The NART-I series turbo chiller, developed and marketed by Mitsubishi Heavy Industries, Ltd. (MHI), utilizes as a base a highly efficient turbo chiller that uses the refrigerant HFC134a, which does not affect the ozone layer. An inverter has also been introduced in the drive source of the NART-I series turbo chiller to improve aerodynamic performance, and a newly designed optimum control capable of coping with variable speed is incorporated that has made it possible to realize remarkable savings in energy. The inverter also facilitates variable speed control of the NART series turbo chiller, thereby helping to make it highly efficient. The turbo chiller has achieved a COP of 6.1 by high performance aerodynamic characteristics, low mechanical loss, and a highly efficient heat exchanger. In addition, advanced calculation control has been incorporated in the NART series to realize optimum acceleration/deceleration and load control based on the aerodynamic characteristics of the compressor. As a result, a remarkable increase in performance could be obtained for NART series turbo chiller during periods of partial load and when cooling water temperature is low. These features have made it possible for the series to attain the highest level of efficiency in the world, achieving a COP as high as 17.8 max. (chilled water outlet temperature of 7 °C). Trial-calculations of the annual energy saving effect of this machine under general plant load patterns have shown remarkable reductions in electric power consumption of as high as 45 % compared with the level of power consumption of a fixed speed machine.