INTRODUCTION:

In cooling towers, as in any device that utilizes mechanical equipment, there is a need to identify, control and monitor sources of vibration. Our primary source of vibration comes from the axial fan which is used to move air through the tower. In past years, the planning involved was limited to primary modes of vibration reported by the fan manufacturer as it related to the velocity of the fan. This is used to calculate the blade pass frequency and fan selection. Monitoring consists of a mechanical vibration switch that is set up to shut the fan off in the case of excessive vibration. These switches do not monitor vibration as much as protect from a catastrophic failure.

THE CHANGING ENVIRONMENT:

With the introduction of the VFD (variable frequency drives), which allow the fan to be run at any speed rather than the one or two speed fans greater focus on vibration prevention has become important. The modes are now used to select fans where blade pass is considered through the entire speed range of the fan rather than simply near the operating speeds. Along with vibration caused directly by the fan, the greater flexibility in fan speed has caused vibration in the fan stacks. This is the term used for the 6 to 12 foot tall duct that surrounds the fan. These stacks have been reinforced to prevent the vibration caused by varying air velocity.

There is also a revolution underway in vibration monitoring. Many manufacturers are offering devices that do not simply offer protection during failure, but also allow the user to monitor vibration during operation for analysis of equipment wear. These devices are beginning to become available at a price that is competitive to other options. Many customers are choosing these to replace the mechanical switches. With the increase in monitoring we have been able to identify problems sooner and prevent expensive failures from occurring.
Cooling Tower Depot, along with many other manufacturers, are currently working with Cooling Tower Institute (CTI), the entity which publishes standards for design, construction and operation of cooling towers to redefine the standards in cooling tower vibration. There will be recommendations based upon recent changes in technology included in upcoming releases of their standards and guidelines.

CONCLUSION:

All cooling tower manufacturers will need to increase their focus on vibration as new technology becomes more widely available and is implemented. Cooling Tower Depot is working hard to ensure we are designing and building our towers to fit the needs of customers now and for years to come.

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