Cooling Towers

What Is It?

A cooling tower is a heat rejection device that cools water by evaporating a small portion of it. It is used to reject waste heat from industrial processes, refrigeration systems, and power plants. Cooling towers are the most efficient method of rejecting low-grade heat to the atmosphere.

How Does It Work?

Warm water from the process is distributed over a fill media at the top of the tower. Air is drawn (induced draft) or forced (forced draft) through the fill, contacting the water. A small portion of the water evaporates — the latent heat of evaporation cools the remaining water. The cooled water collects in the basin and is pumped back to the process. The warm, moist air exits through the top of the tower.

Common Failures

• Scale on fill: Hardness deposits on the fill reduce heat transfer and airflow — from inadequate water treatment.
• Biological growth (Legionella): Algae, bacteria, and biofilm grow in the warm water — health risk and efficiency loss.
• Corrosion: Structural steel, basin, and piping corrode from oxygenated, high-TDS water.
• Drift eliminator failure: Broken eliminators allow water droplets to escape, carrying bacteria and chemicals.
• Fan and motor failure: Bearings, belts, and blades fail from wear, corrosion, and imbalance.

Maintenance

• Water treatment: Maintain scale inhibitor, biocide (alternating oxidising and non-oxidising), and corrosion inhibitor dosing.
• Legionella control: Test for Legionella per local regulations. Maintain biocide residual. Clean the tower at least twice per year.
• Cycles of concentration: Control blowdown to maintain TDS within limits. Too many cycles → scaling. Too few → water waste.
• Fill inspection: Inspect fill during scheduled cleaning — look for scale, biofilm, and damage.
• Fan maintenance: Lubricate bearings, check belts, inspect blades for fouling and balance.

Typical Lifespan

A well-maintained cooling tower (fibreglass or stainless construction) can last 20–30 years. Galvanised steel towers may last 10–15 years before corrosion requires replacement. Fill media needs replacement every 5–10 years. Drift eliminators last 5–8 years.

Selection Criteria

• Heat load: Determine the heat rejection requirement (kW) and design wet-bulb temperature.
• Approach temperature: The difference between cold water outlet and wet-bulb temperature — typically 3–5°C.
• Type: Crossflow or counterflow; induced draft or forced draft. Counterflow induced draft is most efficient.
• Material: Galvanised steel (cheapest, shortest life), fibreglass (good corrosion resistance), stainless steel (best, most expensive).
• Location: Position upwind of buildings, away from air intakes, with adequate clearance for airflow.