A cooling tower is a heat exchanger that cools hot water by bringing it into direct contact with a moving air stream. Evaporation of a small portion of the water removes heat from the remainder, and the cooled water is collected and returned to the plant to absorb heat again.
Cooling towers are used wherever a process or building generates more heat than it can dissipate on its own, including power plants, refineries, petrochemical and chemical plants, steel and cement plants, and the water-cooled chillers of large buildings. A typical industrial cooling tower lowers the water temperature by 5–10 °C per pass, while only a small fraction of the circulating water is lost to evaporation.
This guide explains what a cooling tower is, how cooling towers work (with a labelled diagram), their principal components and types, the parameters used to size them and the criteria for selecting one.
What Is the Purpose of a Cooling Tower?
Most industrial processes that compress, melt, react, condense steam or generate power produce waste heat. This heat is absorbed by circulating water flowing through condensers, heat exchangers, cooling jackets or moulds. A cooling tower rejects this heat to the atmosphere so that the same water can be cooled and reused, rather than drawing fresh water once and discharging it at an elevated temperature.
- Stable process temperatures: condensers, compressors, reactors, furnaces and moulding machines stay within their design limits.
- Lower energy use: a turbine condenser or chiller supplied with colder water works more efficiently.
- Water saving: a recirculating system uses a small fraction of the water a once-through system needs.
- Less thermal pollution: hot water is not discharged into rivers, lakes or the sea.
How Does a Cooling Tower Work?
Cooling towers operate on the principle of evaporative cooling. Water absorbs approximately 2,400 kJ of latent heat for every kilogram evaporated at typical tower temperatures, and this heat is drawn from the water that remains. The air stream accelerates evaporation and discharges the warm, saturated air from the tower. A smaller share of the heat is transferred directly from the warm water to the cooler air as sensible heat.
How a cooling tower works, step by step
- Hot water enters. Warm water returning from the process or condenser is pumped to the top of the tower.
- Water is distributed. A header with spray nozzles (counterflow towers) or a gravity hot-water basin (crossflow towers) spreads the water evenly over the fill.
- Water flows through the fill. The fill breaks the water into thin films or droplets, creating a large wetted surface and keeping the water in contact with air for longer.
- Air moves through the tower. Fans (mechanical draft) or the chimney effect of a tall shell (natural draft) draw outside air in through the air inlet louvers and through the fill.
- Heat is removed. A small portion of the water evaporates into the air, removing latent heat and lowering the temperature of the remaining water.
- Droplets are captured. Drift eliminators above the water distribution system catch the fine droplets carried by the air before it leaves the tower.
- Cold water returns. The cooled water collects in the cold water basin and is pumped back to the process, completing the cycle.
Evaporation rate: approximately 1% of the circulating water evaporates for every 5.5 °C of cooling range, which is why a continuous supply of makeup water is required. Further detail is available in our guide to evaporation loss in cooling towers and how to calculate it.
Minimum achievable temperature: a cooling tower cannot cool water below the wet-bulb temperature of the ambient air, as this is the lowest temperature attainable through evaporation. Water can be cooled below the dry-bulb air temperature, but not below the wet-bulb temperature. In humid locations such as Mumbai and Chennai, the design wet-bulb temperature is high and the tower must be sized accordingly.
Key Cooling Tower Terms: Range, Approach and Wet Bulb
The following parameters are used to specify and compare cooling towers:
- Range: the difference between the hot water temperature entering the tower and the cold water temperature leaving it.
- Approach: how close the cold water temperature gets to the wet-bulb temperature of the air.
- Wet-bulb temperature (WBT): the lowest temperature air can reach by evaporating water into it. It is measured with a wet-bulb thermometer or psychrometer.
- Heat load: the amount of heat the tower must remove, set by the water flow and the range.
- Circulating water flow: the volume of water pumped through the tower, usually in m³/h.
- Effectiveness: the share of the theoretically possible cooling that the tower actually achieves.
Range = Hot water temperature − Cold water temperature
Approach = Cold water temperature − Wet-bulb temperature
Effectiveness (%) = Range ÷ (Range + Approach) × 100
Example: hot water at 40 °C is cooled to 32 °C on a day with a wet-bulb temperature of 28 °C. The range is 8 °C, the approach is 4 °C and the effectiveness is 8 ÷ 12 × 100 = 67%.
