Evaporative and adiabatic cooling for data centers

Evaporating water absorbs a large amount of heat. Evaporative and adiabatic cooling systems use this effect to cool air or water toward the outdoor wet bulb temperature, which is often well below the dry bulb. In data centers, this extends free cooling hours and reduces or eliminates compressor operation, but it also consumes water and introduces water treatment and microbial control obligations.

This guide explains the three main approaches, where they fit in Canada, and how to weigh energy savings against water use.

The physics in brief

When water evaporates into an airstream, the air's dry bulb temperature drops while its moisture content rises. The process is close to adiabatic, meaning the total energy of the air stays roughly constant. The lowest temperature achievable is the wet bulb temperature. Real equipment gets part of the way there; the fraction is called saturation or wet bulb effectiveness, commonly in the range of about 70 to 90 percent for well-designed media.

The drier the air, the bigger the gap between dry bulb and wet bulb, and the more cooling evaporation provides. That is why evaporative cooling is especially effective in dry climates such as the southern Prairies and the BC Interior.

Direct evaporative cooling

Outdoor air passes through wetted media or a fine mist and is supplied directly to the data hall. This lowers supply temperature but also raises humidity, which must stay within the ASHRAE TC 9.9 envelope. Direct evaporative cooling is usually combined with a direct airside economizer.

• Very energy efficient
• Adds moisture to the data hall
• Outdoor contaminants enter with the air, so filtration is important
• Requires careful control to avoid exceeding humidity limits

Indirect evaporative cooling

Evaporation happens on the outdoor (scavenger) side of an air-to-air heat exchanger. Data hall air is cooled through the heat exchanger without gaining moisture or contaminants. Indirect evaporative units are packaged air handlers that often include DX or chilled water trim coils for the hottest, most humid hours.

• Data hall air stays isolated from outdoor air
• No moisture added to the white space
• Large footprint and weight; usually on roofs or beside the building
• Popular for hyperscale and colocation facilities

Adiabatic precooling on dry coolers and condensers

Dry coolers and air-cooled condensers can be fitted with adiabatic pads or spray systems that cool incoming air only on hot days. The rest of the year they run dry. This gives dry cooler simplicity with a boost during peak conditions, and uses much less water than a cooling tower because water is only used for a limited number of hours.

Comparison

Water use and WUE

Evaporative systems lower energy use but raise water consumption. The water usage effectiveness (WUE) metric captures this trade-off. In regions with water restrictions or high water costs, the energy savings must be weighed against water impact. Water consumed includes evaporation plus blowdown or bleed water used to control mineral build-up.

Owners should also consider the water used at the power plant. Where the grid relies on thermal generation, saving electricity also saves water upstream. In provinces with predominantly hydroelectric grids, that indirect relationship is different. This is a site-specific calculation.

Water treatment and Legionella

Any system that wets media or sprays water can support microbial growth, including Legionella bacteria. Risk management should follow ASHRAE Standard 188 and ASHRAE Guideline 12, along with provincial and municipal requirements. Some jurisdictions have specific rules: Quebec, for example, requires registration and maintenance programs for cooling towers. Check local requirements.

Good practice includes:

1. A written water management plan identifying hazards and control points
2. Water treatment to control scale, corrosion and biological growth
3. Regular inspection and replacement of media
4. Draining and drying wetted components when not in use
5. Avoiding stagnant water in sumps and piping
6. Documented testing and record keeping

For related guidance on towers, see cooling tower maintenance and Legionella control.

Canadian climate considerations

Winter

Evaporative stages are not needed in winter because outdoor air alone provides cooling. Wet components must be drained and protected from freezing. Indirect units need control strategies to prevent frost on the heat exchanger when very cold outdoor air meets warm, moist exhaust.

Summer

Humid summers in southern Ontario, Quebec and the Maritimes narrow the wet bulb depression, so evaporative cooling delivers less. Facilities there usually need mechanical trim on the warmest humid days. The Prairies and interior BC typically see larger depressions and greater benefit.

Wildfire smoke

Smoke events affect air quality and can load filters and wetted media quickly. Indirect systems are less exposed than direct systems. Plans should cover filter changes, temporary mode changes and monitoring during smoke events.

Design checklist

• Use hourly weather data for the site to estimate evaporative hours and water use.
• Confirm water supply capacity, quality and cost; consider municipal restrictions during drought.
• Plan water treatment, drainage, blowdown discharge permits and backflow prevention.
• Size mechanical trim for the worst humid conditions, typically using ASHRAE or NBC design wet bulb values.
• Specify freeze protection and seasonal draining procedures.
• Integrate evaporative staging into controls with humidity limits for direct systems.

Frequently asked questions

Is evaporative cooling worth it in Canada?

Often yes, mainly as a summer supplement. Because Canadian winters already provide plenty of free cooling, the evaporative stage mainly reduces peak compressor capacity and summer energy. Whether it pays off depends on water cost, climate and how high your supply temperatures are.

Can I avoid water use entirely?

Yes, with dry coolers, airside economizers without evaporative stages, or warm-water liquid cooling that allows dry heat rejection. The trade-off is usually larger equipment or more compressor hours in summer.

Key takeaways

• Evaporative cooling uses water evaporation to approach the outdoor wet bulb temperature.
• Direct systems add moisture to the data hall; indirect and adiabatic systems do not.
• Energy savings come with water consumption, treatment and Legionella management obligations.
• In Canada, evaporative stages mainly help in summer, especially in dry regions.
• Use site weather data and WUE alongside PUE to judge whether evaporative cooling fits.