Boiler Combustion and Draft Systems: Stoichiometry, Burners, and Draft Balance
What this domain tests
This domain covers the combustion triangle, theoretical air and excess air calculations, flue gas analysis, burner types and turndown, and the draft systems (natural, forced, induced, balanced) that move air and products of combustion through the unit. Expect calculation items on excess air and O2/CO2 relationships alongside equipment-identification questions.
Core rules and concepts
- Combustion requires fuel, oxygen, and ignition in correct proportion and temperature. Theoretical (stoichiometric) air is the minimum for complete combustion; practical firing uses excess air to limit CO and unburned fuel.
- Typical excess air: about 15% for natural gas, 15-20% for light oil, and higher for solid fuels. Too much excess air wastes heat up the stack; too little produces CO and smoke.
- Flue gas relationship: as excess O2 rises, CO2 falls. A flue gas O2 of about 3% on a gas-fired boiler corresponds roughly to 15% excess air; 6% O2 is roughly 35-40%.
- Draft is the pressure difference that drives air in and gases out. Natural draft comes from the density difference between hot stack gas and ambient air. Forced draft uses an FD fan upstream; induced draft uses an ID fan downstream; balanced draft uses both, holding the furnace slightly negative.
- Turndown ratio is the maximum firing rate divided by the minimum stable firing rate.
Worked example
A gas-fired boiler shows 4.5% O2 in the flue gas. Estimate the excess air using the approximation EA (%) = O2 / (20.9 - O2) x 100.
Reasoning: 4.5 / (20.9 - 4.5) = 4.5 / 16.4 = 0.274, so excess air is about 27%. If the burner was designed for 15%, the excess air is high, and the likely causes are an oversized FD fan damper opening, air in-leakage on the ID side, or a mis-set O2 trim. Excess air reduction lowers stack loss and raises efficiency.
Traps that cost marks
- Reporting excess air from O2 without noting the formula assumes dry flue gas and negligible CO.
- Confusing an ID fan failure (furnace goes positive, gases leak out) with an FD fan failure (furnace goes more negative, air-starved).
- Treating CO2 as a fixed value; it changes with fuel hydrogen content and excess air.
- Ignoring that a balanced-draft furnace is held slightly negative, typically a few tenths of a kilopascal, not strongly negative.