Cooling Towers: Types, Approach & Range, and Selection Guide

Quick Answer

A cooling tower rejects waste process heat to the atmosphere by exposing warm water to an air stream. In a wet (evaporative) tower — the dominant industrial type — a small fraction of the water evaporates, and because evaporation absorbs large amounts of latent heat, the remaining water is cooled efficiently; the theoretical limit is the ambient wet bulb temperature, not dry bulb, which is why wet towers can cool water below air temperature. Dry towers use finned coils with no evaporation (no water loss, no plume, but limited by dry bulb and much larger), and hybrid/adiabatic towers combine both. Towers are classified by air movement — natural draft (huge hyperbolic concrete shells, power stations) versus mechanical draft, which splits into induced draft (fan at the top, pulling air through — most common industrially) and forced draft (fan at the inlet, pushing air) — and by flow arrangement: counterflow (air moves vertically against falling water; more thermally efficient, smaller footprint, taller) versus crossflow (air moves horizontally across falling water; lower pump head, easier maintenance access, larger footprint). Performance is defined by range (hot water temperature minus cold water temperature — set by the process heat load) and approach (cold water temperature minus ambient wet bulb — set by tower size; a tighter approach needs a much bigger tower, and approaches below about 3°C become uneconomic). Water losses occur through evaporation, drift, and blowdown, and controlling cycles of concentration is central to both water economy and scale/corrosion control.

Every industrial process that generates heat has to put that heat somewhere, and for most large plants the destination is the atmosphere via a cooling tower. Refineries, power stations, chemical plants, steel mills, and data centres all depend on them. The tower is rarely the most technically glamorous item on the plot plan, but it is often the item that limits plant capacity on the hottest day of the year — because when ambient wet bulb rises, cooling water temperature rises with it, condenser pressure climbs, and throughput falls.

That relationship is what makes cooling tower selection a genuine engineering decision rather than a catalogue pick. Two numbers govern it: range, which the process sets, and approach, which the tower size sets. Specify a tight approach and the tower becomes large and expensive; specify a loose one and the plant loses capacity in summer. Everything else — draft type, flow arrangement, fill selection — follows from getting those two right for the site's design wet bulb.

The second theme is water. A wet tower consumes water continuously through evaporation, drift, and blowdown, and concentrates dissolved solids as it does so. Manage that badly and the result is scale on heat exchanger tubes, corrosion, biological fouling, and in the worst case a Legionella incident. In water-scarce regions — much of the Middle East, North Africa, and Central Asia — the water consumption question often decides whether a wet tower is viable at all.

For plant engineers, utilities specialists, and procurement teams — this guide covers cooling towers: types and configurations, the approach/range relationship, fill and component selection, water management, and how to specify.

For related heat rejection equipment, see Air Cooled Heat Exchangers and Heat Exchangers.

How Cooling Towers Work

Warm water from the process is distributed over a fill (packing) that spreads it into a thin film or droplets, maximising the surface area in contact with air. Air moves through the fill, and two heat transfer mechanisms operate:

• Sensible heat transfer — direct heat exchange between water and air
• Latent heat transfer (evaporation) — a small portion of the water evaporates, absorbing a large amount of latent heat from the remaining water

Evaporation does most of the work. Roughly 1% of the circulating water evaporating cools the remainder by about 5.5°C (10°F).

Because cooling is driven by evaporation, the theoretical minimum temperature is the ambient wet bulb temperature, which is lower than dry bulb whenever the air is not saturated. This is the fundamental advantage of wet towers over dry air cooling.

Cooled water collects in the basin and returns to the process.

Tower Types

Wet (Evaporative) Cooling Towers

Open-circuit towers where process water contacts the air directly.

• Most thermally efficient and most common industrially
• Can cool water below ambient dry bulb temperature
• Consumes water; produces a visible plume in cool weather
• Requires water treatment and Legionella management

Dry Cooling Towers

Water circulates inside finned tube coils; air passes over the outside. No contact, no evaporation.

• No water consumption, no plume, no Legionella risk from the tower
• Limited by dry bulb temperature — cannot cool below ambient air temperature
• Much larger and more capital-intensive for the same duty
• Used where water is scarce or unavailable, and in some environmental jurisdictions

For dry heat rejection, see also Air Cooled Heat Exchangers.

Hybrid / Adiabatic Towers

Combine dry operation for most of the year with evaporative assistance on hot days (pre-cooling the inlet air or wetting the coil).

• Dramatically reduced water use compared with a wet tower
• Better hot-weather performance than a pure dry system
• Higher complexity and capital cost

Closed-Circuit (Fluid Cooler)

Process fluid stays inside a coil while a separate spray water circuit evaporates over the outside.

