Direct Liquid Cooling System Challenges in Data Centers

Overview

The paper describes eight common challenges when deploying DLC at scale, typically 500 kW or more and at least 10 racks, where floor-mounted L-L CDUs and a dedicated technology cooling system (TCS) become applicable. The challenges span specification, installation and operation: material incompatibility, conflicting liquid and air temperature needs, tight coupling of servers to infrastructure, lack of CDU efficiency standards, provisioning space for unknown future loads, contamination control, unclear warranty boundaries, and slow DLC response to GPU power transients.

Numeric examples: typical TCS supply is 30 to 45 C (ASHRAE S30 to S45 classes), and a 25% propylene glycol mixture is common. Chiller efficiency improves about 1 to 2% per 0.6 C (1 F) of higher chilled-water temperature (2 to 4% for variable-speed chillers). The heat capture ratio defines the air share: 80% liquid capture leaves 20% for air cooling. A stranding example: a 300 kW CDU serving 10 racks at 30 kW instead of 3 racks at 100 kW strands 70% of pipe and manifold capacity.

The transient challenge is especially important: GPUs may peak to 150% or more of TDP for milliseconds (for example a 700 W chip to 1,050 W for 50 ms), and the DLC loop responds more slowly than the power transient. Each challenge comes with mitigation guidance; examples include dedicated high-temperature plants, flushing and filtration procedures, and sizing piping for realistic density ranges.

Key points

1. Guidance targets DLC deployments of about 500 kW or more and at least 10 IT racks, where floor-mounted L-L CDUs and a dedicated TCS become applicable.
2. Chiller efficiency improves about 1 to 2% per 0.6 C (1 F) of higher chilled-water temperature (2 to 4% for variable-speed chillers).
3. Typical TCS supply 30 to 45 C (ASHRAE S30 to S45 classes); a 25% propylene glycol mixture is common.
4. Heat capture ratio defines the air share: 80% liquid capture leaves 20% for air cooling.
5. Stranding example: a 300 kW CDU serving 10 racks at 30 kW instead of 3 racks at 100 kW strands 70% of pipe/manifold capacity.
6. GPUs may peak to 150% or more of TDP for milliseconds (700 W chip to 1,050 W for 50 ms); the DLC loop responds more slowly than the power transient.
7. Eight challenges: material compatibility, liquid/air conflict, server-infrastructure coupling, no CDU efficiency standard, unknown future IT, contamination, warranty clarity, transient response.

Using it in design

• Use the eight challenges as a DLC readiness checklist in the commissioning and design-review module.
• Defaults: TCS supply 32 C for compatibility and 40 to 45 C for efficiency, PG25 coolant, CDU N+1.
• Size pipes and manifolds for the expected range of rack densities and warn on stranding.
• Add a materials-compatibility check on wetted parts (copper, stainless; avoid aluminum mixing).
• Require a flushing and filtration step in the commissioning plan.

Who should read it

Engineers and project managers who design, commission and operate large-scale DLC projects.

Related standards

• ASHRAE TC9.9 Liquid Cooling Guidelines (S-classes)
• OCP Advanced Cooling Facilities
• ASHRAE Guideline 0

Publication details

Authors

Maria A. Torres Arango; Victor Avelar; Stefano Lena; David McGlocklin

Publisher

Schneider Electric

Year

2025

Document number

WP 210

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