In March 2026, a major aerospace company contacted data center cooling systems provider OptiCool requesting a factory witness test and product evaluation of OptiCool’s new 120 kW rear-door heat exchanger (RDHx). With power levels at the rack pushing 80 kW, the company needed to increase cooling capacity without a major overhaul of its cooling system and the resulting service disruptions. Company engineers visited the OptiCool lab to evaluate the recently introduced 120 kW RDHx. They were surprised at what they saw.
The test simulated an 80 kW heat load and a system ambient temperature set point of 75°F. Under the simulated workloads, the system maintained an exit temperature of 70°F four feet from the rack while only consuming 550 W (steady-state power draw). This meant that the new RDHx solution not only extracted the 80 kW heat load from the racks but also provided additional cooling capacity to offset heat loads elsewhere in the room. In the customer’s live environment with 20 of these racks, the OptiCool solution would exponentially add supplemental cooling, enabling the customer to continue adding other network equipment while decreasing the workload of their traditional air cooling equipment.
The expanded cooling capability, a significant advancement over conventional airflow-based cooling approaches, was enabled by a breakthrough in the core technology behind the 120 kW RDHx platform known as Delta4T™.
AI Workloads Lead to Cooling Breakthrough
Prior to the development of the Delta4T technology, OptiCool’s success was based on its third-generation two-phase refrigerant cooling process that leverages the expanded thermal capacity created when liquid refrigerant is briefly converted to gas. The phase change eliminates the need to move water faster or over a larger surface area while achieving the same thermal capacity. This approach reduces power usage by up to 90% compared to traditional airflow management, while increasing the capacity compared to a single-phase rear-door heat exchanger.
The fundamentals of two-phase refrigerant cooling have existed since the 1960s, when early computing systems operated at thermal loads that were modest by today’s standards. Over time, improvements in refrigerants, controls, and heat exchanger design steadily increased practical cooling capacity. Building on these foundational principles, OptiCool engineers refined several mechanical and thermodynamic aspects of the traditional two-phase process, ultimately increasing rear-door cooling capacity to 30 kW prior to the development of the Delta4T platform.
Within AI data centers, rack densities are rapidly increasing, pushing thermal loads to record levels; a fully configured NVIDIA DGX H100 stack can instantly demand up to 132 kW of thermal dissipation per rack. With traditional airflow management systems topping out at 40 kW, operators are left with two options: direct-to-chip technology or air-assisted liquid cooling. For those data centers that can’t justify the high deployment costs of direct-to-chip, the only viable solution is one that delivers two-phase liquid cooling via RDHx. Currently, OptiCool is the only manufacturer that offers this solution.
As early as 2023, OptiCool engineers realized that the company’s third-generation technology provided a solid framework that, with incremental but critical modifications, could substantially improve cooling performance. On-going discussions with customers, system integrators and other stakeholders provided important insights that helped inform the company’s efforts.
The core technical breakthrough was a modification of the internal mechanics of the system that enabled a significant increase in PUE performance. The re-engineering process began with approximately nine months of in-house simulations using proprietary software. Following simulations, approximately six iterations of mechanical configurations were designed, developed, and lab-tested to identify the optimal balance of cooling performance, operational efficiency, size, and scalability. Key to the breakthrough was the team’s 14 years of experience in understanding the limiting factors in heat extraction.
The Debut of Delta4T Fourth-Generation Technology
The result of the re-engineering process was OptiCool’s fourth generation two-phase cooling technology, Delta4T, which enabled the company to double cooling capacity—from 30 kW to 60 kW. Delta4T debuted in October 2024 as the enabling technology behind the new 60 kW RDHx.
This innovative approach provides roughly 2x the capacity of single-phase rear-door heat exchangers in the same footprint. Its hyper-efficient operation enables a Power Usage Effectiveness (PUE) as low as 1.02, making it comparable to much more expensive direct-to-chip cooling technologies. Less than a year after introducing the 60 kW RDHx, OptiCool revealed the 120 kW RDHx in September 2025, effectively quadrupling the cooling capacity of two-phase refrigerant technology.
It is important to note that the RDHx doors are just one component in a highly adaptable modular solution. Other components include a closed-loop, low-pressure refrigerant distribution networkthat circulates dielectric refrigerant to the racks, and low-energy pumps that can be positioned in the white or gray space. The pumps can connect to a facility’s chilled water system or be paired with outdoor direct expansion (DX) units, making it suitable for water-constrained regions. By mixing and matching components and capabilities, the modular architecture easily scales from single hotspots to hyperscale environments.
Market Momentum and the Future
Beginning with the 60 kW RDHx, market acceptance of the new technology was slow at first, as data center operators required OptiCool to demonstrate its performance. T-Mobile and Comcast were the first to do so and the first to deploy the Delta4T technology. After that, the 60kW door began gaining industry attention, becoming the company’s most frequently quoted solution in 2025. So far in 2026, the 120 kW RDHx has represented about half the company’s quoted projects.
More importantly perhaps is how the Delta4T technology is changing the perceived role of data center cooling systems. For example, when a 120 kW door is deployed for loads below its max capacity (e.g., 70 kW to 90 kW), it adds cooling back into the data center space. When multiple cabinet doors are deployed in the containment area, they effectively operate as a distributed air conditioning system, creating redundancy or lowering capacity demands on other cooling systems.
Looking ahead, OptiCool leadership envisions hybrid deployments, as heat loads approach 1 mW per rack. They are also working on future designs that will continue to push the limits of what is currently available from non-invasive cooling solutions going well above the 120-kW range.
For more information on the OptiCool Delta4T technology and the two-phase refrigerant cooling systems it enables, visit the OptiCool Two-Phase Liquid Cooling Product Page.