AIRFOIL FAns - HTS

Accessories
Volume Control Devices
Outlet dampers, variable inlet vanes, and variable
frequency drives are three popular devices used to
control volume for fan systems.
Outlet Dampers
The closing of the damper adds to the resistance that
the fan is working against. This moves the operating
point to the left of the initial rating point. The savings
in horsepower depends on the relative position on the
fan curve and is usually much less than offered by
other methods. Outlet dampers are typically the least
expensive option and should be considered when
infrequent operation at lesser capacity is desired or
when handling hot, humid or particulate laden air.
There are two types of outlet dampers: parallel
blade and opposed blade.
Parallel blade dampers are recommended for
systems where air volume is modulated between fullopen
to about 75% of open.
Opposed blade dampers cost about 10% more
and are recommended for systems where volume is
modulated over the entire range. Opposed blades
reduce air volume in a closer relationship to the control
arm movement.
Variable Inlet Vanes
Nested
Inlet
Vanes
10
8
Performance Curves
at Different Vane Settings
50
100
75
200
160
STATIC PRESSURE
6
4
2
0
0
25
25
50
75
100
120
80
40
BRAKE HORSEPOWER
0 2 4 6 8 10
CFM IN 10000's
External Inlet Vanes
Variable inlet vanes cause the entering air to spin
in the direction of wheel rotation, resulting in reduction
in volume, static pressure and brake horsepower
and thus providing an infinite number of fan curves
approximately parallel to the original fan curve.
Variable inlet vanes cost about 50% to 80% more
than outlet dampers but offer significant savings in
energy. Because of their simplicity, inlet vanes can be
more reliable when compared to variable frequency
drives.
There are two types of variable inlet vanes: nested
(internal type) and bolted on (external type).
System Curve
SYSTEM CURVE
SP @ Vane Setting
SP @ VANE SETTING
BHP @ Vane Setting
BHP @ VANE SETTING
Nested inlet vanes are built into the fan inlet cone
and offer the advantage of saving space and lower
cost as opposed to the external type, and they will
be provided on all fan sizes 165 and larger. Twin City
Fan & Blower offers cantilevered vanes to size 730
Class II fans to minimize insertion losses and noise
associated with center hub design.
External inlet vanes are bolted to the inlet of the
fan and are available as standard to size 150. Use of
external vanes should be considered for hostile environments
since operating linkages are shielded from
the airstream. Both types of inlet vanes are available
to 600°F construction.
Variable Frequency Drive (VFD) Fan Motors
VFD changes the fan speed and can provide the
greatest potential for energy savings, although at
highest initial cost. VFD should be considered for
extended operation at part load conditions, especially
below 70% of the full volume operation.
8
Twin City Bulletin 370