In a boiler, the draft is provided either by chimney alone (natural draft) or by fans located either after the boiler (induced draft) or before the boiler (forced draft).
Natural Draft:
Natural Draft is provided by a chimney when hot gases pass through it. Hot gases when formed are lighter and raise up the chimney creating a suction behind them, thus allowing cold air to be drawn through. A high chimney creates a stronger draft than a shorter one.
Induced Draft:
The induced draft is an additional draft created by positioning a fan between the boiler and the chimney. It assists the chimney by sucking air and the gases through the combustion equipment and discharging them into the Forced Draft.
Forced Draft:
The forced draft is also is an additional draft created by positioning the fan, which supplies air under pressure to the combustion chamber. The pressure forces air through combustion equipment. In a system, when both the fans are used, they can adjust to produce a balanced draft in a boiler.
Combined Draft:
A combined draft is produced inside the boiler when both the ID fan and FD fan are used.
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