Industries that require process heat rejection often use field-erected cooling towers to keep pace with the heat load. Cooling towers have large fans and motors that can present worksite challenges, including excessive sound and vibration. Gearboxes are employed to address these challenges.
Gearboxes are used as speed reducers to slow the rotational speed from the incoming motor to the outgoing fan of a cooling tower. Companies that operate high capacity production plants such as those in the chemical process and power industries require cooling towers with large amounts of heat-rejection capacity.
Without gearbox technology, cooling tower motors would be massive to directly handle the torque required by the fan. Something so large and heavy would be impractical. Instead, the speed reduction from the gearbox acts as a torque multiplier, keeping the motor a reasonable size and the overall mechanical system more cost effective.
The speed reduction from gearboxes also optimizes the performance of the cooling tower fan. As fan diameters increase, the fan speed must decrease to maintain acceptable sound and vibration levels as well as to ensure the structural integrity of the fan itself. As it relates to the motor and fan, improper gearbox sizing can result in excessive vibration, loud operation and structural damage to the tower.
In large cooling tower applications, the fan typically operates at a speed between 100 and 200 rpm. The most common motor speed is 1800 rpm, requiring the average gearbox to reduce motor speed by approximately nine to 18 times to achieve the desired fan speed. The exact combination of this gearbox ratio, fan-blade design and fan-pitch contributes to the specific performance and energy efficiency of the cooling tower in a given application.
To address high operating temperatures, select a gear drive with integral cooling fins designed into the housing.
Solutions to Common Gearbox Problems
Cooling tower operators and maintenance engineers cite several common pain points associated with gearbox operation and service.
Issue 1: High operating temperatures, which can contribute to shorter operating cycles, premature gearbox failure and unplanned downtime.
Issue 2: Excessive sound and vibration levels, which can result in noise violations, employee safety issues, equipment fatigue and premature failure.
Issue 3: Extreme temperature and humidity conditions within the cooling tower, which can quickly degrade mechanical systems, affecting sound level and life of gear sets.
Issue 4: Premature bearing failure and excessive wearing of the pinion shaft.
Issue 5: Few gearbox options and choices to support scheduled or unscheduled maintenance and replacement.
Cooling tower gearboxes reduce motor size, optimize fan performance and protect structural integrity.
Fortunately, cooling tower gearbox developers continue product research-and-development activities. Among the design innovations and product enhancements, engineers have identified solutions to address the five gearbox operational issues most often cited by plant managers and maintenance engineers.
High Operating Temperatures. To address high operating temperatures, select a gear drive with integral cooling fins designed into the housing. In lieu of a smooth casting, the cooling fins increase surface area, allowing better heat rejection as air is pulled over the gearbox by the cooling tower fan. Gearboxes that incorporate large internal oil ports help to keep oil well circulated and contribute to overall cooler operating temperatures. As a result, oil life is extended, fewer oil changes are required, and gears and bearings are lubricated properly to maximize performance and service life.
Excess Sound Levels. To dampen excessive sound and vibration levels associated with gearboxes, select for metal castings. Choose a gearbox engineered to minimize the case deflection associated with the torque and thrust loads specific to cooling tower duty. Gearboxes purpose-built for cooling tower service with thicker castings result in quieter, safer working conditions, less metal fatigue and longer service life.
Extreme Humidity and Temperature Conditions. To counteract the extreme temperature and humidity conditions that cooling tower gear drives are subjected to, steel shims at case-connection points offer advantages. Plastic shims exposed to temperature and humidity conditions within the tower can creep over time, causing changes in tolerance that affect the sound and life of the gear sets. Steel shims maintain proper gear settings and control gear sound under extreme operating conditions.
Premature Bearing Failures. To guard against premature bearing failure and excessive wearing of the pinion shaft, use heavy-duty double-row interstage bearings. Bearing life of 100,000 hours is routinely achievable. A premium-grade isolator-type bearing oil seal with fixed stator sealing surface to prevent shaft wear also is recommended in lieu of a lip-type seal that wears against the pinion shaft. Features such as an Inpro seal bearing isolator can extend seal life without excess shaft wear. The use of heavy-duty double-row bearings and oil seals can protect gearboxes from premature wear and extend service life.
Gearboxes are used as speed reducers to slow the rotational speed from the incoming motor to the outgoing fan of a cooling tower.
Plan for Maintenance. To expand gearbox repair or replacement options and choices for either scheduled or unscheduled maintenance, choose a reliable supplier with specific knowledge of gear drives and an extensive inventory of cooling tower componentry. Some gearbox manufacturers offer a dedicated in-house gearbox evaluation team experienced in the inspection, repair and rebuilding of the most common gearbox designs.
Plant operators and maintenance engineers must effectively manage a variety of cooling tower service issues, whether scheduled or unplanned. Finding common-sense, long-term solutions to the most common pain-points not only simplifies routine maintenance and repair operations but goes a long way to alleviate service anxiety. Reliable gearboxes and dependable suppliers provide a sense of security that maintenance and service issues can be managed methodically instead of in crisis mode. PC