COOLING TOWER WATER TREATMENT

Purpose & Importance

Water treatment is critical to:

1. Prevent Scale Formation (mineral deposits)
2. Control Corrosion
3. Inhibit Biological Growth (bacteria, algae, Legionella)
4. Minimize Fouling (silt, mud, organic matter)
5. Optimize Water & Energy Efficiency

Untreated water leads to reduced heat transfer, increased energy consumption, equipment damage, downtime, and health hazards.

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. COMMON PROBLEMS IN COOLING SYSTEMS

A. Scaling

• Cause: Precipitation of dissolved minerals (Calcium Carbonate, Calcium Sulfate, Silica, Magnesium Silicate) when water becomes concentrated (high COC) and/or heated.
• Effect: Insulating layer on heat transfer surfaces → Reduced efficiency, increased energy use, flow restriction.
• Most Common: Calcium Carbonate (CaCO₃) scale – inversely soluble (less soluble in hot water).

B. Corrosion

• Cause: Electrochemical reaction between water and metal surfaces (steel, copper, galvanized steel).
• Forms: General attack, pitting, galvanic corrosion, under-deposit corrosion.
• Effect: Equipment failure, leaks, shortened system life, introduction of metal ions into water.

C. Biological Fouling & Growth

• Microorganisms: Bacteria, algae, fungi.
• Major Risk: Legionella pneumophila (causes Legionnaires’ disease).
• Effect: Biofilms (slime) reduce heat transfer, increase corrosion under deposits, block distribution nozzles, health hazard.

D. Fouling (Suspended Solids)

• Cause: Dirt, silt, clay, sand, corrosion products, organic debris.
• Effect: Deposits in low-flow areas, under biofilm, reduces efficiency, increases corrosion.

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2. WATER TREATMENT PROGRAM COMPONENTS

A comprehensive program addresses all four problems simultaneously.

A. Scale Inhibitors

• Function: Modify crystal growth or keep minerals dissolved.
• Types:
  • Phosphonates (e.g., HEDP, PBTC): Threshold inhibitors, stabilize calcium carbonate.

1. Polymeric Dispersants: Prevent crystal agglomeration and settling.
   1. Chelants (e.g., EDTA): Bind and solubilize metal ions (less common in towers due to cost/corrosion risk).
2. Application: Continuous feed, low dosage (5-20 ppm).

B. Corrosion Inhibitors

• Function: Form a protective film on metal surfaces.
• Types:
  • Anodic Inhibitors (Passivators): Molybdates, Nitrites, Orthophosphates. Form oxide layer on anode.
  • Cathodic Inhibitors: Zinc, Polyphosphates. Form film on cathode.
  • Film-Forming Inhibitors: Azoles (for copper), Silicates, Phosphonates. Adsorptive organic films.
  • pH Control: Maintaining pH 7.5-9.0 is critical for corrosion control (alkaline).
• Application: Continuous feed.

C. Biocides

Two types are required for effective microbial control: Oxidizing + Non-Oxidizing.

1. Oxidizing Biocides

• Mechanism: Destroy cell walls and enzymes.
• Types:
  • Chlorine (Gas, Sodium Hypochlorite, Calcium Hypochlorite): Traditional, low cost, but corrosive at high levels.
  • Bromine/Bromochlorodimethylhydantoin (BCDMH): More effective at high pH, less corrosive.
  • Chlorine Dioxide: Effective against biofilms, less pH dependent, good for Legionella.
  • Ozone: Powerful, no residual discharge, capital-intensive.
  • Electrochlorination (On-site generation): Produces hypochlorite from salt.
• Application: Continuous or slug feed. Maintain residual (e.g., 0.5-1 ppm free chlorine).

2. Non-Oxidizing Biocides

• Mechanism: Interfere with metabolism, reproduction, or cell integrity.
• Types:
  • Isothiazolinones (e.g., DBNPA): Fast-acting, degradable.
  • Quaternary Ammonium Compounds (Quats): Cationic surfactants.
  • Glutaraldehyde: Broad-spectrum.
  • Carbamates, Bronopol.
• Application: Shock dose (slug feed) weekly or bi-weekly to penetrate biofilms. Rotated to prevent resistance.

D. Dispersants & Antifoams

• Dispersants: Keep suspended solids (clay, silt) in suspension for blowdown removal.

Antifoams: Control foam caused by high organic loading or surfactants.