A cooling tower chemical program works when it treats three problems as one: scale that blocks heat transfer, corrosion that thins tube walls, and biofilm that coats surfaces while raising microbiological risk. All three respond to the same two decisions — cycles of concentration and blowdown rate — so raising inhibitor dose without managing blowdown moves cost around rather than solving anything.
One number drives the program
Cycles of concentration (COC) is the ratio of dissolved solids in the circulating water to those in the make-up. In practice it is read from the conductivity or chloride ratio between blowdown and make-up. When evaporation rises and blowdown does not follow, COC climbs, calcium carbonate solubility is exceeded, and scale begins on the hottest tubes.
Blowdown follows a simple balance: at a given COC, the blowdown required is proportional to evaporation divided by (COC − 1). Raising COC from 3 to 5 therefore cuts both blowdown and make-up water, but it also concentrates corrosive ions and feeds biofilm. Choosing a COC is a chemistry decision, not just a water-savings decision.
Three problems, three control mechanisms
Scale. Calcium carbonate has inverse solubility: the hotter the surface, the more readily it deposits. Risers and the hottest tubes show the problem first. Control works on three levels — hold COC down through blowdown, keep pH away from the carbonate-dominant range, and dose a polymer or phosphonate scale inhibitor that disrupts crystal growth. On high-COC systems a saturation index (LSI or RSI) is more useful than a single ppm target, because it combines pH, calcium, alkalinity and temperature.
Corrosion. Concentrated circulating water raises chloride and sulphate, while low pH accelerates attack on carbon steel. Common programs hold operating pH in the 7.5–9.0 band, combine anodic and cathodic inhibitors, and verify the result with corrosion coupons or probes. The corrosion rate on a coupon is evidence that the program is working, not an appendix to a report. If the rate rises after COC is increased, the cause is usually in the blowdown, not the dose.
Biofilm. Biofilm shelters bacteria and holds a stagnant water layer that drives under-deposit corrosion. Control depends on biocide type: oxidisers such as hypochlorite, bromine or chlorine dioxide suit routine organic load, while non-oxidisers such as isothiazolinone, glutaraldehyde or DBNPA are alternated to prevent resistance. Contact time and injection point matter as much as dose. Where the tower serves occupied buildings or a sensitive process, the microbiological control plan has to account for aerosol risk, not just fouling.
Decisions before changing the program
| Decision | Data to review | Consequence of getting it wrong |
|---|---|---|
| Target COC | Make-up and blowdown conductivity or chloride, evaporation rate, chemical limits | Scale on hot tubes, or excess blowdown wasting water and money |
| Inhibitor basis | Full water analysis, LSI/RSI, system metallurgy, operating pH | Correct dose applied to the wrong water, or overfeed raising waste load |
| Biocide scheme | Organic load, temperature, contact time, injection point, aerosol risk | Biofilm persists, or oxidiser spend rises with no result |
| Blowdown route | Local effluent limits, phosphate and zinc content, discharge destination | Program passes technically but fails its discharge permit |
| Measurement points | Sampling location, frequency, instruments, and who reads them | Trends are missed until scale or a leak has already formed |
Technical checklist
- Record actual make-up, evaporation and blowdown rates; calculate COC from measured data, not design assumptions.
- Test calcium, magnesium, total alkalinity, chloride, sulphate, silica, pH and conductivity on both make-up and circulating water.
- Calculate LSI or RSI at operating temperature, not ambient.
- Fit corrosion coupons on both the process and return sides, and set a reading interval the team will actually keep.
- Fix inhibitor and biocide injection points away from dead legs, with mixing before the heat exchanger bundle.
- Define biocide contact time and the rotation schedule, and write both into the operator log.
- Keep readings in the same log as pump and heat exchanger maintenance so trends can be correlated.
- Review the program whenever process load changes; a program that is correct at 60% load can be wrong at full load.
When the program needs review
Three triggers usually precede failure: coupon corrosion rate rising across two consecutive intervals, heat exchanger approach temperature tightening at the same load, or microbiological counts that do not fall after a biocide cycle. Each signals that the COC assumption or the dosing scheme no longer matches actual conditions.
Where blowdown must meet phosphate and zinc limits, the inhibitor choice changes with it. That constraint, and the pretreatment design that satisfies it, is covered in cooling tower blowdown carrying zinc and phosphate. For a microbiological control plan that includes aerosol risk, see keeping Legionella out of cooling towers.
Next steps
- Cooling tower chemical program for scale and corrosion inhibitors selected from the water analysis.
- Biocides for oxidising and non-oxidising schemes.
- Scale inhibitors where calcium carbonate deposition at high COC is the main problem.
- Corrosion inhibitors for carbon steel and copper alloy protection.
- Dosing pumps for metered feed that can be calibrated and logged.
To build a program, bring the make-up and circulating water analyses, operating rates, target COC, system metallurgy and the applicable effluent limits. Those decide the inhibitor selection, not the product name.