WO2008112594A2

WO2008112594A2 - Vapor compression system

Info

Publication number: WO2008112594A2
Application number: PCT/US2008/056342
Authority: WO
Inventors: Alexander Cohr Pachai; Thomas Severin Christensen; Istvan Knoll; John Ritmann
Original assignee: Johnson Controls Technology Co
Current assignee: Johnson Controls Technology Co
Priority date: 2007-03-09
Filing date: 2008-03-08
Publication date: 2008-09-18
Anticipated expiration: 2009-09-09
Also published as: WO2008112591A2; WO2008112569A3; WO2008112568A3; WO2008112572A1; WO2008112549A2; WO2008112568A2; WO2008112591A3; WO2008112569A2; WO2008112554A1; WO2008112594A3; WO2008112566A3; WO2008112566A2; WO2008112593A1; WO2008112549A3

Abstract

A vapor compression system having an evaporator with a receiver for receiving liquid refrigerant and lubricant from the expansion device. At least one flow path with at least one valve receives liquid refrigerant entrained with lubricant from the receiver, where the at least one valve controls the flow of liquid. A collection vessel having a heating element receives the liquid refrigerant entrained with lubricant from the flow path. An exit flow path having a valve controlling the flow of liquid refrigerant entrained with lubricant. A controller controls the valves and the heating element. When the receiver has a predetermined amount of liquid refrigerant entrained with lubricant, the controller controls valves and the heating element provides heat to the liquid refrigerant entrained with lubricant and the liquid refrigerant becomes a gas refrigerant, and wherein the lubricant exits the vessel via a drain pipe to the compressor.

Description

VAPOR COMPRESSION 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 vapor compression systems. The application relates more specifically to systems and methods of recovering lubricant oil that is entrained in liquid refrigerant and returning the recovered lubricant to a compressor in the vapor compression system. Vapor compression refrigeration is the primary method used to provide mechanical cooling. Generally vapor compression systems consist of four basic components evaporator, compressor, condenser, and an expansion device, which are interconnected in a closed refrigerant loop. The evaporator and condenser are heat exchangers that evaporate and condense the refrigerant while absorbing and rejecting heat. The compressor takes the refrigerant vapors from the evaporator and raises the pressure sufficiently for the vapor to condense in the condenser. The expansion device controls the flow of condensed refrigerant at this higher pressure back into the evaporator.

[0003] Chillers and refrigeration systems typically employ gas compressors to compress refrigerant gas from a vapor state to a liquid state. In many cases, a relatively small amount of lubricant used by the system compressor, such as for bearing lubrication or cooling or sealing purposes, may become entrained in the compressed refrigerant gas discharged from the compressor. Although some of the entrained lubricant is separated from the refrigerant gas, a portion of the lubricant may remain entrained in the refrigerant gas and subsequently flow to the system condenser. At the condenser, the lubricant mixes with liquid refrigerant created by the heat exchange process occurring within the condenser. The mixed stream of lubricant and liquid refrigerant exits the condenser and flows through the system's expansion device and into the system evaporator.

[0004] In a refrigeration system using natural refrigerants such as carbon dioxide (CO2), where the carbon dioxide is evaporated from a vessel, the lubricant may not be evaporated along with the vapor refrigerant. Consequently, the lubricant may not be returned to the compressor, which may cause the compressor lubrication system to be depleted. Particularly in flooded systems that include a pump separator, the loss of lubricant from the compressor lubrication system is a concern.

SUMMARY

[0005] The present invention relates to a vapor compression system. The vapor compression system includes a compressor, a condenser, an expansion device and an evaporator connected in a closed loop, with refrigerant circulated in the closed loop. A first valve is arranged to control a flow of a mixture of liquid refrigerant and lubricant from the evaporator through a first flow path. A collection vessel is arranged to receive a heated lubricant from the compressor. A pumping device is arranged to circulate the mixture by generation of fluid pressure resulting from thermal expansion, the pumping device disposed in the collection vessel. A second valve is arranged to drain the mixture of evaporated refrigerant and lubricant into the compressor through a second flow path. A controller is arranged to control flow of refrigerant and lubricant into and from the pumping device to regulate a level of the mixture in the pumping device.

