RDHX
An RDHX is a chilled-water heat exchanger mounted on the rear door of a server rack that rejects rack heat to a water loop instead of room air.
Definition
A rear-door heat exchanger (RDHX) is a passive or fan-assisted chilled-water heat exchanger mounted on the back of a server rack. As hot exhaust air leaves the rack, it passes through the heat exchanger's coils and is cooled before re-entering the room. The cooling load moves from the room's air-handling system to a chilled-water loop.
RDHX is a primary tool for converting a legacy 5–15 kW per rack air-cooled facility into a heavy-density 30–60 kW per rack environment. It is a less invasive option than full immersion or direct-to-chip liquid cooling: the IT remains air-cooled, only the heat removal pathway changes. Most modern rack and chassis form factors support an RDHX without modification.
RDHX engineering work touches the chilled-water plant, branch piping, leak detection, structural floor loading, hot-aisle containment integrity, and the controls that match cooling capacity to live load. Done well, RDHX is the single highest-leverage retrofit pathway for facilities trying to support AI workloads without rebuilding the hall. Done poorly, it produces leak risk, balancing problems, and capacity assumptions that don't survive a hot day.
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