Understanding Actual vs. Ideal Rankine Cycle Differences

The Rankine cycle is a fundamental model for vapor power plants. The ideal Rankine cycle assumes perfect, reversible processes, while the actual Rankine cycle accounts for real-world imperfections such as irreversibilities and pressure losses.

Analyzing Statement A: Steam Pressure Leaving the Boiler

Statement A says: "Steam leaves the boiler at a somewhat lower pressure in the actual cycle than the ideal one."

• In an ideal Rankine cycle, heat addition in the boiler occurs at a constant pressure. There are no pressure drops assumed in the piping or the boiler itself.
• In an actual Rankine cycle, there are always pressure drops due to friction as the working fluid flows through pipes, the economizer, the boiler tubes, and the superheater.
• Therefore, the pressure at which steam exits the boiler system (and enters the turbine) in an actual cycle is lower than the peak pressure inside the boiler or the pressure at which water enters the boiler sections.
• Compared to an ideal cycle designed to operate at the same peak pressure, the pressure at the turbine inlet in the actual cycle will be lower due to these pressure drops.
• Thus, Statement A, suggesting lower steam pressure leaving the boiler in the actual cycle compared to the ideal, is correct.

Analyzing Statement B: Pump Discharge Pressure

Statement B says: "Water must be pumped to a sufficiently higher pressure in the actual cycle than the ideal one."

• In an ideal Rankine cycle, the pump increases the water pressure from the condenser pressure directly to the boiler pressure.
• In an actual Rankine cycle, in addition to the pump being irreversible (requiring more work), there are pressure drops between the pump outlet and the point where the target high pressure is reached within the boiler.
• To achieve a comparable high pressure level within the boiler in the actual cycle, the pump must discharge water at a pressure high enough to overcome the pressure drop in the piping and boiler sections leading up to the main evaporation or superheating sections.
• This means the pump outlet pressure in the actual cycle is necessarily higher than the pressure that would be required in an ideal cycle operating at the same peak boiler pressure level, just to compensate for losses.
• Thus, Statement B, stating that water must be pumped to a higher pressure in the actual cycle, is correct.

Conclusion

Based on the analysis of pressure drops and irreversibilities inherent in real vapor power systems:

• Statement A is correct because pressure losses reduce the steam pressure entering the turbine in the actual cycle compared to the ideal.
• Statement B is correct because the pump needs to overcome system pressure drops to achieve a desired high pressure level in the boiler, requiring a higher discharge pressure than in the ideal case.

Both statements accurately describe deviations of an actual Rankine cycle from an ideal Rankine cycle.