The Vortex Powerfan VMF Series is an in-line duct fan specifically designed to maximize the airflow performance with minimal noise levels in a small, compact housing. These fans incorporate a powerful mixed flow impeller and internal air vanes located at the discharge end of the fan housing. This impeller and guide vane combination provides a smooth laminar air flow which in turn minimizes turbulence and noise and generate an excellent air flow to static pressure performance ratio. The VMF400 is suitable with 4'' duct.