As digital infrastructure expands to support AI, high-performance computing (HPC), and cloud services, data centers are operating under unprecedented thermal demands. Processor power densities are climbing rapidly, while sustainability pressures drive the need for higher cooling efficiency and lower energy use.
Against this backdrop, rear door heat exchangers (RDHXs) are emerging as a vital solution, extending the capabilities of air cooling while complementing the rise of liquid cooling technologies. A recent webinar, presented by OTS Research & Development (OTS R&D) and the Copper Development Association (CDA), examined how optimized RDHX designs, including innovations such as 5 mm copper tube technology, can help data centers achieve both performance and efficiency targets in an evolving cooling landscape.
The surge in data-driven applications and AI workloads has caused a steady increase in server heat output. According to industry data, average rack power density has more than doubled in the last decade, yet roughly 80% of data centers still rely on air cooling.
While next-generation facilities are exploring liquid cold plates, two-phase systems, or immersion cooling for the most heat-intensive applications, the majority of installations remain air-cooled, with RDHX adoption growing rapidly. In fact, 29% of operators currently use RDHXs, and 83% of new builds plan to deploy them within the next year.
This transition reflects an important truth: air cooling isn’t going away, it’s evolving.
Even in liquid-cooled environments, many components such as memory modules, power electronics, and storage devices continue to rely on air. That’s where RDHX systems come in.
Mounted directly to the rear of a server rack, an RDHX captures exhaust air from IT equipment and cools it via a liquid-to-air heat exchanger before it re-enters the room.
This close-coupled cooling approach provides several key advantages:
As the presenters note, “It’s not a question of liquid or air cooling, both will be needed.” RDHXs enable operators to manage this balance effectively.
At the core of RDHX performance lies the heat exchanger, where airflow, water flow, and material selection interact to determine efficiency.
Designers must balance multiple objectives, including maximizing capacity, minimizing fan and pump power, reducing costs, and ensuring mechanical simplicity. Tube diameter, fin spacing, and circuit geometry all play a role in achieving these targets.
The study by OTS R&D focused on one particular variable with significant impact, tube diameter, comparing conventional 9.53 mm (3/8”) copper tubes to smaller 5 mm designs
Using 5 mm copper tubes increases the airside surface area per unit volume, enhancing convective heat transfer while reducing coil mass. These smaller tubes also enable tighter packing and more flexible design configurations.
However, the tradeoff is managing higher pressure drops, a challenge that can be overcome through careful optimization of tube spacing, fin density (FPI), and the number of tube banks.
The OTS team’s modeling revealed remarkable gains:
These improvements translate to lighter, more efficient coils that require less material without sacrificing performance, aligning perfectly with copper’s sustainability and recyclability advantages.
To refine performance, OTS R&D conducted an extensive parametric study of multiple RDHX configurations. Designs were optimized for either the lowest air pressure drop (for energy efficiency) or the highest cooling capacity (for thermal performance).
Results showed that optimized 5 mm designs could save up to 28% in fan power while delivering equal or greater cooling capacity compared to the baseline system. Similarly, mass reductions of approximately 27% were achieved in configurations that maintained equivalent capacity.
This parametric approach demonstrates how precision design and copper-based engineering can adapt to evolving thermal loads without compromising reliability or efficiency.
Beyond physical design, operating temperature has a profound impact on data center efficiency.
The study examined the impact of varying inlet water and air temperatures, key parameters that influence Power Usage Effectiveness (PUE). Under typical conditions (17°C inlet water), lowering water temperature can increase cooling capacity but reduce chiller efficiency. Conversely, raising inlet water temperature to 27°C allows chillers to operate more efficiently, cutting compressor power by 32% and improving overall PUE by 7%
Crucially, if IT equipment can tolerate warmer inlet air, capacity gains can triple under the same airflow conditions. RDHX systems offer the flexibility to operate efficiently across a wide range of variable temperatures.
Temperature management depends as much on airflow control as on heat exchanger performance.
In fixed-airflow scenarios, extremely hot server exhaust can push equipment into thermal risk zones. However, by dynamically adjusting airflow while maintaining a constant temperature differential, RDHX systems can scale capacity safely and prevent overheating, even at higher rack densities.
This adaptability makes RDHXs especially valuable for retrofit projects and hybrid cooling environments, where flexibility and rapid response are crucial.
To quantify the benefits, OTS modeled a 1 MW data center using optimized 5 mm RDHX designs. Compared to a baseline configuration:
These results confirm that while smaller tubes may require higher fan energy, the total system efficiency improves significantly due to reduced chiller load, a favorable tradeoff in modern data center operations.
As with all thermal systems, practical factors must be managed for long-term reliability:
These considerations underscore the importance of careful system integration and material compatibility, areas where copper continues to excel due to its exceptional thermal conductivity, durability, and antimicrobial properties.
The findings from OTS R&D and the Copper Development Association illustrate how RDHX technology is helping bridge the gap between traditional air cooling and emerging liquid-based systems.
With 83% of new data centers planning to deploy RDHXs, the momentum is clear. The integration of 5 mm copper tube technology represents a significant step forward, offering higher capacity, reduced material usage, and improved energy efficiency across a wide range of operating conditions.
As data centers evolve to meet the dual challenges of performance and sustainability, rear-door heat exchangers are proving that innovation in copper-based design can deliver the flexibility, resilience, and efficiency that the next generation of digital infrastructure demands.
References:
Based on the presentation “Optimizing Rear Door Heat Exchangers (RDHXs) for an Evolving Data Center Cooling Landscape,” by Dennis Nasuta and Shankhinee Deshpande, OTS R&D, in collaboration with the Copper Development Association, October 2025.