Dust outlet

The collection efficiency of cyclones varies as a function of particle size and cyclone design. Cyclone efficiency generally increases with (1) particle size and/or density, (2) inlet duct velocity, (3) cyclone body length, (4) number of gas revolutions in the cyclone, (5) ratio of cyclone body diameter to gas exit diameter, (6) dust loading, and (7) smoothness of the cyclone inner wall. Cyclone efficiency will decrease with increases in (1) gas viscosity, (2) body diameter, (3) gas exit diameter, (4) gas inlet duct area, and (5) gas density. A common factor contributing to decreased control efficiencies in cyclones is leakage of air into the dust outlet (EPA, 1998). 

Particulate material is forced to the outside of the tapered segment and collected in a drop-leg located at the dust outlet. Most cyclones have a rotor-lock valve affixed to the bottom of the drop-leg. This is a motor-driven valve that collects the particulate material and discharges it into a disposal container. 

A cyclone will operate equally well on the suction or pressure side of a fan if the dust receiver is airtight. Probably the greatest single cause of poor cyclone performance, however, is the leakage of air into the dust outlet of the cyclone. A slight air leak at this point can result in a tremendous drop in collection efficiency, particularly with fine dusts. For a cyclone operating under pressure, air leakage at this point is objectionable primarily because of the local dust nuisance created. 

A disk or cone baffle located beneath the gas outlet duct may be beneficial if air in-leakage at the dust outlet cannot be avoided. A heavy chain suspended from the gas outlet duct has been found beneficial to minimize dust buildup on the cyclone walls in certain circumstances. Such a chain should be suspended from a swivel so that it is free to rotate without twisting. Substantially all devices that have been reported to reduce pressure drop do so by reducing spiral velocities in the cyclone chamber and consequently result in reduced collection efficiency. 

Reducing dust reentrainment. This opportunity is not shown in the above equation, but is based on proprietary dust outlet designs. 

Cyclone dust-outlet diameter, ft Gas rate, ftVs Gas density, Ib/ft ... 

Above the top limit the total efficiency no longer increases with increasing pressure drop and it may actually decline due to re-entrainment of dust from the dust outlet orifice. It is, therefore, wasteful of energy to operate cyclones above the limit. At pressure drops below the bottom limit, the cyclone represents... 

We now concentrate on centrifugal devices. In these, the dust-laden gas is initially brought into a swirling motion. The dust particles are slimg outward to the wall, and are transported downward to the dust outlet by the downwardly directed gas flow near the wall. 

Fig. 3.1.1. Sketch of a tangential-inlet cyclone with the flow pattern indicated. The coordinate directions are shown, normally the 2-axis coincides with the axis of the cyclone or swirl tube. To the right, the radial distributions of the axial and tangential gas velocity components are sketched. It is understood that the dust outlet may be the Uquid outlet for the case of a demisting cyclone...

To the right in Fig. 3.1.1 the radial profiles of the axial and tangential gas velocity components are sketched. The former shows the outer region of downwardly directed axial flow and the inner one of upwardly directed flow. As mentioned, the downward velocity at the wall is the primary mechanism for particle transport out the dust outlet. The axial velocity often shows a dip aroimd the center hne. Sometimes this is so severe that the flow there is downwardly directed. The tangential velocity profile resembles a Rankine vortex a near loss-free swirl surrounding a core of near solid-body rotation. 

Since the dust outlet is smaller than the gas outlet, CS intersects the physical cone near its lower end, and Hcs is shorter than the physical height (H — S). It can be shown from simple geometric considerations that ... 

Particle mass balances are performed in the differential elements indicated in the figure under the assumption that the particles in each separate region (but not between the regions) are completely mixed in the radial direction. Particles reaching the cyclone wall in sections 1 and 2 are considered captured. Region 3 allows for reentrainment at the dust outlet, but this can often be neglected in practice. 

Obermair and Staudinger have performed measurements on a cyclone having a variety of dust outlet configurations. For illustration purposes, one was selected for simulation herein and its geometry and dimensions are shown in Fig. 6.A.5. Physical property and flow data at test conditions follow. 

When determining the efficiency of cyclones and swirl tubes, samples can be taken at three positions the inlet, the gas outlet and the dust outlet (see Figure 10.4.1). 

Careful design of the dust outlet section of the cyclone can mean the difference between a cyclone working satisfactorily and one emitting coarse particles reentrained into the vortex from the material already separated. Figure 15.1.9 shows four possible configurations of the dust outlet. 

Obermair and Staudinger (2001) studied the gas flow, pressure drop and separation efficiency in cyclones with the dust outlet configurations shown in Fig. 15.1.9. The corresponding performance data is presented in Table 15.1.2. Upon comparison, we see that configuration a is not as efficient a design as the others. This is probably due to re-entrainment from the solids hopper. It does exhibit a comparatively low pressure drop, however. Its two vortex-stabilized... 

---