REFRIGERATION SYSTEM CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No, 60/894,052, entitled SYSTEMS AND METHODS OF USING CO2 IN REFRIGERATION AND AIR CONDITIONING APPLICATIONS, filed March 9, 2007, and U.S. Provisional Application No. 60/917,175, entitled SYSTEMS AND METHODS OF USING NATURAL REFRIGERANTS, filed May 10, 2007, which are hereby incorporated by reference.

BACKGROUND

[0002] The application generally relates to lubrication systems. The application relates more specifically to systems and methods of lubricating multistage and single refrigeration systems that use carbon dioxide (CO2),

[0003] Multistage refrigeration systems (also referred to as cascade refrigeration systems or multi-pressure refrigeration systems) can be used when several evaporators are needed to provide various temperatures for a single application. For example, a multistage refrigeration system can be used to provide the necessary cooling for both refrigerated cases and freezer cases in a supermarket. A multistage refrigeration system can also be used to provide an evaporator temperature lower than that attainable by a single-stage system, e.g., a vapor compression system. For example, a multistage refrigeration system can be used in an industrial process to provide temperatures of between -20 deg C and - 50 deg C or colder, as may be required in a plate freezer application.

[0004] One type of multistage refrigeration system can involve the interconnection of two or more closed loop refrigeration systems in which the heat-absorbing stage, e.g., evaporator, of one system is in a heat exchange relationship with the heat-rejecting stage, e.g., condenser, of the other system. One of the purposes of a multistage refrigeration system having the heat-absorbing stage of one system in a heat exchange relationship with the heat-rejecting stage of the other system is to permit the attaining of temperatures in the heat-rejecting or heat-absorbing stage of one of the systems that exceeds that which

can be attainable if only a single system is used with conventional heat-rejecting or heat- absorbing loads,

[0005] Multistage refrigeration systems have various components that require lubrication for proper operation. One such component is a compressor, which may have shaft bearings and a rotor lubricated by oil. Typically, lubricant is directed to the bearings first to supply lubrication to the compressor. Lubricant can leak outwardly of the bearings at each end toward the motor and may come into contact with the rotor, which is rotating during operation. During rotation, the rotor directs a portion of this lubricant into the flow of refrigerant heading toward the compression chamber(s).

SUMMARY

[0006] The present invention relates to a multistage refrigeration system having a first stage system that circulates a first refrigerant through a first compressor, a first condenser, and a first evaporator. The system also has a second stage system that circulates a second refrigerant through a second compressor, a second condenser, and a second evaporator. The first refrigerant in the first evaporator exchanges heat with the second refrigerant in the second condenser. The first stage system circulates a first lubricant and the second stage system circulates a second lubricant. The first lubricant exchanges heat with the second lubricant and the first lubricant provides lubrication to the first compressor and the second lubricant provides lubricant to the second compressor.

|0007] The present invention also relates to A lubrication system having a compressor with suction bearings and discharge bearings, a receiver that receives lubricant, a pump that circulates lubricant and an oil filter that filter the lubricant. The compressor, receiver, pump, and oil filter are in fluid communication to circulate lubricant and lubricate the suction bearings and discharge bearings in the compressor.

[0008] The present invention further relates to a method for operating a multistage refrigeration system having the steps of providing a second lubricant of a second stage system, directing the second lubricant through a heat exchanger to exchange heat with a

first lubricant of a first stage system, and circulating the second lubricant through a compressor in the second stage system.

BRIEF DESCRIPTION OF THE FIGURES

[0009] FIGS. 1 and 2 show exemplary embodiments of commercial and industrial applications incorporating a refrigeration system.

[0010] FIG. 3 shows a perspective view of an exemplary embodiment of a refrigeration system.

[0011] FIG. 4 shows a side elevational view of the refrigeration system shown in FlG. 3.

[0012] FIG. 5 schematically shows an exemplary embodiment of a multistage refrigeration system.

[0013] FIG. 6 schematically shows an exemplary embodiment of a lubrication circuit for a compressor.

[0014] FlG. 7 schematically shows another exemplary embodiment of a lubrication circuit for a compressor.

