Hi JDD, nice comments, but we are talking big box retail.
I do agree a server room in the big box would be a separate unit (usually very small). Further you can zone central systems rather easily to solve particular exposures or load concentration. All buildings have diversity where the building load moves during the course of a day and the central system can account for this rather easily. With zoning individual schedules can also be set and zones changed easily and less expensively than with individual units.
Also, I've suggested redundancy to a degree. For fan systems typically the larger systems are not as restricted OEM product like with packaged rooftops. I'd suggest 100% redundant fan and partial redundant cooling. I think in the larger motors there is significantly more longevity to the fan section than with packaged rooftop equipment.
For big box retail I don't see the need ever for a centrifugal chiller and the towers. I think you are way overshooting design with that. Think more along the lines of multiple air cooled condensing units to DX coils in the equipment. Right now big box retail is multiple air cooled packaged units.
If you are using proprietary controls first shame on you, and next make the controls 100% redundant on the central plant as we are talking one system, not 25.
I'm contending PM issues and problems over time are significantly lower, operating costs go down, roof leaks diminish. Further, on construction roof installation costs go down with much less penetrations, structural support steel and first cost electrical feeds to multiple equipment. The indoor unit lasts significantly longer than an outdoor packaged unit with 1/2" insulation that loses 6F just from circulating air through it in winter and summer. EDIT: Also, higher parapet walls or rooftop unit screening to hide equipment from street view in many areas.
What I'm really waiting for is a variable coil system similar to a Sanyo or Mitsubishi heat pump system sans the small fan units. Then you could zone off H/C with a single fan system and lower install costs.
I really don't think the big boxers are looking at their repair cost overhead properly to evaluate the system, or maybe they are taking first costs and simply charting out the building life accordingly full well planning on exiting the building for a new one and hanging replacement on the next guy. (I have a whole series of issues with that- different topic regarding local municipalities and blight).
