Understanding Fan Operation and
Performance
Walter Hilbish
Perry Nuclear Power Plant
FirstEnergy
1
Fan Definition
What is a “fan”? Defined by ASHRAE
“A fan is an air pump that creates a pressure
difference and causes airflow. The impeller
does work on the air, imparting to it both
static and kinetic energy, which vary in
proportion, depending on the fan type.”
2000 ASHRAE Systems and Equipment Handbook Chapter 18
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Fans
A fan is a constant volume machine. For a fixed
speed, the fan will move the same volume of gas
irregardless of gas density.
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Fans
• Most manufacturer’s
rate the fan based
upon tests conducted
using AMCA 210.
• Testing uses ideal
configurations (i.e
straight duct runs, 10
duct diameters, no
obstructions, etc.).
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AMCA Installation Types
•Four basic test
installations are defined.
•AMCA Certified Ratings
require that the test
installation be identified.
Example:
“Performance certified is for
installation Type A: Free inlet, Free
outlet. Performance ratings do not
include the effects of appurtenances
(accessories). Performance ratings
do not include the effects of cross
winds.”
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Fan Performance Tables
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Fan Curves
Unless otherwise specified the curve is
at STP .
Graphical depiction
of pressure-volume
performance of given
fan size and speed
operating at various
points from shutoff
to free delivery.
Graph may also
include power,
efficiency, system,
and stall curves.
STP - standard temperature (70° F) and pressure (29.92 in Hg) with gas
density of 0.075 lb/ft3.
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Fan Testing
8
Fan Laws
9
Fan Laws
10
Fan Laws
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Types of Fan
Two basic types of fan are:
Centrifugal - rotating impeller to increase
velocity of an airstream.
Axial - air is pressurized by the aerodynamic
lift of the fan blade.
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Centrifugal Fan Components
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Axial Fan Components
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Axial Fan Components
Vaneaxial fan
shown
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Centrifugal Fans - Blade Designs
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Centrifugal - Airfoil
• Blade curves away from direction of
rotation
• Highest efficiency
• Relatively low noise
• High structural strength
• Power reaches max near peak
efficiency, then reduces twoards free
delivery (self-limiting).
• For given duty, has highest speed of
centrifugal design
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Centrifugal - Backward Inclined
• Efficiency slightly less than
airfoil design
• Pronounced region of instability
where low efficiency and
pulsations (stall, surge, bi-stable
flow)
• Power reaches max near peak
efficiency, then reduces t0wards
free delivery (self-limiting).
• Consideration for use in
applications where airfoil blade
erosion concern
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Centrifugal - Backward Curved
• Power reduces towards free
delivery (self-limiting).
• Promote smoother flow than BI
fan
• Operate at higher speeds and
therefore require sturdier
construction than FC
• Less susceptible to flow/fan
instabilities due to less
pronounced instability region
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Centrifugal - Radial
Rugged blade design, self-cleaning
• Lowest efficiency centrifugal
• Used for material handling due to
blade strength
• Moderate to high pressure industrial
applications
• Power rises continually to free
delivery (overloading).
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Centrifugal - Radial Tip
• Blade is radial at outer edge of wheel,
curved in direction of flow as it
moves inward
• More efficient than Radial blade
• Used for mildly erosive environments
• Higher pressure than Airfoil
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Centrifugal - Forward Curved
Also called “Squirrel Cage”
• Blade curved forward in direction of
rotation
• Low pressure HVAC applications
(residential, etc.)
• Lower speed than other centrifugals
• Single thickness blade, lightweight
construction
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Axial Fans - Types
Propeller - generally operate at
< 1/2 inch water gauge,
inefficient, steeply rising power
curve (overloading).
Tubeaxial - propeller mounted
in cylindrical tube, ranges up to
~ 4 inches water gauge,
overloading.
Vaneaxial - utilizes guide
vanes to recover rotational
energy, up to 10 inches (single
stage), overloading.
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Axial Fans - Curves
Tubeaxial
Propeller
Vaneaxial
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Axial Fan - Guide Vanes
Function - reduce the
helical flow pattern of
the fan.
•Inlet -
vanes guide airflow
into a rotational pattern opposite
to fan rotation to provide
neutralize deflection by blades
and result in axial flow
•Outlet -
vanes take
rotational pattern from blades
and redirects it to an axial flow,
converting some rotational
velocity into static pressure.
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AMCA Class
Definition AMCA 99 An established
minimum
performance level for
fan aerodynamic
performance in terms
of pressure and
airflow rate.
Simply put in general terms - the higher
the Class, the greater mechanical
strength (and price).
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Gaps/Clearances
•Ensure gaps and
clearances set
per Vendor
Manual
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Gaps/ Clearances continued
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Centrifugal Fan Volume Control
Generally 3 methods:
• Variable speed – changing fan speed results in a
change in fan flow.
• Outlet damper – functions by changing system
resistance, forcing the fan to operate against
higher backpressure and reducing the flow.
• Inlet damper – combination of resistance change
and potential flow characterisitic change (spin,
reduced turbulence) at fan inlet.
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Centrifugal Fan Vortex
The inlet box damper
(positioned properly) and
both types of inlet vane
dampers pre-spin the air
in the direction of fan
rotation. By creating spin
in the direction of fan
rotation, the airflow,
pressure and horsepower
of the fan are reduced,
resulting in a new fan
performance curve.
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Recognition
Following references were used in the
generation of this presentation for technical
or pictorial support:
CML Northern Blower Fanfacts (1991)
Improving Fan System Performance: A
Sourcebook for Industry - DOE/AMCA
(2003)
Engineering Papers - Moore Fans
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Recognition (continued)
CML Northern Blower Fanfacts (1991)
Fan Handbook: Selection, Application,
and Design - Frank P. Bleier (1997)
ASHRAE Systems and Equipment
Handbook Chapter 18, Fans (2000)
AMCA Fan Application Manual,
Publications 200-98 & 201-2002
AMCA Standards Handbook Publication
99
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Recognition (continued)
Fan Engineering, 9th Edition - Howden
Buffalo
Value in the Air: Why Direct Drive
Backward Curved Plenum Fans? - AAON
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Additional Information may be found at the following websites:
•http://www.moorefans.com/
•http://www.aaon.com/literature.aspx?id=1
•http://www.chicagoblower.com/service/default.asp
•http://www.cincinnatifan.com/catalogs/EngData-203-internet.pdf
•http://www.northernblower.com/download/fanfacts.pdf
•http://www.nyb.com/eng_letters.asp
•http://www.greenheck.com/library/articles?c=2
•http://www.hartzellfan.com/lib_pdf/Bulletin%20A-108-N.pdf
•http://www.tcf.com/TCFBlower/literature.asp
•http://www.howden.com/en/Library/HowThingsWork/
•http://www.lorencook.com/PDFs/cookbook.pdf
For some history of fans, see the following:
Benjamin Franklin Sturtevant
•http://www.sturtevantfan.com/index.html
History of the Vaneaxial Fan
•http://www.jba-1.com/HistoryOfVaneaxialFan.htm
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