Engineering the Next Era of Data Center Cooling

Two-Phase CDUs Are Here—The Backbone of High-Performance Data Center Cooling— Engineered for What’s Next

As data centers evolve to support extreme-density compute loads driven by AI, ML, and advanced HPC clusters, traditional cooling strategies are straining both in thermal limits and cost-efficiency. Rack power densities for AI workloads are already exceeding 100kW per rack with projections of 500kW+ racks within the next two years—making legacy air cooling and even conventional single-phase liquid systems increasingly impractical. The need for more efficient, scalable, and sustainable thermal solutions is now mission critical.

Two-phase liquid cooling architectures—particularly those employing two-phase direct-to-chip Coolant Distribution Units (CDUs)—offer a technically validated path forward. This article outlines the operational mechanics, integration strategies, and performance advantages of two-phase CDUs, while highlighting how ACT is helping data centers engineer resilient thermal infrastructure for the next generation of computing.

Why Liquid Cooling Is No Longer Optional

Air cooling remains viable for rack loads under ~20–25 kW, but its thermal conductivity and specific heat limitations make it unsuitable for modern high-performance environments. Liquid cooling—both single- and two-phase—offers a 1,000–3,000× improvement in thermal conductivity and significantly higher volumetric heat capacity.

However, single-phase systems typically require high coolant flow rates (often >1500 LPM per rack), larger infrastructure, and active circulation. Two-phase systems overcome many of these constraints through latent heat transfer, absorbing significant heat during vaporization and enabling greater efficiency with lower mechanical complexity.

Understanding Coolant Distribution Units (CDUs)

In any liquid cooling architecture, the CDU is the thermal hub between the IT-side cooling loop and the facility’s heat rejection system. It regulates coolant flow, temperature, and pressure to ensure the reliable operation of high-power compute devices.

All CDUs perform several core functions:

Temperature and Pressure Regulation: Maintains stable coolant conditions for consistent performance and safe operation.
Flow Rate Control: Adjusts coolant flow to match dynamic thermal loads and ensures uniform distribution.
Loop Isolation: Provides hydraulic separation between facility water and IT-side fluid circuits to simplify service and prevent contamination.
Monitoring and Diagnostics: Enables real-time tracking of pressure, flow, temperature, and fluid levels via integrated sensors and PLCs. Interfaces such as Modbus, SNMP, and BACnet support seamless integration with data center and building management systems.

How Two-Phase CDUs Raise the Bar

Two-phase CDUs enhance these functions by leveraging phase change dynamics:

Precise Regulation of Saturation Pressure and Temperature: Ensures fluid enters the cold plate in the correct thermodynamic state for optimal vaporization and heat transfer.
Latent Heat Transfer at Lower Flow Rates: Achieves high thermal efficiency with coolant flows as low as 0.3 LPM/kW.
Dielectric Fluid Safety: Enables direct contact with electronics while reducing electrical risk in the event of a leak.

What Makes a CDU Two-Phase?

Unlike single-phase systems that rely on sensible heat, two-phase CDUs circulate dielectric refrigerants that absorb heat via phase change—from liquid to vapor—near the chip surface. This dramatically increases heat transport capability and enables system-level benefits:

Lower coolant flow rates
Reduced pump energy requirements
Enhanced heat transfer and thermal uniformity
Improved safety due to non-conductive fluids

Once vaporized, the refrigerant returns to the CDU, where an integrated condenser rejects the heat to the facility loop. An accumulator manages fluid inventory and system pressure, while N+1 pumps ensure continuous circulation.

A Closer Look: ACT’s High-Performance Two-Phase Direct-to-Chip CDU Platform

ACT’s two-phase CDUs are built on decades of thermal innovation and incorporate best-in-class components:

High-Efficiency Condenser: Brazed-plate or microchannel heat exchangers designed for high heat rejection in compact footprints.
Accumulator: Accommodates fluid expansion and regulates system pressure to maintain vapor-liquid balance to stabilize performance under fluctuating loads.
Redundant Pumping System (N+1): Enables uninterrupted cooling with automatic switchover to backup pumps in case of fault or maintenance.
Integrated Sensors & Controls: Delivers real-time system visibility, control, and remote integration via industry-standard protocols.

Integration Strategies: Deploying at Scale

ACT’s CDUs are available in flexible form factors for a range of thermal architectures:

In-Rack: Mounted within the server cabinet to support direct-to-chip cooling with minimal plumbing for a single rack.
In-Row / End-Row: Centralized cooling between server racks for multi-cabinet deployments.

Selection depends on:

Rack-level thermal density
Facility cooling infrastructure
Redundancy and serviceability needs

ACT’s engineering team provides thermal modeling and consultative integration design to ensure optimal deployment.

Performance Comparison: Two-Phase vs. Single-Phase Cooling

5 Technical Benefits of ACT’s Two-Phase CDU Platform

High Heat Flux Management: Cools localized chip heat fluxes >500 W/cm². Current CDU systems support up to 200 kW; 1 MW systems are in development.
Isothermal Operation: Maintains near-constant temperatures (~±1°C), reducing thermal cycling and extending component lifespan.
Redundant, Safe Operation: N+1 pump redundancy and dielectric fluids ensure uptime and electrical safety. Leak detection and fail-safe features enhance reliability.
Reduced Infrastructure: Load Lower flow rates mean smaller pipes, pumps, and less pressure drop—reducing erosion concerns and minimizing CapEx and installation time.
Lower Energy and Water Use: High thermal and electrical efficiency can reduce or eliminate reliance on water-cooled chillers, supporting sustainability targets.

Leading the Frontier: ACT’s Commitment to Emerging Technology

Two-phase liquid cooling is emerging as the definitive solution for an ultra-dense, high-performance compute. ACT is driving this transition with proven engineering and thermal management expertise.

Cross-Industry Experience: Our two-phase systems power aerospace, defense, and energy platforms—domains where failure is not an option.
Lab-Validated Designs: All CDUs undergo stress testing under simulated AI/HPC workloads to ensure stability and longevity.
Support for Early Adopters: From thermal modeling to field deployment, ACT provides consultative support to reduce risk and maximize ROI.

Conclusion: Engineering for Thermal Resilience

Two-phase CDUs represent a breakthrough in data center cooling—delivering high-efficiency heat transfer, dielectric safety, and scalable thermal performance. As computing density rises and sustainability targets grow more urgent, they offer a resilient, future-ready foundation for next-generation IT infrastructure.
ACT is more than a provider—we’re your engineering partner. With over 10 years of delivering pumped two-phase systems across diverse industries, we bring unmatched expertise and a product roadmap purpose-built for the AI and HPC era. Let’s design the thermal systems of the future—together.

Ready to future-proof your data center? Let ACT’s engineering team help you evaluate your cooling requirements and explore the benefits of two-phase liquid cooling.