What happens to cycle efficiency if the feedwater heating increases?

What happens to cycle efficiency if the feedwater heating increases?

• A. Increases plant efficiency
• B. Decreases plant efficiency
• C. No change
• D. Decreases turbine efficiency

Answer: (A)

Increasing feedwater heating raises the average temperature at which heat is added in the cycle. When feedwater is preheated to a higher temperature before entering the boiler, the boiler has to supply less heat to produce the same amount of steam. Since cycle efficiency is the net work output divided by the heat input, reducing the heat input for the same power output raises efficiency. Using extracted steam to preheat the feedwater also cuts irreversibilities from large temperature differences during heat addition, further boosting efficiency. In short, more feedwater heating generally improves cycle efficiency, within practical limits due to other losses and design constraints.

Increasing feedwater heating raises the average temperature at which heat is added in the cycle. When feedwater is preheated to a higher temperature before entering the boiler, the boiler has to supply less heat to produce the same amount of steam. Since cycle efficiency is the net work output divided by the heat input, reducing the heat input for the same power output raises efficiency. Using extracted steam to preheat the feedwater also cuts irreversibilities from large temperature differences during heat addition, further boosting efficiency. In short, more feedwater heating generally improves cycle efficiency, within practical limits due to other losses and design constraints.