In a multi-stage centrifugal compressor, which statement is true about the stages?

• A. Each stage has identical cooling requirements
• B. Each stage operates at increasing ambient temperature
• C. Each stage is identical in volume to the other stages
• D. Each stage uses a different lubricant

Answer: (C)

The thing being tested is why centrifugal compressors use repeatable, identical stage hardware. In a multi-stage setup, each stage is built as the same module with the same rotor, diffuser, casing geometry, and internal volume. This makes the physical volume of every stage the same, which simplifies manufacturing, balancing, maintenance, and predictable overall performance.

Remember that while the hardware volume is identical, the gas itself changes as it moves from stage to stage: the mass flow stays roughly constant, but density increases as pressure rises, so the actual gas volume flowing through each downstream stage is reduced. This distinction is why the statement about identical stage volumes is true even though the gas volume per stage isn’t the same throughout the train.