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L2L CDU (Liquid-to-Liquid CDU) Heat from IT secondary coolant transfers via plate heat exchanger to the facility primary chilled water loop (chiller / cooling tower / dry cooler). Two independent liquid loops.
L2A CDU (Liquid-to-Air CDU) Built-in finned-tube coil + EC fans. IT secondary coolant rejects heat directly to data center room air. No facility chilled water loop required.
Item | L2L CDU | L2A CDU |
|---|---|---|
Heat rejection method | Plate heat exchanger → facility water loop | Internal air coil + fans → room air |
Required infrastructure | Need facility chilled water / cooling tower piping | No facility water; rely on existing CRAC/CRAH |
Max cooling capacity | High, single unit up to MW level | Medium, typical 30–200kW |
Rack density support | 80–250kW/rack, for high-density GPU / 3.2T optical modules | 30–120kW/rack, density limited by room air handling |
PUE performance | Better, can use free cooling, lower overall energy consumption | Worse, extra fan power + room AC load increases PUE |
Internal main heat exchanger | Plate heat exchanger | Tube-fin air coil |
Deployment scenario | New build AI data center, large cluster, long-term expansion | Retrofit old air-cooled room, edge, pilot project, colo |
Installation & civil work | Complex, site water pipe construction needed | Simple, plug & play, minimal civil modification |
Floor footprint | Compact for large heat load | Larger footprint due to air coil and fans |
Noise | Lower (no large air fans on CDU) | Higher, from CDU built-in EC fans |
Scalability | Excellent, easy to expand to thousands of racks | Limited, room air capacity becomes bottleneck |
Compatibility with RDHX / cold plate | Fully compatible with RDHX, GPU cold plate, optical module cold plate | Also compatible, but total heat cannot exceed room AC capacity |
Hot glycol coolant returning from RDHX / cold plates → enter plate heat exchanger → transfer heat to facility chilled water → secondary coolant cooled down, pumped back to racks. Facility water carries heat outdoors to cooling tower.
Hot coolant from racks flows into internal fin coil. CDU built-in fans blow room air over the coil to cool the liquid. Heat is dumped into the data center room air, then removed by existing CRAC units.
✅ Pros
High cooling capacity & excellent efficiency, suitable for ultra-high density AI racks
Stable temperature control, low noise, great for large-scale permanent deployment
Supports free cooling, achieves low PUE
No heat load dumped into room air, no room thermal bottleneck
❌ Cons
Requires facility chilled water piping, higher upfront infrastructure investment
Longer construction period
✅ Pros
No facility water needed, fast deployment for legacy air room retrofit
Low initial investment, ideal for liquid cooling PoC / pilot
Independent unit, easy to move or reconfigure
❌ Cons
Heat is released into room air, constrained by existing CRAC cooling capacity
Built-in fans consume extra power, higher PUE
Capacity ceiling; not suitable for massive 200kW+ rack clusters
Choose L2L CDU: New AI compute hall, rack power >80kW, large-scale permanent deployment, pursue low PUE.
Choose L2A CDU: Retrofit existing air-cooled data center, edge site, small pilot, cannot build facility water piping.
CDU Specifications And Selection (board Replacement, Circulation Pump, Redundant Design)
What are the specific functions of the CDU in the liquid cooling system?
How does the CDU interact with other components in the liquid cooling system?
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