Thermodynamics: Rankine vs Brayton Cycles, Reheat & Regenerative Feed Heating
Thermodynamic principles, T-s and P-h charts, steam turbine expansion, and combined cycle gas turbines (CCGT).
In-Depth Interview Questions & Model Solutions
Q1How does Reheating and Regeneration improve the thermal efficiency of a Rankine Power Cycle?
Reheating: Steam expands in the High Pressure (HP) turbine, is returned to the boiler reheater at constant pressure to increase temperature back to boiler inlet levels, then expands in Low Pressure (LP) turbine. This increases mean temperature of heat addition (Tm1) and significantly reduces moisture content at turbine exhaust (preventing blade erosion). Regeneration: Bleeding steam from intermediate turbine stages to pre-heat boiler feed-water reduces irreversibility during heat addition in the economizer, boosting overall cycle efficiency toward Carnot levels.
- Reheating prevents turbine blade cavitation and erosion by keeping steam quality > 88%.
- Regeneration increases the mean temperature of heat addition.
- Combined Cycle Power Plants (CCGT) reach thermal efficiencies exceeding 60%.
Q2Explain the Brayton Cycle with intercooling, reheat, and regeneration for gas turbines.
The ideal Brayton cycle consists of: (1-2) Isentropic compression, (2-3) Constant pressure heat addition, (3-4) Isentropic expansion in turbine, and (4-1) Constant pressure heat rejection. Multi-stage compression with Intercooling reduces compressor work input. Multi-stage expansion with Reheating increases turbine work output. A Regenerator (recuperator) transfers heat from turbine exhaust gases to compressed air before entering combustion chamber, saving fuel.
- Pressure ratio (rp) determines maximum efficiency and specific work output.
- Intercooling + Reheat increases specific work output but requires regenerator to maintain high efficiency.
Technical Panel Interview Strategy Tips
- NTPC boards frequently ask about super-critical and ultra-super-critical boilers (> 22.1 MPa, > 374°C) where water flashes directly to steam without boiling bubble formation.