Energy Efficiency in Thermal Utilities
Energy Efficiency in Thermal Utilities
This domain covers boilers, furnaces, steam distribution, insulation and heat recovery. The exam tests combustion calculations, direct and indirect efficiency, excess air, flue-gas losses, blowdown, steam-trap management and waste-heat recovery options.
Core rules
- Boiler direct efficiency = useful heat absorbed by steam ÷ heat input in fuel. Indirect efficiency subtracts stack, radiation, blowdown and unburnt losses from 100%.
- Excess air raises dry flue-gas loss. Measure O2 or CO2, convert to excess air, and watch CO for incomplete combustion.
- Steam traps that fail open waste flash steam; failed closed traps cause waterlogging. Insulation savings depend on surface area, temperature difference and emissivity.
- Heat recovery follows the pinch: use the hottest available waste stream for the highest-grade demand first.
Worked example
Fuel oil has GCV 10,000 kcal/kg. Stoichiometric air is 14 kg/kg fuel; excess air is 20%, so flue-gas mass is 1 + 14 × 1.2 = 17.8 kg/kg fuel. Flue gas leaves at 220°C and ambient is 30°C; Cp = 0.24 kcal/kg°C. Dry flue-gas loss = 17.8 × 0.24 × (220 − 30) ÷ 10,000 × 100 = 8.1%. If stack temperature were cut to 170°C by an economizer, loss becomes 17.8 × 0.24 × 140 ÷ 10,000 × 100 = 6.0%, a 2.1 percentage-point efficiency gain. State that the calculation is on GCV and ignores radiation and moisture.
Traps
Mixing GCV and NCV; treating O2 percentage directly as excess air; claiming 100% combustion; forgetting radiation and convection losses; and ignoring that reducing excess air too far raises CO and unburnt fuel.