What happens to cycle efficiency if steam temperature at the turbine entrance increases?

• A. Increases available work that can be extracted from turbine
• B. Decreases available work
• C. No change
• D. Reverses cycle direction

Answer: (A)

Raising the temperature of steam at the turbine inlet increases the energy content of the steam going into the turbine. With the condenser pressure fixed, this higher-energy steam can expand and do more work in the turbine, so the turbine work per unit mass increases. The pump work is small in comparison, so the net work produced by the cycle rises.

Thermal efficiency is the ratio of net work to the heat added in the boiler. If more work is extracted from the same cycle without proportionally increasing the heat input (or even with a modest increase in heat input that isn’t as large as the gain in turbine work), the efficiency improves. In practice, increasing turbine inlet temperature raises the cycle’s efficiency (up to material and design limits) because more of the added heat ends up as useful work rather than being lost to the condenser.

So, higher turbine inlet temperature leads to more work available from the turbine, and that generally translates to higher cycle efficiency. This is why superheating and pushing TIT higher is a common way to boost Rankine-cycle performance.