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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