AMCA International Technical
Seminar 2009
PLENUM FANS
Terminology, History, Testing,
Application and Trends
Presented by:
John Cermak, Acme Engineering &
Manufacturing Corp.
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
•
•
•
•
•
•
Understand terminology and classification
Learn plenum fan characteristic features
Understand air and sound performance
testing
Understand how to apply the test data
Understand principle advantages and
limitations in application
Learn the trends in development of
applications
TERMINOLOGY
• Plenum fans are a part of the group of fans
called centrifugal fans
• All centrifugal fans have centrifugal
impellers but some subgroups have
housings, some have not
• The centerpiece of centrifugal fans are fans
with scroll housing and they are called
centrifugal fans
AMCA CERTIFIED RATING
PROGRAM
recognizes as centrifugal fans also:
• In-line centrifugal fans
• Plug fans
• Plenum fans
SW CENTRIFUGAL
IMPELLERS
• With Airfoil Blades
SW CENTRIFUGAL
IMPELLERS
• With single thickness backward curved blades
IN-LINE CENTRIFUGAL FAN
• With cylindrical housing
IN-LINE CENTRIFUGAL FAN
• With box housing
PLUG FAN
• Plug fan may or may have not scroll housing
(here without housing)
PLENUM FAN
• Horizontal Arr.3, Flanged Inlet
PLENUM FAN
• Horizontal Arr3. With Upper Frame
ARRANGEMENT 1
Can include
vibration
islolators
ARRANGEMENT 3
Can include
vibration
islolators
ARRANGEMENT 4
• Arrangement 4 (direct drive) is not
specifically mentioned in the CRP but
the CRP8 form has a provision for this
case
HISTORY
• Centrifugal fan with scroll housing inside
of a cabinet presented a temptation for
cost reduction
• Scroll housing was relatively expensive,
cost reduction substantial
• Doubts about a good air performance of
plenum fans led to a slow pace in their
applications
HISTORY
• Usage of wide centrifugal impellers - reduction
in fan size and low fan efficiency – this
consequence was more devastating for plenum
fans than for centrifugal fan with scroll housing
• Impellers for plenum fans are mostly identical to
those used in centrifugal fans with scroll
housings yet in plenum fans there is no
housing and the air leaves the fan at
substantially higher velocity
HISTORY
• Plenum fans opened the opportunity for
application of centrifugal impellers with
backward curved blades in air
handling equipment - higher efficiency
and better sound performance
• In the last few years the fan users pay
more and more attention to low sound
INSTALLATION TYPE A
Free Inlet/Free Outlet
INSTALLATION TYPE C
Ducted Inlet/Free Outlet
AIR PERFORMANCE TESTING
Typical setup with plenum fan outside of the test
chamber
TESTED
FAN
AUXILIARY
FAN
AIR TEST
CHAMBER
FLOW
SOUND TESTING
Testing of outlet sound
SOUND TEST
CHAMBER
SILENCERS
FLOW
TESTED
FAN
SOUND TESTING
Testing of inlet sound
SOUND TEST
CHAMBER
TESTED
FAN
SILENCERS
FLOW
APPLICATION
• Important condition for successful application
of plenum fans is the knowledge of:
•
Test configuration
•
Test uncertainties and tolerances
•
Application uncertainties
• O.E.M applications are generally safer since
the equipment is tested as a part of R&D
process
APPLICATION OF TEST
DATA
The air performance and sound data based
on tests can be applied to the fans only if the
fan/ducts configuration of tests and in the
application are the same
APPLICATION OF TEST
DATA
• Free or Ducted Inlet do not create any controversy,
Free Outlet does
• During the test the fan discharges air into a room,
usually large, but smaller than the free space. The
shape and the minimum size of that room are not
specified
• Plenum fans are mostly used inside of a cabinet
which size is much smaller than the room used for
testing
• Incomplete and unreliable application correction data
are used
AIR PERFORMANCE
Centrifugal vs. Plenum Fan (same size and speed)
5
Centrifugal Fan
Static Pressure (in.wg)
Plenum Fan
4
Stall
3
2
1
0
0
2000
4000
6000
8000
10000
Flow (cfm)
12000
14000
16000
AIR PERFORMANCE
Centrifugal vs. Plenum Fan (same size and speed)
100
Centrifugal Fan
Max Efficiency
90
Plenum Fan
Static Efficiency (%)
80
BHP Up By 15%
70
60
50
40
30
100%
55%
20
70%
10
0
0
2000
4000
6000
8000
10000
