Most models on the working of cyclones are based on cylinders and cones whereby the vertical tube and dustbin should also be included in their geometries. These separation models of conventional cylinder-on-cone cyclones do not consider the effect of the dust collection system on the flow field. In actual fact, the inclusion of the dust collection system had a considerable effect on the flow pattern in the gas cyclone as well as on the separation efficiency. In this paper, an extended model is presented, which is based on the time-of-flight approach. In this model, the gas residence time is modified so that the entire cyclone geometry is used. ne lift force of a particle in the boundary layer is considered because of the larger velocity gradient here. The availability of the extended model is verified by comparison of the calculated grade efficiency with experimental data and theoretical counterparts in the literature.