Two conventional cyclones with different vortex finder diameters and a double cyclone were used to evaluate and compare their collection efficiencies and the effects of varying the vortex finder lengths and flow rates. In addition, a series of experiments were conducted to investigate the characteristics of the collection efficiencies of the cyclones when different voltages were applied to the vortex finder. The collection efficiency of the conventional cyclone with a smaller vortex finder diameter was higher, and that with a smaller vortex finder length was also higher when no voltage was applied. However, the collection efficiency properties were reversed when a voltage was applied and electric field was formed. The collection efficiency of the double cyclone increased greatly with increases in the applied voltage and decreases in the flow rate, and was the highest of all the cyclones when the same voltage and flow rates were applied. However, the collection efficiency of the double cyclone, with no applied voltage, was lower than that of the conventional cyclone with the 7-mm vortex finder diameter. (C) 2003 Elsevier Ltd. All rights reserved.