1. A data center mounting rack for mounting and supporting a plurality of computer enclosures and port expansion chassises in a stacked arrangement, comprising:a shelving arrangement comprising a plurality of spaced apart vertical end supports supporting a plurality of horizontal mounting trays, each of the horizontal mounting trays supporting at least a pair of computer enclosures;
each of the horizontal mounting trays including one or more rows of first openings positioned at a proximal end of each horizontal mounting tray for permitting cooling air flow in a vertical direction, said first openings positioned in a pair of spaced apart computer enclosure mounting regions positioned immediately below one of said computer enclosures when mounted above the first openings;
each of the horizontal mounting trays including one or more rows of second openings positioned at a distal end of each horizontal mounting tray for permitting cooling air flow in a vertical direction through the horizontal mounting tray;
a central dividing rib extending along the length of each horizontal mounting tray, dividing the spaced apart computer enclosure mounting regions;
attachment surfaces affixed to each horizontal mounting tray, for attaching each horizontal mounting tray to the vertical end supports;
at least one slidably-removable horizontal sled for supporting one of said computer enclosures in a slidable arrangement relative to a horizontal mounting tray, each horizontal sled capable of assuming a closed position for operation and an open position positioned outwardly of the proximal end of said horizontal mounting tray to allow access to a supported computer enclosure and/or port expansion chassis mounted to the horizontal sled;
each horizontal sled comprising a generally planar lower surface, a vertically extending sled face at a proximal end that includes an opening for exposing a portion of a supported computer enclosure, a pair of elongate edge ribs extending along the length of each horizontal sled; and
each horizontal sled further including at least one support rail positioned in a central region of the horizontal sled, for supporting one of said computer enclosures and for defining an opening in the horizontal sled that is positioned immediately above the first openings of one of the computer enclosure mounting regions of a horizontal mounting tray when a horizontal sled is in the closed position, for facilitating the flow of air through the first openings and around a computer enclosure supported on the at least one support rail mounted to the horizontal sled.