Rear-door heat exchanger
A liquid-cooled coil mounted on the back of a cabinet that captures exhaust heat before it enters the room.
Also called: RDHx, Rear door cooler
In practice
A rear-door heat exchanger is the gentlest step into liquid cooling. The cabinet keeps its normal air-cooled equipment and airflow direction; a coil on the back of it removes heat from the exhaust before that air reaches the hot aisle. From the room’s point of view, the cabinet contributes less heat than its power draw would suggest.
Passive versions rely on the equipment’s own fans to push air through the coil, which means the coil’s resistance has to be within what those fans can overcome. Active versions add their own fans, which handles higher resistance and higher densities but introduces another powered component in the cooling path — one that now needs its own supply and its own failure consideration.
The appeal in a retrofit is that it raises the density a room can support without redesigning the air distribution, and it can be applied cabinet by cabinet rather than hall by hall. The requirement it introduces is liquid at the rack: supply and return, connections, leak detection and a maintenance approach for working on a cabinet that now has fluid attached to it.
It does not remove the air path. Heat is still rejected into air first and captured second, so the room still has to handle whatever the coil does not, and the cabinet still depends on its own fans. Designs that state a capture fraction and plan explicitly for the remainder are the ones that behave as expected.
Scope of this definition
This is an educational summary of how the term is used in data center practice. It is not a standard, a specification or engineering advice, and where a real decision depends on it, the current adopted standards, verified site information and qualified professional review are the correct sources.