Mechanical Engineering - Thermal Engineering (Power Plant Cycles, Boilers, Turbines)
Thermal Engineering covers Rankine cycles, boilers, steam turbines, condensers and feedwater systems. The exam tests enthalpy-based cycle efficiency, boiler efficiency, turbine staging and heat-rate calculations.
Cycles
Rankine efficiency η = (W_t − W_p)/Q_in, with Q_in = h₁ − h₄. Reheat raises the average heat-addition temperature; regeneration reduces feedwater-heating irreversibility; superheat reduces turbine exhaust wetness. Specific steam consumption = 3600/W_net kg/kWh.
Boilers and turbines
Boiler efficiency = Q_absorbed/Q_fuel. The indirect method uses stack, moisture and radiation losses. Impulse turbines use pressure drop in nozzles; reaction turbines use fixed and moving blades. Governing methods include throttle, nozzle and bypass governing.
Worked example
Steam enters a turbine at h₁ = 3200 kJ/kg and exits ideally at h₂s = 2200 kJ/kg. Feedwater enthalpy is 200 kJ/kg and pump work is 10 kJ/kg. Turbine work = 1000 kJ/kg; net work = 990 kJ/kg; heat input = 3200 − 210 = 2990 kJ/kg. Cycle efficiency = 990/2990 = 33.1%. If turbine isentropic efficiency is 0.85, actual turbine work = 850 kJ/kg and net work = 840 kJ/kg, so efficiency falls.
Traps
Using temperatures instead of enthalpies; omitting pump work; mixing HHV and LHV; ignoring condenser vacuum effects on h₂; and treating boiler efficiency as cycle efficiency.