Cooling Tower Failures During Summer Operation

During summer operation, thermal load on the cooling tower is at, or can even exceed, design capacity.  If the cooling tower motor fails during summer operation, the entire facility will be left without mechanical cooling, leading to swampy, and uninhabitable tenant spaces, or unplanned process disruption.  It is for this reason that every individual responsible for, or relying on, the cooling tower be aware of how and why cooling tower components can fail suddenly during summer operation.

Failed Fan Motors

Cooling tower fan motors typically have a life expectancy of 12-15 years, depending on operation and maintenance.  Once a motor has reached or exceeded its lifespan, it will fail suddenly, but often not without warning.  A common failure point for fan motors are the internal bearings.  If the fan motor is exhibiting a grinding or whining sound in operation, the internal motor bearings are likely soon to fail, requiring either an overhaul, or a replacement of the motor.

There are several other factors that can cause a fan motor to fail during peak season such as overheating, voltage imbalances, power surges, poor electrical connections, overloading, or even excessive vibration from the cooling tower.  Addressing these issues through regular maintenance, proper installation, and monitoring of operating conditions can help extend the lifespan of the motor, however; the best protection against lengthy down time is to keep a spare motor on site ready to install.

Belt Driven Assemblies Forced Draft Towers (Horizontal & Vertical Shafts)

Belt driven assemblies can be configured either horizontally (forced draft) or vertically (induced draft), however both require the fan motor to spin a motor sheave (small pully) which is connected to a larger fan sheave (large pully) by tensioned belt(s).  The larger fan sheave is also attached to a fan shaft at one side, and either fan wheels (forced draft/horizontal), or fan blades (induced draft/vertical) on the other, in order to blow (forced draft) or pull (induced draft) air through the cooling tower.

Fan shaft assemblies rely on fan bearings to keep the assembly aligned, and running smoothly without vibration.  Fan shaft bearings require regular lubrication (typically a remote grease line is installed) and maintenance in order to ensure consistent, uninterrupted operation.  In the event the cooling tower mechanical assembly is neglected, the fan bearings will begin to exhibit grumbling and grinding sounds as a warning that they will soon fail.  Once the fan bearings begin to exhibit these failure signs, it is critical that they be replaced, so as to avoid fan collapse.

Cooling tower fan shafts can fail for many reasons, most commonly wear and tear.  Over the years, moisture collects on the fan shaft assembly, causing it to rust and pit.  The fan shaft bears the brunt of the torque on startup, and if the shaft is rusted and pitted across too much of its surface area eventually it will break.  It is important to have a qualified cooling tower specialist review the condition of the fan shaft to identify the difference between “rusting, but still solid” and “pitting through”.

Fan sheaves also rust and wear, which can cause belts to slip/break more frequently.  If the tower is belt driven, slipped belts will mean a spinning motor sheave, but no spinning motor.  Have a qualified cooling tower specialist review the condition of the fan and motor sheaves to ensure the grooves are in an acceptable condition for operation prior to start up.

Gearbox Assemblies

Many cooling towers operate with a gearbox (transmission or gear reducer) assembly in lieu of the belt/sheave design.  The gearbox sits directly below the axial fan assembly, and is connected via the crown shaft of the gearbox to a bushing and coupling assembly connected to the fan.  The crown shaft of the gearbox is connected to the top gear internally, which is grooved into the bottom gear below, also connected to a shaft running perpendicular out the side of the gearbox called the pinion shaft.  Both shafts are seated with internal bearings for stability and submerged in oil.

A gearbox assembly is slightly more efficient to operate than a belt assembly, and does not require for belts to be changed, however the gear oil must be changed in accordance with manufacturers guidelines in order to avoid failure.  It should be no surprise that the most common gearbox failure is due to lack of oil change.  If the oil is not changed at least per manufacturers requirements the assembly will begin to exhibit excessive vibration and noise before eventually collapsing internally.

A second common point of failure for gearbox assemblies are the internal bearings.  All bearings eventually fail over time, and the most common failure for gearbox assemblies is worn out bearing internally.  When the bearings begin to fail the shaft will start to exhibit play (movement) which causes vibration across the assembly.  Ensure that prior to start up a qualified cooling tower technician inspect the shaft assembly for play along the gearbox bearings as these can be replaced without replacing the entire assembly.

A third common point of failure for gearbox assemblies are the seals.  The pinion shaft, which runs along the bottom portion of the gearbox, is attached to the lower gear and submerged in oil.  This shaft is separated from the oil basin by a seal, called the pinion seal.  Over time, these seals can dry out and eventually begin to leak.  If a pinion seal leak remains unattended to, eventually the gearbox will run dry of oil and the entire assembly will fail.  Prior to startup, ensure that a qualified cooling tower technician inspect the gearbox assembly for pinion seal leaks, as these seals can be replaced without replacing the entire assembly.

To ensure a safe and comfortable operating season, call CTM today at 1-800-311-1200 or visit us at www.coolingtowerinc.com