System and method for controlling an economizer circuit

1. A method for controlling an economizer circuit in a chiller system, the method comprising the steps of:

providing an economizer circuit for a chiller system having a flash tank, an inlet line to the flash tank and an outlet line from the flash tank connected to an economizer port of a compressor of the chiller system, the outlet line including a valve to control flow of refrigerant in the outlet line;

determining whether a level of liquid in the flash tank is less than a predetermined level;

determining whether an operating parameter of the compressor is greater than a first predetermined value related to the operating parameter of the compressor; and

actuating the valve to engage the economizer circuit in response to a determination that the liquid level in the flash tank is less than the predetermined level and a determination that the operating parameter of the compressor is greater than the first predetermined value related to the operating parameter of the compressor.

2. The method of claim 1 further comprising the steps of:

determining whether an operating parameter of the compressor is less than a second predetermined value related to the operating parameter of the compressor; and

actuating the valve to disengage the economizer circuit in response to a determination that the operating parameter of the compressor is less than the second predetermined value related to the operating parameter of the compressor.

3. The method of claim 2 wherein the step of determining whether an operating parameter of the compressor is less than a second predetermined value related to the operating parameter of the compressor is executed in response to a determination that the liquid level in the flash tank is greater than the predetermined level or a determination that the operating parameter of the compressor is less than the first predetermined value related to the operating parameter of the compressor.

4. The method of claim 2 wherein the first predetermined value is a first predetermined output frequency from a variable speed drive and the second predetermined value is a second predetermined output frequency from a variable speed drive.

5. The method of claim 4 wherein the first predetermined output frequency is 120 Hz and the second predetermined output frequency is 100 Hz.

6. The method of claim 1 further comprising the steps of:

determining whether the compressor of the chiller system is operation; and

actuating the valve to disengage the economizer circuit in response to a determination that the compressor is not in operation.

7. The method of claim 1 further comprising the steps of:

determining whether an outdoor ambient temperature is less than a predetermined temperature;

determining whether an operating time for the compressor is less than a predetermined time period; and

actuating the valve to engage the economizer circuit in response to a determination that the liquid level in the flash tank is less than the predetermined level and a determination that the outdoor ambient temperature is less than a predetermined temperature and a determination that the operating time for the compressor is less than a predetermined time period.

8. The method of claim 7 wherein the predetermined time period is 5 minutes and the predetermined temperature is 40° F.

9. The method of claim 7 further comprising the step of determining whether the economizer circuit is engaged.

10. The method of claim 9 further comprising, in response to a determination that the economizer circuit is engaged, the steps of:

determining whether an operating parameter of the compressor is less than a second predetermined value related to the operating parameter of the compressor;

determining whether a temperature of a motor for the compressor is greater than a predetermined temperature; and

actuating the valve to disengage the economizer circuit in response to a determination that the operating parameter of the compressor is less than the second predetermined value related to the operating parameter of the compressor or a determination that the temperature of a motor for the compressor is greater than a predetermined temperature.

11. The method of claim 10 wherein the chiller system comprises a plurality of refrigerant circuits each having a corresponding compressor and economizer circuit and the step of determining whether a temperature of a motor for the compressor is greater than a predetermined temperature is repeated for each compressor of the plurality of refrigerant circuits.

12. The method of claim 9 wherein the steps of determining whether a level of liquid in the flash tank is less than a predetermined level and determining whether an operating parameter of the compressor is greater than a first predetermined value related to the operating parameter of the compressor are executed in response to a determination that the economizer circuit is not engaged.

13. The method of claim 12 further comprising the steps of:

determining whether a temperature of a motor for the compressor is less than a predetermined temperature;

determining whether a timer for the economizer circuit has completed; and

the step of actuating the valve to engage the economizer circuit in response to a determination that the liquid level in the flash tank is less than the predetermined level and a determination that the operating parameter of the compressor is greater than the first predetermined value related to the operating parameter of the compressor is executed in response to a determination that the temperature of a motor for the compressor is less than a predetermined temperature and a determination that the timer for the economizer circuit has completed.

14. The method of claim 12 wherein the steps of determining whether an outdoor ambient temperature is less than a predetermined temperature and determining whether an operating time for the compressor is less than a predetermined time period are executed in response to a determination that the operating parameter of the compressor is less than a first predetermined value related to the operating parameter of the compressor.

15. The method of claim 14 further comprising the steps of:

determining whether a temperature of a motor for the compressor is less than a predetermined temperature;

determining whether a timer for the economizer circuit has completed; and

the step of actuating the valve to engage the economizer circuit in response to a determination that the liquid level in the flash tank is less than the predetermined level and a determination that the outdoor ambient temperature is less than a predetermined temperature and a determination that the operating time for the compressor is less than a predetermined time period is executed in response to a determination that the temperature of a motor for the compressor is less than a predetermined temperature and a determination that the timer for the economizer circuit has completed.

