Introduction To ID, FD, PA And SA Fans

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Introduction To ID, FD, PA And SA Fans
Gary Martin Writer at big-topics.com, made many
article contributions to http//big-topics.com
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Industrial fans like induced draft(ID), forced
draft (FD), primary air(PA) and secondary air(SA)
fans play a key role in providing critical
support to the various process activities and
maintaining the efficiency of the factory
operations. Draught fans are a type of industrial
fans and are typically heavy duty centrifugal
fans. Draught fans are an essential and
irreplaceable component of the industry. They are
divided in two types Forced Draught or Forced
Draft (FD Fan) and Induced Draught or Induced
Draft (ID Fan). The major difference between the
two is that the FD fan pushes the outside air
into the heating system whereas the ID fan pulls
out flue gases from the system and releases them
into the atmosphere. Both forced draft and
induced draft fan operate in such a way that it
balances the air system in the factory.
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A typical arrangement of FD fan uses inlet and
outlet dampers to maintain and control the air
pressure needed inside the system. These fans
have a wheel at the center that is hung on the
shaft along with filter, silencer, rain hood, and
inlet boxes. The most frequently used centrifugal
fans as FD fans are airfoil and backward curved
blade centrifugal fans. The FD fan builds
positive pressure as it helps push the air in the
systems. They are widely used in industries of
oven, furnaces, boilers and also in dust
collection equipment. FD fans are easier to
maintain and have cleaner operating conditions as
compared to ID fans. An induced draft fan is
usually located at the outlet between the dust
collector and the chimney. All gaseous combustion
products or flue gases are exhausted from the
machines such as boilers or furnaces and released
into the outer atmosphere.
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The ID fans withdraw the air from the systems,
therefore they produce the negative pressures in
the systems. They are more susceptible to
corrosion and erosion problems as ID fans handle
hot flue gases. The most frequently used
centrifugal fans as ID fans are radial and
backward inclined blade centrifugal fans. The
Primary Air (PA) fan and Secondary Air (SA) fan
are also the types of industrial fans. Both
primary and secondary air have a crucial role in
fuel combustion and must be provided in different
ratios according to the fuel quantity. Primary
air is the basic amount of air required for
complete combustion of fuel and it depends on the
required quantity and composition of fuel. The
fuel is fed either manually or through the screw
feeder into the furnace and primary air is
provided from the bottom through the Air box.
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A primary air fan helps in segregating the fuel
particles so that each fuel particle could come
in direct contact with the air which allows
efficient combustion of fuel. Secondary air is
supplied to compensate for the primary air that
went unreacted. Generally, it is said to be extra
air that must be supplied for complete combustion
of fuel. A secondary air fan supplies secondary
air through the nozzles installed on the walls of
the furnace. Steam boilers with external furnaces
are mostly provided with both PA and SA fans. A
forced draft fan which is designed for a
particular boiler must be able to supply a
sufficient quantity of air which is the sum of
both primary and secondary air.
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