Data centers keep equipment within its operating limits by continuously moving heat away from servers and rejecting it outdoors—or, where practical, reusing it. The path may run through room air, chilled water, a cooling tower, or liquid loops close to the IT equipment. The right design depends on site climate, equipment density, water and energy constraints, and the facility’s ability to operate and maintain the system.
How a data center moves heat
Servers use electricity and turn much of it into heat. Fans move that heat out of the equipment, and cooling systems carry it away so server inlets remain within the equipment’s acceptable conditions. A useful way to understand the infrastructure is to follow the heat from the rack to its final destination.
A typical chilled-water and cooling-tower path
- Servers: IT equipment warms air as it processes workloads. Server fans push the hot exhaust into a return-air path.
- Room cooling equipment: Computer-room air-conditioning (CRAC) or air-handler (CRAH) equipment removes heat from the room air. A CRAC may use direct expansion; a CRAH commonly transfers heat into chilled water.
- Chiller: In a typical chilled-water system, the chiller moves heat from the chilled-water loop into a separate condenser-water loop.
- Heat rejection: The condenser-water loop carries heat to a cooling tower. Evaporation commonly helps transfer that heat to the outdoor atmosphere.
This is a common arrangement, not a universal blueprint. Facilities may use direct-expansion systems, air-cooled heat rejection, evaporative towers, economizers, liquid cooling, or combinations of these. The U.S. Department of Energy’s Federal Energy Management Program (FEMP) describes the tower-based heat path in its data-center cooling water guide.
Which cooling approaches do data centers use?
Mechanical room-air cooling
CRAC and CRAH systems cool the room or the air entering server equipment. In a CRAC installation, heat may be removed through a direct-expansion refrigeration cycle. A CRAH typically uses chilled water to absorb heat from room air, after which that heat travels through the facility’s water loops to a heat-rejection system. The choice depends on facility design and the required capacity, controls, and redundancy.
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Air-side economizers
An air-side economizer uses suitable outdoor air to cool the data-center space, reducing reliance on compressor-based cooling when conditions allow. Data centers may be able to operate with warmer inlet air than offices, which can increase the periods when outdoor air is useful. Whether it is appropriate depends on local weather and careful control of air quality and moisture.
- Filtration and contaminants: Outdoor air can introduce particulates or gaseous contaminants that need to be considered in the design.
- Humidity and dewpoint: Seasonal humidity changes and dewpoint conditions may require controls or economizer lockouts.
- Climate: Outdoor conditions vary by location and season, so an economizer does not eliminate the need for mechanical cooling in every operating condition.
FEMP’s 2024 Best Practices Guide for Energy-Efficient Data Center Design recommends evaluating these factors for the specific site rather than assuming outdoor air is suitable everywhere.
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Water-side economizers
A water-side economizer uses a heat exchanger to transfer heat between water loops. When outdoor conditions and the system design permit, cooling-tower water can help cool the chilled-water loop, reducing or bypassing chiller compressor operation. The heat exchanger’s placement and system configuration affect the opportunity; water use, treatment, climate, and energy savings must be considered together.
Direct liquid cooling
Direct liquid cooling captures heat closer to the IT equipment and transports it in a circulating liquid loop rather than relying first on room air to carry that heat away. A coolant distribution unit (CDU) can transfer heat from the equipment loop to another loop or a heat-rejection system. Some facilities still need room-air cooling for residual equipment or facility loads, so liquid and air systems can coexist.
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Liquid cooling is relevant to high-density AI and high-performance computing designs, but it brings operational requirements of its own: monitoring, controls, water-quality management, and planned switchover sequences. FEMP discusses potential energy and water benefits in some configurations, while ASHRAE’s AI Data Center Energy Performance Framework addresses thermal classes and related management considerations. Specific temperature limits depend on the equipment and applicable guidance; check current ASHRAE material and manufacturer specifications rather than assuming one limit applies to every server.
How airflow management helps
In a typical rack layout, cool supply air enters the fronts of servers and warmed exhaust leaves their backs. If the two streams mix, cooling equipment may have to work harder to keep server inlets within range. Cold-aisle and hot-aisle layouts, containment, and attention to gaps in racks help keep supply and exhaust air on separate paths.
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Airflow management is an operational practice as well as a facility design choice. It involves how air is delivered and returned, whether barriers are intact, and whether conditions are checked where equipment actually takes in air. A single room sensor may not reveal the temperature at every server inlet; Lawrence Berkeley National Laboratory identifies temperature measurement as a data-center thermal-management concern in its thermal guidelines and measurement resource.
FEMP’s 2019 water-efficiency discussion says hot/cold air separation practices can enable higher chilled-water temperatures and reduced airflow, which “can result in 20% less energy consumption at the chiller.” This is a stated potential associated with those practices, not a guaranteed saving for every facility. The same source discusses rack isolation and airflow management; a blanking panel that closes an unused rack opening can be one small part of that work, not a substitute for containment, commissioning, or airflow measurement.
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What happens to the heat after it leaves the equipment?
The immediate goal is to remove heat safely, but the facility can also consider whether some of it has a useful destination before rejecting the remainder outdoors. Heat recovery is practical only when there is a dependable nearby use, the recovered heat is at a useful temperature, and the necessary connections, controls, and economics make sense.
Heat rejection choices also affect water use. Cooling towers commonly rely on evaporation; some designs can use dry heat rejection when it suits the site and water goals. Neither heat recovery nor a particular rejection method is automatically suitable for every data center. FEMP’s 2024 guide discusses heat recovery and prioritizing reuse where possible, alongside dry heat rejection where appropriate.
How to compare data-center cooling designs
There is no single best cooling system independent of its site and workload. DOE FEMP’s 2024 guide states: “No design guide can offer ‘the most energy-efficient’ data center design, but these guidelines can provide efficiency benefits for a wide variety of data center scenarios.” Compare complete system designs against the conditions they must serve.
- Climate suitability: How often do outdoor conditions make air- or water-side economizing possible, and when will mechanical cooling still be needed?
- IT load and thermal envelope: What are the rack density and equipment inlet requirements? Does the workload require liquid cooling at the equipment level?
- Energy: Include chiller compressors, fans, pumps, and facility overhead rather than judging only one component.
- Water: Consider evaporative cooling, cooling-tower make-up, treatment needs, and local water availability alongside energy performance.
- Air quality and humidity: Account for filtration, contaminants, humidity control, and conditions that may require economizer lockouts.
- Operations and reliability: Assess controls, sensors, water quality, maintenance capacity, switchover sequences, and redundancy.
- Heat recovery: Identify a credible heat user, distance, useful temperature, and the controls and economics needed to serve it.
What PUE and WUE tell you—and what they do not
Power usage effectiveness (PUE) is total facility energy divided by IT equipment energy. It describes facility energy overhead relative to IT energy, but does not by itself report water use or how much heat is reused. FEMP’s 2019 page characterizes PUE 2.0 as average efficiency and PUE 1.0 as the theoretical minimum approached by highly efficient facilities; those figures are the guide’s comparison, not a current census of all data centers.
Water usage effectiveness (WUE) is useful alongside PUE when comparing water impacts. A low PUE alone does not show whether a system relies heavily on evaporative water use. FEMP reported PUE 1.06 and WUE 0.7 for the National Laboratory of the Rockies data center using a hybrid cooling system. Those figures describe that named installation, not a forecast for other facilities. For current thermal guidance, consult ASHRAE’s publication updates and errata as well as the equipment manufacturer’s specifications.
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