Range tells you how much heat is being rejected; approach tells you how well the tower is actually performing. A cooling tower can only ever approach the ambient wet-bulb temperature, never reach or beat it — that's the real thermodynamic limit this checks against.
Verified against published standard
Water & Ambient Temperatures
Wet-bulb, not dry-bulb — evaporative cooling is limited by wet-bulb temperature, and substituting dry-bulb will meaningfully misstate the tower's real effectiveness.
Enter temperatures to calculate.
Water Balance — Makeup Water Demand
Circulating Water & Cycles of Concentration
Evaporation loss is derived from a genuine heat balance (heat rejected ÷ latent heat), not an empirical correlation. Drift loss and latent heat are typical values for a modern tower with drift eliminators in the ~25–45°C range — both are adjustable; a tower without drift eliminators can drift 10–20× higher.
Enter circulating flow, range, and cycles of concentration to calculate.
About This Calculation
A cooling tower can only ever approach the ambient wet-bulb temperature — never reach or beat it — since evaporative cooling is fundamentally limited by how much moisture the surrounding air can still absorb, which is exactly what wet-bulb temperature measures. Range (how much heat is being rejected) and approach (how close the leaving water gets to that theoretical wet-bulb limit) are genuinely different metrics: range reflects the process heat load and water flow rate, while approach is the real indicator of the tower's own condition and performance.
Frequently Asked Questions
Why can’t a cooling tower ever reach the ambient wet-bulb temperature exactly?
As leaving water temperature approaches wet-bulb, the driving force for further evaporation (the humidity gradient between the water surface and the air) shrinks toward zero — the closer you get, the slower additional cooling happens, which is a diminishing-returns effect that makes reaching true wet-bulb temperature require infinite tower size and contact time.
If range is not a performance indicator, why is it useful at all?
Range directly reflects the process heat load and water flow rate through the tower — it’s a genuinely useful operational number for tracking heat rejection duty, just not a measure of how WELL the tower itself is performing, since the same tower in perfect condition shows different range at different heat loads without that meaning anything changed about the tower.
What does a rising approach at constant range and flow actually indicate?
Since approach measures how close leaving water gets to wet-bulb at a GIVEN heat load and flow, a rising approach with everything else unchanged points to the tower itself degrading — fouled or scaled fill reducing air-water contact surface, poor air distribution, or reduced fan or water flow — not a change in ambient conditions, which are already accounted for by comparing against wet-bulb specifically.
Why use wet-bulb temperature specifically, not the more commonly quoted dry-bulb temperature?
Evaporative cooling capacity is fundamentally set by how much additional moisture the air can absorb, which is what wet-bulb temperature measures directly — dry-bulb (ordinary air temperature) says nothing about humidity, and substituting it for wet-bulb in a cooling tower performance check would meaningfully misstate the tower’s real effectiveness, especially in humid climates.