Patent Publication Number: US-2020283667-A1

Title: Refrigerant

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2019 105 682.9 filed Mar. 6, 2019. The contents of this application are hereby incorporated by reference as if set forth in its entirety herein. 
     The disclosure relates to a refrigerant for a cooling device and to a test chamber with the refrigerant and to a use of a refrigerant, the refrigerant for a cooling device which comprises a cooling circuit with at least one heat exchanger in which the refrigerant undergoes a phase transition consisting of a refrigerant mixture composed of a mass fraction of carbon dioxide, a mass fraction of difluoromethane and a mass fraction of at least one other component. 
     Refrigerants of this kind typically circulate within a closed cooling circuit of cooling devices and undergo a sequence of different changes in state of matter. Refrigerants should be of such a nature that they can be used in a cooling circuit within a predefined temperature difference. Single-component refrigerants and refrigerant mixtures of at least two components are known from the state of the art. The refrigerants are classified according to the latest version of German industry standard DIN 8960 Section 6 as at the priority date. 
     As per statutory regulations, a refrigerant must not significantly contribute to the depletion of ozone in the atmosphere or to global warming. 
     This means that essentially no fluorinated or chlorinated substances are to be used as refrigerants, which is why natural refrigerants or gasses are an option. Moreover, a refrigerant should be nonflammable in order to not complicate filling, shipping and operation of a cooling circuit because of any safety regulations that may have to be observed. Also, production of a cooling circuit becomes more expensive if a flammable refrigerant is used because of the constructional measures required in that case. Flammability refers to the refrigerant&#39;s property of reacting to ambient oxygen by releasing heat. A refrigerant is flammable in particular if it is classified in fire class C of European standard EN2 and DIN 378 classes A2, A2L and A3 in their latest versions as at the priority date. 
     Moreover, a refrigerant should have a relatively low CO 2  equivalent; i.e., a relative global warming potential (GWP) should be as low as possible in order to avoid indirect damage to the environment in case the refrigerant is released. The GWP indicates how much a defined mass of a greenhouse gas contributes to global warming, carbon dioxide serving as the reference value. The value describes the mean warming effect over a specific period, 100 years being set here for the sake of comparability. For a definition of the relative CO 2  equivalent or GWP, reference is made to Intergovernmental Panel on Climate Change (IPCC), Assessment Report, Appendix 8.A, Table 8.A.1 in the latest version as at the priority date. 
     Refrigerants with a low GWP, such as &lt;2500, have the disadvantage that these refrigerants tend to have a significantly lower cold capacity in the temperature ranges relevant for a cooling circuit than refrigerants with a comparatively higher GWP. A lower GWP can be achieved with refrigerant mixtures that have a comparatively high mass fraction of carbon dioxide; however, these refrigerant mixtures may have zeotropic properties due to the different substances mixed, which is undesirable in many cooling circuits. 
     In a zeotropic refrigerant mixture, a phase transition happens across a temperature range which is known as the temperature glide. The temperature glide refers to a difference between the boiling temperature and the dew point temperature at constant pressure. Zeotropic refrigerant mixtures typically contain a high mass fraction of a nonflammable component, which is characterized by a comparatively high GWP, however. At first glance, carbon dioxide appears to be a suitable component for a refrigerant mixture because it is nonflammable and has a low GWP. In a mixture of carbon dioxide with another component, however, it is essential that a mass fraction of a carbon dioxide has to be comparatively large if the other component is flammable. This is disadvantageous, however, because carbon dioxide has a freezing temperature or freezing point of −56.6° C., which hardly allows temperatures of up to −60° C. to be achieved at a high carbon dioxide concentration. 
     Also, the use of refrigerants should be as simple as possible, i.e., not require extensive technical restructuring of a cooling device. With refrigerants having a temperature glide of &gt;3 K in particular, an expansion element and a heat exchanger or evaporator of the cooling circuit in question have to be adjusted to the evaporation temperature of the refrigerant and corresponding control has to be provided. Furthermore, a distinction must be drawn between refrigerants that are designed for static operation of a cooling device, i.e., a cooling device having a temperature at the heat exchanger or evaporator that is substantially constant over a longer period of time, and refrigerants that are designed for a dynamic cooling device, which exhibits comparatively quick temperature changes at the heat exchanger. Dynamic cooling devices of this kind are integrated in test chambers, for example, which means that a refrigerant used has to be usable within a large temperature range. 
     Test chambers are typically used to test physical and/or chemical properties of objects, in particular devices. For instance, temperature test chambers or climate test chambers in which temperatures in a range of −60° C. to +180° C. can be set are known. In climate test chambers, desired climatic conditions can additionally be set, to which the device or the test material is then exposed for a defined period of time. Test chambers of this kind are often or sometimes realized as mobile devices which are merely connected to a building via required supply lines and comprise all modules needed to control the temperature and climate. The temperature of a test space holding the material to be tested is typically controlled in a circulating air duct within the test space. The circulating air duct forms an air treatment space in the test space, in which heat exchangers for heating or cooling the air flowing through the circulating air duct and the test space are disposed. A fan or ventilator aspirates the air located in the test space and directs it to the respective heat exchangers in the circulating air duct. In this way, the test material can be temperature-controlled or exposed to a defined temperature change. During a test interval, a temperature can repeatedly change between a maximum temperature and a minimum temperature of the test chamber. A test chamber of this kind is known from EP 0 344 397 A2, for example. 