[0015] FIG, 8 schematically shows a further exemplary embodiment of a lubrication circuit for a compressor.

[0016] FIG. 9 schematically shows an exemplary embodiment of a lubrication circuit for a compressor.

[0017] FIG. 9A schematically shows the alternate exemplary embodiment of lubrication circuit of FIG. 9.

[0018] FIG. 10 schematically shows another exemplary embodiment of a lubrication circuit for a compressor.

[0019] FIG. 1OA schematically shows the alternate exemplary embodiment of lubrication circuit of FIG. 10.

[0020] FlG. 1 1 schematically shows a further exemplary embodiment of a lubrication circuit for a multistage refrigeration system.

[0021] FIG. 12 schematically shows another exemplary embodiment of a lubrication circuit for a multistage refrigeration system.

[0022] FlG. 13 shows an exemplary embodiment of a pump that may be used with a lubrication circuit for a compressor.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0023] FIGS. 1 and 2 illustrate several exemplary applications for a multistage refrigeration system (also referred to as a cascade refrigeration system or a multi-pressure refrigeration system). Multistage refrigeration systems can include a first stage system (also referred to as a high side system) and a second stage system (also referred to as a low side system) that are interconnected by a heat exchanger and can be used to provide different levels of cooling capacity and/or achieve low temperatures that are difficult to achieve with a single vapor compression cycle.

[0024] FIG. 1 shows an application of an exemplary multistage refrigeration system 10 that can provide both refrigeration and freezing capacity for a supermarket 12 in a commercial setting. The second stage system of multistage refrigeration system 10 can have evaporators incorporated into refrigerated cases or displays 14 and freezer cases or displays 16 that are accessible by a person shopping in supermarket 12. According to an exemplary embodiment, refrigerated cases or displays 14 can be used to keep produce or dairy products at a preselected temperature and can be operated at a temperature between about 2 deg C and about 7 deg C. According to an alternate exemplary embodiment, freezer cases or displays 16 can be used to keep frozen items at a preselected temperature and can be operated at a temperature between about -20 deg C and about -30 deg C. The second stage system of multistage refrigeration system 10 can have an evaporator 18 in a freezer storage area 20 of supermarket 12 and can have an evaporator 22 in a refrigerated storage area 24 of supermarket 12. According to yet another exemplary embodiment, freezer storage area 20 can be used to store items to be subsequently placed in freezer

cases or displays 16 at a preselected temperature and can be operated at a temperature between about -20 deg C and about -30 deg C. According to another exemplary embodiment, refrigerated storage area 24 can be used to store items to be subsequently placed in refrigerated cases or displays 14 at a preselected temperature and can be operated at a temperature between about 2 deg C and about 7 deg C.

[0025] FIG, 2 shows the use of a multistage refrigeration system 10 as a plate freezer 28 in a factory or industrial setting 26. Plate freezer 28 may have horizontal or vertical plates 30 to freeze flat products, such as pastries, fish fillets, and beef patties, as well as irregular-shaped vegetables that are packaged in brick-shaped containers, such as asparagus, cauliflower, spinach, and broccoli. The product may be firmly pressed between metal plates 30 that are cooled to subfreezing temperatures by internally circulating refrigerant from the second stage system through thin channels within plates 30. A high rate of heat transfer can be obtained between the product and plates 30. According to an exemplary embodiment, plate freezers 28 provide cooling temperatures of between about -20 deg C and about -50 deg C or colder and can be used when rapid freezing is desired to retain product flavor and freshness. Once the product is frozen between plates 30, the product may be difficult to remove from plate freezer 28 because the product may be frozen to plates 30. A defrost system that warms plates 30 but does not thaw the product between plates 30 is used to assist in the removal of the product from between plates 30. FIGS. 1 and 2 illustrate exemplary applications only and multistage refrigeration systems are used in many other environments as well.

