At constant RPM, the compressor pressure ratio and the temperature rise across the compressor do what as height increases?

• A. remains constant irrespective of height.
• B. increases with height.
• C. decreases with height.
• D. varies with height in a random way.

Answer: (A)

Holding RPM constant fixes the compressor head, so the pressure rise the compressor can produce stays essentially the same as you climb. The compressor’s pressure ratio is determined mainly by its speed and blade design, not by altitude, even though ambient pressure falls with height. Since the pressure ratio across the compressor remains about the same, the temperature rise that goes with that compression follows the same isentropic relation, so the discharge–inlet temperature behavior remains the same in the sense used for this question.

In practice, the absolute temperature rise in degrees can seem to depend on the inlet temperature, but for a fixed rpm the compressor delivers a fixed pressure rise, and the associated temperature change (relative to the inlet) is not expected to vary with height in the way the other options suggest. Thus, the best answer is that the compressor’s pressure ratio and the resulting temperature rise stay effectively constant with altitude at a given RPM.