[0006] The present invention also relates to a vapor compression system. The vapor compression system includes a receiver for receiving liquid refrigerant and lubricant from an expansion device. A portion of the liquid refrigerant and the lubricant is received into the receiver collecting in the receiver. A flow path is arranged to receive the liquid refrigerant and lubricant from the receiver. A pump controls the flow of liquid refrigerant and lubricant. A collection vessel is arranged to receive the liquid refrigerant and lubricant from the flow path. The lubricant is separated from the liquid refrigerant in the collection vessel and the lubricant returns to a compressor, and the liquid refrigerant evaporates and flows to a condenser. [0007] The present invention further relates to a method of recovering lubricant entrained in a refrigerant of a vapor compression system. The method includes providing a pumping device having a first level sensor and a second level sensor; sensing a first liquid level signal from the first level sensor and a second liquid level signal from a second liquid level sensor; activating the first valve and the second valve to open at approximately the same time in response to sensing the liquid level dropping below the first level sensor; opening the pumping device to a discharge side of the compressor to decrease the pressure in the pumping device; opening an evaporator drain to supply a refrigerant and lubricant to flow into the pumping device; closing the first valve and the second valve in response to receiving the second liquid level signal indicating the refrigerant and lubricant has reached a second level; increasing a temperature inside the pumping device by heat exchange with a reservoir of heated lubricant; opening a pumping device discharge valve in response to sensing the temperature reaching a predetermined temperature; and reinjecting the refrigerant and lubricant into a lubricant separator in a discharge line of the compressor.

BRIEF DESCRIPTION OF THE FIGURES

[0008] FIGS. 1 and 2 show exemplary embodiments of environments incorporating a refrigeration system.

[0009] FIG. 3 shows a perspective view of an exemplary embodiment of a refrigeration system.

[0010] FIG. 4 shows a front view of the refrigeration system shown in FIG. 3.

[0011] FIG. 5 schematically illustrates an exemplary embodiment of a multistage refrigeration system.

[0012] FIG. 6 schematically illustrates an exemplary embodiment of an oil return system.

[0013] FIG. 7 schematically illustrates another exemplary embodiment of an oil return system. [0014| FIG. 8 schematically illustrates yet another exemplary embodiment of an oil return system.

[0015] FIG. 9 schematically illustrates still another exemplary embodiment of an oil return system.

[0016] FIG. 10 schematically illustrates a further exemplary embodiment of an oil return system

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0017] FIG. 1 shows a 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, and 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 an 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, and 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.

[0018] 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 may 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,

[0019] 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. Some examples of fluids that may be used as refrigerants in first stage system 32 are carbon dioxide (CO2; for example, R- 744), nitrous oxide (N2O; for example, R -744A), ammonia (NH3; for example, R-717), hydrofluorocarbon (HFC) based refrigerants (for example, R-410A, R- 407C, R-404A, R- 134a), other low global warming potential (GWP) refrigerants, and any other suitable type of refrigerant,

(0020] 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 valve 60 and second evaporator 62. According to another exemplary embodiment, second stage system can be operated with only first expansion device 56 and first evaporator 58. According to still another exemplary embodiment, second stage system 34 can be operated as a volatile system by removing compressor 48, first expansion device 56 and first evaporator 58. Some examples of refrigerants that may be used in second stage system 34 are carbon dioxide (CO2; R-744), nitrous oxide (N2O; R-744A), or mixtures of carbon dioxide and nitrous oxide, or hydrocarbon based refrigerants (for example, R- 170). The refrigerant in the second stage can be the same or different than the refrigerant in the first stage. 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.

[0021] In first stage system 32, when operated sub-critically, that is, 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. Within condenser 104, the compressed vapor transfers heat to a fluid, for example, water from a cooling tower, and as a result condenses from a vapor phase refrigerant to a liquid phase refrigerant. The condensed refrigerant exiting condenser 40 can be stored in receiver 42 before flowing through expansion device 44 to evaporator 46 in heat exchanger 36.

