Rear door heat exchangers (RDHx)

Rear door heat exchangers (RDHx) bolt to the back of a standard rack, catch the exhaust air with a liquid-cooled coil, and return neutral-temperature air to the room. For 40–70 kW racks that don't (yet) justify direct-to-chip, they are the cheapest path to liquid.

What a rear door heat exchanger is

Mechanically, an RDHx is a finned-tube coil with a chilled-water inlet and a warmer outlet, mounted on a hinged frame that replaces the standard rear door. The rack fans push server exhaust through the coil; the coil dumps heat into the facility loop.

Passive vs active

• Passive RDHx. Uses only the server fans to move air. Simple, silent, and good to ~35 kW.
• Active RDHx. Adds EC fans that respond to coil ΔT. Handles up to ~70 kW per rack and evens out hot spots.

Sizing rules that hold up in the field

1. Flow: 0.03–0.05 L/s per kW of rack load at 10 K ΔT.
2. Supply temp: 18–24 °C for passive, up to 30 °C for active variants.
3. Coil pressure drop: keep total loop head under CDU pump curve at 110% design flow.
4. Access: leave 900 mm of clear aisle behind — the doors swing.

Combining RDHx with direct-to-chip

In GB200-class rooms we routinely deploy DLC + RDHx together: cold plates catch the GPU/CPU heat (70–85%) and the RDHx catches the residual airborne 15–30% (PSUs, NVSwitch trays, top-of-rack switches). Result: near-zero air-cooling load on the CRAHs, which can be dialled back to humidity-management duty.

FAQ

Do RDHx units introduce a single point of failure?

Yes — a stuck valve or coil leak takes a row out. Standard mitigation: dual supply/return with isolation valves per door and a documented rollback to emergency airflow.

How does RDHx compare to in-row cooling?

RDHx captures heat closer to the source, so it beats in-row on both efficiency and containment integrity. In-row wins where door swing clearance is impossible.