16. A method for controlling an economizer circuit in a chiller system, the method comprising the steps of:

providing an economizer circuit for a chiller system having a flash tank, an inlet line to the flash tank and an outlet line from the flash tank connected to an economizer port of a compressor of the chiller system, the outlet line including a valve to control flow of refrigerant in the outlet line;

determining whether an outdoor ambient temperature is less than a predetermined temperature;

determining whether an operating time for the compressor is less than a predetermined time period;

determining whether a level of liquid in the flash tank is less than a predetermined level; and

actuating the valve to engage the economizer circuit in response to a determination that the liquid level in the flash tank is less than the predetermined level and a determination that the outdoor ambient temperature is less than a predetermined temperature and a determination that the operating time for the compressor is less than a predetermined time period.

17. The method of claim 16 further comprising the steps of:

determining whether an operating parameter of the compressor is greater than a first predetermined value related to the operating parameter of the compressor;

determining whether current in a motor for the compressor is less than a predetermined current value;

determining whether a temperature of the motor for the compressor is less than a predetermined temperature; and

actuating the valve to engage the economizer circuit in response to a determination that the liquid level in the flash tank is less than the predetermined level and a determination that the operating parameter of the compressor is greater than the first predetermined value related to the operating parameter of the compressor and a determination that a temperature of a motor for the compressor is less than a predetermined temperature and a determination that current in a motor for the compressor is less than a predetermined current value.

18. The method of claim 16 further comprising the step of determining whether the economizer circuit is engaged.

19. The method of claim 18 further comprising, in response to a determination that the economizer circuit is engaged, the steps of:

determining whether an operating parameter of the compressor is less than a second predetermined value related to the operating parameter of the compressor; and

actuating the valve to disengage the economizer circuit in response to a determination that the operating parameter of the compressor is less than the second predetermined value related to the operating parameter of the compressor.

20. The method of claim 19 further comprising, in response to a determination that the operating parameter of the compressor is greater than the second predetermined value related to the operating parameter of the compressor, the steps of:

determining whether an operating parameter of the compressor is less than a third predetermined value related to the operating parameter of the compressor;

determining whether a temperature of a motor for the compressor is greater than a predetermined temperature; and

actuating the valve to disengage the economizer circuit in response to a determination that the operating parameter of the compressor is less than the third predetermined value related to the operating parameter of the compressor or a determination that the temperature of a motor for the compressor is greater than a predetermined temperature.

21. The method of claim 20 wherein the chiller system comprises a plurality of refrigerant circuits each having a corresponding compressor and economizer circuit and the step of determining whether a temperature of a motor for the compressor is greater than a predetermined temperature is repeated for each compressor of the plurality of refrigerant circuits.

22. The method of claim 21 wherein the step of actuating the valve to disengage the economizer circuit in response to a determination that the operating parameter of the compressor is less than the second predetermined value related to the operating parameter of the compressor is repeated for each economizer circuit.

23. The method of claim 16 further comprising the steps of:

determining whether the compressor of the chiller system is operation; and

actuating the valve to disengage the economizer circuit in response to a determination that the compressor is not in operation.

24. A chiller system comprising:

a refrigerant circuit comprising a compressor, a condenser arrangement, an expansion valve and an evaporator arrangement connected in a closed refrigerant loop;

an economizer circuit connected to the refrigerant circuit, the economizer circuit comprising a flash tank having a first outlet line in fluid communication with the expansion valve and a second outlet line in fluid communication with the compressor, the second outlet line comprising a valve to control flow of refrigerant from the flash tank to the compressor; and

a control panel to control the valve to activate and deactivate the economizer circuit, the control panel being configured to open the valve to activate the economizer circuit in response to a liquid level in the flash tank being less than a predetermined level and an operating parameter of the compressor being greater than a first predetermined value related to the operating parameter of the compressor.

25. The chiller system of claim 24 wherein the refrigerant circuit further comprises a variable speed drive and motor to drive the compressor and the operating parameter of the compressor being an output frequency of the variable speed drive.

26. The chiller system of claim 24 wherein the compressor is a screw compressor having a slide valve and the operating parameter of the compressor being a position of the slide valve in the screw compressor.

27. The chiller system of claim 24 wherein the valve is a solenoid valve.

28. The chiller system of claim 24 further comprising:

at least one additional refrigerant circuit;

at least one additional economizer circuit connected to the at least one additional refrigerant circuit, the at least one additional economizer circuit including at least one additional valve to control refrigerant flow from the at least one additional economizer circuit;

each at least one additional valve in each at least one additional economizer circuit being controlled by the control panel to activate and deactivate the corresponding at least one additional economizer circuit; and

wherein the control panel is configured to close the valve and each at least one additional valve to deactivate the economizer circuit and the at least one additional economizer circuit in response to an operating parameter of the compressor being less than a second predetermined value related to the operating parameter of the compressor.

29. The chiller system of claim 28 wherein the control panel is configured to close the valve to deactivate the economizer circuit in response to a temperature of a motor for the compressor of the refrigerant circuit or a temperature of a motor for the compressor of each of the at least one additional refrigerant circuit being greater than a predetermined temperature.

30. The chiller system of claim 24 wherein the control panel is configured to open the valve to activate the economizer circuit in response to a liquid level in the flash tank being less than a predetermined level, an outdoor ambient temperature being less than a predetermined temperature and an operating time of the compressor being less than a predetermined time value.