Abstract:
An object is to provide a freezing device in which a safely-treatable incombustible mixed refrigerant can be used and which can realize an extremely low temperature of −85° C. or less in chamber by a simple structure. The freezing device comprises a single refrigerant circuit in which the refrigerant discharged from a compressor is condensed and thereafter evaporated to exert a cooling function and which allows heat exchange between the evaporated refrigerant and the condensed refrigerant, wherein there is introduced into the refrigerant circuit a non-azeotropic mixed refrigerant containing R245fa, R600, R23 and R14; a non-azeotropic mixed refrigerant containing R245fa, R600, R116 and R14; a non-azeotropic mixed refrigerant containing R245fa, R600, R508A and R14; or a non-azeotropic mixed refrigerant containing R245fa, R600, R508B and R14.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a freezing device constituted of a single refrigerant circuit in which a refrigerant discharged from a compressor is condensed and then evaporated to exert a cooling function.  
         [0002]     Heretofore, a so-called Freon has broadly been used as a refrigerant for use in a refrigerator or a freezer. However, a specific Freon containing chlorine destroys ozone in an upper region of the atmosphere, many Freons influence global warming as a greenhouse-effect gas, and the Freon is an object whose use is restricted. Therefore, there is demanded the development of a replaceable refrigerant composition capable of maintaining the performance of the refrigerant without any danger that the ozone layer is destroyed and without changing a conventional freezing circuit.  
         [0003]     The refrigerant which can be used is required to have characteristics that physical properties such as its composition and boiling point do not change during vaporization or condensation of the gas and that compatibility with an oil (an alkyl benzene) for use as a lubricant is high. In addition, it is required that the boiling point of the refrigerant be sufficiently low in consideration of a relation between the boiling point and a targeted temperature in chamber and that a critical temperature be high for a smooth operation under a room-temperature environment. Therefore, it is very difficult that such requirements are satisfied by the refrigerant gas of a single component.  
         [0004]     To solve the problem, a mixed refrigerant constituted of two or more of components has heretofore been used, and properties such as the boiling point of the mixed refrigerant are adjusted by selecting the components to be mixed. Especially, to make it possible to realize an extremely low temperature below −80° C. in chamber, a non-azeotropic mixed refrigerant constituted of two or more of components is used. It is difficult to liquefy components each having a low boiling point and a low critical temperature by a capability of a condenser which operates under the room-temperature environment, and therefore, a multistage system or the like is employed in which the refrigerant components are condensed in multiple stages.  
         [0005]     However, in the multistage freezing system, a structure is complicated and enlarged, and maintenance becomes difficult, which leads to a problem that a running cost remarkably increases.  
         [0006]     To solve the problem, heretofore, there have been developed a non-azeotropic mixed refrigerant containing butane, ethylene and R14 (carbon tetrafluoride: CF 4 ) (see Japanese Patent Application Laid-Open No. 2003-13049), and a non-azeotropic mixed refrigerant containing butane, ethane and R14 (see Japanese Patent Application Laid-Open No. 2003-13050). According to these non-azeotropic mixed refrigerant gases, a treating property of the refrigerant in the freezer is secured by an operability of butane having a high boiling point at normal temperature, and the extremely low temperature is realized by ethane or ethylene having a remarkably low boiling point. In consequence, the temperature in chamber can be set to −60° C. or less without using any complicated multistage freezing system.  
         [0007]     However, in the above-described conventional non-azeotropic mixed refrigerant, a combustible gas such as ethylene or ethane is used. Especially, to realize an extremely low temperature of −60° C. or less in chamber, at least a mixture ratio of ethane or ethylene in a mixture of butane and ethane or ethylene has to be 10% or more, and a state of the non-azeotropic mixed refrigerant remains in a combustible region. Therefore, there has been a problem that the combustible mixed refrigerant has an unsatisfactory reliability in respect of safety, and has a very unsatisfactory treating property.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been developed to solve the conventional technical problem, and an object is to provide a freezing device in which a safely-treatable incombustible mixed refrigerant can be used and which can realize an extremely low temperature of −85° C. or less in chamber by a simple structure.  
