Economized refrigeration system

1. A refrigeration system comprising:

a condenser, an evaporator, an economizer, and a main compressor fluidly connected by a main refrigerant line to form a main refrigerant circuit;

an expansion device connected to the main refrigerant line intermediate the condenser and the economizer;

an auxiliary compressor, the auxiliary compressor being separate and distinct from the main compressor and independently controllable at all times with respect to the main compressor, the auxiliary compressor being capable of being operated at a different speed than the main compressor;

an auxiliary refrigerant line fluidly connecting the economizer to the auxiliary compressor and fluidly connecting the auxiliary compressor to the main refrigerant line at a location intermediate the main compressor and the condenser to form an economizer refrigerant circuit, wherein the auxiliary compressor is configured to compress refrigerant flowing through the economizer refrigerant circuit; and

a controller configured to adjust an economizer pressure based on a comparison of an estimated power usage of a version of the refrigeration system without the economizer and an estimated power usage of the refrigeration system with the economizer;

wherein the economizer is positioned at a junction between the main refrigerant line and the auxiliary refrigerant line, the junction in fluid communication with the main refrigerant line and the auxiliary refrigerant line;

wherein all refrigerant flowing in the main refrigerant line from the condenser flows through the expansion device and enters the economizer in a single flow prior to a first portion of the refrigerant entering the evaporator, and prior to a remaining portion of the refrigerant entering the auxiliary compressor, each of the first portion of the refrigerant and the remaining portion of the refrigerant being greater than zero at all times during operation of the system.

2. The refrigeration system of claim 1 , wherein the auxiliary compressor is configured to discharge compressed refrigerant at a pressure substantially equal to a discharge pressure of the main compressor.

3. The refrigeration system of claim 1 , wherein the main compressor comprises a single-stage compressor.

4. The refrigeration system of claim 3 , wherein the main compressor comprises a centrifugal compressor.

5. The refrigeration system of claim 1 , wherein the main compressor comprises a plurality of compressors connected in parallel.

6. The refrigeration system of claim 1 , wherein the auxiliary compressor comprises a screw compressor.

7. The refrigeration system of claim 6 , wherein the screw compressor has a slide valve and a variable speed drive.

8. The refrigeration system of claim 1 , wherein the auxiliary compressor comprises centrifugal compressor.

9. The refrigeration system of claim 8 , wherein the centrifugal compressor has a control feature selected from the group consisting of: a variable speed drive, prerotation vanes, suction throttling, a variable geometry diffuser, and combinations thereof.

10. The refrigeration system of claim 1 , wherein the economizer comprises a flash tank.

11. The refrigeration system of claim 1 , wherein the expansion device comprises a valve.

12. The refrigeration system of claim 1 , wherein the controller is configured to control the expansion device and the auxiliary compressor in response to refrigeration system operating conditions.

13. The refrigeration system of claim 1 further comprising an additional expansion device intermediate the economizer and the evaporator.

14. A method for operating an economized refrigeration system comprising:

providing a main refrigerant circuit comprising

a condenser, an evaporator, an economizer, an expansion device intermediate the condenser and the economizer, and a main compressor fluidly connected by a main refrigerant line;

providing an economizer refrigerant circuit comprising

an auxiliary compressor that is separate and distinct from the main compressor and an auxiliary refrigerant line fluidly connecting the economizer to the auxiliary compressor system and fluidly connecting the auxiliary compressor to the main refrigerant line at a location intermediate the main compressor and the condenser, the auxiliary compressor being capable of being operated at a different speed than the main compressor, wherein the economizer is positioned at a junction between the main refrigerant line and the auxiliary refrigerant line, the junction in fluid communication with the main refrigerant line and the auxiliary refrigerant line, wherein all refrigerant flowing in the main refrigerant line from the condenser flows through the expansion device and enters the economizer in a single flow prior to a first portion of the refrigerant entering the evaporator, and prior to a remaining portion of the refrigerant entering the auxiliary compressor, each of the first portion of the refrigerant and the remaining portion of the refrigerant being greater than zero at all times during operation of the system;

selecting an economizer operating pressure, wherein selecting the economizer operating pressure comprises adjusting the economizer operating pressure based on a comparison of an estimated power usage of a version of the economized refrigeration system without the economizer and an estimated power usage of the economized refrigeration system with the economizer;

operating the economizer at the selected operating pressure independent of changes in the main refrigerant circuit;

controlling a flow rate of refrigerant passing through the auxiliary compressor independently at all times from a flow rate of refrigerant passing through the main compressor; and

controlling a rise in pressure across the auxiliary compressor independently from a rise in pressure across the main compressor based on the selected operating pressure.

15. The method of claim 14 wherein the step of operating the economizer at the selected operating pressure comprises adjusting the expansion device to modify the economizer operating pressure to the selected operating pressure.

16. The method of claim 14 wherein the step of selecting an economizer operating pressure further comprises selecting an economizer operating pressure greater than or equal to evaporator pressure.

17. The method of claim 14 wherein the step of controlling a rise in pressure across the auxiliary compressor comprises:

establishing a common discharge location for intermixing refrigerant compressed by the main compressor and refrigerant compressed by the auxiliary compressor;

determining a pressure of refrigerant compressed by the main compressor at the common discharge location;

determining a pressure of refrigerant compressed by the auxiliary compressor at the common discharge location;

adjusting the rise in pressure across the auxiliary compressor; and

discharging refrigerant from the auxiliary compressor at the common discharge location at a pressure substantially equal to the discharge pressure of the main compressor.

18. The method of claim 14 wherein the step of controlling a flow rate of refrigerant passing through the auxiliary compressor comprises modifying the flow rate to compress only gaseous refrigerant from the economizer in the auxiliary compressor.

19. The method of claim 18 wherein the step of controlling a flow rate of refrigerant passing through the auxiliary compressor comprises modifying the flow rate to compress only saturated gaseous refrigerant from the economizer in the auxiliary compressor.

20. An economized refrigeration system comprising:

a condenser, an evaporator, an economizer, and a main compressor fluidly connected by a main refrigerant line to form a main refrigerant circuit, the main compressor consisting of a single-stage centrifugal compressor;

an expansion device connected to the main refrigerant line intermediate the condenser and the economizer;

an auxiliary compressor, the auxiliary compressor being separate and distinct, and independently controllable with respect to the main compressor, the auxiliary compressor being capable of being operated at a different speed than the main compressor;

an auxiliary refrigerant line fluidly connecting the economizer to the auxiliary compressor and fluidly connecting the auxiliary compressor to the main refrigerant line at a location intermediate the main compressor and the condenser to form an economizer refrigerant circuit, wherein the auxiliary compressor is configured to compress refrigerant flowing through the economizer refrigerant circuit; and

a controller configured to adjust an economizer pressure based on a comparison of an estimated power usage of a version of the economized refrigeration system without the economizer and an estimated power usage of the economized refrigeration system with the economizer;

wherein the economizer is positioned at a junction between the main refrigerant line and the auxiliary refrigerant line, the junction in fluid communication with the main refrigerant line and the auxiliary refrigerant line;

wherein all refrigerant flowing in the main refrigerant line from the condenser flows through the expansion valve and enters the economizer in a single flow prior to a first portion of the refrigerant entering the evaporator, and prior to a remaining portion of the refrigerant entering the auxiliary compressor, each of the first portion of the refrigerant and the remaining portion of the refrigerant being greater than zero at all times during operation of the system.