     The refrigerant circulating in a cooling circuit must be of such a nature that it can be used in the cooling circuit within the aforementioned temperature difference. In particular, a dew point temperature of the refrigerant cannot be higher than a minimum temperature of the temperature range of the cooling circuit that is to be achieved because the minimum temperature would not be achievable otherwise when the refrigerant is evaporated in the heat exchanger serving to cool the test space. The dew point temperature of azeotropic refrigerants is reached immediately behind the expansion element in the heat exchanger. Straight cooling circuits for test spaces require a very high spatial temperature stability to precisely control the temperature of the test chamber, which cannot be achieved at all or only to a limited degree using zeotropic refrigerants. High temperature stability cannot be achieved in this case because the dew point temperature or a dew point of the zeotropic refrigerant may locally shift as a function of a temperature in the test space in the area of the heat exchanger in the test space because of temperature differences. Hence, a use of zeotropic refrigerants, i.e., of refrigerants having a temperature glide, in cooling circuits of test chambers is avoided. 
     Furthermore, cooling devices in which a zeotropic refrigerant mixture is successively evaporated are known. This means that components of the refrigerants are evaporated one after the other by means of an expansion element. Cooling devices of this kind are also referred to as mixed fluid cascade systems and are suitable for realizing a substantially static cryogenic temperature. 
     WO 2017/157864 A1 discloses a refrigerant which contains carbon dioxide and pentafluoroethane among other components. Ranges of 30 to 70 wt % for carbon dioxide and 20 to 80 wt % for pentafluoroethane are indicated for the refrigerant, for example. Difluoromethane as a mixing partner is also disclosed. 
     DE 41 16 274 A1 relates to a refrigerant which contains carbon dioxide and difluoromethane as mixing partners. Mass fractions of 5 to 50 wt % of carbon dioxide and 25 to 70 wt % of difluoromethane are indicated, for example. 
     Hence, the object of the present disclosure is to propose a refrigerant for a cooling device, a test chamber with a refrigerant, and a use of a refrigerant by means of which temperatures up to at least −60° C. can be achieved in an environmentally friendly and safe manner. 
     This object is attained by a refrigerant having the features of claim  1 , a test chamber having the features of claim  16  and a use of a refrigerant having the features of claim  17 . 
     In the refrigerant according to the disclosure for a cooling device comprising a cooling circuit with at least one heat exchanger, the refrigerant undergoes a phase transition in the heat exchanger, the refrigerant being a refrigerant mixture composed of a mass fraction of carbon dioxide, a mass fraction of difluoromethane and a mass fraction of at least one other component, wherein the mass fraction of carbon dioxide in the refrigerant mixture is 9 to 75 mass percent, the mass fraction of difluoromethane being up to 70 mass percent, the other component being 2,3,3,3-tetrafluoropropene, the mass fraction of 2,3,3,3-tetrafluoropropene being up to 47 mass percent. 
     Carbon dioxide (CO 2 ) is also known as a refrigerant or component under the designation R744, pentafluoroethane (C 2 HF 5 ) is known under the designation R125, difluoromethane (CH 2 F 2 ) is known under the designation R32, 2,3,3,3-tetrafluoropropene (C 3 H 2 F 4 ) is known under the designation R1234yf, 1,3,3,3-tetrafluoropropene (C 3 H 2 F 4 ) is known under the designation R1234ze, 1,1,1,2-tetrafluoroethane (C 2 H 2 F 4 ) is known under the designation R134a, fluoromethane (CH 3 F) is known under the designation R41, ethane (C 2 H 6 ) is known under the designation R170, 1,1-difluoroethene (C 2 H 2 F 2 ) is known under the designation R1132a, ethene (C 2 H 4 ) is known under the designation R1150), fluoroethene (C 2 H 3 F) is known under the designation R1141, propane (C 3 H 8 ) is known under the designation R290, propene (C 3 H 6 ) is known under the designation R1270 and fluoroethane (CH 2 FCH 3 ) is known under the designation R161 according to the latest version of German industry standard DIN 8960 as at the priority date of the application. 
     The disclosure provides a refrigerant mixture of carbon dioxide and one or more fluorinated refrigerants which have a low GWP and are nonflammable or flammable to a limited degree only. A fraction of carbon dioxide has to be as low as possible because otherwise a freezing point of the refrigerant mixture would rise with an increasing mass fraction of carbon dioxide. However, a lower mass fraction of carbon dioxide reduces a GWP-reducing effect of the carbon dioxide. This is why partly fluorinated refrigerants have a significantly higher GWP than carbon dioxide, while also having an improved flame-retardant effect. Pentafluoroethane, difluoromethane and 2,3,3,3-tetrafluoropropene in particular contain significant amounts of fluorine atoms, which leads to an undesirably high GWP. As was surprisingly found, however, a sufficiently low GWP, i.e., &lt;1597, for example, can be achieved with a refrigerant mixture containing a mass fraction of carbon dioxide of 9 to 75 mass percent with up to 70 mass percent of difluoromethane and up to 47 mass percent of 2,3,3,3-tetrafluoropropene. As was also found, a flame-retardant effect of difluoromethane is comparatively greater than that of carbon dioxide. Moreover, the negative properties of difluoromethane and of carbon dioxide can be reduced by adding 2,3,3,3-tetrafluoropropene as a third component of the refrigerant mixture. Thus, a refrigerant mixture containing difluoromethane and 2,3,3,3-tetrafluoropropene can be classified as nonflammable. At the same time, difluoromethane and 2,3,3,3-tetrafluoropropene have a lower freezing temperature with carbon dioxide than with pentafluoroethane alone. 
     Consequently, a mixture of difluoromethane, 2,3,3,3-tetrafluoropropene and carbon dioxide can achieve a lower freezing temperature than pentafluoroethane and carbon dioxide alone. 2,3,3,3-Tetrafluoropropene thus lowers the freezing point of the refrigerant mixture significantly, a certain mass fraction of carbon dioxide being required in order for the refrigerant mixture to be nonflammable. At the same time, however, 2,3,3,3-tetrafluoropropene leads to a high final compression temperature and is flammable, which is why 2,3,3,3-tetrafluoropropene is suitable only within limits as a sole mixing partner for carbon dioxide. Difluoromethane cannot lower a freezing point of the refrigerant mixture as far as 2,3,3,3-tetrafluoropropene, but has a greater flame-retardant effect than carbon dioxide, which is advantageous. 
