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Cold-Plate and Immersion CDU Architectures in Data Centers

Thermal-Fluid Design Considerations and Operational Requirements for Modern Data Center CDUs

 

As AI and high-performance computing workloads drive unprecedented power densities, liquid cooling in data centers has transitioned from a niche solution to a mainstream infrastructure strategy. At the center of every liquid cooling deployment is the Coolant Distribution Unit (CDU) — the core system responsible for transferring heat from IT equipment to the facility heat-rejection loop.

In modern data center CDU architectures, two dominant approaches have emerged:

• Cold-Plate (Direct-to-Chip, D2C) CDUs
• Immersion Cooling CDUs

While both are classified under data center liquid cooling, their thermal-fluid design logic, operational constraints, and heat exchanger requirements differ significantly.

This article examines the engineering boundary conditions, system architecture differences, and plate heat exchanger design considerations in both Cold-Plate and Immersion CDUs for data centers.

1. What Is a Data Center CDU?

A CDU in a data center establishes and maintains a controlled liquid-side heat transport path. Its primary function is to:

• Transfer thermal energy from the Technology Cooling System (TCS)
• Deliver that heat to the Facility Water System (FWS)
• Maintain hydraulic and chemical isolation between loops

Regardless of cooling architecture, a typical data center CDU consists of four core functional blocks.

1.1 Liquid-to-Liquid Heat Transfer and Loop Isolation

At the heart of every CDU is a liquid-to-liquid heat exchanger, often a plate heat exchanger. It must:

• Transfer thermal duty reliably under load fluctuations
• Maintain chemical and pressure separation between TCS and FWS
• Sustain long-term performance without cross-contamination

1.2 Hydraulic Conveyance and Flow Distribution

The CDU pump system ensures:

• Required flow rate delivery
• Sufficient differential head
• Stable flow distribution across parallel server branches

Hydraulic stability is critical in high-density data center environments.

1.3 Fluid Condition and Cleanliness Management

Long-term CDU reliability in data centers depends on:

• Filtration control
• Make-up fluid management
• Degassing or venting
• Corrosion prevention
• Online monitoring

Fluid degradation directly impacts thermal performance and pressure drop.

1.4 Instrumentation, Controls, and Protective Interlocks

Modern data center CDUs integrate closed-loop control using:

• Temperature sensors
• Flow meters
• Pressure and differential pressure sensors
• (In immersion systems) level sensors

Redundancy, bypass strategies, and fault isolation logic are designed to meet uptime targets such as N+1 or 2N configurations.

2. Core Engineering Drivers in Data Center CDU Design

Although Cold-Plate and Immersion CDUs share structural similarities, their system behavior is governed by two fundamental variables:

1. Scale of the cooled object
   • Chip-level hotspot (Cold Plate)
   • Whole-server or tank-level environment (Immersion)
2. TCS working fluid
   • Water-based coolant
   • Dielectric fluid

These two factors define:

• Allowable pressure drop
• Heat exchanger selection
• Filtration strategy
• Control bandwidth
• Maintenance requirements

3. Cold-Plate (Direct-to-Chip) CDU in Data Centers

3.1 Thermal-Fluid Characteristics

A Cold-Plate CDU removes heat directly at the chip level using forced convection. The TCS side typically employs:

• Deionized water with corrosion inhibitors
• Water-glycol mixtures (ethylene or propylene glycol)

A typical Cold-Plate CDU loop in a data center:

CDU pump → Rack manifold → Cold plates → Return header → Plate heat exchanger → Recirculation

Primary design objectives include:

• Tight supply temperature control
• Stable branch flow distribution
• Controlled temperature rise across devices

3.2 Key Engineering Constraints for Cold-Plate CDUs

1. Materials Compatibility and Water Chemistry

Compatibility must be verified for:

• Copper, aluminum, stainless steel
• Brazing alloys
• Elastomers
• Additive packages

Water quality parameters such as conductivity, pH, and inhibitor concentration must be defined as operational standards.

2. Cleanliness and Clogging Sensitivity

Cold plates and brazed plate heat exchangers feature narrow passages.

Therefore:

• Filtration rating is a primary design parameter
• Bypass and flush provisions must be integrated
• Differential pressure monitoring is critical

In Cold-Plate data center CDUs, contamination is a system-level risk — not just a maintenance issue.

