Cooling Tower: Working Principle, Types and Diagram

What is a cooling tower? A cooling tower is a heat rejection device that cools warm water by bringing it into contact with moving air, so that a small part of the water evaporates and carries heat away. The cooled water returns to a chiller condenser or industrial process. Most HVAC plants use induced draft towers.

In a water-cooled chiller plant, the cooling tower is where the building’s heat finally leaves. Power stations, refineries, steel, chemical, pharma and plastics plants use cooling towers for the same reason. This guide explains the cooling tower working principle with a diagram, the types of cooling tower, the parts, key terms such as range and approach, water use and maintenance, with notes for Indian conditions.

Key Highlights

• A cooling tower cools water mainly by evaporation; it cannot cool water below the ambient wet-bulb temperature.
• Roughly 1 percent of the circulating water evaporates for every 5.5°C of cooling (range), as a rule of thumb.
• Main types: natural draught and mechanical draft (forced or induced), counterflow or crossflow, open or closed circuit.
• Induced draft towers, with the fan on top, are the most common choice for HVAC chiller plants.
• Key parts are the fan, drift eliminator, distribution system, fill, air inlet louvers and cold water basin.
• Water treatment controls scale, corrosion, fouling and Legionella; ASHRAE Standard 188 requires a water management plan.

How Does a Cooling Tower Work?

The cooling tower working principle is evaporative cooling, the same effect that cools your skin when sweat dries. Water needs a large amount of heat to change into vapour. When a little of the circulating water evaporates into the air, that heat is taken from the water that stays behind, so its temperature drops.

1. Hot water arrives: Warm water from the chiller condenser or process enters the top of the tower through a header.
2. Water is spread out: Nozzles or a hot water basin distribute it evenly over the fill, which breaks it into thin films or droplets with a very large surface area.
3. Air flows through: A fan (or natural draught) moves outdoor air through the fill, either upward against the falling water (counterflow) or across it (crossflow).
4. Heat and moisture transfer: A small part of the water evaporates and some heat passes directly to the cooler air. The air leaves warm and nearly saturated.
5. Cold water collects: Cooled water falls into the basin, passes through a strainer and is pumped back to the condenser to pick up heat again.

Because evaporation depends on how dry the air is, a cooling tower’s performance is governed by the wet-bulb temperature, not the ordinary dry-bulb temperature. This is why water-cooled plants stay more efficient than air-cooled ones on hot, dry afternoons, and why towers in humid coastal cities need more surface area for the same duty.

Cooling Tower Diagram and Parts

Part	Function	Common materials and notes
Fan and motor	Moves air through the tower; the main energy user	Axial fan with FRP or aluminium blades; belt, gear or direct drive; VFD for capacity control
Fan stack / cylinder	Shapes airflow and discharges air upward	FRP; height helps reduce recirculation
Drift eliminator	Catches water droplets carried by the air	PVC cellular or blade type; cuts water loss and aerosol release
Distribution system	Spreads hot water evenly over the fill	PVC or FRP header with spray nozzles (counterflow) or a hot water basin (crossflow)
Fill (packing)	Creates surface area for air and water contact	PVC film fill for clean water; splash fill for dirty or high-solids water
Air inlet louvers	Let air in, keep water and sunlight out	PVC or FRP; reduce algae growth and splash-out
Cold water basin	Collects cooled water	FRP, concrete or steel; with strainer, overflow, drain and make-up float valve
Casing and structure	Holds everything together	FRP, galvanised or stainless steel, or concrete for large field-erected towers

Each part affects performance. Clogged fill or blocked nozzles reduce the wetted area, and damaged drift eliminators raise water loss. Fill and drift eliminators are often the first parts to replace when an older tower stops reaching its design temperature.

Types of Cooling Tower

There are several ways to classify cooling towers. In practice a tower is described by a combination, for example an induced draft counterflow open-circuit tower.

Classification	Type	Key feature	Typical use
By airflow	Natural draught (hyperbolic)	Tall concrete shell; warm air rises by buoyancy, no fan	Large thermal power stations
By airflow	Mechanical forced draft	Fan at the air inlet pushes air through	Smaller towers, indoor or ducted installations
By airflow	Mechanical induced draft	Fan at the top pulls air through	Most HVAC and industrial towers
By air and water path	Counterflow	Air moves up, water falls down	Compact footprint; common in round FRP towers
By air and water path	Crossflow	Air moves horizontally across falling water	Easy access to distribution; lower pump head
By water contact	Open circuit	Process water touches the air directly	Most chiller condenser loops
By water contact	Closed circuit (fluid cooler)	Process fluid stays inside a coil; spray water cools the coil	Where the loop must stay clean, such as water-source heat pumps, furnaces, data centres
By construction	Factory-assembled or field-erected	Packaged modules or towers built on site	Packaged for buildings; field-erected for big plants

In India, round bottle-shaped FRP towers (induced draft counterflow) are widely used for small and medium chiller plants, while rectangular FRP or concrete cell towers serve larger buildings and industry. For a broader look at plant equipment, see our HVAC equipment list.

