Development of annular-type virtual impactor

Annular jet-type virtual impactor was developed and tested with the use of an industrial powder. The new virtual impactor was designed based on an impactor having a rectangular jet. It was expected that the new impactor has a superior performance compared to the impactor with a rectangular jet because the annular jet has no end, which causes the deterioration of the performance. The classification experiments, however, showed lower performance than expected. Observation of the deposited pattern of the classified particles revealed that the flow was asymmetric, i.e. the flow had a three-dimensional structure in azimuthal direction. Therefore, the collection nozzle was improved so as to divide the classification zone into four parts and to accelerate the flow in each part. As a result of the improvement. 50% cut-size of the annular-type virtual impactor was well-fit to the theoretical value and the separation efficiency at the diameter smaller than 50% cut-size was close to the theoretical curve. However, the separation efficiency at the diameter larger than 50% cut-size was unchanged. From the comparison of configurations of the impactors used in a previous work and in this work. we presume that the shape of the nozzle collecting a finer aerosol affects the classification performance. In order to confirm the prediction, classification experiments were carried out by small-scale rectangular jet virtual impactors having different nozzle angles. The results revealed that the smaller nozzle angle deteriorates the classification performance even for the same configuration of the classification zone. The result implies that the performance of the annular-type virtual impactor can further be improved if the nozzle shape is properly designed. (C) 2001 Elsevier Science B.V. All rights reserved.