Most industrial fans are reliable, designed by knowledgeable manufacturers for long life under a multitude of conditions. However, break downs do happen. They can be caused by incorrect installation, lack of maintenance or even improper fan selection. Also, it may be that years of strenuous service have simply worn out the fan.
This checklist outlines the major symptoms of fan problems as well as the more common causes and suggested remedies. Because this is only a guide and not a technical service manual, more complex causes and remedies are not included. Also, some ailments may have more than one cause.
VibrationIf your fan is experiencing excessive vibration, there are several possible causes.
Out of Balance Fan Wheel or Rotor. Check the wheel for any dirt or foreign material, especially in hard-to-see places like the back side of the wheel and the underside of the blades.
Airfoil blades usually are hollow. When exposed to rain or excessive moisture, water can get inside. Drilling a 0.1875" drain hole in the upper surface of each blade near the trailing edge should cure the problem. Rebalancing usually is not necessary.
Inspect the wheel for corrosion or erosion. Usually, wheel erosion will occur at the leading edge of the blade. On a paddle wheel-type fan, the outer blade tip also may be worn. An airfoil wheel exposed to sand or abrasive dust actually can develop pin holes in the leading edge of the blades. Do whatever is possible to eliminate these damaging conditions, then rebalance the wheel. If the wheel is seriously damaged, it will have to be replaced.
Improper or Loose Mounting. Foundation bolts and bearing mounting bolts can loosen themselves. Make sure they are tight.
Loose Set Screws that Hold the Wheel to the Shaft. Again, tighten the screws, but first be certain the wheel has not shifted on the shaft or is rubbing on the inlet cone or drive side of the housing.
Bent Fan Shaft. Check the shaft with a dial indicator. If it is bent, it should be replaced as soon as possible to avoid replacing the entire fan.
Misaligned V-Belt Drive. Another common problem on belt-driven fans is a misaligned drive. To solve it, realign the assembly so the fan and motor shaft are parallel and the faces of the sheaves (pulleys) are flush to a straight edge. Often, a taut string will work just fine for this.
Fan Wheel Turbulence due to the Rotor Running Backwards. Because blade angles and shapes vary greatly, it is easy to misread rotor direction. Check for correct wheel rotation, clockwise or counterclockwise, as seen from the drive side.
Air Pulsation. The fan may be operating in the stall area of its performance curve. That means it is oversized for your particular system or the system resistance is higher than intended. You can lower system resistance by cleaning the filters or opening the dampers.
NoiseAnother common complaint about fans is excessive noise. Here are a few possible causes and solutions.
Foreign Material in the Fan Housing. This could be anything from a loosened bolt to somebody's lunch bag. Inspect the wheel and inside of fan housing and clean thoroughly.
Squealing V-Belts. Squealing belts are a sign that the belts are either loose or misaligned. If belts show wear, you are better off to replace them now and avoid a future breakdown.
Worn Ball or Roller Bearings. If you hear howling, screeching or clicking, the problem may be worn ball or roller bearings. Change the bearings immediately before they cause additional damage. Failing bearings tend to wear the shaft, so you will want to be absolutely certain the shaft is full size before installing new bearings. Using a micrometer, measure the shaft both under the bearing and next to it, and compare the two readings. If they do not match, replace the shaft. New bearings installed on a worn shaft will not last long.
Bearing Seal Misaligned. If you hear a high pitch squeal, the bearing seal may be misaligned. Realign the face of the bearing so it is perpendicular to the shaft.
If the fan housing has a metal shaft seal, it could be misaligned and rubbing on the shaft. Loosen seal plate bolts, recenter the seal on the fan shaft and tighten the bolts. If the seal is fiberglass, cork or rubber, be sure the metal backing plate does not touch the shaft.
Overheated BearingsBall or roller bearings tend to heat up when they have been overgreased and will cool down to their normal running level when the excess grease oozes out. The normal operating temperature of a bearing may be well above 140oF (60oC), which is hot to touch. Temperatures above this must be read with instruments, and anything above 180oF (82oC) should be questioned. If you place a drop of water on the bearing and it sizzles, the bearing is in distress and should be changed before it seizes and ruins the shaft.
Bearings May Be Worn and Failing. Replace the bearings. Remember to also check the shaft. (refer back to Noise section)
Improper Grease. Use a lithium base, high speed, channeling-type grease. Do not use high temperature or general purpose grease.
Overgreasing. If you allow the bearing to run for a few hours, it normally will purge itself of extra grease. You can simply remove excess grease from split roller bearings by lifting the top half of the block for access.
Heat Soak From an Oven or Dryer After Shutdown. Heat soak occurs when a fan is idle and its shaft cooling wheel can no longer cool the inboard bearing. Heat from inside the fan can actually cook the grease. A 15 min fan run after the oven heat is turned off will cool the fan shaft and protect the bearing.
Slippage and Friction. Loose V-belts may cause belt slippage and friction heating, resulting in hot bearings, shafts or sheaves. To solve this problem, tighten belt to proper tension. A good rule of thumb is you should be able to depress the belt the same distance as the thickness of the belt.
Excessive V-Belt Tension. Excessive belt tension also can overheat the bearings. Adjust the belt to the correct tension.
Poor Air PerformanceIf your problem is poor air performance, here are some possible causes.
Fan Rotation Incorrect. As explained under vibration, it is easy to misread fan blade orientation. One way to change rotation on most three-phase motors is to reverse any two motor leads.
Poor Duct Design. Abrupt turns in the duct close to the fan discharge can cause pressure loss, or ductwork elbows at the inlet can cause air pre-spin. To address either of these problems, install turning vanes or elbow splitters in the duct. If air performance still is inadequate, the discharge position may have to be changed.
Improperly Installed Inlet Volume Control. If fan has an inlet volume control (IVC), check to be sure it is installed with pre-spin of the air in direction of wheel rotation when the IVC is partially closed (figure 1).
Off-Center Wheel. This can occur on double-width, double-inlet fans. Center the fan between the inlet cones to avoid overloading one side of the wheel while starving the other.
Fan Horsepower Unexpectedly Low. Correct one or more of the following conditions: air pre-spin into the fan inlet; fan drive sheaves set for too low a fan speed; resistance to airflow, such as that caused by a closed damper, is much higher than calculated.
Fan Horsepower Unexpectedly High. Be sure fan speed is not too high. There are three possible reasons why fan horsepower is unexpectedly high. First, the fan may be operating without ductwork at low resistance, so too much air is flowing. Second, the fan may be handling ambient air when it was intended for hot, less dense air. Third, the fan may be running backwards.