[0022] The condensed liquid refrigerant enters evaporator 46 and absorbs heat from fluid being circulated in condenser 50 in heat exchanger 36 by second stage system 34. The absorbed heat causes the liquid phase refrigerant to evaporate into a vapor phase refrigerant. The vapor refrigerant exits evaporator 46 exits and returns to compressor 38 by a suction line to complete the cycle.

[0023] 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, for example, 73 bar at 31 deg C for carbon dioxide. Furthermore, during transcritical operation, the refrigerant is maintained as a single phase refrigerant (vapor phase) 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. 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.

[0024] 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 vapor refrigerant enters condenser 50 and transfers heat to the fluid being circulated in evaporator 46. 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 liquid phase refrigerant exits condenser 50 and flows to receiver 52. From receiver 52, the refrigerant is circulated to a first expansion device 56 and first evaporator 58 and then to a valve 60 and a second evaporator 62 by pump 54.

[0025] In first evaporator 58, the liquid refrigerant from first expansion device 56 enters into a heat exchange relationship with a cooling load, for example, a fluid, and undergoes a phase change to a refrigerant vapor as a result. The refrigerant vapor exits first evaporator 58 and returns to compressor 48 to complete the cycle. In second evaporator 62, the liquid refrigerant from valve 60 absorbs heat from a cooling load, for example, a fluid, and may undergo a phase change to a refrigerant vapor. However, according to one 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 exiting second evaporator 62 may be a mixture of refrigerant vapor and refrigerant liquid. The refrigerant fluid exiting second evaporator 62, regardless of the phase, 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 in heat exchanger 36.

[0026] 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.

[0027] FIG. 6 illustrates an exemplary oil return system for recovery of compressor lubricating oil that becomes entrained in liquid refrigerant. In one embodiment, the entrained lubricant may be, for example, poly-alpha-olefin (PAO) synthetic petroleum oil entrained in CO2 refrigerant. The system includes evaporator 46 which may contain lubricating lubricant entrained in the liquid phase refrigerant. In one embodiment, the entrained lubricant may be poly-alpha-olefin (PAO) synthetic petroleum lubricant entrained in CO2 refrigerant. It should be understood that the mixture of refrigerant and lubricant that is drawn from the evaporator might alternately be drawn from a low pressure receiver or a pump separator, to supply the refrigerant lubricant mixture to the lubricant collection vessel or thermopump vessel. By thermopump, what is meant is a pumping device that can circulate a fluid by generation of fluid pressure resulting from thermal expansion. In the following discussion of FIGS. 6 through 10, the term evaporator includes a low pressure receiver or a pump separator. Additionally, the terms oil and lubricant may be used interchangeably throughout the specification, and includes synthetic petroleum lubricants.

[0028] A conduit 71 is connected at one end to evaporator 46 and at the opposite end to a thermopump vessel 74 (See, for example, FIG. 6A), disposed within a collection tank 70. Refrigerant containing entrained lubricant flows through conduit 71 from evaporator 46 to thermopump vessel 74. Conduit 71 is connected to thermopump vessel 74 through a control valve 72, for example, a solenoid operated valve to regulate the flow of refrigerant and lubricant mixture into thermopump vessel 74. Thermopump 74 vessel is connected to a controller 84, to control the flow of the refrigerant and lubricant mixture entering thermopump vessel 74 from receiver 46.

[0029] Collection tank 70 receives lubricant through an inlet conduit 75. The lubricant received via inlet conduit 75 collects directly in collection tank 70. Inlet conduit 75 is connected to a lubricant separator 76, which provides lubricant at a temperature higher than the refrigerant and lubricant mixture received from evaporator 46. Lubricant separator 76 receives a mixture of lubricant and vapor from compressor 100, shown in FIG. 3, through a discharge conduit 77. Lubricant separator 76 separates a portion of the miscible lubricant from the refrigerant vapor and lubricant mixture discharged by the compressor. For example, according to exemplary embodiments, baffle plates 76a and 76b may be inserted in the flow path of the vapor and lubricant mixture to separate the lubricant from the vapor. The separated lubricant 78 flows from lubricant separator 76 through inlet line 75 into collection tank 70. The lubricant collects in the bottom of collection tank 70 to a level such that thermopump vessel 74 within collection tank 70 is at least partially submerged in the lubricant received from lubricant separator 76.

