The evaporation of water takes heat from the air; the total energy of the air does not change, but its temperature falls and its humidity rises. The word adiabatic describes this change, which takes place without any exchange of heat with the outside. Wet cellulose pads placed at the air inlet of the condenser, or nozzles spraying a fine water mist, cool the air before it reaches the coil surface. In the pad arrangement the condenser itself stays dry; this is what distinguishes it from a cooling tower. The water evaporates not on the hot metal surface but in the air; the fins do not scale up and do not corrode.
The air can be cooled down to the wet-bulb temperature at most; pad systems achieve 70–90% of this difference. In a dry climate the gain is large: at a temperature of 36 °C and 20% relative humidity the wet bulb is around 19 °C and the air leaves the pad at 22–24 °C. The condensing temperature falls by 10–12 K and compressor consumption by 20–30%. In a humid coastal climate the same system gains only 3–5 K. The gain is highest in the hottest hours, that is, at exactly the moment it is needed.
The system supplies water only when the outside air exceeds a certain temperature (usually 25–28 °C); for most of the year the condenser runs dry. That is why annual water consumption is a small fraction of that of a cooling tower, and because there is no standing warm water the hygiene risk is low. In regions with hot, dry summers such as Central Anatolia it is effective especially on transcritical CO₂ gas coolers and on condensing units that lose capacity because of high condensing temperature. It can also be retrofitted to existing condensers.
Water quality is decisive. In arrangements where the mist is sprayed directly onto the fins, hard water coats the fins within a few seasons and permanently weakens the condenser; in these arrangements softened or reverse osmosis water and fins coated against corrosion are used. In pad systems the scale stays in the pad, and the pad is replaced every few years. At the end of the season the lines are drained. Because the pad creates air resistance even when dry, the fan selection is made accordingly. A filter is fitted in the water line and the nozzles are cleaned regularly.