Applied Thermodynamics and Heat Transfer
Core concepts
A Third Class candidate must be fluent with the first and second laws, sensible and latent heat, specific heat, enthalpy, entropy, and the steam tables (saturated and superheated). Know the relationship between absolute and gauge pressure, and be able to convert between them without hesitation. Understand the three modes of heat transfer - conduction (Fourier's law), convection (Newton's law of cooling), and radiation (Stefan-Boltzmann) - and how each is applied in boiler and heat-exchanger contexts. Familiarity with LMTD for heat exchangers and the concept of thermal resistance in series is expected.
Traps to watch
- Mixing gauge and absolute pressure in steam-table lookups (a classic error).
- Forgetting that specific heat changes between phases; using cp where latent heat applies.
- Sign conventions in energy balances (work in vs. work out).
- Confusing entropy (a property) with enthalpy (also a property, but different meaning).
- Ignoring heat losses when the problem states "adiabatic" - that word is a strong hint.
Worked example / rule of thumb
Sensible heat: Q = m·cp·ΔT. For water, cp ≈ 4.18 kJ/(kg·°C). Heating 100 kg of water from 20 °C to 80 °C requires Q = 100 × 4.18 × 60 = 25,080 kJ ≈ 25.1 MJ. Rule of thumb: 1 kW ≈ 1 kJ/s, so this is about 7 kW·h of energy. Always check units before plugging numbers in.