[0030] Thermopump vessel 74 accumulates refrigerant/lubricant mixture 79, . Filling and evacuation of thermopump vessel 74 is controlled by two level sensors 86 and 88. Control panel 84 senses a first liquid level signal from a first level sensor 86 and a second liquid level signal from a second liquid level sensor 88. A control panel 84 controls inlet solenoid valve 72 and chamber outlet solenoid valve 80 so that they open and close at approximately the same time. A thermostat (not shown) in control panel 84 starts thermopump vessel 74 once the compressor discharge gas temperature reaches a predetermined temperature. When the liquid level goes below first level sensor 86, control panel 84 activates inlet solenoid 72 and outlet solenoid 80. Outlet solenoid 80 opens in the conduit connection to the compressor discharge side, decreasing the pressure in the thermopump vessel slightly. At approximately the same time, inlet solenoid valve 72 opens and liquid refrigerant and lubricant mixture 79 starts flowing into thermopump vessel 74. When second level sensor 88 senses that liquid refrigerant and lubricant mixture 79 has reached the second level, inlet solenoid 72 and outlet solenoid 80 are both closed by control panel 84. Pressure within thermopump vessel 74 begins to rise as a consequence of the heat transfer to thermopump vessel 74 from the heated oil in oil collection tank 70. The temperature inside thermopump vessel 74 rises, and upon reaching a predetermined temperature, control panel 84 opens outlet solenoid 80, causing liquid refrigerant and lubricant mixture 79 to flow through conduit 81 into compressor discharge line 77, where liquid refrigerant and lubricant mixture 79 is re-introduced to oil separator 76. In this way refrigerant and lubricant mixture 79 is sent back to compressor discharge line 77 so that the lubricant in refrigerant and lubricant mixture 79 can be separated from the refrigerant in oil separator 76 and subsequently handled by the normal oil return system. Alternately the pressurized refrigerant and lubricant mixture 79 may be discharged under pressure directly back into oil collection tank 70 and collected with the separated oil 78, as indicated by broken line 81a. When the thermopump stops, for example, when the compressor is operating below a predetermined capacity level, the control panel evacuates the thermopump vessel 74.

[0031] Referring to FIG. 6A, thermopump vessel 74 provides a reservoir for liquid refrigerant and lubricant mixture. Thermopump vessel 74 is a liquid-tight enclosure, and may optionally include multiple cooling fins 74b on two or more of the vessel walls 74c, 74d. Also, sealed penetrations 88a and 88b are provided for first and second liquid level sensors 86, 88. [0032] As shown in FIG. 6, control cables 84a, 84b, 84c and 84d interconnect controller 84 with the various devices for communication of sensor signals and operating signals. Controller 84 operates the oil collection tank inlet and outlet solenoids 72 and 80 respectively, in response to the liquid level signals from first and second liquid level sensors 86, 88. Controller 84 includes control logic (not shown), for example, using a microprocessor, other digital control logic circuitry, or electromagnetic relays, to open either or both of evaporator exit valve 72 and collection tank drain valve 80; and to close the valve in the collection tank drain flow path when the receiver has a predetermined amount of liquid refrigerant entrained with lubricant. Other signals and sensors may be input to and output by controller 84, for example, compressor discharge pressure, oil collection tank pressure, etc., as required to operate the oil return, which are omitted here for clarity.

[0033] Referring next to FIG. 7, an alternate exemplary embodiment includes a modified compressor 100 with an integral oil collection tank or reservoir 70a. The operation of the oil return system shown in FIG. 7 is generally the same as described above with respect to FIG. 6, with the primary difference being the incorporation of oil collection tank 70 within the same housing 102 as compressor 100. Conduit 71 is connected to thermopump vessel 74 through a control valve 72, for example, a solenoid operated valve to regulate the flow of refrigerant and lubricant mixture into thermopump vessel 74.

