Questions: Rankine Cycle Improvements: Reheat and Regenerative Feedwater Heating

Question 1

In a regenerative Rankine cycle with an open feedwater heater, a fraction of steam is extracted from an intermediate turbine stage and used to heat the feedwater. What is the primary thermodynamic reason this improves cycle efficiency?

• A. The extracted steam reduces the load on the condenser, so less cooling water infrastructure is needed
• B. The extracted steam preheats the feedwater so that heat is added to the boiler at a higher average temperature, reducing the irreversibility of low-temperature heat addition
• C. The bleed reduces total mass flow through the turbine, allowing the remaining steam to expand more efficiently
• D. The extraction removes the wettest, lowest-quality steam from the cycle, preventing turbine blade erosion

Answer: (B)

The thermodynamic mechanism is raising the mean temperature of heat addition. In a simple Rankine cycle, cold feedwater (near condenser saturation temperature, ~40–50°C) must be heated through the entire subcooled liquid range in the boiler — a low-temperature process with high irreversibility. Regeneration uses bled steam to do this preheating, so the boiler only receives feedwater that is already near the bleed-point saturation temperature. The boiler then adds heat over a narrower, higher-temperature range, closer to Carnot's ideal. Option D describes the benefit of reheat, not regeneration.

Question 2

A student claims that reheat improves Rankine cycle efficiency because it reduces the total amount of heat that must be added to the cycle. This reasoning is:

• A. Correct — reheating mid-expansion uses steam work already extracted, reducing boiler duty
• B. Correct — splitting expansion into two stages reduces the irreversibility of each stage, requiring less total heat input
• C. Incorrect — reheat actually increases total heat input; efficiency improves because the additional turbine work gained from the second expansion exceeds the additional heat cost of reheating
• D. Incorrect — reheat improves efficiency only by eliminating moisture, not by any thermodynamic cycle improvement

Answer: (C)

Reheat adds more fuel (more heat input to reheat the steam at intermediate pressure). It improves efficiency because the additional turbine work from the second LP expansion exceeds the heat cost of reheating — the work/heat ratio improves. The thermodynamic justification is that reheating raises the mean temperature at which heat is added: you are adding heat at an intermediate turbine pressure (a higher temperature than the condenser) rather than allowing inefficient expansion deep into the wet region. It is NOT a net reduction in heat input.

Question 3

Both reheat and regeneration improve Rankine cycle efficiency by the same underlying thermodynamic principle: raising the mean temperature at which heat is added to the working fluid.

• T. True
• F. False

Answer: True

True — both modifications move the actual heat addition profile closer to the Carnot ideal of a constant high-temperature source. Reheat adds heat at intermediate turbine pressure (rather than allowing expansion all the way into low-quality wet steam territory). Regeneration eliminates the inefficient low-temperature heating of cold feedwater by substituting bled steam. In both cases, the cycle average T_H increases, and by the Carnot relationship, efficiency improves. This shared principle is why modern plants implement both simultaneously.

Question 4

In a regenerative Rankine cycle, the steam bled from the turbine for feedwater heating reduces total net work output compared to a simple Rankine cycle operating at the same turbine inlet conditions.

• T. True
• F. False

Answer: False

False — while the bled steam does bypass the lower turbine stages (some potential work is forgone), the cycle efficiency increases, meaning more net work is obtained per unit of heat input. The mass flow through the LP turbine decreases, but the boiler heat input decreases by more than proportionally (because the preheated feedwater requires less heat addition). The result is a higher work-to-heat ratio — higher efficiency — not less net work per unit heat. Regeneration is economically beneficial precisely because it improves the conversion efficiency.

Question 5

Explain in one to two sentences the thermodynamic principle shared by both reheat and regeneration that explains why each improves Rankine cycle efficiency.

Model answer: Both modifications raise the mean temperature at which heat is added to the working fluid, which improves thermal efficiency because Carnot efficiency increases with T_H. Reheat adds energy at an intermediate turbine pressure (a higher temperature than the condensation range) instead of continuing expansion into the wet region, while regeneration uses extracted steam to preheat feedwater, eliminating the inefficient addition of heat to cold water at low temperatures.

The underlying principle is the Carnot relationship: η = 1 − T_L/T_H. Anything that raises T_H (the mean temperature of heat addition) while holding T_L fixed (condenser temperature is constrained by the environment) improves efficiency. Neither reheat nor regeneration changes the condenser temperature — they both act on the heat-addition side of the cycle, making the temperature profile of that process more favorable.