What Is an Induced Draft Cooling Tower?

An induced draft cooling tower places the fan at the top of the tower, on the air outlet. The fan creates a slight negative pressure inside, drawing outdoor air through the inlet louvers and up (or across) through the fill. Warm, moist air leaves at high velocity through the fan stack.

Point	Induced draft	Forced draft
Fan position	Top, on discharge side	Bottom or side, on inlet side
Air distribution	Even through the fill	Can be uneven near the fan
Recirculation of hot exit air	Low, because air leaves fast and upward	Higher, because exit velocity is low
Fan exposure	Fan handles warm, moist air	Fan handles dry inlet air
Typical use	Most HVAC and industrial towers	Indoor towers, ducted inlets, smaller units

Range, Approach, Wet-Bulb and Tonnage

• Range: hot water temperature in minus cold water temperature out. For a chiller plant it is often about 5 to 5.5°C.
• Approach: cold water temperature out minus ambient wet-bulb temperature. Towers are commonly selected for an approach of about 3 to 5°C; a smaller approach needs a much larger tower.
• Wet-bulb temperature: the lowest temperature air can reach by evaporating water into it. It sets the limit for any cooling tower. Use ISHRAE design weather data for the project city.
• Cooling tower tonnage: by common industry convention, one cooling tower ton is 15,000 BTU/h of heat rejection, more than a refrigeration ton (12,000 BTU/h), because the tower must also reject the chiller compressor’s heat.
• Effectiveness: range divided by (range + approach). It shows how close the tower gets to the theoretical limit.

Example: if hot water enters at 37°C, cold water leaves at 32°C and the design wet-bulb is 28°C, the range is 5°C and the approach is 4°C.

How Much Water Does a Cooling Tower Use?

Make-up water must replace three losses. In water-stressed Indian cities, these numbers matter as much as energy.

Loss	What it is	How to estimate or reduce it
Evaporation (E)	Water that turns to vapour and does the cooling	About 1% of circulating flow per 5.5°C of range (rule of thumb); cannot be avoided
Blowdown (B)	Water drained to stop dissolved solids building up	B = E / (COC – 1); raise cycles of concentration (COC) with good treatment
Drift (D)	Droplets carried out with the air	A very small fraction of flow with modern drift eliminators; replace damaged eliminators

Make-up = evaporation + blowdown + drift. Cycles of concentration (COC) compare dissolved solids in the tower water with those in the make-up water. Going from 3 to 6 cycles, for example, cuts blowdown by more than half. Many Indian sites also consider treated sewage or recovered condensate as make-up, subject to local rules and treatment.

Water Treatment and Legionella Control

An open cooling tower is a large air scrubber: it collects dust, pollen and organic matter, and evaporation concentrates the dissolved salts. Without treatment, four problems follow.

• Scale from calcium and magnesium salts coats condenser tubes and raises chiller power. Hard groundwater in many Indian cities makes this common.
• Corrosion attacks steel piping, pumps and heat exchangers.
• Biological fouling (algae, slime, bacteria) blocks fill and nozzles and insulates heat transfer surfaces.
• Legionella bacteria can grow in warm, stagnant water and spread in aerosols. ANSI/ASHRAE Standard 188 requires buildings with cooling towers to have a water management plan, and ASHRAE Guideline 12 describes control measures.

A typical programme combines scale and corrosion inhibitors, oxidising and non-oxidising biocides, automatic conductivity-based blowdown, side-stream filtration and periodic cleaning. Monitor COC, pH, conductivity and microbiological counts as your treatment supplier specifies.

How to Select a Cooling Tower in India

1. Fix the heat load: chiller cooling capacity plus compressor heat, or process heat rejection.
2. Set water temperatures and flow in line with the chiller condenser design and the range you want.
3. Use the right design wet-bulb for the city from ISHRAE data, not a generic value. Bangalore, Chennai, Mumbai and Delhi have very different design conditions.
4. Choose type and material: FRP induced draft for most buildings; closed circuit where the loop must stay clean; splash fill for dirty water.
5. Check the site: free air around inlets, distance from fresh air intakes to limit drift and recirculation, structural load on the roof, noise near neighbours.
6. Plan energy and water: VFD fans, multiple cells for part load, and a water treatment plan from day one.
7. Ask for certified performance: thermal performance certified or tested to Cooling Technology Institute (CTI) procedures gives more confidence than catalogue figures.

