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Cooling water is lost from an open recirculating system primarily through evaporation (typically 1–3% of circulating flow per day) and drift (aerosol carryover from cooling tower, typically 0.001–0.002% of flow). As water evaporates, dissolved solids remain, concentrating the chemistry relative to the make-up water. The ratio of dissolved solids in the circulating water to those in the make-up is the cycles of concentration (CoC). If CoC is not controlled by blowdown (intentional discharge of concentrated water and replacement with fresh make-up), dissolved solids accumulate indefinitely, eventually causing precipitation of calcium carbonate, sulphate, or silica scale on heat-exchanger surfaces.
The Langelier Saturation Index (LSI) predicts whether a water is likely to precipitate (scale) or dissolve (corrode) calcium carbonate: LSI = pH_measured − pH_sat, where pH_sat = pCa + pAlk + C (temperature-dependent constant). LSI > +0.5 indicates scaling tendency; LSI < −0.5 indicates corrosive tendency. Cooling-water chemical treatment targets LSI in the range −0.5 to +0.5 by controlling CoC (blowdown rate), pH, and dosing antiscalants and corrosion inhibitors. Destratification plays a supporting role by homogenising temperature, which reduces localised scale precipitation in warm zones.
Blowdown rate calculation: Blowdown (m³/h) = Evaporation rate (m³/h) / (CoC − 1). For a 1,000 m³/h recirculating system with 2% evaporation loss (20 m³/h) and target CoC = 4: Blowdown = 20 / (4−1) = 6.67 m³/h. Make-up = 20 + 6.67 = 26.67 m³/h. Increasing CoC from 4 to 5 reduces blowdown to 5 m³/h and make-up to 25 m³/h — a 6% saving in water consumption.
| Parameter | Typical Make-Up | At CoC = 3 | At CoC = 5 | Treatment Target |
|---|---|---|---|---|
| Total hardness (mg/L as CaCO₃) | 150 | 450 | 750 | < 600 (antiscalant); < 400 preferred |
| Chloride (mg/L) | 30 | 90 | 150 | < 200 (mild steel); < 500 (SS304) |
| Sulphate (mg/L) | 50 | 150 | 250 | < 500 (Cl + SO₄ limit depends on inhibitor) |
| Silica (mg/L SiO₂) | 10 | 30 | 50 | < 150 (below amorphous silica saturation at pH 8.5) |
| pH (circulating water) | 7.5 (make-up) | 8.0–8.5 (CO₂ stripping) | 8.5–9.0 | 7.5–9.0 (programme-specific) |
| TDS (mg/L) | 400 | 1,200 | 2,000 | < 3,000 (system-specific) |
Characterise make-up water quality (seasonal samples: spring and summer minimum): hardness, alkalinity, chloride, sulphate, silica, TDS, conductivity, pH. These data establish the maximum achievable CoC before chemical limits are exceeded. UK mains water is typically 200–500 mg/L TDS; borehole may be 500–2,000 mg/L.
Determine maximum CoC for each limiting ion: CoC_max = (limit concentration) / (make-up concentration). The governing ion (lowest CoC_max) sets the system maximum. Design blowdown programme to maintain CoC 10–15% below this maximum. Automatic conductivity controller with motorised blowdown valve provides the most precise CoC control.
Calculate LSI at target CoC and operating temperature range. If LSI > +0.5 without treatment: dose phosphonate antiscalant at 5–10 mg/L to suppress CaCO₃ precipitation. If LSI < −0.5: apply tolyltriazole or benzotriazole corrosion inhibitor for copper alloys; adjust pH upward. Confirm chemical programme by laboratory evaluation at CoC extremes.
Install online conductivity probe at cooling tower basin. Automated PLC: when conductivity exceeds setpoint (corresponding to target CoC), open blowdown valve for timed duration. Log conductivity and blowdown events continuously. Compare measured CoC against theoretical (TDS-based) monthly to verify conductivity probe calibration and blowdown valve operation.
Cooling-water blowdown may require Environmental Permit (EP) under the Water Resources Act 1991 / Environmental Permitting Regulations 2016 if discharged to surface water. Check EP conditions for temperature, pH, suspended solids, and metals limits. Biocide residuals (free chlorine) must be within EP limits before discharge — neutralise with sodium thiosulphate if free Cl₂ > 0.1 mg/L at discharge.
Calculate annual water consumption: make-up volume (m³/yr), blowdown volume (m³/yr), evaporation (m³/yr). Report against water efficiency targets (Water Abstraction Licence conditions). Review chemical spend per m³ of make-up water. Evaluate whether increasing CoC target (with upgraded antiscalant) would reduce water cost more than it increases chemical requirement — typically CoC 4→5 saves money in high-cost water regions.
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