Open recirculating cooling systems concentrate everything that enters with the make-up water. At five cycles of concentration, a feedwater with 120 mg/L calcium hardness becomes 600 mg/L in the tower; at eight cycles it doubles again. Scale and fouling are therefore not accidents, they are the predictable result of an economic decision about how much water to save – and the polymers in the treatment programme are what make high cycles viable.
This guide covers how AA-HPA copolymer (CAS 55719-33-0) is used in cooling water: where it acts, how it fits with phosphonates and corrosion inhibitors, how to dose it, and how to tell whether it is working.
The four foulants in a cooling system
| Foulant | Origin | Where it deposits | Best control route |
|---|---|---|---|
| Calcium carbonate | Hardness plus alkalinity, concentrated by evaporation | Hot heat-transfer surfaces, condenser tubes | Threshold inhibition (phosphonate) plus dispersion (polymer) |
| Calcium phosphate | Phosphate-based corrosion programmes, or phosphate in make-up | High-flow and high-heat areas | Polymer dispersion – where AA-HPA outperforms PAA |
| Iron oxide | Corrosion transport, make-up water | Low-flow zones, tube sheets, under deposits | Dispersion – the hydroxyl comonomer helps here |
| Biological deposits and silt | Airborne contamination through the tower | Fill, distribution decks, basin | Biocide programme plus dispersant |
The polymer’s job is the second, third and fourth rows. It rarely works alone: a complete programme pairs a threshold inhibitor for carbonate with a dispersant polymer for phosphate, iron and silt, plus corrosion inhibitors and biocides.
Where AA-HPA acts in the programme
AA-HPA contributes three things that a phosphonate cannot:
- Particulate dispersion. Phosphonates inhibit crystal growth but do not keep suspended solids apart; the polymer does, which is why iron and silt stay mobile instead of settling in low-flow zones.
- Calcium phosphate control. Alkaline programmes with phosphate corrosion inhibitors are prone to calcium phosphate deposition; the hydroxyl-functional comonomer improves dispersion of exactly this species.
- Chlorine stability. Phosphonates are consumed by free chlorine; polycarboxylates are more tolerant, so the dispersant continues to work on days when the halogen residual is high.
Typical programme composition
| Component | Function | Typical dose (mg/L product) |
|---|---|---|
| AA-HPA copolymer (30%) | Dispersant, calcium phosphate and iron oxide control | 5 – 20 |
| HEDP or ATMP | Threshold inhibition of calcium carbonate | 2 – 10 |
| Zinc or phosphate | Corrosion inhibition (anodic/cathodic) | 1 – 5 (zinc) or 3 – 10 (phosphate) |
| Azole (BTA/TTA) | Copper and brass protection | 1 – 3 |
| Oxidising biocide | Biological control | Programme dependent |
| Non-oxidising biocide | Resistance management | Alternating programme |
Product doses must be separated from active doses: at 30% solids, 10 mg/L of product delivers about 3 mg/L of polymer.
Setting the dose from water chemistry
The dose is set by the saturation state of the water, not by habit. The three inputs that matter are calcium hardness, total alkalinity and pH, because together they determine the calcium carbonate saturation index.
| Condition | Implication for the polymer dose |
|---|---|
| High cycles with high hardness and alkalinity | Increase both phosphonate and polymer; consider a calcium carbonate inhibitor with better tolerance |
| Phosphate-based corrosion programme | Raise AA-HPA to control calcium phosphate; monitor phosphate residual |
| High iron in make-up or heavy corrosion transport | Raise dispersant; treat the corrosion source rather than simply increasing polymer |
| Elevated suspended solids or turbidity | Increase dispersant and consider side-stream filtration |
| Low hardness, low alkalinity | Reduce dose; over-dosing a dispersant is wasted money and can add to turbidity |
How to tell whether it is working
Cooling water programmes are judged on deposit control and on heat transfer, not on the appearance of the water. Four measurements cover most of it:
- Deposit weight on a corrosion coupon or heat-transfer monitor. The most direct evidence; a mild steel or copper coupon tells you whether the programme holds deposits below the target rate.
- Polymer residual. A measurable residual confirms the product is reaching the system and is not being consumed or precipitated at the dosing point.
- Iron and turbidity trends. Rising iron with stable cycles suggests corrosion transport rather than scale; falling turbidity with a stable dose suggests the dispersant is working.
- Heat transfer efficiency and approach temperature. A slowly rising approach temperature is the earliest sign that the programme is losing ground.
Common failure modes
| Symptom | Likely cause | Fix |
|---|---|---|
| White deposit on hot tubes only | Carbonate scale – threshold inhibitor underdosed for the saturation state | Recheck cycles and hardness; raise phosphonate; consider acid feed |
| Phosphate-rich deposit | Calcium phosphate with insufficient dispersant | Raise AA-HPA; review phosphate programme pH window |
| Red-brown deposit in low-flow zones | Iron oxide settling where velocity is low | Raise dispersant; consider flow correction and filtration |
| Polymer precipitating at the dosing point | Stream incompatibility – cationic biocide or high-calcium stream | Move dosing point; dilute; sequence the additions |
| Deposits despite correct dose | Wrong polymer molecular weight for the duty | Confirm limiting viscosity number is in the 0.065-0.095 dl/g range |
Frequently asked questions
Can I run a cooling system on AA-HPA alone? For carbonate scale control alone you need a threshold inhibitor; the polymer is a dispersant. In practice, programmes use both.
How much does the 30% concentration affect dosing? It changes everything: 10 mg/L of product is only 3 mg/L of active polymer. Confirm the basis before calculating.
Is AA-HPA compatible with chlorine? Better than phosphonates, but not immune. Avoid feeding polymer into a concentrated halogen slug.
What happens if the system freezes? The product can separate irreversibly; keep drums above 5 deg C and do not use frozen material without checking appearance and viscosity.
Contact PolyBlueChem for AA-HPA samples, COA and dosing support.