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For most new, high-density AI servers, direct-to-chip liquid cooling is the best default. Cold plates remove heat directly from GPUs and CPUs, allowing dense racks without forcing the entire room to move extreme volumes of air. Keep air cooling for lower-density, legacy, storage, and networking zones. Choose immersion cooling when density or space constraints justify tanks, dielectric fluid, compatible hardware, and a different service process.
The right answer is set by rack heat load, facility infrastructure, hardware compatibility, service requirements, and lifecycle cost—not by the word “AI” on the equipment label.
What determines the best cooling method?
Cooling selection starts with heat density. A rack drawing modest power can often use conventional room air, while a rack packed with high-power accelerators may overwhelm room-based cooling even when the surrounding data center appears adequately conditioned.
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The U.S. Department of Energy notes that high-performance-computing facilities adopted direct liquid cooling as rack densities reached about 60 kW per compute rack in 2013 and later exceeded 125 kW per compute rack. Those figures are historical industry milestones, not universal cutoffs, but they show why rack-level heat load matters more than a simple server category.
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ENERGY STAR guidance likewise explains that rack- and row-based cooling can become preferable to room-based CRAC approaches as density rises. ASHRAE’s AI Data Center Energy Performance Framework recommends adopting “liquid or liquid-assisted architectures … for AI clusters while retaining air for lower-density zones.”
Before choosing a technology, establish these inputs:
- Continuous and peak rack power: Include GPUs, CPUs, memory, storage, networking, fans, and power-conversion losses.
- Heat captured at the server: Determine which components can accept cold plates or immersion and which will continue rejecting heat to air.
- Facility heat rejection: Check available chilled water, warm-water loops, dry coolers, cooling towers, pumps, heat exchangers, and electrical capacity.
- Retrofit constraints: Record floor loading, ceiling height, containment options, pipe routes, drainage, leak detection, and shutdown windows.
- Service model: Decide whether technicians can work on liquid manifolds, tanks, wet hardware, and coolant treatment safely and repeatably.
- Hardware and warranty rules: Verify approved cold plates, fluids, quick disconnects, boards, drives, seals, and vendor support.
A useful design often combines methods. Liquid removes heat from the highest-wattage chips, while air handles memory, storage, network adapters, power supplies, and residual heat. A rear-door heat exchanger or contained airflow path can finish the job without converting the whole facility to immersion.
Air cooling: where it still makes sense
How air cooling works
Air-cooled data centers use server fans, room air handlers, CRAC or CRAH units, containment, and heat exchangers to move heat from components to the facility’s cooling plant. The architecture is familiar: servers remain dry, technicians use conventional procedures, and the building does not need a liquid loop at every rack.
Best-fit deployments
- Low- to moderate-density CPU or GPU racks.
- Existing rooms designed around raised-floor or overhead air distribution.
- Legacy equipment that has no liquid-cooling option.
- Storage, networking, management, and other lower-heat racks surrounding a liquid-cooled compute area.
- Sites where adding pumps, manifolds, leak detection, or facility water is impractical.
Advantages
- Lowest facility change: Existing CRAC or CRAH equipment, containment, and maintenance practices can often remain in place.
- Simple server service: There is no coolant connection to disconnect before removing a server.
- Broad hardware compatibility: Standard air-cooled servers and add-in cards can be deployed without board or fluid approvals.
- No liquid leak path at the rack: Risk is concentrated in the building’s established cooling plant rather than in server manifolds.
Where air becomes the constraint
Air has relatively low volumetric heat capacity, so removing more heat requires more airflow, larger heat exchangers, tighter pressure control, or all three. Dense accelerator racks can create high fan power, hot spots, recirculation, and uneven inlet temperatures. Containment reduces mixing, but it does not change the amount of heat the air must carry.
As density rises, a room may have enough total cooling capacity yet still fail locally because one rack cannot receive or exhaust the necessary airflow. In-rack heat exchangers and rear-door units can extend air cooling, but those additions increase cost, weight, maintenance, and dependence on careful airflow balancing.
Air-cooling checks
- Measure the sustained and transient power of each rack, not just the nameplate rating.
- Verify that supply-air temperature, humidity, and pressure remain within every server manufacturer’s limits at peak load.
