Cooling the AI datacenter (2/4): air and its limits

Contents

The first article drew the map of the thermal challenge and laid out the spectrum of solutions. Now for the bottom rung, the one almost everybody already has installed: air cooling. There is a temptation to treat it as legacy technology, a leftover of the pre-AI datacenter that liquid will sweep away in a couple of years. The data says otherwise. The Uptime Institute’s 2025 cooling systems survey puts perimeter air, the classic room CRAC or CRAH, in 75 % of installations, against 22 % reporting direct liquid cooling. Air is the base case, and it will remain so across most of the installed base for years.

This article has two goals. The first is to understand air well enough to squeeze it: where the energy goes, which measures pay back, and how much headroom is left in a poorly tuned room. The second is to understand exactly why it breaks, because the limit of air is not set by a liquid-cooling vendor’s catalogue: it is set by room geometry and by an exponent.

Inside the air-cooled room

The equipment that cools a traditional room comes in two variants that are often confused. A CRAC (Computer Room Air Conditioner) is a direct-expansion machine: it carries its own compressor and its own refrigerant circuit, and runs autonomously without depending on anything external. A CRAH (Computer Room Air Handler) has no compressor: it receives chilled water from a central plant and passes it through a coil while the fan moves room air. The difference matters for two reasons. The CRAH is usually more efficient, because compression work is concentrated in large, well-optimised chillers instead of being spread across dozens of small machines. And above all, the CRAH integrates naturally with an economiser: if the water can be cooled with outside air, the whole room runs without compressors. The CRAC needs its compressor whatever the weather.

Behind those units sits the final heat rejection equipment, and there the picture of the installed base is revealing. According to the same Uptime survey, rejection splits between air-cooled chillers (35 %), evaporative cooling towers (22 %), direct expansion (11 %), dry coolers and fluid coolers (6 %), free cooling with outside air (4 %) and adiabatic misting (3 %). It is a base dominated by the mechanical chiller, which is exactly the piece modern design tries to keep switched off for as many hours as possible.

Between the room and the rack there is an intermediate step, close-coupled cooling: in-row units placed between cabinets, shortening the air path and raising the density that can be handled. A typical row unit today, such as the Vertiv CoolPhase Row range, moves 30 to 40 kW per unit. Some 32 % of surveyed installations already have some form of this cooling, almost always coexisting with perimeter air.

The geometry of airflow

The formula that governs any fluid cooling system already appeared in the previous article:

$$\dot{Q} = \dot{m} \cdot c_p \cdot \Delta T$$

Solved for air with a realistic server temperature rise, it yields a rule of thumb every room designer knows: roughly 140 to 160 CFM per kW dissipated. Schneider works through an example with an 18 kW rack needing 2,500 CFM (1,180 l/s) at the inlet; an air-cooled DGX B200 asks for 1,550 CFM on its own. Those figures are not large in the abstract. They become a problem when crossed with the other half of the system, which is where that air comes in.

A perforated raised-floor tile delivers around 300 CFM (142 l/s). A grate-type tile reaches 700 CFM. Going beyond 500 CFM per tile takes special pieces and a very carefully designed plenum, because pressure under the floor is not infinite and tiles compete with each other. That is where the most quoted and least disputed limit of classic air comes from: a conventional distribution with one vented tile per rack does not sustain much more than 6 kW per rack. With containment and serious airflow management you reach around 10 kW sustained, and highly optimised designs stretch into the twenties.

The arithmetic of an AI rack breaks that beyond argument. At 140 CFM/kW, a 50 kW rack asks for about 7,000 CFM. With standard 300 CFM tiles that would take twenty-three of them under a cabinet whose footprint is worth two. With 700 CFM grates, ten. There is no floor area through which to push that flow, nor plenum pressure to distribute it. Air fails on geometry before it fails on thermodynamics: the room runs out of places to deliver the fluid long before the fluid runs out of capacity to absorb heat.

And there is a second, subtler problem that shows up when someone tries to force the machine.

The cube law, friend and executioner

The power a fan draws grows with the cube of its rotational speed. The US Department of Energy illustrates it with a direct case: a fan drawing 6.5 kW at full speed drops to 3.3 kW at 80 % of revolutions, because \( 0.8^3 \approx 0.51 \). The same relation seen from the air circuit says the required pressure grows with the square of the flow, \( P = Z \cdot Q^2 \), so doubling the flow quadruples the pressure and multiplies fan power by eight.

