Home / Articles / 19 October 2026
Theme of the month: Data centresPower density: where air cooling runs out
Up to 10 kW per rack the question is airflow management. Above 30 kW air physically runs out. And AI workloads now ask for over 100 kW per rack — at which point the conversation moves from air conditioning to pipework.
Power density is the number that determines everything else in the hall: ceiling height, the type of cooling, rack layout and whether there is a raised floor at all. It is stated in kilowatts per rack and often written into designs out of habit — “8 kW per rack” — without checking what will actually go into one.
The check is elementary and worth doing early. Air cooling carries heat by Q = m · cp · ΔT. At a typical supply-to-return difference of 12 °C, removing 1 kW of heat takes roughly 240 m³/h of air. Multiply by the rack power and it becomes clear where the numbers run out.
The three regimes
- Up to 5–8 kW per rack. A classic hall with hot and cold aisles, supply under a raised floor, perimeter precision units. It works reliably, provided the aisles do not mix.
- 8–20 kW per rack. Aisle containment becomes mandatory — cold or hot, depending on the layout. Without it you get recirculation: hot air from the rear travels around the rack and enters at the front, and the servers breathe their own exhaust.
- 20–40 kW per rack. In-row coolers or rear-door heat exchangers. Air still works, but its path has to be short — the fans already consume a noticeable share of total power.
- Above 40–50 kW per rack. Direct-to-chip liquid cooling. Air remains only for the residual 10–20 % of the heat.
The four errors a thermal camera finds
Before changing technology there is usually something to fix in what exists. Most halls run below their capability for the same reasons:
- Missing blanking panels. Every free U in a rack without a blank is an opening through which hot air returns to the front. It is the cheapest measure with the largest effect and almost never completed.
- Unsealed cable openings. Cut-outs in the raised floor where cables pass let cold air out where it is not wanted. Brush seals cost nothing.
- Misplaced perforated tiles. Placed in the hot aisle, they mix the streams directly. A sheet of paper finds them.
- Supply temperature set too low. Halls are widely held at 18 °C “to be safe”. ASHRAE recommends a range of 18–27 °C at the equipment inlet for classes A1–A4, and the allowable range for class A2 extends to 35 °C. Every degree upward is energy saved directly on cooling.
Raising the supply temperature is the most underrated measure. But it is done after containment, not before — in a hall with mixing, the hottest point is well above the average and the margin is needed. Separate the streams first, then raise the setpoint.
What AI workloads changed
A few years ago 20 kW per rack was high density. Today’s model-training systems work in an entirely different order of magnitude — a rack of current-generation accelerators draws over 100 kW, and announced platforms head towards several hundred. At those figures air cooling is not inefficient but physically impossible: the required flow cannot pass through the rack.
From there the hall stops being an air-conditioned room and becomes a hydraulic installation. There is a primary and a secondary loop separated by a coolant distribution unit, manifolds along the row, quick couplings to each server and — mandatory — leak detection with automatic isolation. The designer of such a hall solves problems of flow, pressure and temperature difference, not air change.
This has a direct consequence for the power side too. At 100 kW per rack the supply is no longer a few individual feeders but an overhead busway with tap-off boxes, in a configuration matching the hall’s redundancy. Conductor sizes, short-circuit currents and selectivity are all recalculated — the values have nothing in common with those of a 5 kW-per-rack hall.
The practical conclusion
Density is set at the start of design and checked against what will actually be installed, not against an average. A hall designed for 8 kW per rack is not upgraded to 30 kW by replacing the cooling units — the room, the distribution and the routes all change.
Where the intended use is unclear, the sensible approach is a mixed hall: the greater part at moderate density and a defined zone prepared for high density — with reinforced power and a liquid-cooling connection, even if it is not used immediately. The cost difference at construction is small; at retrofit it is several times over.