Combustion, Fuels, Burners and Draft Systems
Core concepts
Combustion requires fuel, oxygen, heat and mixing time. Learn stoichiometric (theoretical) air, excess air, air-fuel ratio, flue gas analysis, CO₂, O₂, CO and smoke number. Know the three T's: time, temperature, turbulence.
Fuels include natural gas, LPG, light and heavy fuel oil, and coal. Understand flash point, viscosity, calorific value, specific gravity, atomisation and oil preheating. Burner types include atmospheric, forced draught, pressure jet, rotary cup and dual fuel.
Draft systems: natural draft, forced draft, induced draft, balanced draft. Understand furnace pressure (slightly negative in balanced draft), draft loss, stack effect and air/fuel ratio control with O₂ trim.
Traps
- Confusing excess air with excess oxygen; both are linked but not identical.
- Believing more excess air always improves efficiency; it lowers efficiency by stack loss.
- Mixing up forced and induced draft fan locations.
- Ignoring that heavy oil needs heating to achieve correct atomisation.
Worked rule of thumb
Approximate excess air from flue O₂: excess air % ≈ O₂ % × 100/(21 − O₂ %). At 3 % O₂, excess air ≈ 300/18 ≈ 17 %.