Boilers Chimney


A steam boiler assembly, be it any fuel (solid/liquid/gaseous) used for combustion, the flue gases coming out of the Induced draft fan, coming from air preheaters and dust collectors, enters the chimney to conclude this process before getting released to the atmosphere.

Self-supported type Chimneys can be provided suiting the boiler size. It can be designed and manufactured as per IS-6533 standard to ensure compliance.


The chimney’s working is based on the natural draft or stack effect. As we know, the density of a gas reduces, with rise in its temperature, so because of this reason when the flue gases enter the chimney, the buoyancy forces of air come in to action and helps in carrying the flue gases out of the chimney. In case of a Boiler assembly employing an Induced Draft Fan, the flue gases are brought to the chimney by this fan and then buoyancy forces deliver them to the atmosphere. These buoyancy forces depend upon the weight of the air displaced by the flue gases in the chimney. Greater the weight, greater are the buoyancy forces present. And this weight can be increased by increasing the height of the chimney. Back in time, when Lancashire Boilers were in fashion, induced draft fans were not used. The reason was the extremely high temperature of flue gases at the chimney inlet, this high temp. of flue gases reduced the density of flue gases to a great extent. Now they were even lighter and the buoyancy forces produced were of greater magnitude. This removal of flue gases in the chimney, created a vacuum at the chimney inlet. So, because of this vacuum (pressure difference between boiler furnace and chimney inlet), the flue gases traveled all the way from the boiler to the chimney, eliminating the need of an Induced Draft Fan.

Buoyancy forces produced, are given by relation as given below:
Fb = (Pa  – Pf )xgxh

Fb = Buoyancy force (N)
Pa = Density of Ambient Air (kg/m3)
Pf   = Density of Flue gases at chimney inlet temperature (kg/m3)
g = Acceleration due to gravity (m/s2)
h = Height of chimney (m)
We, at thermodyne have successfully designed, manufactured, supplied and erected both structure and self-supported Chimneys in compliance with IS-6533 (part-1&2) to many of our clients.