         [0009]     In a first invention of the present application, a freezing device comprises: a single refrigerant circuit in which a refrigerant discharged from a compressor is condensed and then evaporated to exert a cooling function and which allows heat exchange between the evaporated refrigerant and the condensed refrigerant, wherein there is introduced into the refrigerant circuit: a non-azeotropic mixed refrigerant containing R245fa, R600, R23 and R14; a non-azeotropic mixed refrigerant containing R245fa, R600, R116 and R14; a non-azeotropic mixed refrigerant containing R245fa, R600, R508A and R14; or a non-azeotropic mixed refrigerant containing R245fa, R600, R508B and R14.  
         [0010]     In a second invention of the present application, the freezing device of the above invention is characterized in that with respect to a total weight of the non-azeotropic mixed refrigerant, a total weight ratio of the refrigerants R245fa and R600 is set to a range of 40 wt % to 80 wt %, a weight ratio of the refrigerant R23, R116, R508A or R508B is set to a range of 15 wt % to 47 wt %, and a weight ratio of the refrigerant R14 is set to a range of 3 wt % to 20 wt %.  
         [0011]     In a third invention of the present application, the freezing device of the above invention is characterized in that with respect to the total weight of the non-azeotropic mixed refrigerant, the total weight ratio of the refrigerants R245fa and R600 is set to a range of 49 wt % to 70 wt %, the weight ratio of the refrigerant R23, R116, R508A or R508B is set to a range of 21 wt % to 42 wt %, and the weight ratio of the refrigerant R14 is set to a range of 9 wt % to 20 wt %.  
         [0012]     In a fourth invention of the present application, the freezing device of the above invention is characterized in that with respect to the total weight of the non-azeotropic mixed refrigerant, the total weight ratio of the refrigerants R245fa and R600 is set to 64 wt %, the weight ratio of the refrigerant R23, R116, R508A or R508B is set to 24 wt %, and the weight ratio of the refrigerant R14 is set to 12 wt %.  
         [0013]     According to the first invention of the present application, in the freezing device comprising the single refrigerant circuit in which the refrigerant discharged from the compressor is condensed and then evaporated to exert the cooling function and which allows the heat exchange between the evaporated refrigerant and the condensed refrigerant, there is introduced into the refrigerant circuit: the non-azeotropic mixed refrigerant containing R245fa, R600, R23 and R14; the non-azeotropic mixed refrigerant containing R245fa, R600, R116 and R14; the non-azeotropic mixed refrigerant containing R245fa, R600, R508A and R14; or the non-azeotropic mixed refrigerant containing R245fa, R600, R508B and R14. Accordingly, it is possible to realize an extremely low temperature of −80° C. or less in chamber as a cooling object by various types of comparatively inexpensive refrigerants without using any Freon refrigerant whose use is restricted. In consequence, when the temperature of −80° C. or less is realized in chamber, long-period storage of a food, a living tissue, a specimen or the like can further be stabilized, and reliability can be enhanced.  
         [0014]     Especially, according to the present invention, even when a refrigerant composition is changed, a performance of a conventional freezing circuit can be maintained without changing the freezing circuit. Moreover, it is possible to cope with an environmental problem such as destruction of the ozone layer. In the present invention, the extremely low temperature can be realized by a single-stage type freezing system without using the so-called multistage freezing system. In consequence, the device can be simplified, and a production cost can be reduced.  
         [0015]     Furthermore, since the non-azeotropic mixed refrigerant for use in the present invention is incombustible, it can be used safely. A treating property of the refrigerant is enhanced. Moreover, it is possible to avoid a disadvantage that the mixed refrigerant burns in a case where the refrigerant leaks owing to breakage of refrigerant piping or the like.  
         [0016]     Especially as in the second invention, with respect to the total weight of the non-azeotropic mixed refrigerant, the total weight ratio of the refrigerants R245fa and R600 is set to the range of 40 wt % to 80 wt %, the weight ratio of the refrigerant R23, R116, R508A or R508B is set to the range of 15 wt % to 47 wt %, and the weight ratio of the refrigerant R14 is set to the range of 3 wt % to 20 wt %. It is more preferable that as in the third invention, with respect to the total weight of the non-azeotropic mixed refrigerant, the total weight ratio of R245fa and R600 is set to the range of 49 wt % to 70 wt %, the weight ratio of R23, R116, R508A or R508B is set to the range of 21 wt % to 42 wt %, and the weight ratio of R14 is set to the range of 9 wt % to 20 wt %. It is further preferable that as in the fourth invention, with respect to the total weight of the non-azeotropic mixed refrigerant, the total weight ratio of R245fa and R600 is set to 64 wt %, the weight ratio of R23, R116, R508A or R508B is set to 24 wt %, and the weight ratio of R14 is set to 12 wt %. In consequence, it is possible to stably realize a temperature of −80° C. or less in chamber. The long-period storage of the food, the living tissue, the specimen or the like can further be stabilized, and the reliability can be enhanced.  
