When does an AI data centre need liquid cooling?

Liquid cooling is a question about specific equipment in a specific room; no universal rack density settles it. An AI data centre needs it when the equipment being bought requires it, or when a proper assessment shows that air cannot remove the heat at the rack positions available, or can only do so with unacceptable penalties in energy, space or noise.

What has changed is the direction of travel. ASHRAE's data centre committee, TC 9.9, warned in a 2021 white paper that rising chip power is making high-temperature air and water cooling harder to sustain, and recommended that any future data centre include the capability to add liquid cooling. The US Department of Energy's 2024 design guide draws on that paper, and observes that high-performance computing sites adopted direct liquid cooling early because of their rack densities. It also notes that ASHRAE has introduced an air-cooling class, H1, for high-density equipment, and advises operators to consider direct liquid cooling for high-density servers.

Start with the equipment

Some accelerated systems are designed for air cooling. Others are offered only with liquid cooling, or in both variants. The exact model and configuration matter: its supported cooling method, the inlet temperatures or liquid supply conditions it requires, and whether the next generation you might buy will be the same. A cooling strategy chosen for today's server can be wrong for its replacement.

Capacity in the right place

A facility may have ample cooling in total and still be unable to deliver it to a handful of dense racks. Short of converting to liquid, the usual air-side measures (containment, blanking panels, sealing cable openings, spacing racks) may solve a moderate problem. If they do not, liquid becomes the realistic option.

The main approaches

The DOE guide groups liquid cooling into three classes.

The guide treats cold plates and immersion as direct liquid cooling. Some solutions are hybrid, with part of the heat still removed by air; others capture practically all of it, which the guide describes as the most energy-efficient.

The CDU is the interface between the facility water loop and the IT liquid loop, supplying liquid at the temperature, pressure and chemistry the equipment needs. ASHRAE's water classes, renamed in 2021 (W17, W27, W32, W40, W45 and W+), state the upper supply temperature in degrees Celsius. Higher classes make it easier to use economisers or dry coolers, saving energy and water.

Efficiency in principle and in practice

The DOE guide explains why liquid can be more efficient: water carries heat far better than air, warmer supply temperatures allow more economiser hours, and fan power falls. That describes what liquid cooling can do; the saving at your own site has yet to be measured. Whether it lowers operating cost depends on the climate, the existing plant, tariffs and the maintenance regime, and the capital cost of the conversion has to be weighed against it.

Designing the whole liquid path

Once liquid cooling is chosen, the design has to cover the equipment interface, CDUs, facility connections, water treatment, isolation valves, leak detection, drainage and alarms, and explain how each part is maintained without shutting down live equipment. The mechanical engineers and the IT team need to agree who responds to a leak alarm, and how. Operating staff should have procedures, spares and training by the time the first rack is filled.

Questions to settle before committing

• Which equipment, now and at the next refresh, requires or benefits from liquid?
• Can air-side improvements meet the need at lower cost and risk?
• Which liquid approach suits the equipment, and what facility water temperature is available?
• Who operates and maintains the liquid loop, and with what training?