Selecting the right coolant distribution units (CDUs) is essential for data centres to optimise liquid cooling, enhance leak-proof reliability, and maintain high efficiency under AI-driven heat demands, writes Abhishek Gupta, Director of Engineering for Liquid Cooling and Networking at nVent.

For data centre managers designing liquid cooling systems, selecting the right coolant distribution unit (CDU) is paramount. CDUs pump cooled fluid to racks and chips in a closed loop, with precision control adjusting fluid temperature and flow rates to maximise efficiency. Hot or cold liquid circulates through hoses and manifolds to reach IT equipment, then back to the CDU where it is cooled using a line of facility water and recirculated. The entire system is a completely closed loop, so risks of leaks or liquid coming into contact with electrical infrastructure are minimised. However, reliable liquid systems are a requirement to minimise the risk of leaks.

CDUs are at the core of driving the efficiencies that liquid cooling can bring to data centres, so choosing the right one is critical. While CDUs may all offer similar features and benefits, data centre managers need to look beyond the obvious.

Examine testing-based performance of the unit versus theoretical projections

To meet the increasing demand and to accommodate high pressure drops in the cooling system loop in/across the IT racks, CDUs are often pushed for the maximum thermal and hydraulic performance possible, and rightfully so. However, due to the physics of flow, hydraulic and associated thermal losses can start to creep in with high flow velocities within the plumbing and heat exchangers.

Engineering tools such as network modelling, HX selection software, computational fluid dynamics and digital twins are great resources when selecting the optimal components and building prototypes. However, testing the complete CDU system to the top end of limits across the range of applications where it will be deployed is where rubber hits the road.

This testing will expose the nuances in the efficiencies of the components and complete system under extreme flow conditions. The hydraulic and thermal performance can degrade hugely and can affect the rating of the unit from prototype to testing, so data centres must work with vendors who are equipped to test and customise CDUs to their needs.

Suitability of critical CDU parts for the specific application

Data centres are a specific application and need to be serviced with units that are truly designed with that in mind. A small leak in residential and industrial segments may be more forgiving compared to a much higher safety and cost risk around IT server racks packed with sophisticated high performing chips. Critical CDU parts need to go through a higher rigor of selection and design verification compared to components in other applications.

For pump selection specifically, the material compatibility of all wetted parts across different use case fluids should be based on verification of existing manufacturing processes. Additionally, the hydraulic failure mode of pumps in case of internal damage to components is critical. Since CDU applications use closed TCS loops, they are sensitive to debris from the early commissioning phase. This debris can compromise the seal integrity of mechanical seal-based pumps, making the pumps susceptible to higher maintenance instances and the associated downtime and cost of replacements. Magnetically coupled pumps are a good alternative for CDU applications.

Reliability for leak integrity

As we have covered earlier, the components and systems in data centres are very leak sensitive due to the high risk, associated liability and cost impacts of any potential leaks. Every joint in the whole facility water system and technology cooling system loops represent potential failure points if not vetted through thoughtful and reliability tests. The learnings from components and system level testing for leak integrity and pressure decay is critical to make CDUs and the entire hydraulic system reliable for longer life of operations.

Designing for manufacturability

There are different design solutions for a given data centre application. However, a solution which is designed to keep manufacturability in the main path of the design process will prove the most beneficial to meet the short- and long-term quantity needs of our industry. As AI and ML creates more demand for data centres, the industry needs to be able to produce different types of products in a wide range of quantities. Manufacturability intent keeps the design of equipment and components accountable to production methods. A great theoretical design is not worthwhile if it cannot be manufactured or scaled.

Serviceability of field replaceable components and true cost of ownership

CDUs are electro-mechanical devices and need to be maintained as such. However, improved serviceability comes not only from how well a particular component fits within a system to start with, but also how friendly the design is to maintain and service. This includes replacing certain parts within the unit during its lifetime. When designing CDUs, a list of such parts should be reviewed and evaluated for a true cost of ownership. As discussed earlier, if critical components such as pumps are not selected for the application and have obvious failure modes, there will be more service requirements and hence cost of ownership will be higher during the unit’s course of operation.