### Fan Selection Basics

```Fan Selection Basics
1
4
Fan Selection Basics
• Centrifugal Fan Selection
• Fan Laws
• Specific Speed
• Multiple Fan Arrangements
• Fan Features and Considerations
2
Fan Selection Basics
Centrifugal Fan Selection
Centrifugal Fan
• Air Enters the Impeller Axially,
iiiThen Passes Through the
• The Blades Drag the Air in a
Image area
______
>
iiiCircular Motion and Centrifugal
iiiForces Accelerate the Airflow
• The Airflow is Then Collected in
iiiThe Volute Casing and
iiiDischarged at Right Angles to
iiithe Inlet Flow
3
Fan Selection Basics
Centrifugal Fan Selection
Motion of Air
• The Increase of Centrifugal
Image area
• The Blades Drag the Air in a
iiiCircular Motion and Centrifugal
iiiForces Accelerate the Airflow
4
Fan Selection Basics
Centrifugal Fan Selection
Motion of Impeller
• The Motion of the Rotating
iiiMomentum to the Flow,
iiiIncreasing the Velocity of Flow
iiiin the Tangential Direction
Image area
• 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
5
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
Image area
• The Fan Casing is Usually
iiiVolute Shaped, Allowing
iiiDiffusion of the Air Leaving the
iiiImpeller to Take Place
6
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 -
7
Fan Selection Basics
Fan Laws - Volume
8
Fan Selection Basics
Fan Laws - Pressure
9
Fan Selection Basics
Fan Laws - Power
10
Fan Selection Basics
Fan Laws - Summary
Fan Law Summary
11
Fan Selection Basics
Fan Laws - Example
12
Fan Selection Basics
Fan Laws - Example
13
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
Image area
Low Specific Speed
______
>
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
14
Fan Selection Basics
Impeller Styles and Specific Speed
Impeller Ranges
Abridged Fan Range
(with lines of constant specific speed plotted)
9
I Metre dia. fans
1910 rpm
1 kg/m3
8
Ns=750
Ns=1500
Ns = N * V 1/2
P 3/4
7
Ns=2000
Ns=3000
Pressure kPa
6
Image area
5
4
Ns=5000
3
2
Ns=7500
1
0
0
5
10
15
25
20
30
35
40
45
Volume m3/s
Axial Impellers, various hub sizes
15
Fan Selection Basics
Multiple Fan Arrangements
Multiple Fan Arrangements
• Fans in Series
• Fans in Parallel
• Start Up in Series or Parrallel
16
Fan Selection Basics
Multiple Fan Arrangements
Fans in Series
Image area
17
Fan Selection Basics
Multiple Fan Arrangements
Fans in Parallel
Image area
18
Fan Selection Basis
Multiple Fan Arrangements
Start Up for Fans in Series
Image area
19
Fan Selection Basics
Multiple Fan Arrangements
Start Up for Fans in
Parallel
Image area
20
Fan Selection Basics
Fan Features and Considerations
Fan Features and
Considerations
Image area
21
Fan Selection Basics
Fan Features and Considerations
Fan Features and
Considerations
Image area
22
```