• Keeps the process fluid clean and isolated from atmospheric contamination
• Useful for glycol loops, closed cooling water systems, and equipment jackets
• Lower thermal efficiency than an open tower for the same size

Draft Type

Natural Draft

The tall hyperbolic concrete shell familiar from power stations. Buoyancy of warm, moist air inside the shell creates the draft — no fans.

• Very large capacity
• No fan power — low operating cost
• Very high capital cost and civil works
• Used almost exclusively for power station-scale duties

Mechanical Draft

Fans move the air. The industrial standard.

Induced draft — fan mounted at the top, drawing air through the tower and discharging it upward.

• High discharge velocity reduces recirculation of humid exhaust back into the inlet
• Most common industrial configuration
• Fan operates in the hot, humid exhaust stream (motor usually outside via drive shaft)

Forced draft — fan mounted at the inlet, pushing air through.

• Fan handles cool dry air — easier on the fan and motor, better for freezing climates
• Lower discharge velocity means higher recirculation risk
• Common on smaller packaged units

Flow Arrangement

Counterflow

Air moves vertically upward against water falling downward.

• Higher thermal efficiency — the coldest water meets the driest air
• Smaller footprint for the same duty
• Taller, and requires higher pump head (water sprayed from a pressurised distribution system)
• Enclosed distribution system reduces algae growth from sunlight

Crossflow

Air moves horizontally across the falling water.

• Lower pump head — water is gravity-distributed from open basins at the top
• Easier access to the distribution system for maintenance while running
• Larger footprint and generally lower thermal efficiency
• Open hot water basins are more exposed to sunlight and debris

Range and Approach

These two parameters define tower performance and drive its size.

Range = hot water temperature − cold water temperature.
Range is set by the process heat load and the circulation rate: Q = m × cp × range. It is a process decision, not a tower decision.

Approach = cold water temperature − ambient wet bulb temperature.
Approach is set by the tower size and design. It is where the money goes.

The key relationship: approach cannot be zero — that would require an infinitely large tower. As approach tightens, tower size and cost rise steeply. Approaches below about 3°C (5°F) become uneconomic; typical industrial design approaches sit around 3–8°C.

Worked logic: if design wet bulb is 28°C and you specify a 5°C approach, cold water leaves at 33°C. Ask for a 3°C approach and you get 31°C water — but the tower may be 40–50% larger. The question is always whether the process genuinely benefits from the colder water.

Design wet bulb selection matters enormously. Choosing the site's 1% or 5% exceedance wet bulb (rather than the absolute maximum) is standard practice, but in hot humid regions the difference between design points can change tower size significantly.

Fill (Packing)

Fill maximises air-water contact area and is the heart of tower performance.

Film fill — thin corrugated PVC sheets over which water spreads as a film.

• Highest thermal efficiency per volume
• Prone to fouling and blockage with suspended solids, biological growth, or oily water
• Requires clean water and good treatment

Splash fill — bars or grids that repeatedly break the falling water into droplets.

• Much more tolerant of dirty water, solids, and biological fouling
• Lower efficiency per volume, so a larger tower
• Preferred for poor water quality, high-suspended-solids service, and steel/process applications

Low-clog / anti-fouling film fill — larger flute geometry, a compromise between the two.

Material: PVC is standard (temperature limit around 50–55°C); CPVC or PP for hotter water.

Rule: if water quality is uncertain or poor, choose splash or low-clog fill. Fouled film fill is the single most common cause of gradual cooling tower performance loss.

Other Components

• Drift eliminators — capture entrained water droplets in the exhaust; modern designs limit drift to around 0.001–0.005% of circulating flow. Important for water economy, plume nuisance, and preventing dispersal of biologically contaminated droplets.
• Water distribution — spray nozzles (counterflow) or gravity basins with orifices (crossflow)
• Fans — axial, with gearbox or belt drive; VFDs give large energy savings at part load
• Basin — concrete or steel; includes strainers, makeup and blowdown connections, and often side-stream filtration
• Structure and casing — timber (legacy), galvanised steel, stainless steel, FRP, or concrete

Water Management

A wet tower loses water three ways:

1. Evaporation — the useful loss; roughly 1% of circulation per 5.5°C of range
2. Drift — entrained droplets carried out with the air; minimised by drift eliminators
3. Blowdown (bleed) — water deliberately discharged to control dissolved solids concentration

Cycles of concentration (CoC) = the ratio of dissolved solids in circulating water to those in makeup water. Because evaporation removes pure water and leaves salts behind, solids concentrate.

