Wet cooling towers or open-circuit Cooling Tower or evaporative cooling towers operate on the principle of evaporative cooling. The working coolant (usually water) is the evaporated fluid, and is exposed to the elements.
Closed circuit cooling towers (also called fluid coolers) pass the working coolant through a large heat exchanger, usually a radiator, upon which clean water is sprayed and a fan-induced draft applied. The resulting heat transfer performance is close to that of a wet cooling tower, with the advantage of protecting the working fluid from environmental exposure and contamination.
Adiabatic cooling towers spray water into the incoming air or onto a cardboard pad to cool the air before it passes over an air-cooled heat exchanger. Adiabatic cooling towers use less water than other cooling towers but do not cool the fluid as close to the wet bulb temperature. Most adiabatic cooling towers are also hybrid cooling towers.
Dry cooling towers (or dry coolers) are closed circuit cooling towers which operate by heat transfer through a heat exchanger that separates the working coolant from ambient air, such as in a radiator, utilizing convective heat transfer. They do not use evaporation and are air-cooled heat exchangers.
Hybrid cooling towers or wet-dry cooling towers are closed circuit cooling towers that can switch between wet or adiabatic and dry operation. This helps balance water and energy savings across a variety of weather conditions. Some hybrid cooling towers can switch between dry, wet, and adiabatic modes. Thermal efficiencies up to 92% have been observed in hybrid cooling towers.