Care is needed for cooling tower fans

The main safety issue is with the fan blades themselves coming out of the fan: large projectiles can be sent many metres across the plant area. In one incident, a solid aluminium fan blade escaped from the plenum chamber and flew across a plant. The cause was down to the fan blades catching on the fan cowl and the fan blades then having a fatigue failure due to the overload.

In another accident, corrosion from the cooling water and atmospheric conditions caused pitting corrosion of the sheet aluminium cooling tower fan blade that then failed by fatigue. The failure resulted in the fan blade being ejected from the cooling tower.

In both cases, a "simple" piece of machinery posed a significant safety risk from a flying missile that could have caused serious injury, or worse. Taking care of these types of fans is, therefore, needed to prevent unscheduled breakdowns as well as to improve plant safety.

The fans are usually arranged in banks, so if one fan fails catastrophically, neighbouring fans can be damaged. In one known instance, five fans were put out of action in a bank of 20 following a bad failure on a single fan.

The starting point for reliable operation is with design. Many years ago, heavy steel couplings were commonly used on cooling tower fans. The usual design arrangement of the fan employs a central gearbox driven by a motor outside the fan chamber.

In this design, the coupling shaft needs to be very long, which is good for coping with misalignment, but the steel coupling itself requires a central bearing - typically a plumber block - otherwise the coupling shaft suffers from critical speed vibration problems.

A grease-lubricated bearing in the middle of the flume, where it is always moist and blowing a gale from the fan, is not a recipe for reliability. Failure of intermediate bearings can cause the coupling to fail and the coupling shaft to smash around inside the fan chamber, with an obvious risk of severe blade failure. Fortunately, single-piece carbon fibre couplings have been developed that are light and stiff enough not to need the intermediate bearing, largely eliminating this problem.

Cooling tower fan blades were originally made of wood, with issues of rot and natural deterioration. GRP blades are now widely used. Aluminium blades are still offered, but have disadvantages over GRP, such as poor fatigue resistance, which can lead to cracking and parts of blades coming out of the fan chamber. With correct water droplet protection, GRP blades can give long service and because of the ability to form a correct aerofoil, an improvement in air flow can also be achieved.

Where aluminium construction comes into its own is for fin fans. Here the atmosphere is dry and there is little risk of corrosion-induced fatigue. Sheet aluminium aerofoils can give good service and, due to their low weight, put much less stress on the bearings and support framework. Belt drives also benefit from the low inertia of aluminium fans, with less slip at start-up and as the fan runs up to full speed. Another advantage of sheet aluminium in fin fans is that, in the event of failure, the light material can be contained more easily within the plenum chamber and not thrown out as a missile.

Maintenance of cooling tower fans can be complicated due to difficult access issues. Monitoring of the motor outside the cooling tower flume is easy, but not so the central gearbox. During normal operation the gearbox cannot be seen or touched, so routine condition monitoring must be done by fitting a remote accelerometer onto the gearbox. Even so, cooling towers are needed most during the summer months and failures can happen within a few weeks of detecting a problem. A routine winter service routine on each of the cooling tower cells, therefore, does not interrupt plant production and can ensure smooth running during the summer.

Fin fans, in contrast, can at least be seen when they are running and on-line. Static checks can help diagnose problems with any loosening parts of the structure, as well as aid monitoring of the condition of the drive belts. A new EEMUA Guide (see panel, p37) provides a suggested checklist for both cooling towers and fin fans that has been built up from operational experience.

Operation of cooling towers is reasonably straightforward, but fin fans create much more of a debate. One operational tactic is to apply water to the fin fan in the summer, with the belief that the evaporative cooling from the fan bank will improve the duty period. Although some short-term success may be had from this tactic, the real issue is more likely to be dirt blocking the air flow between the tubes. All that the water does is to attract any passing debris, which then adheres to the tube bank until hardly any air gets through at all. Only efficient winter-time washing by a specialist contractor will clean the fan bank without damaging the fins and enable the cooler to work effectively when it is really needed.

Cooling tower fans and fin fans are often regarded as "ancillaries" that are there to make the real equipment work, but when they fail to perform, the results can be just as dramatic as major process equipment problems. The failures described above are few and far between, but good care and attention can help to prevent them happening and ensure reliable and effective operation.