Types of Cooling Towers in Power Plants
Power plants use the largest and most efficient cooling towers available.
1. Natural Draft Cooling Towers:
- How they work: The iconic hyperbolic (hourglass) shape. The tall stack creates a natural buoyancy effect: the warm, moist air inside the tower is less dense than the cooler, drier air outside, causing it to rise rapidly. This draws air in at the bottom without the need for large mechanical fans.
- Use Case: Common in very large nuclear and coal-fired power plants where the immense scale justifies the high construction cost. Their biggest advantage is extremely low operating costs since they use almost no electrical power for fans.
2. Mechanical Draft Cooling Towers:
- How they work: Use large electric fans to force or induce air flow through the tower. They are more compact and cheaper to build than natural draft towers but have higher operating costs due to fan energy consumption.
- Induced Draft: Fan at the top of the tower pulls air through. (Most common mechanical type).
- Forced Draft: Fan at the bottom pushes air into the tower.
- Use Case: Common in many fossil fuel plants and smaller power stations, or as a supplement to natural draft systems.
Considerations and Challenges
- Water Consumption: While they reduce water withdrawal, cooling towers are responsible for significant water consumption (lost to evaporation). This is a major consideration in water-scarce regions. A typical 500 MW coal plant can evaporate thousands of gallons per minute.
- Visible Plume: The saturated warm air exiting the tower mixes with cooler ambient air, often condensing into a highly visible white plume of water droplets. While mostly an aesthetic issue, it can cause icing on nearby roads and structures in cold weather.
Drift and Water Treatment: “Drift” is the minute droplets of water carried out of the tower. This water contains treatment chemicals, so drift eliminators are used to minimize
- it. Rigorous water treatment is essential to prevent scaling, corrosion, and biological growth (like Legionella bacteria).
- Maintenance: Towers must be regularly inspected and maintained. Their large, concrete structures can suffer from aging, and the internal “fill” (which increases water-to-air contact surface area) can degrade or clog over time.