Which one of the following modifications of the simple ideal Rankine cycle increases the thermal efficiency and reduces the moisture content of the steam at the turbine outlet?

To determine which modification of the simple ideal Rankine cycle increases the thermal efficiency and reduces the moisture content of the steam at the turbine outlet, let us analyze the effects of the proposed changes:

1. Decreasing the condenser pressure:
Lowering the condenser pressure increases the net work output and thermal efficiency because the expansion process in the turbine continues to a lower pressure. However, it shifts the turbine exit state further into the two-phase (wet steam) region, thereby increasing the moisture content (decreasing quality) of the steam at the turbine outlet.

2. Increasing the boiler pressure:
Raising the boiler pressure increases the average temperature at which heat is added, which increases the thermal efficiency of the cycle. However, for a constant turbine inlet temperature, increasing the boiler pressure shifts the turbine expansion line to the left on a T-s diagram. This results in a higher moisture content (lower quality) at the turbine exit.

3. Decreasing the boiler pressure:
Lowering the boiler pressure decreases the average temperature of heat addition, which decreases the thermal efficiency.

4. Increasing the turbine inlet temperature (Superheating):
Superheating the steam to a higher temperature before it enters the turbine increases the average temperature at which heat is added to the cycle, which increases the thermal efficiency. Additionally, on a T-s diagram, starting the expansion from a higher temperature at the same boiler pressure shifts the entire expansion process to the right. This shifts the state at the turbine exit closer to the saturated vapor line (or even into the superheated region), which significantly reduces the moisture content of the steam at the turbine outlet.

Therefore, increasing the turbine inlet temperature is the modification that achieves both desired effects.