[0026] FIGS. 3 through 5 illustrate a multistage refrigeration system (shown schematically in FIG. 5). The multistage refrigeration system can include a first stage system 32 and a second stage system 34 that are interconnected by a heat exchanger 36. Heat exchanger 36 can be a plate heat exchanger, a shell and tube heat exchanger, a plate and shell heat exchanger or any other suitable type of heat exchanger. First stage system 32 can be a vapor compression system that circulates a refrigerant through a compressor 38, a condenser 40, a receiver 42 (optional), an expansion device 44, and an evaporator 46 that is incorporated into heat exchanger 36. Fluids that can be used as refrigerants in first stage system 32 are carbon dioxide (CO2; R-744), nitrous oxide (N2O; R-744A),

ammonia (NH3; R-717), hydrofluorocarbon (HFC) based refrigerants (R-410A, R-407C, R-404A, R- 134a), other low global warming potential (GWP) refrigerants, and any other suitable type of refrigerant.

[0027] Second stage system 34 can be a vapor compression system that circulates a refrigerant through a compressor 48, a condenser 50 that is incorporated into heat exchanger 36, a receiver or separator 52, a pump 54, and a first expansion device 56 and a first evaporator 58 that can be in parallel with a second device 60, such as a valve, and second evaporator 62. In another embodiment, second stage system can be operated with only first expansion device 56 and first evaporator 58. In still another embodiment, second stage system 34 can be operated as a volatile system by removing compressor 48, first expansion device 56 and first evaporator 58. Fluids that can be used as refrigerants in second stage system 34 are carbon dioxide (CO2; R-744), nitrous oxide (N2O; for example, R-744A), or mixtures of carbon dioxide and nitrous oxide, or hydrocarbon based refrigerants (R-170). When second stage system 34 is operated as a volatile system, the refrigerant circulating through the system can be replaced with a glycol solution or a brine solution.

[0028] In first stage system 32, when operated sub-critically, i.e., below the critical pressure for the refrigerant being circulated in first stage system 32, compressor 38 compresses a refrigerant vapor and delivers the compressed vapor to condenser 40 through a discharge line. Compressor 38 can be a screw compressor, reciprocating compressor, centrifugal compressor, rotary compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable type of compressor. The refrigerant vapor delivered by compressor 38 to condenser 40 enters into a heat exchange relationship with a fluid, e.g., water from a cooling tower, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser 40 can be stored in receiver 42 before flowing through expansion device 44 to evaporator 46 in heat exchanger 10.

[0029] The condensed liquid refrigerant delivered to evaporator 46 in heat exchanger 36 enters into a heat exchange relationship with the fluid being circulated in condenser 50 in

heat exchanger 36 by second stage system 34, and undergoes a phase change to a refrigerant vapor as a result. The vapor refrigerant in evaporator 46 exits evaporator 46 and returns to compressor 38 by a suction line to complete the cycle.

[0030] First stage system 32 can be operated as a transcritical or supercritical system. During transcritical operation, first stage system 32 can be operated partly below (sub- critical) and partly above (supercritical) the critical pressure of the refrigerant circulated in first stage system 32. The discharge pressure of compressor 38 (or high side pressure) can be greater than the critical pressure of the refrigerant, e.g., 73 bar at 31 deg C for carbon dioxide. Furthermore, during transcritical operation, the refrigerant is maintained as a single phase refrigerant (gas) in the high pressure side of first stage system 32 and is first converted into the liquid phase when it is expanded in expansion device 44. When operated as a transcritical system, the refrigerant from compressor 38 flows to a gas cooler (which can operate as a condenser in low ambient temperatures permitting the system to operate sub-critical) that cools the refrigerant by heat exchange with another fluid. The cooling of the refrigerant gradually increases the density of the refrigerant. The refrigerant in the second stage can be the same or different than the refrigerant in the first stage. During transcritical operation of first stage system 32, the high side pressure can be modulated to control capacity or to optimize the coefficient of performance by regulating the refrigerant charge and/or by regulating the total internal high side volume of refrigerant.

[0031] In second stage system 34, compressor 48 compresses a refrigerant vapor and delivers the compressed vapor to condenser 50 through a discharge line. Compressor 48 can be a screw compressor, reciprocating compressor, centrifugal compressor, rotary compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable type of compressor. The refrigerant vapor delivered by compressor 48 to condenser 50 in heat exchanger 36 enters into a heat exchange relationship with the fluid being circulated in evaporator 46 by first stage system 32, and undergoes a phase change to a refrigerant liquid as a result. The condensed liquid refrigerant from condenser 50 is circulated to receiver 52. The liquid refrigerant in receiver 52 is circulated to first

expansion device 56 and first evaporator 58 and then to second device 60 and second evaporator 62 by pump 54.

