Subzero: CDUs The Brains Behind Direct Liquid Cooling

As data centres evolve to meet the exponential power and cooling demands of AI and high-performance computing (HPC), traditional air-cooling methods are reaching their limits. Direct Liquid Cooling (DLC) has emerged as the next frontier in thermal management, offering the precision and efficiency needed for today’s high-density workloads.

At the heart of this innovation lies the Coolant Distribution Unit (CDU), the intelligent control layer that ensures safety, stability and sustainability in liquid-cooled environments.

In this Q&A, Gordon Johnson, Senior CFD Manager at Subzero Engineering, explores the pivotal role CDUs play in transforming data centre operations. From maintaining system integrity and adapting to variable AI workloads to driving energy efficiency and future scalability.

What initiated the transition towards DLC and what makes CDUs crucial for development?

The increase in AI and high-performance computing (HPC) demands has pushed rack power densities beyond the capabilities of conventional air-cooling strategies.

High-end server CPUs and GPUs are approaching or even exceeding 700–1000 watts per socket, especially in AI and HPC deployments and air-cooling can’t remove the generated heat amounts effectively. Direct Liquid Cooling (DLC), especially Direct-to-Chip (DTC) cooling, delivers accuracy and thermal capacity needed at these densities. However, DLC works effectively with a control layer, which is the CDU, allowing for regulations on pressure, flow and temperature to guarantee consistent thermal performance. It transforms liquid cooling from a mechanical method into an adaptive system to accommodate high-density workloads.

In what ways does the CDU maintain safety and stability in a liquid-cooled data centre?

The CDU acts as a safeguard between the facility's water system (FWS) and the IT equipment. It separates the technology cooling system (TCS) loop from pollutants, pressure variations and chemical substances present in the FWS loop, guaranteeing that the fluid supplied to the cold plates attached directly to the CPUs and GPUs is pure, regulated and non-conductive.

By utilising integrated sensors, pumps and heat exchangers, the CDU regulates the ideal coolant temperature and pressure, avoiding condensation and safeguarding hardware from thermal fluctuations. This separation protects IT resources as well as improving predictability and operational performance throughout the cooling system.

AI and HPC tasks are recognised for quick changes in power consumption. How do CDUs handle this uncertainty?

AI training and inference tasks are highly variable. GPUs can either increase or decrease performance quickly, resulting in immediate temperature surges. CDUs address these changes by adjusting pump speeds, flow rates, temperature and valve positions to evenly distribute the load.

By continuously adapting to the workflow demand, CDUs maintain stable pressure and temperature throughout all racks. This degree of regulation guarantees that even in unstable environments, operations can run with limited risks and maintain a prolonged equipment lifecycle.

Are there different types of CDUs for varying facility sizes and requirements?

CDUs are usually classified into two primary groups:

• Liquid-to-Liquid CDUs: These utilise heat exchangers to move heat from the TCS (IT coolant loop) to the FWS and are best deployed for large-scale or HPC data centres with existing chilled water infrastructure. 
• Liquid-to-Air CDUs: These expel heat straight into the ambient air within the data centre via an internal exchanger, rendering them ideal for smaller or edge facilities and where a FWS or chilled water is unavailable.

Both offer similar control, isolation and safety advantages, but the decision typically relies on the site's existing infrastructure and cooling capability.

In what ways do CDUs help achieve sustainability and energy efficiency goals?

CDUs play a crucial role in sustainable thermal management as they help avoid excessive pumping and cooling, which are significant causes of energy wastage in data centres. Smart flow and temperature regulation enhance Power Usage Effectiveness (PUE) and Water Usage Effectiveness (WUE), with PUE figures at times even reaching below 1.1.

Also, CDUs facilitate the integration of waste heat recovery systems, enabling operators to utilise surplus heat for district heating or industrial applications. This converts a byproduct into a sustainability resource, decreasing overall energy use and promoting circular energy approaches.

How do CDUs enhance scalability for future liquid cooling installations?

A significant benefit of the CDU is its flexibility, employing mixing and bypass control to adjust liquid coolant for particular IT loads. This adaptability allows operators to gradually implement liquid cooling, without the need for complete infrastructure overhauls. Therefore, CDUs render liquid cooling attainable, expandable and secure, enabling data centres to progress alongside workload requirements and sustainability objectives.

What part do CDUs play in influencing the future of thermal management?

CDUs serve as the thermal regulation system in contemporary data centres. They provide intelligence, isolation and efficiency to Direct Liquid Cooling, ensuring scalability and sustainable functioning of high-density settings and create an accurate framework that is purposefully designed for the AI era.