Capacity control for chillers having screw compressors

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.

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.