OVERSIZED FANS
Fans like all
equipment can be oversized in the design phase to accommodate
variances within the process. Oversizing a fan is expensive as the
capital cost is higher and it takes more energy to operate. In
addition, oversized fans require more frequent maintenance as it is
not operating at it optimum levels, it may experience cyclic bearing
and drive train stresses. Oversize fans can cause high levels of
noise and vibration which may lead to worker discomfort, structure
problems in weld and mechanical joints.
Indications of
Oversized Fans
-
Check the
frequency of the dampers and inlet vanes – if they are often
minimized or shut
-
Refer to the
fan curve and pinpoint its current operating conditions and
compare that to it optimum operating limits – If it way off the
fan optimum limits, the fan size is not suitable for the purpose
-
Compare the
pressure at the end use and to the pressure generated by the fan
-
Obtain the motor power consumption by measuring the amps and
voltage of the motor.
Solutions for
Oversized Fans
Here are some
suggested typical solutions to overcome the issue of oversize fans:
|
# |
Solution |
Solution Description |
|
1 |
Fan Speed* |
One way to
decrease the fan speed by adjusting the ratio of the pulley
diameters of the motor and the fan. Do not adjust the fan
speed this way if the motor load is < 40%, as the efficiency
declines. |
|
2 |
Replacing
motor |
If
feasible, consider switching to a smaller motor |
|
3 |
ASDs |
If the fan
is used at different speeds consider installing a type of
Adjustable Speed Drive (ASD) (i.e. one type of ASD is a
Variable Speed Drive (VSD). Note: Fans with severe
instability regions should not use with seeking professional
attention. |
|
4 |
CPFs |
Controllable Pitch Fans (CPFs) allow adjustment of the fan’s
blade angle of attack according to airflow requirements. The
adjustment of the angle of attack changes the load on the
motor; and hence potentially reduces the amount of energy
used. This is only applicable to axial fans. |
* Only suitable
for belt driven fans
PREVENTATIVE
MAINTENANCE
The goal of
preventative maintenance is to avoid unscheduled downtime of the fan
systems. The intervals of these preventative maintenance practices
depend on the type and make of the fan, refer to the manufacturer’s
recommendations.
|
# |
Item |
Maintenance Description |
|
1 |
Belts |
When belts
lose tension they tend to reduce their power transmission
efficiency. Ensure belt tightening is included in the
routine maintenance schedule. It’s best to change the belts
at the same time to avoid uneven loading. Over-tightening of
belts can lead to higher radial bearing loads, accelerated
wear and shorter belt life spans. Consult the belt
manufacturer if unsure. |
|
2 |
Fan blades |
Particulate can build up on the fans over time and as the
build up is not even all over the surfaces; it can lead to
imbalance and cause performance issues, unnecessary wear and
higher operating costs. Ensure the fans are cleaned and
maintained on a regular basis as recommended by the
manufacturer. |
|
3 |
Ducts &
Piping |
If there
is a decline in system performance check the duct work for
leaks and malfunctions. Looks for tears in flexible joints
and use temporary pressurization equipment to identify the
leaks and also determine the system’s integrity. |
|
4 |
Bearing
Lubrication |
Refer to
the manufacturer’s recommendations for lubrication, have a
formalized system to ensure the lubrication is done on a
regular basis. Be careful not to over lubricate the system
as it can cause overheating. |
|
5 |
Motors |
To rewind
or to replace motors? This decision depends on the size,
hours used, operating duration, motor size, motor type,
number of times it’s been rewound and cost of electricity.
Generally, larger motors should be rebuilt and smaller ones
replace. If replacing motor, look into replacing it with an
efficient motor. |
|
6 |
Vibration
Analysis |
Organize
to have vibration amplitude and frequency tests to be
performed on a regular basis. Monitor the changes that these
tests detect, the trend will potentially detect pending
bearing, voltage imbalances, mechanical imbalances, impeller
erosion, and coupling problems. |
NOTE: Adequate
maintenance records should be kept to ensure future problems with
certain parts of the systems are diagnosed. Details of observations,
inspections results history of repairs and repairs should be logged.
