Working of Draft Fans

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• Working of Draft Fans

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• Draft fans are extensively used in boilers to
  improve the flow rate efficiency of air and
  gases. The working of forced draft fan is
  different from the induced draft fan. One can use
  either forced draft fan or induced draft fan
  based on the industrial need. The basic working
  of any draft fan is to either exhaust the burnt
  gases or increase the air flow.
• Working of Induced Draft Fan
• Induced draft fans are placed at the outlet of
  the exhaust fans.
• Induced system consists of a fan which pulls the
  burnt flue gases through the boiler and pushes
  them into stack or chimney.
• These fans are operated by electric motor.
• Induced draft system uses negative pressure which
  means, the pressure inside the boiler is slightly
  lower than the atmospheric pressure.
• Boiler casing and openings are sealed to prevent
  the air leaking into the boiler which will result
  in lower capacity and lower efficiency of the
  induced draft fans.

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• The boiler casing is built of strong material in
  order to withstand the external pressure of the
  atmosphere.
• The working can be explained in brief by the flow
  direction of the exhaust gases.
• Combustion occurs inside the boiler or furnace
  and then the exhaust gases are moved over the
  induced draft fans where it contains series of
  vanes or blades that makes the pressure of the
  gases lower than the atmospheric pressure.
• Then the gases are moved in to the chimney from
  where it gets discharged outside.
• Working of Forced Draft Fan
• Forced draft system are placed at inlet of the
  air to the furnace.
• They are run by either steam turbines or by using
  electric motor.
• In forced draft system fans used are commonly
  centrifugal fans, where the air flows along the
  radial direction of the blades or vanes.

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• The air which flows into the boiler must have an
  higher pressure than the pressure inside the
  furnace.
• So centrifugal fans are used which will increase
  the pressure of the air that is entering into the
  furnace.
• While using a forced draft system the whole
  boiler is under positive pressure.
• Every opening of the furnace must be plastered
  well in order to stop the contamination of the
  air from the exhaust gases or any dust particles.
• In this type of fans the air flow direction
  explains the working of the forced draft fans.
• In the boiler, the induced draft fan and forced
  draft systems can be used with series blades
  mounted on it. The series of blades will make the
  pressure difference inside the boiler system,
  through which the air and the exhaust gases move.

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