Patent Publication Number: US-10309704-B2

Title: Compressor with an oil separator between compressing stages

Description:
TECHNICAL FIELD 
     The present application and the resultant patent relate generally to refrigeration systems and more particularly relate to refrigeration systems using carbon dioxide as the refrigerant and having a two-stage compressor with an oil separator therein. 
     BACKGROUND OF THE INVENTION 
     Modern refrigeration systems provide cooling, ventilation, and humidity control for all or part of an enclosure. Such enclosures may include a refrigerator, a cooler, a vendor, a dispenser, and other types of light commercial or household appliances. 
     Because of environmental, financial, and other reasons, these modern refrigeration systems are increasing moving away from the use of synthetic refrigerants such as hydrofluorocarbons. Given such, there is an increased interest in the use of natural refrigerants such as carbon dioxide and the like. The use of carbon dioxide as the refrigerant may have the advantages of being relatively inexpensive, readily available, non-toxic, nonflammable, and environmentally friendly. Moreover, carbon dioxide generally has a higher volumetric capacity as compared to most common synthetic refrigerants. 
     Generally described, a carbon dioxide refrigeration cycle may be similar to other types of refrigeration cycles but may operate at higher pressures and may not involve a change in state. The typical supercritical carbon dioxide refrigeration cycle may include compressing the flow of carbon dioxide within a compressor at a high pressure and a high temperature. Second, the compressed carbon dioxide may be cooled within a gas cooler or other type of heat exchanger by heat exchange with the surrounding environment. Third, the carbon dioxide may pass through an expansion device that reduces both the pressure and the temperature. Fourth, the carbon dioxide may be pumped to an evaporator or a further heat exchanger where the carbon dioxide may absorb heat from an enclosure so as to provide cooling therein. The flow of carbon dioxide then may be returned to the compressor so as to repeat the cycle. Many variations on such a carbon dioxide refrigeration cycle may be known. 
     One way to improve the efficiency of a carbon dioxide refrigeration system is to use a two-stage compressor. The miscibility of oil in carbon dioxide in such carbon dioxide refrigeration systems, however, may be greater as compared to typical synthetic refrigerants at high operating pressures. Moreover, the miscibility of oil in carbon dioxide may increase as the pressure increases. Such an increase in the oil content of the refrigerant may present a challenge at the evaporator and elsewhere. Specifically, oil may begin to accumulate in the evaporator as the temperature of the refrigerant is reduced. 
     Moreover, the viscosity of the oil may increase so as to lead potentially to increased maintenance needs, premature failure of the components, clogging, and other types of ongoing maintenance issues. 
     Although different types of oil separators are known, such systems generally require pumps and/or complex valve arrangements due to the pressure differential between the inlet and outlet of the compressor. Moreover, such known oil separators may be ineffective with respect to two-stage compressors given that the oil is needed within a shell casing at an intermediate pressure. 
     There is thus a desire for an improved carbon dioxide refrigeration system for use with light commercial or household appliances and the like. Such an improved carbon dioxide refrigeration system may accommodate the increased miscibility of oil in the carbon dioxide refrigerant at higher pressures for an increase in overall system performance and efficiency with a reduction in maintenance requirements. 
     SUMMARY OF THE INVENTION 
     The present application and the resultant patent thus provide a compressor for use with a flow of carbon dioxide. The compressor may include a first stage compression mechanism for compressing the flow of carbon dioxide from a low pressure to an intermediate pressure, an oil separator downstream of the first stage compression mechanism, and a second stage compression mechanism positioned downstream of the oil separator for compressing the flow of carbon dioxide from the intermediate pressure to a high pressure. 
     The present application and the resultant patent further provide a method of compressing a flow of carbon dioxide for use in a refrigeration system. The method may include the steps of compressing the flow of carbon dioxide from a low pressure to an intermediate pressure in a first stage compressor, passing the flow of carbon dioxide at the intermediate pressure through an oil separator, and then compressing the flow of carbon dioxide from the intermediate pressure to a high pressure in a second stage compressor. 
     The present application and the resultant patent thus provide a compressor for use with a flow of a refrigerant. The compressor may include a shell casing, a first stage compression mechanism for compressing the flow of the refrigerant from a low pressure to an intermediate pressure positioned within the shell casing, an oil separator downstream of the first stage compression mechanism and positioned outside the shell casing, a second stage compression mechanism downstream of the oil separator for compressing the flow of refrigerant from the intermediate pressure to a high pressure positioned within the shell casing, and a motor to drive the first stage compression mechanism and the second stage compression mechanism positioned within the shell casing. The oil separator may include an expansion chamber and/or a J-tube positioned within the expansion chamber and/or an oil drain in communication with a shell casing. The refrigerant may include a flow of carbon dioxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic diagram of a known carbon dioxide refrigeration system. 
         FIG. 2  is a pressure/enthalpy chart showing the work savings in a two-stage compressor. 
         FIG. 3  is a schematic diagram of a known two-stage compressor for use in the refrigeration system of  FIG. 1 . 
