1 Fan Selection Basics
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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
0
1
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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
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