JMT

NFPA 496 deals with purged and pressurized equipment intended to prevent ignition in rooms or enclosures when combustible gasses may be present. Clause 5.4.1 requires at least 4 volume changes, but where motors or such are present, then 10 volume changes are required before the equipment may be energized. I'd want to use that as a guide. From your dimensioins, you have about 6730 cu feet per exchange, and so a 100 cfm system would take about an hour to do one exchange. So it sounds like you would need a much more efficient ventilation system to do this effectively.

JMT,

I use Broan ventilation equipment for quiet efficiency, recommend 'ceiling mount model L400' for your application.