Calculate range, approach, and cooling tower efficiency from hot water, cold water, and wet bulb temperatures.
Enter the hot water inlet, cold water outlet, and ambient wet bulb temperatures.
Enter temperatures and hit calculate
A cooling tower works by rejecting heat from a process, typically a chiller condenser, an industrial process, or a power plant condenser, into the atmosphere by evaporating a portion of the circulating water. The performance of a cooling tower is judged not by how cold it makes the water in absolute terms, but by how close it gets the outgoing cold water temperature to the theoretical best possible limit, which is the ambient wet bulb temperature. This is why cooling tower efficiency is always expressed relative to wet bulb temperature rather than dry bulb (ordinary air) temperature — the wet bulb reading already accounts for the cooling effect of evaporation, which is exactly the mechanism a cooling tower relies on.
Two terms describe how a cooling tower is performing. The range is simply the temperature drop the tower achieves: the difference between the hot water entering the tower and the cold water leaving it. A larger range generally means the tower is rejecting more heat. The approach is the difference between the cold water leaving the tower and the wet bulb temperature of the surrounding air, and this is the figure that really tells you how close the tower is running to its theoretical limit. A smaller approach means a more efficient tower, since it means the outgoing water temperature is very close to the lowest temperature physically achievable given the current weather conditions; a larger approach signals fouled fill media, poor air distribution, undersized fans, or a tower that is simply oversized or undersized for the current heat load.
Cooling tower efficiency combines the range and the approach into a single percentage: it expresses the actual temperature drop achieved as a fraction of the maximum theoretically possible temperature drop, which would occur if the cold water left the tower at exactly the wet bulb temperature. This calculation is used constantly by facility and HVAC engineers to benchmark how a tower is performing against its design specification, to decide when fill media needs cleaning or replacement, and to verify that a tower is correctly sized for the building's actual cooling load, especially during commissioning or after a maintenance shutdown.
Worked example: Suppose a cooling tower receives hot water at 38°C and returns it cooled to 30°C, while the ambient wet bulb temperature on that day is 27°C. First, the range: Range = 38 − 30 = 8°C. Next, the approach: Approach = 30 − 27 = 3°C. Applying the efficiency formula: Efficiency = 8 ÷ (8 + 3) × 100 = 8 ÷ 11 × 100 ≈ 72.7%. This tells the engineer the tower is currently operating at roughly 73% of its theoretical maximum cooling potential for the prevailing weather conditions, a reasonable but improvable figure, since well-maintained towers with clean fill and properly functioning fans commonly achieve efficiencies in the 80–90% range under similar wet bulb conditions.
Because a cooling tower cools water primarily through evaporation, and the wet bulb temperature is the lowest temperature air can reach through evaporative cooling alone. It represents the true physical limit the tower is working toward, unlike dry bulb temperature which ignores humidity.
A smaller approach is better. Well-designed and well-maintained towers commonly run with an approach in the 3–5°C range, while a rising approach over time, without any change in weather, usually points to fouled fill media, reduced airflow, or a fan or pump issue.
Not necessarily. Range tells you how much heat the tower is rejecting, which depends on the process load, not just tower performance. Approach is the better indicator of how close the tower is running to its theoretical best for the current weather, which is why efficiency combines both terms together.
Because wet bulb temperature itself changes with ambient humidity and dry bulb temperature. The same tower running the same load can show a different efficiency percentage on a humid summer day versus a dry winter day, simply because the theoretical limit it's being measured against has shifted.
This calculator uses the standard range/approach/efficiency formulas taught in HVAC and mechanical engineering coursework. For commissioning reports, warranty claims, or performance guarantees, always cross-check readings against calibrated instruments and, where required, have results reviewed by a qualified engineer.