The air cooled packaged chiller is a very popular choice for HVAC designers in capacities up to and slightly above 1,900 kW (550 tons). Until recently, the only compressor types commonly available in air cooled packaged chillers across that entire range were of the positive displacement type. By contrast, for nearly 100 years, the efficiency and other inherent advantages of centrifugal compressors have made them a popular compressor choice in water cooled packaged chillers with capacities as low as 350 kW (100 tons). This article explores the history of centrifugal compressors when applied in air cooled chiller systems.