The Science Behind Induced Draft Fan Design

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The Science Behind Induced Draft Fan Design
Neel Rao Writes on Induced draft fans, Forced
draft fan
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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 systems outlet, which is in the middle of
  the chimney and the dust collector.

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• 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. Its 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.

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• 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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