Parallel swirl tubes

Fig. 16.2.3. Left a Cyclo-TrelF Multicyclone unit featuring a closely packed arrangement of parallel swirl tube type cyclones. Courtesy Hamon Research Cottrell. Right Simplified view of a multicyclone showing upper and lower tubesheets...

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. 

There are many situations wherein one cyclone or swirl tube is inadequate for the separation task at hand. In such situations, it is often feasible to use multiple units either in series or in parallel or both. 

There are two common reasons for choosing to install more than one cyclone or swirl tube in parallel. One is that one device handling the whole gas stream... 

Additionally, Eqs. (4.3.18) and (4.3.19) indicate that the Euler number and, therefore, the pressure drop, are independent of the cyclone size for geometrically similar cyclones or swirl tubes if the inlet velocity is kept constant. We can thus gain in efficiency without increasing pressme drop by splitting up the solid-laden process stream over two or more cyclones or swirl tubes and operating them in parallel. 

One commercially available system comprising a parallel arrangement of cyclones, called swirl tubes , is the multicyclone unit illustrated in Fig. 16.2.2. The solids-bearing feed stream enters the separator vessel via a centrally located pipe at the top of the vessel. The feed exits this inlet pipe near its bottom end from where it flows radially outwards - between two tube sheets - and into the feed chamber or plenum. The solids-bearing gas then enters the individual swirl tubes wherein it is split into a solids-laden underflow stream and a clean-gas overflow stream. 

Fig. 16.2.2. Shell third-stage separator with swirl tubes working in parallel. Courtesy Shell Global Solutions International...

---