Types of Draft Fans
                     Types of Draft Fans
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Boiler system has many types of fans such as primary air fans, gas respiration 
fans and draft fans. Draft fans are broadly classified into two main types based 
on the area of application in the industry. The type of arrangement used may 
vary based on the application need.
Forced Draft Fans
Forced draft fan is a kind of pressurized fan that gives positive energy inside a 
system. They are mostly used in industries in order to improve the thermal 
efficiency of the boiler system.
In a typical boiler system inlet and outlet dampers are utilized in order to 
maintain the pressure inside the system. A forced draft fan uses a wheel at the 
center that is hanged on the shaft along with inlet boxes.
By the use of forced draft fans, regulation of air flow in large and small industries 
can be achieved more effectively. Resistance to impact, slip and corrosion are 
some of the additional benefits of using forced draft fans. Forced draft fans are 
easier to maintain in air coolers because of the ease operation with the belt, 
fans and motors.
Advantages of Forced Draft Fans
Forced draft requires lower horsepower
Easy maintenance
Good accessibility of bundles for replacements
Can bear high inlet temperatures
Disadvantages of Forced Draft Fans
Distribution of air is not uniform
Higher chance of hot air recirculation which result from low discharge velocity 
from bundles and high intake velocity from fan rig
When the fan fails natural draft capability is less.
The parts are exposed to the sun, rain which result in poor process control.
Induced Draft fans
Induced draft fans creates negative pressure inside the boiler so that the burnt 
fuel gases exhaust out from the chimney. Induced fan sits on the path of the hot 
products of the combustion. They pull things out from the furnace. Introduction 
of induced draft fans proved the improvement of efficiency by wasting less air 
from outside. These fans no longer rely on the gravity of the exhaust gases. 
Heat exchangers are designed not to slow the products of combustion as they 
moved by convection inside the furnace. If the exhaust products are slowed 
down it makes the heat exchanger less efficient and we loose the draft up the 
chimney. Induced draft system eliminates the problem in the heat exchanger. If 
induced draft system is not used, it requires extra dilution of air to burn the 
natural gas. If induced draft fan is utilized, then there is no need for automatic 
vent damper anymore because when the fan is off, it eliminates the airflow 
through the venting system. 
Advantages of Induced Draft Fans
Better distribution of air.
The hot air is discharged upward at a higher velocity than the intake velocity.
Increased capability in fan off or fan failure situation, due to greater natural draft 
effect.
Disadvantages of Induced Draft Fans
Requires high horsepower if the air supplied is very hot.
Supplied air is limited to lower temperatures.
Fans are less accessible for maintenance.
Depending upon the need one can use either forced or induced, axial or 
centrifugal fan. It is also important to consider the direction of the air or exhaust 
gas flow while selecting the types of  draft fans.
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