The "PRV" Series fans are designed to be economical, low profile roof exhausters.  The pleasing appearance of its low profile is accomplished by recessing the fan unit into the curb opening. Consequently, a mounting sleeve or higher-than standard curb may be required for clearance between fan motor and damper when back draft damper is used.  The PB PRVs incorporate Style H fan components for models in the 24- to 48- inch range.  Exhaust - belt drive - 2,000 to 78,000 CFM - to 3/4" static pressure.