Which of the following is NOT a type of positive displacement compressor?

Centrifugal compressors operate on a different principle than positive displacement compressors. They are classified as dynamic compressors, which utilize the principles of centrifugal forces to compress gas. In a centrifugal compressor, the gas enters the impeller, where it gains kinetic energy as it is thrown outward by the rotation of the impeller, subsequently being converted to pressure energy as it passes through the diffuser. This method differs from the positive displacement principle, where a fixed volume of air or gas is trapped and compressed.

In contrast, the other types mentioned—screw, piston, and rotary vane compressors—all function by trapping a specific volume of refrigerant within the chamber and reducing that volume to effectuate compression. This fundamental operational difference highlights why the centrifugal compressor is not categorized as a positive displacement compressor.