How does a reciprocating compressor move gas?

• A. As a positive displacement machine that moves a specific volume of gas with each stroke.
• B. By centrifugal action of an impeller.
• C. By rotating a screw to push gas.
• D. By vibrating to shake gas through.

Answer: (A)

Reciprocating compressors move gas using positive displacement. A piston inside a cylinder is driven by a crankshaft to perform a suction stroke that draws a fixed volume of gas into the cylinder, followed by a compression stroke that reduces the cylinder volume and raises the gas pressure, and then a discharge stroke that pushes the gas out into the system through a valve. Because the amount of gas moved with each complete piston stroke is essentially fixed by the cylinder bore and length of the stroke, the flow is pulsating and the pressure is high at discharge. This mechanism is different from other types: a centrifugal compressor relies on spinning an impeller to impart velocity and then convert it to pressure, not by moving a specific volume per stroke; a screw compressor uses intermeshing screws to transport gas in a rotary, continuous manner rather than in discrete strokes; and vibrating methods do not provide the controlled compression and discharge cycle that defines these machines.