The PDA measurements in the downcomer tube of a gas cyclone provide the first in-situ experimental proof of agglomerated particles in a cyclone. The formation of agglomerates in the cyclone explains how particles of less than 20-50 mu m are able to settle in the dust bin, in the presence of high velocities. The gas flow in the downcomer tube of the cyclone induces circulation of the particles in the downcomer tube and in the dust bin leading to agglomeration, and thus improves particle separation. The measurements confirm circulation of particles in the dust outlet geometry, as described earlier by Obermair. The particles transported upwards from the dust bin are centrifuged to the wall, and due to the increasing tangential velocities reach the downward flow at the wall. Due to the additional particle separation in the downcomer tube a cyclone with a downcomer tube obtains the best separation efficiency. Further, the measurements prove that due to the dust load there is a change in vortex from a potential vortex to a solid body vortex within the downcomer tube. While the effect of particles on velocity and the associated deceleration of the vortex was well-known in the literature, the change of vortex type had not been observed before. (c) 2005 Elsevier B.V. All rights reserved.