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Cooling towers, dry coolers and adiabatic coolers compared

Updated 23 October 2026 · SEO Dons Editorial

What do the terms mean?

This page is about heat rejection for building and process cooling, such as the condenser water circuit of a water-cooled chiller, not power-station towers. The law and the government's Energy Technology List (ETL) define the equipment in plain terms.

  • Cooling tower: the 1992 notification regulations define it as a device whose main purpose is to cool water by direct contact between that water and a stream of air. That is the open tower.
  • Evaporative condenser: a device whose main purpose is to cool a fluid by passing it through a heat exchanger that is itself cooled by contact with water passing through a stream of air. That wording describes the coil-based, closed-circuit arrangement.
  • Dry cooler: the ETL describes a self-contained system that cools a single-phase liquid by rejecting sensible heat via a heat exchanger to air circulated by integral fans.
  • Adiabatic cooler: the ETL describes a dry cooler that can also present water into the path of the air, reducing the air temperature through evaporative cooling before it reaches the heat exchanger.

The ETL notes that industry also calls these units dry adiabatic coolers or hybrid coolers, so the names overlap. HSE uses 'hybrid' and 'adiabatic' for systems designed to run in both dry and wet modes.

How do the three compare?

QuestionCooling towerDry coolerAdiabatic cooler
How is heat rejected?By water evaporating into the air streamSensible heat to air onlyDry, plus evaporative pre-cooling of the air when hot
Coldest fluid it can supplyClosest to the wet-bulb temperature, so generally the most efficientAlways warmer than the air, by the approachBetween the two, depending on how humid the air is
Water useContinuous evaporation, plus treatmentNone in operationOnly in wet mode
Notifiable to the council?Yes, as a notifiable deviceNo water exposed to air, so outside the definitionDepends on construction, so check
Visible plumePossibleLower riskDepends on design

The efficiency row comes from CIBSE Journal, which notes that cooling towers generally deliver more efficient operation than air-cooled condensers. The King's Cross district cooling scheme, described in the same journal, runs air-cooled chillers below 17 °C ambient because that reduces the risk of pluming, and notes they are less energy efficient. The 'approach' is the gap between the fluid leaving the cooler and the air entering it.

For electricity, the ETL sets a listing minimum for dry and adiabatic coolers of an energy efficiency ratio of 90 or more at a 5 K liquid temperature difference and a 15 K approach, with hybrid coolers tested dry. Fan power is small against the heat moved, but that is a floor, not a typical figure.

How much water does evaporative heat rejection use?

Evaporation is what makes towers efficient and thirsty. CIBSE Journal states that every litre of water evaporated requires about 2,500 kilojoules of energy. One kilowatt-hour is 3,600 kilojoules, so rejecting a kilowatt-hour entirely by evaporation takes roughly 1.4 litres.

Illustrative example: a plant rejecting 500 kW entirely by evaporation for 1,000 hours would evaporate about 720,000 litres, or 720 cubic metres. That excludes blowdown and drift, which add to the total. An adiabatic cooler evaporates water only in its wet hours, so its annual figure depends on how often the weather forces wet mode.

What happens on a hot UK day?

A dry cooler can only return fluid warmer than the air entering it. At the ETL's 15 K approach, 35 °C air would give fluid leaving at about 50 °C, which is illustrative arithmetic, not a model rating. The Met Office confirmed a UK record of 40.3 °C at Coningsby, Lincolnshire, in July 2022, so the design ambient is a decision to take deliberately, not copy from an old specification.

Adiabatic pre-cooling narrows the gap. CIBSE Journal describes adiabatic cooling as evaporating water into the air so that it cools to a temperature close to the wet-bulb temperature, and says it is most effective in high dry-bulb, low-humidity conditions. In a worked example for condenser air, 30 °C dry-bulb and 20 °C wet-bulb air leaves the wetted media at approximately 23 °C. The more humid the air, the smaller the drop. For how heat rejection fits each chiller type, see the types of chillers guide.

What are the Legionella and legal differences?

The notification duty is the clearest line. Regulation 3 of the 1992 regulations makes each person with control of premises responsible for ensuring no notifiable device is situated there unless it has been notified in writing to the local authority. A notifiable device is a cooling tower or evaporative condenser, except where it contains no water that is exposed to air, among other exceptions. A true dry cooler has no water exposed to air.

HSE says that if you have a cooling tower or evaporative condenser you should put in place measures to prevent or control legionella as described in its Approved Code of Practice, and that if you have another type of cooling system you need other measures that are equally effective. Its guidance on dry/wet systems says these can have a wide range of risk profiles, and it points to a Water Management Society document endorsed by HSE. The technical guidance, HSG274 Part 1, was published as a second edition in March 2024.

In practice, treat every adiabatic unit as needing its own legionella risk assessment, and ask the local authority whether a given model is notifiable. The duties for towers themselves are set out in the cooling tower legionella guide.

How do you choose between them?

  1. Start with the fluid temperature the process or chiller needs. CIBSE notes that higher supply temperatures allow more efficient heat rejection using dry coolers or cooling towers.
  2. Compare that with the design dry-bulb and wet-bulb temperatures for the site, and check the capacity on the hottest days you must cover.
  3. Count the hours of use. A year-round load keeps a tower evaporating, while a summer-only load may suit adiabatic assistance.
  4. Weigh water supply and treatment capacity, plume, noise and the planning position.
  5. Cost the legal workload: notification, risk assessment, monitoring and records for wet systems.

Evaporative cooling of supply air for comfort is a different subject, covered in the guide to evaporative cooling versus air conditioning. If you are selecting heat rejection for a chiller project, the commercial chillers page explains the process, and you can send an enquiry through the site's form.

Sources

Regulations and guidance are revised, so confirm the current text before relying on a figure.

Common questions

Is an adiabatic cooler the same as a cooling tower?

No. The ETL describes an adiabatic cooler as a dry cooler that can also wet the air before it reaches the heat exchanger. A cooling tower rejects heat by bringing water and air into contact, and carries the notification duty for notifiable devices.

Does a dry cooler need a Legionella risk assessment?

A true dry cooler has no water exposed to air, so it is outside the notification definition. The site's wider water systems still need a competent assessment, which should record what the dry cooler contains.

Can a dry cooler simply replace a cooling tower on an existing chiller?

Not automatically. A tower can supply condenser water closer to the wet-bulb temperature, so a dry cooler gives warmer water on hot days. The chiller must be rated for that at the design ambient, which its maker can confirm.

Does an adiabatic cooler use less water than a tower?

It uses water only when it runs wet, so annual use is normally lower than a tower evaporating all year. The saving depends on how many hours the local weather forces wet mode, so ask for a calculation using your site's climate.

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