Which of these compressors is not a positive displacement type?

• A. Reciprocating
• B. Rotary screw
• C. Scroll
• D. Centrifugal

Answer: (D)

The key idea is the difference between positive-displacement and dynamic compression. Positive-displacement compressors trap a fixed amount of refrigerant and compress it by shrinking the trapped volume, using mechanisms like pistons, screws, or scrolls. Reciprocating compressors push a piston to reduce the volume in a chamber, rotary screw compressors trap refrigerant between intermeshing screws, and scroll compressors form shrinking pockets with a moving and a stationary scroll to compress the gas. In all of these, compression comes from decreasing a fixed volume per cycle.

Centrifugal compressors, on the other hand, compress by dynamic means. They spin a high-speed impeller to impart velocity to the refrigerant and then slow it in diffusers to raise pressure. There isn’t a fixed chamber whose volume is reduced during each cycle. This dynamic action makes centrifugal compressors not positive-displacement types, even though they can move large volumes of refrigerant at moderate pressures.