In cooling systems that use cooling towers, hot water (typically 80 to 90°F [27 to 32°C]) is circulated from the condenser/heat exchanger to the tower, where it is cooled by direct means and by evaporation. The tower’s cooling capacity depends on the amount of both air and water flowing through the tower.

Airflow usually is controlled with a fan. When the cooling demand is low, the fan often is stopped, and cooling is achieved only by evaporation as the water circulates. When the cooling demand increases, the fan restarts to provide additional cooling. In some cases, the fan might be speed-controlled.

The water flow through the cooling tower usually is controlled with a single- or three-way valve and a pump (non-controlled). The valve controls the flow through the cooling tower and thus the cooling power according to the temperature of the return water.

In the case of three-way valves, the pump operates with an almost constant flow, regardless of the load in the tower. However, if two-way valves are used, the pump will be throttled when the cooling demand is reduced. A more energy-efficient solution would be to use an intelligent pump, which would reduce power consumption when the system is only partially loaded and would make the control valve superfluous.

An intelligent pump that has a built-in PID controller can be connected directly to a temperature transmitter, thereby allowing the pump to provide closed loop control of the cooling system. It should be noted, though, that to ensure a sufficient function of the nozzles in the cooling tower, a minimum flow is required. If the flow is too low, the nozzles cannot atomize the water satisfactorily. Therefore, the minimum pump speed must be set to ensure that this requirement is always fulfilled.