Fan Selection Basics

Report
Fan Selection Basics
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Fan Selection Basics
• Centrifugal Fan Selection
• Fan Laws
• Specific Speed
• Multiple Fan Arrangements
• Fan Features and Considerations
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Fan Selection Basics
Centrifugal Fan Selection
Centrifugal Fan
• Air Enters the Impeller Axially,
iiiThen Passes Through the
iiiImpeller Radially
• The Blades Drag the Air in a
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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 Basics
Centrifugal Fan Selection
Motion of Air
• The Increase of Centrifugal
iiiForce With Radius Produces a
iiiPressure Gradient, Whereby the
iiiStatic Pressure in the Blade
iiiIncreases With Radius
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• The Blades Drag the Air in a
iiiCircular Motion and Centrifugal
iiiForces Accelerate the Airflow
iiiRadially Outward
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Fan Selection Basics
Centrifugal Fan Selection
Motion of Impeller
• The Motion of the Rotating
iiiBlades Adds Rotational
iiiMomentum to the Flow,
iiiIncreasing the Velocity of Flow
iiiin the Tangential Direction
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• 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 Basics
Centrifugal Fan Selection
Air Flow Through The
Casing
• A Proportion of the Dynamic
iiiPressure From the Air Leaving
iiithe Impeller is Transferred Into
iiiMore Useful Static Pressure
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• The Fan Casing is Usually
iiiVolute Shaped, Allowing
iiiDiffusion of the Air Leaving the
iiiImpeller to Take Place
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Fan Selection Basics
Fan 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 – N
Fan 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 Basics
Fan Laws - Volume
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Fan Selection Basics
Fan Laws - Pressure
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Fan Selection Basics
Fan Laws - Power
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Fan Selection Basics
Fan Laws - Summary
Fan Law Summary
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Fan Selection Basics
Fan Laws - Example
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Fan Selection Basics
Fan Laws - Example
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Fan Selection Basics
Specific Speed
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
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Low Specific Speed
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High Specific Speed
• 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
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Fan Selection Basics
Impeller Styles and Specific Speed
Impeller Ranges
Abridged Fan Range
(with lines of constant specific speed plotted)
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I Metre dia. fans
1910 rpm
1 kg/m3
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Ns=750
Ns=1500
Ns = N * V 1/2
P 3/4
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Ns=2000
Ns=3000
Pressure kPa
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Ns=5000
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2
Ns=7500
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0
0
5
10
15
25
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30
35
40
45
Volume m3/s
Radial Impeller
Curved plate blades
Radial Impeller
Curved plate blades
Radial Impeller
Airfoil blades
Axial Impellers, various hub sizes
Airfoil blades
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Fan Selection Basics
Multiple Fan Arrangements
Multiple Fan Arrangements
• Fans in Series
• Fans in Parallel
• Start Up in Series or Parrallel
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Fan Selection Basics
Multiple Fan Arrangements
Fans in Series
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Fan Selection Basics
Multiple Fan Arrangements
Fans in Parallel
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Fan Selection Basis
Multiple Fan Arrangements
Start Up for Fans in Series
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Fan Selection Basics
Multiple Fan Arrangements
Start Up for Fans in
Parallel
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Fan Selection Basics
Fan Features and Considerations
Fan Features and
Considerations
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Fan Selection Basics
Fan Features and Considerations
Fan Features and
Considerations
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