         [0017]     Furthermore, since the non-azeotropic mixed refrigerant is surely constituted to be incombustible, it is possible to more effectively avoid a disadvantage that the refrigerant burns when it leaks.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a front view of an extremely-low-temperature freezer to which a freezing device is applied;  
         [0019]      FIG. 2  is a side view of  FIG. 1 ;  
         [0020]      FIG. 3  is a plan view of  FIG. 1 ;  
         [0021]      FIG. 4  is a refrigerant circuit diagram in the present embodiment;  
         [0022]      FIG. 5  is a refrigerant circuit diagram in another embodiment;  
         [0023]      FIG. 6  is a graph concerning data in a case where a weight of a mixed refrigerant of R245fa and R600 and a weight of R14 are set to be constant, and a weight of R23 is changed; and  
         [0024]      FIG. 7  is a graph concerning data in a case where the weight of the mixed refrigerant of R245fa and R600 and the weight of R23 are set to be constant, and the weight of R14 is changed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Embodiments of the present invention will be described hereinafter in detail with reference to the drawings. An extremely-low-temperature storage  1  of the present embodiment is used in, for example, storing a frozen food to be stored at a low temperature over a long period or storing a living tissue, a specimen or the like at an extremely low temperature. A main body of the storage is constituted of an insulating box article  2  having its top opened.  
         [0026]     This insulating box article  2  is constituted of: an outer box  3  made of a steel plate and an inner box  4 , the boxes having tops opened; breakers  5  made of a synthetic resin and connecting upper ends of the box  3  to those of the box  4 , respectively; and a polyurethane resin insulating material  7  with which a space enclosed by the outer box  3 , the inner box  4  and the breakers  5  is filled by an on-site foaming system. The inside of the inner box  4  is a storage chamber  8  having its top opened.  
         [0027]     In the present embodiment, to set a targeted temperature in the storage chamber  8  (hereinafter referred to as the temperature in chamber) at, for example, −80° C. or less, the insulating box article  2  which separates the inside of the storage chamber  8  from outside air requires a great insulating capability as compared with a low-temperature storage having the temperature in chamber set in the vicinity of 0° C. Therefore, to secure the insulating capability by the only polyurethane resin insulating material  7 , the material has to be formed into a considerable thickness, and there is a problem that a sufficient storage amount of the storage chamber  8  cannot be secured with a limited main-body dimension. To solve the problem, in the insulating box article  2  of the present embodiment, a vacuum insulating material of a glass wool is disposed on an inner wall surface of the outer box  3 , and a thickness dimension of the polyurethane resin insulating material  7  is reduced in accordance with the insulating capability of the vacuum insulating material.  
         [0028]     Moreover, the tops of the breakers  5  are formed into staircase-like shapes, and an insulating door  9  is attached to the breakers via packing members  11  so as to be rotatable centering on one end, that is, a rear end in the present embodiment. Accordingly, an opening in the top of the storage chamber  8  is openably closed by the insulating door  9 . A handle portion  10  is disposed on the other end of the insulating door  9 , that is, a front end thereof in the present embodiment. When the handle portion  10  is operated, the insulating door  9  is opened or closed.  
         [0029]     Furthermore, an evaporator (refrigerant pipe)  13  constituting a refrigerant circuit of a freezing device R is heat-exchangeably attached to the peripheral surface of the inner box  4  on the side of the insulating material  7 . A mechanical chamber (not shown) is constituted in a lower part of the insulating box article  2 . In this mechanical chamber, a compressor  14 , a condenser  15 , a blower (not shown) for air-cooling the compressor  14  and the condenser  15  and the like are arranged to constitute a refrigerant circuit  12  of the freezing device R. Moreover, the compressor  14 , the condenser  15 , a drier  17 , a heat exchanger  16 , a capillary tube  18  as a pressure reducing unit and the evaporator  13  are successively annularly connected to one another by piping as shown in  FIG. 4  or  5 , thereby constituting the refrigerant circuit  12  of the freezing device R. It is to be noted that the heat exchanger  16  is disposed in the insulating material  7 .  