[0032] In first evaporator 58, the liquid refrigerant from first expansion device 56 enters into a heat exchange relationship with a cooling load, e.g., a fluid, and undergoes a phase change to a refrigerant vapor as a result. The refrigerant vapor in first evaporator 58 exits first evaporator 58 and returns to compressor 48 to complete the cycle. In second evaporator 62, the liquid refrigerant from second device 60 enters into a heat exchange relationship with a cooling load, e.g., a fluid, and may undergo a phase change to a refrigerant vapor as a result. According to an exemplary embodiment, the amount of refrigerant liquid provided to second evaporator 62 may exceed the heat exchange capabilities of the cooling load causing less than all of the liquid refrigerant to undergo a phase change. Thus, the refrigerant fluid leaving second evaporator 62 may be a mixture of refrigerant vapor and refrigerant liquid. The refrigerant fluid exiting second evaporator 62, whether refrigerant vapor or a mixture of refrigerant vapor and refrigerant liquid, returns to receiver 52. Receiver 52 can also have a connection to the discharge line from compressor 48 to provide refrigerant vapor from receiver 52 to the discharge line and subsequently to condenser 50.

[0033] Compressor 38 of first stage system 32 and compressor 48 of second stage system 34 can each be driven by a motor or drive mechanism. The motor used with compressor 38 or compressor 48 can be powered by a variable speed drive (VSD) or can be powered directly from an alternating current (AC) or direct current (DC) power source. The VSD, if used, receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source and provides power having a variable voltage and frequency to the motor. The motor used with compressor 38 or compressor 48 can be any type of electric motor that can be powered by a VSD or directly from an AC or DC power source. For example, the motor used with compressor 38 or compressor 48 can be a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or any other suitable motor type. In an alternate embodiment, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive the motor used with compressor 38 or compressor 48.

[0034] In multistage refrigeration systems, and other refrigeration systems with a compressor, the compressor is lubricated with a lubricant or oil, for example, miscible oil or oil entrained with refrigerant. During operation of the compressor, refrigerant is mixed with lubricant in the compressor and is circulated to an oil separator. Although the oil separator removes refrigerant from the lubricant, the oil separator typically may not remove a sufficient amount of the refrigerant from the lubricant before the lubricant is circulated back to the compressor. Lubricant diluted with refrigerant has a reduced viscosity, causing the lubricant to ineffectively lubricate the compressor. Lubricant diluted with refrigerant expands within the compressor, releasing the refrigerant from the lubricant. The refrigerant is released in a vapor form and may cause corrosion of the internal components of the compressor, such as the bearings. The compressor may operate at low temperatures with cold lubricant that easily absorbs refrigerant. Lubricant may be heated to substantially prevent refrigerant from absorbing into the lubricant and damaging the compressor. Lubrication circuits may be incorporated into multistage refrigeration systems, or other refrigerant systems, to substantially improve issues with lubrication.

[0035] FlG. 6 illustrates an exemplary low-pressure lubrication circuit 64 that uses miscible lubricant or oil in compressor 48. Lubrication circuit 64 conditions the lubricant by limiting the amount of refrigerant absorbed into the lubricant and controlling the temperature of the lubricant to achieve a predetermined viscosity for improved compressor performance. While any suitable compressor may be used, an exemplary embodiment may include a screw compressor. The lubricant is circulated from receiver 68 through an expansion device 66 to a low-pressure receiver 70. Receiver 68 has a pressure substantially equal to the discharge pressure of compressor 48 and receiver 70 has a pressure substantially equal to the suction pressure of compressor 48. The discharge pressure of compressor 48 may be substantially higher than the suction pressure. Low- pressure receiver 70 discharges vapor refrigerant to compressor 48. Lubricant from low- pressure lubricant receiver 70 is pumped from the discharge of low-pressure lubricant receiver 70 by pump 72 to compressor 48. The lubricant circulates through the bearing circuit (not shown) of compressor 48 and is discharged back to low-pressure lubricant receiver 70. In the bearing circuit, some of the lubricant may leak through the bearings

