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8/13/2019 2a. Fan Selection Basics R4B
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Fan SelectionSelection by Performance
Requirements
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Fan Selection FactorsFactors to be considered in selecting a fan include:
Fan performance
characteristics
Variable airflow/pressure
How the fan is to be installed(Duct orientation, location,
mounting, etc.)
Acoustics
This part focuses on the first two areas
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Fan Performance Fan CurvesThe performance of a fan is described by its fan curve:
Describes the fans
airflow-to-resistance
behaviour
Curve of best fit from
multiple test points
Can also show power
absorbed, efficiency,
noise level etc.varying with airflow
Plotted at one speed
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Airflow m/s
Press
ure
Drop,
Pa
1 m/s = 1000 L/s
Fan urve Animated Example
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Fan urves Resistance and pressure Resistance on fan
measured as a
pressure difference
before & after fan
Pressure can be
defined as Static or
Total Pressure
Fantech Data usually
references Fan StaticPressure
VelocityPressure Curve
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System urves Duty Point Required airflow and
resulting duct losses
expressed as a duty
point Resistance on fan
estimated by
calculating the
pressure loss of the
ductwork at an airflow
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System urves Load on the Fan If duct calculations
are perfect, duty point
must be on the fan
curve for fan tooperate on the curve
The pressure loss
usually varies to 2ndpower of flowrate
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Fan Operating Points If duty point is not on
fan curve, actual
operating pointis at
intersection of fan and
system curves
Fan curve valid at one
speed
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0.5 m/s
100 Pa
0.6 m/s at 150 Pa
354D required toachieve 0.5 m at
100 Pa.
Operating Point Example
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Fan Selection Is the fan suitable?Other criteria: Extremities of fan curve
Good practice to
avoid selecting fans
near (or above!!)
peak pressuredevelopment
Frequent mistake is
to assess this against
required duty pointnot actual fan
operating point
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Fan Selection Is the fan suitable?Other criteria: Lack of resistance/pressure
Applicable to
centrifugal style fans
Increased noise
levels at bottom offan curve (near free
air)
Bottom 10%of Pressure
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Fan urves Power Consumption Fan power output function of airflow and fan total
pressure
Other factors are pitch angle, speed etc.
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Variable Duty Speed ChangesFan Characteristics Varying with Speed
100% 100% 100% 100%
66% 66%
44%
29%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Speed Airflow Pressure Power
%o
fOriginal
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Variable Duty Fan Curves High Speed and low
speed fan curves
plotted as per
previous bar graph(66% Speed, 44%
Pressure)
Low
Speed
HighSpeed
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Variable Duty Changing System If system resistance
changes differently to
fan characteristics
Common cause oftrouble are constant
pressure applications
High and low duty
points must be withinoperating window80%Pressure
Line
Operating
Window
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Fan Selection Recap Fan curve shows airflow-pressure
Fan operates where system & fan curves
match
Fan curves can show a lot of otherinformation such as power etc.
Varying airflow duties must fall within fans
performance envelope