Speed Drives for Capacity Control

Cooling towers are sized for the maximum anticipated heat rejection. As a consequence, they are 'too big' most of the time. The best way to quell this excess capacity is to manipulate airflow through the tower. Two-speed motors were the answer years ago but today's approach is to apply speed drives. Close capacity control and energy savings are the result.

Keep in mind that reduced water temperatures can be most desirable elsewhere in the system. Low condensing water temperatures, for example, typically lead to even more energy savings at the chiller. But at some point, the cooling tower's leaving water temperature will have to be controlled and variable speed drives are the best way to go.

After the successful application of speed drives to the fan motors, don't expect to apply them to the condenser water pumps as well. This is not a good idea because cooling towers are fitted with spray nozzles that are expected to operate in a fairly narrow flow range. Inadequate flow will cause localized scale buildup on the wet deck and increased freeze potential.

The only time I've seen a speed drive used on a condenser water pump to any advantage was on a small tower that was installed incorrectly with most of its piping overhead. The drive was programmed to 'ramp up' to speed over a period of 10-15 minutes to give the make up valve enough time to keep the basin full of water. It slowed down the same way to lessen the flood of water from the draining piping. Hardly an elegant solution.

Don Davis

Expert Cooling Tower Repair, LLC