Combustion, Fuels, Burners, Draft, and Combustion Controls
What this domain tests
This domain checks combustion fundamentals, fuel and burner safety, draft measurement, and the controls that prove purge, pilot, main flame, and safe shutdown. Expect questions on excess air, stoichiometric air, flame safeguard timing, draft gauges, and the difference between forced, induced, natural, and balanced draft.
Core rules and concepts
- Combustion basics: fuel, oxygen, and heat are required. Theoretical or stoichiometric air is the minimum for complete combustion; excess air is added to avoid CO and unburned fuel. Too little excess air produces CO and smoke; too much lowers efficiency and raises stack loss.
- Fuels: natural gas is roughly 1,000 Btu per cubic foot and needs about 10 volumes of air per volume of gas at stoichiometric conditions. No. 2 fuel oil is near 140,000 Btu per gallon and must be atomized for clean burning.
- Burner sequence: prepurge the furnace, prove pilot or ignition, establish main flame, prove flame by scanner, and go to modulation only after stable combustion. Loss of flame must trip fuel within the flame-failure response time.
- Draft: natural draft uses a chimney; forced draft uses a supply fan; induced draft uses a downstream fan; balanced draft uses both. The furnace is normally held slightly negative to prevent hot gas leakage.
- Controls: high and low fire limits, fuel pressure switches, air proving, flame safeguard, and combustion interlock prevent unsafe light-off or firing.
Worked example
A flue-gas analyzer reads 3.0% O₂. Using the common approximation, excess air ≈ O₂ ÷ (20.9 − O₂) × 100 = 3.0 ÷ 17.9 × 100 ≈ 16.8%. That is plausible for many gas-fired boilers, but the operator still checks CO, stack temperature, and burner stability rather than tuning to oxygen alone.
Traps that cost marks
Confusing purge time with post-purge; treating a flame scanner as a substitute for fuel-train integrity; chasing low O₂ without checking CO; and assuming more draft always means better combustion.