Fan failures: five typical problems and what causes them

An industrial HVAC fan system. PHOTO: Thinkstock

Centrifugal fans and fan systems used in industrial HVAC applications can fail in many ways. To fix failures it’s useful to know what causes them. Engineering manager Marcel Kamutzki from fan and blower manufacturer Daltec Canadian Buffalo Manufacturing Ltd. in Guelph, Ont. detailed five malfunctions and their symptoms during a workshop for maintenance pros in Hamilton.

1. Fan is too noisy. When an impeller hits the inlet or housing, it’s likely not centred; the inlet or housing is damaged; the impeller is crooked or damaged; there’s a loose shaft or loose bearing; a shaft is bent; or there’s a misaligned shaft and bearing.

If the impeller is hitting the cut-off, the cut-off is not secure in the housing, it’s improperly positioned, or it’s damaged.
There are numerous causes if the problem is the belt drive: the sheave is not tight on the shaft; the belts are hitting the belt tube; belts are too loose or too tight; belts have the wrong cross section; belts are not matched in length on multi-belt drives; variable pitch sheaves are not adjusted so each groove has the same pitch diameter; the sheaves are misaligned or the belts are worn; the motor anchoring, base or fan is not secure; improper drive selection; loose key; or oily and dirty belts.
Probable causes of coupling problems are a loose key or an unbalanced, misaligned, loose or wrongly lubricated coupling.
Noisy bearings are caused by a defect, loose support, inadequate lubrication, loose shaft, misaligned seals, foreign material, or fretting corrosion between the inner race and the shaft.

When the shaft seal squeaks, the culprit may be inadequate lubrication, misalignment or a bent shaft.

Impeller noise can be caused by a defect, looseness on the shaft, unbalance, excessive wear as a result of abrasive or corrosive material moving through flow passages, or blades rotating close to a structural member.

Motor symptoms are an insecure lead-in cable, an AC hum in the motor or relay, starting relay chatter, noisy motor bearings, the cooling fan striking a shroud, or low voltage and single-phasing of a three-phase motor.

If the symptom is high air velocity, probable causes are ductwork that’s too small for the application or the fan is too large, the registers or grilles are too small, or the heating or cooling coil has an insufficient face area.

When obstructions cause a rattle or whistle, the reasons may be dampers, sharp elbows, a sudden expansion or contraction in ductwork, leaks in ductwork, turning vanes, or fins on coils.

Pulsation or surge traces to a restricted system that causes a fan to operate left of the peak, a fan that’s too large for the application, ducts that vibrate at the same frequency as fan pulsations, distorted inlet flow, inlet vortex surge or rotating stall.

Rattle or rumbles are caused by vibrating ductwork, vibrating cabinet parts, or vibrating parts not isolated from the building.

2. Fan is too quiet. When the fan is not operating at all, it’s likely straightforward mechanical and/or electrical problems routinely analyzed by service personnel. Reasons include blown fuses, broken belts, loose pulleys, wrong voltage and/or excessive line drop or inadequate wire size, a load inertia that’s too great for the motor or a seized bearing.

3. Fan performance is wrong. Causes of inadequate airflow to the fan include a backwards installed or running impeller, improper blade angle setting, missing cut-off, an impeller not centred with inlet collars, slow fan speed, dirty or clogged inlet, improper running clearance or improper inlet cone-to-wheel fit.

Not enough airflow to the duct system indicates the actual system is more resistant to airflow than expected. Other probable causes are closed dampers or registers, loose insulating duct liner or leaks in the supply ducts.

If airflow to the filters or coils is inadequate, look for dirty or clogged filters, replacement filters with greater than specified pressure drop, or install a replacement coil with tighter fin spacing.

Airflow issues arising from poor inlet conditions may be elbows, cabinet walls or other obstructions. Inlet obstructions cause more restrictive systems but don’t cause increased negative pressure readings near the fan inlet. Increase fan speed to counteract the effect of restricted inlets, but not beyond the manufacturer’s recommendations.

Poor airflow due to poor outlet conditions results from a sudden expansion or contraction at the fan outlet, or a duct bend. If it’s not practical to install a straight section of duct at the fan outlet, increase the fan speed to overcome the pressure loss, but don’t increase fan speeds beyond the manufacturer’s recommendations.

Too much airflow is caused by oversized ductwork, damper set to bypass, filters not in place, too fast a fan speed, low system resistance or an open access.

Remember pressure density will be less with high temperature gases or at high altitudes.

High airflow indicates the system has less resistance to flow than expected.

When airflow is low, the impeller is likely installed backwards or it’s running backwards. Other probable causes are improper blade angle setting, missing cut-off, too slow fan speed, dirty or clogged impeller or inlet, improper running clearance, or improperly set inlet vane or damper. It’s also possible that the system is more resistant to flow than designed, dampers are closed, coil fin spacing is too close, or replacement filters are too restrictive.

4. Fan is vibrating. Ask yourself whether the fan impeller is properly balanced, motor and sheaves are balanced, key lengths are correct, fan shaft seals are rubbing, and does the motor have the right bearings.

If the fan foundation is inadequate, are isolators properly sized and levelled; shims properly installed; anchor bolts properly sized, intact and installed on all anchor points; and is ducting twisting or rubbing?

Check bearings: are they upgraded for a higher load capacity? Do you use the correct type and amount of grease? Is one bearing free to float?

Finally, check for changes in the system such as higher or lower pressure. Have expansion joints or dampers failed? What’s the duct loading?

5. When the fan fails. This is the worst-case scenario. Higher trending vibration and rapidly increasing bearing temperatures indicate bearing ring failure. Other reasons include failing belt drive or coupling, shaft failure and impeller failure due to abrasive or corrosive wear.

Knowing the reason for fan failures and making required repairs correctly in the first place will ultimately save you time
and money.

This article is condensed from a technical paper presented at an education workshop convened by the Hamilton section of the Society of Tribologists and Lubrication Engineers (STLE).