Applied Thermodynamics
Applied thermodynamics tests your ability to convert steam tables, cycle diagrams and measured flows into efficiency, heat rate and power. The exam favours the Rankine cycle, reheat and regenerative feedwater heating, throttling and nozzle behaviour, and the difference between ideal and actual processes.
Core rules
- First Law for a steady-flow component: energy in = energy out. For a turbine, work = mass flow x enthalpy drop; for a pump, work = mass flow x enthalpy rise.
- Second Law sets the maximum conversion between heat and work. Entropy increases in an irreversible adiabatic process; isentropic efficiency compares actual work with ideal work.
- Rankine cycle efficiency = net work / heat input = (turbine work - pump work) / boiler heat input. Heat rate in kJ/kWh = 3,600 / thermal efficiency.
- Reheat raises average heat-addition temperature and reduces exhaust moisture; regenerative extraction heating raises feedwater temperature and cycle efficiency.
- Supercritical units operate above the critical point, where latent heat disappears and the fluid changes density continuously.
Worked example
A simple Rankine cycle has turbine inlet enthalpy 3,000 kJ/kg, turbine exhaust enthalpy 2,200 kJ/kg, condenser outlet enthalpy 200 kJ/kg and pump outlet enthalpy 210 kJ/kg. Turbine work = 3,000 - 2,200 = 800 kJ/kg. Pump work = 210 - 200 = 10 kJ/kg. Net work = 790 kJ/kg. Boiler heat input = 3,000 - 210 = 2,790 kJ/kg. Cycle efficiency = 790 / 2,790 = 28.3%. The corresponding heat rate is 3,600 / 0.283 = 12,720 kJ/kWh. If the turbine were only 90% isentropic, actual exhaust enthalpy would be higher and net work lower.
Traps that cost marks
Using gauge pressure in steam-table lookups; treating enthalpy and internal energy as interchangeable; omitting pump work when it is explicitly requested; assuming a real turbine is isentropic; and mixing kJ/kg with kJ/kWh. Also watch dryness fraction: wet steam enthalpy is hf + x hfg, not saturated-vapour enthalpy. In nozzle problems, velocity increases as enthalpy drops, but actual exit velocity is reduced by the nozzle coefficient.
Key points: use absolute pressure for steam properties; turbine work is mass flow times actual enthalpy drop; cycle efficiency excludes or includes pump work as defined; heat rate is inversely related to efficiency; reheat and regeneration improve cycle performance for different reasons.