     Consequently, the refrigerant mixture may be a ternary or quaternary mixture. 
     Advantageously, a mass fraction of carbon dioxide in the refrigerant mixture may be 25 to 55 mass percent. 
     Particularly advantageously, a mass fraction of carbon dioxide in the refrigerant mixture may be 35 to 45 mass percent. 
     A mass fraction of carbon dioxide in the refrigerant mixture may be 10 to 70 mass percent, a mass fraction of difluoromethane may be 10 to 70 mass percent, and a mass fraction of 2,3,3,3-tetrafluoropropene may be up to 45 mass percent, wherein the refrigerant may have a mass fraction of pentafluoroethane, in which case the mass fraction of pentafluoroethane may be up to 35 mass percent. With this mass fraction of 2,3,3,3-tetrafluoropropene, the carbon dioxide is mixable with pentafluoroethane in a particularly advantageous manner. A freezing point of the refrigerant mixture can be significantly reduced by adding the mentioned components. This reduction can be set in such a manner that the freezing point of the refrigerant mixture is lower than the intended evaporation temperature and, at the same time, the vapor pressure associated with the evaporation temperature may be higher or only slightly lower than the ambient pressure. 
     Advantageously, a mass fraction of carbon dioxide in the refrigerant mixture may be 25 to 55 mass percent, a mass fraction of difluoromethane may be 25 to 55 mass percent, a mass fraction of 2,3,3,3-tetrafluoropropene may be up to 30 mass percent, and a mass fraction of pentafluoroethane may be up to 20 mass percent. 
     Particularly advantageously, a mass fraction of carbon dioxide in the refrigerant mixture is 35 to 45 mass percent, a mass fraction of difluoromethane is 35 to 45 mass percent, a mass fraction of 2,3,3,3-tetrafluoropropene is 11 to 21 mass percent, and a mass fraction of pentafluoroethane is up to 10 mass percent. 
     Alternatively, a mass fraction of carbon dioxide in the refrigerant mixture may be 9 to 69 mass percent, a mass fraction of difluoromethane may be up to 53 mass percent, a mass fraction of 2,3,3,3-tetrafluoropropene may be up to 40 mass percent, and a mass fraction of pentafluoroethane may be up to 50 mass percent, wherein the refrigerant may have a mass fraction of 1,1,1,2-tetrafluoroethane, in which case the mass fraction of tetrafluoroethane may be up to 40 mass percent. Pentafluoroethane is nonflammable, which means that all mixtures containing pentafluoroethane and carbon dioxide in the minimum amounts indicated are nonflammable. The freezing point is not reduced as far compared to difluoromethane alone. Its GWP of 3150 is significantly higher than that of other possible components. Hence, it may also be partly replaced with other substances in the refrigerant mixture in order to reduce the GWP of the refrigerant mixture. The flame-retardant effect of pentafluoroethane is greater than that of carbon dioxide, which means that a mass fraction of carbon dioxide in the refrigerant mixture can be reduced, which lowers the freezing point further and still ensures non-flammability but increases the GWP. 
     Advantageously, a mass fraction of carbon dioxide in the refrigerant mixture may be 24 to 54 mass percent, a mass fraction of difluoromethane may be 8 to 38 mass percent, a mass fraction of 2,3,3,3-tetrafluoropropene may be up to 24 mass percent, a mass fraction of pentafluoroethane may be 5 to 35 mass percent, and a mass fraction of tetrafluoroethane may be up to 24 mass percent. 
     Particularly advantageously, a mass fraction of carbon dioxide in the refrigerant mixture is 34 to 44 mass percent, a mass fraction of difluoromethane is 18 to 28 mass percent, a mass fraction of 2,3,3,3-tetrafluoropropene is 4 to 14 mass percent, a mass fraction of pentafluoroethane is 15 to 25 mass percent, and a mass fraction of tetrafluoroethane is 4 to 14 mass percent. In this case, a GWP of the refrigerant can be reduced even further. 
     Thus, a mass fraction of refrigerant R410A in the refrigerant mixture may be used at least partially. Refrigerant R410A contains equal mass fractions of pentafluoroethane and difluoromethane. Refrigerant R410A is a ready-made refrigerant mixture easily available for purchase, which means that the refrigerant can be prepared in a cost-effective and simple manner by simply partially mixing carbon dioxide with R410A and 2,3,3,3-tetrafluoropropene. 
     Alternatively, a mass fraction of carbon dioxide in the refrigerant mixture may be 15 to 75 mass percent, a mass fraction of difluoromethane may be 8 to 68 mass percent, and a mass fraction of 2,3,3,3-tetrafluoropropene may be up to 47 mass percent. Advantageously, a mass fraction of carbon dioxide in the refrigerant mixture may be 30 to 60 mass percent, a mass fraction of difluoromethane may be 23 to 53 mass percent, and a mass fraction of 2,3,3,3-tetrafluoropropene may be up to 32 mass percent. 
     Particularly advantageously, a mass fraction of carbon dioxide in the refrigerant mixture is 40 to 50 mass percent, a mass fraction of difluoromethane is 33 to 43 mass percent, and a mass fraction of 2,3,3,3-tetrafluoropropene is 9 to 22 mass percent. 
     In this case, the refrigerant mixture may consist solely of carbon dioxide, difluoromethane and 2,3,3,3-tetrafluoropropene. A mass fraction of carbon dioxide of at least 35 mass percent is sufficient for the refrigerant mixture to be classified as nonflammable. 
     Furthermore, the refrigerant may have a mass fraction of 1,3,3,3-tetrafluoropropene, in which case a mass fraction of 1,3,3,3-tetrafluoropropene may be up to 35 mass percent, preferably up to 20 mass percent, particularly preferably up to 10 mass percent. 
     Thus, it is also particularly advantageous if the refrigerant mixture contains up to 30 mass percent, preferably up to 20 mass percent, particularly preferably up to 10 mass percent, each of fluoromethane, ethane, 1,1-difluoroethene, ethene, fluoroethene, ethyne, propane, propene and/or fluoroethane as an additional component. Improved properties of the refrigerant can be achieved even with this comparatively low mass fraction of said component(s). 
     In Tables 1, 2 and 3, examples of refrigerants according to the embodiments described above are indicated. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 R744 
                 R32 
                 R125 
                 R1234yf 
                   
