The Role of Coolant Distribution Units (CDUs) in Data Center Liquid Cooling Systems

As data centers grow in density and processing power, so does the challenge of thermal management. Traditional air cooling—though still widely used—is fast approaching its limits, especially in high-performance computing environments. Enter the Coolant Distribution Unit (CDU): a linchpin in the shift toward liquid cooling technologies designed for modern server cooling demands.

In this article, we’ll explore what CDUs are, how they work, their benefits, and why they’re becoming critical infrastructure in high-density server cooling environments.

What Is a CDU?

A Coolant Distribution Unit (CDU) is a purpose-built system that enables liquid cooling for servers without exposing delicate IT hardware to potentially corrosive or impure facility water systems (FWS). It acts as a thermal and hydraulic interface between the facility cooling system and the technology cooling system (TCS) used to directly cool IT equipment such as cold plates, rear door heat exchangers, or direct-to-chip cooling systems[*1].

Why CDUs Are Essential in Modern Data Centers

Here’s what makes a CDU invaluable in today’s data center environment:

• Heat Exchange: CDUs transfer heat from the TCS (loop that cools IT hardware) to the FWS (loop that supplies chilled water from cooling towers or chillers).

• Water Quality Management: They isolate the sensitive internal TCS loop from the often-variable water quality of facility systems.

• Condensation Control: CDUs manage coolant temperatures to remain above dew point, protecting hardware from moisture.

• Scalability: One CDU can support multiple racks, making expansion more manageable.

In essence, CDUs create a controlled, closed-loop environment tailored to the performance, material compatibility, and reliability needs of liquid-cooled IT gear[*1][*2].

CDU vs. Non-CDU Liquid Cooling Systems

CDU-Based Architecture

CDU-based systems are by far the most common form of liquid cooling implementation. They use liquid-to-liquid heat exchangers to isolate the primary (FWS) and secondary (TCS) cooling loops. Benefits include:

• Use of engineered fluids (e.g., glycol-based coolants)

• Independent control of coolant temperature

• Fine-tuned filtration and water chemistry management

• Compatibility with microchannel cold plates and narrow flow channels

CDUs typically reside outside the rack (floor-standing) or within the rack itself, depending on the data center layout.

Non-CDU Liquid Cooling Systems

In non-CDU implementations, facility water is routed directly to the IT hardware. While simpler and more space-efficient, this design has challenges:

• No buffer against poor water quality

• Increased risk of condensation

• Greater material compatibility constraints

• Fewer options for flexible coolant choice or flow control

For data centers targeting highly optimized thermal performance or dealing with elevated ambient humidity, the CDU remains the gold standard.

Design Considerations for CDU Installations

Designing for a CDU-based cooling system requires careful attention to:

1. Water Quality

• FWS and TCS loops must use compatible materials and inhibitors.

• TCS loops demand stricter water purity: <5 ppm chloride, <10 ppm sulfate, and <3 ppm total suspended solids[*1].

2. Filtration

• Plate heat exchangers in CDUs have narrow channels (2–8 mm).

• Filters on both primary and secondary sides are necessary to prevent fouling and scale buildup.

3. Temperature Regulation

• CDUs maintain TCS loop temperatures 2°C (3.6°F) above dew point to avoid condensation.

• Variable speed pumps and control valves adjust flow based on real-time conditions.

4. Pressure Rating

• TCS loop components are typically designed to operate at lower, more stable pressures than direct FWS systems.

• CDUs reduce risk of cold plate deformation or hose rupture.

When Is a CDU Most Valuable?

Use a CDU when:

• Scaling to high-density workloads like AI, machine learning, or scientific computing

• Deploying cold plate or direct liquid cooling systems

• Requiring precise thermal control or wish to use alternative fluids

• Operating in regions with variable ambient humidity or hard water supply

• Seeking reliable, isolated cooling loops for IT gear longevity

Real-World Trend: Water-Cooled Server Deployment

Despite early hesitation, OEMs are slowly expanding their water-cooled product lines. Data from ASHRAE shows that from 2010 to 2017, water-cooled server rack products supported an average heat density significantly higher than air-cooled counterparts[*1].

Many of these designs require CDUs for proper operation. For instance, 75% of water-cooled racks during that period used a combination of direct and indirect water cooling, with CDUs central to their design.

Conclusion

As air cooling nears its limits, Coolant Distribution Units (CDUs) are emerging as the backbone of next-gen data center cooling strategies. By offering thermal stability, water quality isolation, and scalable architecture, CDUs enable operators to unlock higher densities, lower PUEs, and greater operational reliability.

Whether you’re engineering for the future of hyperscale, outfitting a modular edge site, or upgrading existing infrastructure, integrating a CDU-based cooling loop should be at the top of your checklist.

References

[*1]: ASHRAE Technical Committee 9.9. (2019). Water-Cooled Servers: Common Designs, Components, and Processes.

[*2]: ASHRAE. (2014). Liquid Cooling Guidelines for Datacom Equipment Centers, Second Edition.

[*3]: The Green Grid. TCO Models for Liquid Cooling in Data Centers. www.thegreengrid.org