Which type of compressor is not classified as a positive displacement compressor?

A centrifugal compressor is not classified as a positive displacement compressor because it operates based on the principle of dynamic compression rather than the direct volumetric changes associated with positive displacement compressors. Positive displacement compressors, such as reciprocating, screw, and rotary compressors, work by trapping a specific volume of gas and then reducing its volume to create pressure, effectively displacing the gas from the intake to the discharge.

In contrast, a centrifugal compressor uses a rotating impeller to impart kinetic energy to the gas, increasing its velocity. The gas then enters a diffuser where this kinetic energy is converted into pressure. This continuous flow and energy conversion process does not involve trapping and compressing a fixed volume of gas like positive displacement compressors do.

Understanding the operation of centrifugal compressors is crucial in fields such as HVAC and industrial processes, where different compression methods can significantly impact efficiency and application suitability.