• Higher CoC = less blowdown = less makeup water and lower chemical cost
• But higher CoC also means higher scaling and corrosion risk
• Typical operation runs at 3–6 cycles, depending on makeup water quality and treatment programme

Treatment programme must address:

• Scale — calcium carbonate and sulphate deposition (see Heat Exchanger Cleaning & Maintenance)
• Corrosion — inhibitors matched to system metallurgy
• Biological growth — biocides; a wet tower is an ideal environment for bacteria
• Suspended solids — side-stream filtration

Legionella deserves specific mention. Cooling towers create warm water and aerosols — the conditions Legionella bacteria need to grow and disperse. A documented water safety plan covering temperature control, biocide dosing, drift limitation, regular cleaning, dead-leg elimination, and sampling is a legal requirement in many jurisdictions and good practice everywhere.

Selection Process

1. Heat load (Q) and required water flow rate
2. Hot and cold water temperatures → gives range
3. Site design wet bulb temperature → with cold water temperature gives approach
4. Water availability and quality → wet, hybrid, or dry; film or splash fill
5. Draft and flow arrangement → induced/forced, counterflow/crossflow, based on footprint, height limits, pump head, and maintenance access
6. Plot space, height restrictions, prevailing wind → orientation and spacing to avoid recirculation between cells and from adjacent structures
7. Materials → for water chemistry, ambient corrosion, and fire code (FRP/steel rather than timber in many jurisdictions)
8. Noise limits → low-noise fans, attenuators where near boundaries
9. Controls → VFD fan control, basin level, blowdown conductivity control, chemical dosing

Common Specification Mistakes

After 15+ years supplying industrial equipment to process and utility projects:

Mistake 1: Unrealistically Tight Approach

3°C or lower approach specified without evaluating cost. The tower is far larger and more expensive than the process benefit justifies.

Prevention: Evaluate the process value of colder water against tower cost. Approaches below about 3°C are rarely economic; 3–8°C is the normal design band.

Mistake 2: Wrong Design Wet Bulb

Tower designed to an average or optimistic wet bulb. On hot humid days the tower cannot make design cold water temperature and plant throughput falls.

Prevention: Use site-specific design wet bulb at the appropriate exceedance level, and check summer performance explicitly.

Mistake 3: Film Fill with Poor Water Quality

High-efficiency film fill specified where makeup water carries suspended solids or the system fouls biologically. The fill blocks, airflow drops, and performance degrades progressively.

Prevention: Match fill to water quality — splash or low-clog fill for dirty water, oily service, or unreliable treatment.

Mistake 4: Ignoring Recirculation

Towers sited too close together or downwind of obstructions. Humid exhaust is drawn back into the inlet, raising the effective wet bulb and destroying performance.

Prevention: Follow manufacturer spacing rules, orient to prevailing wind, and prefer induced draft where recirculation risk is high.

Mistake 5: Under-Managing Cycles of Concentration

CoC pushed high to save water without adequate treatment. Scale forms on exchanger tubes and corrosion accelerates.

Prevention: Set CoC from makeup water chemistry and the treatment programme, with automatic conductivity-controlled blowdown.

Mistake 6: No Legionella Management Plan

Tower installed without a documented water safety regime. Biological growth develops and creates a serious health risk.

Prevention: Implement a water safety plan — biocide programme, drift eliminators, cleaning schedule, dead-leg elimination, and routine sampling.

Mistake 7: Choosing Wet Where Water Is Scarce

Conventional wet tower specified in an arid region without evaluating makeup water availability and cost.

Prevention: In water-scarce locations, evaluate hybrid/adiabatic and dry alternatives on lifecycle cost including water.

Supply from Kasko Makine

Kasko Makine supplies cooling towers, components, and heat rejection equipment for refinery, petrochemical, power, chemical, and industrial projects:

Tower types:

• Induced and forced draft mechanical towers
• Counterflow and crossflow configurations
• Field-erected and packaged units
• Closed-circuit fluid coolers
• Hybrid / adiabatic systems
• Dry cooling and air cooled heat exchangers

Components and spares:

• Film, splash, and low-clog fill
• Drift eliminators
• Spray nozzles and distribution systems
• Fans, gearboxes, drive shafts, and motors
• Structural components, casing, and louvres
• Basin strainers and side-stream filtration
• Fan stacks and velocity recovery

Circulation equipment: cooling water pumps, valves, and piping (see Centrifugal Pumps)

Engineering support:

• Thermal selection from heat load, range, approach, and site wet bulb
• Wet vs hybrid vs dry evaluation for water-scarce sites
• Fill selection against water quality
• Layout and recirculation review
• Water balance (evaporation, drift, blowdown) and cycles of concentration
• Materials selection for water chemistry and ambient conditions
• Energy analysis including VFD fan control

Certification: thermal performance data, material test certificates (EN 10204 Type 3.1), fan and motor certification, drift eliminator performance data, third-party thermal certification where required

Logistics: Cooling tower components shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Spares and fill 4-10 weeks; packaged towers 12-20 weeks; field-erected towers by project schedule.

