- Artificial Draught Definition: Artificial draught is a method using steam jets or forced air to create pressure differences, reducing the need for tall chimneys.
- Induced Draught: Utilizes a fan to suck flue gases from the furnace, creating a pressure difference that draws fresh air into the system.
- Forced Draught: Uses a blower fan to push air through the furnace, creating positive pressure and efficiently driving flue gases towards the chimney.
- Balanced Draught: Combines forced and induced draught methods, using fans at both the furnace entry and exit to optimize airflow and combustion.
- Efficiency and Cost-Effectiveness: Artificial draught systems enhance efficiency and reduce costs by optimizing heat extraction and minimizing the need for tall chimneys.
The pressure difference from a chimney includes chimney height. Enough natural draught therefore needs a taller stack, which is not always practical. In those cases the plant uses artificial draught instead of stack height alone. Artificial draught comes from fans, blowers or a steam jet. Steam-jet draught is an older method. Fan draught is the usual method on modern plant. With fans in the air or flue-gas path, the chimney can be shorter and still send flue gases to the atmosphere.
Induced Draught
An induced draught fan sits in the flue-gas path, often at the chimney base or after the last heat-recovery surface, and draws gases from the furnace. Furnace pressure then sits a little below atmosphere, so air enters through the grate, burners or leakage paths. Heat can be taken from the gases in an economiser or air heater because the fan, not stack temperature, keeps the flow moving. Cooler gas still has volume and the fan still has to overcome duct and filter resistance. The chimney can be shorter than a natural-draught stack sized for the same boiler.
Forced Draught
Forced draught uses a blower on the air inlet, often ahead of a grate or burner windbox. The fan raises air pressure so the furnace sits above atmosphere and gases are pushed toward the chimney. Air may later pass through an air heater. The fan itself does not heat the air. Because the furnace is under positive pressure, leakage goes outward. Casing joints and inspection doors need to stay tight or hot gas and dust escape into the boiler house.
Balanced Draught
Balanced draught uses both a forced-draught fan on the air side and an induced-draught fan on the gas side. The forced-draught fan supplies combustion air. The induced-draught fan takes the gases away. Operators set the two fans so furnace pressure stays close to atmosphere, often slightly negative. That reduces outward leakage compared with forced draught alone and still lets the plant recover heat without relying on a tall chimney.
