Calculate cooling tower capacity, required flow or range, and makeup water from water flow, hot and cold temperatures, cycles, and drift.
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Cooling Tower Capacity Formula
The calculator uses three core formulas, one per mode.
Capacity mode converts water flow and temperature drop into heat load and tower tons.
Q = 500 × GPM × (T_hot − T_cold) Tower Tons = Q / 15,000
Flow or Range mode rearranges the same heat balance to solve for the unknown.
GPM = Q / (500 × Range) Range = Q / (500 × GPM)
Water Use mode estimates evaporation, blowdown, and makeup.
E = 0.00085 × GPM × Range D = GPM × (Drift% / 100) B = E / (Cycles − 1) − D Makeup = E + D + B
- Q: heat load in BTU/hr
- GPM: circulating water flow in gallons per minute
- T_hot: hot water temperature entering the tower (°F)
- T_cold: cold water temperature leaving the tower (°F)
- Range: T_hot − T_cold (°F)
- Approach: T_cold − wet-bulb (°F)
- 500: 8.33 lb/gal × 60 min/hr × 1 BTU/lb·°F for water
- Tower Tons: nominal cooling tower tons at 15,000 BTU/hr each
- E: evaporation rate (gpm)
- D: drift loss (gpm)
- B: blowdown rate (gpm)
- Cycles: cycles of concentration (dimensionless, > 1)
The Capacity tab takes flow and the two water temperatures, computes range, then heat load, then tons. If you add a wet-bulb temperature, it also returns approach. The Flow or Range tab accepts a heat load in BTU/hr, kW, nominal tower tons, or refrigeration tons, then solves for either flow or range using the value you supply for the other variable. The Water Use tab uses the standard 0.00085 evaporation factor and your cycles and drift inputs to estimate makeup water.
Reference Tables
Use these typical values to sanity check inputs before you trust a result.
| Parameter | Typical Range | Notes |
|---|---|---|
| Cooling range | 10 to 20 °F | HVAC design is often 10 °F; process can be higher. |
| Approach | 5 to 10 °F | Below 5 °F sharply increases tower size. |
| Flow per nominal ton | 3 gpm | CTI nominal: 3 gpm at 95/85/78 °F. |
| Cycles of concentration | 3 to 6 | Higher cycles need stronger water treatment. |
| Drift rate | 0.001 to 0.02 % | Modern drift eliminators target ≤ 0.005 %. |
| Heat Load Unit | Equivalent in BTU/hr |
|---|---|
| 1 nominal tower ton | 15,000 BTU/hr |
| 1 refrigeration ton | 12,000 BTU/hr |
| 1 kW | 3,412 BTU/hr |
| 1 boiler horsepower | 33,475 BTU/hr |
Worked Examples
Example 1: Capacity from flow and temperatures. A tower circulates 600 gpm with 95 °F hot water and 85 °F cold water. Range is 10 °F. Q = 500 × 600 × 10 = 3,000,000 BTU/hr. Nominal tower tons = 3,000,000 / 15,000 = 200 tons. With a 78 °F wet-bulb, approach is 85 − 78 = 7 °F, which is a standard selection.
Example 2: Makeup water. Same 600 gpm tower, 10 °F range, 4 cycles, 0.005 % drift. Evaporation = 0.00085 × 600 × 10 = 5.1 gpm. Drift = 600 × 0.00005 = 0.03 gpm. Blowdown = 5.1 / (4 − 1) − 0.03 = 1.67 gpm. Makeup = 5.1 + 0.03 + 1.67 = 6.8 gpm, or about 9,792 gallons per day.
FAQ
Why is a tower ton 15,000 BTU/hr instead of 12,000? A refrigeration ton accounts for evaporator load only. A cooling tower also rejects compressor heat, so the industry uses 15,000 BTU/hr per nominal tower ton at the standard 95/85/78 °F rating.
What if my range is in °C? Pick °C in the unit dropdown. The calculator converts the temperature difference using a factor of 9/5 before applying the BTU/hr equation.
Can I use this for a closed-circuit or dry cooler? The heat balance Q = 500 × GPM × Range still applies to the process water. The water-use formulas do not, since closed loops do not evaporate process water.
Why does blowdown sometimes show as zero? When drift alone removes enough dissolved solids to hold the target cycles, the formula returns a negative blowdown and the calculator floors it at zero.