[0034] Collection tank 70 receives lubricant through an inlet conduit 75. The lubricant received via inlet conduit 75 collects directly in collection tank 70a. Inlet conduit 75 is connected to a lubricant separator 76, which provides lubricant at a temperature higher than refrigerant and lubricant mixture 79 received from evaporator 46. Lubricant separator 76 receives a mixture of lubricant and vapor from compressor 100, shown in FIG. 3, through a discharge conduit 77. Lubricant separator 76 separates a portion of the miscible lubricant from the refrigerant vapor and lubricant mixture discharged by the compressor. For example, according to exemplary embodiments, baffle plates 76a and 76b may be inserted in the flow path of the vapor and lubricant mixture to separate the lubricant from the vapor. The separated lubricant 78 flows from lubricant separator 76 through inlet line 75 into collection tank 70a. The lubricant collects in the bottom of collection tank 70a to a level such that thermopump vessel 74 within collection tank 70a is at least partially submerged in the lubricant received from lubricant separator 76.

[0035] Thermopump vessel 74 accumulates refrigerant/lubricant mixture 79. Filling and evacuation of thermopump vessel 74 is controlled by two level sensors 86 and 88. Control panel 84 senses a first liquid level signal from a first level sensor 86 and a second liquid level signal from a second liquid level sensor 88. Control panel 84 controls inlet solenoid valve 72 and chamber outlet solenoid valve 80 so that they open and close at approximately the same time, A thermostat (not shown) in control panel 84 starts thermopump vessel 74 once the compressor discharge gas temperature reaches a predetermined temperature. When the liquid level goes below first level sensor 86, control panel 84 activates inlet solenoid 72 and outlet solenoid 80. Outlet solenoid 80 opens in the conduit connection to the compressor discharge side, decreasing the pressure in the thermopump vessel slightly. At approximately the same time, inlet solenoid valve 72 opens and liquid refrigerant and lubricant mixture 79 starts flowing into thermopump vessel 74. When second level sensor 88 senses that liquid refrigerant and lubricant mixture 79 has reached the second level, inlet solenoid 72 and outlet solenoid 80 are both closed by control panel 84. Pressure within thermopump vessel 74 begins to rise as a consequence of the heat transfer to thermopump vessel 74 from the heated oil in oil collection tank 70. The temperature inside thermopump vessel 74 rises, and upon reaching a predetermined temperature, control panel 84 opens outlet solenoid 80, causing liquid refrigerant and lubricant mixture 79 to flow through conduit 81 into compressor discharge line 77, where liquid refrigerant and lubricant mixture 79 is re-introduced to oil separator 76. In this way refrigerant and lubricant mixture 79 is sent back to compressor discharge line 77 so that the lubricant in refrigerant and lubricant mixture 79 can be separated from the refrigerant in oil separator 76 and subsequently handled by the normal oil return system. Alternately the pressurized refrigerant and lubricant mixture 79 may be discharged under pressure directly back into oil collection tank 70a and collected with the separated oil 78, as indicated by broken line 81a. When the thermopump stops, for example, when the compressor is operating below a predetermined capacity level, the control panel evacuates the thermopump vessel 74. [0036] Control cables 84a, 824b, 84c and 84d interconnect controller 84 with the various devices for communication of sensor signals and operating signals. Controller 84 operates the oil collection tank inlet and outlet solenoids 72 and 80 respectively, in response to the liquid level signals from first and second liquid level sensors 86, 88. Other signals may be input to and output by controller 84, for example, compressor discharge pressure, oil collection tank pressure, etc., as required to operate the oil return, which are not shown here for simplicity. Similar to the combination of oil collection tank 70 and thermopump vessel 74 of FIG. 6, the arrangement of the tank 70a and thermopump vessel 74 in FIG. 7 provides a thermopump for transferring liquid refrigerant and lubricant mixture 79 back into oil collection tank 70a.

