Fan Selection Criteria
and
Efficiency
by
John Magill
The Air Movement and Control Association
International (AMCA), has met the standards
and requirements of the Registered
Continuing Education Providers Program.
Credit earned on completion of this program
will be reported to the RCEPP. A certificate of
completion will be issued to each participant.
As such, it does not include content that may
be deemed or construed to be an approval or
endorsement by NCEES or RCEPP.
Learning Objectives
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•
•
•
List available fan types
Know fan characteristics that are required
Understand tradeoffs when selecting a fan
Define fan efficiency
Outline
 Fan Types
 Basic Fan Curve
 Applications
 Performance Characteristics
 Fan Selection
 Efficiency, low noise, size, space and cost
considerations
 Mechanical considerations for a given application
including balancing and vibration levels,
construction, arrangements, ruggedness, spark
resistance, corrosion resistance, high temperature
resistance, bearings, motors, drives etc.
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Basic Fan Types
 Centrifugal
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Backward Inclined Airfoil-blade
Backward Inclined Flat-blade
Forward Curved Blade
Radial Blade
Radial Tip
 Axial
 Propeller / Panel Fan
 Tubeaxial
 Vaneaxial
 Special Designs
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Power Roof Ventilators
Tubular Inline Centrifugal
Mixed Flow
Plenum/ Plug
Centrifugal:Backward Inclined Airfoil-Blade
 Name is derived from the “airfoil” shape of blades
 Developed to provide high efficiency
 Used on large HVAC and clean air industrial systems
where energy savings are of prime importance
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Centrifugal:Backward Inclined or Curved Flat-Blade


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Backward inclined or curved blades are single thickness or “flat”
Efficiency is only slightly less than airfoil blade
Similar characteristics as airfoil blade
Same HVAC applications as airfoil blade
Also for industrial applications where airfoil blade is not
acceptable because of corrosive or erosive environment
Backward Inclined or Curved Flat & Airfoil-Blade
 High volume at moderate
pressure
 Non-overloading power
characteristic
 Stable performance
characteristic
 Low noise
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Centrifugal: Forward Curved Blade
 Blades are curved forward in the
direction of rotation
 Must be properly applied to avoid
unstable operation
 Less efficient than Airfoil and
Backward Inclined
 Requires the lowest speed of any
centrifugal to move a given amount of
air
 Used for low pressure HVAC systems
 Clean air and high temperature
applications
 Typically smallest size selection
 Rising power overloading
characteristic
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Centrifugal: Radial Blade
 The blades are ‘radial’ to the fan shaft
 Generally the least efficient of the
centrifugal fans
 For material handling and moderate to
high pressure industrial applications,
rugged construction
 Low volume at high pressure
 Large wheel diameter for a given volumehigher cost
 Material handling, self cleaning
 Easy to maintain
 Rising Power overloading characteristic
 Suitable for dirty airstream, high pressure,
high temperature and corrosive
applications
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Centrifugal:
Radial Tip
The blades are radial to the fan shaft at the
outer extremity of the impeller, but gradually
slope towards the direction of wheel rotation
More efficient than the radial blade but less
than backward inclined
Offers wear resistance in mildly erosive air
streams
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Axial: Propeller or Panel Fan
 One of the most basic fan designs
 For low pressure, high volume
applications
 Often used for ventilation through a
wall
 Available in square panel or round
ring fan
 Maximum efficiency is reached near
free delivery
 Reversible blade for reversible flow
applications like jet tunnel fans
 Many axial fans can overload at
shutoff
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Tubeaxial Fan
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
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
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More efficient than the panel fan
Cylindrical housing fits closely to outside diameter of blade tips
For low to medium pressure ducted HVAC systems
Also used in some low pressure industrial applications
Performance curve sometimes includes a dip to the left of peak
pressure which should be avoided
Vaneaxial Fan
 Highest efficiency axial fan
 Cylindrical housing fits closely to outside diameter of blade tips
 The straightening vanes allow for greater efficiency and pressure
capabilities
 For medium to high pressure HVAC systems. More compact than
centrifugal fans of same duty
 Aerodynamic stall causes the performance curve to dip to the left of
peak pressure which should be avoided. However anti-stall options
available for both unidirectional and reversible axials
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Power Roof Ventilators
 A variety of backward inclined centrifugal wheels or axial
impeller designs
 Also available in upblast damper design to discharge air away
from the building
 For low pressure exhaust systems of all building types (roof
mounted)
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Inline Centrifugal Fan
 Cylindrical housing is similar to a vaneaxial fan
 Wheel is generally an airfoil or backward inclined type
 Housing does not fit close to outer diameter of wheel
 For low and medium pressure HVAC systems or industrial
applications when an inline housing is geometrically more
convenient than a centrifugal configuration
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Mixed Flow Fan
 Specific Speed between a centrifugal and axial fan
 Cylindrical housing is similar to a vaneaxial fan
 High volume advantages of axial fans
 Low sound, high efficiency advantages of tubular
centrifugal fans
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PLENUM / PLUG FAN
This is basically a centrifugal
wheel and inlet in a frame
without a scroll or housing. The
‘housing’ is the AHU box.