         [0030]      FIG. 4  is a refrigerant circuit diagram in which the rotary compressor  14  is used. The compressor  14  is connected to a sub-cooler  20 , and is constituted to discharge, to a refrigerant discharge tube  21 , a refrigerant which has once released heat in the outside and then again returned into a shell of a sealed container to be compressed again. The compressor  14  on a discharge side is connected to the condenser  15  via the refrigerant discharge tube  21 , and the condenser  15  on an outlet side is successively connected to the drier  17 , the heat exchanger  16  and the capillary tube  18  as pressure reducing means. The capillary tube  18  on the outlet side is connected to the evaporator  13 . The evaporator  13  on the outlet side is connected to the compressor  14  on a suction side via a return pipe  22  and the heat exchanger  16 .  
         [0031]     In the present embodiment, the refrigerant circuit  12  is filled with a mixed refrigerant of R245fa and R600 and a non-azeotropic mixed refrigerant of R23 and R14. The refrigerant R245fa is pentafluoropropane (CHF 2 CH 2 CF 3 ) having a boiling point of +15.3° C., and R600 is a butane (C 4 H 10 ) having a boiling point of −0.5° C. The refrigerant R600 has a function of feeding a lubricant of the compressor  14  and a mixed moisture that cannot be absorbed by the drier  17  back into the compressor  14  in a state in which the lubricant and the moisture are dissolved in the refrigerant. However, R600 is a combustible substance. Therefore, when R600 is mixed with incombustible R245fa at a predetermined ratio of R245fa/R600=70/30 in the present embodiment, the mixed refrigerant can be treated as an incombustible refrigerant. Moreover, R23 is trifluoromethane (CHF 3 ) having a boiling point of −82.1° C., and R14 is tetrafluoromenthane (CF 4 ) having a boiling point of −127.9° C.  
         [0032]     Furthermore, in a composition of these mixed refrigerants in the present embodiment, the mixed refrigerant of R245fa and R600 occupies 64 wt % of the whole composition, R23 occupies 24 wt %, and R14 occupies 12 wt %.  
         [0033]     In the above constitution, a high-temperature gas-like refrigerant discharged from the compressor  14  is once discharged from the sealed container to the sub-cooler  20  via the refrigerant discharge tube on the side of the sub-cooler  20 . After releasing its heat, the refrigerant again returns into the shell of the sealed container, and is discharged to the condenser  15  via the refrigerant discharge tube  21 .  
         [0034]     The high-temperature gas-like refrigerant which has flowed through the condenser  15  is condensed to release its heat, and liquefied. The moisture contained in the refrigerant is then removed by the drier  17 . The refrigerant then flows through the heat exchanger  16  to allow the heat exchange between the refrigerant and a low-temperature refrigerant in the heat-exchangeably disposed return pipe  22 . Accordingly, an uncondensed refrigerant is cooled, condensed and liquefied in the heat exchanger  16 . Therefore, the pressure of the mixed refrigerant passed through the heat exchanger  16  is reduced by the capillary tube  18 . Subsequently, when the mixed refrigerant flows through the evaporator  13 , the refrigerants R14, R23 evaporate. The refrigerant performs its cooling function in the evaporator  13 , and an ambient temperature around this evaporator  13  is set to −85° C. to realize an extremely low temperature of −80° C. in chamber. The refrigerant passed from the heat exchanger  16  returns to the compressor  14  by the return pipe  22 .  
         [0035]     At this time, a capability of the compressor  14  is 425 W, and a temperature to be finally reached by the evaporator  13  being operated is −100° C. to −60° C. At such a low temperature, the boiling point of R245fa in the refrigerant is +15.3° C., and the boiling point of R600 is −0.5° C. Therefore, the refrigerant remains in a liquid state in the evaporator  13  without being evaporated, and therefore hardly contributes to the cooling. However, R600 performs a function of feeding the lubricant of the compressor  14  and the mixed moisture that cannot be absorbed by the drier  17  back to the compressor  14  in a state in which they are dissolved in the refrigerant, and a function of lowering the temperature of the compressor  14  by the evaporation of the liquid refrigerant in the compressor  14 .  