That exponent hands out very unequal prizes. At low density it is the greatest gift air has, and the measured numbers back it up. Variable-speed fan retrofits documented by the DOE at three facilities cut cooling system consumption by 22 % to 32 %, with payback under two years. The LBNL demonstration at a Digital Realty site in El Segundo, with 72 CRAHs and about 2.6 MW of IT load, is even more emphatic: swapping scroll fans for plug fans dropped CRAH power from 439 to 233 kW, a 47 % cut; adding intelligent speed control brought it to 147 kW. The combined effect was a 66 % reduction in CRAH-associated power, 2.9 million kWh a year, with payback again under two years.

At high density that same exponent presents the bill. Every attempt to compensate for missing airflow by raising revolutions is paid at the cube, and not only in room units. The same happens inside the server, and there Uptime has a clean data point from HPE: two single-socket servers of practically identical configuration, one in 1U and one in 2U, differ by 30 to 60 W of consumption, and at low utilisation the 1U burns around 30 % more power to do the same work. The cause is geometric: a 1U chassis forces small fans that have to spin fast. In an air-cooled AI node, where fans in high-performance systems can take 10 % to 20 % of system power, that penalty stops being anecdotal and becomes a cost line.

Containment: cold aisle or hot aisle

Before buying anything, almost every room has headroom in airflow management. The enemy goes by two names: recirculation, hot exhaust air finding its way back to server inlets, and bypass, cold air reaching the return without passing through any equipment. Without airflow management, the installed cooling capacity needed can reach two or three times what theory requires, because the designer compensates for poor mixing by lowering the setpoint of the whole room.

The metrics to diagnose it have existed for years and remain underused. The RCI (Rack Cooling Index) measures how effectively inlet temperatures stay within specification, with two separate indices for the high and low ends; a well-designed room should be at 90 % or above. The RTI (Return Temperature Index) compares the temperature rise of the cooling units with that of the IT equipment and separates the two ills: below 100 % there is bypass, above it there is recirculation. A couple of measurement sessions and a handful of blanking panels in the rack gaps recover more efficiency than any catalogue promises.

The next step is physical containment, and here the choice between containing the cold aisle (CACS) or the hot one (HACS) has a consequence that gets overlooked. Schneider’s model of a 700 kW, 100-rack facility in Chicago, comparing both under the realistic condition that the uncontained zone stays at 24 °C so staff can work, shows a large difference: hot-aisle containment consumes 43 % less cooling energy and drops annualised PUE from 1.98 to 1.69. The reason lies in the setpoint, not in the physics of the exchange. Contain the heat and the whole room lives at 24 °C, equipment draws air at 24 °C and the economiser can run 5,319 hours a year. Contain the cold and the room is the hot aisle, and keeping it habitable at 24 °C forces supply air at 10 °C, which leaves the economiser at zero hours.

Figures published by ENERGY STAR point the same way with similar orders of magnitude: hot and cold aisle layout without containment already saves 10 % to 35 %; cold-aisle containment reaches up to a 30 % reduction in cooling cost; hot-aisle containment adds around 40 % on top of that. Typical payback on containment is under two years.

The thermal envelope and class H1

All of the above rests on a prior question: at what temperature can the equipment work. ASHRAE TC 9.9 answers with its air classes, and the ranges are less demanding than many rooms actually practise.

ClassRecommendedAllowableMaximum dew point
A118-27 °C15-32 °C17 °C
A218-27 °C10-35 °C21 °C
A318-27 °C5-40 °C24 °C
A418-27 °C5-45 °C24 °C
H118-22 °C18-25 °C17 °C

The recommended range is identical for every A class: 18 to 27 °C equipment inlet temperature. What changes between classes is how far you can stray from it without losing warranty. A decade of PUE improvement was built on that table, raising setpoints and widening the hours in which outside air is enough.

The fifth edition of the guidelines, published in 2021, added a class that deserves attention: H1, intended for high-density air-cooled equipment. Its recommended range is 18 to 22 °C and its allowable limit drops to 25 °C, against A1’s 32 °C. ASHRAE’s stated reason is physical and direct: in some dense systems there is no room for heatsinks or fans capable of keeping components within their limit if the incoming air is warm. Class H1 is the formal acknowledgement, by the body that wrote the thermal envelope, that high density on air no longer fits inside it.

