Cooling Tower Water Use: Cycles of Concentration Explained

A cooling tower is a giant evaporator. Every kW of heat it rejects boils off about 1.2 kg of water. Multiply that by 6,000 operating hours and you get numbers most site managers do not want to see. The single biggest lever to reduce that water use — often by half — is cycles of concentration (COC). This guide covers the math and the practical limits.

1. The three components of makeup water

Makeup = Evaporation + Blowdown + Drift

Evaporation

A fixed function of the heat rejection duty. For open recirculating towers:

E (GPM) ≈ 0.001 × Recirc(GPM) × ΔT (°F)

The factor 0.001 assumes ~1,000 Btu/lb latent heat and water density near 8.34 lb/gal. It is remarkably consistent across tower brands and climates.

Blowdown

Evaporation removes pure water and leaves dissolved solids behind. Left unchecked, TDS climbs until scale forms. Blowdown deliberately dumps some concentrated water and refills with fresh:

B = E / (COC − 1)

Where COC = TDS_blowdown / TDS_makeup. At COC = 2, blowdown equals evaporation. At COC = 6, blowdown is only 20% of evaporation. That is the entire game.

Drift

Water droplets carried out with the exiting air. Modern drift eliminators achieve 0.001–0.005% of recirculating flow. Older towers can lose 0.1% — a real number in humidity-sensitive plants.

2. Why COC matters so much

Consider a 1,000 GPM tower with 10°F range operating 6,000 hr/yr:

COC	Blowdown	Makeup	Annual water
2	10.0 GPM	20.2 GPM	7.3 M gal
4	3.3 GPM	13.5 GPM	4.9 M gal
6	2.0 GPM	12.2 GPM	4.4 M gal
8	1.4 GPM	11.6 GPM	4.2 M gal

Moving from COC 2 to COC 4 saves 2.4 million gallons per year. Going further to COC 6 saves another 0.5 M — diminishing returns kick in fast.

3. The treatment limits you cannot ignore

COC is capped by whichever species will scale first:

• Calcium hardness — CaCO₃ scale forms above LSI ≈ +0.5.
• Silica — precipitates above ~150 ppm as SiO₂.
• Sulfate — CaSO₄ above ~1,500 ppm.
• Chloride — corrosion accelerates above ~500 ppm on carbon steel.

Calculate the ratio: max allowable / makeup concentration. The lowest ratio is your maximum COC. Push past it and you trade water savings for cleaning bills.

4. Getting to high COC

Most plants stuck at COC = 3 can safely reach COC = 5+ with modest changes:

1. Softened makeup — remove hardness before it enters the tower.
2. Acid feed — controls alkalinity, prevents CaCO₃.
3. Side-stream filtration — removes suspended solids that concentrate.
4. Real-time TDS/conductivity control — replaces timed blowdown, keeps COC near the limit.

Each of these has payback under 2 years at typical water and sewer rates.

5. Drift is often forgotten

Drift matters for two reasons: it is legionella-relevant, and old eliminators can hide massive water loss. If your tower predates 2000 and has no efficiency label, assume 0.05–0.1% drift and budget replacement. New eliminators are inexpensive and pay back through both water and chemical savings.

Run the numbers

Use the Cooling Tower Makeup Water Calculator to model your tower at different COC values, then check whether your makeup water chemistry allows it. For steam-side condensate recovery — the other high-ROI water lever — see the Condensate Return Calculator.