Need a cooling tower or replacement components? Send us the heat load or circulating water flow, hot and cold water temperatures, site design wet bulb, makeup water analysis, available plot space and height limits, and any noise or plume restrictions to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll size the tower, recommend the configuration and fill for your water quality, and provide a quotation within 72 hours.

Continue Reading: Heat Transfer Guides

• Heat Exchangers: Complete Guide — Process heat transfer equipment
• Air Cooled Heat Exchangers — Dry heat rejection
• Heat Exchanger Cleaning & Maintenance — Scale and fouling control
• Centrifugal Pumps — Cooling water circulation
• Chemical Factory Setup — Plant utilities

Frequently Asked Questions

Q: How does a cooling tower work?
A: A cooling tower rejects waste heat to the atmosphere by exposing warm process water to an air stream. Water is distributed over a fill (packing) that spreads it into thin films or droplets to maximise contact area with air. Two mechanisms transfer heat: sensible heat transfer directly between water and air, and latent heat transfer through evaporation. Evaporation does most of the work — roughly 1% of the circulating water evaporating cools the remainder by about 5.5°C. Because cooling is evaporation-driven, the theoretical minimum achievable temperature is the ambient wet bulb temperature rather than dry bulb, which is why a wet cooling tower can cool water below the surrounding air temperature. Cooled water collects in the basin and returns to the process.

Q: What is the difference between approach and range in a cooling tower?
A: Range is the difference between the hot water temperature entering the tower and the cold water temperature leaving it. Range is determined by the process heat load and circulation rate — it is a process decision, not a tower design decision. Approach is the difference between the cold water temperature leaving the tower and the ambient wet bulb temperature. Approach is determined by tower size and design, and it is where capital cost concentrates. Approach can never be zero because that would require an infinitely large tower, and as approach tightens, tower size and cost rise steeply. Approaches below about 3°C become uneconomic, with typical industrial design approaches falling in the 3–8°C band.

Q: What is the difference between counterflow and crossflow cooling towers?
A: In a counterflow tower, air moves vertically upward against water falling downward, so the coldest water meets the driest incoming air. This gives higher thermal efficiency and a smaller footprint, but the tower is taller and requires higher pump head because water is sprayed from a pressurised distribution system; the enclosed distribution also limits algae growth. In a crossflow tower, air moves horizontally across the falling water. Water is gravity-distributed from open basins at the top, which reduces pump head and gives easier maintenance access to the distribution system while running, but the footprint is larger, thermal efficiency generally lower, and the open hot water basins are more exposed to sunlight and debris.

Q: What is the difference between induced draft and forced draft cooling towers?
A: Both are mechanical draft towers, differing in fan location. An induced draft tower has the fan mounted at the top, drawing air through the tower and discharging it upward at high velocity. The high discharge velocity reduces recirculation of humid exhaust back into the air inlet, making induced draft the most common industrial configuration. A forced draft tower has the fan mounted at the air inlet, pushing air through the tower. The fan handles cool dry air, which is easier on the fan and motor and better suited to freezing climates, but the lower discharge velocity increases the risk of recirculation. Forced draft is common on smaller packaged units.

Q: What are cycles of concentration in a cooling tower?
A: Cycles of concentration (CoC) is the ratio of dissolved solids in the circulating cooling water to those in the makeup water. Because evaporation removes pure water and leaves dissolved salts behind, solids progressively concentrate in the system. Blowdown — deliberately discharging a portion of circulating water — controls this concentration. Running at higher cycles means less blowdown, so less makeup water and lower chemical consumption, but it also increases scaling and corrosion risk as solids concentrate further. Typical industrial operation runs at 3–6 cycles depending on makeup water quality and the treatment programme. Automatic conductivity-controlled blowdown is standard practice for maintaining the target.

Q: What type of cooling tower fill should be used?
A: Fill choice depends on water quality. Film fill uses thin corrugated PVC sheets over which water spreads as a film, giving the highest thermal efficiency per unit volume — but it is prone to fouling and blockage from suspended solids, biological growth, and oily water, so it requires clean water and reliable treatment. Splash fill uses bars or grids that repeatedly break falling water into droplets; it is far more tolerant of dirty water, solids, and biological fouling, but has lower efficiency per volume so the tower must be larger. Low-clog film fill with larger flute geometry is a compromise. If water quality is poor or uncertain, choose splash or low-clog fill — fouled film fill is the most common cause of gradual cooling tower performance loss.