Natural Draft Cooling Tower

What is a Natural-draft cooling tower?

A natural-draft cooling tower is a very tall, hyperbolic concrete shell that cools water without a fan. Warm, moist air inside the tower is lighter than the air outside, so it rises by itself and pulls fresh air in at the base.

How a natural-draft cooling tower works

A power station or a large plant has a lot of heat to get rid of every hour of the day. The natural-draft cooling tower does it with water, air and gravity.

1. Hot water arrives. Water that has passed through the steam condenser or process heat exchangers is carried to the tower in large pipes or ducts and fed to the distribution system inside the shell.
2. It is spread over the fill. Spray nozzles or troughs spread the water evenly across the fill, which breaks it into thin films and droplets and gives it a very large surface.
3. Air enters at the base. The shell stands on columns, so outside air flows in all the way around the bottom edge and moves up through the fill.
4. Some water evaporates. Evaporation takes heat from the rest of the water, which cools it. The air leaves the fill warmer and almost saturated with moisture.
5. The warm, moist air rises. It is lighter than the cooler air outside, so it climbs the shell and pulls new air in behind it. The flow keeps itself going as long as there is heat to reject.
6. Cold water goes back. The cooled water falls into the cold-water basin at the foot of the tower and is pumped back to the plant. The moist air leaves the top as a white plume.

A wet natural-draft cooling tower is an open-circuit tower: the water that cools the plant is the water that touches the air. It cools in the same way as an open-circuit mechanical tower. The difference is that the height of the shell does the job of the fan.

A tall hyperbolic shell stands on columns. Hot water enters from the left and is sprayed over fill near the base. Air enters through the open base, rises through the fill and the shell, and leaves at the top as a plume. Cooled water collects in a basin and leaves on the right.

1. Hyperbolic concrete shell
2. Rising warm, moist air
3. Water distribution system
4. Fill (packing)
5. Open air inlet and support columns
6. Cold-water basin

Why height creates airflow

The airflow in a natural-draft cooling tower comes from a difference in density. Air inside the shell is warmer and holds more water vapor, so it weighs less per cubic meter than the air outside. The taller the column of light air, the bigger the pressure difference that pushes air through the tower.

In simple terms, the available draft is about g × H × (ρ_outside − ρ_inside), where H is the effective height of the column of warm air and ρ is air density.

This is why natural-draft cooling towers are so tall. Height is the only way to get enough draft without a fan. It is also why they perform best when the air outside is cool and the air inside is much warmer. On hot, humid days the density difference shrinks and the draft weakens.

Main parts of a natural-draft cooling tower

• Hyperbolic shell. A thin reinforced-concrete wall, often only a few tens of centimeters thick, that forms the chimney and carries wind loads. It is usually stiffened with a ring at the top.
• Support columns and air inlet. The shell stands on inclined columns around its edge. The open space between them lets air flow in from all sides.
• Fill (packing). A block of film or splash fill placed above the rain zone. It gives the water a large surface so air and water can exchange heat and vapor.
• Water distribution system. A network of pipes, nozzles or troughs that spreads hot water evenly across the fill.
• Drift eliminators. Baffles above the distribution system that catch droplets before they leave with the air.
• Hot-water ducts or pipes. Large-diameter lines that bring water from the condenser or process into the tower.
• Cold-water basin. A concrete basin or pond at the foot of the tower that collects the cooled water for return to the plant.

Draft

The pressure difference that moves air through the tower. In a natural-draft cooling tower it comes from buoyancy, not from a fan.

Stack effect

Warm air rising in a tall column and pulling cooler air in behind it. Chimneys work the same way.

Throat

The narrowest part of the shell, about three quarters of the way up. Air speeds up as it passes through.

Rain zone

The open space between the bottom of the fill and the basin, where the cooled water falls through the incoming air.

Range

The drop in water temperature across the tower: hot water in minus cold water out.

Approach

Cold water temperature minus the ambient wet-bulb temperature. A smaller approach needs a bigger tower.

Drift

Fine droplets carried out of the tower by the air. It is separate from evaporation, which leaves as vapor.

Why the shape is hyperbolic

The curved, hourglass shape is not for looks. A hyperbolic shell has double curvature, which makes a thin concrete wall very strong and stiff. It reduces the amount of concrete needed, and it resists wind well. The narrowing at the throat also speeds up the air, which helps the cooling. The widening above the throat helps the exhaust air leave smoothly.

Types of natural-draft cooling towers

By cooling method

• Wet (evaporative) natural draft. The most common type. Water meets the air directly in the fill and cools by evaporation. This is the tower most people mean by “natural draft cooling tower”.
• Dry natural draft. Water circulates inside finned-tube heat exchangers around the base, and air passes over them. There is no evaporation, so water use is minimal and there is no plume. Heller-type systems work this way. Capital cost is higher and the water cannot be cooled as far, so dry towers are chosen mainly where water is scarce.
• Fan-assisted natural draft. A hybrid with fans at the base that add airflow when the natural draft is weak, which allows a shorter shell.

By flow arrangement

• Counterflow. Air moves up against falling water. This is the usual arrangement for large natural-draft cooling towers and is shown in the diagram.
• Crossflow. Air moves sideways through fill arranged around the base while water falls through it.

Natural draft vs mechanical draft cooling tower

Both cool water by evaporation. The difference is what moves the air.

Advantages and disadvantages

Where natural-draft cooling towers are used

They suit sites that need a large, constant cooling duty for many years.

• Thermal power stations. Coal-fired plants are the most familiar users, and in India most large coal-fired stations have hyperbolic towers.
• Nuclear power plants. Many have large natural-draft cooling towers to cool the condenser water.
• Large combined-cycle plants where the steam cycle needs a big condenser cooling system.
• Very large industrial complexes with a steady, heavy heat load, where the long life and low running cost justify the investment.

The size of these structures is striking. The tallest cooling tower listed on Wikipedia is at the Pingshan II power station in China, at about 210 m. India’s Kalisindh thermal power station in Rajasthan has towers about 202 m tall.

Is the white cloud smoke?

No. The white plume rising from a natural-draft cooling tower is mostly water vapor that condenses into tiny droplets as the warm, moist air meets cooler air. The tower itself burns nothing. The plume is larger and lasts longer on cold, humid days, and it can nearly disappear in hot, dry weather.

Water treatment and upkeep

A wet natural-draft cooling tower is an open system, so the water has to be looked after just like in a mechanical tower. Even without fans, a good routine matters.

• Water chemistry. Control scale, corrosion and biological growth with treatment and blowdown. Higher cycles of concentration save water but raise the risk.
• Fill and nozzles. Check for blockage, fouling and damage. Uneven distribution reduces cooling.
• Drift eliminators. Keep them clean and in place to limit water loss and mist.
• Concrete shell. Inspect for cracks and spalling, and repair them early.
• Basin. Clean it on schedule and keep strainers clear.
• Legionella control. Follow a written water management plan, such as the approach in ASHRAE Standard 188, and local rules.

Frequently asked questions