Abstract: Apparatus for separating particles from a stream of gas and entrained parles comprising an annular duct having at one end an inlet to direct the stream into the duct such that the stream passes through the duct with swirl and at the other end outlet means to divide the inner part of the flow from particles in the outer region of the flow. The inlet and duct are shaped so that the swirl within the duct is that of a potential vortex in which the velocity is perpendicular to the axis of the vortex and is inversely proportional to the radius from this axis. This enables substantially streamline flow to be maintained. The annular duct may be conically tapered.