In the tower the warmed water is sprayed from above over the fill, and the fan draws air from below or from the side. A small part of the water evaporates and takes its heat of evaporation from the remaining water; the water cools, falls into the basin and goes back to the condenser by pump. Cooling by evaporation depends not on the dry-bulb temperature of the air but on the wet-bulb temperature; that is why a tower gives a lower condensing temperature than an air-cooled condenser. It is common with large-capacity chillers and in industrial plants.
The water leaving the tower can be cooled to within 3–5 K above the wet-bulb temperature; this difference is called the approach. On a summer day when the wet bulb is 22 °C the water returns at 26–27 °C and the condensing temperature is around 35 °C; on the same day, in dry air at 36 °C, an air-cooled condenser condenses at 46–50 °C. The difference between them means 25–35% in compressor consumption. The cooling range of the water in the tower is usually 5 K. In selecting a tower the design wet-bulb temperature of the location is taken as the basis.
What it costs in return is water consumption. For every 1 kW of heat rejected, about 1.5 litres of water an hour evaporate. Because the water that evaporates is pure, lime and salts build up in the basin; to limit the concentration part of the water is drained continuously (blowdown). Total consumption comes to 1.3–2 times the evaporation: a tower rejecting 500 kW of heat needs 25–35 m³ of water a day at full load. Water softening, chemical treatment and automatic blowdown based on conductivity are required.
In an open tower the condenser water is in direct contact with the air; in a closed-circuit tower the water passes through a coil and stays clean. An evaporative condenser, for its part, condenses the refrigerant directly in the coil and does away with the water circuit in between. Maintenance cannot be neglected: warm, standing water is favourable to the growth of legionella bacteria. The basin is cleaned regularly, biocide is applied and the drift eliminators are kept in good condition. Against freezing in winter a basin heater or draining is provided. The cost of water and maintenance is assessed together with the energy gain.