Working of Draft Fans
                     Working of Draft Fans
Lavy Johnson
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Draft fans are extensively used in boilers to improve the flow rate efficiency of air 
and gases. The working of forced draft fan is different from the induced draft fan. 
One can use either forced draft fan or induced draft fan based on the industrial 
need. The basic working of any draft fan is to either exhaust the burnt gases or 
increase the air flow.
Working of Induced Draft Fan
 Induced draft fans are placed at the outlet of the exhaust fans.
 Induced system consists of a fan which pulls the burnt flue gases through the 
boiler and pushes them into stack or chimney.
 These fans are operated by electric motor.
 Induced draft system uses negative pressure which means, the pressure inside 
the boiler is slightly lower than the atmospheric pressure.
 Boiler casing and openings are sealed to prevent the air leaking into the boiler 
which will result in lower capacity and lower efficiency of the induced draft 
fans.
 The boiler casing is built of strong material in order to withstand the external 
pressure of the atmosphere.
 The working can be explained in brief by the flow direction of the exhaust 
gases.
 Combustion occurs inside the boiler or furnace and then the exhaust gases are 
moved over the induced draft fans where it contains series of vanes or blades 
that makes the pressure of the gases lower than the atmospheric pressure.
 Then the gases are moved in to the chimney from where it gets discharged 
outside.
Working of Forced Draft Fan
 Forced draft system are placed at inlet of the air to the furnace.
 They are run by either steam turbines or by using electric motor.
 In forced draft system fans used are commonly centrifugal fans, where the air 
flows along the radial direction of the blades or vanes.
The air which flows into the boiler must have an higher pressure than the pressure 
inside the furnace.
 So centrifugal fans are used which will increase the pressure of the air that is 
entering into the furnace.
 While using a forced draft system the whole boiler is under positive pressure.
 Every opening of the furnace must be plastered well in order to stop the 
contamination of the air from the exhaust gases or any dust particles.
 In this type of fans the air flow direction explains the working of the forced 
draft fans.
In the boiler, the induced draft fan and forced draft systems can be used with series 
blades mounted on it. The series of blades will make the pressure difference 
inside the boiler system, through which the air and the exhaust gases move.
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