The Science Behind Induced Draft Fan Design

By Author: Neel Rao
Total Articles: 113
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Induced draft (ID) fans, together with forced draft (FD) fans, are elements of paramount relevance in many plants. The presence of both these fans is essential for carrying out the combustion process. This particular article will discuss the design and engineering principles followed for making and installing ID fans.

The basic function of an induced draft fan is to “pull” out the hot flue gasses and other pollutants that are there in the combustion chamber. The fan then expels these gasses and substances into the outside environment.
An ID blower should be able to effectively suck out air from the chamber and push it out through the chimney. It has to be designed accordingly. Also, it is imperative that these blowers are made from materials like stainless steel and carbon steel. Science clearly shows that fans manufactured from only these types of materials have the ability to withstand the harsh conditions in the chamber.
Blowers used chiefly as exhaust fans have to be deployed closer to the chimney/opening through which the pollutants are released into the atmosphere. Thus, the ID fans are fixed near ...
... the boiler system’s outlet, which is in the middle of the chimney and the dust collector.
Devices that have suction as their major operation should be able to generate negative pressure. And, induced draft fans do exactly that to bring out all toxic substances from the chamber. These blowers should be able to create just a slight positive pressure near the lowermost area of the chimney. That is required to make sure that the gasses get discharged from the chimney.
The principles of engineering clearly state that when adequate ventilation is there in boiler systems, there is a rise in the rate of heat transfer. The induced draft blowers provide this ventilation; as a result, the heat level inside the chamber never goes out of control.
Most plants make use of induced draft blowers of either the backward inclined or the backward curved category. It’s indispensable that the fans come with one of these two designs. It is only then that they will be able to appropriately exercise the “pull” effect to suck out gases from the chamber.
The laws of physics unambiguously state that, along with design, the size and capacity of the fans are equally important, as well. Both the size and operational capacity of the ID blowers need to be as per the boiler system capacity. Here, installing fans of both lower or higher specifications will seriously disrupt the processes. The technical aspects of the devices should perfectly match the demands of the system.
If the processes to be handled are more harsh than what is normally seen in many plants, then special care has to be taken. Here, the regular ID blowers might be unable to do the job.
The concerned facility is better off selecting induced draft fans that have a few added accessories.
For instance, if the process involves excessive temperature fluctuations, it will be useful to choose models equipped with flex connectors. In the same way, variants having spring vibration isolators are of immense value in processes that cause excess vibrations in the fans.

The outlet and inlet ducts should have the correct size and must be in the right configuration. Any deviation, in this regard, will alarmingly lower the performance levels of the fans. And that, in turn, will tell upon the total efficiency of the process.

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