Questions: The Rankine Cycle and Steam Power Plants

Question 1

An engineer wants to improve the thermal efficiency of a Rankine cycle power plant. Which modification directly increases efficiency by raising the maximum temperature at which heat is added?

• A. Lower the condenser pressure so more work is extracted in the turbine
• B. Increase boiler pressure and superheat the steam to raise the peak steam temperature
• C. Increase the pump work to raise the water to a higher pressure
• D. Reduce the mass flow rate of steam through the cycle

Answer: (B)

Raising boiler pressure and superheating increases T_hot, pushing the cycle's efficiency closer to the Carnot limit η = 1 − T_cold/T_hot. Option A (lowering condenser pressure) also improves efficiency by reducing T_cold, but the question specifically asks about raising the heat addition temperature. Option C is wrong because pump work is a small parasitic loss, not a source of output. Option D changes power output, not efficiency.

Question 2

In an ideal Rankine cycle, the turbine expansion is modeled as isentropic. In a real steam turbine, what actually happens to the entropy of the steam during expansion, and what is the consequence for power output?

• A. Entropy decreases slightly, meaning the real turbine extracts more work than the ideal
• B. Entropy stays constant but the process is slower, reducing power
• C. Entropy increases due to irreversibilities, so the actual enthalpy drop is less than ideal and output is reduced
• D. Entropy increases, but this is corrected by the condenser and does not affect net output

Answer: (C)

Real turbines have friction and heat losses, making expansion irreversible — entropy increases. On the h-s diagram (Mollier diagram), the actual exit state lies to the right of the ideal isentropic exit, at a higher enthalpy. Since turbine work = h_in − h_out, a higher actual h_out means less work extracted. Turbine isentropic efficiency η_t = (h₃ − h₄_actual)/(h₃ − h₄_ideal) quantifies this. The condenser (Option D) only rejects heat; it cannot recover lost turbine work.

Question 3

Superheating the steam above the saturation temperature before it enters the turbine both improves efficiency and protects the turbine blades.

• T. True
• F. False

Answer: True

Both benefits are real. Superheating raises the average temperature at which heat is added, increasing efficiency toward the Carnot limit. It also keeps the steam drier throughout expansion — wet steam (liquid droplets) would erode turbine blades. The two motivations reinforce each other.

Question 4

Pump work is negligible in the Rankine cycle and is typically set to zero when calculating net cycle efficiency.

• T. True
• F. False

Answer: False

Pump work is small relative to turbine output — because liquid water is nearly incompressible, compressing it to boiler pressure requires far less work than expanding high-pressure steam. But it is not zero, and omitting it overestimates net work W_net = W_turbine − W_pump. In detailed cycle analyses (using steam tables), pump work must be included. The misconception 'pump work ≈ 0' is acceptable as a rough estimate, not a rigorous assumption.

Question 5

Why does the Rankine cycle use water as a two-phase working fluid rather than a purely gaseous working fluid like an ideal gas?

Model answer: Water's liquid-vapor phase transition allows isothermal heat addition (during boiling) and rejection (during condensation) at high heat transfer rates in compact equipment. The latent heat of vaporization stores and releases enormous amounts of energy at constant temperature and pressure, enabling efficient, compact boilers and condensers. A purely gaseous working fluid would require larger heat exchangers and cannot exploit this high-capacity isothermal exchange.

The Brayton cycle (gas turbines, jet engines) does use gaseous working fluid, but it requires very high temperatures to achieve good efficiency because there is no isothermal latent-heat phase. The Rankine cycle trades mechanical simplicity (all gas) for thermodynamic advantage (phase change). The condenser's ability to reject huge amounts of heat at nearly constant temperature — while the steam condenses — is central to the cycle's practical success in large power plants.