Is liquid cooling only used for GPUs, or the whole server?

Most current direct-to-chip deployments target only the highest-power components — GPUs and often CPUs — with cold plates, while memory, storage, networking, and voltage regulators remain air-cooled by rack-level fans. This hybrid split (commonly 60-80% liquid, 20-40% residual air by heat load) is typical; full immersion cooling is one of the few approaches that cools essentially everything with liquid.

Does liquid cooling eliminate the need for CRAH/CRAC units entirely?

Not usually. Even a heavily liquid-cooled facility typically still needs air handling for the residual air-cooled load (the fraction of heat not captured by cold plates), general room conditioning, and any non-liquid-cooled equipment — so most real deployments run both systems in parallel rather than replacing air cooling outright.

Why is water so much more effective than air at carrying heat?

Water has a much higher specific heat capacity and density than air, so a given volume of water moving at the same velocity as air can absorb roughly 3,500 times more heat energy. That means far smaller pipes, far less pumping power, and far less physical space are needed to move the same amount of heat compared to the ductwork and fan power air cooling would require at the same load.

Does moving to liquid cooling always lower PUE?

Not automatically — it depends on implementation, but liquid cooling commonly improves PUE because pumping a liquid loop is more energy-efficient per watt of heat removed than moving the equivalent air volume, and higher achievable coolant temperatures can extend free-cooling hours. A poorly designed liquid system could still underperform a well-optimized air system, so the cooling approach and its execution both matter.

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Liquid Cooling vs. Air Cooling

Why a 10 kW GPU server can't simply be cooled by more fans — air runs out of heat-carrying capacity long before GPU racks do, which is the real reason liquid cooling went from a niche HPC technique to the default architecture for AI infrastructure.

Ask why AI data centers are moving to liquid cooling and you'll often hear "because GPUs run hotter." That's true but incomplete — plenty of hot components have been air-cooled for decades. The real constraint is volumetric heat capacity: water carries roughly 3,500 times more heat per unit volume than air moving at the same velocity. A traditional CRAH/CRAC air-cooling system was designed around 5-10 kW/rack, where moving enough air through a rack is straightforward. A GPU training rack drawing 40-130+ kW needs to reject far more heat in the same physical footprint — and beyond a certain density, no amount of additional fan power and airflow can move it fast enough without impractical noise, power draw, and hot-aisle temperatures. Liquid cooling exists because water can carry that heat away in a fraction of the flow volume.