The fluid to be cooled (usually water or glycol 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 – from the fluid is conducted through the tube walls to the water, which is sprayed over the coil.
The fan at the base of the tower forces air in counterflow to the water, causing a small proportion of the spray water to be evaporated. The evaporative process cools the remaining water causing heat from the coil surface to be transferred from the spray water to the atmospheric air passing through the tower. The remaining water is re-circulated by the spray water pump from the basin to the spray nozzles. (Secondary circuits). A small quantity of heat is also transmitted by sensible heat exchange to the atmospheric air in a similar manner to an air cooler..