Cooling tower chemicals

Scale, corrosion, fouling and biology are controlled by a programme of chemicals, blowdown and monitoring, not by chemicals alone.

What goes wrong in a cooling system

As water evaporates in a cooling tower, the salts left behind concentrate, so hardness, silica, chloride and sulphate rise with every cycle of concentration. Warm, aerated water with nutrients also grows algae and bacteria, including Legionella. Without treatment this gives scale on heat-exchange surfaces, corrosion of steel and copper alloys, biofilm that insulates tubes and fouling of fill and nozzles. The result is lower heat transfer, higher energy use and shorter equipment life. See also cooling tower water treatment.

Scale control

Phosphonates: Scale inhibitors that stop calcium carbonate and related scales from forming at the surface. Common types include HEDP and PBTC, chosen for the water chemistry and chlorine exposure.
Polymeric inhibitors and dispersants: Polyacrylates and similar polymers keep fine solids and scale crystals suspended so they leave with the blowdown instead of depositing.
pH control: In acid-fed programmes sulphuric acid lowers alkalinity to hold scale-forming salts in solution, at a higher control and safety requirement.

Corrosion control

Phosphate and phosphonate-based inhibitors: Form a protective film on carbon steel. Used in alkaline and all-organic programmes.
Zinc and molybdate: Added to some programmes to improve film strength. Their use depends on local discharge limits.
Azoles for copper alloys: Tolyltriazole and similar chemicals protect copper and brass heat exchangers and reduce copper pick-up that can attack steel elsewhere.
Closed-loop chilled and hot water systems: Use different inhibitors, such as nitrite or molybdate based products, because the water does not evaporate and concentrate.

Microbiological control

Oxidising biocides: Sodium hypochlorite, stabilised bromine products and chlorine dioxide kill organisms quickly and keep the system clean. Dosed on timer or controlled by ORP.
Non-oxidising biocides: Isothiazolinone, glutaraldehyde, DBNPA and quaternary products work differently and are alternated or used as shock doses so organisms do not become tolerant.
Biodispersants: Help loosen and remove biofilm so that the biocide can reach the organisms.
Algaecides: Control algae on sunlit decks and fill.
Legionella risk: Cooling towers can harbour Legionella. A written risk assessment, regular biocide dosing, cleaning and periodic testing are part of the programme.

Other chemicals

Defoamers: Control foam in the basin when it appears.
Cleaning and pre-passivation chemicals: Used at start-up and after shutdowns to clean the system and form the first protective film on metal surfaces.
Side-stream filter aids: Where side-stream filtration is used, good dispersant control helps keep it working.

The programme: chemicals plus control

Element	What it does
Cycles of concentration	Set by conductivity-controlled blowdown to balance water use and scaling risk. Higher cycles save water but need better scale control.
Make-up water quality	Softened or RO make-up allows higher cycles. See RO.
Inhibitor dosing	Fed in proportion to make-up or blowdown flow so the concentration stays steady.
Biocide dosing	Oxidising biocide on timer or ORP control, with non-oxidising shocks on a schedule.
Monitoring	Conductivity, pH, ORP, corrosion coupons or online corrosion probes, and microbial counts.
Side-stream filtration	Removes dust and debris that feed biology and fouling.

Chemical programme types

Alkaline programme: Runs at higher pH with phosphonate and polymer inhibitors. Lower safety burden than acid, common in many plants.
Acid-fed programme: Uses sulphuric acid to hold alkalinity down. Allows high cycles but needs reliable pH control and safe acid handling.
All-organic programme: Uses organic inhibitors without phosphate or zinc where discharge limits restrict them.

How we supply and support

Water analysis and cycle selection: We check make-up and recirculating water, set a target cycle of concentration and select the programme.
Chemical supply: Inhibitor, dispersant, biocides, algaecide, defoamer and cleaners, supplied on schedule with a certificate of analysis for each batch.
Dosing and control: Dosing pumps, conductivity-controlled blowdown and biocide timers or ORP control, as a skid or on site. See dosing systems.
Monitoring and review: Regular sampling and corrosion monitoring, with the dose reviewed against results.
Cleaning: Chemical cleaning of fouled exchangers and towers, and start-up pre-treatment.

Common questions

What chemicals are used in a cooling tower?

A scale inhibitor and dispersant, a corrosion inhibitor, oxidising and non-oxidising biocides, sometimes an algaecide and defoamer, and cleaning chemicals for start-up and shutdowns.

What are cycles of concentration?

The ratio of dissolved salts in the circulating water to the make-up water. Higher cycles use less water but increase the scaling risk, so inhibitor and blowdown control must be tight.

How is Legionella controlled in a cooling tower?

By a written risk assessment, a steady biocide programme, regular cleaning and periodic testing, following the rules that apply in your country.

Why alternate biocides?

Organisms can become tolerant to one biocide. Alternating oxidising and non-oxidising biocides keeps the control effective.

Can you supply chemicals together with dosing and monitoring?

Yes. We supply the chemicals, dosing skid, conductivity-controlled blowdown and monitoring as one package or under an O&M contract.