As we know, combustion is a necessary process to generate steam in a boiler. Regardless of the type of fuel used (solid, liquid, gaseous), the presence of three elements: Fuel, Heat, and Air are undeniable for the proper combustion process. Boiler manufacturers use Mechanical Draught Fans to facilitate the air requirement in the boiler and to ensure an appropriate combustion process. Steam boiler mainly utilizes Forced Draft Fan (FD Fans) or Induced Draft Fans (ID Fans) in their operations.
Forced Draft Fan and Induced Draft Fan are both commonly termed as Centrifugal Fan or Centrifugal Boilers. The main difference between both the fans is that the Forced Draft Fan pushes air into the furnace for combustion while the Induced Draft Fan pulls the flue gases out of the steam boiler after the combustion process. Forced Draft Fans are found at the inlet of a boiler as they push high-pressure air in the combustion chamber, whereas Induced Draft Fans are located at the outlet of the boiler between dust collector and chimney.
Forced Draft Fans:
The primary function of Forced Draft Fans is to supply air for combustion in steam boilers. FD Fans create air pressure mechanically in the combustion chamber of a boiler.
One of the most preferred ways of accumulating more energy through combustion with solid fuels is through fluidized bed combustion. However, this process would be unsuccessful without a Forced Draft Fan. The occurrence of fluidization is possible solely due to the Forced Draft Fan as it supplies adequate pressurized air as per the weight of the bed and the quantity of fuel that aids in creating fluidization of the sand in the furnace of the boiler.
It is essential to calculate the pressure head as well as the quantity of air supplied for combustion.
In some cases, Secondary Air Fan fulfills the demand of excess air required for the process of complete combustion. Primary Air Fan and Secondary Air Fan both fall under Forced Draft Fan and are often installed together in a steam boiler. However, they both operate at varying flow levels and head generation.
Induced Draft Fans:
The primary function of Induced Draft Fans in a boiler is the suction or creating negative pressure to dispense the flue gases or air from the system.
Induced Draft Fans or ID Fans removes flue gases from the combustion chambers and creates a vacuum of negative air pressure with a blower to suck the air out of the combustion chamber. These air or flue gases are discharged from the boiler through a stack or chimney.
Induced Draft Fans ensures the regulation of pressure in the steam boiler and paves the way for the combustion process in the boiler by dispensing the flue gases.
Notwithstanding the many benefits of Induced Draft Fan in a boiler, including its capability of handling hot flue gases, they face a drawback of frequent corrosion and erosion in the steam boilers.
Difference between Forced Draft Fan and Induced Draft Fan:
Apart from the push and pull factors in FD Fans and ID Fans, there are some noteworthy differences in these two centrifugal fans.
- Forced Draft Fans require less power whereas, Induced Draft Fans require more.
- The pressure within flue gases is somewhat more than atmospheric pressure in FD Fans while, in ID Fans, the pressure within flue gases is a bit less.
- The flow of flue gases is more uniform in the Forced Draft Fans and less uniform in Induced Draft Fans.
- Lastly, the risk of fire in case of any kind of leakages is high with Forced Draft Fans whereas, there is no danger of fire through leakages in Induced Draft Fans.
The Collateral Relation between Boilers and Centrifugal Fans:
Although it is a well-known fact that Forced Draft Fans and Induced Draft Fans work together to ensure proper combustion and thereby the overall operation of a steam boiler, the performance and load on the FD Fans and ID Fans depend on the maintenance of the boiler. In simple words, Clinker formation in the boiler mightily affects the functioning of FD and ID Fans.
Clinkers or slag consist of the non-combustible minerals and elements found in fuel that fuse and melt together to form lumps. It either falls on the floor or sticks to the wall of the boiler. When clinkers are gathered in large amount in the pass of the flue gases, it obstructs the air passage and creates back pressure. It results in the FD and ID fans working harder to maintain the expected draught pressure. Clinkers increase the FD and ID fans consumption and reduce the heat transfer area by settling on the boiler tubes.
Boiler manufacturers have noticed that the most common reason for clinker formation in the boilers are due to the usage of poor quality fuels and neglect in maintenance of the furnace. The fuel does not burn completely when the air-to-fuel ratio in the boiler is incorrect. These unburnt particles fuse with the high-temperature zones of the boiler that result in the formation of clinkers.
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