Cooling Water Management: Cycles of Concentration and Water Chemistry

What is cycles of concentration in a cooling tower?

An evaporative cooling tower rejects heat by evaporating a fraction of the circulating water. Pure water leaves as vapour, so the dissolved solids left behind concentrate over time. Cycles of concentration (CoC) is the ratio of dissolved-solids concentration in the recirculating water to that in the makeup water — equivalently, the ratio of makeup to blowdown flow.

Because CoC is a ratio of a conserved tracer, conductivity of the recirculating water divided by conductivity of the makeup gives a fast field estimate. Chloride or silica ratios give a more robust value where the makeup already carries treatment chemicals that skew conductivity. The higher the cycles, the less water you bleed off — but the more scale-forming and corrosive the concentrated water becomes, so CoC is the single number a cooling programme is built around.

How do you write the cooling water balance?

Every steady-state cooling system obeys a simple mass balance. Makeup water (M) must replace three losses: evaporation (E), blowdown or bleed (B), and drift (D) — the fine droplets entrained in the exhaust air.

Only the blowdown carries dissolved solids out of the system, so it is the lever that fixes CoC. Drift is normally under 0.02% of circulation on a modern drift eliminator and is often folded into the blowdown term for a conservative estimate. The equation B = E / (CoC − 1) is the single most useful relationship in cooling water management: it shows that blowdown falls sharply as CoC rises, but with diminishing returns beyond about 5–6 cycles.

Worked example: makeup and water saved by raising CoC

Take a tower rejecting 3,500 kW of heat, running with a 5.5 °C range. Work the balance at CoC = 3, then at CoC = 6, and compare.

• Evaporation: E = 1.8 L/h/kW × 3,500 kW = 6,300 L/h = 6.3 m³/h. Evaporation is set by the heat load and does not change with CoC.
• At CoC = 3: blowdown B = E / (CoC − 1) = 6.3 / 2 = 3.15 m³/h. Makeup M = E + B = 6.3 + 3.15 = 9.45 m³/h.
• At CoC = 6: blowdown B = 6.3 / 5 = 1.26 m³/h. Makeup M = 6.3 + 1.26 = 7.56 m³/h.
• Water saved: makeup falls by 9.45 − 7.56 = 1.89 m³/h, and blowdown falls by 3.15 − 1.26 = 1.89 m³/h. Over 8,000 operating hours that is ≈ 15,100 m³/yr of makeup avoided and the same volume of effluent no longer discharged.

Doubling cycles from 3 to 6 cut blowdown by 60% and makeup by 20% for the same cooling duty. That is a large saving in both purchased water and trade-effluent charges — but it doubles the concentration of scale-forming ions, so it is only safe with the right chemical programme. Reusing a treated stream as makeup, discussed under industrial water reuse, compounds the benefit.

How do cycles of concentration change blowdown?

The table below, computed from B = E / (CoC − 1) and M = E × CoC / (CoC − 1) for the worked example (E = 6.3 m³/h), shows how quickly the returns diminish.

Most of the achievable water saving is captured by CoC 5–6. Beyond that, each extra cycle saves little water but sharply raises the risk of scaling and corrosion, so the economic optimum is usually set by water chemistry, not by the balance equation.

What limits how high you can push cycles?

The upper bound on CoC is chemical, not hydraulic. As the water concentrates, sparingly soluble salts — principally calcium carbonate, but also calcium sulphate, silica and calcium phosphate — approach and then exceed saturation. Scaling potential is quantified with stability indices derived from the carbonate equilibrium.

Concentrating the water raises calcium, alkalinity and TDS together, driving LSI positive — so the maximum safe CoC is the point at which the treatment programme can still hold LSI within band. The practical ceiling is set by whichever salt saturates first, commonly CaCO₃ or silica (often capped near 150 mg/L SiO₂ in the recirculating water).

How do you interpret scaling and corrosion indices?

The Langelier and Ryznar indices give a quick read on whether concentrated cooling water will scale or corrode. Use them to set an operating envelope, then confirm with corrosion coupons and deposit monitoring.

Indices signal tendency, not rate, and say nothing about biofouling — so they are one input among several. A system can sit at a benign LSI yet still fail through microbial deposits, which is why a biocide programme is non-negotiable.

What chemical and filtration programmes keep the water in band?

Running high cycles safely depends on a coordinated treatment programme:

• Scale inhibitors / dispersants — phosphonates (e.g. HEDP, PBTC) and polyacrylate/polymaleic copolymers hold calcium salts in suspension above saturation and disperse silt, extending the safe CoC.
• Corrosion inhibitors — molybdate, zinc, phosphate or organic film-formers (tolyltriazole for copper alloys) to keep steel and admiralty brass passive when the water runs mildly corrosive.
• Biocides — an oxidising biocide (chlorine, bromine, or stabilised halogen) as the base programme with periodic non-oxidising doses, controlling biofilm and Legionella. UK operators should follow HSE ACoP L8 / HSG274 for legionella risk.
• Side-stream filtration — filtering 1–5% of the circulation (media, cartridge or self-cleaning screens) removes suspended solids that otherwise accumulate at high cycles and shelter microbes under deposits.
• Alternative makeup — softened, dealkalised, RO-permeate or treated effluent makeup lowers the incoming hardness and alkalinity, letting you cycle higher for the same LSI.

Where water is scarce or discharge is constrained, blowdown itself can be recovered and recycled; at the extreme, a zero liquid discharge treatment train evaporates the concentrated blowdown to a solid and returns distillate as makeup, closing the loop entirely.

How does cooling water management cut a site water footprint?

Evaporative cooling is often the single largest consumptive water use on an industrial site, because the evaporated fraction never returns to the watercourse. Cooling water management attacks the non-evaporative losses — blowdown and drift — which are the only parts you can realistically reduce.

Raising CoC, recovering blowdown, and substituting treated effluent for fresh makeup can each cut freshwater intake by double-digit percentages, as the worked example showed. Because the evaporated fraction is fixed by the heat duty, the correct benchmark for progress is not total water use but the ratio of blowdown to evaporation — a well-managed system pushes that ratio down toward the chemical limit rather than wasting water on excessive bleed. These measures feed directly into a site's industrial water footprint accounting and, where cycles are pushed to their chemical limit, into a wider reuse or ZLD strategy. If you need a programme benchmarked against your makeup chemistry and discharge consent, talk to our process engineers about a water-balance audit.