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Air cooling moves heat into room air, evaporative cooling uses water evaporation to reject heat, and liquid cooling carries heat away from IT equipment in a circulating fluid loop. They are not always mutually exclusive: a data center can use liquid cooling at high-density racks, air cooling for residual room heat, and evaporative cooling in the facility’s heat-rejection plant.
How the three approaches differ
Cooling a data center involves three stages: capturing heat at the IT equipment, transporting it, and rejecting it outside. “Air,” “evaporative,” and “liquid” can describe different stages of that chain rather than three complete, competing facility designs. The U.S. Department of Energy (DOE) illustrates this in its cooling-water guidance for federal data centers.
Air cooling: move heat into room air
Fans move air across servers and other IT equipment, carrying their heat into the data-center room. Air handlers then transfer that heat to a cooling loop or reject it. Airflow containment, temperature management, and economizers—which use favorable outdoor conditions to reduce mechanical cooling—can affect how much energy the system needs.
Air cooling is a familiar option for lower-density equipment and can remain part of a facility that also has liquid-cooled zones. Its limits become more relevant as rack power density rises: air’s ability to carry heat can constrain the design, while server fans and mechanical cooling add to facility energy use. Good airflow management still matters.
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Evaporative cooling: use water to reject heat
Evaporative cooling uses the heat absorbed by evaporating water to carry heat into the ambient air. In a common arrangement, heat passes from IT equipment to room air, then through a chilled-water system to a cooling tower, where evaporation rejects it outdoors. In that arrangement, evaporative cooling is a heat-rejection method; it does not necessarily describe how heat is captured at the servers.
Cooling towers use water through evaporation and related operation. Water availability, treatment, blowdown, and local operating conditions all affect whether this approach is suitable. Non-evaporative heat rejection may reduce on-site water use, but it brings other design trade-offs. A change that saves water can also increase energy demand, so assess both measures rather than assuming one improvement guarantees the other.
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Liquid cooling: capture heat in a fluid loop
Liquid cooling transfers heat from IT equipment into a circulating fluid loop rather than first transferring it to room air. Depending on the design, the fluid may be treated water, a glycol mixture, or a dielectric fluid. A coolant distribution unit (CDU) commonly connects the equipment loop with facility cooling.
Because liquid can capture heat close to high-density equipment, it can suit racks that are difficult to cool with air alone. It may also reduce server-fan and room-cooling loads and allow warmer facility loops. Those benefits depend on the system design; liquid cooling does not by itself specify whether the facility ultimately uses chillers, cooling towers, or another heat-rejection method.
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Comparison at a glance
| Approach | Where heat is captured or rejected | Potential strengths | Important constraints | Questions to ask |
|---|---|---|---|---|
| Air cooling | Fans move heat from IT equipment into room air; air handlers transfer or reject it. | Familiar architecture; can serve lower-density equipment and coexist with liquid-cooled zones. Containment and airflow management can reduce cooling demand. | Air’s heat-carrying capacity can constrain high-density racks. Fans and mechanical cooling use facility energy. | What is the rack density? Are hot and cold aisles contained? Can ambient conditions support economizer hours? |
| Evaporative heat rejection | Water evaporates to carry heat to ambient air, often at a cooling tower after heat has passed through IT air and a chilled-water system. | Can reject heat effectively with an appropriate climate and plant design. | Uses water; water availability, treatment, blowdown, and operating context matter. Water and energy outcomes may move in different directions. | Is evaporation direct or at the heat-rejection stage? What are the site’s water constraints and its defined energy and water metrics? |
| Liquid cooling | A fluid loop captures heat at IT equipment and carries it to a CDU or another facility interface. | Suited to capturing heat from high-density IT; may reduce server-fan and room-cooling loads and enable warmer facility loops. | Often hybrid, with air still handling residual heat. Fluid chemistry, pressure, temperature, CDUs, and facility-loop integration need operational attention. | What share of IT heat is captured? Which fluids and temperatures are supported? Does the facility still use chillers or evaporative heat rejection? |
How to choose for a particular data center
There is no context-free winner. Compare the equipment and the whole facility, including its heat-rejection plant. DOE’s 2024 Best Practices Guide for Energy-Efficient Data Center Design and the National Renewable Energy Laboratory’s 2024 guide discuss cooling choices and liquid-cooling integration.
Start with rack density and workload
Identify the rack power density and the workload’s cooling needs. Higher rack density can strengthen the case for liquid or liquid-assisted cooling, but it does not establish that an entire facility should convert. Lower-density equipment may remain well served by air, and different zones in one building may need different approaches.
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Account for climate, water, and heat rejection
Local ambient conditions affect economizer opportunities and heat rejection. Water availability and site constraints matter where cooling towers are used. Determine whether evaporation occurs at the equipment stage or in the facility plant, and identify the actual heat-rejection equipment before comparing systems.
Measure water and energy separately. DOE notes that reverse-osmosis water reuse can lower water consumption while negatively affecting power usage effectiveness (PUE) because of its energy demand. A single metric therefore cannot establish the overall environmental or operating result.
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Check retrofit and operating requirements
For liquid cooling, establish the equipment’s supported fluid and operating temperatures, the share of heat the loop captures, how its CDU interfaces with facility cooling, and how staff will manage fluid chemistry and pressure. For air systems, evaluate containment and airflow. For evaporative systems, include water treatment and blowdown in the operating picture. Existing infrastructure and the site team’s ability to maintain the design can rule out an otherwise attractive option.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Compare performance with defined boundaries
Use site-specific measurements and state what each one includes. ASHRAE’s AI Data Center Energy Performance Framework calls for multiple measures, including PUE and water usage effectiveness (WUE), rather than treating one number as a complete sustainability verdict. PUE relates total facility energy to IT energy; WUE measures water use against IT energy. Comparisons are meaningful only when the facility boundary, reporting period, and calculation method are clear.
DOE’s 2019 federal data-center guidance describes PUE 2.0 as the average-efficiency data center and says highly efficient facilities can approach the theoretical minimum of 1.0. Those are contextual reference points, not predicted outcomes for a specific cooling retrofit. ASHRAE Standard 127-2020, described on its Titles, Purposes, and Scopes page, establishes uniform test methods for rating cooling equipment; an equipment rating is not, by itself, a whole-facility comparison.
Does liquid cooling replace air conditioning?
Usually not entirely. Liquid cooling often removes most of the IT equipment’s heat while air systems handle residual heat in the room. It may reduce room-cooling demand, but it does not guarantee that server fans, air handling, or air conditioning can be eliminated. The DOE and NREL guides describe liquid cooling as an integration choice, while ASHRAE’s framework addresses high-density liquid cooling alongside airflow and economization.
Further technical reading
For readers planning a detailed design review, ASHRAE’s Datacom Series lists books and guidance on data-center topics, including liquid cooling. Check the current edition and availability when selecting a manual.
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