Cooling Water Treatment Chemical Program Design

The Three Pillars

Every cooling water program must address three threats simultaneously:

1. Scale — mineral deposits on heat transfer surfaces
2. Corrosion — metal loss from pipes and heat exchangers
3. Microbiological growth — biofilm, algae, Legionella

Scale Control Program

Scale Type	Primary Inhibitor	Dosage	Notes
CaCO₃	HEDP or PBTCA	2–5 ppm	PBTCA preferred if using chlorine
CaSO₄	ATMP or AA/AMPS	3–8 ppm	Higher dose needed at high hardness
Silica	Polymer dispersant	5–15 ppm	Critical when SiO₂ > 150 ppm in makeup

Corrosion Control Program

Metal	Inhibitor	Dosage	Mechanism
Mild steel	HEDP + zinc sulfate	2–5 ppm HEDP + 1–2 ppm Zn	Anodic + cathodic protection
Copper alloys	Tolyltriazole (TTA)	1–3 ppm	Forms Cu-TTA film
Stainless steel	Molybdate (if needed)	50–100 ppm MoO₄	For closed systems

Biocide Program

Combine oxidizing + non-oxidizing biocides:

• Continuous: SDIC or TCCA tablets at 0.3–0.5 ppm free chlorine residual
• Shock: DBNPA 50 ppm or glutaraldehyde 100 ppm, 2–3x per week
• Legionella control: Maintain > 0.5 ppm free chlorine at all times

Monitoring Parameters

Parameter	Frequency	Target
pH	Daily	7.0–9.0
Conductivity/TDS	Daily	< 2500 µS/cm (varies by CoC)
Free chlorine	Daily	0.3–0.5 ppm
HEDP/phosphonate residual	Weekly	2–5 ppm
Iron (corrosion indicator)	Weekly	< 1 ppm
Bacteria count	Monthly	< 10⁴ CFU/mL

Blowdown Control

Cycles of concentration (CoC) = Makeup conductivity / Blowdown conductivity. Target 3–5 CoC for most systems. Higher CoC saves water but increases scaling risk.