Cyclone inlet velocity

Table 7. Dust Loadings and Cyclone Inlet Velocities above Which Erosion Is Excessive ...

The cyclones are typically designed with diameters of 100—160 cm for ease of maintenance. Cyclone inlet velocities are usually restricted to 18—21 m /s in the first stage and to 20—26 m/s in the second stage to achieve satisfactory pressure drop and erosion characteristics (62). The number of sets of two-stage cyclones thus depends on the total gas flow. Finding room to house all the necessary cyclones within the regenerator frequently requires considerable ingenuity (62). 

Average cyclone inlet velocity, based on area m/s ft/s ft/s... 

V,- = Cyclone inlet velocity, average, based on area a) ft/sec... 

Fev = Friction loss (inlet-velocity heads) A pev = Pressure drop through the cyclone (inlet-velocity heads)... 

These devices are generally designed to meet specific pressure-drop limitations. For ordinary installations operating at approximately atmospheric pressure, fan limitations dictate a maximum allowable pressure drop corresponding to a cyclone inlet velocity in the range of 20 to 70 feet per second. Consequently, cyclones are usually designed for an inlet velocity of 50 feet per second. 

More recently, Reppenhagen and Werther (1997) have conducted a comprehensive study ofthe attrition mechanism in a cyclone. Their experimental set-up is shown in Fig. 16. The 0.09 m ID cyclone is operated in the suction mode. The solids (spent FCC, surface mean diameter 105 microns) are introduced via a vibrating feeder into the cyclone inlet tube which allows an independent variation of cyclone inlet velocity Ue and solids loading ll... 

According to the model described above the cyclone attrition rates Ra c obtained under steady-state conditions have been plotted against the square of the cyclone inlet velocity in Fig 18. Although the number of experiments is not large the general relationship between Rac and Ue predicted by Eq. (19) is confirmed. 

Figure 15. Changes in the size distribution of FCC catalyst due to attrition in a 5" cyclone, (inlet velocity 35ft/s). (Zenz andKelleher, 1980.)...

The cyclone inlet velocity is thus shown to be the most significant factor influencing the solids loss due to attrition in the cyclone. 

Collection efficiency increases as the gas throughput increases. Kalen and Zenz [14] reported that collection efficiency increases with increasing gas inlet velocity to a minimum tangential velocity. This reaches the point where the dust is re-entrained or not deposited because of saltation. Koch and Licht [12] showed that saltation velocity is consistent with cyclone inlet velocities in the range of 50 to 90ft/sec. 

Calealate die optimum value of the cyclone inlet velocity Krby Eq, (3.5-4). 

Cyclone inlet velocity not only affects efficiency of separation but also reflects in pressure loss and possible erosion. Gas viscosity has an important effect on particle efficiency, and so it is advisable to check its dependency with temperature and consider those cases in which a gas different from air is involved in the process. Smaller cyclone diameters increase overall efficiency, but also promote erosion. In addition to this, it is sometimes necessary to consider possible attrition of solids in the cyclone, which will result in pr< uction of fines and considerable losses. Erosion occurs primarily where the particles first impact the cyclone wall, but also occur at the bottom of cyclones too short to accommodate the length of the naturally occurring vortex. 

Figure 19 Influence of the cyclone inlet velocity on the cyclone attrition rate at different solids loadings measured by Reppenhagen and Werther (1999a) in a 90 mm ID cyclone. Material spent FCC catalyst cyclone inlet velocity solids-to-gas loading ratio. 

However, for a consistent description of the overall proeess it is reasonable to substitute the orifice velocity orj the solids loading the cyclone inlet velocity... 

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