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Fan Selection Basics

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• Centrifugal Fan Selection• Fan Laws• Specific Speed• Multiple Fan Arrangements• Fan Features and Considerations

Fan Selection Basics4

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Fan Selection BasicsCentrifugal Fan Selection

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Centrifugal Fan• Air Enters the Impeller Axially,

iiiThen Passes Through the iiiImpeller Radially

• The Blades Drag the Air in a iiiCircular Motion and Centrifugal iiiForces Accelerate the Airflow iiiRadially Outward

• The Airflow is Then Collected in iiiThe Volute Casing and iiiDischarged at Right Angles to iiithe Inlet Flow

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Fan Selection BasicsCentrifugal Fan Selection

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Motion of Air• The Increase of Centrifugal

iiiForce With Radius Produces a iiiPressure Gradient, Whereby the iiiStatic Pressure in the Blade iiiIncreases With Radius

• The Blades Drag the Air in a iiiCircular Motion and Centrifugal iiiForces Accelerate the Airflow iiiRadially Outward

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Fan Selection BasicsCentrifugal Fan Selection

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Motion of Impeller • The Motion of the Rotating

iiiBlades Adds Rotational iiiMomentum to the Flow, iiiIncreasing the Velocity of Flow iiiin the Tangential Direction

• As the Air Moves Radially iiiThrough the Impeller, the iiiAbsolute Velocity of the Flow iiiWill Increase, With the Absolute iiiMaximum Velocity at the iiiImpeller Outlet

• The Increase in the Absolute iiiVelocity of the Air Leads to an iiiIncrease in the Kinetic Energy iiiand Dynamic Pressure Across iiithe Impeller

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Fan Selection BasicsCentrifugal Fan Selection

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Air Flow Through The Casing • A Proportion of the Dynamic

iiiPressure From the Air Leaving iiithe Impeller is Transferred Into iiiMore Useful Static Pressure

• The Fan Casing is Usually iiiVolute Shaped, Allowing iiiDiffusion of the Air Leaving the iiiImpeller to Take Place

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Fan Selection BasicsFan Laws

“The Fan Laws”It is not practical to test every fan at every variable which may be encountered. Fortunately, by use of a series of equations, commonly referred to as the “Fan Laws” it is possible to predict with good accuracy the performance of a fan at other variables.

The variables include : Fan Size – D Fan Speed – NFan Air Density - Fan Flow Rate -

Fan Total Pressure - Fan Velocity Pressure – Fan Static Pressure – Fan Input Power – Fan Total Efficiency – Fan Static Efficiency –Compressibility Coefficient –Sound Power Level -

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Fan Selection BasicsFan Laws - Volume

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Fan Selection BasicsFan Laws - Pressure

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Fan Selection BasicsFan Laws - Power

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Fan Selection BasicsFan Laws - Summary

Fan Law Summary

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Fan Selection BasicsFan Laws - Example

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Fan Selection BasicsFan Laws - Example

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Fan Selection BasicsSpecific Speed

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Fan Specific Speed• The Ability of a Fan To Generate

iiiPressure and Flow With Impeller iiiSpeed. It is the Speed at Which a iiiFan of Indeterminate Diameter iiiWill Run at to Produce Unit Flow iiiat Unit Pressure

• Low Specific Speed Fans iiiProduce High Pressures at Low iiiFlow Volume

• High Specific Speed Fans iiiProduce Lower Pressures at High iiiFlow Volumes

• Specific Speed is Proportional to iiiImpeller Inlet Diameter and iiiImpeller Width for a Given (Fixed) iiiWheel Diameter

Low Specific Speed High Specific Speed______>

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Impeller Ranges

Fan Selection BasicsImpeller Styles and Specific Speed

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Abridged Fan Range

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3

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0 5 10 15 20 25 30 35 40 45

Volume m3/s

Pres

sure

kPa

Ns=2000

Ns=1500Ns=750

Ns=3000

Ns=5000

Ns=7500

I Metre dia. fans1910 rpm1 kg/m3

(with lines of constant specific speed plotted)

Ns = N * V 1/2 P 3/4

Radial ImpellerCurved plate blades

Radial ImpellerCurved plate blades

Radial ImpellerAirfoil blades

Axial Impellers, various hub sizes Airfoil blades

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Fan Selection BasicsMultiple Fan Arrangements

Multiple Fan Arrangements• Fans in Series

• Fans in Parallel

• Start Up in Series or Parrallel

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Fans in Series

Fan Selection BasicsMultiple Fan Arrangements

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Fans in Parallel

Fan Selection BasicsMultiple Fan Arrangements

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Start Up for Fans in Series

Fan Selection BasisMultiple Fan Arrangements

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Start Up for Fans in Parallel

Fan Selection BasicsMultiple Fan Arrangements

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Fan Selection BasicsFan Features and Considerations

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Fan Features and Considerations

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Fan Selection BasicsFan Features and Considerations

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Fan Features and Considerations