3. Control and Dynamic Requirements

Critical control variables include:

• Supply temperature stability
• Total loop flow
• Branch differential pressure

Lower heat exchanger approach temperatures improve economizer operation and free-cooling hours, but often increase pressure drop. This creates a system-level tradeoff between:

• Pumping energy consumption
• Temperature control margin

4. Immersion CDU Architecture in Data Centers

Immersion cooling in data centers transfers heat from IT components directly into a dielectric fluid.

Thermal duty is then transferred to facility water via heat exchange.

4.1 Immersion CDU Topologies

Two common data center immersion CDU configurations are:

1. In-Tank Heat Exchanger

• Facility water flows through an in-tank heat exchanger
• Dielectric fluid circulates via natural convection or guided flow

1. External Dielectric Loop

• Dielectric fluid is pumped out of the tank
• Heat exchange occurs externally
• CDU integrates pumps, filtration, and controls

Two-phase immersion systems add:

• Boiling and condensation recovery
• Non-condensable gas management
• Enhanced sealing and level control requirements

4.2 Engineering Constraints in Immersion CDUs

1. Material Compatibility with Dielectric Fluids

Dielectric fluids can cause:

• Elastomer swelling
• Plastic extraction
• Long-term aging

Compatibility testing and defined replacement strategies are mandatory.

2. Fluid Property Control

Long-term performance depends on monitoring:

• Viscosity
• Moisture content
• Dissolved contaminants
• Particulate load

These properties directly influence heat transfer efficiency and hydraulic resistance.

3. Operational and Safety Considerations

Immersion CDUs require:

• Level control systems
• Make-up and recovery processes
• Spill and leak handling procedures
• Fire safety compliance

Two-phase systems additionally require careful condensation recovery and non-condensable gas management.

5. Plate Heat Exchangers in Data Center CDUs

Plate heat exchangers are widely used in data center liquid cooling CDUs due to:

• High heat transfer coefficients
• Compact footprint
• Scalability

However, design constraints differ between Cold-Plate and Immersion applications.

5.1 Plate Heat Exchangers in Cold-Plate CDUs

In most data center Cold-Plate CDUs, brazed plate heat exchangers (BPHE) serve as the primary TCS–FWS interface.

Key performance indicators:

• Approach temperature
• Total thermal duty
• Pressure-drop

Design constraints include:

• Sensitivity to particulate fouling
• Tradeoff between low approach temperature and increased ΔP
• Material compatibility with water chemistry

5.2 Plate Heat Exchangers in Immersion CDUs

In immersion data center systems, plate heat exchangers may be used for:

• Dielectric-to-water heat transfer
• Intermediate stages in multi-loop systems

Engineering focus shifts toward:

• Viscosity compatibility
• Fouling risk management
• Cleaning accessibility
• Long-term fluid contamination control

In two-phase systems, exchanger placement must also accommodate:

• Condensate return paths
• Gas holdup management

6. Design Inputs for Data Center CDU Engineering

Cold-Plate CDU – Key Design Inputs

1. TCS supply temperature and allowable fluctuation band
2. Total flow rate and pressure drop budget
3. Filtration rating and water quality specification
4. Heat exchanger approach temperature target
5. Redundancy level (N+1 / 2N)
6. Quick disconnect and leak detection standards

Immersion CDU – Key Design Inputs

1. Single-phase or two-phase architecture
2. Dielectric fluid selection and property envelope
3. Tank thermal load and circulation method
4. Heat transfer topology (in-tank vs external)
5. Fluid purification and lifecycle management strategy
6. Spill response and fire safety compliance requirements

Conclusion: Choosing the Right Data Center CDU Architecture

Cold-Plate and Immersion CDUs in data centers are not competing technologies — they are responses to different thermal and operational boundary conditions.

• Cold-Plate CDUs prioritize temperature precision and hydraulic control.
• Immersion CDUs prioritize dielectric fluid management and long-term stability.

For both architectures, the CDU and its heat exchanger design must be engineered within clearly defined fluid, pressure, and reliability envelopes.

As data center power density continues to increase, understanding the thermal-fluid engineering logic behind CDU architecture selection becomes essential for system designers, operators, and equipment manufacturers alike.