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Air cooling moves server heat into room air; direct liquid cooling carries heat from components through a coolant loop; immersion cooling transfers heat from submerged equipment into dielectric fluid. None is universally best. For a data center project, the choice depends on rack heat density, compatible IT equipment, the facility’s heat-rejection system, energy and water constraints, climate, maintenance, retrofit scope, reliability requirements, and whether recovered heat can be used.
How each cooling method moves heat
The key difference is where heat is captured first. In an air-cooled design, server heat enters the room’s air. Direct liquid systems collect heat in a liquid loop at or near IT components. Immersion puts equipment in contact with dielectric fluid. In all three cases, the heat must ultimately leave the facility through a heat-rejection path; changing the server-side method does not, by itself, determine a site’s total energy or water use.
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Air cooling: components to room air
Server components transfer heat to air moving through the equipment. Fans push the warmed air out of the server, often toward a contained hot aisle. Room-level cooling equipment then removes heat from the air and transfers it to chilled water, refrigerant, or another facility system. The U.S. Department of Energy (DOE) describes a typical arrangement in which computer-room air-conditioning equipment transfers IT exhaust heat to chilled water, with a cooling tower often providing the final heat-rejection stage.
Air-side performance depends on airflow volume and direction, rack arrangement, containment, temperature set points, and preventing hot exhaust from recirculating into server intakes. Separating hot and cool zones helps avoid spending cooling capacity on air that has already been heated. Where outdoor conditions and equipment operating limits allow, air-side economizing can use suitable ambient conditions to reduce mechanical cooling. The opportunity depends on climate, the equipment’s environmental limits, and control settings; it is not available to the same extent at every site.
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Direct liquid cooling: components to a circulating loop
Direct liquid cooling captures heat from IT equipment in a recirculating liquid loop rather than first moving the main IT heat into room air. In a common arrangement, the equipment loop transfers heat at a coolant distribution unit (CDU) to a facility-side loop. That facility loop then rejects heat through equipment such as a chiller, cooling tower, dry cooler, or another configured system. Some designs can operate without compressors under suitable conditions; others still rely on a chiller.
Cold plates attach to heat-generating components and carry their heat away in coolant. This reduces the load that must be removed from the room air, but does not necessarily eliminate room cooling: unconnected components and residual heat may still require it. ASHRAE’s 2023 handbook chapter, Chapter 20. Data Centers and Telecommunication Facilities, says: “Except for immersion, the datacom room needs to support a hybrid of air and liquid cooling.”
Direct-to-chip or cold-plate cooling and immersion are both forms of direct liquid cooling, but they capture heat differently. A cold plate contacts selected heat-generating components, leaving other equipment and residual room heat to be handled as needed. Immersion submerges the IT equipment in dielectric fluid. The distinction matters when checking hardware support and deciding what room-air systems remain part of the design.
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Immersion cooling submerges IT equipment in dielectric fluid, allowing heat to transfer directly from the equipment to the fluid. DOE classifies immersion and cold-plate cooling as direct-liquid-cooling technologies. The collected heat still has to move through heat exchangers and a facility heat-rejection system. Immersion describes how heat is captured at the IT equipment; it does not specify whether the overall site uses a chiller, evaporative cooling, dry heat rejection, or how much water or energy the complete design consumes.
Comparison at a glance
| Approach | Where heat is captured | Typical next heat-transfer step | Important design consideration |
|---|---|---|---|
| Air cooling | Server airflow carries component heat into room air. | Room cooling equipment transfers heat to a facility system, such as chilled water. | Airflow management, containment, and room cooling capacity are central. |
| Direct liquid with cold plates | Cold plates collect heat from connected components into coolant. | An equipment loop commonly transfers heat through a CDU to a facility loop. | Room air cooling may still be needed for residual heat and components outside the liquid loop. |
| Immersion | Dielectric fluid surrounds the submerged IT equipment and absorbs heat. | Heat passes from the fluid through heat exchangers to facility heat rejection. | Equipment must be compatible with the immersion arrangement; the site still needs a heat-rejection design. |
The table describes heat paths, not guaranteed comparative efficiency. DOE’s guidance discusses the potential for liquid systems to improve power usage effectiveness (PUE), but configurations vary. The DOE and ASHRAE materials do not establish an apples-to-apples universal percentage for energy or water savings across air, direct-to-chip, and immersion systems.
For a fair comparison, use the same workload, operating conditions, and system boundary for each proposal. Count the whole cooling path: server fans, pumps, room cooling, chillers where used, and final heat rejection. A rack-level or component-level efficiency figure cannot alone establish how much energy the complete facility will use. Likewise, identify whether the facility loop uses evaporative or dry heat rejection before drawing conclusions about water.
When air and liquid cooling work together
Hybrid cooling is not necessarily a temporary compromise. In a cold-plate deployment, liquid may remove much of the heat from selected high-load components while room air continues to cool other equipment and manage remaining room heat. That means project teams need to size and coordinate both paths rather than treating a liquid loop as proof that room cooling can be removed.
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For construction planning, identify the intended IT configuration early enough to coordinate equipment layout, room air management, liquid-loop interfaces, and facility heat rejection. A late change in server cooling method can affect more than the rack: it can alter the role and capacity expected of the room and facility systems. The required scope is project-specific; DOE cautions against treating one design as the most efficient across every scenario.
How to compare proposals fairly
Compare complete systems serving the same workload, not isolated pieces of cooling equipment. A proposal that counts only a server-side cooler, for example, may omit pumps, room cooling, chillers, and final heat rejection. Use these questions to make the boundary and assumptions visible.