- Map cold-aisle and hot-aisle containment, bypass airflow, blanking panels, and cable openings.
- Check fan energy and acoustic limits; high-speed server fans can become a significant electrical and service burden.
- Confirm that the cooling plant can reject the added heat during worst-case outdoor conditions and maintenance outages.
Direct-to-chip liquid cooling: the practical default for dense AI
How the system works
Direct-to-chip systems attach cold plates to high-heat packages, typically GPUs and CPUs. A coolant distribution unit (CDU) pumps liquid through a rack or row loop, controls temperature and pressure, and transfers heat to facility water or a liquid-to-air heat exchanger. Components that do not have cold plates continue to use air.
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This arrangement targets heat at its source. The server fans still move air across memory, storage, voltage regulators, network adapters, and other residual loads, but they no longer need to carry the full thermal output of the processors.
Why it is the general recommendation
- High rack density: Heat is removed at the GPU and CPU instead of relying on room airflow alone.
- Incremental deployment: Liquid can be introduced for a new AI row while neighboring racks remain air-cooled.
- Familiar rack service: Technicians still work with conventional servers, with added procedures for manifolds, quick disconnects, and leak checks.
- Hybrid flexibility: Rear-door air cooling or room air can handle components that do not justify a cold plate.
- Future expansion: A properly sized CDU and facility loop can support additional accelerator generations without redesigning every room air path.
Capacity examples
Vendor specifications illustrate the range possible with liquid-assisted designs, but they are not an apples-to-apples performance comparison:
| System example | Published capacity | What the figure means |
|---|---|---|
| Vertiv SmartIT direct-to-chip system | Up to 142 kW per rack | Rack support using direct-to-chip liquid cooling with supplemental rear-door air cooling; current product specification. |
| CoolIT AHx180 CDU | 180 kW | Published CDU cooling capacity on the current product page; actual deployment depends on loop temperatures and configuration. |
| CoolIT CHx 3000 CDU | 3 MW | Published system capacity for a larger plant-scale CDU; site hydraulics and redundancy still determine usable capacity. |
Design and commissioning requirements
- Confirm cold-plate compatibility: Match the server, GPU carrier, CPU socket, board layout, tubing, and approved coolant to the manufacturer’s design.
- Define the liquid boundary: Document facility water, CDU, rack manifold, quick disconnects, cold plates, drains, vents, and isolation valves.
- Set coolant quality controls: Specify chemistry, filtration, conductivity limits, freeze protection where relevant, sampling frequency, and treatment responsibilities.
- Install detection and isolation: Use leak sensors, drip trays where appropriate, automatic or manual shutoff procedures, and clearly labeled valves.
- Provide CDU redundancy: Decide whether pumps, controls, power feeds, and heat exchangers are N, N+1, or another resilience level.
- Commission under load: Test flow, pressure, temperature stability, alarm behavior, failover, and recovery before accepting production workloads.
- Document maintenance: Include draining, refilling, venting, filter changes, connector inspection, and safe server removal.
Limitations
Direct-to-chip liquid introduces a leak path and a dependency on facility-water integration. A server can be electrically healthy yet unavailable if a CDU, pump, valve, or building loop is down. The design also has to accommodate components that remain air-cooled; ignoring those residual loads can produce hot spots even when the GPU cold plates are operating correctly.
Immersion cooling: maximum density with a different operating model
How immersion works
In immersion cooling, servers are submerged in electrically non-conductive dielectric fluid. The fluid contacts heat-generating components directly, absorbs heat uniformly, and transfers it through a tank heat exchanger or external loop. Because the fluid removes heat from much of the board, conventional server-fan airflow can be reduced or eliminated.
When immersion is justified
- Extreme rack or enclosure density that exceeds practical air and direct-to-chip layouts.
- Small or constrained facilities where compact thermal infrastructure is more valuable than conventional rack access.
- Specialized deployments that can standardize hardware and service procedures.
- Sites prepared to manage tanks, fluid inventory, filtration, lifting equipment, and wet-hardware handling.
Published capacity example
Vertiv reports CoolCenter Immersion systems in capacities from 25 to 240 kW per system (2025). That range describes the system offering, not a guaranteed rack density for every server configuration. Tank dimensions, fluid temperature, heat-rejection equipment, and hardware layout determine the practical result.
