# Rear Door Heat Exchangers A rear door heat exchanger (RDHx) replaces the standard rear door of a server rack with a door containing a water-cooled coil. Hot exhaust air from the servers passes through the coil before exiting, rejecting heat to a water loop instead of into the room. **Why it matters at high density:** At 30-40kW per rack, traditional air cooling struggles. You'd need massive airflow volumes and oversized CRAH units. An RDHx intercepts heat at the source — right at the rack exhaust — before it becomes a room-level problem. **How Delta uses it:** The AI CDC container uses rear door heat exchangers as the primary cooling mechanism for its 30-40kW racks. The water loop connects to an external heat rejection system (dry cooler or cooling tower). This keeps the container self-contained — no dependency on the MDC's DX cooling system. **Trade-offs:** - Adds water to the data center environment (leak risk, maintenance) - Requires plumbing infrastructure that DX-cooled MDCs don't have - But avoids the full cost and complexity of chilled water plant infrastructure - Works as a middle ground between pure DX and full chilled water **Key point:** RDHx is a rack-level intervention, not a room-level one. It lets you run high-density racks in an enclosure that would otherwise top out at 20kW/rack with air cooling alone. --- See also: [[Delta AI CDC Container]] | [[DX vs Chilled Water Crossover]] | [[Modular Data Centers MoC]]