Understanding Data Center Liquid Cooling Options and Infrastructure Requirements

Overview

The paper states that air cooling loses effectiveness above about 20 kW per rack, and that racks above 30 kW effectively require liquid. The reason is fluid physics: volumetric heat capacity is 4,182 J/L-K for water, about 1,855 for a hydrocarbon dielectric, about 1,831 for a fluorocarbon and only about 1.3 for air. Moving the same heat with air takes thousands of times the volume.

The solution family is introduced as five options: rear-door heat exchangers, single-phase and two-phase direct-to-chip cold plates, and single-phase and two-phase immersion. It stresses that cold plates remove only 70-75% of rack heat, so the rest must still be taken by air and a hybrid air/liquid design is unavoidable. Room cooling therefore cannot be dropped even when liquid is chosen. The CDU's role gets detailed treatment: it creates an isolated secondary loop and holds supply temperature above the room dew point to prevent condensation. A single CDU can exceed 1 MW of capacity. Indoor chillers, immersion tanks and heat-rejection plant complete the infrastructure list.

In the sustainability section, return water is said to reach 60 degC (140 degF) or higher, which makes heat reuse possible; the claim that liquid cooling can push PUE toward 1.0 is a vendor assertion. The paper dates from 2021 and is introductory: it maps the options rather than giving a sizing method, so more recent density levels need later documents.

Key points

1. Air cooling loses effectiveness above ~20 kW per rack; >30 kW racks effectively require liquid
2. Direct-to-chip cold plates typically remove 70-75% of rack heat; the remainder must be air-cooled
3. Volumetric heat capacity: water 4,182 J/L-K; hydrocarbon dielectric ~1,855; fluorocarbon ~1,831; air ~1.3
4. CDU keeps secondary supply temperature above room dew point
5. Single CDUs are available above 1 MW
6. Liquid return water can reach 60 degC (140 degF) or higher, enabling heat reuse
7. Liquid cooling can drive PUE toward ~1.0 (vendor claim)

Using it in design

• Rule: rack kW > 20 suggests containment plus in-row cooling or rear-door heat exchangers; rack kW > 30-40 recommends liquid.
• When direct-to-chip is chosen, default the air fraction to 25-30% and size room air cooling for it.
• Use the volumetric heat-capacity table in the coolant catalog; flow is Q[L/s] = P[kW]/(rho x cp x dT).
• Add a validation that keeps CDU secondary supply temperature above the room dew point.

Who should read it

Designers and managers evaluating the move from air to liquid cooling for the first time.

Related standards

• ASHRAE TC9.9 Liquid Cooling Guidelines
• OCP Immersion Design Guidelines

Publication details

Authors

Vertiv

Publisher

Vertiv

Year

2021

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