Evaporative cooling explained

Aug 28, 2026

Summary

Evaporative cooling is a low-energy cooling approach that uses the natural evaporation of water to reduce air temperature. Because it does not rely on a compressor or refrigerant-based cooling circuit, it can support lower energy consumption and reduced refrigerant dependency in suitable applications. Evaporative cooling is particularly effective in hot, dry regions, where the air has greater capacity to absorb moisture.

What is evaporative cooling?

Evaporative cooling units are designed to use water evaporation to cool air. These systems do not use refrigerant or rely on a compressor, which can make them a low-energy and refrigerant-free cooling option when specified for the right conditions. They are particularly effective in hot, dry regions – the drier the climate, the greater the cooling capacity – where they are typically used for air conditioning in buildings. However, they can also be effective in humid climates, and in specialist applications such as industrial buildings, logistics spaces, data centres, production areas and ventilation systems. For industry professionals, the key considerations are climate suitability, indoor humidity control, water consumption, hygiene management, accurate performance data and whole-system design.

What are the main types of evaporative cooling systems?

In a direct evaporative cooling (DEC) system, warm unsaturated air is drawn into the unit and passed across a wetted medium or water distribution system. As the water evaporates, it absorbs heat from the air, lowering the air temperature before it is supplied to the building or process. DEC units are typically self-contained, with an integrated fan and motor, and can use either recirculating or non-recirculating water. Because moisture is added directly to the supply air, good ventilation balance and humidity control are essential.

Indirect evaporative cooling (IEC) uses evaporation in one airstream to cool a separate supply airstream through a heat exchanger. The two airstreams do not mix, so the primary supply air can be cooled without adding humidity. This makes IEC a useful option for applications where lower supply air temperature is required, but humidity addition must be limited or avoided. In the Eurovent Certified Performance programme, IEC products are classified under Case A and Case B, covering systems with primary outside air and systems with separation of external and room air.

Case A refers to an indirect evaporative cooler with integrated primary and secondary air passages and air-moving devices for both airstreams. Depending on the product configuration, one air-moving device may serve both primary and secondary airflow. The unit includes the water distribution, collection and, where applicable, recirculation system, including pump and piping. It may also include provisions for additional heat and mass transfer devices, such as direct evaporative coolers or auxiliary coils, although these are outside the scope of the certification programme. Primary air is always drawn from outside, and some discharged primary air may or may not be used as secondary air.

Case B refers to a packaged indirect evaporative cooling unit with integrated primary and secondary air passages, dedicated air-moving devices, and a complete water distribution, collection and recirculation system, including pump and piping. Like Case A, the unit may be designed to accommodate additional devices such as direct evaporative coolers or auxiliary heating and cooling coils, but these additional components are not covered by the certification.

Evaporative cooling equipment refers to the device or product that provides the evaporative cooling effect within a wider host system. Depending on the application, this equipment can include water spray systems, wetted pads or media, or ultrasonic units.

A water spray system connects to the water supply through an automatic or manually actuated flow control cabinet, with or without pressurising equipment such as a pump. Water is delivered through pipework to specially designed nozzles, which distribute it across the required area to support evaporation and cooling.

Wet pads or media are made from corrugated sheets of glass fibre paper, cellulose paper or other suitable materials. As incoming air passes through the wetted surface, water evaporates into the airstream, helping to cool and humidify the supply air.

An ultrasonic unit uses one or more piezoelectric transducers immersed in water. The transducer converts an electronic signal into a mechanical oscillation, creating a fine mist at the water surface that can then be introduced into the airstream.

It is important to note that the Eurovent Certified Performance programme for Evaporative Cooling covers equipment designed to cool air for air-conditioning purposes. It does not cover adiabatic coolers used for heat rejection to ambient air, such as cooling tower applications.

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What are the benefits of evaporative cooling?

