Fluid Dynamics: Liquid Cooling's Role in the Future of Data Centers
Burns & McDonnell · Sam Allen
A five-page Burns & McDonnell white paper describing three liquid cooling approaches at the IT level (rear-door heat exchangers, direct-to-chip and immersion) and their effect on facility design.
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Overview
This five-page 2024 Burns & McDonnell white paper by Sam Allen describes three liquid cooling approaches at the IT level: rear-door heat exchangers (RDHX), direct-to-chip and immersion. It explains the split between the technology cooling system (TCS) and the facility water system via CDUs, and why facility water is not piped straight to IT equipment.
On density, liquid-cooled halls are being built at about 10x the density of air-cooled ones; some solutions reach 80-120+ kW per rack versus a typical 8-12 kW. Liquid gives roughly 3,200x more heat capacity per unit volume than air. An RDHX transfers about 100% of server heat to the TCS and discharges air at about cold-aisle temperature, so containment matters less.
Even with 100% liquid-cooled IT, about 20-50% of the IT load still needs air cooling (convection losses plus air-cooled network racks). Facility water is kept off the IT because of purity requirements and to limit the water volume per circuit if a leak occurs. If dry coolers can reject heat in all hours for the climate, WUE tends to zero.
In energy accounting, CDU pumps take over server fan energy, moving consumption from the PUE denominator to the numerator, and TUE is suggested as a fairer comparison. The paper also touches on fluid choice, the change in staff skills and the reuse of higher-grade waste heat. As a limitation, it is a five-page overview and offers no calculation method.
Key points
- Liquid-cooled halls are being built at about 10x the density of air-cooled ones.
- Some solutions reach 80-120+ kW per rack versus a typical 8-12 kW.
- Liquid gives roughly 3,200x more heat capacity per unit volume than air.
- An RDHX transfers about 100% of server heat to the TCS and discharges air at about cold-aisle temperature.
- Even with 100% liquid-cooled IT, about 20-50% of the IT load still needs air cooling.
- Facility water is kept off the IT because of purity requirements and to limit water volume per circuit on a leak.
- CDU pumps take over server fan energy, moving consumption from the PUE denominator to the numerator; TUE is suggested.
- If dry coolers can reject heat in all hours for the climate, WUE tends to zero.
Using it in design
- For liquid-cooled racks, always keep a residual air-cooling load (default 20-50% of IT, user-adjustable) and size CRAH or fan-wall capacity for it.
- Model CDUs as a TCS/facility-water boundary with fluid type and loop volume fields.
- Show a warning when PUE comparisons between air and liquid designs ignore server fan power.
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Who should read it
Data centre mechanical designers and decision makers planning a move to liquid cooling.
Related standards
- ASHRAE TC 9.9 Liquid Cooling Guidelines
Publication details
- Authors
- Sam Allen
- Publisher
- Burns & McDonnell
- Year
- 2024
- Document number
- 673770
- Document language
- English
- Source type
- Expert / community
- Access
- Free to read
- Licence
- Copyright retained; citation only — the publisher’s terms apply to the original document
Citation
Sam Allen (2024). Fluid Dynamics: Liquid Cooling's Role in the Future of Data Centers. Burns & McDonnell. 673770. https://info.burnsmcd.com/hubfs/Collateral/White%20Papers/GFS_KCM_LiquidCoolingforDataCenters_673770_WP/Fluid-Dynamics-Liquid-Coolings-Role-in-Future-of-Data-Centers-white-paper-burns-mcdonnell-673770.pdf?hsLang=en
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