AMCA In Motion - Fall 2016 - 14
Outlet damper/system damper
FTP - 1180 rpm
9
Fan total pressure (inches WG)
10
HP - 1180 rpm
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
100
200
300
400
500 600
CFM
700
800
Horsepower
10
0
900 1000
Figure 3. Effect of resistance control devices on
centrifugal fan performance. Image courtesy of Howden.
Variable inlet vanes
FTP - 0% VIV closure
FTP - 25% VIV closure
FTP - 50% VIV closure
FTP - 75% VIV closure
HP - 0% VIV closure
HP - 25% VIV closure
HP - 50% VIV closure
HP - 75% VIV closure
9
Fan total pressure (inches WG)
8
7
6
10
9
8
7
6
5
5
4
4
3
3
2
2
1
1
0
0
100
200
300
400
500 600
CFM
700
800
Horsepower
10
0
900 1000
SPEED CONTROL
Figure 4. Effect of spin control devices on centrifugal
fan performance. Image courtesy of Howden.
Variable blade pitch
FTP - 0% blade closure
FTP - 25% blade closure
Fan total pressure (inches WG)
9
FTP - 50% blade closure
FTP - 75% blade closure
8
HP - 0% blade closure
HP - 25% blade closure
HP - 50% blade closure
HP - 75% blade closure
7
6
10
9
8
7
6
5
5
4
4
3
3
2
2
1
1
0
0
100
200
300
400
500 600
CFM
700
800
Horsepower
10
0
900 1000
Figure 5. Effect of blade pitch on axial fan performance.
Image courtesy of Howden.
14
Summer 2016
attack between the incoming airflow and the blade.
Reducing the angle of attack reduces both the airflow and
the load on the motor. Consequently, variable pitch blades can
keep fan efficiency high over a range of operating conditions
(Figure 5).
Variable pitch blades can be a very efficient flow control
option. They offer several performance advantages. Because
variable pitch blades maintain their normal operating speed,
they avoid resonance problems that can be problematic for
certain fan types. Additionally, when properly selected, variable pitch blades can operate from a no-flow to a full-flow
condition without stall problems. During startup, the fan
blades can be shifted to a low angle of attack, reducing the
torque required to accelerate the fan to normal operating
speed.
There is a difference between where the area of peak efficiency occurs for axial fans and where it occurs for centrifugal
fans. Peak efficiency for axial fans occurs with the blades in
a partially closed position, meaning the fan can be selected
to have its maximum efficiency at normal operating conditions
while still being able to achieve test block conditions at a
somewhat lower efficiency. Peak efficiency for a centrifugal
fan occurs with spin control in the open position at design
speed, meaning the maximum efficiency for a centrifugal fan
occurs at test block conditions if using variable inlet vane
(VIV) dampers or parallel bladed inlet dampers (Figure 6).
This means that variable pitch axial fans can maintain higher
levels of efficiency at lower fan loads than centrifugal fans
with spin control devices (Figure 1).
Disadvantages of this flow control option include the higher
maintenance costs associated with the moving parts of the
operating mechanism.
AMCA
Speed control provides the most efficient means of controlling fan flow. By reducing fan rotational speed, less energy
is imparted to the airstream, which means less energy must
be dissipated by the system airflow control devices (Figure
7).
There are two primary devices used to control fan speed:
two-speed motors and adjustable speed drives (ASDs). Although
both directly control fan output, two-speed motors and ASDs
typically serve separate applications.
ASDs allow fan rotational speed adjustments over a continuous range, avoiding the need to jump from speed to speed as
required by multiple-speed fans. ASDs include several different
types of mechanical and electrical systems. Mechanical ASDs
are typically adjustable fill fluid couplings. Electrical ASDs
include eddy current clutches, wound rotor motor controllers
and variable frequency drives (VFDs). VFDs are by far the
most popular type of ASD, largely because of their proven
effectiveness in reducing energy costs. VFDs have advantages
and disadvantages that are described in the sidebar "More
About VFDs."
Two-speed motors contain a different set of windings for
inmotion
W W W. A M C A . O R G
http://WWW.AMCA.ORG
Table of Contents for the Digital Edition of AMCA In Motion - Fall 2016
Contents
AMCA In Motion - Fall 2016 - BB1
AMCA In Motion - Fall 2016 - BB2
AMCA In Motion - Fall 2016 - Cover1
AMCA In Motion - Fall 2016 - Cover2
AMCA In Motion - Fall 2016 - Contents
AMCA In Motion - Fall 2016 - 2
AMCA In Motion - Fall 2016 - 3
AMCA In Motion - Fall 2016 - 4
AMCA In Motion - Fall 2016 - 5
AMCA In Motion - Fall 2016 - 6
AMCA In Motion - Fall 2016 - 7
AMCA In Motion - Fall 2016 - 8
AMCA In Motion - Fall 2016 - 9
AMCA In Motion - Fall 2016 - 10
AMCA In Motion - Fall 2016 - 11
AMCA In Motion - Fall 2016 - 12
AMCA In Motion - Fall 2016 - 13
AMCA In Motion - Fall 2016 - 14
AMCA In Motion - Fall 2016 - 15
AMCA In Motion - Fall 2016 - 16
AMCA In Motion - Fall 2016 - 17
AMCA In Motion - Fall 2016 - 18
AMCA In Motion - Fall 2016 - 19
AMCA In Motion - Fall 2016 - 20
AMCA In Motion - Fall 2016 - 21
AMCA In Motion - Fall 2016 - 22
AMCA In Motion - Fall 2016 - 23
AMCA In Motion - Fall 2016 - 24
AMCA In Motion - Fall 2016 - 25
AMCA In Motion - Fall 2016 - 26
AMCA In Motion - Fall 2016 - 27
AMCA In Motion - Fall 2016 - 28
AMCA In Motion - Fall 2016 - 29
AMCA In Motion - Fall 2016 - 30
AMCA In Motion - Fall 2016 - 31
AMCA In Motion - Fall 2016 - 32
AMCA In Motion - Fall 2016 - 33
AMCA In Motion - Fall 2016 - 34
AMCA In Motion - Fall 2016 - 35
AMCA In Motion - Fall 2016 - 36
AMCA In Motion - Fall 2016 - Cover3
AMCA In Motion - Fall 2016 - Cover4
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