Range, approach and heat
Heat rejected equals mass flow times the specific heat of water times the range, the drop from entering to leaving water temperature: Q = m x cp x (T in minus T out). The gap between leaving water and the outdoor wet bulb is called the approach.
This teaching model holds the approach fixed at 4 °C, a common design value, so cold water always leaves 4 °C above wet bulb. In a real tower the approach grows when load rises or the fan slows.
Try it: keep flow at 100 L/s and water entering at 35 °C, then raise wet bulb from 18 to 26 °C. Heat rejected falls from about 5,400 kW to under 2,100 kW because the water cannot get as cold.
Where the water goes
Evaporating one kilogram of water absorbs about 2,400 kJ, so the evaporation readout divides heat rejected by that value. Towers also lose water to blowdown, which flushes out minerals that concentrate as water evaporates, and federal water guidance focuses on managing that.