A closer approach delivers colder water, but tower size and cost increase significantly as the approach narrows. Capacity is often quoted in TR; however, conventions differ between manufacturers, so towers should be compared on flow, range, approach and design wet-bulb temperature. Further detail is available in our article on cooling tower capacity.
Parts of a Cooling Tower and Their Functions
The numbered labels in the diagram correspond to the principal cooling tower components listed below. Each component serves one function: moving air, distributing water, increasing the air-water contact surface or supporting the structure.
| Part | What it does |
|---|---|
| Fan | Moves air through the tower; the blade profile and pitch determine the airflow. |
| Fan stack | The cylindrical shroud around the fan, which guides the air and recovers velocity pressure to improve fan efficiency. |
| Motor, gearbox, drive shaft and coupling | The drive train. The gearbox reduces motor speed to the required fan speed, while the shaft and couplings transmit power. |
| Drift eliminator | Captures fine water droplets entrained in the discharge air, reducing water loss and spray around the tower. |
| Spray nozzles, sprinklers and flow control valves | The water distribution system, which spreads hot water evenly over the fill and balances flow between cells. |
| PVC fill, trickle fill and PVC splash bars | The heat-transfer media, which spread the water into films or droplets to create a large air-water contact surface. Refer to the types of cooling tower fills. |
| GRP grid and SS grid | Hold the fill and splash bars in position. |
| Casing | The outer enclosure, which contains the fill and water and directs air through the tower. |
| Air inlet louvers | Admit air while retaining water and shading the basin from sunlight, which limits algae growth. |
| Cold water basin | Collects the cooled water at the base of the tower before it is returned to the process. |
| Structure (treated wood, FRP or RCC) | Carries the weight of the fill, water, fan deck and machinery. |
The complete range of cooling tower parts is available, along with a detailed guide to cooling tower components and types.
Types of Cooling Towers
Cooling towers are classified in four independent ways: by draft (how air is moved), by airflow direction relative to the water, by construction material and by whether the process water is exposed to air.
1. By draft: natural draft vs mechanical draft
- Natural draft cooling towers have no fans. Their tall hyperbolic concrete shell functions as a chimney: the warm, moist air inside is less dense than the ambient air, so it rises and draws fresh air in at the base. They are used in large thermal and nuclear power plants. Read more about natural draft cooling towers.
- Mechanical draft cooling towers use fans, allowing airflow to be controlled independently of weather conditions. The majority of industrial and HVAC cooling towers are of the mechanical draft type.
- Induced draft towers have the fan mounted at the top, drawing air through the tower and discharging the warm exhaust at high velocity. This minimises recirculation of exhaust air, making it the most widely used design.
- Forced draft towers have the fan at the base or side, pushing air into the tower. The fans are easily accessible, but the low-velocity exhaust is more prone to recirculating into the air inlets.
2. By airflow: crossflow vs counterflow
In a crossflow tower the air moves horizontally across falling water. In a counterflow tower the air moves vertically upward against the falling water.
| Feature | Crossflow cooling tower | Counterflow cooling tower |
|---|---|---|
| Air direction | Horizontal, across the falling water | Vertical, upward against the falling water |
| Water distribution | Gravity flow from an open hot-water basin | Pressurised spray nozzles on a header |
| Pump head | Lower | Higher |
| Footprint | Larger plan area | Smaller plan area for the same duty |
| Maintenance access | Distribution basin and fill are easy to reach | Nozzles and fill are inside the tower |
| Typical use | Where pumping energy and easy maintenance matter | Where space is limited and higher thermal performance is needed |
Read the difference between crossflow and counterflow cooling towers in detail →
3. By construction material
- FRP cooling towers: fibre-reinforced plastic is lightweight, corrosion resistant and quick to install, making it suitable for most industrial and HVAC duties.
- Wooden cooling towers: treated timber structures are field-erected, easy to repair and resistant to aggressive water chemistry.
- RCC cooling towers: reinforced concrete towers are built for large flows and a long service life at power plants and heavy industry.
4. By water circuit: open vs closed-circuit
- Open (direct-contact) cooling towers cool the process water directly by exposing it to air. They are the most common and most thermally efficient type.
- Closed-circuit cooling towers carry the process fluid inside a coil. Spray water evaporating on the outside of the coil cools it, so the process fluid remains clean and is never exposed to air.
Where Are Cooling Towers Used?
