Five Reasons to Adopt Liquid Cooling

Overview

The paper pushes back on the common view that liquid cooling is only for high-density chips. It names five reasons: chip density, low PUE targets, space constraints, harsh environments and water restrictions. They are treated as separate decision triggers, so even a low-density site may have a reason to evaluate liquid cooling. The practical limit of air cooling is taken from The Green Grid's guidance: 15 to 25 kW per rack, with 20 kW per rack as its air-cooled reference.

Beyond that, active rear-door heat exchangers (RDHx) support up to about 40 kW per rack, and row-based units up to about 67 kW in HPC cases. Liquid cooling supports racks of 100 kW and more. Direct-to-chip (DTC) removes 70 to 80% of IT heat and immersion more than 95%. On energy and water, fan removal lowers server energy by about 4 to 15% (8% is typically used in analysis).

Warm water (above 30 C, up to 45 C) allows simple dry coolers and full economization in most climates, eliminating cooling-tower water. The paper also covers footprint compaction and sealed immersion for dusty or corrosive edge sites. This is a justification paper: it lays out reasons and limits rather than computing cost or PUE for a particular facility. On DTC heat capture this paper says 70 to 80%, WP 265 says 50 to 80% and WP 133 says 60 to 90%; that spread needs to be handled explicitly in design.

Key points

1. The Green Grid suggests 15 to 25 kW per rack as the practical limit for air cooling, with 20 kW per rack as its air-cooled reference.
2. Active rear-door heat exchangers support up to about 40 kW per rack; row-based units up to about 67 kW in HPC cases.
3. Liquid cooling supports racks of 100 kW and more.
4. Fan removal lowers server energy by about 4 to 15% (8% typically used in analysis).
5. DTC removes 70 to 80% of IT heat; immersion more than 95%.
6. Warm water (above 30 C, up to 45 C) allows simple dry coolers and full economization in most climates, eliminating cooling-tower water.

Using it in design

• Use 20 kW per rack (the Green Grid midpoint) as the default threshold at which the wizard recommends evaluating liquid cooling.
• Recommend liquid cooling strongly above 40 kW per rack, the RDHx limit.
• Add non-density triggers: PUE target below 1.2, a water-restricted site (WUE target near 0), a harsh or edge environment, or a space-constrained site.
• Note the conflict on DTC heat capture (WP 279: 70 to 80%, WP 265: 50 to 80%, WP 133: 60 to 90%); default to 0.75 and allow a user override.

Who should read it

Designers and decision makers weighing liquid cooling for reasons beyond density, such as PUE, water, space or environment.

Related standards

• The Green Grid liquid cooling guidance
• ASHRAE TC9.9 Liquid Cooling Guidelines

Publication details

Authors

Paul Lin; Tony Day

Publisher

Schneider Electric

Year

2019

Document number

WP 279

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

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

Citation