Cooling Tower vs Chiller Water Treatment
Open evaporative cooling towers and closed-loop chilled water systems need very different water treatment - here is how they differ and what each one requires.
An open evaporative cooling tower loses water to atmosphere and is a high Legionella risk, so its treatment centres on biocide dosing, Legionella sampling and biannual cleaning. A closed-loop chilled water (CHW) system is sealed, so its treatment centres on corrosion and scale inhibition, passivation and - where frost is a risk - glycol, with little Legionella exposure.
Two systems, two very different problems
"Cooling" covers two quite different system types, and confusing their treatment is a common and costly mistake. An open cooling tower rejects heat by evaporating water to atmosphere; a closed-loop chilled water system circulates the same sealed water round and round. Because one is open to the air and constantly topped up, and the other is sealed, they fail in different ways and need different treatment programmes.
Side by side
| Open cooling tower | Closed-loop chilled water (CHW) | |
|---|---|---|
| System | Evaporative, open to atmosphere | Sealed, recirculating |
| Water loss | Significant (evaporation, drift, blowdown) | Minimal once filled |
| Legionella risk | High - legally high-risk | Low (sealed, no aerosol) |
| Main treatment focus | Biocide, Legionella control, cleaning | Corrosion & scale inhibition, passivation |
| Key extras | Quarterly sampling, biannual clean, notification | Flushing, glycol frost protection |
| Compliance driver | HSG274 / HPSC 2009 Legionella duties | Asset protection & efficiency |
Open cooling tower treatment
Because a cooling tower is open, aerated and warm, it is a high Legionella risk and is regulated accordingly. Treatment focuses on biocide dosing to control bacteria, corrosion and scale inhibitors, routine Legionella sampling, biannual cleaning and full compliance documentation. See our Cooling Tower Water Treatment hub.
Closed-loop chilled water treatment
A sealed CHW system has little Legionella exposure, so treatment protects the asset instead: pre-clean flushing and passivation of the metals, then a treatment programme guarding against corrosion, scale, fouling and bio-contamination. Chilled systems often also need glycol for frost protection during cold periods. This is specialist closed-loop work - see our LPHW & CHW page and Water-Cooled Plant for chiller systems.
Cooling vs chiller FAQs
A sealed CHW loop carries little Legionella risk because it is not open to atmosphere and does not generate aerosol, so it is not treated as a high-risk system like an open cooling tower. Its treatment focuses on corrosion and scale control instead. Your risk assessment confirms what each system needs.
Because it is open and evaporative, a cooling tower can release water aerosol to the air and is legally high-risk for Legionella - driving quarterly sampling, biannual cleaning and notification. A sealed chilled loop does not, so its regime is about protecting the system.
Glycol provides frost protection in chilled water systems during cold periods, preventing freeze damage. Halpin & Hayward supplies, pumps in, circulates, tests and certifies glycol as part of closed-loop treatment.
Not sure what your systems need?
We will assess your cooling towers and closed loops and set the right programme for each. Call Halpin & Hayward on +353 1 296 7188 or email sales@halpinandhayward.com to arrange a site survey and quotation.