datacenter cooling question....

hi folks,
we are building a computer datacenter. room is 18' x 16', concrete walls and 2/3 below ground level. average temperature is about 65-70 degrees F with fairly low humidity (Northern California).
initially we will have two 42U server racks that pull a total of 13000 Watts under full load. electrical power feed is three phase to an in room distribution box and dual 208V/30A twistlock feeds to the racks.
we plan to expand the datacenter 200-300% in the next 2-3 years (maybe 30-40,000 watts). how much cooling should i be looking at given the growth i am expecting? i have researched this topic and folks caution not to over-estimate your need otherwise your cooling unit will short cycle and such. of course underestimating could also be a serious issue.
based upon my limited knowledge, i am thinking a 4 ton Mitsubishi split ductless unit would be a good fit. can anybody offer me a little advice?
best, alex
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. Don't forget heat load from lighting, people, and any adjacent spaces. . Consider buying two (or more) small machines rather than one big one.     What will you do when 1 machine fails or needs maintenance? . As has been suggested, get machines designed for mostly sensible (dry heat) loads, rather than machines designed for human comfort. . I assume you'll be UPS'ing your servers; do you need extended run time beyond what batteries provide? If you then switch over to diesel (or similar), will the A/C restart automatically? If the diesel starts in just a few seconds, the A/C should have     anti-slugging (delayed auto-restart) provisions. . Are you doing raised floor? or more generally, how will you distribute the air in the room? . Have room built with a vapor barrier (or have it painted with vapor barrier paint) to make humidity control easier. . Take a look at > http://www.tricare.osd.mil/tmis_new/Policy%5CFederal%5Cfips31.pdf a somewhat outdated (1974) but still valuable guide to setting up a          data center.
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