Water Treatment
Cooling Tower Water Chemistry: From Readings to Reliability
By Ghattas Banoub · 8 min read
Cooling water chemistry is easy to treat as a list of numbers on a test report. In a critical facility it's much more than that — every reading is an early warning about what will happen to the condenser, the chiller and, eventually, the load it protects.
Water chemistry → Scale / Corrosion / Biological growth → Heat transfer ↓ → Condenser performance ↓ → Chiller kW ↑ → Reliability ↓
Why the water changes
A cooling tower rejects heat by evaporating water. The water leaves as vapor, but the dissolved minerals stay behind — so the remaining water gets more concentrated every hour. Two flows keep that in balance:
- Make-up water replaces what evaporates, drifts and is drained.
- Blowdown drains concentrated water on purpose, so fresh make-up can dilute it.
The parameters that matter
| Parameter | What it tells you | Main risk |
|---|
| Conductivity / TDS | How concentrated the dissolved minerals are | Scale and deposits when high; wasted water and chemicals when low |
| pH | Acid / alkaline balance | Corrosion when too low; scaling tendency when too high |
| Total & calcium hardness | Scale-forming minerals | Calcium carbonate scale on condenser tubes |
| Alkalinity | Buffering capacity; works with calcium to form scale | Scale |
| Chlorides | Aggressive ions | Localized / pitting corrosion |
| Dissolved oxygen | Oxygen available to react with metal | Corrosion |
| Iron & copper | Metal showing up in the water | Evidence that steel or copper parts are corroding |
| Silica & phosphate | Other deposit-forming species | Hard, difficult-to-remove deposits |
| ORP & free chlorine | How well the oxidizing biocide is working | Biological growth when too low |
| Bacteria / Legionella | Biological activity | Biofilm, fouling, corrosion and a health hazard |
Cycles of concentration (COC)
COC tells you how many times the minerals in the make-up water have been concentrated in the tower. A quick field estimate uses conductivity:
COC ≈ Tower water conductivity ÷ Make-up water conductivity
Example: 2,000 µS/cm ÷ 500 µS/cm = 4 cycles
- COC too high → not enough blowdown → scale, deposits and corrosion risk.
- COC too low → too much blowdown → wasted water and treatment chemicals.
Langelier Saturation Index (LSI)
LSI = pH − pHs
- LSI > 0 — water tends to deposit calcium carbonate scale.
- LSI ≈ 0 — near equilibrium.
- LSI < 0 — water is undersaturated with calcium carbonate.
Don't oversimplify: LSI only describes calcium-carbonate saturation. A negative LSI doesn't automatically mean the whole system is corrosive — and there's no single ORP number that fits every tower either. Always work to the site's own limits.
What scale does to the chiller
Scale and biofilm add thermal resistance between the refrigerant and the condenser water:
Refrigerant → Tube wall → Scale → Condenser water
The chain reaction that follows: heat transfer drops, condenser approach rises, condensing pressure climbs, compressor lift and amps increase, and kW/ton goes up. A "water problem" quietly becomes an energy problem — and, at the limit, a high-head-pressure trip.
Troubleshooting an abnormal reading
Example: condenser-water conductivity suddenly reads much higher than normal.
- Safety first — is there an immediate risk to people or equipment?
- Verify — check the probe, calibration and a hand test before trusting the number.
- Trend — compare with history and with make-up water conductivity.
- Inspect — blowdown valve and controller, make-up valve, chemical feed pumps, basin, strainers and fill.
- Check equipment impact — condenser water supply/return temps, condenser approach, head pressure, compressor amps and kW, BMS alarms.
- Correct per SOP — fix the mechanical or controls cause using the approved procedure.
- Escalate — chemical or setpoint changes go to the water-treatment vendor or engineer.
- Document — reading, trend, cause, action and final result.
Know your limits: don't add chemicals or change treatment setpoints on your own. Follow the site's water management program, biocide program and escalation path — especially where Legionella control is involved.
The mindset
One reading is never the whole story. Verify it, understand its trend, connect it to what the equipment is doing, fix the cause safely, escalate what isn't yours to change, and write it down. That's what turns water chemistry into reliability.
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