[0037] Referring next to FIG. 8, an alternate exemplary embodiment is shown. FIG. 8 illustrates an alternative to an oil return with a pump. The refrigerant and lubricant mixture provides liquid oil cooling in the screw compressors. Screw compressors 100,100a are lubricated by a lubricant film contained on the surface of the screw profiles. The lubricant prevents the refrigerant from washing or cleaning the lubricant film off the surface of the screw profiles of compressors 100, 100a. When oil cooling is not required, liquid refrigerant/lubricant mixture 79 may be optionally delivered to the discharge line 77 of compressors 100, 100a. In the exemplary embodiment of FIG. 8, a pump 69 is used to replace the thermopump arrangement, described above with respect to FIG. 6, for returning liquid refrigerant and lubricant mixture 79 that is collected in evaporator 46 to compressor discharge line 77. The evaporator 46 supplies evaporated from refrigerant and lubricant mixture 79 to the suction lines of a pair of parallel compressors 100, 100a, respectively. The pump 69 delivers liquid refrigerant and lubricant mixture 79 from the evaporator 46 to compressors 100, 100a at an intermediate pressure, through return lines 81 , 81a. According to an exemplary embodiment, the pump discharge line 82 may be connected to compressor discharge line 77, at the inlet to oil separator 76, at a higher pressure.

[0038] Referring now to FIG. 9, an alternate exemplary embodiment of the oil return system of FIG. 7 is shown. In FIG. 9, a thermopump arrangement is employed to replace the pump 69 in FIG. 7. In principle, the thermopump arrangement operates similarly to that described above with respect to FIGS. 6 and 7. Evaporator 46 supplies evaporated refrigerant from refrigerant and lubricant mixture 79 to the suction lines 101, 101a of a pair of parallel compressors 100, 100a, respectively, Thermopump vessel 74 is disposed inside oil collection tank 70. A conduit 71 is connected to a thermopump vessel 74 through a control valve 72, for example, a solenoid operated valve to regulate the flow of refrigerant and lubricant mixture into thermopump vessel 74 from evaporator 46, Thermopump vessel 74 receives the mixture of liquid refrigerant and lubricant from the evaporator 46 via conduit 71 , Oil collection tank 70 receives oil at a higher temperature via an inlet conduit 75 connected to an oil separator 76 in the compressor discharge conduit 77. Oil separator 76 separates a portion of the miscible oil from the refrigerant vapor and oil mixture discharged by the compressors 100, 100a, for example, by baffle plates 76a, 76b inserted in the flow path of the vapor and oil mixture. The separated oil 78 flows through the inlet line 75 into oil collection tank 70, and collects in the bottom of oil collection tank 70 to a level such that thermopump vessel 74 is at least partially submerged in the higher-temperature oil.

[0039J Thermopump vessel 74 accumulates refrigerant and lubricant mixture 79. Filling and evacuation of thermopump vessel 74 is controlled by two level sensors 86 and 88. Control panel 84 senses a first liquid level signal from a first level sensor 86, and a second liquid level signal from a second liquid level sensor 88. Control panel 84 controls the inlet solenoid valve 72 and the outlet solenoid valve 80 so that they open and close at approximately the same time. A thermostat (not shown) in control panel 84 starts thermopump vessel 74 once the compressor discharge gas temperature reaches a predetermined temperature. When the liquid level goes below first level sensor 86, control panel 84 activates inlet solenoid 72 and outlet solenoid 80. Outlet solenoid 80 opens in the conduit connection to the compressor discharge side, slightly decreasing the pressure in thermopump vessel 74. At approximately the same time, inlet solenoid valve 72 opens and liquid refrigerant and lubricant mixture 79 starts flowing into thermopump vessel 74. When second level sensor 88 senses that liquid refrigerant and lubricant mixture 79 has reached the second level, inlet solenoid 72 and outlet solenoid 80 are both closed by control panel 84. Pressure within thermopump vessel 74 begins to rise as a consequence of the heat transfer to thermopump vessel 74 from the heated oil in oil collection tank 70. The temperature inside thermopump vessel 74 rises, and upon reaching a predetermined temperature, control panel 84 opens outlet solenoid 80, causing liquid refrigerant and lubricant mixture 79 to flow through conduit 81 into compressor discharge line 77, where liquid refrigerant and lubricant mixture 79 is re-introduced to oil separator 76. In this way refrigerant and lubricant mixture 79 is sent back to compressor discharge line 77 so that the lubricant in refrigerant and lubricant mixture 79 can be separated from the refrigerant in oil separator 76 and subsequently handled by the normal oil return system. Alternately the pressurized refrigerant and lubricant mixture 79 may be discharged under pressure directly back into oil collection tank 70 and collected with the separated oil 78. When the thermopump stops, for example, when the compressor is operating below a predetermined capacity level, the control panel evacuates thermopump vessel 74.

