Boiler Auxiliaries and Combustion Equipment
What this domain tests
This domain covers the equipment that supports combustion and flue gas flow: burners, fuel oil atomisers, gas trains, forced and induced draft fans, air preheaters, sootblowers, and the combustion control hardware that ties them together. Expect questions on draft, excess air, and the interaction between burner and fan performance.
Core rules to have at your fingertips
- Draft: natural draft depends on stack height and gas/air density difference. Balanced draft uses forced draft (FD) fan to supply air and induced draft (ID) fan to remove flue gas, keeping furnace pressure slightly negative (typically −20 to −50 Pa).
- Burners: atomisation is essential for liquid fuels - pressure-jet, rotary-cup, and steam/air atomising types. Poor atomisation produces smoke and high CO.
- Excess air: controlled by the O₂ trim in flue gas. Too little excess air → CO and smoke; too much → heat lost up the stack.
- Air preheater: recovers heat from flue gas; must be protected against cold-end corrosion by keeping metal temperature above the acid dew point.
- Sootblowers: remove ash deposits from tubes using steam or air; fouling reduces heat transfer and raises exit gas temperature.
- Gas train: includes pressure regulators, safety shut-off valves with proof-of-closure, and a low-gas-pressure switch.
Worked example
A furnace operates with 12 % excess air. Stoichiometric air for the fuel is 14 kg air/kg fuel. Actual air supplied = 14 × 1.12 = 15.68 kg/kg fuel.
If the FD fan delivers 20 kg/s of air, fuel rate = 20 / 15.68 = 1.276 kg/s.
If the ID fan must handle the products of combustion plus excess air, the flue gas mass ≈ fuel + air = 1.276 + 20 = 21.276 kg/s (plus moisture). Checking fan capacity against this mass flow at the operating temperature confirms whether the ID fan is the limiting factor.
Traps that cost marks
- Confusing FD and ID fan roles - FD supplies air, ID removes gas.
- Ignoring the effect of air preheater leakage on excess air readings.
- Assuming a burner can operate below its turndown ratio without instability.
- Forgetting that sootblowing must not be done while the boiler is at low load without checking drum level.
- Treating excess air as a fixed value - it must be adjusted with load.
Exam strategy
For draft and fan questions, sketch the air/gas path, label pressures, and state which fan controls furnace pressure. For burner questions, name the fuel, the atomisation method, and the failure symptom.