The efficiency consequence is the part that interests an architect. Going from an allowable of 32 °C to one of 25 °C, and from a recommended of 27 °C to 22 °C, destroys much of the annual economiser hour budget. Facilities designed around free cooling and adiabatic cooling do not sit well in that envelope, and Uptime says so without hedging: the way out is to deploy liquid cooling for the dense part. Air does not run out of thermal capacity when density rises. It runs out of efficient thermal capacity, which is what made the model attractive.

Free cooling and a climate on the move

What the economiser is worth depends on where you are. Vertiv’s analysis of a 1 MW datacenter gives three figures per city that serve as a quick reference:

CityAir-cooled chillerFree coolingAdiabatic free cooling
LondonpPUE 1.21pPUE 1.09pPUE 1.06
MadridpPUE 1.22pPUE 1.12pPUE 1.07
DubaipPUE 1.31pPUE 1.24pPUE 1.18

In annual operating modes, London spends 81 % of the year in free cooling or adiabatic free cooling, and Madrid 74 %, which is on the order of 7,100 and 6,480 hours without mechanical compression. Adiabatic savings against a standard air-cooled chiller at full load land between 60 % and 65 %.

That window is closing, and it has been measured. A paper published in Scientific Reports in June 2026 quantifies out-of-specification hours (inlet above 27 °C, or dew point above 15 °C, or relative humidity above 70 %) for the 1980-2024 period and projects to 2050. Northern Europe remains comfortable, with about two hours a day on average out of range. The Mediterranean and the Middle East add more than four hours a day per decade. Dallas-Fort Worth exceeds 20 % of the year; Singapore, 85 %. The fraction of sites in the optimal tier falls 6 % per decade while the most constrained tier grows 9.1 %. For a design with a fifteen-year life in southern Europe, the 2026 economiser hour budget is not the 2040 one.

What it costs in water

Air has a cheap route to better energy efficiency: evaporate water. And there lies one of the most uncomfortable trade-offs in physical infrastructure. Vertiv’s numbers settle it in a single sentence. Adding an adiabatic misting system on top of an air-cooled chiller improved pPUE by 0.01 points and raised WUE from 0 to 0.165 gallons per kWh (about 0.62 l/kWh). At a 3 MW facility that is 4.3 million gallons a year, on the order of 16,000 m³, the consumption of about forty homes. Indirect evaporative cooling sits around one litre per kWh. An air-cooled chiller without adiabatic assist, or a pumped-refrigerant direct-expansion system, has zero WUE and pPUE of 1.12 to 1.13.

One hundredth of a pPUE point in exchange for four million gallons a year was for years an obvious trade in water’s favour, because energy was paid for and water almost was not. In 2026, with moratoria and public opposition around several projects, that sum no longer always works out, and in some jurisdictions it is not even legal. The reference case for evaporative air done well is still Prineville, where Meta reported a WUE of 0.22 l/kWh with 100 % outside-air economisation, direct evaporative misting and evaporation efficiency around 85 %, with no chillers and no towers.

The ceiling of air, and who reaches it

Well-designed air goes a long way. Google reports a fleet PUE of 1.09, and its best site, in central Ohio, sustains 1.04. Yahoo’s Computing Coop in Lockport was designed for a PUE of 1.08 using ambient air 99 % of the time, with the roof acting as a chimney and cooling dropping from 25 % to 1 % of total power, although that figure is design rather than measurement. Kyoto-type thermal wheels, with 4.6 metre rotors turning at 1 to 6 rpm and around 300 kW per unit, have taken whole facilities in Chicago to pPUE 1.19 with up to ten months of free cooling a year.

Against that, the industry average measured by Uptime has been stuck around 1.54 for six years: 1.59 in 2020, 1.55 in 2022, 1.54 in 2025. The gap between 1.09 and 1.54 is not technological. It is about design, scale and the ability to build from scratch in the right place. Air has a very high efficiency ceiling that almost nobody reaches, and that is the best news for an operator with an existing room: there is nearly always headroom before touching infrastructure.

