Cooling Tower Calculations

Range, approach and effectiveness

Range is the cooling the tower does (hot water minus cold water temperature); approach is how close the cold water gets to the inlet wet-bulb temperature. Effectiveness = range / (range + approach). A tower whose approach has crept up at the same load and wet-bulb is losing capability.

Water balance: evaporation, blowdown and makeup

E ≈ 0.00153 × range(°C) × circulation B = E/(N − 1) − D M = E + B + D

E is evaporation, B blowdown, D drift, M makeup and N the cycles of concentration. The evaporation factor (about 0.15 % of circulation per °C of range) varies with climate; the PlantCalcKit tool offers the Perry, Marley and GPSA forms or your own. Example. 5000 m³/h circulation, 10 °C range, 5 cycles, drift neglected: E = 76.5 m³/h, B = 76.5/4 = 19.1 m³/h, M = 95.6 m³/h. Raising cycles from 5 to 7 cuts blowdown to 12.8 m³/h — the water balance calculator puts a yearly cost on that.

Cycles of concentration and water chemistry

Cycles are the ratio of a dissolved species in the circulating water to the same species in the makeup — conductivity, chloride or silica. How high you can go is set by scaling and corrosion: the Langelier saturation index LSI = pH − pH_s and the Ryznar stability index RSI = 2 pH_s − pH screen for calcium carbonate scale; silica, calcium sulfate and the Larson–Skold index for chloride and sulfate corrosion set further limits.

Performance testing

A cooling tower test compares the water flow the tower can cool, at the test conditions, with the design flow — the tower capability, using the maker's performance curves and a fan-power correction (the method of CTI ATC-105). A capability below about 95 % usually points to fill, distribution or air-flow problems.

Thermal rating by the Merkel method

KaV/L = ∫ c_p dT / (h_w − h_a)

The Merkel integral gives the tower characteristic KaV/L from the water temperatures and the air enthalpy line. Fitting it through the design point lets you predict the cold water temperature at any wet-bulb, flow, heat load or fan power — and compare fill or fan upgrades.

Calculators for this

Free calculators

Water balance — Evaporation, blowdown, makeup and yearly water cost — and what higher cycles would save.
Performance test — Tower capability from three curve readings, with the fan-power correction.
Water chemistry — LSI, RSI, PSI, Larson-Skold and the screening maximum cycles for your makeup water.
Thermal rating — Predicted cold water at any wet-bulb, flow, heat load or fan power (Merkel method).
Psychrometric calculator — Relative humidity, wet bulb, dew point, humidity ratio and enthalpy of moist air at any altitude (ASHRAE).
Chemical dosing rate — ppm to L/h and mL/min, pump stroke, drums a month and yearly cost — once-through or cooling tower dosing.

Online tools and Excel tools

Cooling Tower Water Balance — Evaporation, drift, blowdown, makeup and water cost for open recirculating towers — three cases side by side. Use online · Excel tool, USD 29
Cooling Tower Performance Test — Tower capability from a field test by the performance-curve method, with cell-by-cell results and screening diagnostics. Use online · Excel tool, USD 49
Cooling Water Chemistry and Cycles — Scaling and corrosion indices, safe cycles, acid feed and the full cost of your treatment programme. Use online · Excel tool, USD 49
Cooling Tower Thermal Rating and Upgrade — Predict cold water for any weather, flow, load or fan condition — and compare fill, fan and extra-cell upgrades with payback. Use online · Excel tool, USD 49

Field guide

Cooling Towers Made Simple — A 121-page plain-language field guide for operators and young engineers: how towers work, monitoring, testing, troubleshooting and cooling water chemistry.

Questions

How do I calculate cooling tower evaporation?

A common estimate is about 0.15 % of the circulating flow for every °C of range (0.085 % per °F). It varies with climate, so the PlantCalcKit tool lets you choose the method.

How do I calculate blowdown?

B = E/(N − 1) − D, where E is evaporation, N the cycles of concentration and D the drift loss.

What are cycles of concentration?

The ratio of the concentration of a dissolved species (conductivity, chloride, silica) in the circulating water to its concentration in the makeup water.

More calculation guides

These guides and calculators give engineering estimates for screening, troubleshooting and training. Check important decisions against site data, your equipment maker and your plant's approved procedures.