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A vapor compression system includes a first compression mechanism that compresses the working fluid from a low suction pressure to an intermediate pressure, a second compression mechanism that compresses intermediate pressure working fluid to a higher discharge pressure, and a fluid circuit circulat

A vapor compression system includes a first compression mechanism that compresses the working fluid from a low suction pressure to an intermediate pressure, a second compression mechanism that compresses intermediate pressure working fluid to a higher discharge pressure, and a fluid circuit circulating the working fluid discharged from the second compression mechanism to the first compression mechanism. The fluid circuit includes, in serial order, a first high pressure heat exchanger, an expansion device and an evaporator. The system may be operated as a transcritical system employing carbon dioxide as the working fluid. An intermediate pressure vessel is in communication with the system between the first and second compression mechanisms and working fluid at an intermediate pressure is communicated with the vessel. The system may be regulated by controlling the mass of working fluid contained in the intermediate pressure vessel, e.g., by regulating the temperature or storage volume of the vessel.

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1. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid com

1. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the second intermediate pressure to a third discharge pressure;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein said first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism; andan intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, wherein a single fluid conduit communicates both inflows and outflows of the working fluid between said vessel and said system at a location between said first and second compression mechanisms.2. The vapor compression system of claim 1 wherein a fluid conduit providing communication of working fluid between said vessel and said system between said first and second compression mechanisms defines an unregulated fluid passage.3. The vapor compression system of claim 1 wherein the discharge pressure of the working fluid is greater than the critical pressure of the working fluid.4. The vapor compression system of claim 1 further including at least one fluid conduit providing fluid communication between said vessel and said fluid circuit between said second compression mechanism and said first compression mechanism and at least one valve controlling fluid flow through said at least one fluid conduit.5. The vapor compression system of claim 1 further comprising a third heat exchanger disposed in said system between said first and second compression mechanisms.6. The vapor compression system of claim 1 wherein the quantity of liquid phase working fluid contained within said vessel varies as a function of the temperature of said vessel.7. The vapor compression system of claim 1 wherein said intermediate pressure vessel has a selectively adjustable storage volume.8. The vapor compression system of claim 1, further comprising at least one additional fluid conduit communicating working fluid between said vessel and at least one location in said system other than between said first and second compression mechanisms.9. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the second intermediate pressure to a third discharge pressure;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein said first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism; andan intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, wherein all working fluid communicated to and from said vessel is communicated from and to said system between said first and second compression mechanisms.10. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the second intermediate pressure to a third discharge pressure;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein said first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism;an intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, wherein the quantity of liquid phase working fluid contained within said vessel varies as a function of the temperature of said vessel; andmeans for regulating the temperature of said vessel.11. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the second intermediate pressure to a third discharge pressure;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein said first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism; andan intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, wherein the quantity of liquid phase working fluid contained within said vessel varies as a function of the temperature of said vessel, and wherein the temperature of said vessel is regulated by the selective exchange of thermal energy between said vessel and one of working fluid diverted from said fluid circuit, a secondary fluid, a heating element, and an external temperature reservoir.12. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the second intermediate pressure to a third discharge pressure;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein said first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism;an intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid; andan intermediate pressure heat exchanger cooling intermediate pressure working fluid and positioned between said first compression mechanism and said intermediate pressure vessel.13. A transcritical vapor compression system having a working fluid, said system comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a low pressure to an intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the intermediate pressure to a discharge pressure wherein the discharge pressure is above the critical pressure of the working fluid;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein the first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism; andan intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to an from said vessel and said vessel contains a variable quantity of liquid phase working fluid, said quantity of liquid phase working fluid varying as a function of the temperature of said vessel, wherein a single fluid conduit communicates working fluid between said vessel and said system, said single fluid conduit communicating both inflows and outflows of the working fluid between said vessel and said system between said first and second compression mechanisms.14. A transcritical vapor compression system having a working fluid, said system comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a low pressure to an intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the intermediate pressure to a discharge pressure wherein the discharge pressure is above the critical pressure of the working fluid;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein the first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism; andan intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to an from said vessel and said vessel contains a variable quantity of liquid phase working fluid, said quantity of liquid phase working fluid varying as a function of the temperature of said vessel, wherein all working fluid communicated to and from said vessel is communicated from and to said system between said first and second compression mechanisms.15. A transcritical vapor compression system having a working fluid, said system comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a low pressure to an intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the intermediate pressure to a discharge pressure wherein the discharge pressure is above the critical pressure of the working fluid;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein the first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism;an intermediate pressure vessel in fluid communication with said system between said first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, said quantity of liquid phase working fluid varying as a function of the temperature of said vessel; andtemperature regulator in thermal communication with said vessel.16. The vapor compression system of claim 15 wherein a fluid conduit providing communication of working fluid between said vessel and said system between said first and second compression mechanisms defines an unregulated fluid passage.17. The vapor compression system of claim 15 further including at least one fluid conduit providing fluid communication between said vessel and said fluid circuit between said second compression mechanism and said first compression mechanism and at least one valve controlling fluid flow through said at least one fluid conduit.18. The vapor compression system of claim 15 further comprising a third heat exchanger disposed in said system between said first and second compression mechanisms.19. The vapor compression system of claim 15 wherein the temperature of said vessel is regulated by the selective exchange of thermal energy between said vessel and one of working fluid diverted from said fluid circuit, a secondary fluid, and an external heat source.20. The vapor compression system of claim 15 further comprising an intermediate pressure heat exchanger cooling intermediate pressure working fluid and positioned between said first compression mechanism and said intermediate pressure vessel.21. A method of regulating a transcritical vapor compression system having a working fluid, said method comprising:compressing the working fluid from a low pressure to an intermediate pressure in a first compression mechanism;compressing the working fluid from the intermediate pressure to a discharge pressure in a second compression mechanism, the discharge pressure being greater than the critical pressure of the working fluid;circulating working fluid discharged from the second compression mechanism through a fluid circuit having, in serial order, a first heat exchanger, an expansion device and a second heat exchanger and then returning the fluid to the first compression mechanism wherein the first heat exchanger is positioned in a high pressure side of the circuit between the second compression mechanism and the expansion device and the second heat exchanger is positioned in a low side of the circuit between the expansion device and the first compression mechanism;providing fluid communication of the working fluid between an intermediate pressure vessel and the system at a location between the first and second compression mechanisms wherein intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, the quantity of liquid phase working fluid varying as a function of the temperature of the vessel; andregulating the pressure in the first heat exchanger by controlling the temperature of the vessel.22. The method of claim 21 wherein controlling the temperature of the vessel comprises selectively exchanging thermal energy between the vessel and one of working fluid diverted from the fluid circuit, a secondary fluid, a heating element, and an external temperature reservoir.23. The method of claim 21 wherein providing fluid communication of the working fluid between the vessel and the system includes providing a single fluid conduit between the vessel and the system, the single fluid conduit communicating both inflows and outflows of the working fluid between the vessel and the system between the first and second compression mechanisms.24. The method of claim 21 wherein all working fluid communicated to and from the vessel is communicated from and to the system between the first and second compression mechanisms.25. The method of claim 21 further comprising cooling the intermediate pressure working fluid between the first compression mechanism and the intermediate pressure vessel.26. A method of regulating a transcritical vapor compression system having a working fluid, said method comprising:compressing the working fluid from a low pressure to an intermediate pressure in a first compression mechanism;compressing the working fluid from the intermediate pressure to a discharge pressure in a second compression mechanism, the discharge pressure being greater than the critical pressure of the working fluid;circulating working fluid discharged from the second compression mechanism through a fluid circuit having, in serial order, a first heat exchanger, an expansion device and a second heat exchanger and then returning the fluid to the first compression mechanism wherein the first heat exchanger is positioned in a high pressure side of the circuit between the second compression mechanism and the expansion device and the second heat exchanger is positioned in a low side of the circuit between the expansion device and the first compression mechanism;providing fluid communication of the working fluid between an intermediate pressure vessel and the system at a location between the first and second compression mechanisms, intermediate pressure working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid, all communication of working fluid to and from the vessel being communicated from and to the system between the first and second compression mechanisms; andregulating the pressure in the first heat exchanger by controlling the quantity of liquid phase working fluid within the vessel.27. The method of claim 26 wherein controlling the quantity of liquid phase working fluid within the vessel comprises controlling the temperature of the vessel.28. The method of claim 26 wherein controlling the quantity of liquid phase working fluid within the vessel comprises controlling the storage volume of the vessel.29. The method of claim 26 wherein providing fluid communication of the working fluid between the vessel and the system includes providing a single fluid conduit between the vessel and the system, the single fluid conduit communicating both inflows and outflows of the working fluid between the vessel and the system between the first and second compression mechanisms.30. The method of claim 26 further comprising cooling the intermediate pressure working fluid between the first compression mechanism and the intermediate pressure vessel.31. A vapor compression system having a working fluid and comprising:a first compression mechanism, said first compression mechanism compressing the working fluid from a first low pressure to a second intermediate pressure;a second compression mechanism, said second compression mechanism in fluid communication with said first compression mechanism and compressing the working fluid from the second intermediate pressure to a third discharge pressure;a fluid circuit circulating the working fluid from said second compression mechanism to said first compression mechanism and including, in serial order, a first heat exchanger, an expansion device and a second heat exchanger wherein said first heat exchanger is positioned in a high pressure side of said circuit between said second compression mechanism and said expansion device and said second heat exchanger is positioned in a low pressure side of said circuit between said expansion device and said first compression mechanism; anda working fluid vessel in fluid communication with said system between said second compression mechanism and said first heat exchanger wherein working fluid is communicated to and from said vessel and said vessel contains a variable quantity of liquid phase working fluid varying as a function of the temperature of the vessel; anda temperature regulator in thermal communication with said vessel.