and rotor in compressor 48 to the vapor refrigerant exiting compressor 48. Low-pressure lubricant receiver 70 operates at a pressure identical to the suction pressure. At that operational pressure level of the receiver 70, the lubricant or oil degasses, or is separated from, substantially all of the refrigerant that was entrained in the lubricant. A heating element 74 heats low-pressure lubricant receiver 70, thereby warming the lubricant in low-pressure lubricant receiver 70. In another embodiment, heating element 74 may include a heat exchanger in which warm lubricant received from receiver 68 exchanges heat with lubricant received from low-pressure lubricant receiver 70. Vapor refrigerant is discharged from compressor 48 into receiver 68. The vapor refrigerant may mix with lubricant when discharged from compressor 48. Receiver 68 removes the lubricant from the vapor refrigerant and discharges the vapor refrigerant to the refrigerant circuit. A portion of the lubricant removed from the vapor refrigerant in receiver 68 is discharged back to compressor 48 by a direct flow path and through low-pressure lubricant receiver 70 through expansion device 66. Expansion device 66 may be an expansion valve, but may be any suitable expansion device.

[0036] FIG. 7 illustrates an alternate embodiment of lubrication circuit 64 where lubricant from the bearing circuit 90 is circulated from leakage in the rotor circuit 91. Bearing lubrication circuit 90 may have a pump 72 that supplies lubricant from low- pressure receiver 70 to compressor 48. A portion of the lubricant circulates through heating element 84 and a portion of the lubricant bypasses heating element 84 and is mixed in a mixing valve 76 with the heated lubricant discharged from heating element 84. An oil filter 78 receives the mixture of heated and bypassed lubricant from mixing valve 76 and circulates the lubricant to a temperature sensor 80. The lubricant circulates to bearings 82 within compressor 48. Lubricant is drained from bearings 82 to receiver 70, where heating element 74 heats the drained lubricant collected in the bottom of receiver 70. A portion of the lubricant collected in receiver 70 is supplied to pump 72 for circulation to bearing lubrication circuit 90. A flow control device, or valve 86 may drain excess lubricant from receiver 70 to supply suction of compressor 48. Excess lubricant from rotor 88 is discharged to receiver 68. Vapor discharge from receiver 68 is supplied to the refrigeration circuit (not shown), and the lubricant collected in receiver 68 is returned to rotor 88 for lubrication,

[0037] FIG. 8 illustrates another alternate embodiment of lubrication circuit 64 described with respect to FIG. 6, where refrigerant is removed from the lubricant or oil before entering the compressor bearings. Lubrication circuit 64 removes refrigerant from the lubricant at two separate stages in the circuit to provide a lubrication supply that is free of refrigerant to compressor 48. First, discharge gas from the refrigerant circuit (not shown) and vapor discharge from low-pressure receiver 70 enter compressor 48. The vapor refrigerant mixes with lubricant in compressor 48 and is discharged to oil separator 92. A baffle 94 disposed in oil separator 92 prevents lubricant from being suctioned from oil separator 92 and directs the lubricant to the bottom of oil separator 92 where it is discharged to a high-pressure receiver 96. Vapor refrigerant is suctioned from the oil separator and circulated through the refrigerant circuit (not shown). Lubricant is discharged to low pressure receiver 70 from high-pressure receiver 96 through a flow control device, or throttle valve 98. Vapor refrigerant collected in low-pressure receiver 70 is discharged to the suction of compressor 48. Refrigerant-free lubricant from low- pressure receiver 70 is circulated by pump 72 to compressor 48 through oil filter 78.