         [0036]     An evaporation temperature in the evaporator  13  differs with a composition ratio of the non-azeotropic mixed refrigerant to be introduced into the refrigerant circuit  12 . There will be described hereinafter in detail an evaporator temperature, a temperature in chamber, a high-pressure-side pressure and a low-pressure-side pressure with respect to the composition ratios of the refrigerants based on experiment results.  FIG. 6  is a graph showing an evaporator inlet temperature, the temperature in chamber, the high-pressure-side pressure and the low-pressure-side pressure in a case where a weight of the mixed refrigerant of R245fa and R600 and the weight of R14 are set to be constant, and a weight of R23 is changed.  FIG. 7  is a graph showing the evaporator inlet temperature, the temperature in chamber, the high-pressure-side pressure and the low-pressure-side pressure in a case where the weight of the mixed refrigerant of R245fa and R600 and the weight of R23 are set to be constant, and the weight of R14 is changed.  
         [0037]     According to an experiment result of  FIG. 6 , a weight ratio of R23 is increased from 20.0 wt % to 42.0 wt % with respect to a total weight of the refrigerants to be introduced. According to this result, in a case where the weight ratio of R23 is 20.0 wt % which is regarded as the minimum amount in such an experiment, the inlet temperature of the evaporator  13  is −88.0° C., whereas the temperature in chamber is −71.0° C. On the other hand, when the weight ratio of R23 is 21.3 wt %, the inlet temperature of the evaporator  13  rapidly drops to −95.9° C., and the temperature in chamber also drops to −87.5° C. While the weight ratio of R23 is then increased to 42.0 wt %, the temperature only slightly rises. At any weight ratio, the temperature in chamber can be set to about −85° C. or less.  
         [0038]     Moreover, according to an experiment result of  FIG. 7 , a weight ratio of R14 is increased from 0.0 wt % to 14.1 wt % with respect to the total weight of the refrigerants to be introduced. According to this result, in a case where the weight ratio is 0.0 wt % which is regarded as the minimum amount in such an experiment, that is, R14 is not contained, the inlet temperature of the evaporator  13  is −66.1° C., whereas the temperature in chamber is −66.9° C. On the other hand, when the weight ratio of R14 is 1.8 wt %, the inlet temperature of the evaporator  13  rapidly drops to −80.2° C., and the temperature in chamber also drops to −74.1° C. When the weight ratio of R14 is gradually increased to 14.1 wt % in the present experiment, the inlet temperature of the evaporator  13  drops to −98.90° C., and the temperature in chamber also drops to −90.0° C. Since the boiling point of R14 is −129.7° C., it is expected that when the weight ratio of R14 is then increased, the temperature of the evaporator  13  and the temperature in chamber further drop.  
         [0039]     However, as seen from the graph of  FIG. 7 , when the weight ratio of R14 increases, the high-pressure-side pressure rises. Therefore, when the weight ratio of R14 is further increased to 20 wt % or more, a problem occurs that the high-pressure-side pressure reaches an excessively high pressure of, for example, 3 MPa or more. The rise of the high-pressure-side pressure results in a problem that breakage of a unit such as the compressor  14  is incurred or that a starting property of the compressor  14  is deteriorated. Therefore, in order to set the temperature in chamber to a preferable target temperature of −75° C. or less, it is preferable to set the weight ratio of R14 to 3 wt % to 20 wt % of the total refrigerant amount.  
         [0040]     It is to be noted that as described above, the boiling point of R23 is −82.1° C. Therefore, the temperature of the evaporator  13  below the boiling point cannot be achieved by the only refrigerant R23. However, when a predetermined amount, for example, about 5 wt % or more of R14 having a remarkably low boiling point is added as in the present invention, the cooling function of R14 can regularly realize an extremely low evaporation temperature of −80° C. or less in the evaporator  13 .  
         [0041]     According to the above experiment results, when with respect to the total weight of the non-azeotropic mixed refrigerant to be introduced into the refrigerant circuit  12 , the total weight ratio of the mixed refrigerant of R245fa and R600 is set to 40 wt % to 80 wt %, the weight ratio of R23 is set to 15 wt % to 47 wt %, and the weight ratio of R14 is set to 3 wt % to 20 wt %, an extremely low temperature of −70° C. or less can be realized in chamber by the incombustible non-azeotropic mixed refrigerant.  
         [0042]     Especially, when with respect to the total weight of the non-azeotropic mixed refrigerant to be introduced into the refrigerant circuit  12 , the total weight ratio of the mixed refrigerant of R245fa and R600 is set to 49 wt % to 70 wt %, the weight ratio of R23 is set to 21 wt % to 42 wt %, and the weight ratio of R14 is set to 9 wt % to 20 wt %, an extremely low temperature of −85° C. or less can be realized in chamber by the incombustible non-azeotropic mixed refrigerant.  
