Is a cooling-tower leaving-water temperature of 30°C always the same operating state? Staring at that number alone can treat a normal condition as a fault—or keep fans at high frequency just to chase a few more degrees. Leaving-water temperature and approach are not an either-or choice: the former is the control objective; the latter helps relate that objective to weather. Neither by itself proves the plant is saving energy. Real savings show up in the system total under the same boundary.
What approach temperature actually tells you
Approach is leaving-water temperature minus the wet-bulb temperature of air entering the tower. Use the wet bulb of air the tower actually draws in—not a phone weather app’s dry-bulb air temperature.
Suppose leaving water is 30°C in both cases, while entering wet bulb is 27°C and 24°C: approach is 3°C and 6°C. That only says how close water is to current air conditions. It does not mean 3°C is always better or more efficient.
Tower load, water flow, fan speed, and the number of cells in service all move that difference. Comparing approach under unequal conditions can mistake operating differences for equipment performance. Metering also matters: if the wet-bulb sensor does not see true inlet air, or leaving water is measured on a mixed header, the calculated approach may not represent a single cell. Before comparing, clarify whether points refer to one tower or the whole bank.
Control still needs a water-temperature target
One common approach is: take entering wet bulb, add a target approach, form a candidate leaving-water setpoint, then modulate fans from measured leaving water. For example, with a 4°C target approach, wet bulb 27°C yields a 31°C candidate; wet bulb 24°C yields 28°C. The 4°C figure only illustrates the algorithm—it is not a universal recommendation.
Candidate targets still need checks against chiller allowable water temperature, tower capability, flow, and site op