Centrifugal compressor basics — select the correct statement Which of the following statements correctly characterizes a centrifugal compressor in standard engineering practice?

Introduction / Context:
Centrifugal compressors add kinetic energy to air by high-speed impeller rotation and then convert part of that kinetic energy into static pressure in the diffuser. They are widely used in small turbochargers and industrial air systems where moderate pressure ratios and ruggedness are desired.

Given Data / Assumptions:

• Conventional single- or multistage centrifugal compressor with diffuser and volute.
• Steady, subsonic to transonic relative speeds depending on design.
• Comparison against axial compressors on general traits.

Concept / Approach:
Energy transfer in turbomachinery follows Euler's pump/compressor equation. In a centrifugal compressor, high impeller tip speed imparts swirl and kinetic energy to the air, which the diffuser converts to static pressure. Therefore, the defining correct statement revolves around high rotational speeds of the impeller to add energy.

Step-by-Step Solution:
Check option (a): axial-parallel flow describes axial compressors, not centrifugal.Check option (b): static pressure generally rises from impeller exit into diffuser; the phrasing about ‘‘decreases’’ is incorrect.Check option (c): aligns with Euler's principle; correct.Check option (d): multistage axial compressors typically achieve higher peak efficiencies for large mass flows.

Verification / Alternative check:
Performance maps show typical impeller tip speeds several hundred m/s, confirming the need for high peripheral speed for effective compression.

Why Other Options Are Wrong:
(a) Confuses machine type; (b) misstates pressure behavior; (d) overclaims versus axial machines.

Common Pitfalls:
Equating single-stage pressure rise with overall efficiency; the efficiency comparison depends on application scale and staging.

Final Answer:
The impeller rotates at high peripheral speeds to add energy to the flow.