes thermal contact with the vapor area of the first expansion stage before its expansion, but its inlet is additionally cooled by the expanded gas of the further fluid flowing back. 
         [0046]    In a further preferred refinement, the expanded gas of the further fluid flows in the opposite direction to that before it was expanded, also in order to cool the pressurized further fluid. 
         [0047]    This makes it possible to further reduce the temperature which can be achieved in the vapor area of the first expansion stage. 
         [0048]    Because of the high integral Joule-Thomson coefficients, methane and in particular tetrafluoromethane (also referred to as tetrafluorocarbon) can be used as the further fluid for the first cooler stage. However, argon can also be used for the same reason. Argon has a cooling capacity that is greater than that of nitrogen by a factor of 1.5. 
         [0049]    In an alternative refinement, the mixture as described above and which forms a positive azeotrope, comprises argon or nitrogen as a main component and at least one alkane as a secondary component, is also used for the further fluid, because of the low temperatures that can be achieved and the high cooling capacity. In this case, the described refinements and compositions can likewise preferably be used for the mixture. 
         [0050]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0051]    Although the invention is illustrated and described herein as embodied in method for cooling a detector, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0052]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0053]      FIG. 1  is a schematic view of a Joule-Thomson cooler; 
           [0054]      FIG. 2  is a cross section taken through the technical implementation of a Joule-Thomson cooler; 
           [0055]      FIG. 3  is a graph illustrating the enthalpy profile during the expansion process in an open Joule-Thomson cooler; and 
           [0056]      FIG. 4  is a schematic view of a two-stage Joule-Thomson cooler. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]    Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, the schematic shows the design of an open Joule-Thomson cooler  1  for cooling an IR detector  2 . A pressurized fluid flows from a pressure bottle or pressure tank  4  via an inlet valve  6  to an inlet path  7  to a counter-flow or reverse-flow cooler  10 . The temperature of the fluid is thereby decreased in comparison to the temperature in the pressure bottle  4 , by means of the cooler return  14 . The pressurized fluid is expanded via a restrictor  11  which, in particular, is in the form of a nozzle. The expanding gas enters an expansion area or vapor area  13  where it is cooled down as a consequence of the expansion process. As a result of the temperature reduction, a dynamic equilibrium is created between a gas phase  16  and a liquid phase  17  in the expansion area  13  at the boiling point of the azeotropic composition, cooling down the IR detector which is arranged at the bottom of the expansion area  13 , by means of a thermal contact. In this case, a temperature close to the boiling point is achieved as the low temperature. 
         [0058]    Gas of the composition of the gas phase flows out of the expansion area via a return path  14  through the reverse-flow cooler  10 , cooling the fluid as it flows in. After passing through the return path  14 , the expanded gas is exhausted to the environment through an outlet  18 . 
         [0059]      FIG. 2  shows a cross section of a technical implementation of an open, flow-controlled Joule-Thomson cooler  1 ′. In this case, the IR detector  2  to be cooled adjoins the inner wall of a Dewar vessel  19 . The interior of the Dewar vessel  19  is evacuated, thus providing good thermal insulation with respect to thermal conduction and radiation to the environment. A connecting stub  20  extends into the internal area of the Dewar vessel  19  and is provided with a flange  22  for attachment. A gas supply line  23  is arranged in the connecting stub  20  and is connected to a pressure bottle in order to supply with a pressurized fluid. The pressurized fluid flows along the lines which helically surround the connecting stub  20  and form the inlet path  7 , to the expansion nozzle  11  where the fluid is expanded. The emerging gas expands into the expansion area  13 . 
         [0060]    Gas in the gas phase flows out of the expansion area  13  via the lines which form the inlet flow path, thus forming the return path  14 , and are passed to the exterior of the upper end of the Dewar vessel  19 . The inlet flow is therefore cooled by the flow in the opposite direction. 
         [0061]    The method of operation of an open Joule-Thomson cooler as shown in  FIGS. 1 and 2  will be explained by means of the temperature-entropy graph (for argon as an example) illustrated in  FIG. 3 . The graph shows the states which occur during the expansion process in the Joule-Thomson cooler, annotated with the letters “A” to “D”. The associated points are marked in a corresponding manner in the schematic illustration of the Joule-Thomson cooler in  FIG. 1 . 
         [0062]    The entropy of the system is plotted on the abscissa of the graph. The system temperature or system lines of equal enthalpy are marked on the ordinate. The graph also shows isobars with a pressure of p=1000 bar, p=500 bar, p=300 bar and p=1 bar. The curve profiles of constant enthalpy are also shown on the graph. 