         FIG. 4  is a schematic diagram of a two-stage compressor with an oil separator as may be described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  shows an example of a refrigeration system  10  as may be described herein. The refrigeration system  10  may be used to cool any type of enclosure such as a refrigerator, a cooler, a vendor, a dispenser, and the like. The overall refrigeration system  10  may have any suitable size or capacity. The refrigeration system  10  also may be applicable to air conditioning and/or heating systems. Although primarily directed towards light commercial or household appliances, the refrigeration system  10  also may have other types of commercial, industrial, and/or residential applications. 
     The refrigeration system  10  may include a compressor  15 . The compressor  15  may have any suitable size or capacity. The compressor  15  may compress a flow of refrigerant  20  at a high pressure and a high temperature. In this example, the refrigerant  20  may be a flow of carbon dioxide  25 . The flow of carbon dioxide  25  may be in a supercritical cycle or in a sub-critical cycle depending upon the ambient temperatures in which the compressor  15  operates and other types of operational parameters. 
     The refrigeration system  10  may include a gas cooler  27  or other type of heat exchanger positioned downstream of the compressor  15 . The gas cooler  27  may have any suitable size or capacity. The gas cooler  27  may include a number of coils  30  therein or other type of heat exchange surface. A gas cooler fan  35  may be positioned adjacent thereto. The gas cooler fan  35  may be a single speed fan, a variable feed fan, and the like. The gas cooler  27  may cool the flow of carbon dioxide  25  through heat exchange with the surrounding environment. 
     The refrigeration system  10  may include an expansion device  40  downstream of the gas cooler  27 . The expansion device  40  may have any suitable size or capacity. The expansion device  40  may reduce the pressure and temperature of the flow of carbon dioxide  25 . The expansion device  40  may include a number of capillary tubes and the like therein. 
     The refrigeration system  10  also may include an evaporator  45  or other type of heat exchanger positioned downstream of the expansion device  40 . The evaporator  45  may have any suitable size or capacity. The evaporator  45  may include a number of evaporator coils  50  or other type of heat exchange surface. An evaporator fan  55  may be positioned adjacent thereto. The evaporator fan  55  may be a single speed fan, a variable speed fan, and the like. The flow of carbon dioxide  25  may be pumped to the evaporator  45 . The flow of carbon dioxide  25  may absorb heat with a flow of air blown or drawn across the evaporator coils  50  by the evaporator fan  55  so as to cool an enclosure and the like. The flow of carbon dioxide  25  then may be returned to the compressor  15  so as to repeat the cycle. Other components and other configurations may be used herein. The refrigeration system  10  described herein is for the purpose of example only. Many other types of refrigeration systems, refrigeration components, and refrigerants may be known. 
     As described above, one way to improve the efficiency of the refrigeration system  10  is to use a two-stage compressor  60 . As is shown in the pressure-enthalpy chart of  FIG. 2 , the refrigerant  20  may be input to the compressor  60  at a low pressure P L , may be compressed to an intermediate pressure P M  in a first stage of the compressor, cooled while maintaining the intermediate pressure P M , and then be compressed to a high pressure P H  in a second stage of the compressor. As a result, a savings in the amount of total work required to be performed by the compressor  60  may be realized as is shown in the crosshatched area of the graph. 
       FIG. 3  shows an example of the two-stage compressor  60 . The components of the two-stage compressor  60  may be enclosed within a shell casing  65 . The shell casing  65  may be suitable for enclosing at least an intermediate pressure fluid. The shell casing  65  may have any suitable size, shape, or configuration. A conventional DC motor  70  may be positioned within the shell casing  65 . Other types of motors and other types of drive means may be used herein. 
     The two-stage compressor  60  may include a first stage compression mechanism  75 . The first stage compression mechanism  75  may be driven by the motor  70 . The first stage compression mechanism  75  may compress a fluid via rotary displacement or other types of compression techniques. The first stage compression mechanism  75  may compress an incoming low pressure flow  80  to an intermediate pressure flow  82 . The first stage compression mechanism  75  may discharge the intermediate pressure flow  82  within the shell casing  65 . A second stage pathway  84  may extend between the shell casing  65  and a second stage compression mechanism  86 . The second stage compression mechanism  86  may be driven by the motor  70  or otherwise. 
     The second stage compression mechanism  86  may compress a fluid via rotary displacement or other types of compression techniques. The second stage compression mechanism  86  may compress the intermediate pressure flow  82  into a high pressure flow  88 . The high pressure flow  88  may be discharged towards the gas cooler  25  or elsewhere. Other components and other configurations also may be used. Other types of two stage compressors may be known. 
     Also as described above, although the two-stage compressor  60  improves the efficiency of the overall refrigeration system  10 , the miscibility of oil  90  in the flow of the carbon dioxide refrigerant  25  may increase as the pressure increases. Specifically, the percentage of oil  90  within the flow of refrigerant  25  may increase several times between the low pressure side and the high pressure side of the compressor  15  and the overall refrigeration system  10 . The presence of the oil  90  in the refrigerant  20  thus may present maintenance issues and the like. 
