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Cooling tower calculator

Enter flow, water temperatures and wet bulb to see how much heat the tower rejects, how much water evaporates and how much makeup it needs. Compare annual water cost at two levels of cycles of concentration.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Circulating water
Water balance
Costs

Treatment per unit of makeup; disposal per unit of blowdown sent to sewer or treatment.

—Tower makeup water
—

Makeup and blowdown versus cycles of concentration

Evaporation does not change with cycles; blowdown falls fast up to about 5 cycles and slowly after that.

Tap the chart to see the values.

Annual water cost

Makeup, chemical treatment and blowdown disposal at current and proposed cycles.

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Method, assumptions and references

Equations used

Range: R = Thot − Tcold; approach: A = Tcold − Twb; effectiveness: ε = R ÷ (R + A)
Heat rejected: Q = ṁ · (h(Thot) − h(Tcold)), water enthalpies from IAPWS
Evaporation, rule of thumb: E = 0.00085 · Qc · R [gpm, °F]; energy balance: E = Q ÷ hfg, hfg ≈ 2501 − 2.37 · Tmean [kJ/kg]
Drift: D = Qc × drift %; blowdown: B = E ÷ (C − 1) − D; makeup: M = E + B + D = E · C ÷ (C − 1)

Assumptions and limits

  • The rule of thumb assumes about 85% of the heat leaves by evaporation and the rest as sensible heat to the air; the energy balance assumes all heat is latent and gives the upper bound.
  • Cycles of concentration defined by the dissolved solids (or conductivity) ratio between basin and makeup water; no leaks or other losses.
  • The page does not check the tower’s thermal size (Merkel number, performance curves); use the manufacturer’s selection at the site design wet bulb for that.

Standards and technical documents

  • CTI ATC-105: Acceptance Test Code for Water Cooling Towers
  • CTI STD-201: Standard for the Certification of Water Cooling Tower Thermal Performance
  • ASHRAE Handbook — HVAC Systems and Equipment, chapter on cooling towers
  • Perry’s Chemical Engineers’ Handbook, section on cooling towers (evaporation estimate)
  • ASHRAE Standard 188: Legionellosis — Risk Management for Building Water Systems

U.S. regulations

  • EPA Clean Water Act, NPDES permits (40 CFR Part 122) and local pretreatment rules for blowdown discharge
  • OSHA 29 CFR 1910.1200: hazard communication for treatment chemicals; 1910.28 for fall protection on towers
  • State and local Legionella rules for cooling towers (for example, New York City Rules Title 24, Chapter 8), often referencing ASHRAE 188

Frequently asked questions

How much water does a cooling tower evaporate?

About 0.085% of circulating flow per °F of range (roughly 1% per 12 °F). A 2,000 gpm tower with a 10 °F range evaporates about 17 gpm.

How do I calculate cooling tower blowdown?

Blowdown = evaporation ÷ (cycles − 1) − drift. With 17 gpm evaporation and 5 cycles, blowdown is about 4.2 gpm and makeup about 21 gpm.

What are approach and range?

Range is how much the water cools in the tower; approach is how far cold water stays above wet bulb. With 95 °F to 85 °F water and 78 °F wet bulb, range is 10 °F and approach 7 °F.

Is it worth raising cycles of concentration?

Up to about 5 to 6 cycles the water savings are large; beyond that each extra cycle saves little and raises scaling risk. In the example, going from 5 to 7 cycles saves about 1.4 gpm of makeup.