         [0063]    Starting with a fluid which has been pressurized to a pressure of p=500 bar and is at a temperature of 350 K at the point B, the fluid flows, as shown in  FIG. 1 , through the inlet path  7 , where it is pre-cooled by the expanded and cooled-down gas flowing back in the opposite direction. The pressure along the inlet path  7  to the expansion nozzle  11  can in this case be considered to be constant. In consequence, the system as shown in  FIG. 3  starts from the point B and moves on a curve of constant pressure of p=500 bar to a point C of low temperature. 
         [0064]    The fluid is expanded at the expansion nozzle  11 . The emerging gas expands as shown in  FIG. 1  into the expansion area  13 . During this expansion process, the gas is cooled down along a curve of constant enthalpy. The system state in this case moves as shown in  FIG. 3  from point C to point D in the wet vapor region, with the gas emerging partially in the liquid aggregate state. Based on the lever law, an amount of liquid in the ratio D-D″ and a corresponding amount of gas in the ratio D-D′ are produced. In the expansion area, the liquid phase exists in accordance with the state point D′ in an equilibrium with the gas phase D″. The detector  2 , which makes thermal contact with the expansion area  13 , is cooled down to a temperature of below 100 K largely by the amount of liquid. 
         [0065]    The gas flows from the gas phase D″ at a normal pressure of about p=1 bar outwards by the return path  14 . During the process, the gas flowing out in the return path  14  is heated by heat dissipation from the fluid flowing in the inlet path  7 . As shown in  FIG. 3 , the system moves in a corresponding manner on a curve of constant pressure from P=1 bar to the point A at the ambient temperature of 350 K. 
         [0066]    If the curve of constant enthalpy is considered, starting from the point B, then this results in the point E. After the gas emerges from the Joule-Thomson cooler, the overall system has an increased enthalpy of the point A. The reversible cooling power of the Joule-Thomson cooler is calculated from the enthalpy difference at the points A and E. This enthalpy difference is in the ideal case taken from the detector as cooling power and from the environment as dissipated energy. 
         [0067]    A number of experiments were carried out using a Joule-Thomson cooler as shown in  FIG. 2 , with different fluid mixtures in a temperature range between −54° C. and +70° C. In this case, a pressure tank was used with a volume of 415 ccm at an initial pressure of 345 bar, and at a temperature of 220. A fluid I with 30% by volume of nitrogen, 30% by volume of methane, 20% by volume of ethane and 20% by volume of propane, as well as a fluid II with a proportion of 30% by volume of nitrogen, 35% by volume of methane and 35% by volume of ethane were investigated as fluid mixtures. In contrast to argon and air as pure cooling gases, the behavior of the fluid mixtures was now investigated in terms of the running time of the Joule-Thomson cooler. The running time was in this case investigated with a pressure bottle at temperatures of −54° C., +22° C. and +70° C. A glass Dewar was used as the Dewar vessel  19  as shown in  FIG. 2 , in order to analyze the processes in the expansion area  13 . 
         [0068]    The same experiments were carried out by a fluid III with a composition of 56% by volume of argon and 44% by volume of methane, as well as a fluid IV composed of a mixture of 70% by volume of nitrogen and 30% by volume of methane. 
         [0069]    The result of these experiments is that it can be stated that a running time extension is found in all the investigated temperature ranges with the fluids I, II, III and IV that were used, with an achieved cooling temperature below 100 K, in comparison to air and argon. In this case, the fluid I exhibited the greatest running time extension. The extension factor was in this case 2.6; 4.4 and 4.4, respectively, in comparison to air and 1.9; 2.7 and 2.9, respectively, in comparison to argon at the temperatures −45° C., +22° C. and +70° C. At the investigated temperature of 22° C., in comparison to argon, the fluid II resulted in a running time extension by a factor of 2.4, with a factor of 4.0 in comparison to air. 
         [0070]    Overall, it was possible to achieve running times of between 4 and 8 hours using the reference cooler comprising a pressure vessel of only 415 cm 3  and with an initial pressure of 340 bar at room temperatures, and of between 4 and 11 hours at temperatures of +70° C. The running times could be increased even further by higher initial pressures and temperature-stabilized pressure vessels. 
         [0071]      FIG. 4  shows, schematically, the design of a two-stage Joule-Thomson cooler  38  with a fluid which cools an IR detector  80  by means of expansion and comprises a mixture forming a positive azeotrope being initially cooled by expansion cooling of a further fluid. 
         [0072]    The Joule-Thomson cooler  38  illustrated in  FIG. 4  is split, in order to assist understanding, into two coolers  40  and  42 , but these should not be confused with the expansion stages. 