                 Boiling 
                   
               
               
                 Refrig- 
                 [mass  
                 [mass  
                 [mass  
                 [mass  
                 GWP 
                 point  
                 Glide 
               
               
                 erant 
                 %] 
                 %] 
                 %] 
                 %] 
                 [−] 
                 [° C.] 
                 [° C.] 
               
               
                   
               
             
            
               
                 1 
                 35-45 
                 35-45 
                 0-10 
                 11-21  
                 237-655  
                 −75.8  
                 16.8-18.3 
               
               
                   
                   
                   
                   
                   
                   
                 to  
                   
               
               
                   
                   
                   
                   
                   
                   
                 −78.8 
                   
               
               
                 2 
                 25-55 
                 25-55 
                 0-20 
                 &gt;0-30   
                 170-1072 
                 −75.1  
                 15.2-18.3 
               
               
                   
                   
                   
                   
                   
                   
                 to  
                   
               
               
                   
                   
                   
                   
                   
                   
                 −80.6 
                   
               
               
                 3 
                 10-70 
                 10-70 
                 0-35 
                 &gt;0-45   
                  69-1597 
                 −67.3  
                 10.4-17.8 
               
               
                   
                   
                   
                   
                   
                   
                 to  
                   
               
               
                   
                   
                   
                   
                   
                   
                 −83.5 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 R744 
                 R32 
                 R125 
                 R1234yf 
                 R1234a 
                   
                 Boiling 
                   
               
               
                 Refrig- 
                 [mass  
                 [mass 
                 [mass  
                 [mass 
                 [mass 
                 GWP 
                 point  
                 Glide 
               
               
                 erant 
                 %] 
                 %] 
                 %] 
                 %] 
                 %] 
                 [−] 
                 [° C.] 
                 [° C.] 
               
               
                   
               
             
            
               
                 4 
                 34-44 
                   18-28  
                 15-25  
                 4-14 
                 4-14 
                 738-1231 
                 −79.1  
                 18.8-22   
               
               
                   
                   
                   
                   
                   
                   
                   
                 to  
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 −80.2 
                   
               
               
                 5 
                 24-54 
                   8-38 
                 5-35 
                 &gt;0-24   
                 0-24 
                 291-1684 
                 −77.3  
                 16.6-29.8 
               
               
                   
                   
                   
                   
                   
                   
                   
                 to 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 −81.1 
                   
               
               
                 6 
                  9-69 
                 &gt;0-53 
                 0-50 
                 &gt;0-40   
                 0-40 
                 2-2329 
                 −62.8  
                 11-31 
               
               
                   
                   
                   
                   
                   
                   
                   
                 to  
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 −83.3 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 R744  
                 R32  
                 R1234yf  
                 R1234ze  
                   
                 Boiling  
                 Glide  
               
               
                 Refrig- 
                 [mass  
                 [mass 
                 [mass  
                 [mass 
                 GWP 
                 point  
                 point  
               
               
                 erant 
                 %] 
                 %] 
                 %] 
                 %] 
                 [−] 
                 [° C.] 
                 [° C.] 
               
               
                   
               
             
            
               
                 7  
                 40-50  
                 33-43  
                 12-22  
                   
                 223-292  
                 −77.9  
                 17.4-17.6 
               
               
                   
                   
                   
                   
                   
                   
                 to 
                   
               
               
                   
                   
                   
                   
                   
                   
                 −80.1 
                   
               
               
                 8  
                 30-60  
                 23-53  
                 2-32  
                   
                 156-359  
                 −75    
                 16.5-17.2  
               
               
                   
                   
                   
                   
                   
                   
                 to  
                   
               
               
                   
                   
                   
                   
                   
                   
                 −81.8 
                   
               
               
                 9  
                 15-75  
                 8-68  
                 &gt;0-47    
                   
                  55-460  
                 −67.2  
                 12.3-16.2  
               
               
                   
                   
                   
                   
                   
                   
                 to 
                   
               
               
                   
                   
                   
                   
                   
                   
                 −84.3 
                   
               
               
                 10  
                 40-50  
                 33-43  
                 9-19  
                 0-10  
                 223-292  
                 −78    
                 17.4-22.1  
               
               
                   
                   
                   
                   
                   
                   
                 to 
                   
               
               
                   
                   
                   
                   
                   
                   
                 −79.9 
                   
               
               
                 11  
                 30-60  
                 23-53  
                 &gt;0-30    
                 0-20  
                 156-360  
                 −75.5  
                 16.5-27.2  
               
               
                   
                   
                   
                   
                   
                   
                 to 
                   
               
               
                   
                   
                   
                   
                   
                   
                 −81.7 
                   
               
               
                 12  
                 15-75  
                 18-68  
                 &gt;0-45    
                 0-35  
                 122-461  
                 −67.6  
                 12.3-30.5  
               
               
                   
                   
                   
                   
                   
                   
                 to 
                   
               
               
                   
                   
                   
                   
                   
                   
                 −84.3 
               
               
                   
               
            
           
         
       
     
