An optimum design approach to reverse-flow cyclones based on the concept of optimizing cyclone dimensions is introduced in this paper. This approach involves optimizing cyclone dimensions by categories, calculating cyclone performance by correlating similitude numbers and optimizing the combination of four cyclone parameters, D, KA, dr and vi, which has been proven to be applicable not only for single-stage cyclone, but also for multistage cyclone separators. Applications of the designed cyclones in FCC units and acrylonitrile reactors are also presented. An optimum design approach to reverse-flow cyclones based on the concept of optimizing cyclone dimensions by categories, categories cyclizing performance by correlating similitude numbers and optimizing the combination of four cyclone parameters, D, KA, dr and vi, which has been proven to be applicable only for single-stage cyclone, but also for multistage cyclone separators. Applications of the designed cyclones in FCC units and acrylonitrile reactors are also presented.