Offers tremendous flexibility for
inlet and discharge in a AHU
application
More efficient than a scroll
centrifugal for high flows and low
SP. All SP rise occurs in the blade
passage
Wall clearance rules must be
followed to avoid significant
system effect losses
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Housed vs plenum fan
SO YOU HAVE ALL THESE CHOICES
OF FANS TYPES AVAILABLE…WHAT
SHOULD YOU DO TO PICK THE
RIGHT FAN FOR YOUR
APPLICATION?
Let’s consider a couple of examples to illustrate the
selection process from an efficiency, sound, cost and
available space perspective
All Air tests based on AMCA std 210, and Sound tests
based on AMCA std 300
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All fans selected at peak SE (Static Efficiency) for
Airflow=10,000 cfm, Static Pressure (SP)~2 iwc
Type
Dia (in)
Spd (rpm)
BHP
SE %
LwiA
(Static
Efficiency)
(Inlet
Sound
Power ‘A’)
1
Forward
Curved- SW
(Centrifugal)
30
476
5.09
61.7
89
2
Backward
Airfoil – SW
(Centrifugal)
36.5
650
3.82
80.0
77
3
4
Plenum
33
800
4.25
74.0
80
Tubular
Mixed Flow
27
1074
4.48
70.2
81
5
Tubular Vane
Axial
28
1438
4.77
65.9
86
6
Propeller
(Axial)
30
1998
4.92
54.4
103
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Narrowing in after main Fan Type Selection..........
FT-1
FT-1
LESS
EFFICIENT
LESS COST
MORE
NOISY
In general, for all fan types, as first cost goes down, operating costs (BHP)
and noise go up…trade off!
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Tone at Blade Pass Frequency (Blade Tone)
 Blade Pass Frequency, bpf= #blades * rpm / 60
 Sound Power level, Lw, at bpf is a distinct audible tone. This
aerodynamic tone can be very annoying and is usually the worst
for radial bladed fans, followed by plenums and housed
centrifugals.
 Axial fans have a high pitched tone which is not as annoying.
 The bpf tone is a spike in Lw over the surrounding broadband
noise spectra.
 Blade Tone Prominence is defined as the dominant energy level
of the blade tone integrated over a narrowband region of the
sound spectrum surrounding the blade tone.
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FT-2
FT-2
Blade Tone
prominence
Acoustic Engineers do not like blade tone prominence to exceed 6dB in
addition to low Sound Power Levels (Lw)
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Fan Selection based on Specific
Speed
Dimensional Specific Speed, is the fan
speed required to raise the SP by 1 iwc
with 1 cfm airflow.
Ns = N * (Q)^0.5/(SP)^0.75
Where, N = Speed (rpm)
Q = Airflow (cfm)
SP = Static pressure (iwc)
Density = 0.075 lbm/cu ft
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All fans selected at peak SE (Static Efficiency) for Specific
Speed, Ns
Type
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Specific
Speed, Ns
Max Static
Efficiency (SE%)
1
Forward Curved-SW
(Centrifugal)
26,300
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2
Backward Airfoil-SW
(Centrifugal)
40,000
80
3
Plenum
50,000
75
4
Tubular Mixed Flow
65,800
70
5
Tubular Vane Axial
90,000
65
6
Propeller (Axial)
126,000
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Summary
 Fan selection is not a trivial process for a given application.
 Example shown applies to one design operating point. The
selections will change for other operating points.
 There is no magic fan that will result in least cost, best efficiency
and low noise for a wide range of operating points.
 Compromises should be well understood upfront.
 Direct Drive (DD) selection speeds may further limit selections.
Varying width options can optimize DD selections.
 Mechanical design requirements like balancing and vibration
levels, spark and high temp resistance, corrosion resistance,
arrangements, motors, bearings, drives can further challenge
the selection process.
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