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A refrigerant system is configured to alternatingly run in an economized mode and a standard mode. A control system shifts the refrigerant system between the economized mode and standard mode responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic eff

A refrigerant system is configured to alternatingly run in an economized mode and a standard mode. A control system shifts the refrigerant system between the economized mode and standard mode responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic efficiency; condenser efficiency; evaporator efficiency; efficiency of hardware mechanically powering the compressor; and a mode-associated cycling efficiency. In a bypass mode, a bypass refrigerant flow from an intermediate port may return to the suction port. Shifting into the bypass mode may be similarly controlled based upon the determined efficiency.

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1. An apparatus (20) comprising: a compressor (22) having a suction port (24), a discharge port (26), and an intermediate port (28);a condenser (32);an evaporator (44);an economizer heat exchanger (40);a conduit system: coupling the condenser to the discharge port;coupling the economizer heat exchan

1. An apparatus (20) comprising: a compressor (22) having a suction port (24), a discharge port (26), and an intermediate port (28);a condenser (32);an evaporator (44);an economizer heat exchanger (40);a conduit system: coupling the condenser to the discharge port;coupling the economizer heat exchanger to the condenser;cooperating with the economizer heat exchanger and the evaporator to define a first flow path between the economizer heat exchanger and the suction port;cooperating with the economizer heat exchanger to define a second flow path between the economizer heat exchanger and the intermediate port, bypassing the evaporator;having one or more valves for selectively blocking and unblocking the second flow path; anda control system (70): coupled to the one or more valves and configured to alternately operate the apparatus in a plurality of modes including: a standard mode essentially wherein a refrigerant flow from the condenser passes along the first flowpath and not the second flowpath; andan economized mode essentially wherein a refrigerant flow splits into: a first portion passing along the first flow path; anda second portion extending through the second flow path section to return to the intermediate port; andconfigured to shift the apparatus between the modes responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency. 2. The apparatus of claim 1 wherein: the control system is configured to determine the efficiency reflecting a combination of at least three of said: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency. 3. The apparatus of claim 1 wherein: the plurality of modes further includes: a bypass mode essentially wherein a refrigerant flow passes along the first flow path and a bypass flow of refrigerant from the intermediate port returns to the suction port. 4. The apparatus of claim 1 wherein: said efficiency of hardware mechanically powering the compressor comprises a combination of electric motor efficiency and variable frequency drive efficiency. 5. The apparatus of claim 1 wherein: the controller is configured to determine a refrigerant density ratio and determine said compressor isentropic efficiency responsive to the determined refrigerant density ratio. 6. The apparatus of claim 5 wherein: the controller is configured to determine said refrigerant density ratio based upon a combination of: compressor suction temperature; compressor suction pressure; compressor discharge temperature; and compressor discharge pressure. 7. The apparatus of claim 1 wherein: at least a first of the one or more valves is a solenoid valve. 8. The apparatus of claim 1 wherein: the one or more valves are bistatic. 9. The apparatus of claim 1 wherein: the compressor is a screw compressor. 10. A method for operating a cooling system, the system having: a compressor having a suction port, a discharge port, and an intermediate port;a condenser having an inlet and an outlet, the condenser inlet coupled to the discharge port;an evaporator having an inlet and an outlet, the evaporator outlet coupled to the compressor suction port; andan economizer first and second flow path sections;the method comprising: determining a most efficient mode of a plurality of modes, the determining including determining efficiency factors associated with at least two of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency; andresponsive to the determining, at different times: running the system in an economized mode wherein a refrigerant flow from the discharge port proceeds essentially through the condenser, splitting into a first portion extending through the first flow path section and evaporator to return to the suction port and a second portion extending through the second flow path section to return to the intermediate port; andrunning the system in a non-economized mode wherein a refrigerant flow from the discharge port proceeds essentially through the condenser, first flow path section and evaporator to return to the suction port. 11. The method of claim 10 further comprising: running the system in a bypass mode wherein a refrigerant flow from the discharge port proceeds essentially through the condenser, first flow path section and evaporator to return to the suction port and a bypass flow of refrigerant from the intermediate port returns to the suction port. 12. The method of claim 10 wherein: the determining includes determining at least three said efficiency factors. 13. The method of claim 10 further comprising: sensing least one operational parameter selected from the group consisting of: saturated evaporating temperature;saturated evaporating pressure;air temperature entering or leaving the evaporator;saturated condensing temperature;saturated condensing pressure;air temperature entering or leaving the condenser;compressor current;compressor voltage; andcompressor power; andselecting one of said modes responsive to the at least one operational parameter. 14. A system comprising: a compressor;a condenser;an evaporator;an economizer;first means coupling the evaporator and economizer to the compressor and condenser for alternatingly operating the system in: a standard mode; andan economized mode; andsecond means for determining respective efficiencies of the standard mode and economized mode and coupled to the first means to shift between said standard mode and said economized mode responsive to said determined efficiencies. 15. The system of claim 14 wherein: the second means is configured to control the first means to alternatingly operate the system in said standard mode, said economized mode, and a bypass mode. 16. The system of claim 15 wherein: the second means is configured to determine said respective efficiencies reflecting a combination of at least two of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency. 17. The system of claim 14 wherein: the second means is configured to determine said respective efficiencies reflecting a combination of at least two of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency. 18. The system of claim 14 wherein: the second means is configured to determine said respective efficiencies reflecting a combination of at least three of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency. 19. The system of claim 14 wherein: the second means is configured to determine said respective efficiencies reflecting a combination of at least four of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency. 20. The system of claim 14 wherein: the second means is configured to determine said respective efficiencies reflecting a combination of at least all of: compressor isentropic efficiency;condenser efficiency;evaporator efficiency;efficiency of hardware mechanically powering the compressor; anda mode-associated cycling efficiency.