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The fan blades can be damaged in the operation of cooling
tower due to one of the following reasons.
1) Margin of Resonance Frequency
This is very important point
in deciding the number of fan blade and the speed of
fan. Fan blades have their natural critical speed, which
will start to vibrate with a definite frequency when
a body or system is given with an initial displacement
from its equilibrium position and is released. In other
word, if you were to suspend a slender article and you
begin vibrating it at varying frequencies, beginning
at zero hz or at some point, it would begin to vibrate
dramatically as its resonant frequency is reached. This
is a point where its natural frequency to vibrate is
exited by an equal applied source and it would resonate.
The fan must not be operated
near the resonance speed. We recommend minimum 10% of
separation of fan operating frequencies from the critical
frequency of fan. There are two operating frequencies
relevant to the fan running. First is a blade passing
frequency obtained from a formula of "Number of
Fan Blades x Fan Speed / 60". Second is a beam
passing frequency resulted from an equation of "Number
of Supporting Beam x Fan Speed / 60".
2)BHP per Blade
This is another important
factor in deciding the number of fan blades. Fans are
limited in operating. The mechanical strength of fan
blade is based on the maximum centrifugal force and
air loading. So, the tip speed of fan must not exceed
12,800 FPM. BHP per Blade which is a result of Fan BHP/Number
of Fan Blade is chosen by the fan diameter.
High blade air loading result
in fatigue, vibration and noise problem. So, we recommend
4 BHP less than the maximum BHP per Blade. This is trouble
free in most application considering the unusual air
loading influences due to wind, or fan stack rigidity.
3) Pressure Margin
If the operation of fan closes
to the stall area, the fan shows an unstable flow condition
and can cause the damage of fan blade due to the resultant
dynamic load. So, we recommend the minimum 15% of pressure
margin at the given pitch angle for preventing a possible
operation at the stall limit area due to the influence
of wind. The pressure margin can be obtained from the
fan performance curve and it is not easy to determine
the pressure margin.
4) Tow Speed Motor
We have experienced that
the fan blade and the output shaft of gear reducer were
damaged when the motor is switched from high speed to
low speed without pausing. Note that the motor must
be switched from high speed to low speed after the motor
is full stopped.
Theoretically, the BHP of
fan at 2/3 of motor full load speed is reduced to (2/3)3
at the constant air density and constant system, since
the air flow is proportionally reduced by the fan speed
ratio and the static pressure is reduced by square of
fan speed ratio, too. However, this is only applicable
to the assumption of the constant air density and constant
system. Actual fan BHP at 2/3 of motor full load speed
must be consulted by cooling tower thermal design engineers.
While BHP at 2/3 speed is
theoretically calculated 40.6 BHP (= 136.9 x (2/3)3),
the fan BHP at 2/3 speed actually obtained from the
thermal design (variations in the air density and static
pressure) and constant pitch is 39.7 HP.
Considering the required
fan BHP at full and 2/3 speed, the use of variable torque
motors are common sense. The mistakes in selecting the
type of load results in the fan blade brokerage. Some
customers use the constant torque motor. This is a general
fault in selecting the motor load type and can occur
the severe damages in the fan blades when the motor
is accelerating to the 2/3 speed from the rest or from
reverse wind milling conditions.
5) Reverse Rotation of Fan
The fans can be reversibly
rotated even under the no power into the electric motor.
This is due to the wind induced when the water is falling
onto the fill package or due to the ambient wind through
the fan stack. If the power during the fan is reversibly
windmilling, the instant high reverse torque can be
applied to the fan. There is much possibility in giving
a damage on the fan blade. So, we recommend to mount
the back stop onto the gear reducers in case of using
the relatively large diameter of fan.
6) Fan Leading Edge Erosion
Note that fan leading-edge
erosion can occur as fan blades continually impact water
droplets in the discharge air stream. If left unattended,
this erosion can severely reduce fan performance and
may eventually lead to structural failure.
7) Vibration
This is another factor to
damage the fan blades. The vibration can be occurred
by the fan imbalance, fan shaft misalignment, and blade
passing frequency interaction with fan stack or cooling
tower structure. The fan imbalance, in general, could
be corrected as follows;
- ● Minimize the tip track
variation which is a major source of dynamic imbalance.
- ● Check the proper assembly
of fan including hardware tightness, blades in the
proper position, and blades at the equal pitch. After
finishing this work, you may analyze the vibration
with vibration-frequency meter. If we get the plot
chart for vibration-frequency, it is easy to find
the vibration problem.
- ● Check the static moment
value in the label attached in the area of fan neck.
Hudson is recording the value of static moment on
the label. In case of 30H, the maximum balance tolerance
is 35 in-pound. The minimum static moment for 30H
is 9,767 in-lb and the maximum static moment is 9,802
lb-in.
The only normal radial force would be due to the minimum
and maximum balance tolerance on the blades. That
is, if one blade has a maximum tolerance and the opposite
blade has a minimum tolerance, there would be a small
net imbalanced centrifugal force acting horizontally
at the fan centerline. If possible, relocate the fan
blade to minimize the balancing tolerance after checking
the balancing value of static moment.
Note: This issue was written
by Mr. Oick Kwon of Chungrok ENC Comapny in Seoul, Korea.
If you have any questions, please feel free to mail
to criok@hananet.net
or contact me at 822-786-4237.
References:
1. R.C. Monroe's paper for
Fan keys to optimum cooling tower design
2. R.C. Monroe's paper for Improving cooling tower fan
system efficiencies
3. A. Pinkerton? paper for Environmental Noise Control
4. Technical correspondences between author and R.C.
Monroe and K. Won of Hudson
5. Technical correspondences between author and Jack
Newton of PMC/BETA Corp.
6. Technical correspondences between author and Marcel
R. Lefevre of M.R.L. Corp.
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