Open-system cooling towers reject waste heat from the steam cycle by exposing the cooling water directly to the atmosphere. The majority of heat removed is due to evaporation and the remaining cooled water drops into a collection basin. Level control applications include a high level switch to avoid overflow conditions in the cooling tower basin. In a once-through cooling system, the water intake structure is often a vertical wet pit pump which requires high and low level sensing and possible pump control.
Water infeed and basin levels of the cooling tower require level sensing and control. In frigid climates, a level switch can work in tandem with a resistance heater to protect standing water in the cooling tower basin against freezing.