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.