     In another embodiment, the refrigerant may have a temperature glide in a range of 10.4 K to 31 K at evaporation pressures around 1 bar. As described above, a mixture of carbon dioxide with difluoromethane and 2,3,3,3-tetrafluoropropene has proven particularly advantageous. Furthermore, this refrigerant mixture leads to a concentration-dependent reduction of the freezing point. Hence, flammable and nonflammable refrigerant mixtures for different temperature applications may arise when mass fractions deviate from the indicated mass fractions. 
     The refrigerant may have a relative CO2 equivalent of &lt;2329, preferably &lt;1597, particularly preferably &lt;460, over 100 years. Consequently, the refrigerant may be of little harm to the environment. 
     The refrigerant may be nonflammable. If the refrigerant is nonflammable, the cooling circuit and a test chamber in particular can be designed more cost-effectively because no special safety measures in terms of flammability of the refrigerant will have to be observed. In this case, the refrigerant may at least not be classified in fire class C and/or refrigerant safety group A1. Moreover, shipping and transport of the cooling circuit is easier because the cooling circuit can be filled with the refrigerant before being transported, irrespective of the mode of transport. If a flammable refrigerant is used, filling may not be possible until start-up at the installation site. Furthermore, use of the nonflammable refrigerant in the presence of ignition sources is possible. 
     The test chamber according to the disclosure for conditioning air comprises a test space which serves to receive test material and which can be sealed against an environment and is temperature-insulated, and a temperature control device for controlling the temperature of the test space, a temperature in a temperature range of −70° C. to +180° C. being establishable within the test space by means of the temperature control device, the temperature control device having a cooling device comprising a cooling circuit with a refrigerant according to the disclosure, a heat exchanger, a compressor, a condenser and an expansion element. Regarding the advantages of the test chamber according to the disclosure, reference is made to the description of advantages of the refrigerant according to the disclosure. 
     By means of the temperature control device, a temperature in a temperature range of −80° C. to +180° C., preferably −90° C. to +180° C., particularly preferably −100° C. to +180° C., may be established within the test space. Unlike in a mixed fluid cascade system, the refrigerant with all components contained in the refrigerant can be evaporated at once by means of the expansion element. Since a freezing point of the carbon dioxide is −56.6° C., refrigerant mixtures that contain a large mass fraction of carbon dioxide are no longer suitable for achieving temperatures below −56.6° C., on principle. The use of the refrigerant according to the disclosure, however, allows a dew point temperature of the refrigerant of less than −70° C. to be achieved. 
     The cooling circuit may have an internal heat exchanger, and the internal heat exchanger may be connected to a high-pressure side of the cooling circuit upstream of the expansion element and downstream of the condenser and to a low-pressure side of the cooling circuit upstream of the compressor and downstream of the heat exchanger. By use of the internal heat exchanger and cooling of the liquefied refrigerant of the high-pressure side by means of the internal heat exchanger, temperatures below −56° C. can be reached easily. The evaporation temperature of the refrigerant cooled by means of the internal heat exchanger can be reduced at the expansion element relative to an evaporation temperature of an uncooled refrigerant. The cold capacity transferred from the low-pressure side to the high-pressure side via the internal heat exchanger can thus be used at least in part, preferably exclusively, to reduce the evaporation temperature of the refrigerant at the expansion element. Furthermore, use of a zeotropic refrigerant having a temperature glide is made possible in the first place because the location of the dew point temperature of the refrigerant or the dew point of the refrigerant can be shifted into the internal heat exchanger in this case. As a consequence of the temperature glide of the zeotropic refrigerant, the achieved dew point temperature of the refrigerant may be comparatively high and thus prevent the heat exchanger from cooling further. 
     Hence, only part of the refrigerant may be evaporated in the heat exchanger and the unusable part of the wet vapor portion of the refrigerant can be shifted into the internal heat exchanger. On the whole, this allows refrigerants which contain a mass fraction of carbon dioxide and which, while being environmentally friendly, have zeotropic properties to be used to establish low temperatures in a test space. Moreover, if part of the temperature glide or part of the wet vapor of the refrigerant is shifted from the heat exchanger in the test space into the internal heat exchanger, a comparatively improved temperature stability can be achieved with the zeotropic refrigerant. In this case, a cold capacity output via the heat exchanger can be generated in a section of the temperature glide only, which means that a shift of the dew point of the refrigerant in the cooling circuit has hardly any impact on a temperature stability of the heat exchanger. Furthermore, a single heat exchanger may be used to cool a fluid, i.e., the air in the test space, in this case. 
     The heat exchanger may be dimensioned in such a manner that only part of the refrigerant can evaporate in the heat exchanger. This results in the advantage that the dew point or the location of the dew point temperature of the refrigerant can be shifted out of the heat exchanger into the internal heat exchanger. Because of a temperature glide of the zeotropic refrigerant, partial evaporation of the refrigerant in the heat exchanger achieves a lower temperature in the heat exchanger than the following remaining evaporation of the refrigerant in the internal heat exchanger. 
     In one embodiment of the test chamber, the heat exchanger may be disposed in the test space. In this case, the heat exchanger may also be disposed in an air treatment space of the test space so that air circulated by a fan can come into contact with a heat exchanger. In this way, a circulated amount of air of the test space can be cooled directly in the test space by means the cooling device via the heat exchanger. The test chamber may have the cooling circuit as a sole, single cooling circuit. In this case, the cooling circuit is connected directly to the test space. 
     In another embodiment of the test chamber, the condenser may be realized as a cascade heat exchanger of another cooling circuit of the cooling device. Accordingly, the test chamber may have at least two cooling circuits, in which case the cooling circuit may form a second stage of the cooling device and another cooling circuit, which is disposed upstream of the cooling circuit, may form a first stage of the cooling device. In this case, the condenser serves as a cascade heat exchanger or a heat exchanger for the cooling circuit. This embodiment of a test chamber allows particularly low temperatures to be established in the test space. 
