Rear-door heat exchanger
Beginner
A radiator on the back door of a rack. Hot air blows through it, and cool water inside carries the heat away.
Novice
An air-to-water coil mounted on the back of a rack. The servers still use fans and air, but the air is cooled right as it leaves the rack instead of mixing into the room. Passive doors rely on server fans; active doors add their own fans.
Expert
Removes heat close to the source with ordinary IT hardware, so it suits mixed or retrofit sites. Capacity depends on water temperature, flow and server airflow; one vendor’s published sizing curves span 20–40 kW per rack, and supply water must stay above the room dew point.
Explained in Power and cooling (Systems).
See also: Direct-to-chip liquid cooling (cold plates).