I've found that in a nice clear rack with plenty of space and good fans, the hot air exiting the rear of any rack-mounted equipment pretty much goes straight up and out. Just as I was starting to pack my racks much more densely I saw a rack full of Apple gear in a JPL scientist's lab, and it was arranged quite differently to all of Apple's promotional material. In fact it looked awful! He had about half a dozen Xserves sitting at the bottom of his 42U rack, spaced 1U apart with blanking plates filling the gaps, then there was a bit of a gap, filled with blanks, then three or four Xserve RAIDS at the top, all with a 1U spaces between them covered by blanks. The bit that stood out was that he'd used heaps of duct tape to seal off the gaps where blanks met equipment, between the vertical mounting rail and the walls of the cabinet, and he'd completely covered up all the mounting holes with tape, so that the only way for air in the front of the cabinet to get to the back was to go through one of the machines. He must have used at least 4 or 5 rolls of tape, it was everywhere! I've since copied his technique, and it is very effective. While the fans have to work a little harder to start with to overcome the pressure heat doesn't accumulate nearly as quickly, and if the fans have spun up under load, they return to normal much faster once the task is complete.