### Fan Velocity Pressure

```Fan & Systems
Chris Carr PE
H. Clay Moore & Associates
Rod Furniss
Howden North America
Key Account Manager - Nuclear
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Aerodynamics
• Pressure & Flow Definitions and Measurement
• Development of Fan Curves
• Performance of Various Blade Shapes
• Fan Controls
• System Effects
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HNA Profile
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Aerodynamics
What is a Fan?
• Any device for producing a current of air by the movement
of a broad surface or a number of such surfaces.
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Aerodynamics
Pressure & Flow Definitions and Measurement
Fan Flow Rate
• Often called inlet volume
• Measured at the fan inlet, by convention
• Measured in cubic feet per minute (cfm)
• May be converted to scfm (standard cubic feet per minute), or mass flow (lb/hr)
• Fan manufacturers commonly use acfm (actual cubic feet per minute)
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Aerodynamics
Pressure & Flow Definitions and Measurement
Pressure
• Force per Unit Area
• Commonly measured in psi or inwg
Absolute Pressure
• Pressure when the datum is absolute zero
• iiAlways positive
Barometric Pressure
• Absolute pressure exerted by the atmosphere
• Always positive
Gauge Pressure
•
•
Pressure when the datum is barometric pressure
May be positive or negative
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Aerodynamics
Pressure & Flow Definitions and Measurement
Static Pressure
•
•
Pressure due to degree of compression and density only
May be positive or negative
Velocity Pressure
• Pressure due to rate of motion and density only
• Always positive
Total Pressure
•
•
Algebraic sum of static and velocity pressures at a point
May be positive or negative
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Aerodynamics
Pressure & Flow Definitions and Measurement
Fan Total Pressure
• Difference in Total Pressure between the fan outlet and fan inlet
• FTP = TPo - TPi
Fan Velocity Pressure
• Pressure corresponding to the average velocity at the fan outlet
• FVP = VPo
Fan Static Pressure
• Fan Total Pressure minus Fan Velocity Pressure
• FSP = SPo – TPi
Static Pressure Rise
• Difference in Static Pressure between the fan outlet and fan inlet
• SPR = SPo - SPi
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Aerodynamics
Pressure & Flow Definitions and Measurement
Fan Pressure Terms
• For a Given Fan, SPR is
iiiAlways Greater Than FSP
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• If the Specified Pressure is
iiiAssumed to Be FSP, the Fan
iiiSelected Will Be Smaller
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Aerodynamics
Pressure & Flow Definitions and Measurement
Pitot – Static Tube
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Aerodynamics
Pressure & Flow Definitions and Measurement
Fan Laboratory Test Setup
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Aerodynamics
Development of Fan Curves
Fan Curve
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Aerodynamics
Development of Fan Curves
System Curve
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Aerodynamics
Development of Fan Curves
P-V and Peak Fan
Pressure
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Aerodynamics
Development of Fan Curves
Selection Range
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Aerodynamics
• Airfoil
• Backward Curved / Backward Inclined
• Forward Curved
• Axial
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Aerodynamics
Airfoil
• Highest Efficiency Centrifugal
DDesign
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•Low Noise
•Usually for Clean Applications
•Largest Diameter Centrifugal
•Highest Cost Centrifugal
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Aerodynamics
Backward Curved /
Backward Inclined
• Essentially a Single
TThickness Airfoil
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• Efficiency slightly less than
AAirfoil
• Low Noise
• For Clean or Dirty
AApplications
• Lower Cost than Airfoil
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Aerodynamics
• Lower Efficiency
• Continuously Rising HP ..
