Centrifugal (turbo) refrigerant compressors are most suitable for refrigerants and duty conditions that require which combination of displacement and condensing pressure?

Introduction / Context:
Centrifugal compressors are dynamic machines that deliver high flow rates at modest pressure ratios per stage. This question asks which refrigerant duty combination best suits centrifugal compressors.

Given Data / Assumptions:

• Focus on single-stage or few-stage centrifugal machines.
• Typical refrigeration pressure ratios (per stage) are limited compared with positive-displacement machines.

Concept / Approach:
Dynamic compressors excel at moving large volumes at relatively low to moderate pressure rises per stage. In refrigeration, they pair well with refrigerants and systems where volumetric flow is large and the required condensing pressure (and thus pressure ratio) per stage is not extreme. Positive-displacement compressors are preferred when very high pressure ratios or small displacements are required.

Step-by-Step Solution:

Verification / Alternative check:
Industrial centrifugal chillers commonly serve large tonnage with moderate pressure ratios, confirming the selection.

Why Other Options Are Wrong:

• Small displacements: Better matched to reciprocating or screw compressors.
• High condensing pressures: Drive up pressure ratio per stage, favoring PD machines or multistaging.
• Negligible displacement: Not meaningful for practical refrigeration duty.

Common Pitfalls:
Assuming centrifugal compressors suit any pressure ratio; ignoring surge and efficiency penalties at off-design conditions.

Final Answer:
Large displacements and low condensing pressures