Centrifugal-chiller and method for controlling the same

A method for controlling a centrifugal-chiller including

• a variable-speed centrifugal-compressor whose rotational frequency can be varied by an inverter and which compresses a refrigerant;
• a constant-speed centrifugal-compressor connected in parallel with the variable-speed centrifugal-compressor and operated at a constant rotational speed to compress the refrigerant;
• a condenser that condenses the refrigerant compressed by the variable-speed centrifugal-compressor and/or the constant-speed centrifugal-compressor into a liquid using cooling water supplied thereto;
• an expansion valve that expands the refrigerant that is condensed into a liquid by the condenser;
• and
• an evaporator that evaporates the refrigerant expanded by the expansion valve,
• the method controlling the operations of the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor, wherein
• in the case where a required load factor of the centrifugal-chiller is less than a predetermined value, the control unit selects a variable-speed-compressor priority operating mode in which only the variable-speed centrifugal-compressor is activated, and in the case where the required load factor of the centrifugal-chiller is the predetermined value or greater, the control unit selects a combined-use operating mode in which the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor are activated.