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16 / October 2020 powderbulk.com PBE Ray Cocco, president, Particulate Solid Research Inc. Understanding particle flow in reverse-flow cyclones PARTICLE PROFESSOR relatively compact and quiet as they have no moving parts. A cyclone requires low capital investment and, for a properly designed cyclone, relatively low maintenance costs (for most applications). Cyclones can be constructed of almost any material and lined with refractory, which makes them suitable for cost-effec- tive, high-temperature applications. Cyclones also are forgiving and can continue to operate when damaged, even with holes, but at reduced collection efficiencies. Cyclones aren't without their issues and limitations, however. Cyclones can have a low collection efficiency for particles smaller than 10 microns; low-loading cyclones can be crippled by erosion in the cone region; and particle attrition in a cyclone can be significant. Understanding particle hydrody- namics is crucial when designing and optimizing a cyclone system. Particle flow is more than the cen- trifugal force on the cyclone's wall, and deviations in this simple con- cept are at the root of the cyclone's issues and limitations. In an earlier PBE column, we discussed cyclone design and operation, 2 with an emphasis on the macroscopic flows. In this column, we'll discuss microscopic physics and how these features translate to cyclone failure and limitation. T he first patent of a cyclone was issued in 1885 to John M. Finch of the Knickerbocker Company. 1 Instead of using large, stagnant settling chambers, as was the case with the gunpowder indus- try, dust-laden air could now be sent to a small cylindrical housing with a tangential inlet. The tangen- tial inlet caused a swirling flow in the cyclone, where centrifugal force instead of gravity provided the gas-solids separation. By adjusting the inlet velocity and cylindri- cal housing (barrel) diameter, a significant centrifugal force devel- oped. This force could be 100 times greater than the force of gravity. The fundamental physics describing a cyclone's performance were laid out more than 150 years ago by Isaac Newton and George Stokes. Reasons why the reverse- flow cyclone worked so well, however, weren't totally understood until the 1930s to 1960s, when con- tributions from individuals such as Eugen Feifel, J. Stairmand, Walter Barth, and Edgar Muschelknautz provided that understanding. Spe- cifically, these scientists showed that the cyclone had outer and inner vor- texes that provided two-stage solids separation for dust collection. Cyclone pros and cons Indeed, cyclones offer many advan- tages in particle processing. They're Feeding a cyclone The basic design of a cyclone is sim- ple, as shown in Figure 1. To review, a tangential inlet feeds the gas- solids flow into a cylindrical barrel, creating a swirling gas flow. Cen- trifugal force causes the solids to migrate to the barrel wall. A conical section below the barrel confines this swirling flow and directs the solids down to the dipleg, which extends from the bottom of the cyclone, to be either discharged or recycled. The cleaned gas reverses its flow (typically in the conical region) and flows upward and out through the gas outlet tube (some- times called the vortex finder). While the cyclone's basic design is simple, the resulting FIGURE 1 Parts of a reverse-flow cyclone Outlet Vortex finder or outlet tube Barrel Cone Dust hopper (optional) Dipleg Tangential inlet