How to dispose of old rack-mount servers

If your servers use front to back flow-through cooling, as most rack mounted servers do, leaving gaps can actually hurt cooling. You don't want the cold air to have any way to get to the hot aisle except through the server itself. If you need to leave gaps (for power concerns, floor weight issues, etc) you should use blanking panels so air can't pass between the servers.

When you rack mount the servers you end up creating what are called hot and cold isles. The hot isle is the where all the servers vent their hot air into, and the cold isles are where you pump the chilled air from the AC into. In a server room it isn't about how many fans you can put inside the machine, it is about how much cold air can you get to the machine. Rack mount servers are configured to allow for the maximum amount of cold air to move through the machine as quickly as possible.

Rack mounted servers also make it much easier to get into the machines as you just slide the machine out on the rails do what ever needs to be done, then slide it back into place. When done correctly there is no need to unplug a single cable when doing this.

You mentioned space, but I'll talk a little about that. When towers they come in all shapes and sizes. If you start putting towers in a rack, you can probably fit 3-6 machines in a rack, depending on tall they all, and if you can put two on a shelf and have them fit. With rack mounted servers you can easily get 42 machines in a rack (assuming 1U servers and a 42U rack). That is a much more efficient use of space, and space costs money.