Evaporator water temperature control for a chiller system

A method of controlling the capacity of a chiller system, the chiller system including a circularly linked compressor, condenser, expansion device, and evaporator, where the evaporator receives entering water having an entering water temperature and provides leaving water having a leaving water temperature and where the compressor compresses refrigerant having a measured superheat, the method comprising the steps of:

• measuring the leaving water temperature and the refrigerant superheat;
• determining a leaving water temperature error as a function of the difference between the leaving water temperature and a setpoint;
• providing a first mode of capacity control wherein the compressor is modulated in response to the leaving water temperature error and the expansion valve is modulated in response to the measured superheat;
• and
• providing a second mode of capacity control where the compressor is maintained at its minimum capacity and the expansion valve is modulated in response to the leaving water temperature error.

A method of controlling the capacity of a chiller system comprising the steps of:

• determining a needed capacity for the chiller system;
• adjusting a compressor flow control means to meet the needed capacity;
• repositioning an expansion valve in response to the adjusting step;
• determining if the compressor flow control means is at an end of range capacity;
• and
• modulating the position of the expansion valve to control chiller system capacity whenever the flow control means has been determined to be at an end of capacity range.

A method of controlling the capacity of a water chiller system having a variable capacity compressor, a variable capacity expansion device, and an evaporator having a leaving water temperature, the method comprising the steps of:

• measuring the leaving water temperature;
• comparing the leaving water temperature to a setpoint so as to determine an error;
• modulating the compressor capacity to minimize the error unless the compressor capacity is at a minimum capacity position;
• maintaining the compressor at the minimum capacity position;
• and
• modulating the expansion device to minimize the error.

A method of selecting between first and second modes of capacity control for a water chiller system including an evaporator having an entering fluid temperature and a leaving fluid temperature, the method comprising the steps of:

• determining the leaving fluid temperature;
• determining the entering fluid temperature;
• determining a differential temperature as a function of the difference between the leaving fluid temperature and the entering fluid temperature;
• determining a temperature error as a function of the difference between the leaving fluid temperature and a setpoint;
• and
• selecting either the first or second mode of capacity control as a function of the temperature differential and the temperature error.