Benefits
- Very high heat-transfer capability: Fluid surrounds the board rather than relying on a small contact area at a CPU or GPU package.
- Reduced fan power: Most server airflow hardware is unnecessary or operates at a lower level.
- Uniform thermal environment: Properly designed tanks reduce component-to-component air-temperature gradients.
- Compact deployment potential: More heat can be handled in a smaller footprint than a conventional air-cooled room.
Operational trade-offs
- Hardware compatibility: Boards, drives, cables, coatings, seals, thermal interface materials, and power components must be approved for the selected fluid.
- Service complexity: Technicians must lift wet servers, drain or manage fluid, prevent contamination, and clean equipment before bench work.
- Fluid management: Dielectric fluid needs inventory control, filtration, sampling, spill response, and a disposal or reclamation plan.
- Facility redesign: Tanks add concentrated floor loads, access clearances, fluid-handling equipment, and different fire and safety considerations.
- Support and warranty: Every hardware supplier must confirm that immersion operation is supported for the exact configuration and fluid.
Immersion is therefore not simply a more powerful version of direct-to-chip cooling. It is a dedicated architecture. Use it when the density or space benefit outweighs the cost of changing hardware logistics and service practices.
Air vs direct-to-chip liquid vs immersion
| Decision axis | Air | Direct-to-chip liquid | Immersion |
|---|---|---|---|
| Best fit | Existing, lower-density racks | New high-density AI racks | Extreme-density or specialized sites |
| Retrofit difficulty | Lowest | Medium to high | Highest |
| Heat capture | Room and rack air | GPU and CPU cold plates | Entire board or server in dielectric fluid |
| Service model | Familiar IT service | Rack service plus liquid controls | Tank and fluid service |
| Density potential | Lowest | High | Very high |
| Main risks | Airflow, hot spots, and fan energy | Leaks, CDU failures, and facility-water integration | Hardware compatibility, fluid handling, and warranty limits |
There is no universal winner on efficiency. The available evidence does not provide one controlled, apples-to-apples percentage comparing all three methods under identical AI-server conditions. Vendor capacity numbers are system-specific, so compare a complete design rather than one headline figure.
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What to use for 100 kW-plus GPU racks
A 100 kW-plus rack should begin with a liquid-based concept unless a detailed airflow model proves that air can meet inlet-temperature, fan-power, and service requirements. Direct-to-chip liquid is usually the first architecture to evaluate because it can remove GPU and CPU heat while retaining air for the rest of the server.
Recommended architecture for most sites
- Place high-power GPU servers on a dedicated liquid-ready row.
- Use a CDU sized for the planned rack count, diversity, supply and return temperatures, and growth margin.
- Connect cold plates through standardized manifolds and quick disconnects.
- Add rear-door heat exchangers or contained airflow for memory, storage, networking, and residual heat.
- Keep adjacent lower-density racks on room air where that is economical and operationally simpler.
- Provide independent power and, where required, redundant CDUs and facility-water paths.
When to consider immersion instead
Move immersion to the front of the evaluation when the site cannot provide sufficient water or airflow, the footprint is severely constrained, or the target density would require an impractical number of liquid manifolds and rear-door units. Confirm tank floor loading, hardware support, fluid logistics, and technician training before selecting it.
When air can still handle the rack
Air remains viable when measured rack power is moderate, server manufacturers publish adequate airflow at the intended inlet conditions, and the room has enough supply and return capacity at peak outdoor conditions. Do not infer suitability from an average rack number if accelerator workloads produce sustained peaks.
How to choose: a practical decision process
Step 1: Build a rack heat-load schedule
List every rack, its sustained and peak electrical load, the percentage expected to be captured by liquid, and the residual air load. Include network switches, storage, power supplies, fans, and conversion losses. Separate production, standby, and future expansion loads.
Step 2: Classify the facility
Record available chilled or warm water, dry-cooler and cooling-tower capacity, pump redundancy, water quality, heat-rejection temperatures, and electrical service. If no suitable water loop exists, price a liquid-to-air heat exchanger and the required airflow before assuming direct liquid is impossible.
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Identify pipe routes, CDU locations, floor loading, service clearances, drainage, leak detection, containment, and shutdown windows. A technically efficient system can be a poor retrofit if installation requires unacceptable downtime or structural work.