Cooling tower selection is part of the whole condenser water system design. Read more about how an HVAC system works and HVAC design basics, or talk to Envigaurd’s HVAC contracting team.

Cooling Tower Maintenance Checklist

Frequency	Task
Daily	Check basin water level, make-up float valve, fan running and vibration, water treatment readings, leaks and overflow
Weekly	Inspect distribution nozzles and spray pattern; check blowdown operation and chemical stock
Monthly	Clean strainers; check belt tension or gearbox oil; inspect louvers and drift eliminators; review microbiological test results
Half-yearly / yearly	Drain, clean and disinfect basin and fill; inspect fill for scaling and sagging; check structure, fasteners and motor insulation; align fan

Frequently Asked Questions

What is a cooling tower in simple words?

A cooling tower is a heat rejection device that cools warm water by letting a small part of it evaporate into moving air. The cooled water is pumped back to a chiller condenser, a heat exchanger or a process, picks up heat again, and returns to the tower. It is how many large buildings and factories get rid of the heat they remove.

What is the working principle of a cooling tower?

A cooling tower works on evaporative cooling. Warm water is sprayed over fill media so it spreads into thin films and droplets, while air flows through the fill. A small fraction of the water evaporates, and the latent heat needed for evaporation comes from the remaining water, which becomes cooler. Some sensible heat also passes directly to the air.

What are the main types of cooling tower?

Cooling towers are classified by airflow (natural draught or mechanical draft, with mechanical draft split into forced draft and induced draft), by the direction of air and water (counterflow or crossflow), and by water contact (open circuit or closed circuit). Induced draft counterflow and crossflow towers are the most common in HVAC plants.

What is an induced draft cooling tower?

An induced draft cooling tower has its fan at the top, on the discharge side, so it pulls air in through the louvers at the bottom or sides and pushes warm, moist air out vertically. The high exit velocity reduces the chance of hot air recirculating back into the inlets, which is why it is the most common type for chiller plants.

What is range and approach in a cooling tower?

Range is the difference between the hot water entering and the cold water leaving the tower. Approach is the difference between the cold water leaving and the ambient wet-bulb temperature. A tower can never cool water below the wet-bulb temperature, and a smaller approach needs a larger, more costly tower.

How much water does a cooling tower use?

Make-up water replaces evaporation, blowdown and drift. A common rule of thumb is that about 1 percent of the circulating flow evaporates for every 5.5°C (10°F) of range, before allowing for local humidity. Blowdown depends on the cycles of concentration the water treatment allows. Higher cycles mean less blowdown and lower water use.

Why is Legionella a concern in cooling towers?

Cooling towers hold warm water and release fine droplets, which can spread Legionella bacteria if the water is not managed. ASHRAE Standard 188 requires a water management plan for buildings with cooling towers, and ASHRAE Guideline 12 gives practical control measures such as biocide dosing, cleaning, efficient drift eliminators and regular testing.

How often should a cooling tower be cleaned?

Daily or weekly checks of water level, fan and water treatment readings are normal, with strainer cleaning and inspection monthly. Most operators clean the basin, nozzles and fill and disinfect the system at least once or twice a year, and more often if water quality is poor. Follow the manufacturer’s manual and your water management plan.

Sources and Further Reading

• Cooling Technology Institute (CTI): thermal performance certification and acceptance test codes for cooling towers
• ASHRAE: Standard 188 (Legionellosis: Risk Management for Building Water Systems), Guideline 12 and the ASHRAE Handbook chapter on cooling towers
• US EPA WaterSense at Work: best management practices for cooling tower water efficiency
• Bureau of Energy Efficiency (BEE): energy efficiency guide books for cooling towers and the Energy Conservation Building Code
• ISHRAE: Indian weather data (design wet-bulb temperatures) and HVAC resources

Conclusion

A cooling tower is a simple machine with a big job: it uses a small amount of evaporation to reject the heat of an entire building or plant. Knowing how it works, which type suits your site, and how range, approach and wet-bulb set its limits helps you size it correctly. Good water treatment and regular cleaning then keep it efficient and safe for decades.

If you are designing a new water-cooled chiller plant or your tower no longer delivers design temperatures, talk to Envigaurd’s HVAC team for design, supply, installation, commissioning and AMC support.