The fluid to be cooled (usually water) flows through the tubes of the coil without coming into direct contact with the external air, preventing dirt or pollution from entering the primary water circuit. The heat is transmitted from the fluid through the tube walls to the water, which is being continuously sprayed over the coil.
The fan situated at the top of the tower intakes air in counterflow to the water, thereby evaporating a small part of the re-circulating water, drawing off the necessary heat for evaporation and releasing it into the atmosphere. The rest of the water is re-circulated with a spray water pump from the basin to the spray nozzles. (Secondary circuits). A small quantity of heat is transmitted by convection to the external air, just as for an air cooler.