Modern data centers are no longer simple server rooms. They are complex energy systems integrating cooling, power management, and waste heat recovery. As shown in the system diagram, heat exchangers form the core thermal interface between IT equipment and external energy infrastructure.
Rather than acting as isolated components, heat exchangers operate as thermal bridges, connecting internal liquid cooling loops with facility water systems, free cooling units, and heat reuse networks.
1) Cooling System — BPHE (Brazed Plate Heat Exchanger) Inside CDU
In the cooling section, heat exchangers are located inside the Coolant Distribution Unit (CDU). The BPHE separates two independent circuits:
- IT coolant loop (directly connected to servers)
- Facility water loop (chilled water or cooling water)
The BPHE transfers heat from server liquid to building water without mixing fluids.
Typical parameters
- IT loop inlet: 30–45°C
- IT loop outlet: 20–30°C
- Cooling capacity per BPHE: 200–1500 kW
- Design pressure: 10–25 bar
- Approach temperature: 2–4 K
Here, the heat exchanger ensures stable server operation, electrical isolation, and high heat transfer efficiency in a compact space.
2) Free Cooling — Heat Exchanger as Environmental Interface
In the free cooling section, heat exchangers connect internal cooling loops to dry coolers, cooling towers, or ambient water sources. Instead of using chillers, the system uses outside air or water directly.
Typical operation
- Cold water temperature: 12–18°C
- Free cooling window: 40–70% of annual operating hours
- Energy savings: 15–30%
In this scenario, the heat exchanger becomes the boundary between the data center and the natural environment.
3) Waste Heat Recovery — Heat Exchanger as an Energy Export Device
The lower part of the diagram shows how heat exchangers enable waste heat reuse. Recovered warm water is transferred to office buildings, district heating systems, or domestic hot water networks.
Typical heat recovery parameters
- Warm water temperature: 30–55°C
- Annual reuse time: 6000–8000 hours
- CO₂ reduction: up to 40%
In this role, the heat exchanger transforms the data center from a pure energy consumer into an energy supplier.
Engineering Conclusion
From cooling to free cooling and heat recovery, heat exchangers are no longer auxiliary devices. They act as:
- Thermal interfaces
- Energy transfer hubs
- System integration components
In next-generation data centers, heat exchangers are not just part of the cooling system — they are core infrastructure for digital energy management.
FAQ — Heat Exchangers in Data Centers
1) What type of heat exchanger is typically used inside a CDU?
Inside a Coolant Distribution Unit (CDU), the most commonly used type is a Brazed Plate Heat Exchanger (BPHE).
Reasons:
- Compact size for rack-level or room-level integration
- High heat transfer coefficient
- Low approach temperature (2–4 K)
- High pressure capability (10–25 bar typical)
- No gasket leakage risk
For higher capacities (above ~1.5 MW), gasketed plate heat exchangers (GPHE) or plate-and-shell heat exchangers (PSHE) may be used.
2) When should a gasketed plate heat exchanger be used?
A Gasketed Plate Heat Exchanger (GPHE) is typically used when:
- Cooling capacity exceeds 1–2 MW
- Maintenance accessibility is required
- Water quality may require mechanical cleaning
- Long-term serviceability is critical
GPHE units allow plate removal and cleaning, making them suitable for large central plant installations.
3) Are shell-and-tube heat exchangers used in data centers?
Yes, but less frequently. They are used when:
- Extremely high reliability is required
- Water quality is poor
- Fouling risk is high
- Very large flow rates exist
However, they typically have a larger footprint, lower efficiency vs plates, and higher approach temperatures.
For modern high-density data centers, plate heat exchangers are generally preferred.
4) What type is best for free cooling systems?
Common options:
- GPHE — most common for building-side integration
- BPHE — for modular or smaller systems
- Welded plate — for higher pressure or glycol concentration
Selection depends on water chemistry, maintenance strategy, approach temperature, and capacity.
5) What type is used for waste heat recovery?
- GPHE / PSHE for district heating integration
- Plate-and-shell for higher pressure networks
- Double-wall exchangers when separation safety is required
When exporting heat to district heating systems, design pressures can exceed 16–25 bar, requiring reinforced plates or welded structures.
6) Why are plate heat exchangers preferred?
- Very high heat transfer efficiency
- Low approach temperature (critical for liquid cooling)
- Compact footprint
- Modular scalability
- Lower refrigerant/coolant charge
- Easier integration with energy recovery systems
7) What materials are typically used?
Plates: 316L (most common), 304 (low corrosion), Titanium (special water)
Brazing: Copper (standard), Nickel (aggressive fluids)
Gaskets (for GPHE): EPDM (water), NBR (oil), FKM (high temperature)
8) How do heat exchangers impact PUE?
Efficient heat exchanger design reduces chiller load, improves free cooling window, enables higher supply temperatures, and supports waste heat reuse — directly lowering cooling energy consumption and improving PUE.
In liquid-cooled data centers, the heat exchanger is one of the primary determinants of thermal efficiency.
9) What design factors are most critical?
- Cooling capacity (kW or MW)
- Approach temperature
- Pressure drop limitations
- Water chemistry
- Maintenance access
- Redundancy requirements
- Future scalability
Proper sizing is critical because undersizing increases approach temperature, while oversizing increases capital cost.
10) Are double-wall heat exchangers required?
Double-wall heat exchangers may be required when:
- There is risk of cross-contamination
- Fluids must remain absolutely isolated
- Compliance standards demand leak detection
They provide enhanced safety but increase cost and footprint.
Pillow Plate Heat Exchangers in Data Centers
Are pillow plate heat exchangers used in data centers? Yes — but not typically inside CDUs.
Pillow plate heat exchangers are more commonly used in:
- Rear door heat exchangers (RDHx)
- Immersion cooling tank walls
- Waste heat recovery modules
- Structural cooling panels
Where Pillow Plates Are Applied
Typical placement
- Rear-door cooling panels: large surface area, uniform distribution, low pressure drop
- Immersion tanks: large welded panels integrated into tank walls or internal cooling surfaces
- Heat recovery skids: warm water transfer modules with large flat heat transfer surfaces
Why Pillow Plates Are Not Common in CDUs
| Factor | BPHE | Pillow Plate |
|---|---|---|
| Compactness | Very High | Moderate |
| Heat Transfer Coefficient | Very High | Lower |
| Approach Temperature | 2–4 K | Higher |
| Size per kW | Small | Larger |
| Pressure Capability | High | High |
When Pillow Plate Is the Right Choice
- Large flat surfaces are available
- Structural integration is required
- Moderate heat flux density is acceptable
- Fully welded construction is preferred
- Fouling tolerance is needed
Engineering perspective: In modern high-density AI data centers, BPHE/GPHE dominate CDU applications; pillow plates support rack-level and immersion cooling structures; welded plate or plate-and-shell designs are used for district heating export.