     The temperature control device may have a heating device comprising a heater and a heating heat exchanger in the test space. The heating device may be an electric resistance heater which heats the heating heat exchanger in such a manner that the temperature in the test space can be raised by means of the heating heat exchanger. If the heat exchanger and the heating heat exchanger can be specifically controlled by means of a control device to cool or heat the air circulated in the test space, a temperature in the temperature range indicated above can be established within the test space by means of the temperature control device. A temperature stability over time of ±1 K, preferably ±0.3 K to ±0.5 K or less than ±0.3 K, may be established in the test space during a test interval irrespective of the test material or of an operating state of the test material. A test interval is a segment of a full test period in which the test material is exposed to a substantially constant temperature or climatic condition. The heating heat exchanger may be combined with the heat exchanger of the cooling circuit in such a manner that a shared heat exchanger body through which the refrigerant can flow and which has heating elements of an electric resistance heater can be realized. The condenser may be cooled with air, water or another coolant. In principle, the condenser can be cooled using any suitable fluid. The essential aspect is that the thermal load generated at the condenser is discharged via the cooling air or the cooling water in such a manner that the refrigerant can condense until it is completely liquefied. 
     A first bypass having at least one controllable second expansion element may be realized in the cooling circuit, in which case the first bypass may be connected to the cooling circuit upstream of the internal heat exchanger and downstream of the condenser and the first bypass may be realized as a controllable additional internal cooling system. The first bypass may thus form a re-injection device for refrigerant. Accordingly, refrigerant can be recycled from the controllable second expansion element in the internal heat exchanger on the low-pressure side. In this case, the first bypass may be connected to the low-pressure side of the cooling circuit upstream of the internal heat exchanger and downstream of the heat exchanger. The refrigerant cooled or having its temperature level lowered by the second expansion element may be led through the internal heat exchanger and intensify cooling of the refrigerant on the high-pressure side of the internal heat exchanger. Also, a cooling capacity of the internal heat exchanger can be controlled even more precisely in this way. 
     A second bypass comprising at least one third expansion element may be formed in the cooling circuit, in which case the second bypass bypasses the expansion element downstream of the condenser and upstream of the internal heat exchanger and the refrigerant can be metered by means of the third expansion element in such a manner that a suction gas temperature and/or a suction gas pressure of the refrigerant can be controlled upstream of the compressor on the low-pressure side of the cooling circuit. In this way, potential overheating and damage of the compressor, which may be a compressor device, for example, can be prevented among other things. Consequently, gaseous refrigerant located upstream of the compressor can be cooled via the second bypass by actuation of the third expansion element by adding still-liquid refrigerant. The third expansion element can be actuated by means of a control device which itself is coupled to a pressure and/or temperature sensor in a cooling circuit upstream of the compressor. Particularly advantageously, a suction gas temperature of &lt;30° C. can be set via the second bypass. Also, the refrigerant can be metered in such a manner that an operating time of the compressor can be controlled. On principle, it is disadvantageous for the compressor or compressor device to be switched on and off repeatedly. A service life of a compressor can be prolonged if the compressor operates for longer periods of time. A refrigerant can be led past the expansion element or the condenser via the second bypass in order to delay an automated deactivation of the compressor and to prolong an operating time of the compressor, for example. 
     Another bypass comprising at least one other expansion element may be formed in the cooling circuit, the other bypass bypassing the compressor downstream of the compressor and upstream of the condenser in such a manner that a suction gas temperature and/or a suction gas pressure of the refrigerant can be controlled upstream of the compressor on the low-pressure side of the cooling circuit and/or that a pressure difference between the high-pressure side and a low-pressure side of the cooling circuit can be equalized. The second bypass may additionally be equipped with a settable or controllable valve, such as a magnetic valve. Connecting the high-pressure side and the low-pressure side via the other expansion element ensures that the gaseous refrigerant thus compressed gradually flows from the high-pressure side to the low-pressure side of the cooling circuit in the event of a system standstill. This also ensures gradual pressure equalization between the high-pressure side and the low-pressure side even when the expansion element is closed. A cross-section of the other expansion element may be dimensioned in such a manner that the refrigerant flowing from the high-pressure side to the low-pressure side has only a marginal impact on the normal operation of the cooling device. At the same time, a gaseous refrigerant located upstream of the compressor may be cooled by adding the liquid refrigerant via the other bypass. 
     Furthermore, the internal heat exchanger may be realized as a sub-cooling section or a heat exchanger, in particular a plate heat exchanger. The sub-cooling section may simply be realized by two line sections of the cooling circuit that are in contact with each other. 
     The expansion element may have a throttle and a magnetic valve, in which case refrigerant can be metered via the throttle and the magnetic valve. The throttle may be a settable valve or a capillary via which refrigerant is routed by means of the magnetic valve. The magnetic valve itself may be actuated by means of a control device. 
     Also, the temperature control device may comprise a control device comprising at least one pressure sensor and/or at least one temperature sensor in the cooling circuit, in which case a magnetic valve can be actuated by means of the control device as a function of a measured temperature and/or pressure. The control device may comprise means for data processing which process sets of data from sensors and control the magnetic valves. In this case, a function of the cooling device may also be adjusted to the refrigerant used via an appropriate computer program, for example. Furthermore, the control device may signal a malfunction and initiate a shut-down of the test chamber, if necessary, in order to protect the test chamber and the test material from damage due to critical or undesirable operating states of the test chamber. 
     When a refrigerant consisting of a refrigerant mixture composed of a mass fraction of carbon dioxide of 9 to 75 mass percent, a mass fraction of difluoromethane of up to 70 mass percent and a mass fraction of 2,3,3,3-tetrafluoropropene of up to 47 mass percent is used according to the disclosure, the refrigerant is used to condition air in a test space of a test chamber, the test space serving to receive test material and being sealable against an environment and temperature-insulated, a cooling device of a temperature control device of the test chamber comprising a cooling circuit with the refrigerant, a heat exchanger, a compressor, a condenser and an expansion element is used to establish a temperature in a temperature range of −60° C. to +180° C., preferably −70° C. to +180° C., particularly preferably −80° C. to +180° C., within the test space. 