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1. Define the workload, rack density, and IT compatibility
- Record the heat load to be served and its distribution across racks and components. Ask whether the proposal addresses the actual equipment layout, not just a peak number for a different deployment.
- Confirm which server designs support the proposed cold plates or immersion arrangement, and what equipment remains outside that cooling path.
- Put density figures in historical context. DOE’s July 2024 Best Practices Guide for Energy-Efficient Data Center Design reports that DOE observed 60 kW per compute rack in 2013 as an early high-performance-computing example, and describes more than 125 kW per compute rack as a density that had recently been surpassed. These are examples reported by the guide, not a current average or a design target for every facility.
2. Set one energy boundary
Ask for the energy accounting boundary and the operating assumptions behind any efficiency claim. Include server fans, pumps, room cooling, chillers, and heat-rejection equipment in the comparison. A compressor-free configuration may still use other equipment, and another liquid design may retain a chiller. Without a stated baseline and system boundary, a claimed saving cannot be compared reliably with a different proposal.
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3. Check water use at the heat-rejection stage
Trace the heat after it leaves the IT equipment. If the facility rejects heat through an evaporative cooling tower, water use remains relevant even when heat was captured in liquid at the rack. A dry heat-rejection design changes that part of the water picture, but does not make “liquid cooling” itself synonymous with zero water use. Compare the actual heat-rejection choices against local water constraints.
So, does liquid cooling use less water? The cooling label alone cannot answer that. The answer depends in part on whether the facility uses evaporative cooling towers or a dry system, as well as on the rest of its design and operating conditions. Ask vendors to state the heat-rejection assumptions behind any water-use comparison, rather than comparing a rack cooling method with a facility-wide water figure.
4. Account for climate and economizer conditions
Outdoor temperature and humidity influence how often a site can use ambient conditions and how much mechanical cooling it needs. Equipment environmental limits and the controls strategy also determine whether those conditions are usable. DOE’s guidance treats air management and economizing as design opportunities where climate and equipment limits permit, not as benefits available in every location or operating period.
5. Include reliability, service, and retrofit scope
- For liquid systems, establish how the design will monitor loops, maintain coolant quality, detect leaks, and provide safe service access. Obtain maintenance procedures and support commitments specific to the chosen equipment; the reviewed guidance does not provide a neutral, quantified failure-rate comparison between technologies.
- For any approach, identify the building work needed: floor and rack layout, power distribution, piping, CDUs, facility loops, room cooling, and any changes to existing equipment.
- Separate new-build decisions from phased-retrofit constraints. A design that suits a new facility may require different construction scope when installed around an operating room or existing distribution systems. DOE’s design guide emphasizes that the answer depends on the scenario rather than naming a universal best system.
6. Evaluate heat reuse as a site-specific opportunity
DOE includes heat recovery among its data-center design topics. Before assigning value to it, determine whether the temperature and continuity of the rejected heat match a real downstream use. Heat recovery is not automatically a cooling-cost offset: its usefulness depends on the application and the facility’s ability to use the heat when it is available.
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Thermal guidance and design references
ASHRAE’s official data-center resources list the fifth edition of Thermal Guidelines for Data Processing Environments, along with other technical resources on liquid cooling. DOE’s July 2024 design guide reports ASHRAE water classes as W17, W27, W32, W40, W45, and W+, renamed to show the maximum server supply-water temperature. These classes are equipment and design guidance; they are not a complete specification for a facility’s cooling architecture or heat rejection.
For a project team, the class information is a reference point for server supply-water conditions, not a substitute for checking the specific equipment and full facility design. Confirm applicable thermal requirements against current ASHRAE resources and the selected hardware rather than treating a class label as a complete system prescription.
ASHRAE’s publication updates page lists the second edition of Liquid Cooling Guidelines for Datacom Equipment Centers as updated September 11, 2014. That update date is useful context when consulting the publication; it should not be mistaken for a newer edition. Design teams should check ASHRAE’s current online resource and edition information when selecting guidance for a project.
DOE’s 2024 guide makes the broader limitation explicit: “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.” The practical implication is to evaluate the proposed IT equipment, building, climate, water availability, and full heat path together, rather than select a cooling label in isolation.
Frequently Asked Questions
What is the difference between air cooling and liquid cooling in a data center?
Air cooling carries heat from server components into room air, which room-level equipment then cools. Direct liquid cooling captures heat in a circulating loop at or near IT components; a CDU commonly transfers that heat to a facility loop for rejection. Both still require a complete heat-rejection design.
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Is immersion cooling better than direct-to-chip cooling?
Neither is universally better. Direct-to-chip cold plates collect heat from selected components, while immersion submerges compatible IT equipment in dielectric fluid. Compare supported hardware, workload and rack density, room and facility requirements, energy and water assumptions, maintenance, and retrofit scope for the specific project.
Does liquid cooling use less water than air cooling?
Not necessarily. Water use depends in part on the facility’s heat-rejection system: a liquid-cooled rack can still be connected to a facility using evaporative cooling towers, while dry heat rejection changes that part of the water picture. Compare complete system designs and stated operating assumptions.
Can air and liquid cooling work together?
Yes. Cold-plate systems commonly work alongside room-air cooling, which can serve components outside the liquid loop and manage residual heat. ASHRAE’s 2023 handbook chapter says that, except for immersion, the datacom room needs to support a hybrid of air and liquid cooling. Even immersion systems still require facility heat rejection.
The Bottom Line
Choose by comparing complete systems serving the same workload, not by cooling label alone. Rack density, supported hardware, room and facility heat paths, climate, water and energy constraints, maintenance, retrofit scope, and possible heat reuse all affect the result. Air, direct liquid, immersion, and hybrid designs each fit different scenarios; no one approach is best for every data center.
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