[0038] FIG. 9 illustrates yet another embodiment of lubrication circuit 64, where two separate lubrication circuits are used to supply lubricant or oil to compressor 48. Compressor 48 may be any suitable type of compressor, for example a screw compressor or a scroll compressor. Discharge lubrication circuit 108 may include a reservoir 1 10 at discharge pressure, a pump 1 12 and an oil filter 1 14. Suction lubrication circuit 100 may include a reservoir 102, a pump 104 and an oil filter 106. Discharge lubrication circuit 108 and suction lubrication circuit 100 are separate and provide lubrication to bearings 82 within compressor 48. A third lubrication circuit (not shown), a rotor lubrication circuit, may lubricate the rotor 88 of compressor 48. Discharge lubrication circuit 108 provides lubrication to the discharge side of compressor 48. Lubricant is circulated from bearings 82 to reservoir 1 10. Liquid lubricant is then circulated by pump 1 12 from reservoir 1 10 through oil filter 1 14 and back to bearings 82. Suction lubrication circuit 100 provides lubricant to bearings 82 on the inlet side of compressor 48. Lubricant is circulated from bearings 82 to reservoir 102. Liquid lubricant is circulated by pump 104 from reservoir 102 through oil filter 106 and back to bearings 82. In both lubrication circuits 100 and 108, pump 104 and pump 1 12 provide circulation of lubricant to bearings 82.

[0039] FlG. 9A illustrates yet another embodiment of lubrication circuit 64, where one lubrication circuit is used to supply lubricant or oil to compressor 48. Lubrication circuit

107 may include a reservoir 102, a pump 104 and an oil filter 106. Lubricant is circulated from bearings 82 to reservoir 102. Liquid lubricant is then circulated by pump !04 from reservoir 102 through oil filter 106 and back to bearings 82.

[0040] FIG. 10 illustrates an alternate embodiment of lubrication circuit 64 as shown in FIGS. 9 and 9 A. Discharge lubrication circuit 108 may include a reservoir 1 10 at discharge pressure, a pump 1 12 and an oil filter 1 14. Suction lubrication circuit 100 may include a reservoir 102, a pump 104 and an oil filter 106. Discharge lubrication circuit

108 provides lubrication to the discharge side of compressor 48. Reservoir 1 10 is disposed beneath compressor 48 and bearings 82 to facilitate gravitational flow of the lubricant from bearings 82 to reservoir 1 10. Lubricant is discharged from reservoir 1 10 through oil filter 1 14 and circulated by pump 1 12 to bearings 82. Suction lubrication circuit 100 provides lubrication to the discharge side of compressor 48. Reservoir 102 is disposed beneath compressor 48 and bearings 82 to facilitate gravitational flow of the lubricant from bearings 82 to reservoir 102. Lubricant is discharged from reservoir 102 through oil filter 106 and circulated by pump 104 to bearings 82.

[0041] FIG. 1 OA illustrates an alternate embodiment of lubrication circuit 64. Lubrication circuit 109 may include a reservoir 102, a pump 104 and an oil filter 106. Reservoir 102 is disposed beneath compressor 48 and bearings 82 to facilitate gravitational flow of the lubricant from bearings 82 to reservoir 102. Lubricant is discharged from reservoir 102 through oil filter 106 and circulated by pump 104 to bearings 82.

[0042] FIG. 1 1 illustrates a lubrication circuit 64 that uses heat from first stage system 32 to heat lubricant circulating to compressor 48 in second stage system 34. First stage system 32 may use ammonia as a refrigerant and may include compressor 38, oil separator 92, condenser 40, heat exchanger 74, cooler 1 18, valve 120 and evaporator 46, which is incorporated in heat exchanger 36. Second stage system 34 may use carbon dioxide as a refrigerant and may include compressor 48, oil separator 122, valve 78,

evaporator 58, and condenser 50, which is incorporated in heat exchanger 36. Discharge refrigerant mixed with lubricant from heat exchanger 36 is supplied to compressor 38, and then discharged from compressor 38 to oil separator 92 where refrigerant is separated from lubricant. Refrigerant is circulated to condenser 40 before returning to heat exchanger 36. Lubricant collected at the bottom of oil separator 92 is circulated to heating exchanger 74. After being cooled in heating element 74, lubricant is circulated through cooler 1 18 to further reduce the temperature of the lubricant before entering compressor 38 to lubricate compressor 38. Cold lubricant form compressor 48 is circulated to heat exchanger 74 to exchange her and be heated with heat from lubricant from oil separator 92. The lubricants are contained within separate lubrication circuits for each of compressors 48 and 38. After circulating through heating exchanger 74, lubricant returns to compressor 48 to lubricate compressor 48. Refrigerant mixed with lubricant is discharged to oil separator 122 where the refrigerant is removed from the lubricant and is discharged to heat exchanger 36.