     The refrigerant can be cooled by means of an internal heat exchanger of the cooling circuit, which is connected to a high-pressure side of the cooling circuit upstream of the expansion element and downstream of the condenser and to a low-pressure side of the cooling circuit upstream of the compressor and downstream of the heat exchanger, of the high-pressure side, the cooling of the refrigerant of the high-pressure side by means of the internal heat exchanger being usable to lower an evaporation temperature at the expansion element. During lowering of the evaporation temperature of the refrigerant of the high-pressure side, a suction pressure of the refrigerant of the low-pressure side can be kept constant. A greater system complexity, such as in the form of additional control of the suction pressure and control of the expansion element as a function of the suction pressure, is not necessarily required in that case. In particular, the compressor may also be operated at constant output irrespective of an operating state of the cooling circuit. When piston pumps are used as compressors in particular, it is essential for them to be in operation for long periods of time and at a constant speed in order to achieve a long service live. 
     The refrigerant of the high-pressure side may be cooled by the refrigerant of the low-pressure side at a constant suction pressure on the low-pressure side by means of the internal heat exchanger. Consequently, the refrigerant can evaporate at constant suction pressure on an evaporation section of the cooling circuit from the expansion element up to and including the internal heat exchanger. If the suction pressure or evaporation pressure of the refrigerant is constant, the refrigerant can evaporate from the expansion element at a low evaporation temperature to the internal heat exchanger at a high evaporation temperature according to the temperature glide of the refrigerant. The dew point temperature resulting from the temperature glide may be higher than the temperature of the fluid to be cooled or of the air in the test space. Once an evaporation temperature of the refrigerant is equal to the temperature of the air to be cooled in the test space at the same suction pressure, the air cannot be cooled any further. However, the dew point temperature reached in the other heat exchanger is lower than the liquid temperature of the refrigerant on the high-pressure side of the internal heat exchanger, which means that a liquid temperature of the refrigerant can be reduced further. Accordingly, an evaporation temperature downstream of the expansion element can be lowered without changing the suction pressure, allowing further cooling of the air in the test space to be achieved. 
     Thus, a first portion of the refrigerant routed via the expansion element can be evaporated in the heat exchanger and a second portion of the refrigerant can be evaporated in the internal heat exchanger. An evaporation section of the cooling circuit within which the refrigerant evaporates may extend from the expansion element as far as to the internal heat exchanger. The evaporation section may run through the internal heat exchanger, in which case a dew point of the refrigerant may be located at an exit of the internal heat exchanger upstream of the compressor. A first portion/second portion ratio may change during operation of the cooling circuit as a function of a temperature in the test space or at the heat exchanger. For example, a comparatively large temperature difference between the temperature of the heat exchanger and a temperature in the test space may lead to accelerated heating of the refrigerant in the heat exchanger, which results in a shift of the dew point of the refrigerant toward an entry of the internal heat exchanger or an exit of the heat exchanger upstream of the compressor. This kind of shift of the dew point can be tolerated as long as no comparatively low temperature or target temperature has been established in the test space yet. When the temperature of the heat exchanger approaches the temperature in the test space, the dew point shifts and the second portion thus grows relative to the first portion of the refrigerant. 
     The evaporation temperature of the refrigerant of the high-pressure side can be lowered in a self-controlled manner. Depending on the temperature at the heat exchanger, refrigerant no longer evaporating can be discharged from the heat exchanger in the flow direction because the temperature at the heat exchanger is no longer sufficient to cause a phase transition of the refrigerant in this case. Thus, wet vapor or liquid refrigerant is re-evaporated in the internal heat exchanger because here a temperature difference between the high-pressure side and the low-pressure side can always be greater than at the heat exchanger. If a temperature of the liquid refrigerant upstream of the expansion element is reduced by means of the internal heat exchanger by the heat exchange at the internal heat exchanger, the energy density of the refrigerant upstream of the expansion element and the temperature difference thus achievable at the heat exchanger increase. The interaction of the expansion element, the heat exchanger and the internal heat exchanger does not have to be controlled, in principle. 
     Particularly advantageously, the cooling device is operated exclusively below the critical point of the refrigerant. If the cooling device is operated below the triple point of the refrigerant, reaching of a supercritical state of the refrigerant can be precluded. Thus, the cooling device does not have to be configured for operation in the supercritical state, which saves costs for production of the cooling device. 
     In particular, the constant suction pressure may also be maintained during lowering of the evaporation temperature of the refrigerant of the high-pressure side by means of the internal heat exchanger. Accordingly, the cooling of the refrigerant of the high-pressure side via the internal heat exchanger can also be exploited in part or exclusively to lower an evaporation temperature of the refrigerant at the expansion element. 
     A dew point temperature of the refrigerant may be higher than a minimum temperature of the temperature range. In the test chambers known from the state of the art, the minimum temperature of the temperature range can no longer be established with a refrigerant of this kind in that case, but a comparatively higher minimum temperature, which substantially corresponds to the dew point temperature of the refrigerant. In the test chamber according to the disclosure, however, a refrigerant whose dew point temperature is higher than an achievable minimum temperature of the temperature range can be used because the liquefied refrigerant on the high-pressure side can be cooled by means of the internal heat exchanger, which means that an evaporation temperature of the refrigerant at the expansion element can be comparatively lower. 
     The refrigerant can be evaporated absolutely at a suction pressure or evaporation pressure in a pressure range of 0.3 to 5 bar. Use of the refrigerant within that pressure range allows cost-effective production of the cooling circuit because no special pressure-resistant modules and components have to be used to construct the low-pressure side of the cooling circuit. 
     Also, the refrigerant can be condensed absolutely at a condensation pressure in a pressure range of 5 to 35 bar. Here, too, the high-pressure side can be constructed using modules and components that do not have to be adapted to comparatively higher pressures. 
     Other embodiments of a use are apparent from the description of features of the claims depending on device claim  1 . 
    
