Vortex Finder Geometries

A vortex finder—or vortex tube —normally consists of a simple hollow cylinder centered within the cyclone and extending down to the approximate level of the bottom of the inlet duct. It serves several important functions including that of defining (or controlling) the diameter of the inner vortex core —at 

As shown in Fig. 15.1.12 (and Fig. 15.1.1), vortex tubes come in a variety of shapes and sizes, depending primarily on design objectives and, not the least, on tradition, preferences, or experiences of the designer or design organization. We will briefly introduce and discuss some of their geometries below. 

The restricted vortex tube designs shown in frames d, e and f each feature some sort of reducing cone or cone-and-cylinder at their inlet openings. They can usually be found in cyclones that have been modified to improve separation performance by reducing the effective vortex tube diameter. However, they may also be included as part of the original design in situations where there is an expectation of a significant future increase in gas volumetric flow rate. In this case, the restriction section can be removed and, if necessary, replaced with a more open cylindrical section. 

Pressure-recovery type vortex tubes, along with pressure-recovery type diffusers set atop the roof of the cyclone, are occasionally used to convert some of the rotational energy of the exiting gas back into static pressure. Based on data presented by Muschelknautz and Bruimer (1967), a modest amount of pressure recovery (15 to 20% reduction in vortex core pressure loss) can be achieved with a simple conically shaped vortex tube, such as that shown in frame g. More efficient recovery (35 to 40% loss reduction) is possible with a well-designed internal conical insert, such as that shown in frame h. Normally, such a vortex tube is directly connected to a wide-bodied outlet diffuser or exit scroll which sits atop the cyclone roof. 

Muschelknautz (1980) investigated the effect of a number of pressure-recovery configurations including an exit scroll, a simple diffuser, an aimular diffuser with exit scroll, and a central body mounted with its bottom flush with the vortex finder lip without and with rectifying vanes. The last fom configurations were combined with a smoothing of the vortex finder lip. He found that these modifications reduced the pressme drop by 12, 31, 40, 44 and 

---

For a properly designed and operated cyclone, the sharpness iadex is constant, typically 0.6. The cut size and apparent bypass are a function of the cyclone geometry, the volumetric feed rate, the material relative density, the feed soflds concentration, and the slurry rheology. The relationship for a standard cyclone geometry, where if is the cylinder diameter ia cm and inlet area = 0.05 vortex finder diameter = 0.35 ... 

The fourth type of inlet we wish to describe is that of swirl vanes. As shown in Fig. 1.3.8 d, a swirl-vane assembly allows the gas to enter the cyclone parallel to the axis of the cyclone The swirl-vane assembly is positioned between the vortex finder (or, in case of a straight-through device, see below, a central solid body) and the outer (body) wall of the cyclone. This type of inlet is often inserted in cylindrical-bodied cyclones rather than in cylinder-on-cone or conical-bodied geometries. When it is, we refer to the separator as a swirl tube. Swirl tubes are often of small size (by commercial standards) and are most commonly arranged in a parallel array on a common tube-sheet within a pressure-retaining vessel. They are normally fed from, and discharge into, common, but separate overflow and underflow plenums. 

In some geometries the tangential gas velocity at the wall, and in the entire space between the wall and the vortex finder, can be significantly higher than the inlet velocity due to constriction of the inlet jet. In Fig. 4.2.2, the inlet flow pattern in a cyclone with a slot type of rectangular inlet is compared with one with a 360 wrap-around or full scroll inlet. 

As is standard in scaling, we assume that the model and the prototype are geometrically similar. This means that all dimensionless numbers describing the cyclone geometry, for example the ratio of the vortex finder diameter to the body diameter Dx/D, are the same between model and prototype. 

Erosion of the outer surface of the gas outlet tube or vortex finder can occur from several possible causes and we will briefly discuss each of these below. Perhaps the most obvious cause is direct impaction, which can occur if any part of the vortex finder lies in the projected path of the particles entering the cyclone. See Fig. 12.1.3a. As shown in Fig. 12.1.3b, the incoming gas can be expected to constrict, either due to the geometry or due to the effect of the gas already rotating in the cyclone and flow around the gas outlet tube. But this is not always the case for the solids. 

Most significantly, the overall pressme loss across the cyclone was found to decrease by 40% at a penetration of about 75% relative to 0% penetration of the vortex finder into the diffuser chamber. Compared to the case where the gas exhausted directly out the vortex finder (the dashed line), the pressure loss decreased by about 44%. These are very significant reductions, especially in light of the simplicity of the diffuser geometry employed in this study. These results support Dehne s earlier comments, only they indicate a far larger pres-sme drop reduction. They are also very similar to those reported by Idelchik (1986) for the pressure loss coefficient for linear (irrotational) flow of a jet exiting a pipe and impacting a flat baffle plate. See Fig. 15.1.17. 

---