Wayne, having 10yrs experience working on computer room a/c units (Phone company
) I can tell you this. Cleanliness needs to be paramount... also redundancy needs to be built in to your system... What I mean is, you need a lead and a lag unit.... These units will change lead/lag weekly to insure equal run time on the compressors... also, The lead unit only cools, and the lag unit needs to have it's fan running continuously... what this does, is continually circulate air, but also prevents the lead unit from running after cooling has stopped, and redepositing the moisture that it has collected back into the air... If, you are utilizing cool dry air as return air, you run the risk of dropping your humidity too low, to the point that static electricity becomes an issue, and electronics can be knocked out due to these situations... also, if your main unit goes out, you have a back up... as far as the freezing up situation, it is possible that the evaporator is freezing up due to the cold air for one simple reason, usually you have a 10-20 degree split between supply air and discharge air... that being said, if your supply air is 60 degrees and you have a 20 degree split, that means your discharge air would be approximately 40 degrees, which would mean that you coil temperature has dropped below freezing... you may remedy the situation by overcharging it, but run the risk of damaging the valves in the compressor... If I were your a/c guy I would recommend only cooling the air in the room that needs to be conditioned... I would not import or export it... Computer rooms are considered critical for a reason and are generally taken better care of / more expense applied to than personnel occupied spaces....