Cyclone length

The issue of overall cyclone length was already discussed in Chaps. 5 and 9 in connection with the effect of length on pressure drop/efficiency and the natural vortex end. 

As illustrated in Fig. 5, the outer downwardly spiralling gas velocity decays as gas peels off into the inner upwardly spiralling exiting vortex. The length of this inner vortex, the natural vortex length, L, (Alexander, 1949) then represents the distance below the edge of the gas outlet tube below which no feed gas remains to peel off into the inner vortex. If the 

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TABLE 17-4 Required Cyclone Length as a Function of Area Ratio... 

Hoffman et al. [AIChE J. 47(11) 2452-2460 (2001)] studied the effect of cyclone length on cyclone efficiency and showed that the efficiency of a cyclone increases with length. However, they also found that after a certain length, cyclone efficiency decreased. They reported that cyclone efficiency suddenly decreased after a certain cyclone length, which in their cyclone was at a length/diameter ratio of 5.65. 

Height of separation contact element Phenomenological factor Length of absorber contact zone Cyclone length... 

Figure 9 Influence of cyclone length on theoretical electrical efficiency...

Obviously the predictions of the models differ considerably. While the models of Stairmand and Shepherd and Lapple agree reasonably well, the Casal/Mar-tinez and Barth models differ by almost a factor of two. In the following chapter we shall put the models for pressure drop to a test we shall compare their predictions of the effect of cyclone length on pressure drop with experiment. 

The range of cyclone lengths tested, and the effect of length on cut size pressure drop... 

The cut size X50 is seen to decrease slightly with increasing cyclone length, although the last result is atypical (and we will return to this issue below). The pressure drop decreases with increasing cyclone length as we anticipated. 

We could use the experimental Ap s to evaluate the Rietema model rather than the Ap s computed from the Shepherd and Lapple model. This would raise the Rietema points somewhat, causing them to lie almost on top of the Mothes-Loffier points. The trend in the Rietema predictions of a decreasing cut size with increasing cyclone length would be maintained in spite of the decreasing Ap. 

Fig. 7.1.2. Profiles for the tangential and axial velocity profiles at two axial stations in a Stairmand HE cyclone with CFD and LDA (Slack et al., 2000). The cyclone length is 0.82 m, and the axial stations are left 0.17 and right 0.44 m under the...

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... 

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