Combustion, Fuels, and Burner Equipment
The three T's and the air/fuel ratio
Combustion needs fuel, oxygen, time, temperature, and turbulence. Theoretical (stoichiometric) air is the exact quantity for complete combustion; real firing uses excess air - roughly 10–20% for natural gas and 15–25% for fuel oil. Too little excess air produces CO, smoke, and soot; too much carries heat up the stack.
Draft arrangements are natural (chimney), forced (FD fan ahead of the burner), induced (ID fan after the boiler), and balanced (both). Flue gas analysis - O2, CO2, CO, and stack temperature - is the operator's efficiency instrument. Rising CO2 usually means less excess air, but when CO begins to climb with it, you have passed the useful limit.
Burners include atmospheric inshot, power burners, pressure-atomizing, rotary cup, air-atomizing, and combination gas/oil. Oil must be atomized, and heavier grades need heating to reach atomizing viscosity. Gas trains need correct manifold pressure, and the furnace must be purged (about four to five air changes) before every light-off.
Traps
- High CO2 is not proof of efficiency if CO is present - that is unburned fuel.
- A flame safeguard that fails to prove flame must go to lockout, never to a retry loop.
- Distinguish water-vapor dew point from acid dew point; SO3 limits how far flue gas can be cooled.
- Skipping the pre-light-off purge is a classic wrong answer.
Rule of thumb
About 3% O2 in the flue gas corresponds to roughly 15% excess air when firing natural gas.