Cooling Towers Explained
Cooling towers reject heat from process water through evaporation. Learn how they work, types, water treatment requirements, and the key maintenance tasks.
How Cooling Towers Work
A cooling tower rejects heat from water by evaporating a small portion of it. Warm process water is sprayed over a fill medium while air is drawn through. As some water evaporates, it cools the remaining water. The cooled water collects in the basin and is pumped back to the process.
The key principle: evaporation of just 1% of the water cools the remaining water by approximately 5.5 degrees C. This makes cooling towers very efficient for large heat rejection duties.
Types of Cooling Towers
- Crossflow: Air flows horizontally across the falling water. Water flows by gravity over the fill. Simpler design, easier to clean, larger footprint.
- Counterflow: Air flows upward, counter to the falling water. More efficient heat transfer, smaller footprint, but fill is harder to access for cleaning.
- Forced draft: Fans at the air inlet push air through the tower. Simpler fan arrangement but fans are in the moist, potentially corrosive air stream.
- Induced draft: Fans at the air outlet pull air through the tower. More common. Fans are in dry air, reducing corrosion. Better air distribution.
- Natural draft (hyperbolic): No fans. Uses the natural buoyancy of warm air to create airflow. Very large concrete structures. Used in power stations. No fan energy but very high capital cost.
Key Operating Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Approach temperature | 3 to 5 degrees C | Difference between cold water temperature and wet-bulb temperature. Lower approach requires larger tower. |
| Range | 5 to 15 degrees C | Difference between hot inlet and cold outlet water temperatures. |
| Cycles of concentration | 3 to 8 | Ratio of dissolved solids in tower water to makeup water. Higher cycles reduce water usage but increase scaling risk. |
| Evaporation rate | ~1% per 5.5 degrees C range | Water evaporated to reject the heat. |
| Drift rate | 0.001 to 0.005% | Water droplets carried out with the air. Reduced by drift eliminators. |
| Blowdown rate | Varies with cycles | Water discharged to control dissolved solids. |
Water Treatment Requirements
Cooling tower water treatment is critical. The warm, aerated water is an ideal environment for scale, corrosion, and biological growth (including Legionella). A typical treatment program includes:
- Scale inhibitors: Phosphonates and polymers that prevent calcium and magnesium from precipitating as scale on the fill and heat exchangers.
- Corrosion inhibitors: Azoles, phosphates, or molybdates that form a protective film on metal surfaces, preventing corrosion.
- Biocides: Oxidising (chlorine, bromine) and non-oxidising biocides that control bacteria, algae, and fungi. Critical for Legionella prevention.
- Dispersants: Keep suspended solids in solution so they can be removed by blowdown, rather than settling in the tower.
Legionella risk
Cooling towers are a known source of Legionnaires' disease. The warm water (20 to 45 degrees C) and aerosol drift create ideal conditions for Legionella bacteria. A comprehensive biocide program, regular cleaning, and drift eliminator maintenance are essential. See our Cooling Tower Legionella guide.
Tip: Monitor cycles of concentration by comparing the conductivity of tower water to makeup water. If cycles are too low, you are wasting water and chemicals. If too high, scaling and corrosion risk increases. Automatic conductivity-controlled blowdown maintains the correct cycles.