Coolant Distribution Units (CDU)

The Coolant Distribution Unit (CDU) is the isolation valve between the messy facility water and the pristine secondary loop that touches the silicon. It's also the single most misconfigured box in modern AI halls.

What a CDU does

A CDU decouples two hydraulic circuits with a brazed-plate heat exchanger, then controls the secondary side with variable-speed pumps, flow meters, expansion tank, air separator, filtration and a PLC. Its job is constant secondary supply temperature and stable flow regardless of primary noise.

Liquid-to-liquid vs liquid-to-air

• L2L CDU. Rejects heat to a facility chilled/warm water loop. Standard for anything above ~200 kW.
• L2A CDU. Rejects heat back into the room via a coil and fans. Useful for pilots, small clusters, and edge sites where no facility water exists.

Sizing worked example

A row of eight GB200 NVL72 racks at 130 kW each = 1,040 kW. Design for 10 K ΔT secondary. Required flow: 1,040 / (4.18 × 10) ≈ 25 L/s or ~90 m³/h. Add 15% margin for filter fouling and future racks: specify a 1,200 kW CDU with pumps sized to ~110 m³/h at the total system head.

Redundancy and failover

• N+1 pump configuration inside the CDU is table stakes.
• Two CDUs per row (2N) is common for Tier IV; each sized to full row load.
• Auto-failover on primary flow loss must trigger CRAH ramp-up within 30 s to avoid GPU throttle.
• Manual bypass valves for CDU service without draining the row.

Commissioning checklist

1. Hydrostatic pressure test at 1.5× working pressure for 4 hours.
2. Chemical flush and passivation of stainless components.
3. Fill with de-aerated, filtered coolant; run air separator for 24 h.
4. Log ΔT and ΔP at 25/50/75/100% load with a calibrated flow meter.
5. Simulate pump trip and confirm N+1 rollover under load.