Capacity control for chillers having screw compressors

1. A compressor system for a refrigeration circuit, comprising:

a screw compressor including a slide valve selectively actuatable between a first position and a second position to facilitate modulating a capacity of the screw compressor between fully-loaded and fully-unloaded; and

a controller communicably coupled to the slide valve, the controller configured to:

receive a chilled fluid temperature setpoint for a fluid in heat transfer communication with a refrigerant of the refrigeration circuit;

receive temperature data indicative of a chilled fluid temperature of the fluid;

determine a difference between the chilled fluid temperature and the chilled fluid temperature setpoint;

provide one of a load command and an unload command to the slide valve based on the difference between the chilled fluid temperature and the chilled fluid temperature setpoint;

start a timer each time one of the load command and the unload command is provided to the slide valve, wherein the load command causes the timer to count toward a load time threshold indicating that the screw compressor is fully-loaded and the unload command causes the timer to count toward an unload time threshold indicating that the screw compressor is fully-unloaded; and

estimate a current position of the slide valve based on the timer relative to a corresponding one of the load time threshold and the unload time threshold,

wherein the controller does not receive feedback from the screw compressor regarding the current position of the slide valve.

2. The compressor system of claim 1 , wherein the controller is further configured to:

provide the load command to the slide valve to increase the capacity of the screw compressor in response to the chilled fluid temperature being greater than the chilled fluid temperature setpoint; and

stop providing the load command in response to the chilled fluid temperature decreasing to the chilled fluid temperature setpoint.

3. The compressor system of claim 1 , wherein the controller is further configured to:

provide the unload command to the slide valve to decrease the capacity of the screw compressor in response to the chilled fluid temperature being less than the chilled fluid temperature setpoint; and

stop providing the unload command in response to the chilled fluid temperature increasing to the chilled fluid temperature setpoint.

4. The compressor system of claim 1 , wherein the controller is further configured to continue providing the load command for a predetermined amount of time and stop the timer in response to the timer reaching the load time threshold indicating that the screw compressor is fully-loaded.

5. The compressor system of claim 4 , wherein the controller is further configured to stop providing the load command after the predetermined amount of time.

6. The compressor system of claim 1 , wherein the controller is further configured to stop providing the unload command and stop the timer in response to the timer reaching the unload time threshold indicating that the screw compressor is fully-unloaded.

7. The compressor system of claim 6 , wherein the controller is further configured to take the screw compressor offline in response to the screw compressor being fully-unloaded.

8. The compressor system of claim 1 , wherein the controller is further configured to:

receive first pressure data indicative of a suction pressure of the refrigerant entering an inlet of the screw compressor;

receive second pressure data indicative of a discharge pressure of the refrigerant exiting an outlet of the screw compressor; and

determine a load limit for the screw compressor based on the suction pressure and the discharge pressure.

9. The compressor system of claim 8 , wherein the controller is further configured to provide at least one of the load command and the unload command within the load limit for the screw compressor.

10. A method for capacity control of a chiller having a compressor, comprising:

receiving, by a processing circuit, a chilled fluid temperature setpoint for a fluid in heat transfer communication with a refrigerant of the chiller;

receiving, by the processing circuit, temperature data from a temperature sensor indicative of a chilled fluid temperature of the fluid;

providing, by the processing circuit, a load command to a slide valve of the compressor to increase the capacity of the compressor in response to the chilled fluid temperature being greater than the chilled fluid temperature setpoint;

providing, by the processing circuit, an unload command to the slide valve to decrease the capacity of the compressor in response to the chilled fluid temperature being less than the chilled fluid temperature setpoint;

starting, by the processing circuit, a timer each time one of the load command and the unload command is provided to the slide valve, wherein the load command causes the timer to count toward a load time threshold indicating that the compressor is fully-loaded and the unload command causes the timer to count toward an unload time threshold indicating that the screw compressor is fully-unloaded; and

estimating, by the processing circuit, a current position of the slide valve based on the timer relative to a corresponding one of the load time threshold and the unload time threshold,

wherein the processing circuit does not receive feedback from the compressor regarding the current position of the slide valve.

11. The method of claim 10 , further comprising:

continue providing, by the processing circuit, the load command for a predetermined amount of time in response to the timer reaching the load time threshold; and

stopping, by the processing circuit, the timer in response to the timer reaching the load time threshold.

12. The method of claim 11 , further comprising stop providing, by the processing circuit, the load command after the predetermined amount of time.

13. The method of claim 10 , further comprising:

stop providing, by the processing circuit, the unload command in response to the timer reaching the unload time threshold; and

stopping, by the processing circuit, the timer in response to the timer reaching the unload time threshold.

14. The method of claim 13 , further comprising taking, by the processing circuit, the compressor offline in response to the compressor being fully-unloaded.

15. A chiller, comprising:

a compressor configured to provide a refrigerant throughout the chiller, the compressor having a slide valve selectively actuatable to facilitate modulating a capacity of the compressor;

a condenser positioned downstream of the compressor;

an expansion valve positioned downstream of the condenser;

an evaporator positioned downstream of the expansion valve and upstream of the compressor, the evaporator configured to subject the refrigerant to a heat exchange relationship with a fluid; and

a controller configured to:

receive a temperature setpoint for the fluid in heat transfer communication with the refrigerant;

receive temperature data indicative of a temperature of the fluid;

provide a load command to the slide valve of the compressor to increase the capacity of the compressor in response to the temperature of the fluid being greater than the temperature setpoint;

provide an unload command to the slide valve to decrease the capacity of the compressor in response to the temperature of the fluid being less than the temperature setpoint;

start a timer each time one of the load command and the unload command is provided to the slide valve, wherein the load command causes the timer to count toward a load time threshold indicating that the compressor is fully-loaded and the unload command causes the timer to count toward an unload time threshold indicating that the compressor is fully-unloaded; and

estimate a current position of the slide valve based on the timer relative to a corresponding one of the load time threshold and the unload time threshold,

wherein the controller does not receive feedback from the compressor regarding the current position of the slide valve.