Applied Thermodynamics and Mechanics
What this domain tests
Applied Thermodynamics and Mechanics asks you to apply first-law energy balances, the properties of steam and water, gas laws, and basic static/dynamic mechanics to power plant equipment. Expect numerical questions: enthalpy balances across a boiler or feedwater heater, sensible and latent heat, thermal efficiency, expansion work, plus beam, pressure, and torque problems on plant structures.
Core rules to have at your fingertips
- Steam tables rule everything. Subcooled liquid enthalpy is approximated by h ≈ cp·T (cp = 4.187 kJ/kg·K). Saturated mixtures use h = hf + x·hfg, where x is dryness fraction. Superheated steam requires superheat tables, not extrapolation.
- First law, steady flow: Q − W = m[(h2 − h1) + ½(v2² − v1²) + g(z2 − z1)]. In boilers, W = 0; in turbines, Q ≈ 0 (adiabatic).
- Boiler efficiency (input-output method): η = m_steam(h_steam − h_fw) / (m_fuel × LHV).
- Gas laws: P1V1/T1 = P2V2/T2 for a fixed mass; use absolute temperatures (K).
- Mechanics: Stress σ = F/A, strain ε = ΔL/L, Hooke's law σ = E·ε; torque T = F·r; pressure p = F/A; work = force × distance.
Worked example
A boiler produces 10 kg/s of steam at h = 2 800 kJ/kg from feedwater at h = 420 kJ/kg, burning 0.75 kg/s of fuel with LHV = 42 000 kJ/kg.
Heat absorbed = 10 × (2 800 − 420) = 23 800 kW. Heat input = 0.75 × 42 000 = 31 500 kW. η = 23 800 / 31 500 = 0.756, i.e. 75.6 %.
If instead the question gave stack losses of 12 %, moisture loss 4 %, and radiation 2 %, you would apply the indirect (heat-loss) method: η = 100 − (12 + 4 + 2) = 82 %. Both methods should agree within measurement uncertainty; never mix them in one calculation.
Traps that cost marks
- Using gauge pressure where absolute pressure is required in P1V1/T1 problems.
- Forgetting the latent heat term for wet steam - h = hf alone is only valid for saturated liquid.
- Mixing °C into gas-law ratios; always convert to K.
- Using higher heating value (HHV) when the question specifies LHV - the answer will be about 5–10 % low.
- Reporting efficiency as a fraction when the question asks for a percentage.
- Applying beam formulas without checking that the load is within the elastic limit.
Exam strategy
Write the formula, substitute with units, and state the answer with units. Show the intermediate heat-absorbed and heat-input values - partial credit is usually available for correct method.