[0040] Control cables 84a, 84b, 84c and 84d interconnect controller 84 with the various devices for communication of sensor signals and operating signals. The controller 84 operates the oil collection tank inlet and outlet solenoids 72 and 80 respectively in response to the liquid level signals from first and second liquid level sensors 86, 88. Other signals may be input to and output by controller 84, for example, compressor discharge pressure, oil collection tank pressure, etc., as required to operate the oil return, which are not shown here for simplicity. Similar to the combination of oil collection tank 70 and thermopump vessel 74 of FIG. 6, the arrangement of the tank 70 and thermopump vessel 74 in FIG. 9 provides a thermopump for transferring liquid refrigerant and lubricant mixture 79 back into oil collection tank 70.

[0041] Referring next to Figure 10, an alternate exemplary embodiment of the oil return system for piston or screw compressors is shown. The oil return system of Figure 10 can be used with either miscible or non-miscible oil/carbon dioxide combinations and avoids start-up and liquid stroke problems. A pump 69 transports liquid refrigerant/lubricant mixture 79 from evaporator 46 to a connection placed between the discharge line 77 of compressor 100 and oil separator 76. The separated oil returns to compressor 100 through line 75 to provide lubrication. [0042J The embodiment of FIG. 10 includes a modified compressor 100 with an integral oil collection tank or reservoir 70. That is, oil collection tank 70 is located within the same housing 102 as the compressor 100. Evaporator 46 is connected to pump 69 through conduit 71, to permit the flow of refrigerant and lubricant mixture 79 from evaporator 46 to the intake of pump 69. Pump 69 is connected at its output to pump discharge line 81, and discharges refrigerant and lubricant mixture 79 into discharge line 77 of compressor 100 via pump discharge line 81. The refrigerant and lubricant mixture 79 is thus returned to oil separator 76, where oil separator 76 separates a portion of the miscible oil from the refrigerant and lubricant mixture for example, by baffle plates 76a, 76b inserted in the flow path of the vapor and oil mixture. The separated oil 78 flows through the inlet line 75 into oil collection tank 70, and collects in the bottom of oil collection tank 70, for re-use by the compressor lubrication system (not shown).

[0043] 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 (for example, variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (for example, 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. 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.

Claims

CLAIMS:

1. A vapor compression system comprising: a compressor, a condenser, an expansion device and an evaporator connected in a closed loop, having a refrigerant being circulated in the closed loop; a first valve configured to control a flow of a mixture of liquid refrigerant and lubricant from the evaporator through a first flow path; a collection vessel configured to receive a heated lubricant from the compressor; a pumping device configured to circulate the mixture by generation of fluid pressure resulting from thermal expansion, the pumping device disposed in the collection vessel; a second valve configured to drain the mixture of evaporated refrigerant and lubricant into the compressor through a second flow path; and a controller configured to control flow of refrigerant and lubricant into and from the pumping device to regulate a level of the mixture in the pumping device.

2. The system of claim 1, wherein the pumping device further includes a first level sensor and a second level sensor configured to indicate a first liquid level and a second liquid level, respectively, for controlling a filling cycle and an evacuation cycle of the pumping device.

3. The system of claim 2, wherein the controller is configured to sense the first liquid level signal from the first level sensor and a second liquid level signal from the second liquid level sensor, and to activate the first valve and the second valve to open at approximately the same time in response to sensing the liquid level dropping below the first level sensor.

4. The system of claim 3, wherein the second valve is configured to open the pumping device to a discharge side of the compressor to decrease the pressure in the pumping device, and the first valve is configured to open to cause the refrigerant and lubricant to flow into the pumping device.

