For a compressor, the input work decreases as the suction temperature decreases. From which source should air be taken to minimize the work required for compression?

Introduction / Context:The power required by compressors depends strongly on the suction (inlet) state. Lower inlet temperature reduces the specific volume and the enthalpy rise, thereby lowering the compression work. This question checks understanding of how suction conditions influence compressor work.

Given Data / Assumptions:

• Same delivery pressure and mass flow requirement.
• Ideal gas behavior for air.
• Comparison across different air sources with different inlet temperatures.

Concept / Approach:For isentropic or polytropic compression of an ideal gas, the shaft work per unit mass increases with inlet temperature. For a given pressure ratio, temperature rise is proportional to inlet absolute temperature. Therefore, the colder the suction air, the lower the work to reach the same discharge pressure.

Step-by-Step Solution:

Verification / Alternative check:Industrial practice uses inlet chillers or precoolers to reduce compressor power in gas turbines and large air systems, confirming the principle that colder inlet air reduces work and increases density (higher mass flow per unit volume).

Why Other Options Are Wrong:

• The atmosphere: Only correct if it is the coldest option, which is not guaranteed.
• A source at 0°C: Colder than typical ambient, but option (C) is more general and always correct.
• A source of high temperature air: Increases work due to higher T1.
• A source of saturated steam–air mixture: Adds moisture issues and usually higher enthalpy; not work-minimizing.

Common Pitfalls:Confusing volumetric flow with mass flow; colder air has higher density, affecting volumetric throughput and sometimes leading to misinterpretation of power changes.

Final Answer:A source of low temperature air