Selecting a FCU...
1) Run load calculations for the zone to be served by the AHU.
2) Review floor plans and designated area for FCU placement.
3) Determine FCU configuration, floor mounted, suspended, vertical, horizontal, ducted/ductless, etc.
4) Determine the cooling mechanism, Dx, CHW, GSHP, etc.
5) Go to manufacturer's internet catalogs or paper catalogues if you have them, search for units that match the requisite configuration and type.
6) Sort through performance data and select the one that best matches requisite load profile.
and all these years I thought these were the 6 steps:
1) Square foot the load
2) Place the unit is the most inaccessible location for servicing
3) Let the contractor worry about configuration
4) Assume there is adequate chilled water nearby and put "Connect to existing" on drawings
5) Let your vendor make the selection
6) Move on the the next job and don't worry about it.
You should learn psychrometrics. See Carrier Handbook of Air Conditioning System Design. Find space sensible and latent cooling load and heating load. Match with fan coil capacity with building chilled water and heating water supply temperatures.