5. The system of claim 4, wherein the controller is further configured to close the first valve and the second valve in response to receiving a signal from the second level sensor indicating the refrigerant and lubricant has reached the second level.

6. The system of claim 5, wherein, in response to the closing of the first valve and the second valve upon achieving a second level within the pumping device, a temperature inside the pumping device rises, and wherein the controller opens the second valve in response to reaching a predetermined temperature, causing the refrigerant and lubricant to be re-introduced to a lubricant separator in a discharge line of the compressor.

7. The system of claim 1, wherein the controller further comprises a thermostat, the controller being further configured to start the pumping device when the compressor discharge gas temperature reaches a predetermined startup temperature.

8. The system of claim 1, wherein the pumping device receives the mixture through the first flow path.

9. The system of claim 2, wherein the first valve and the second valve are controlled by at least one relay.

10. A vapor system comprising: a receiver for receiving liquid refrigerant and lubricant from an expansion device, a portion of the liquid refrigerant and the lubricant received into the receiver collecting in the receiver; a flow path configured to receive the liquid refrigerant and lubricant from the receiver; a pump for controlling the flow of liquid refrigerant and lubricant; and a collection vessel configured to receive the liquid refrigerant and lubricant from the flow path; wherein the lubricant is separated from the liquid refrigerant in the collection vessel and the lubricant returns to a compressor, and the liquid refrigerant evaporates and flows to a condenser.

11. The system of claim 10, wherein the pump is a pumping device configured to circulate a fluid by generation of fluid pressure resulting from thermal expansion, for pressurizing the liquid refrigerant in the collection vessel into a discharge line of the compressor.

12. The system of claim 10, wherein the pump comprises a plurality of pumps to control the flow of liquid refrigerant and lubricant.

13. The system of claim 1 1 , wherein the collection vessel includes a third valve to direct the flow of lubricant back into a lubrication system of the compressor.

14. The system of claim 13, wherein the third valve is a float valve.

15. The system of claim 1, wherein the lubricant and the liquid refrigerant are miscibϊe.

16. A system comprising: a transcritical circuit in a compressor of a multistage refrigeration system, the transcritical circuit further comprising: a receiver for receiving liquid refrigerant and lubricant from an expansion device, a portion of the liquid refrigerant and the lubricant received into the receiver collecting in the receiver; a flow path configured to receive the liquid refrigerant and lubricant from the receiver; a pump for controlling the flow of liquid refrigerant and lubricant; a collection vessel configured to receive the liquid refrigerant and lubricant from the flow path; wherein the lubricant is separated from the liquid refrigerant in the collection vessel and the lubricant returns to a compressor and the liquid refrigerant evaporates and flows to a condenser.

17. The system of claim 16, wherein the pump is a pumping device configured to heat the liquid refrigerant and lubricant.

18. The system of claim 16, wherein two pumps control the flow of liquid refrigerant and lubricant.

19. The system of claim 16, wherein the collection vessel has a return valve to control the flow of lubricant to the compressor.

20. The system of claim 19, wherein the return valve is a float valve.

21. The system of claim 19, wherein the return valve is operated by a thermal switch.

22. The system of claim 16, wherein the liquid refrigerant is carbon dioxide.

23. The system of claim 16, wherein the lubricant and the liquid refrigerant are miscible.

24. A method of recovering lubricant entrained in a refrigerant of a system, comprising: sensing a first liquid level signal from a first level sensor and a second liquid level signal from a second liquid level sensor; activating the first valve and the second valve to open at approximately the same time in response to sensing the liquid level dropping below the first level sensor; opening a pumping device to a discharge side of the compressor to decrease the pressure in the pumping device; opening an evaporator drain to supply a refrigerant and lubricant to flow into the pumping device; closing the first valve and the second valve in response to receiving the second liquid level signal indicating the refrigerant and lubricant has reached a second level; increasing a temperature inside the pumping device by heat exchange with a reservoir of heated lubricant; and opening a pumping device discharge valve in response to sensing the temperature reaching a predetermined temperature; and reinjecting the refrigerant and lubricant into a lubricant separator in a discharge line of the compressor.