    
     
       Hereinafter, preferred embodiments of the disclosure will be explained in more detail with reference to the accompanying drawings. 
         FIG. 1  is a pressure-enthalpy diagram for a refrigerant; 
         FIG. 2  is a schematic illustration of a first embodiment of a cooling device; 
         FIG. 3  is a schematic illustration of a second embodiment of a cooling device; 
         FIG. 4  is a schematic illustration of a third embodiment of a cooling device; 
         FIG. 5  is a schematic illustration of a fourth embodiment of a cooling device; 
         FIG. 6  is a schematic illustration of a fifth embodiment of a cooling device. 
     
    
    
       FIG. 2  shows a first embodiment of a cooling device  10  of a test chamber (not shown). Cooling device  10  comprises a cooling circuit  11  with a refrigerant, a heat exchanger  12 , a compressor  13 , a condenser  14  and an expansion element  15 . Condenser  14  is cooled by another cooling circuit  16  in the case at hand. Heat exchanger  12  is disposed in a test space (not shown) of the test chamber. Furthermore, cooling circuit  11  has a high-pressure side  17  and a low-pressure side  18 , to which an internal heat exchanger  19  is connected. 
       FIG. 1  shows a pressure-enthalpy diagram (log p/h diagram) for the refrigerant circulating in cooling circuit  11 , the refrigerant being a zeotropic refrigerant. According to a combined view of  FIGS. 1 and 2 , starting from position A, the refrigerant upstream of compressor  13  is aspirated and compressed, whereby a pressure is achieved downstream of compressor  13  according to position B. The refrigerant is compressed by means of compressor  13  and is subsequently liquefied in condenser  14  according to position C. The refrigerant passes through internal heat exchanger  19  on high-pressure side  17 , where it is cooled further, position C′ upstream of expansion element  15  thus being reached. By means of internal heat exchanger  19 , the portion of the wet vapor area (positions E to E′) not usable in heat exchanger  12  can be used to further reduce a temperature of the refrigerant (positions C′ to C). At expansion element  15 , the refrigerant is relaxed (positions C′ to D′) and partially liquefied in heat exchanger  12  (positions D′ to E). Then, the wet vapor of the refrigerant enters internal heat exchanger  19  on low-pressure side  18 , where the refrigerant is re-evaporated until the dew-point temperature or the dew point of the refrigerant is reached at position E′. Hence, a first subsection  20  of an evaporation section  22  of the refrigerant runs through heat exchanger  12 , a second subsection  21  of evaporation section  22  running through internal heat exchanger  19 . The essential aspect is that a suction pressure of compressor  13  on low-pressure side  18  is kept constant on evaporation section  22  even if the evaporation temperature at expansion element  15  changes. 
     The refrigerant is a refrigerant mixture composed of a mass fraction of carbon dioxide of 9 to 75 mass percent, a mass fraction of difluoromethane of up to 70 mass percent and a mass fraction of 2,3,3,3-tetrafluoropropene of up to 47 mass percent. In principle, it is possible for the refrigerants listed in Tables 1, 2 and 3 above to be used in cooling circuit  11  and the cooling circuits described below. 
       FIG. 3  shows a schematic illustration of a simplest embodiment of a cooling device  23 , cooling device  23  being self-controlling. Cooling device  23  comprises a cooling circuit  24  with a heat exchanger  25 , a to compressor  26 , a condenser  27 , an expansion element  28  and an internal heat exchanger  29 . Depending on a temperature at heat exchanger  25 , refrigerant not fully evaporated escapes from heat exchanger  25  because the temperature at heat exchanger  25  or in a test space (not shown) is no longer high enough to cause a phase transition. In this case, refrigerant still liquid is re-evaporated in internal heat exchanger  29  because a temperature difference there has to be greater than at heat exchanger  25  at all times. Once the temperature of the liquid refrigerant upstream of expansion element  28  has been reduced by heat exchange in internal heat exchanger  29 , the energy density and the temperature difference achievable with it at heat exchanger  25  increase. Cooling device  23  does not require elaborate control by way of sensors etc. 
       FIG. 4  shows a cooling device  30  which differs from the cooling device of  FIG. 3  in that it has a first bypass  31  and a second bypass  32 . A controllable second expansion element  33  is disposed in first bypass  31 , first bypass  31  being configured as an additional internal cooling system  34 . First bypass  31  is connected to cooling circuit  24  immediately downstream of condenser  27  upstream of internal heat exchanger  29  and downstream of heat exchanger  25  and upstream of internal heat exchanger  29 . First bypass  31  thus bypasses expansion element  28  with heat exchanger  25 , internal heat exchanger  29  being suppliable with evaporating refrigerant via second expansion element  33 . A suction gas mass flow introduced into internal heat exchanger  29  can be cooled additionally by means of first bypass  31  in case of high suction gas temperatures, which may be caused by heat exchanger  25 . In this way, evaporation of refrigerant upstream of the expansion element can be precluded. Hence, first bypass  31  can be used to react to changing load cases of cooling device  30 . Second bypass  32  has a third expansion element  35  and is connected to cooling circuit  24  downstream of condenser  27  and upstream of internal heat exchanger  29  and downstream of internal heat exchanger  29  and upstream of compressor  26 . This allows a suction gas mass flow upstream of compressor  26  to be reduced far enough via second bypass  32  to avoid inadmissibly high final compression temperatures. 
       FIG. 5  shows a cooling device  36  which differs from the cooling device of  FIG. 4  in that it has another cooling circuit  37 . Other cooling circuit  37  serves to cool a condenser  38  of a cooling circuit  39 . Condenser  38  is realized as a cascade heat exchanger  40  in the case at hand. Furthermore, cooling circuit  39  has another bypass  41  having another expansion element  42 . Other bypass  41  is connected to cooling circuit  39  downstream of compressor  26  and upstream of condenser  38  and downstream of internal heat exchanger  29  and upstream of compressor  26 . Thus, refrigerant not yet liquefied but compressed can flow back to upstream of compressor  26  via other bypass  41 , whereby a suction gas temperature and/or a suction gas pressure of the refrigerant can be controlled. 
       FIG. 6  shows a cooling device  30  having a cooling circuit  44  and another cooling circuit  45  and, in particular, an internal heat exchanger  46  in cooling circuit  44 . In the case at hand, a heat exchanger  47  is disposed in a temperature-insulated test space of a test chamber (not shown).