Comment First, fan out the heat (Score 1) 414
If you effectively duct the heat pushed out by the rack units to the outside, your requirement to cool down the hot exhausted air in the room diminishes quite a lot.
Ok, not usually possible with computers, but I used to do that for racks of audio power amplifiers used in a permanent install. They drew air in from the front of the racks and blew out the back which was fanned outside, thus the room didn't require much to keep it cool.