Spun aluminum roof, direct driven, downblast centrifugal exhaust ventilator.  Fan shall be listed by Underwritters Laboratories (UL 705).  The spun aluminum structural components shall be constructed of minimum 16 gauge marine alloy aluminum, bolted to a rigid aluminum support structure.   The motor shall be enclosed in a weather-tight compartment, separated from the exhaust airstream. Wheel shall be centrifugal backward inclined, constructed of 100% aluminum, including a precision machined cast aluminum hub. Motor shall be heavy duty type with permanently lubricated sealed ball bearings and furnished at the specified voltage, phase and enclosure.