Cooling Tower Evaporation Calculator
Calculate water loss due to evaporation based on circulation rate and temperature differential.
Imperial (GPM, °F)
Metric (m³/h, °C)
Typical range: 3.0 – 6.0
Error: Inlet temperature must be higher than Outlet temperature.
Calculation Results
Temperature Range (°F)
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Evaporation Rate (GPM)
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Blowdown/Bleed Rate (GPM)
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Total Make-up Water Required (GPM)
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Percent Evaporation
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Understanding Cooling Tower Evaporation
Evaporation is the primary method by which open recirculating cooling towers dissipate heat. As a small portion of the circulating water evaporates, it removes latent heat from the remaining water, lowering its temperature. While this process is efficient, it represents a significant consumption of water that must be continuously replaced.
How is Evaporation Rate Calculated?
The rate of evaporation in a cooling tower is determined primarily by the amount of heat that needs to be rejected. The standard industry calculation relies on the recirculation flow rate and the temperature drop (Range) across the tower.
The formulas used in this calculator are:
1. Determine the Range (ΔT)
The "Range" is the difference between the hot water entering the tower and the cold water leaving it.
- Formula: Range = Tinlet – Toutlet
2. Calculate Evaporation (E)
Industry rules of thumb suggest that for every 10°F (5.56°C) drop in water temperature, approximately 0.85% to 1% of the recirculation water flow evaporates. The precise factors used are:
- Imperial (GPM, °F): E = Flow Rate × Range × 0.00085
- Metric (m³/h, °C): E = Flow Rate × Range × 0.00153
Note: The factor 0.00085 is derived from the latent heat of vaporization of water (approx 1000 BTU/lb).
Water Balance Components
Managing a cooling tower requires understanding the complete water balance, not just evaporation:
- Make-up Water (M): The total fresh water added to the system. M = E + B + D (Drift).
- Blowdown (B): Water intentionally drained to prevent mineral concentration buildup.
- Cycles of Concentration (COC): The ratio of dissolved solids in the tower water to the makeup water. Higher cycles reduce water waste but increase scaling risk.
Why This Matters
Accurate calculation of evaporation rates is essential for sizing make-up water piping, estimating operational costs, and ensuring chemical treatment programs are dosed correctly. An under-sized make-up line can lead to tower basins running dry, causing pump cavitation and system shutdown.