How to design piping systems for data centers that require liquid cooling

Overview

This article, published in Consulting-Specifying Engineer in 2025 and written by Ardas Sabuncuyan, is a freely available expert piece. The author sets out the design elements of direct liquid cooling piping served by coolant distribution units (CDUs). The scope covers CDU size classes, technology cooling system (TCS) temperature limits, filtration, materials, coolant chemistry, leak prevention, monitoring and redundancy. The guidance draws heavily on ASHRAE TC 9.9 water-quality and TC 9.6 velocity and materials recommendations. Practical installation details include isolation valves at racks, dripless quick disconnects, hose restraint and torque control. The article recommends validating the coolant choice by small field trials.

The numerical criteria can be used directly as design checks. CDU size classes are in-rack 60-80 kW, in-row 300-800 kW and large units of 1 MW and above. TCS supply must be held at least 2 C above room dewpoint. CDUs typically filter at 50 micrometres or finer, with plate heat exchanger spacing of 0.08-0.31 in. In pipes larger than 3 in., coolant velocity should not exceed 2.1 m/s.

The materials and leakage rules are also specific: avoid zinc, aluminium, brass with more than 15% zinc and non-stainless iron in wetted parts, and 25% propylene glycol in deionized water is generally preferred. The maximum allowable spillage at a server disconnect is cited as 1 cm3, and isolation valves should be provided at each rack and manifold.

Key points

1. CDU size classes: in-rack 60-80 kW; in-row 300-800 kW; large units 1 MW and above.
2. TCS supply must be held at least 2 C above room dewpoint to avoid condensation.
3. CDUs typically filter at 50 micrometres or finer; plate heat exchanger spacing 0.08-0.31 in.
4. Coolant velocity should not exceed 2.1 m/s in pipes larger than 3 in. to limit erosion (ASHRAE TC 9.6).
5. Avoid zinc, aluminium, brass with >15% zinc and non-stainless iron in wetted parts; 25% propylene glycol in deionized water generally preferred.
6. Maximum allowable spillage at a server disconnect cited as 1 cm3; isolation valves at each rack and manifold.
7. Coolant choice should be validated by small field trials.

Using it in design

• Liquid-loop design rules: check pipe velocity <= 2.1 m/s above 3 in. NPS and TCS supply >= dewpoint + 2 C, and flag incompatible wetted materials.
• Catalogue CDUs by capacity class with filtration rating and redundancy (pump/HX) fields.
• Place leak-detection zones at CDU, critical piping and rack level by default.
• Specify isolation valves and dripless quick disconnects at racks.
• Validate coolant choice with a small field trial before commissioning.

Who should read it

Mechanical engineers and specifiers designing piping for liquid-cooled data centers.

Related standards

• ASHRAE TC 9.9 liquid cooling guidelines
• ASHRAE TC 9.6 Water-Cooled Servers white paper (2019)

Publication details

Authors

Ardas Sabuncuyan

Publisher

Consulting-Specifying Engineer

Year

2025

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