Cooling Tower Design, Operation & Use

Cooling towers remove heat from water by evaporative cooling. Warm process water is sprayed or distributed across fill media while air moves through the tower, causing a small portion of the water to evaporate and carry heat away. This guide explains cooling tower configurations, heat transfer principles, evaporation rate, airflow patterns, fill design, drift elimination, and key operational considerations used in U.S. industrial and HVAC systems.

 

1. Cooling Tower Configurations

Cooling towers are classified by how air moves through the tower and how water is exposed to that air. The two main categories are natural draft and mechanical draft. Natural draft towers rely on the chimney effect of warm, moist air rising through a tall shell. Mechanical draft towers use fans to move air through the fill and are the most common type in industrial and HVAC service.

 

Mechanical draft towers use either forced draft or induced draft airflow. In a forced draft tower, fans push air into the tower at the air inlet. In an induced draft tower, fans pull air out at the air outlet, creating more uniform airflow and reducing recirculation of warm, moist air back into the tower.

Hot water enters at the top of the tower and is broken into droplets by spray nozzles or perforated plates. As the water falls, it contacts upward‑moving air and cools. To increase evaporation, the water passes through fill media—corrugated plastic sheets or splash bars—that break the water into thin films or droplets and increase contact time with the air.

 

At the bottom of the tower, cooled water collects in the sump and is pumped back to the process. To reduce water loss from fine droplets carried out of the tower, drift eliminators are installed at the top.

 

The diagram below shows typical mechanical draft cooling tower configurations.

 

2. How Cooling Towers Remove Heat

Cooling towers rely primarily on evaporative heat transfer. When warm water contacts cooler, dryer air, a portion of the water evaporates. The energy required to convert liquid water into vapor is taken from the water itself, lowering its temperature. This is the same cooling effect that occurs when sweat evaporates from skin.

A smaller amount of cooling occurs through sensible heat transfer—direct contact between warm water and cooler air.

 

3. Fill Media and Water Distribution

Fill media increases the surface area for heat transfer. U.S. cooling towers typically use:

• Film fill: Corrugated sheets that spread water into thin films for maximum surface area.
• Splash fill: Grids or bars that break water into droplets—better for dirty or high‑solids water.

Uniform water distribution is essential. Poor distribution causes:

• Hot spots
• Reduced cooling efficiency
• Increased scaling and fouling

 

4. Airflow Patterns

 

Cooling towers use natural or mechanical draft airflow:

• Counterflow: Air moves upward while water flows downward.
• Crossflow: Air moves horizontally across the falling water.
• Induced draft: Fans pull air through the tower.
• Forced draft: Fans push air into the tower.

 

5. Drift Elimination

Drift eliminators capture water droplets carried by the airflow. Proper drift control:

• Reduces water loss
• Prevents chemical carryover
• Minimizes environmental impact

Modern eliminators can reduce drift to less than 0.0005% of circulating flow.

 

6. Evaporation Rate

Cooling effectiveness depends on several factors:

• Water flow rate (how much water must be cooled)
• Airflow rate (how much cooling is available)
• Incoming water temperature
• Difference between wet‑bulb and dry‑bulb temperatures

The wet‑bulb temperature is the lowest temperature water can reach through natural evaporation. It is measured by wrapping a wet cloth around a thermometer bulb and allowing evaporation to cool it.

The dry‑bulb temperature is the normal air temperature reported by weather services. The greater the difference between dry‑bulb and wet‑bulb temperatures, the more evaporation can occur and the cooler the water can become.

 

7. Energy Transfers

Most cooling in a tower occurs through the latent heat of vaporization. As water evaporates, it absorbs energy from the remaining liquid, lowering its temperature.

As a drop of water falls through the tower:

• At the top, the air is warm and humid, so cooling is slow.
• Lower in the tower, the air is cooler and dryer, so cooling is faster.
• In the sump, only surface cooling occurs as air passes over the water.

 

8. Additional Design & Operational Considerations

 

 

Materials of Construction

Packaged cooling towers are commonly built from fiberglass, plastics, and stainless steel. Larger towers may use concrete, wood, cement sheeting, and stainless steel bracketing. Materials must resist corrosion, moisture, and biological growth.

 

Locating Cooling Towers

Cooling tower placement must consider:

• Prevailing winds and plume direction
• Dust loading from surrounding ground conditions
• Nearby buildings that may restrict airflow
• Recirculation of warm, moist plume air
• Corrosion risk to nearby structures
• Chemical vapors that may be drawn into the tower
• Worker exposure to the plume

 

Cleanliness & Water Treatment

Warm, wet environments inside cooling towers promote bacterial growth. Legionella is the most well‑known risk, but many bacteria can thrive in tower water. U.S. health standards require:

• Regular biocide dosing (slug or metered)
• Periodic tower cleaning
• Bleed‑off to control dissolved solids
• Optional in‑line filtration

Entry into a cooling tower is considered a confined space entry and requires proper training and procedures.

 

Operating Issues

Check the following regularly:

• Water distribution is even—sprays and holes can block.
• Fill is properly installed—gaps allow air to bypass the water.
• Mist eliminators are complete—missing sections increase water loss.
• Fan blades are free of corrosion—inspect frequently.

 

9. Maintenance Best Practices

To maintain efficient operation:

• Inspect fill media for fouling and damage.
• Check drift eliminators for cracks or misalignment.
• Verify fan operation, belt tension, and motor condition.
• Ensure nozzles provide even water distribution.
• Maintain proper water chemistry to control scale and biological growth.

 

Adapted from original work by Mike Sondalini, Maintenance Engineer.