Why does air entry into the main condenser reduce the efficiency of the steam cycle?

Correct Answer: Option D — Low pressure turbine exhaust steam enthalpy value increases

Air ingress blankets condenser tubes, reducing heat transfer efficiency and increasing backpressure. Higher backpressure elevates the exhaust saturation temperature and enthalpy value of the low pressure turbine exhaust steam, degrading overall Rankine cycle thermal efficiency as governed by 46 CFR Subchapter F.