Range and approach. A cooling tower's water temperature drop, and how close its cold water gets to the entering wet bulb; the approach can never be zero. The tower model is rated from one point.
A cooling tower's range is the temperature drop of its water, from hot inlet to cold outlet; its approach is how close the cold water gets to the entering air's wet-bulb temperature. A tower is rated at one point — 35 °C in, 29.4 °C out, at a 25.6 °C wet bulb is a common one, a range of 5.6 K and an approach of 3.9 K — and the approach is the harder number: it shrinks with less load or more air, grows on a humid day, and can never reach zero, since that would take an infinitely large tower. A rating with the cold water at or below the wet bulb is not a rating.
The same word is used of any heat exchanger — a condenser's approach is its condensing temperature over the entering air, a coil's the leaving fluid over the surface — and it is the number a designer buys with surface area. This model's tower takes the certified rating point and derives the tower's transfer capability from it once, then follows the water loading with a fitted exponent held against two makers' published performance tables; a free-cooling plant stages and cycles its tower fans to make the least air that meets the approach the plate exchanger needs.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year · Superheat · Subcooling · Sensible heat ratio (SHR) · Bypass factor · Wet-bulb temperature · Dew point · COP and EER · Compressor map · Evaporating and condensing temperature · Heat-recovery (three-pipe) VRF · Typical meteorological year (TMY)