Cooling
Cooling topics
Thirteen questions about liquid cooling in an AI hall, each with its own individually sourced figures — most are still unverified by design.
| Topic | What it covers |
|---|---|
| Why air stops | Moving heat with air needs airflow and fan power that become impractical above roughly 20 to 30 kW per rack. |
| Direct-to-chip cooling and the two loops | Cold plates bolted to each chip carry coolant through micro-channels. |
| Coolant distribution units | A CDU is pumps plus a heat exchanger. |
| Coolant and water quality | The default coolant is PG25: 25 % propylene glycol in water with an inhibitor package. |
| Water classes and NVIDIA's 45 deg C | ASHRAE classes facility water by maximum supply temperature: W17, W27, W32, W40, W45 and W+ above 45. |
| Heat rejection | A dry cooler can only bring water down to a few degrees above outdoor air. |
| Water use | Water usage effectiveness (WUE) is water consumed per unit of IT energy, in litres per kWh or cubic metres per MWh. |
| Rear-door heat exchangers and in-row coolers | A rear-door heat exchanger is a water coil on the back of a rack: server fans push exhaust through it and the air leaves near room temperature. |
| Immersion | Single-phase immersion, servers in a bath of dielectric fluid, is a working niche. |
| Heat reuse | Return water at 45 to 65 deg C can pre-heat buildings, pools or processes directly. |
| Efficiency metrics | PUE is total facility energy over IT energy. |
| Failure modes and commissioning | A leak shows first as pressure loss or humidity, before temperature moves. |
| Structure and layout | Pipes run under the floor, where they compete with power and network, or overhead, where they add load to the structure. |