Flow (cfm)
12000
14000
16000
AIR PERFORMANCE
Centrifugal vs. Plenum Fan (same size and speed)
5
Centrifugal Fan
Static Pressure (in.wg)
Plenum Fan
4
Max Efficiency
Stall
3
BHP Up By 15%
2
1
0
0
2000
4000
6000
8000
10000
Flow (cfm)
12000
14000
16000
IMPELLER
DISCHARGE VELOCITY
Absolute Velocity
Radial
Tangential
CCW
Rotation
FAN DISCHARGE VELOCITY
Centrifugal vs. Plenum Fan (same size and speed)
Best Efficiency
Discharge
Velocity (fpm)
4000
3500
3000
2500
2000
20
00 0
0
30 000 0
4
0
50 000 0
6
0
70 000
8
00
Flow (cfm)
90 000
10 1000 0
1
00
12
1500
1000
500
0
Ple
Ho
nu
us
m
ed
Fa
n
Fa
n
PLENUM AS MIXING DEVICE
• Well designed plenum and the
position of the plenum fan inside the
plenum can enhance the fan
performance
• Mixing process of the flow from the
wheel and the air inside of the plenum
improves usage of filters and or coils
located downstream of the fan
PRINCIPLE APPLICATION
• Plenum fans are mostly, but not exclusively
used in air handling units
• Plenum fans are short in length compared
to vane-axial or centrifugal fans
• On the down-stream side of the fan the
plenum fans allow length saving installation
of the equipment requiring good air velocity
distribution across their face area
IMPACT OF MIXING
INLET vs. DISCHARGE SOUND
SWL (dB)
92
90
88
86
7 dB
84
82
80
78
76
3
Octave Bands
4
5
6
7
8
Outlet
2
Inlet
1
SOUND vs. PERFORMANCE
SWL (dB)
97
96
95
94
93
92
91
90
89
14000
13640
12000
10000
88
1
2
3
4
Octave Bands
5
6
8000
7
8
Flow (cfm)
SPECIFIC SWL
Specific SWL
Lowest Sound
Highest Efficiency
55.0
50.0
45.0
40.0
35.0
Free Deliv
Low
Mid
Mid
Best SE
Position on
Top
Pressure Curve
30.0
1
2
3
4
Octave Bands
5
6
7
8
PLENUM FAN APPLICATION
5
Static Pressure (in.wg)
Stall
Max Efficiency
Low Sound
4
3
Good For Application
BHP Up By 15%
2
100%
50%
1
70%
0
0
2000
4000
6000
8000
10000
Flow (cfm)
12000
14000
16000
CLASSES FOR PLENUM FANS
• AMCA Class Classification for plenum
fans does not exist
• Industry uses the same impellers for
centrifugal fans with the scroll housing
as for plenum fans and applies the
same speed ranges for both product
lines
CLASSES FOR PLENUM FANS
• Outlet velocity of a centrifugal fan with
the scroll housing is quite different from
the plenum fan of the same size at the
same speed and at the same flow
TRENDS
The newest trend is going towards:
• Direct drives (Arrangment 4)
• Usage of variable frequency drives
• Applications with multiple plenum fans
sometimes called fan walls
DIRECT DRIVE FANS
VFD – Positive Side
•
•
•
•
•
Offers air performance flexibility
Changes the flow as required
Allows for easy monitoring of fan operation
Eliminates cost and maintenance of drives
Offers compact and rigid design
VFD – Negative Side
• Requires good electrical grounding of
motor or motor bearing protection against
arcing
• May require non-stock motor
• May not deliver expected power savings
• Can be a unpleasant source of sound
• Can be an unwanted source of electromagnetic radiation
PLENUM FAN WALL
PLENUM FAN WALLS
Positive Side
•
•
•
•
•
Reduces length of air-handler
Eliminates drives (sheaves and belts)
Improves sound spectrum
Improves operational reliability
Reduces cost, operation and maintenance
costs
PLENUM FAN WALLS
Negative Side
• Original cost of fans and motors is a
concern
• Many small fans in parallel operation may
lead to in-duct fans rather than plenum
fans
• Parallel operation of fans should be
concern
• The set up is very sensitive to non-uniform
velocity distribution over the fan wall
PLENUM FAN WALLS
Negative Side
• Back-draft dampers adding power losses
• Using small sizes of fans leads to
decrease in their efficiency
• Many small fans in parallel operation
may lead to in-duct fans rather than
plenum fans
• Possible vibration problems due to due
to excitation of powerful beats with
acoustical consequences
PLENUM FANS
Conclusion
• Plenum fans is advantage is their mixing
flow downstream of the fan
• Plenum fans are excellent for air
handlers or cabinet fans having a filter or
heat exchanger downstream of the fan
• Plenum fans can develop a substantial
fan pressure compared to propeller fans
• Attractive cost of plenum fans
especially with direct drive
Questions?