As hotter summers become the norm, tighter energy-efficiency requirements come into force and pressure increases to cut greenhouse gas emissions, evaporative cooling is becoming more relevant for HVACR design. In suitable applications, it can help reduce electrical demand compared with compressor-based cooling, support peak-load management and lower refrigerant dependency. However, performance depends heavily on outdoor temperature, relative humidity, ventilation strategy, water quality, maintenance regime and correct product selection.

Unlike conventional vapour-compression cooling systems, evaporative cooling does not use CFC, HCFC or HFC refrigerants in the cooling process. This makes it particularly attractive where building owners, manufacturers and specifiers are seeking lower-carbon cooling solutions, reduced refrigerant-related risk and technologies aligned with decarbonisation strategies.

Evaporative cooling can be competitive in capital cost, operating cost and maintenance cost, especially when used in applications with high fresh-air requirements or high internal heat loads. Typical applications include warehouses, manufacturing facilities, commercial buildings, agricultural spaces, data centres, process cooling support, and comfort cooling where climate conditions are suitable. The technology can also contribute to indoor air quality by supplying high volumes of fresh, filtered air, provided that airflow paths, extract air routes and humidity levels are properly designed and controlled.

What are the design considerations?

Correct ventilation balance is essential. Direct evaporative systems add moisture to the supply air, so buildings need sufficient extract or relief air to prevent excessive indoor humidity. In sealed or process-sensitive spaces, designers may need to use mechanical extract systems or consider indirect evaporative cooling, which cools the supply air through a heat exchanger without adding humidity to the primary airstream.

Water consumption depends on outdoor dry-bulb temperature, relative humidity, airflow, operating hours, system efficiency and control strategy. For responsible specification, industry players should assess both energy and water performance. Efficient evaporative cooling design should include water treatment, bleed-off control, filtration, drainage, access for cleaning and a maintenance plan that reflects local water quality and hygiene requirements.

The drier the climate, the greater the potential cooling effect. In hot, dry conditions, evaporative cooling can significantly reduce supply air temperature. In humid climates, the reduction is smaller, but the technology can still be useful in specialist applications, particularly where ventilation, heat rejection, process cooling support or pre-cooling are required. For every project, designers should evaluate local weather data, indoor comfort requirements, humidity limits and seasonal operating profiles.

In industrial environments, internal heat gains can be high due to machinery, lighting, production processes or dense occupancy. Evaporative cooling can provide a practical and efficient solution when combined with the right ventilation strategy. It may also be used as part of a hybrid system, reducing the load on mechanical cooling equipment and improving overall system efficiency.

Hygiene and maintenance should be considered from the start of any evaporative cooling project. Water systems must be designed, operated and maintained to control microbial risk, scaling, corrosion and fouling, in line with local regulations and standards. Good practice includes appropriate water treatment, regular inspection, cleaning and disinfection, monitoring of water quality and clear responsibility for maintenance.

FAQ: Evaporative cooling

What is evaporative cooling?

Evaporative cooling is a cooling process that uses the evaporation of water to reduce air temperature. Air passes through or across water, and as the water evaporates it absorbs heat from the air, creating a cooling effect.

Where is evaporative cooling most effective?

It is most effective in hot, dry climates where the air can absorb more moisture. It can also be effective in specialist applications, including industrial spaces, logistics buildings, data centres, production environments and hybrid cooling systems, when designed around local climate, airflow and humidity requirements.

What should industry players consider before specifying evaporative cooling?

Key factors include outdoor climate data, indoor humidity limits, airflow and extract strategy, water quality, water treatment, maintenance access, hygiene management, controls, pressure drop, energy consumption and verified product performance. Whole-life cost should consider both energy and water use.

Does Eurovent Certification have an evaporative cooling programme?

Yes. There is a Eurovent Certified Performance programme for Evaporative Cooling. Certified characteristics include Cooling Capacity, Air flow for DEC and IEC units, Efficiency, Water Consumption, EER, and wet and dry pressure drop for evaporative cooling equipment only.