Draft Control and Reading the Stack
Draft is the pressure difference that pulls air into the furnace and pushes flue gas out through the chimney. Too little and the fire starves and smokes; too much and cold air rushes through, carrying heat away. The chimney itself is the last instrument an attendant reads.
You'll learn
- Explain natural, forced, induced and balanced draft
- Calculate the natural draft a chimney produces
- Set and monitor furnace draft
- Read smoke and stack conditions
Before this: Air–fuel ratio and excess air, Flue gas analysis
Moving air and gas through a boiler
- Natural draft: a tall chimney full of hot, light gas draws air through the boiler on its own. Common on small, older plants.
- Forced draft: a fan (FD fan) pushes combustion air into the furnace, which runs slightly above atmospheric pressure.
- Induced draft: a fan (ID fan) after the boiler sucks flue gas out, keeping the furnace below atmospheric pressure.
- Balanced draft: both fans together, controlled so the furnace sits just below atmospheric pressure. Usual for solid-fuel boilers.
A slightly negative furnace means that any leak lets cold air in rather than letting flame and gas out into the boiler house. That is safer, but too much negative pressure draws in air that wastes heat.
How much draft a chimney gives
| Symbol | Meaning | Unit |
|---|---|---|
| , | Density of outside air and of flue gas in the chimney | kg/m³ |
| Gravity, 9.81 | m/s² | |
| Chimney height above the boiler outlet | m |
- Outside air at 30 °C, flue gas at 180 °C, atmospheric pressure; flue gas treated as air for density.
- Densities from the gas law, ρ = p / RT: air 1.164 kg/m³, flue gas 0.779 kg/m³.
- Draft:
Furnace draft is measured with a draft gauge in mm of water column (mmWC), and held by an ID fan damper or speed control at a small negative value set by the boiler maker. A sudden change in draft at steady firing points to a blocked flue, a fouled economizer, a failing fan or a door left open.
Reading the stack
| Appearance | Usual meaning |
|---|---|
| Clear or faint haze | Good combustion |
| Black smoke | Incomplete combustion: too little air, poor mixing, overloaded grate or poor atomization |
| White plume that disperses quickly | Water vapor condensing, normal with wet fuel and in cold or humid weather |
| White, persistent plume that does not disperse | Possible sulfur trioxide or fine particulate; check combustion and dust collector |
| Brown or yellowish plume | Possible high NOx, or burning of certain fuels |
Stack temperature, read alongside O₂, says how much heat is leaving. Rising stack temperature at steady load means fouled surfaces; falling stack temperature with high O₂ can mean air leakage diluting and cooling the gas. A sudden change after sootblowing or cleaning confirms where the fouling was.
Key takeaways
- Natural, forced, induced and balanced draft; solid-fuel boilers usually run balanced, slightly negative.
- Chimney draft = (ρ_air − ρ_gas) × g × H: modest, which is why modern plants need fans.
- A 30 m chimney at 180 °C gas and 30 °C air gives about 11.6 mmWC.
- Black smoke means incomplete combustion; fix air at the fire, not with more chimney draft.