Liquid Cooling Whitepaper - End-to-End Cooling: The Key to Resilient AI Data Centers

Overview

The six-level model runs L1 server, L2 rack subdistribution, L3 row/zone distribution (dual A/B feeds possible), L4 building management, L5 central coolant distribution (N+1 pump groups) and L6 dry coolers. The paper discusses why hydraulic separation at the CDU matters and how tightening approach temperature from 4 K to 2 K at 1,000 kW changes the thermal balance, with TCS flow of 1.5 LPM/kW and external static pressure of 250 kPa. Integration in retrofit buildings is also covered.

Transient load spikes are the main theme. PG25 has a specific heat of about 3.8 kJ/(kg K) against about 4.19 for water. 500 L of PG25 warming 10 K absorbs about 19 MJ: about 38 s of bridging at 500 kW uncovered load or about 10 s at 2 MW; 300 L gives about 23 s and 6 s. The fluid volume of the secondary loop therefore buys only seconds of buffer.

The control proposal is N+1 CDU group control: add a unit when pump speed stays above 80 % for at least 10 min, remove below 40 %, and keep TCS outlet below 35 C, with dual-setpoint PID logic. A planning anchor of about 1.5 litres per minute per kW is used on the TCS. Rack heat densities are cited from about 125 kW today to architectures beyond 500 kW. The paper reflects a manufacturer view.

Key points

1. Six levels: L1 server, L2 rack subdistribution, L3 row/zone (dual A/B possible), L4 building management, L5 central distribution (N+1 pump groups), L6 dry coolers.
2. CDU comparison at 1,000 kW: 4 K vs 2 K approach temperature, TCS flow 1.5 LPM/kW, external static pressure 250 kPa.
3. Planning flow anchor of about 1.5 litres per minute per kW on the TCS.
4. PG25 specific heat is about 3.8 kJ/(kg K) versus about 4.19 for water.
5. 500 L of PG25 warming 10 K absorbs about 19 MJ: about 38 s at 500 kW or about 10 s at 2 MW; 300 L gives about 23 s / 6 s.
6. N+1 CDU staging: add a unit when pump speed stays above 80 % for at least 10 min, remove below 40 %; keep TCS outlet below 35 C.
7. Rack heat densities from about 125 kW today to beyond 500 kW.

Using it in design

• In the liquid-cooling module size TCS flow at about 1.5 L/min/kW by default and store CDU approach temperature and external static pressure.
• Compute secondary loop volume and display ride-through seconds for a given uncovered load (V x rho x cp x dT / P).
• Require N+1 CDUs per hydraulic group and warn if the group lacks staging or redundancy.
• Check the TCS supply limit (e.g. outlet below 35 C) against chip vendor specs, and show the six-level chain as a checklist.

Who should read it

Engineers designing the liquid-cooling chain and CDU control for AI halls.

Related standards

• ASHRAE TC 9.9 Liquid Cooling Guidelines
• OCP ACS

Publication details

Authors

Jonathan Aaron Jürgens; Daniel Ducci

Publisher

STULZ

Year

2026

Document number

Release 01, September 2026 (V1.1 0926)

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