Rack assemblies and methods for cooling one or more rack-based computer systems, as well as data center configurations that utilize the rack assembly. The rack assembly comprises a rack providing support for multiple columns of heat-generating electronic devices and device fans for moving air from an air inlet side of the rack through the devices and through an air outlet side of the rack. The rack assembly also comprises a unitary door having a support frame spanning the air outlet side of the rack and hingedly coupling the door to a rear vertical edge of the rack. The door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet must pass through the heat exchanger panel. The air outlet passage has a cross-sectional area that is substantially equal to or greater than the cross-sectional area of the multiple columns of heat-generating devices.