High-Performance Two-Phase Cooling under Different Cold Plate Orientations (OCP EMEA Summit 2025, Future Technologies Symposium)

Overview

This short technical talk covers pumped two-phase direct-to-chip cooling for very dense AI racks. The motivation is next-generation racks near 600 kW that rotate servers vertically; vendor roadmaps point to such racks around 2027. The source is a conference talk and does not include a thorough method or uncertainty analysis.

The talk reports a 250 kW rack test using a two-phase CDU with 40 C facility water. Two-phase CDU products released in April 2025 cool 250 kW of H100 servers using 40 C facility water. In cold-plate tests with R515B refrigerant at 40 C saturation, 4,500 W was removed from a GPU thermal test vehicle before wall-power limits were reached.

Two-phase cold plates are near-isothermal, so the maximum and average case temperatures coincide. In comparisons of the same cold plate in horizontal and vertical orientations, vertical slightly outperformed at high power and horizontal-up slightly at low power; all orientations performed acceptably.

The results show only small orientation effects. The limit of the document is that the talk reflects one vendor's tests and depends on a particular refrigerant and test setup, so the results should not be generalised directly to other systems. For designers its value is in showing that 40 C facility water can be a valid design point for two-phase CDUs.

Key points

1. Two-phase CDU products released in April 2025 cool 250 kW of H100 servers using 40 C facility water.
2. A cold-plate test with R515B at 40 C saturation removed 4,500 W from a GPU thermal test vehicle before wall-power limits were reached.
3. Two-phase cold plates are near-isothermal, so maximum and average case temperature coincide.
4. Vertical orientation slightly outperformed at high power and horizontal-up slightly at low power; all orientations performed acceptably.
5. Vendor roadmaps point to about 600 kW racks with vertically mounted servers around 2027.
6. Results show only small orientation effects.

Using it in design

• In CDU catalogue entries, distinguish single-phase and two-phase (refrigerant type, saturation temperature, facility-water supply temperature).
• Allow 40 C facility water as a valid design point for two-phase CDUs.
• Record cold plate orientation as a design parameter for high-density racks with vertical server layouts.

Who should read it

Cooling designers evaluating two-phase direct-to-chip cooling for very high-density AI racks.

Publication details

Authors

Richard Bonner; Qingyang Wang

Publisher

Open Compute Project Foundation

Year

2025

Document number

OCP EMEA Summit 2025, FTS Data Center Sustainability

Event

OCP EMEA Summit 2025, Dublin

Document language

English

Source type

Industry body

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation