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Forward Curved Motorised Impeller
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When we have defined the volume flow rate that we require, whether this is to provide
fresh air or process cooling, we need to combine this with the resistance to flow that the <a
href="https://www.longwellfans.com/" target="_self">fan </a>will encounter in the
application. The volume flow rate, (in m3/hr) and the pressure (in Pascals - Pa), are
combined to become the duty point against which the fan must operate. It is important that we
select a fan whose performance characteristic meets the required duty point on or near the
point of peak efficiency. Using the fan at its peak efficiency minimises the power
consumption and noise emitted from the fan whilst delivering the required performance.
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How does Forward Curved Centrifugal Fan work?
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The name, ‘Centrifugal Fan’ is derived from the direction of flow and how the air
enters the impeller in an axial direction and then propelled outwards from the outer
circumference of the fan. The difference in flow direction between a forward and backward
curved centrifugal fan is the direction that the air exits the impeller circumference. With a
backward curved impeller, the air exits in a radial direction whereas with a forward curved
the air exits tangentially from the circumference of the fan.
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A forward curved centrifugal fan is characterised by its cylindrical shape and lots of
small blades on the circumference of the impeller. In the example shown below, the fan
rotates in a clockwise direction.
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Unlike the backward curved impeller, the forward curved impeller requires a housing that
converts high velocity air leaving the tips of the impeller blade into a lower velocity
static force. The shape of the housing also directs the air flow to the outlet. This type of
fan housing is commonly known as a scroll; however, it can also be referred to as a volute or
a sirocco housing. By installing the forward curved impeller in a scroll housing, we usually
refer to it as a forward curved blower.
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There are two types of blowers that employ a forward curved motorised impeller as shown
below…
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The single inlet blower on the left, draws in air from one side of the housing through
the round inlet and directs it to the square outlet, (seen here with a mounting flange). The
double inlet blower has a wider scroll housing drawing air in from both sides of the scroll
delivering it to the wider square outlet.
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As with the backward curved centrifugal fan, the suction side of the impeller blade draws
air from the centre of the <a href="http://www.longwellfans.com/cross-flow-fan/"
target="_self">cross flow fan</a> which results in a directional change of the airflow
between the inlet and the exhaust of 90o.
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Fan Characteristic
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The optimum operating area for a forward curved centrifugal fan is when it is operating
at higher pressure. A <a href="http://www.longwellfans.com/forawrd-curved-centrifugal-fan/"
target="_self">forward curved centrifugal fan</a> works best when high pressures against
lower volume flows are required. The graph below illustrates the optimum working area…
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The volume flow is plotted along the X-axis and the system pressure is plotted on the Y-
axis. When there is no pressure in the system, (the fan is blowing freely), a forward or <a
href="http://www.longwellfans.com/backward-curve-centrifugal-fan/" target="_self">backward
curved centrifugal fan</a> will produce the greatest volume flow. As a resistance to flow is
applied to the suction or exhaust side of the fan, the volume flow rate will drop.
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Caution should be exercised when selecting a forward curved blower to operate at low
pressures and highest volume flow. At this point, the impeller is operating in an aerodynamic
stall in the same manner as an<a href="http://www.longwellfans.com/axial-flow-fan/"
target="_self"> axial flow fan </a>operating in the saddle point of its curve. At this point
noise and power consumption will be at its peak due to turbulence.
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