CCharacteristic
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• Higher Noise Levels
• May have Predominant BFP
TTone
• Smaller Diameter
• Easily Armored for Erosive
SService
• Lower Cost
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Aerodynamics
Forward Curved (Sirocco )
TM
• Lower Efficiency
• Continuously Rising HP
CCharacteristic
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• Smallest Diameter Centrifugal
• Suited for High Volume/Low
PPressure
• Distinctive Dip in P-V Curve
• Seldom Used Today
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Aerodynamics
Axial
• Airflow Parallel to Axis of
RRotation
• High Efficiency
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• Low Noise
• Suited for High Volume/Low
PPressure
• Distinctive Dip in P-V Curve
• Controllable Pitch has Higher
CCost
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Aerodynamics
Fan Controls
Fan Control
• Reasons for Providing Fan Control
• Methods of Control
• Efficiency Comparison
• Mechanical Arrangements
• Control Criteria
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Aerodynamics
Fan Controls
Reasons for Providing Fan Control
• Uncertainty in System Calculations
• Safety Factor or Margin
• Expected Changes in the System with Time
• Normal Process Variations
• Provisions for Extraordinary Events
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Aerodynamics
Fan Controls
System Pressure Loss
and Fan Design Pressure
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Aerodynamics
Fan Controls
Moving the Operating
Point
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Aerodynamics
Fan Controls
System Duty Cycle
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Aerodynamics
Fan Controls
Methods of Control
• None
• Outlet / System Damper
• Inlet Box Dampers
• Variable Inlet Vanes (VIV)
• Variable Speed
• Combinations of the Above
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Aerodynamics
Fan Controls
Outlet Damper / System
Damper
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Aerodynamics
Discharge Damper
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Aerodynamics
Fan Controls
Inlet Box Dampers
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Aerodynamics
Fan Controls
Inlet Box Dampers
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Aerodynamics
Fan Controls
Inlet Box Dampers
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2
FSP - 0% IBD CLOSURE
HP - 0% IBD CLOSURE
FSP - 25% IBD CLOSURE
1
HP - 25% IBD CLOSURE
FSP - 50% IBD CLOSURE
0
0
100
200
300
400
CFM
500
600
700
800
HP - 50% IBD CLOSURE
FSP - 75% IBD CLOSURE
HP - 75% IBD CLOSURE
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IBD
Discharge
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IBD
Discharge
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Aerodynamics
Fan Controls
Variable Inlet Vanes
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Aerodynamics
Fan Controls
Physical Arrangement of
Cantilevered VIV
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Aerodynamics
Fan Controls
Physical Arrangement of
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Aerodynamics
Fan Controls
VIV Fabrication
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Aerodynamics
Fan Controls
Variable Inlet Vanes
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6
5
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FSP - 0%VIV CLOSURE
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HP - 0% VIV CLOSURE
FSP - 25% VIV CLOSURE
HP - 25% VIV CLOSURE
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FSP - 50% VIV CLOSURE
HP - 50% VIV CLOSURE
1
FSP - 75% VIV CLOSURE
HP - 75% VIV CLOSURE
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0
100
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800
CFM
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Aerodynamics
Fan Controls
Variable Speed
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6
5
4
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2
FSP - 1180 RPM
1
HP - 1180 RPM
FSP - 880 RPM
HP - 880 RPM
0
0
100
200
300
400
500
600
700
800
CFM
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Aerodynamics
Fan Controls
Variable Speed- VFD
and Gỳrol Fluid Drive
Rotating
Stall
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Aerodynamics
Fan Controls
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6
5
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3
2
1
0
0
100
200
300
400
500
600
700
800
CFM
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Aerodynamics
Fan Controls
Efficiency Comparison
120
100
% DESIGN HP
80
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60
40
OUTLET DAMPER
INLET BOX DAMPER
VARIABLE INLET VANE
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VARIABLE PITCH
VARIABLE SPEED
0
30
40
50
60
70
80
90
100
% DESIGN FLOW
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Aerodynamics
Fan Controls
Physical Arrangement of
Various Control Devices
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Aerodynamics
Fan Controls
Physical Arrangement of
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Aerodynamics
Fan Controls
Control Criteria
• Turndown / Leakage
• Sensitivity to Change / Stability / Transients
• Accuracy Required
• Repeatability
• Expected Efficiency
• Acoustic Considerations
• Structural Considerations
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System Effects – Poor Flow
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System Effects – Poor Flow
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System Effects – Poor Flow
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System Effects – Poor Flow
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Fan & Systems
Chris Carr PE
H. Clay Moore & Associates
Rod Furniss
Howden North America
Key Account Manager - Nuclear
52
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