Cooling Tower Calculator (Range, Approach, Efficiency and Make-up Water)
Cooling tower range, approach and efficiency, evaporation, drift and blowdown losses, make-up water from cycles of concentration, and heat rejected in kW and tons of refrigeration.
How it works
Range = T_hot − T_cold · approach = T_cold − T_wet-bulb · efficiency = range ÷ (range + approach) · evaporation E = 0.00085 × Q × range(°F) (= 0.00153 × Q × range in °C) · blowdown B = E ÷ (cycles − 1) − drift · make-up = E + B + drift · heat = ṁ·cp·range
A cooling tower cools water by evaporating a small part of it into the air, so it can never cool below the ambient wet-bulb temperature. The range is how much the water is cooled and the approach is how close it gets to the wet bulb; their ratio is the tower’s thermal efficiency, usually 70–75% for a well-designed tower at design conditions. Roughly 1% of the circulating flow evaporates for every 7 °C (12.5 °F) of range. Because evaporation leaves dissolved solids behind, some water is bled off as blowdown to hold the cycles of concentration; make-up water replaces evaporation, blowdown and drift.
Worked example
A tower cools 1,000 m³/h of water from 37 °C to 32 °C with a 27 °C wet bulb: the range is 5 °C, the approach 5 °C and the efficiency 5 ÷ 10 = 50.0%. Evaporation is 0.00085 × 9 °F × 1,000 = 7.65 m³/h; at 4 cycles of concentration the blowdown is 7.65 ÷ 3 − 0.05 = 2.50 m³/h, so the make-up water is 10.20 m³/h. The tower rejects 5,814 kW (1,653 TR).
Sources: Perry’s Chemical Engineers’ Handbook — cooling tower evaporation (summarized by Engineering ToolBox) · US DOE FEMP — Cooling tower water efficiency best management practice
Frequently asked questions
How do you calculate cooling tower efficiency?
Efficiency = range ÷ (range + approach) × 100, where range = hot − cold water temperature and approach = cold water − wet-bulb temperature. A 5 °C range with a 5 °C approach is 50%.
How much water does a cooling tower evaporate?
About 0.00085 × flow × range in °F (0.00153 × flow × range in °C) — roughly 1% of the circulating flow for every 7 °C of cooling.
What are cycles of concentration?
The ratio of dissolved solids in the circulating water to those in the make-up water. Higher cycles mean less blowdown and make-up water but more scaling risk; most treated towers run 3 to 6 cycles.
Why can’t a cooling tower cool below the wet-bulb temperature?
Evaporative cooling stops when the air is saturated at the water temperature, which is the wet-bulb temperature. Towers are usually designed for a 3–5 °C (5–10 °F) approach; a closer approach needs a much bigger tower.
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This calculator is for educational purposes. Double-check important results before acting on them.