Patent Publication Number: US-6339932-B1

Title: Refrigerating cycle using carbon dioxide as refrigerant

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of Japanese Patent Application No. 11-235693 filed on Aug. 23, 1999, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a refrigerating cycle using carbon dioxide as refrigerant. 
     2. Description of the Related Art 
     Lubricating oil (refrigerating machine oil) for a compressor in a refrigerating cycle is generally mixed with refrigerant to be supplied to a sliding part in the compressor. Polyalkylene glycol (PAG) and polyol ester (POE) are widely known as lubricating oil for the refrigerating cycle using, as refrigerant, flon such as  134   a.    
     In a hermetic electric compressor driven by an electric motor, refrigerant must be introduced into a housing of the electric motor for cooling the electric motor. This compressor is difficult to use PAG as lubricating oil because PAG mixed with  134   a  (flon) has extremely small electric insulation resistance. Therefore, when the refrigerating cycle using  134   a  (flon) as refrigerant includes such a hermetic electric compressor, generally, POE having electric insulation resistance larger than that of PAG is used as lubricating oil. Incidentally, PAG is generally used in a refrigerating cycle equipped with an open type compressor which is provided separately from a driving device such as an electric motor. 
     On the other hand, a kind of refrigerating cycle uses carbon dioxide as refrigerant and includes an accumulator for separating lubricating oil from refrigerant discharged from an evaporator and for supplying the separated lubricating oil to a suction side of a compressor together with gaseous phase refrigerant. This kind of refrigerating cycle is referred to as an accumulator cycle below. When POE having high compatibility relative to carbon dioxide is used as lubricating oil in the accumulator cycle, a large amount of liquid phase refrigerant that is dissolved in lubricating oil can be sucked into the hermetic electric compressor together with lubricating oil. Accordingly, the hermetic electric compressor is brought into a liquid compressed state, and the efficiency of the compressor is lessened. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems. An object of the present invention is to provide an accumulator cycle including a hermetic electric compressor, which can prevent efficiency of the compressor from being lessened. 
     According to the present invention, in a refrigerating cycle using carbon dioxide as refrigerant, lubricating oil for a hermetic electric compressor contains as a main component one of polyalkylene glycol (PAG) and poly (vinyl ether) (PVE). 
     Accordingly, in state where lubricating oil is mixed with refrigerant (carbon dioxide), the lubricating oil can exhibit high electric insulation resistance that causes no problems on a practical use. Further, since lubricating oil can have lower compatibility with respect to carbon dioxide than POE, a large amount of liquid phase refrigerant is very difficult to be sucked into the compressor while being dissolved in lubricating oil. Therefore, the efficiency of the compressor is not lessened. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings, in which; 
     FIG. 1 is a schematic diagram showing an accumulator cycle in preferred embodiments according to the present invention; 
     FIG. 2 is a cross-sectional view showing a hermetic electric compressor used in the accumulator cycle; 
     FIG. 3 is a schematic view showing an accumulator used in the accumulator cycle; and 
     FIG. 4 is a table indicating values of electric insulation resistances of refrigerant and lubricating oil. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
     A refrigerating cycle according to the present invention is applied to an accumulator cycle using carbon dioxide as refrigerant. 
     Referring to FIG. 1, the accumulator cycle includes a hermetic electric compressor  100  for sucking and compressing refrigerant (carbon dioxide). The compressor  100  is, as shown in FIG. 2, composed of a scroll type compressing unit  110 , and an electric motor  120  for driving the compressing unit  110 . Refrigerant enters a motor housing  130  of the electric motor  120  from one end side in an axial direction of the motor housing  130 , cools the electric motor  120 , and then is sucked into and compressed in the compressor unit  110  that is provided at the other end side in the axial direction of the motor housing  130 . 
     Further, in FIG. 1, a radiator (gas cooler)  200  performs heat exchange between air and high-temperature and high-pressure refrigerant discharged from the compressor  100 , and a pressure-reducing unit  300  decompresses refrigerant discharged from the radiator  200 . The pressure-reducing unit  300  is a fixed aperture type, which has a fixed opening degree, such as a capillary tube. An evaporator  400  then evaporates refrigerant decompressed by the pressure-reducing unit  300 , which thus exhibits a refrigerating capability. Further, an accumulator  500  receives refrigerant from the evaporator  400  and divides the refrigerant into liquid phase refrigerant and gaseous phase refrigerant. The accumulator  500  further separates lubricating oil for the compressor  100  from refrigerant. The separated gaseous phase refrigerant and lubricating oil are conducted from the accumulator  500  toward the suction side of the compressor  100 . 