       FIG. 4  shows an example of a two-stage compressor  100  as may be described herein. Similar to that described above, the two-stage compressor  100  may include a shell casing  110 . The shell casing  110  may be suitable for enclosing at least an intermediate pressure fluid. The shell casing  110  may have any suitable size, shape, or configuration. A DC motor  120  or other type of drive device may be positioned within the shell casing  110  or elsewhere. Other components and other configurations may be used herein. 
     The two-stage compressor  100  may include a first stage compression mechanism  130 . The first stage compression mechanism  130  may be driven by the motor  120  or otherwise. The first stage compression mechanism  130  may compress a fluid via rotary displacement or other types of compression techniques. The first stage compression mechanism  130  may have any suitable size or capacity. The first stage compression mechanism  130  may have a first stage input  140 . The first stage input  140  may be in communication with a low pressure flow  150  of a carbon dioxide refrigerant  155 . Other types of refrigerants also may be used herein. The first stage compression mechanism  130  may compress the low pressure flow  150  into an intermediate pressure flow  160 . The first stage compression mechanism  130  may have a first stage output  170 . The first stage output  170  may discharge the intermediate pressure flow  160  into the shell casing  110 . Due to the discharge of the carbon dioxide refrigerant  155  within the shell casing  110 , an amount of oil  175  thus may reside in the bottom thereof. Other components and other configurations may be used herein. 
     The shell casing  110  may include a second stage pathway  180 . The second stage pathway  180  may extend from the shell casing  110  to a second stage compression mechanism  190 . The second stage compression mechanism  190  may be driven by the motor  120  or otherwise. The same or different motors may drive the respective stages. 
     The second stage compression mechanism  190  may compress a fluid via rotary displacement or other types of compression techniques. The second stage compression mechanism  190  may have any suitable size or capacity. The second stage compression mechanism  190  may include a second stage input  200  in communication with the second stage pathway  180 . The second stage compression mechanism  190  may compress the intermediate flow  160  of the carbon dioxide refrigerant  155  into a high pressure flow  210 . The second stage compression mechanism  190  may include a second stage output  220 . The second stage output  220  may extend out of the shell casing  110  to discharge the high pressure flow  210  towards the gas cooler  25  or elsewhere. Other components and other configurations may be used herein. 
     The two-stage compressor  100  also may include an oil separator  230 . The oil separator  230  may be positioned about the second stage pathway  180  between the first stage mechanism  130  and the second stage mechanism  190  and outside of the shell casing  110 . An input check valve  240  may be positioned about the second stage pathway  180  upstream of the oil separator  230 . The oil separator  230  may include an expansion chamber  250 . The expansion chamber  250  may have any suitable size, shape, or configuration. The oil separator  230  also may include a J-tube  260 . The J-tube  260  may extend from the second stage pathway  180  into the expansion chamber  250 . A wire mesh  270  also may be positioned about an oil pan  280  in the expansion chamber  250 . The oil separator  250  may include an oil drain  290  positioned about the oil pan  280 . The oil drain  290  may extend from the oil pan  280  back towards the shell casing  110 . An output check valve  300  may be positioned on the oil drain  290 . Other components and other configurations may be used herein. 
     In use, the input check valve  240  may prevent back pressure towards the shell casing  110  due to any pressure fluctuations within the oil separator  230 . The oil separator  230  includes the expansion chamber  250  and the J-tube  260  so as to reduce the velocity of the flow of refrigerant  155 . This reduction in the velocity may promote the separation of the oil  175  from the refrigerant  155 . The wire mesh  270  may facilitate the collection of the oil  175  therein. The oil content within the refrigerant  155  exiting the oil separator  230  thus may be reduced before entry into the second stage compression mechanism  190 . The oil drain  290  permits return of the separated oil  175  back into the shell casing  110 . 
     Because the pressure in the oil separator  230  and the shell casing  110  may be similar, the oil  175  should easily drain back into the shell casing  110 . The output check valve  300  may be biased such that a threshold amount of the oil  175  may accumulate within the oil separator  230  before allowing the oil  175  to drain back into the shell casing  110 . The output check valve  300  also may prevent a secondary flow path from the shell casing  110  into the oil separator  230 . The output check valve  300  thus may prevent the refrigerant  155  from bypassing the oil separator  230  so as to ensure that the oil content within the refrigerant  155  entering the second stage compression mechanism  190  may be sufficiently low. 
     The oil separator  230  thus removes excess oil  175  from the flow of refrigerant  155  before entry into the second stage compression mechanism  190  for an increase in overall efficiency and a reduction in maintenance requirements. Moreover, the oil separator  230  avoids the use of complex pumps and/or valve arrangements and/or any type of parasitic drain on the refrigeration system as a whole. Specifically, the use of the two stage compressor  100  allows for two levels of pressure within the shell casing  110 . The oil separator  230  also may be used with other types of refrigerants.