Gas cleaning (that is, separating solid particles from a gas) isn’t only critically important but also one of the most common unit operations. The reverse-flow cyclone separator is among the most popular pieces of gas-cleaning equipment. The reason is simple: The cyclone has no moving parts or parts to be replaced. Also, its operating costs tend to be low and, if designed and maintained correctly, the cyclone can provide years of reliable service. This is why reverse-flow cyclones are widely used in fluidized-bed and circulating fluidized-bed processes, at the end of pneumatic conveying lines, and even in today’s household vacuum cleaners. Yet much is unknown about how the cyclone works, and its design tends to be more art than science.