Embed Size (px)
Citation preview
1
Fan Selection Basics
2
• Centrifugal Fan Selection• Fan Laws• Specific Speed• Multiple Fan Arrangements• Fan Features and Considerations
Fan Selection Basics4
3
Fan Selection BasicsCentrifugal Fan Selection
Image area
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
______>
4
Fan Selection BasicsCentrifugal Fan Selection
Image area
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
5
Fan Selection BasicsCentrifugal Fan Selection
Image area
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
6
Fan Selection BasicsCentrifugal Fan Selection
Image area
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
7
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 -
8
Fan Selection BasicsFan Laws - Volume
9
Fan Selection BasicsFan Laws - Pressure
10
Fan Selection BasicsFan Laws - Power
11
Fan Selection BasicsFan Laws - Summary
Fan Law Summary
12
Fan Selection BasicsFan Laws - Example
13
Fan Selection BasicsFan Laws - Example
14
Fan Selection BasicsSpecific Speed
Image area
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______>
15
Impeller Ranges
Fan Selection BasicsImpeller Styles and Specific Speed
Image area
Abridged Fan Range
0
1
2
3
4
5
6
7
8
9
0 5 10 15 20 25 30 35 40 45
Volume m3/s
Pre
ssu
re k
Pa
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
16
Fan Selection BasicsMultiple Fan Arrangements
Multiple Fan Arrangements
• Fans in Series
• Fans in Parallel
• Start Up in Series or Parrallel
17
Fans in Series
Fan Selection BasicsMultiple Fan Arrangements
Image area
18
Fans in Parallel
Fan Selection BasicsMultiple Fan Arrangements
Image area
19
Start Up for Fans in Series
Fan Selection BasisMultiple Fan Arrangements
Image area
20
Start Up for Fans in Parallel
Fan Selection BasicsMultiple Fan Arrangements
Image area
21
Fan Selection BasicsFan Features and Considerations
Image area
Fan Features and Considerations
22
Fan Selection BasicsFan Features and Considerations
Image area
Fan Features and Considerations
