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nyb Engineering Letter 6
ENGINEERING LETTER
6
The New York Blower Company 7660 Quincy Street, Willowbrook, Illinois 60521-5530
I N C R E A S I N G F A N P E R F O R M A N C E
INTRODUCTION
Industrial processes and plant-ventilation systems often need
more air than originally designed. Increased production
requirements, process changes, and facility renovations are a
few of the major reasons. Additionally, the lack of adequate
maintenance over time can negatively impact system airflows.
This letter discusses several procedures that can increase airflow.
CHECK THE FANS MECHANICAL CONDITION
Often airflow can be increased by adhering to proper fan
maintenance procedures as outlined in fan installation and
maintenance literature.
Properly aligned and tightened V-belt drives. See Figure 1.
Fan speed can decrease by as much as 10% to 20% when belts
are too loose, with a corresponding loss of airflow.
Clean airstream surfaces. A fan cannot perform as designed if
the air flow surfaces are distorted by contaminants. Even in
large fans, a sixteenth of an inch of build up can reduce
performance.
Check fan rotation. See Figure 2. Centrifugal fans will move
some air even when running backwards. While some types
would use so much horsepower they would trip circuit breakers,
other designs could run for years without being detected.
Check wheel and inlet cone alignment. See Figure 3.
Components may be out of position due to routine cleaning or
painting or the wheel could have shifted during shipment. For
backward inclined fans, the relation of wheel to inlet cone is
very critical. Even a quarter of an inch can have a major
impact. The fans installation and maintenance literature shows
the proper positioning of the wheel to the inlet cone (A
dimension) or inlet plate.
INSPECT THE SYSTEM
The design and maintenance of the system plays a large role in
achieving the overall desired performance. Visual inspections
often reveal some easily rectified problems that can
significantly impair performance.
Check for clogged filters or coils. If the system has not been
properly maintained, clogged filters or obstructed coils will
reduce airflow. The greater the obstruction, the greater the loss
in airflow.
Eliminate System leaks. Any leaks in the ductwork will
contribute to reduced performance, especially leaks around
plenum bulkheads that can lead to recirculation of air. Worn
flexible connectors are a common source of leaks and should
be inspected regularly.
Verify that dampers are installed correctly and operating
properly. If the damper linkage is out of adjustment, the
damper may not be opening completely, thereby reducing
performance. If inlet dampers are used, make sure they are
installed so that the air is pre-spun in the same direction as
wheel rotation. See Figure 4. If the air distribution system
employs balancing dampers, make sure they are set properly.
Figure 1 - Poor Drive Alignment and Belt Tension
Figure 2 Incorrect Wheel Rotation
Figure 3 Wheel to Cone Alignment For all dampers, make sure there is sufficient clearance for the
blades to open and close completely without hitting the
ductwork or other system components. Last, for systems with
either pneumatic or electric controls, make sure damper
actuators are operating properly.
Look for system effect. Sharp changes in the direction of air-
flow at either the fan inlet or outlet will disrupt the flow through
the fan and impair performance. If it is impossible to straighten
the ductwork entering and leaving the fan, the use of inlet boxes
and turning vanes can minimize performance losses as shown in
Figure 5. For a more detailed explanation, refer to Engineering
Letter 5, Fan Performance - The System Effect.
INCREASE THE FAN SPEED
One of the easiest solutions to low airflow problems is speeding
up the fan. While airflow is increased by speeding up the fan, so
too are static pressure, noise, and power requirements. Figure 6
presents this graphically. Therefore, while increasing the fans
speed is an easy procedure with low first cost, the additional
operating expense over time makes it the most costly solution.
See Engineering Letter 2 - Fan Laws and System Curves, for
additional information.
When increasing fan speed, it is necessary to check the
maximum safe speed of the fan and make sure the motor is
capable of the horsepower required to run the fan at the new
speed. Never run a fan beyond its maximum safe speed.
ADD OR REPLACE FAN EQUIPMENT
On a first-cost basis, adding or replacing fan equipment is the
most costly alternative. However, on a life-cycle-cost basis,
considering operating and maintenance expense, it can be the
least expensive, as compared to increasing the speed of an
existing fan.
Sometimes a second fan may be added, either in series or
parallel with the original, although it may be more cost effective
to simply upgrade the system with a new fan capable of the
required airflow and pressure.
Adding another fan in series will increase the airflow because of
the additional pressure. The operating point of the new system
moves further out/up the system curve. Where duct size is
adequate to handle the desired amount of air but the existing fan
doesnt provide sufficient pressure, a second fan in series may
be the best solution. However, make sure the ductwork can
handle the increase in pressure.
Adding another fan in parallel with the first will increase airflow
due to the combined capacities. Because capacities are being
combined instead of pressures, a greater increase in airflow will
result for a given system. However, system pressures will also
increase and caution is required to avoid the unstable operating
area of the combined fan system.
CONCLUSION
When more air is required it is important to investigate the
system on a step-by-step basis, considering the least expensive
possibilities first. For existing systems that seem to have lost
performance, fan and system maintenance is the place to start.
Often, simply improving the efficiency of existing components
will suffice. For systems that require greater airflow and/or
pressure, increased fan speed is generally the first alternative.
However, when large increases in performance are required,
there may be no alternative but to purchase a larger fan.
Form 607 GAW
Figure 4 Inlet Damper/Fan Wheel Rotation
Figure 5 Fan Inlet Connections
Figure 6 The effects on brake horsepower, static pressure and
loudness when fan speed is increased.