The rotational phase separator is a patented technique for separating heavy or light phases and particulate matter from fluids by centrifugation. This technique is used in areas ranging from the treatment in fluids in industrial processes to the filtering of air to protect men from respiratory allergic reactions. In a particular design to separate particles from gas flows the rotating filter element is combined with a cyclone type housing. For this application a sealing impeller is required to prevent uncleaned gas flowing alongside the filter element to the outflow. Since the design of the impeller is critical to the performance of the apparatus, the flow phenomena involved are studied numerically. The results are used to formulate a one-dimensional model for easy use during the design process.