Towers provide a means to eject heat through direct evaporation of the cooling water used in the facility. Water cascades down over a "wet deck" through which air is blown. In the process, heat from the water is rejected to the atmosphere through evaporation.
Because the process water comes in direct contact with the atmosphere, impurities can contaminate the water. Impurities already in the water become more concentrated. Regular maintenance, therefore, is very important. Cooling towers in northern climates require that a tank be installed inside the building to eliminate the possibility of water in the tower freezing, damaging the tower.
There are two types of fluid coolers. In both cases, the coolant from the facility is piped through a coil where its heat is ejected to the outside.
Evaporative coolers use water to dissipate the heat. Water is re-circulated in the unit, dripping across the coil containing the coolant. A fan forces fresh air up through the coil. In the process, heat is transferred from the coolant to the circulating water where it is rejected into the atmosphere through evaporation. A small amount of the cooling water is lost during evaporation and must be replenished. A heater may be required to keep the circulating water from freezing during winter. Since the coolant does not come in direct contact with the atmosphere, no contaminants enter the cooling circuit.
A dry air cooler, on the other hand, has finned coils. Here, fans mounted above the coil draw air across the coils removing the heat into the atmosphere. Dry air coolers tend to be less efficient than evaporative coolers so they require larger coils and fans. They tend to operate at higher temperatures and produce less of a temperature reduction.
In both evaporative coolers and dry air coolers, the coolant is in a closed system, so a glycol mix can be used. The coolers, however, have no tank to store the coolant when not in use, so like cooling towers, an indoor tank-pump set or pumping unit is required.