- Power generation: condensing turbine exhaust steam in captive and thermal power plants and nuclear power plants.
- Oil, gas and chemicals: refineries and petrochemical plants, chemical plants and fertilizer plants.
- Metals and heavy industry: steel and metallurgical plants, heavy engineering and cement plants.
- Process industries: pharmaceutical plants, distilleries, food processing and plastic moulding.
- HVAC: water-cooled chillers in malls, hospitals, hotels, airports and data centres.
View all industries served by Cooltech.
Cooling Tower Water: Evaporation, Drift, Blowdown and Makeup
Because water evaporates while dissolved salts remain, the concentration of solids in the circulating water increases over time. Controlling this concentration is essential for thermal efficiency and equipment life.
- Evaporation: the water converted to vapour, which carries heat away; this is the principal loss.
- Drift: fine droplets entrained in the discharge air. Efficient drift eliminators limit this to a very small fraction of the circulating water.
- Blowdown (bleed-off): water deliberately drained to keep dissolved solids within limits.
- Cycles of concentration (COC): how many times more concentrated the circulating water is than the makeup water. Higher COC means less blowdown and less water use, within the limits of scaling.
- Makeup water: fresh water added to replace losses, where makeup = evaporation + drift + blowdown.
- TDS (total dissolved solids): the measure of salts in the water, used to set blowdown and treatment.
Untreated tower water leads to scale, corrosion, algae and bacterial growth, including Legionella. These problems are prevented by a treatment programme that controls pH, scale, corrosion and biological growth, together with regular cleaning of the basin and fill. Related guides: Legionnaires’ disease in cooling towers and preventing algae in cooling tower systems.
Is a Cooling Tower Environmentally Friendly?
Compared with once-through cooling, yes. A recirculating cooling tower reuses the same water many times, drawing far less fresh water and discharging no heated water to rivers or the sea. Its environmental impact is lowest when the water is treated well enough to run at higher cycles of concentration, drift is controlled with efficient drift eliminators, and fans run on energy-efficient motors or variable-speed drives that match airflow to the heat load.
How to Choose the Right Cooling Tower
Correct sizing of a cooling tower requires the following data:
- Circulating water flow rate (m³/h)
- Hot water inlet and required cold water outlet temperatures
- Design wet-bulb temperature for the site
- Heat load and the process being cooled
- Available space, height limits and site layout
- Water quality (TDS, hardness, suspended solids) or the process fluid
- Preferred material (FRP, timber or RCC), noise limits and power availability
Based on these inputs, Cooltech Engineers recommends the appropriate tower size and type. Contact Cooltech Engineers with these details for a technical recommendation and quotation.
FAQs About Cooling Towers
What is the main purpose of a cooling tower?
To remove waste heat from circulating water by evaporating a small part of it, so the water can be reused to cool equipment, processes or chiller condensers.
What comes out of a cooling tower?
Warm, moist air. The visible white plume is water vapour condensing into fine droplets on contact with cooler ambient air; it is not smoke. A small quantity of fine droplets (drift) may also escape, which drift eliminators are designed to minimise.
Can a cooling tower cool water below the air temperature?
Yes. Because it works by evaporation, a cooling tower can cool water below the dry-bulb air temperature, down towards the wet-bulb temperature, but never below it.
Is a cooling tower better than an AC?
The two serve different functions. An air conditioner cools the air in a room. A cooling tower cools water, rejecting heat from industrial processes or from the condenser of a water-cooled chiller, which then produces chilled water for air conditioning in large buildings.
What is the difference between a cooling tower and a chiller?
A cooling tower cools water by evaporation and can only approach the wet-bulb temperature. A chiller uses a refrigeration cycle to produce water colder than the air. Large installations commonly use both. Further reading: cooling towers vs chillers.
What is TDS in a cooling tower?
TDS (total dissolved solids) is the concentration of salts in the circulating water. It rises as water evaporates and is controlled with blowdown and water treatment to prevent scaling and corrosion.
Why do power plants have cooling towers?
Coal, gas and nuclear power plants condense the steam leaving their turbines back into water. The cooling tower removes this heat from the condenser cooling water so it can be reused.
How often should a cooling tower be maintained?
Routine checks of water quality, fans, drives and nozzles should be part of a regular schedule, with periodic cleaning of the basin, fill and drift eliminators. Refer to the cooling tower maintenance checklist.
