Yes. The draft used is Induced draft. The draft is created
by a ID fan( Induced Draft). This fan sucks the flue gas
from the boiler furnace and forces out through chimney.
Usually the fan is controlled by a VFD where we can reduce
or increase the speed of the fan by adjusting the frequency.
As the power consumption of these fans are high, it is
better advisable to go with VFD( Variable Frequency Drive).
The technical specification of these fans can be googled.
Daught is nothing but the pressure difference between
furnace and atmosphere. there is two types of daught, one is
natural circulation and second is forced circulation. in
natural circulation the flue gases passes due to natural
pressure gradient exist due to temp and density difference.
in forced circulation a mechanical device is used to create
the pressure gradient, it may be fan/blower. and the
position also decide the type. if it is placed after the
furnace then it is termed as induced draught. and if it is
placed before the furnace then it will termed as forced daught.
this daught is to make to escape the flue gases from the
systum. in boiler force daught is used.
The definations given by Mangesh and Santosh are right.
->the problem with the Forced daught boilers is that the
pressure in the Furnace is higher than the outside so when
there is leakage or if you open a inspection window the
flame will burst out.
->the problem with the Induce daught is that the pressure
is negative in the furnace. so there is a chance that
outside air can contaminate the furnace and combustion
through leakage or openings of inspection window.
--> so normaly balanced daught is used in which where both
fans (force daugth & induce daught fan) are provide one
before furnace and one after furnace.
these fans keeps the furnace pressure in balanced condition,
generally the pressure inside the furnace is kept negative
5-10 mm water column.
Draught in Boiler chimney is Induced Draught which is
generated by induced draught fan.
This Fan is mounted after Esp(Electro static precipitator)
and Before Chimney.
Is Funtion is to suck Flue Gas from Boiler , Esp & Flue gas
duct to chiney.
This draught is manual one.
While the natural draught is created by temprature
difference between the flue gas inside the chimney and
outside the chimney.
Hence the Pressure difference is generated.
hence Flue gas Tend to move Upward.
from hight to low temprature.
more over Pressure inside the chimney is more than
atmospheric pressure hence it moves outward.
draught is like a pressure due to which flow of flue gases through chimney is takes place..........in a chimney induced draft is used.
there are two types of draught.1.natural draught,2. artificial draught