[0043J Refrigerant discharged from heat exchanger 36 to valve 78 and circulated to evaporator 58 before entering compressor 48. Heating element 74 maintains lubricant temperature between approximately 60 deg C and 80 deg C without regulation of a temperature control valve or other control device.

[0044] Compressor 48 has a suction end and a discharge end, each end having a pressure level. According to an exemplary embodiment, the pressure level of the suction end and the discharge end may be substantially unequal. These pressure levels contribute to seeping or leaking of refrigerant from the bearing circuit (not shown) within compressor 48 and the rotor circuit (not shown) within compressor 48. Any excess vapor refrigerant that seeps or leaks from rotor 88 (not shown) from first stage system 32 is circulated back into the refrigeration circuit of second stage system 34.

[0045] FlG. 12 illustrates a low temperature lubrication circuit 64 that uses heat from the lubrication circuit in first stage system 32 to heat lubricant in the lubrication circuit of second stage system 34. First stage system 32 may use ammonia as a refrigerant and may include compressor 38, oil separator 92, condenser 40, heat exchanger 74, cooler 1 18,

valve 120 and evaporator 46, which is incorporated in heat exchanger 36. Second stage system 34 may use carbon dioxide as a refrigerant and may include compressor 48, oil separator 122, valve 78, evaporator 58, and condenser 50, which is incorporated in heat exchanger 36. Discharge refrigerant mixed with lubricant from heat exchanger 36 is supplied to compressor 38, and then discharged from compressor 38 to oil separator 92 where refrigerant is separated from lubricant. Refrigerant is circulated to condenser 40 before returning to heat exchanger 36. Lubricant collected at the bottom of oil separator 92 is circulated to heat exchanger 74. After being cooled in heat exchanger 74, lubricant is circulated through cooler 118 to further reduce the temperature of the lubricant before entering compressor 38 to lubricate compressor 38. Cold lubricant from compressor 48 is circulated to heat exchanger 74 to exchange heat and be heated with lubricant from oil separator 92. The lubricants are contained within separate lubrication circuits for each of compressors 48 and 38. After circulating through heating element 74, lubricant returns to compressor 48 to lubricate compressor 48. Refrigerant mixed with lubricant is discharged to oil separator 122 where the refrigerant is removed from the lubricant and is discharged to heat exchanger 36. Refrigerant discharged from heat exchanger 36 to valve 78 and circulated to evaporator 58 before entering compressor 48. Heat exchanger 74 maintains lubricant temperature for the lubricant of second stage 34 between approximately 40 deg C and 60 deg C without regulation of a temperature control valve or other control device.

[0046J FIG. 12 illustrates a separate lubrication circuit for the rotor (not shown) in compressor 48, where lubricant collected in oil separator 122 is discharged to heat exchanger 84 to cool the lubricant. The cooled lubricant is circulated to rotor (not shown) of compressor 48, if necessary.

[0047] FIG. 13 illustrates lubrication system 64 having pump 128. Pump 128 is a rotating pump with a motor 130. Motor 130 operates to rotate the pipe 132 and increase the lubricant pressure. The pressure level of pump 128 is substantially equal to the inlet pressure level in the secondary stage system (not shown). Once the lubricant reaches a predetermined level, the lubricant is circulated through compressor 48. Lubricant may be circulated to bearing 82 and/or rotor 88 in compressor 48. While it has been described

that the rotor circuit and bearing circuit having separate receivers and pumps, the rotor circuit and bearing circuit may share a receiver and a pump.

[0048] In the embodiments described, the lubricant may be heated by a heat exchanger, heating element, or other heating circuits or systems. The lubricant may exchange heat with an inverter, with discharge gas from a diesel engine coolant system exhaust, or other heat producing system.

[0049] While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.