What constitutes a pressure stage in a compressor?

• A. A rotor-stator pair constitutes a pressure stage
• B. A single rotor blade constitutes a price stage
• C. Two stator blades constitute a pressure stage
• D. An impeller alone constitutes a pressure stage

Answer: (A)

The key idea is that a pressure stage is where the gas gets its pressure rise through both energy addition and flow turning. In an axial compressor, that happens with a rotor blade row that imparts energy to the gas, followed by a stator blade row that straightens the flow and converts that energy into additional static pressure. Together, the rotor and stator form a single stage, and stacking stages gives higher overall pressure.

A single rotor blade is only part of the energy-adding process and cannot complete the stage, because without a stator to remove swirl and convert the added energy into pressure, the flow won’t achieve the desired pressure rise. Two stator blades in a row don’t add energy themselves; they merely guide and diffuse the flow. An impeller alone isn’t a complete stage in most compressor types because the diffuser or stator is needed to convert the rotor’s kinetic energy into usable static pressure. Therefore, the rotor-stator pair best defines a pressure stage.