What is the relationship between condenser pressure and cycle efficiency when condenser vacuum increases?

What is the relationship between condenser pressure and cycle efficiency when condenser vacuum increases?

• A. Lower condenser pressure leads to higher cycle efficiency
• B. Higher condenser pressure leads to higher cycle efficiency
• C. No change
• D. Cycle efficiency decreases with vacuum

Answer: (A)

Lower condenser pressure (stronger vacuum) increases cycle efficiency because it allows the steam to expand more in the turbine. With a lower back pressure, the turbine work rises significantly for the same heat input, while the pump work to move condensate back into the boiler only increases modestly. The net result is more useful work produced per unit of heat added, so the thermal efficiency improves. If condenser pressure were higher, the turbine would have less expansion, producing less work and lowering efficiency. Saying there’s no change ignores how the energy balance shifts with condenser pressure, and claiming that efficiency decreases with vacuum contradicts the actual trend.

Lower condenser pressure (stronger vacuum) increases cycle efficiency because it allows the steam to expand more in the turbine. With a lower back pressure, the turbine work rises significantly for the same heat input, while the pump work to move condensate back into the boiler only increases modestly. The net result is more useful work produced per unit of heat added, so the thermal efficiency improves.

If condenser pressure were higher, the turbine would have less expansion, producing less work and lowering efficiency. Saying there’s no change ignores how the energy balance shifts with condenser pressure, and claiming that efficiency decreases with vacuum contradicts the actual trend.