         [0043]     In consequence, long-period storage of a food, a living tissue, a specimen or the like can further be stabilized, and reliability can be enhanced. Since the non-azeotropic mixed refrigerant is incombustible, it can be used safely. A treating property of the refrigerant is enhanced. Moreover, it is possible to avoid a disadvantage that the mixed refrigerant burns in a case where the refrigerant leaks owing to breakage of refrigerant piping or the like.  
         [0044]     Especially, when composition ratios of components of the non-azeotropic mixed refrigerant are set to 64 wt % of the mixed refrigerant of R245fa and R600, 24 wt % of R23 and 12 wt % of R14, it is possible to realize an extremely low temperature of −80° C. or less in chamber. In consequence, the food, the living tissue, the specimen and the like can more stably be stored for a long period, and reliability of the device can be enhanced.  
         [0045]     It is to be noted that the refrigerant of the present invention is not limited to R23. For example, even when R116 (hexafluoroethane: CF 3 CF 3 ) having a boiling point of −78.4° C., or R508A (R23/R116=39/61, boiling point: −85.7° C.) or R508B (R23/R116=46/54, boiling point: −86.9° C.) constituted by mixing R23 and R116 at a predetermined ratio is used, a similar effect can be produced.  
         [0046]     Moreover, in a case where the non-azeotropic mixed refrigerant is used as in the present invention, even when the refrigerant composition is changed, the performance of the conventional refrigerant circuit can be maintained without changing the circuit. Moreover, it is possible to cope with an environmental problem such as destruction of the ozone layer. Further in the present invention, since the extremely low temperature can be realized by a single-stage type freezing system without using a so-called multistage freezing system. Therefore, the device can be simplified, and a production cost can be reduced.  
         [0047]     It is to be noted that in the present embodiment, as the compressor, the rotary compressor  14  shown in  FIG. 4  is used, but a reciprocating compressor  24  shown in  FIG. 5  may be used. That is, as shown in  FIG. 5 , the compressor  24  on a discharge side is connected to a condenser  25  via a refrigerant discharge tube  26 . In a middle stage of this condenser  25 , an oil cooler  27  partially drawn into the compressor  24  is disposed. Moreover, the oil cooler  27  on an outlet side of this condenser  25  is successively connected to a drier  17 , a heat exchanger  16  and a capillary tube  18  as pressure reducing means. The capillary tube  18  on an outlet side is connected to an evaporator  13 , and the evaporator  13  on an outlet side is connected to the compressor  24  on a suction side via a return pipe  22  and the heat exchanger  16 .  
         [0048]     In such a constitution, a high-temperature gas-like refrigerant discharged from the compressor  24  is discharged to the condenser  25  via the refrigerant discharge tube  26 . The refrigerant which has released heat to be partially liquefied in the condenser  25  cools an oil of the compressor  24  in the oil cooler  27 . Then, in the subsequent-stage condenser  25 , the refrigerant releases its heat and is liquefied.  
         [0049]     After the high-temperature gas-like refrigerant is condensed to release its heat and liquefied in the condenser  25 , a moisture contained in the refrigerant is removed by the drier  17 . The refrigerant flows through the heat exchanger  16 , thereby allowing the heat exchange between the refrigerant and a low-temperature refrigerant in the heat-exchangeably disposed return pipe  22 . Accordingly, an uncondensed refrigerant is cooled, condensed and liquefied in the heat exchanger  16 . Therefore, in the same manner as in the above embodiment, the pressure of the mixed refrigerant passed through the heat exchanger  16  is reduced by the capillary tube  18 . Subsequently, the mixed refrigerant flows through the evaporator  13 , and the refrigerants R14, R23 evaporate. The refrigerant performs its cooling function in the evaporator  13 , and an ambient temperature around this evaporator  13  is set to −85° C. to realize an extremely low temperature of −80° C. in chamber. The refrigerant passed through the heat exchanger  16  returns to the compressor  24  by the return pipe  22 .  
         [0050]     Thus, the compressor for use in the present invention is not limited to the rotary type. For example, even the reciprocating compressor can produce a similar effect.  
         [0051]     It is to be noted that when the heat exchanger  16  is not used in each embodiment, the temperature of a compressed gas may be lowered in the above-described temperature range by another well-known cooling means to proceed with a targeted condensing process.