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.