Liquid Cooling - Exponentially increasing power density: At what pace should Liquid Cooling be implemented?
STULZ
This October 2025 STULZ planning paper ties the NVIDIA GPU roadmap to rack densities and to the share of heat that has to leave through liquid. It discusses how to scale cooling infrastructure from today's racks to 600 kW racks.
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Overview
The paper translates the Blackwell, Blackwell Ultra, Rubin, Rubin Ultra and Feynman generations into heat load per rack. Densities have moved from 70 kW to 125-130 kW today, with 600 kW and more expected by the end of 2027 and over 1 MW beyond. GB200 sits around 125-130 kW, GB300 around 135-140 kW, Rubin at 135-200 kW and Rubin Ultra/Kyber up to 600 kW.
The authors' central recommendation is that a hybrid air/liquid infrastructure sized around 125-200 kW per rack is a sound basis today, provided it is modular enough to grow to 600 kW racks that need 100 % liquid cooling. Racks up to about 140 kW are hybrid, with 80-90 % of heat going to liquid and 10-20 % to air.
The physical argument is that air has a specific heat of about 1 kJ/(kg K) at about 1.2 kg/m3, while water offers 4.18 kJ/(kg K) at 1,000 kg/m3. The paper also describes STULZ CDU test procedures (multi-CDU flow stability, filtration down to 25 um to protect cold plates, fluid monitoring against corrosion and microbial growth) and the German rules: new data centres from 1 July 2026 must reach PUE 1.2 within two years, existing ones 1.5 from July 2027 and 1.3 from July 2030.
Key points
- Rack density has moved from 70 kW to 125-130 kW; 600 kW and more is expected by end of 2027 and over 1 MW beyond.
- GB200 about 125-130 kW, GB300 about 135-140 kW, Rubin 135-200 kW, Rubin Ultra/Kyber up to 600 kW.
- 600 kW racks require 100 % liquid cooling.
- Up to about 140 kW the rack is hybrid: 80-90 % of heat to liquid, 10-20 % to air.
- Air is 1 kJ/(kg K) at 1.2 kg/m3; water is 4.18 kJ/(kg K) at 1,000 kg/m3.
- Filtration goes down to 25 um to protect cold plates; CDU tests include multi-CDU flow stability.
- Germany: new data centres reach PUE 1.2 within two years from 1 July 2026; existing ones 1.5 (July 2027) and 1.3 (July 2030).
Using it in design
- Define rack templates per GPU generation with a default kW and liquid/air split, for example 85/15 at 130 kW and 100/0 at 600 kW.
- Above about 140 kW per rack, require a 100 % liquid path including power-shelf cooling.
- Add a filtration-rating field to the technology cooling system (TCS) loop, with 25 um as the reference.
- In pipe and CDU sizing, show the headroom between today's density and the 600 kW class.
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Who should read it
Design engineers and investment decision makers phasing cooling infrastructure for AI clusters.
Related standards
- German Energy Efficiency Act (EnEfG)
Publication details
- Authors
- STULZ
- Publisher
- STULZ
- Year
- 2025
- Document number
- Release 01, October 2025
- 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
STULZ (2025). Liquid Cooling - Exponentially increasing power density: At what pace should Liquid Cooling be implemented?. STULZ. Release 01, October 2025. https://www.stulz.com/fileadmin/user_upload/Documents/Downloads/Whitepaper/Whitepaper_Liquid_Cooling_Planning_en_web.pdf
Events on this subject
- SC26 – International Conference for High Performance Computing, Networking, Storage and Analysis
- NVIDIA GTC
- ISC High Performance
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