Two approaches to managing heat in high-density environments.
As rack densities increase, traditional cooling methods need to evolve. CDUs and rear door heat exchangers (RDHx) both enable higher performance – but serve different roles in the data center.
CDUs (Coolant Distribution Units)
How They Work
CDUs circulate liquid coolant to remove heat from IT equipment – typically through direct-to-chip cooling systems.
Advantages
- Supports very high rack densities (100 kW+)
- Enables direct-to-chip cooling
- Scalable from rack to hyperscale deployments
- Integrates with facility water in a closed loop, reducing water usage
Considerations
- Requires liquid cooling infrastructure
- Higher upfront planning and integration
Rear Door Heat Exchangers (RDHx)
How They Work
RDHx units are mounted on the back of racks and remove heat from exhaust air using water-cooled coils.
Advantages
- Extends air cooling capabilities
- Easier retrofit into existing environments
- Minimal changes to server hardware
Considerations
- Limited density compared to liquid cooling
- Still relies on airflow
- Less efficient at extreme loads
- Requires facility water loop (often chilled) for heat rejection
Key Differences
| Factor | CDU-Based Cooling | Rear Door Heat Exchanger |
|---|---|---|
| Cooling method | Liquid (direct) | Air-assisted |
| Density support | Very high | Moderate |
| Efficiency | High | Moderate |
| Scalability | High (MW+) | Limited |
| Al readiness | Strong | Transitional |
When to Choose Each
Choose CDU-based cooling if:
- You’re deploying AI or high-density workloads
- You need long-term scalability
- You’re moving toward liquid-cooled infrastructure
Choose RDHx if:
- You’re extending existing air-cooled environments
- You need a transitional solution
- Rack densities are moderate
Key Takeaway
Rear door heat exchangers extend air cooling – but CDUs enable the transition to true high-density liquid-cooled infrastructure.
