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I mentioned circulation. It's important to keep the air in the room moving or else a kind of heat bubble can surround servers / computers and the machines will just continue to recycle their already heated air. A cieling fan does the trick nicely.
It's possible to aquire small air conditioners for server rooms. We had a 1 ton unit at a former company which we got for around $400. It kept the room pretty cool even though we had about 30 servers in a 10x10 room.
Good luck.
Can the small air-con units be placed inside?
A/c units move heat from one place to another. So the heat from your server room needs to go somewhere, but not necessarily outside, it could be a false ceiling, corridor or adjacent room etc.
A portable air cooled unit might be your best bet. They have small, typically 6 to 8" diameter hot air exhaust ducts which can be hung out of windows etc.
Alternatively, wall mounted versions are now available which only need a couple of 6" vents on an outside wall. To the casual eye they look like bathroom/toilet extractor grilles.
Both excessive heat and cold can damage a PC.
Also, another risk you run when you cool the air around computers / servers is static electricity. I'd say to contact your machine's manufacturer / look in the literature provided with your machine to determine the machine's effective operating range. A lot of big server rooms I've been in were kept around 60 degrees so you'd probably be safe in that range.