Step 4: Validate the hardware bill of materials
Obtain approved lists for cold plates, manifolds, connectors, fluids, pumps, boards, drives, cables, and seals. Verify warranty language for the exact server model and accelerator configuration. For immersion, perform this validation for every submerged component.
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- Air pumped through to the top exhaust system of the fan
Step 5: Compare total cost of ownership
Include capital equipment, building work, pumps, fans, water treatment, fluid, monitoring, spares, technician training, energy, maintenance labor, downtime risk, and eventual decommissioning. A lower fan load does not automatically offset expensive tanks or a new facility loop; a cheaper air retrofit may cost more to operate over years at high density.
Step 6: Design for failure and maintenance
Define what happens when a fan, pump, CDU, valve, sensor, heat exchanger, or facility-water source fails. Set alarm thresholds, automatic throttling behavior, safe shutdown sequences, isolation procedures, spare-parts levels, and recovery tests. Cooling resilience should match the availability requirement of the AI workload.
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Deploy a representative rack or tank with the intended servers, workload, coolant, monitoring, and service process. Measure temperatures, flow, pressure, fan power, water use, alarm behavior, and technician time through normal operation and fault drills. Use those results to size the production design rather than extrapolating from a vendor’s maximum rating.
Common mistakes and how to avoid them
Treating a rack-power number as a cooling design
A 142 kW rack specification or a 180 kW CDU rating does not mean every site can operate at that level. Supply temperature, return temperature, flow, redundancy, residual air heat, and local conditions determine usable capacity. Model the complete loop.
Cooling only the GPUs
Memory, voltage regulators, network adapters, storage, power supplies, and cabling still produce heat. Direct-to-chip systems need an air-side design for those loads; immersion systems need compatibility checks for all submerged parts.
Ignoring water quality and connectors
Improper chemistry, particles, trapped air, or repeated connector cycles can reduce reliability. Specify coolant treatment, filtration, sampling, venting, connector inspection, and replacement intervals before production.
Assuming immersion preserves normal server service
Wet hardware requires a tank-specific workflow, lifting and drainage equipment, fluid containment, cleaning, and trained personnel. If the operations team cannot support those steps, the theoretical density advantage may not be worth the risk.
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Using one cooling method everywhere
AI compute, storage, and networking rarely have identical heat profiles. A mixed design—liquid for accelerator racks and air for lower-density zones—often reduces retrofit work and avoids overbuilding the whole facility.
Comparing efficiency percentages from different tests
Cooling efficiency depends on outdoor conditions, supply temperatures, workload, redundancy state, and what the measurement includes. Do not compare isolated vendor percentages unless test boundaries and conditions are identical.
Water, energy, and sustainability considerations
Liquid cooling can reduce server-fan energy and enable warmer facility-water operation, but pumps, CDUs, heat exchangers, treatment systems, and heat-rejection equipment consume energy too. Immersion can reduce airflow energy while adding fluid production, handling, filtration, and replacement considerations.
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Use measured power and water data from a representative pilot or a fully specified design. A single efficiency ratio cannot capture reliability, maintenance, embodied equipment, or the cost of retrofitting an existing building.
Bottom-line selection guide
| If your situation is… | Start with… | Reason |
|---|---|---|
| Existing room, moderate rack density, no liquid infrastructure | Air, with containment or targeted rear-door cooling | Lowest disruption and broadest hardware compatibility. |
| New AI cluster with dense GPU racks | Direct-to-chip liquid plus air for residual loads | Captures heat at the processors while preserving hybrid flexibility. |
| Racks around or above 100 kW with limited airflow | Direct-to-chip liquid; evaluate immersion if space or water is severely constrained | Airflow alone becomes difficult to distribute and scale. |
| Extreme density, compact site, standardized hardware | Immersion | Highest density potential when tank and fluid operations are acceptable. |
| Mixed compute, storage, and networking environment | Hybrid liquid and air | Matches cooling effort to each rack’s actual heat profile. |
For a typical new AI deployment, specify direct-to-chip liquid first, retain air for lower-density equipment, and reserve immersion for cases where its density and footprint benefits outweigh the hardware and service changes. Air remains the sensible answer for many existing or moderate-density rooms.
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