Parallel cyclones

As discussed above, theoretically a smaller-diameter cyclone should be able to collect smaller particles because it can develop a higher centrifugal force. However, using smaller cyclones generally means that many have to be used in parallel to accommodate large gas flows. The problem with parallel cyclones (as indicated above) is that it is difficult to get even distribution of solids into all the cyclones. If maldistribution occurs, this can cause inefficiencies that can negate the natural advantage of the smaller cyclones. 

Figure 23 The seesaw phenomenon of area-averaged solid fluxes at the cyclone inlets (Zhang et al., 2010), in accordance with the experimental findings in identical parallel cyclones (Masnadi et al., 2010).

Figure 3. Cutaway view of lead trap showing location of alumina pellets upstream of cyclone separators. The alumina pellets promote agglomeration and the dual parallel cyclones separate and retain the exhaust particulate matter in collection chambers.

Several small cyclones are placed in parallel when it is not possible to fit a single large cyclone into the available height, or when extremely high centrifugal forces are required. It is difficult to ensure equal distribution of gas and solids into parallel cyclones. This can lead to cyclone inefficiencies and increased wear on the cyclones taking the bulk of the soUds flow. 

Typically, when the number of parallel cyclones is small (less than 6 to 8), each cyclone will have an individual inlet and outlet duct. However, when the number of cyclones exceeds this number, the cyclones generally have common inlet and outlet plenums and a common collection bin. Parallel arrangements of this type frequently result in the operating problems of equalizing the gas and solid flow rates into each cyclone, and preventing backflow from the common chamber into one or more cyclones and reentrainment of solids from the common collection hopper back into one or more cyclone outlets. To prevent these two problems, a device is often placed at the outlet of each parallel cyclone. This device (generally proprietary) reduces the area available for flow and acts as a check valve to prevent backflow and dust reentrainment into the cyclone. 

Because of problems in the operation of small parallel cyclones, it is generally found that they result in a collection efficiency less than or (if designed well) equal to that of a large, single cyclone processing the same amount of gas and solids. 

There has been very little work in the literature on the operation of parallel cyclones. However, what work does exist indicates that too low a pressure drop results in poor gas-solids distribution, and that too many parallel cyclones in a common hopper are not recommended. 

Broodryk and Shingles (1995) studied parallel cyclone operation with cyclones in the freeboard above a turbulent fluidized bed. They found that too low an overall pressure drop (caused by inlet gas velocities of less than about 15 m/s) resulted in preferential flow of gas through two of their three cyclones. They also found that poor trickle valve design and/or operation could induce an imbalance in the gas flow into the cyclones. When preferential gas flow occurred, it resulted in significant solids carryover and reduced cyclone efficiency. 

In some parallel cyclone arrays, wherein it is suspected or known that the cyclone system is not performing up to design expectations, it may be possible to inject a tracer, such as helirnn gas, in a common header upstream of the cyclones and measme the time required for the tracer to exit out the... 

Returning now to the much smaller sized cyclones comprising most multicyclone installations, one of their advantages over an external array of parallel cyclones is that the individual cyclones do not have to be equipped with their own individual inlet pipes or ducts. A disadvantage is that, because the individual cyclones are fully enclosed, it is not normally possible to perform diagnostic or repair work on the individual cyclones while they are in service. On the other hand, it is sometimes possible to repair or patch up an... 

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