Rear doors: the last rung

Between room air and liquid on the chip there is a step that has become the big retrofit product of 2025 and 2026: the rear-door heat exchanger, or RDHx. The idea is simple. Instead of cooling the room, hot air is intercepted right at the rack outlet, its heat removed with water, and it is returned to the room at the temperature it came in at. From the rest of the facility’s point of view, that 60 kW rack does not exist thermally. This is what vendors call room neutrality, and it explains why a rear door forces neither a containment redesign nor a plenum recalculation.

There are two families. The passive kind consumes no electricity: it uses the push of the server’s own fans to cross the coil, and sits in the lower capacity band. The active kind adds variable-speed fans in the door, compensating the coil’s pressure drop and responding better to hot spots, at the cost of its own consumption. Capacities declared by serious vendors converge on a reasonable band: Vertiv announces up to 80 kW per rack in its CoolLoop, Motivair up to 75 kW in the ChilledDoor, nVent up to 78 kW with 14 °C water and a maximum draw of 1,500 W per door. Other catalogues publish figures of up to 200 kW per cabinet without specifying water temperature or flow, and those should be read for what they are. The defensible band today is 40 to 80 kW per rack.

The detail that makes the rear door attractive is water temperature. It works with warm water, on the order of 14 °C, well above the 7 °C of a classic chiller plant, which widens free cooling hours instead of reducing them. And it deploys rack by rack, with no general construction and no room shutdown. Adoption reflects this: the 2026 AFCOM report puts operators who have deployed or are evaluating rear doors at 37 %, and Schneider Electric’s purchase of Motivair confirms that the large vendors see them as a strategic product.

The real density of the installed base

Public conversation revolves around 130 kW racks. The real density distribution of the global installed base, measured by Uptime in 2025, tells another story:

Average rack densityFacilities
1-3 kW16 %
4-5 kW30 %
6-7 kW18 %
8-9 kW10 %
10-14 kW14 %
15-19 kW5 %
20-29 kW4 %
30 kW or more4 %

Some 74 % of facilities have an average density below 10 kW per rack, and 82 % do not exceed 30 kW even in their densest cabinet. Asked about the threshold beyond which air stops being enough, operators give a modal answer of 40-49 kW (25 % of responses), and 44 % put it at 40 kW or above, well past the “over 20 kW already needs liquid” repeated in presentations. Uptime marks the impracticable boundary at around 50 kW per rack.

From that comes the correct reading of the moment: air is not being replaced, it is being segmented. The training and dense inference frontier is moving to liquid, and the headline goes with it. The bulk of the installed base, still below 30 kW, is solved with optimised air, containment and, when a GPU island appears, a rear door. The useful question for an operator is not when air dies, but which density percentile their room sits in.

For an inference factory

Three concrete decisions follow from all of the above.

The first is to exhaust airflow management before buying hardware. Measuring RCI and RTI, closing rack gaps with blanking panels, matching tiles to the real load distribution and raising the setpoint to the top of ASHRAE’s recommended range costs little and frees capacity that is already installed. The ventilation retrofits measured by DOE and LBNL, with 22 % to 66 % reductions in cooling power and payback under two years, are the best return available in an existing room.

The second is to choose hot-aisle containment if the goal is the economiser, and to design the system around the highest water temperature the equipment tolerates. The decision chain runs in that order: the setpoint sets free cooling hours, free cooling hours set the PUE and, in many geographies, the absence of compression is what allows the evaporative tower and its water consumption to be dropped. Class H1 pushes the other way for dense equipment, and that tension is what decides when liquid becomes due.

The third is to size by percentiles rather than by the hottest rack in the catalogue. A room about to host two 40 kW GPU racks alongside forty 6 kW cabinets does not need rebuilding: it needs rear doors on those two and serious airflow management on the rest. Moving to liquid makes sense when sustained density forces it, and that moment arrives sooner through efficiency than through capacity.

One idea runs through the rest of the series. Even when a whole room moves to direct liquid to chip, 2 % to 20 % of the heat still leaves through air, and somebody has to remove it. Air does not disappear from the AI datacenter. It changes role: it stops being the cooling system and becomes the system that picks up what the liquid does not capture. The next article goes into that liquid, into the cold plate that carries it to the silicon, and into the CDU that governs the loop.

See also

Sources