     As shown in FIG. 3, the accumulator  500  is composed of a generally columnar accumulator housing  510 , a refrigerant inlet  520  provides at an upper side of the housing  510 , and a refrigerant discharge pipe  530  formed into a J-shape projecting downward and convexly. One end of the refrigerant discharge pipe  530  is positioned at an upper side of a liquid surface of liquid phase refrigerant accumulated in the housing  510 , and open at a lower side of the refrigerant inlet  520  for conducting gaseous phase refrigerant to the suction side of the compressor  100  through the refrigerant discharge pipe  530 . The refrigerant discharge pipe  530  further has a lubricating oil suction port  531  at the lower end thereof for sucking lubricating oil accumulated at the lower side of liquid phase refrigerant. The lubricating oil separated by and accumulated in the accumulator  500  is sucked into the compressor  100  together with the gaseous phase refrigerant flowing in the refrigerant discharge pipe  530 . 
     Here, in the present embodiment, oil containing polyalkylene glycol (PAG) as a main component is used as lubricating oil (refrigerating machine oil). Here, it should be noted that the content of the main component in oil may be 100%. 
     Next, effects and features of the present embodiment are described. As described above, PAG has extremely small electric insulation resistance. As opposed to this, carbon dioxide is generally insulating material. Therefore, PAG mixed with carbon dioxide can exhibit, as shown in FIG. 4, electric insulation resistance of 1 GΩ or more, thereby securing electric insulation resistance that causes no problems on a practical use. That is, since carbon dioxide has extremely large electric insulation resistance, lubricating oil mixed with refrigerant (carbon dioxide) can exhibit a sufficient magnitude of the electric insulation resistance encountering no problems on a practical use regardless of the magnitude of the electric insulation resistance of the lubricating oil. 
     PAG has low compatibility relative to carbon dioxide as compared to POE, it is difficult that a large amount of liquid refrigerant dissolved in lubricating oil is sucked into the compressor  100  together with lubricating oil. Therefore, the efficiency of the compressor  100  is prevented from being lessened. Thus, according to the present embodiment, the efficiency of the compressor can be prevented from being lessened while securing the electric insulation resistance that causes no problems on a practical use. 
     Incidentally, in case where the accumulator cycle in the present embodiment works to remove heat at a heating operation, there is a possibility that the density of liquid phase refrigerant exceeds the density of lubricating oil when the temperature of refrigerant is lowered excessively (for instance, to about −35° C. to −40° C.) at the side of the evaporator  400 . Liquid phase refrigerant having the density larger than that of lubricating oil may move downward to be sucked into the lubricating oil suction port  531  in the accumulator  500 . 
     To prevent this problem, in the present embodiment, various kinds of additives are added to PAG to increase the density of lubricating oil so that the density of liquid phase refrigerant does not exceed the density of lubricating oil even at low temperature of −35° C. to −40° C. Specifically, the density of lubricating oil must be increased to be larger than 1115 kg/m 3  at −35° C. to −40° C. that is the density of refrigerant (carbon dioxide) at −35° C. to −40° C. 
     (Second Embodiment) 
     A second preferred embodiment adopts oil containing poly(vinyl ether) (PVE) as a main component of lubricating oil. The compatibility of PVE is higher than that of PAG, but sufficiently lower than that of POE. Therefore, PVE can exhibit properties as effective as those of PAG. Here, it should be noted that a content of the main component in oil may be 100%. 
     (Third Embodiment) 
     In a third preferred embodiment, lubricating oil can be supplied to the compressor  100  even in state where temperature of refrigerant is lowered excessively. Specifically, oil containing PAG or PVE as a main component is mixed with oil (POE in the present embodiment) having compatibility higher than that of the main component. The compatibility of lubricating oil can be secured at an appropriate level while being prevented from being increased excessively, by mixing lubricating oil with an appropriate amount of POE or the like having high compatibility. 
     Accordingly, even when the temperature of refrigerant is lowered excessively so that liquid phase refrigerant is easily sucked into the compressor  100 , since lubricating oil have compatibility, the lubricating oil is supplied into the compressor  100  together with liquid phase refrigerant. Thus, even in case where the supply amount of lubricating oil is liable to be deceased due to excessive fall in temperature of refrigerant and the like, a sufficient amount of lubricating oil can be supplied into the compressor  100 . 
     While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.