Calculate cooling tower efficiency, makeup water use, and heat load from water temperatures, flow rate, wet-bulb temperature, and cycles.
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Cooling Tower Performance Formulas
The calculator uses three sets of formulas, one per mode.
Performance mode calculates range, approach, and efficiency:
Range = T_hot - T_cold Approach = T_cold - T_wb Efficiency = Range / (T_hot - T_wb) * 100
Water Use mode estimates evaporation, drift, blowdown, and total makeup:
Evaporation (gpm) = 0.00085 * Flow(gpm) * Range(°F) Drift (gpm) = Flow(gpm) * DriftRate%/100 Blowdown (gpm) = Evaporation / (Cycles - 1) - Drift Makeup (gpm) = Evaporation + Drift + Blowdown
Capacity mode calculates the heat rejected by the circulating water:
Heat Load (kW) = 1.163 * Flow(m³/h) * Range(°C) Heat Load (Btu/hr) ≈ 500 * Flow(gpm) * Range(°F) Tons = kW / 3.5169
- T_hot: hot water temperature returning to the tower
- T_cold: cold water temperature leaving the tower basin
- T_wb: ambient wet-bulb temperature
- Range: temperature drop across the tower
- Approach: how close the cold water gets to the wet-bulb limit
- Flow: circulating water flow through the tower
- Cycles: cycles of concentration in the basin water
- DriftRate: percent of flow lost as droplets through the eliminators
Performance mode tells you how well the tower is using the available driving force. Water Use mode breaks down where the makeup water goes. Capacity mode converts flow and range into a heat-rejection number you can match against a chiller or process load.
Reference Tables
Use these values to sanity-check your inputs and your result.
| Efficiency | Approach | Interpretation |
|---|---|---|
| Above 75% | Under 5 °F | Strong performance, near design limit |
| 50–75% | 5–10 °F | Typical operating range |
| Below 50% | Over 10 °F | Fouling, low airflow, or undersized fill |
| Cycles of Concentration | Blowdown vs. Evaporation | Notes |
|---|---|---|
| 2 | 100% | High water and chemical use |
| 3 | 50% | Common minimum target |
| 4 | 33% | Typical for treated systems |
| 6 | 20% | Requires good water quality control |
| 8+ | ~14% | Limited by scaling and corrosion risk |
Example Problems
Example 1: Performance. A tower has hot water at 95 °F, cold water at 85 °F, and a wet-bulb of 78 °F. Range is 10 °F. Approach is 7 °F. Efficiency is 10 / (95 − 78) × 100 = 58.8%. That falls in the typical operating range.
Example 2: Capacity and water use. A 1,000 gpm tower with a 10 °F range rejects about 500 × 1000 × 10 = 5,000,000 Btu/hr, or roughly 417 tons. Evaporation is 0.00085 × 1000 × 10 = 8.5 gpm. At 4 cycles with 0.005% drift, drift is 0.05 gpm and blowdown is 8.5 / 3 − 0.05 ≈ 2.78 gpm. Total makeup is about 11.3 gpm.
FAQ
Why can’t the cold water be colder than the wet-bulb? The wet-bulb temperature is the thermodynamic limit for evaporative cooling. The cold water can approach it but not cross it.
Is the 0.00085 evaporation factor exact? No. It assumes about 1,000 Btu per pound of water evaporated and that essentially all heat rejection is by evaporation. Real towers also lose some heat by sensible transfer, so actual evaporation is slightly lower.
What drift rate should I pick? Modern towers with high-efficiency drift eliminators run around 0.001% to 0.005% of circulating flow. Use 0.02% only for older equipment or worst-case estimates.
How do I increase cycles of concentration? Improve makeup water quality, add chemical treatment for scale and corrosion control, and verify that conductivity-based blowdown is set correctly. Higher cycles cut blowdown but raise dissolved solids in the basin.
