Questions: Rankine Cycle and Power Plant Applications

Question 1

A power plant engineer raises boiler pressure to improve thermal efficiency, but finds that turbine blade erosion increases dramatically. What modification addresses this problem while preserving the efficiency gain?

• A. Increase condenser pressure to raise the turbine exit temperature and reduce moisture
• B. Add superheating to raise the turbine inlet temperature and shift the expansion endpoint to drier steam
• C. Add regenerative feedwater heating to extract steam before it reaches the wet region
• D. Switch to a closed feedwater heater to avoid introducing moisture from extracted steam

Answer: (B)

Higher boiler pressure shifts the turbine expansion path leftward on the T-s diagram, increasing steam moisture at the turbine exit. Wet steam droplets erode low-pressure turbine blades. Superheating the steam beyond saturation moves the expansion starting point into the superheat region, shifting the exit point to higher quality (drier steam) while also raising the average temperature of heat addition — improving efficiency. Increasing condenser pressure would reduce efficiency by raising the rejection temperature.

Question 2

Why does regeneration improve the thermal efficiency of a Rankine cycle, even though it reduces net turbine work output?

• A. Regeneration allows the condenser to reject heat at a lower temperature by preheating feedwater externally
• B. Extracted steam that preheats feedwater would otherwise be condensed and its energy discarded; using it internally reduces the irreversibility of cold feedwater absorbing high-temperature combustion heat
• C. Regeneration increases the mass flow through the turbine, which increases power output proportionally
• D. The feedwater heaters operate as heat pumps, upgrading waste heat to higher temperatures before entering the boiler

Answer: (B)

Without regeneration, cold condensate at ~40°C enters the boiler and absorbs heat from combustion gases at ~1000°C — a massive temperature difference representing a large irreversibility. Feedwater heaters preheat the condensate using extracted turbine steam (which would otherwise be fully expanded and condensed), raising the average temperature at which heat is added and reducing the irreversibility of the heat-addition process. Less turbine work is produced (some steam is extracted early), but less fuel is needed per unit of work — net efficiency improves.

Question 3

A cogeneration (combined heat and power) plant produces more electrical power than a conventional steam power plant of the same fuel input, because the heat output is generated for free.

• T. True
• F. False

Answer: False

False. Cogeneration produces less electricity per unit of fuel than a conventional power plant, because steam is extracted at intermediate pressure (still carrying significant enthalpy) for heat delivery rather than expanding fully through the turbine to generate more work. The advantage is not more electricity — it is that the unavoidable heat rejection is made useful (district heating, industrial processes) instead of discarded. Total useful energy output (electricity + useful heat) is 80-90% of fuel input, versus 35-45% electricity-only efficiency for a conventional plant.

Question 4

Lowering condenser pressure in a Rankine cycle improves thermal efficiency by reducing the temperature at which heat is rejected.

• T. True
• F. False

Answer: True

True. The condenser operates at the saturation temperature corresponding to its pressure. Lower condenser pressure means lower saturation temperature, which means the cycle rejects heat at a lower temperature (closer to the cold reservoir). From a Carnot perspective, η = 1 − T_cold/T_hot — reducing T_cold directly improves the upper bound on efficiency. In practice, condenser pressure is limited by the available cooling medium temperature (river water, cooling towers) and the need to avoid air ingestion at very low pressures (below atmospheric).

Question 5

Explain why reheat improves both thermal efficiency and turbine reliability in a Rankine cycle, and what would happen without it at high boiler pressures.

Model answer: At high boiler pressures, turbine expansion ends in the two-phase (wet steam) region — the T-s diagram shows the expansion path crossing into the dome. Wet steam droplets erode low-pressure turbine blades through droplet impact. Reheat extracts the steam after partial expansion through the high-pressure turbine, returns it to the boiler for reheating back to near inlet temperature, then expands it through the low-pressure turbine. This moves the final exit point to much drier steam (protecting blades) and slightly raises the average temperature of heat addition (improving efficiency). Without reheat at high pressures, blade erosion would severely limit plant life and force lower operating pressures.

Reheat is an example of a modification that simultaneously addresses two independent problems: a thermodynamic limitation (efficiency) and an engineering constraint (blade durability). Most large coal and nuclear plants use one or two stages of reheat. Each stage adds complexity and cost (extra piping, an additional turbine section, boiler reheat pass) but the efficiency and reliability benefits justify it at large scale. The T-s diagram makes the trade-off visible: each reheat step adds a horizontal segment at high temperature, shifting work production to more favorable conditions.