         [0073]    The first cooler  40  is in this case operated with a mixture, forming a positive azeotrope, from a compressed-gas container  44 . The mixture used in the compressed-gas container  44  is at ambient temperature and at a pressure of 200-500 bar. The mixture is passed by a valve  46  and a straight line  48  running through the cooler  42  to an inlet path  50  of a heat exchanger  51  of the cooler  40 . The first cooler  40  is an expansion cooler with an expansion nozzle or restrictor  52 . The restrictor  52  is connected to the output of the inlet path  50  via a high-pressure line  54 . The high-pressure line  54  is provided with thermal insulation  56 . 
         [0074]    The second cooler  42  is operated with tetrafluoromethane from a compressed-gas container  58 . The tetrafluoromethane in the compressed-gas container  58  is likewise of ambient temperature and at a pressure of 200-350 bar. The tetrafluoromethane is passed via a valve to the input  62  of an inlet path  64  of a reverse-flow heat exchanger  66  in the second cooler  42 . A line  70  passes from the output  68  of the inlet path  64  of the reverse-flow heat exchanger  66  straight through the second cooler  40  to a restrictor or expansion nozzle  72 . The restrictor  72  is seated at the end of the first cooler  40  that is remote from the second cooler  42 . The tetrafluoromethane, which is at high pressure, emerges from the restrictor  72 . In the process, it is expanded and is cooled down. The expanded and cooled-down tetrafluoromethane now flows through a return path  74  through the heat exchanger  51  in the first cooler  40  in the opposite direction to the mixture which is flowing in and forms a positive azeotrope. This mixture is therefore precooled in the first cooler  40  by the expanded tetrafluoromethane wet vapor, but not by the expanded mixture itself. The expanded tetrafluoromethane then flows through a return path  76  through the reverse-flow heat exchanger  66  in the second cooler  42 . Here, the tetrafluoromethane which is flowing in and is at high pressure is precooled by the expanded and cooled-down tetrafluoromethane. The expanded tetrafluoromethane emerges from the return path  76 , at an outlet  78 . 
         [0075]    The mixture which flows out, is used for cooling and forms a positive azeotrope, is aimed in a jet at an IR detector  80  which is arranged in a moving mount  82 . The expanding gas from this mixture then emerges from the mount  82  through an aperture  84 . 
         [0076]    The two coolers  40  and  42  are surrounded by a casing  86  which is closed on the object side by an end wall  88 . The thermally insulated high-pressure line  54  is passed through the end wall  88 . 
         [0077]    The fluid III as described above and as investigated, and comprising 56% by volume of argon and 44% by volume of methane is particularly suitable for use as a mixture for cooling down the IR detector  80 . This mixture has a boiling point of about 96 K (at 1 bar) and a melting point of less than 75 K. The cooling power is better than that of argon by a factor of about 2. The second expansion stage (associated with the first cooler  40 ) can also be operated with a mixture comprising 30-70% by volume of nitrogen, 15-35% by volume of propane and 15-35% by volume of ethane. A mixture comprising 40% by volume of nitrogen, 30% by volume of propane and 30% by volume of ethane results, in comparison to nitrogen, in a cooling capacity that is about 3 to 7 times greater with a boiling point of only 78 K (at 1 bar). No freezing of the expansion nozzle was found. In comparison to the argon which was also used, the mixed gas resulted in a somewhat higher boiling point, with a cooling capacity that was better by a factor of 2 to 4.5 times. 
         [0078]    Furthermore, it is also possible to use a mixture comprising 50-64% by volume of nitrogen and 36-50% by volume of methane. A mixture with the same proportions of nitrogen and methane has a boiling point of 82 K (at 1 bar). The mixture remains liquid at the boiling point of nitrogen. As our own measurements have shown, the mixture results in a cooling capacity which is about twice as good as that of pure nitrogen. 
         [0079]    Furthermore, it is also possible to use a mixture comprising 20-70% by volume of nitrogen, 20-40% by volume of methane and 10-40% by volume of ethane. Since methane is soluble in liquid nitrogen, ethane is soluble in liquid methane, and ethane and propane are soluble in one another, this mixture has an even better cooling capacity. In particular, a mixture comprising 30% molar of nitrogen and 35% molar of methane and ethane, respectively, has a cooling capacity which is 4 to 9 times greater than that of nitrogen. The boiling point of this mixture is about 80 K. This mixture behaves like an azeotropic mixture, and has the characteristics of a virtually eutectic mixture, since no freezing occurs at the low boiling point. 
         [0080]    Furthermore, a mixture comprises 20-70% by volume of nitrogen, 10-30% by volume of methane and 10-25% by volume of ethane and propane, respectively, also has good characteristics. The boiling point of a mixture comprising 30% by volume of nitrogen, 30% by volume of methane and 20% by volume of ethane and propane, respectively, is about 80 K (at 1 bar). The cooling capacity is better than that of nitrogen by a factor of 7 to 12.