Swirl tube separation

In process engineering work the viewpoint is often taken that an improved quality of separation or purification is achieved at a correspondingly higher cost. Empirical relationships are developed which relate the quality of separation achieved to the cost. Not surprisingly this includes both cyclones and swirl tube separators. The measure of the quality of separation is the cut size, X50 or the dimensionless cut size, Stk o, and the cost is the pressure drop required to achieve this, or its dimensionless measure Eu. 

We will be returning briefly to the topic of solids loading in swirl tube separators later. Here, we wish to note that a difference exists between the behavior of cyclones with tangential inlets and swirl tubes equipped with inlet vane assemblies, so that the results shown in these figures cannot be applied to swirl tubes. 

The second cyclone body shown in Figure 13.3.4 illustrate a vane-type inlet design that we refer to herein as a swirl tube separator. Vane inlets are the most symmetrical of all inlet designs but are somewhat more comphcated to design and fabricate or, in some cases, to cast. Both the twin inlet and vane inlet cyclone designs described thus far are of the conventional reverse-flow variety. 

Figure 7 Third stage separator, blow-up of swirl tube, emissions improvements.

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. 

We have examined some of the most widely acclaimed and cited cyclone models. There is one more way of predicting the flow pattern, pressure drop and the separation efficiency in cyclones and swirl tubes, however by Computational Fluid Dynamics, or CFD for short. 

In order to formulate the flow equations for a fluid, for instance, for the gas in the cyclone or swirl tube, we must balance both mass and momentum. The mass balance leads to the equation of continuity the momentum balance to the Navier-Stokes equations for an incompressible Newtonian fluid. When balancing momentum, we have to balance the x-, y- and -momentum separately. The fluid viscosity plays the role of the diffusivity. Books on transport phenomena (e.g. Bird et ah, 2002 Slattery, 1999) will give the full flow equations both in Cartesian, cylindrical and spherical coordinates. 

Much less experimental data are available on the effect of solids loading on the separation efficiency of swirl tubes with swirl vanes. If the effect of solids loading is indeed one taking place in the inlet region of tangential inlet cyclones, we might expect it to be quite different in devices with swirl vanes. 

The following sections will show that the vortex end significantly influences the behavior of cyclones and swirl tubes. Its nature, and the factors governing its position, should therefore be well understood by anyone who designs such cyclonic type separators, and this topic should be given high priority in cyclone research at this time. 

Once one is aware of the fact that a vortex has a natural length , one may think it is permissible to simply substitute this length for the physically available length of the cyclone or swirl tube. The assumption being that the end of the vortex simply limits the useful length of the separation space. 

Although this seems to be approximately true in swirl tubes with a cylindrical body, experimental results indicate that the separation performance of cylinder-on-cone cyclones is reduced more than would be expected from the reduction of their effective length when the vortex ends within the conical section. One example is the dramatic reduction in separation efficiency (corresponding to an increase in cut diameter) for the longest cyclone length... 

This chapter presents information about three diverse topics relevant to cyclone technology thus the title, Some special topics . Two of the topics are related to the gas velocity in the separator cyclone erosion and the critical deposition velocity. The last topic is the working of cyclones or swirl tubes under conditions of high vacuum. 

In cyhnder-on-cone cyclones, indications are that the separation performance becomes erratic if the vortex terminates within the conical section. Our experience is that this is less so with cylindrical swirl tubes. It appears that the main effect of the vortex ending in the tube is simply to shorten the effective... 

A swirl tube equipped with a 6-element inlet vane of 5 mm thickness is to be used to separate entrained droplets from a carrier gas. The vane will be fitted into a 150 mm ID cyclone and will have an inner diameter of 100 mm. The vanes trailing edge will be 30° off the horizontal. 

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. 

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...

New material on the end-of-vortex phenomenon in reverse flow cyclones and swirl tubes is given in Chapter 9. Chapters 10 and 11 emphasize the importance of base-line performance of a new cyclone, tracer measurements, and post-separation problems including hopper design. Prudent advice is given regarding suggested focus on underflow mechanics when operational problems arise. Substantial changes were made in Chapter 12, with new material and illustrations added on erosion of the vortex finder s outer wall in view of recent CFD results, and use of wetted walls, sprays, and electrostatic fields in cyclones. 

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