The efficiency of a cyclone collector is related to the pressure drop across the collector. This is an indirect measure of the energy required to move the gas through the system. The pressure drop is a function of the inlet velocity and cyclone diameter. Form the above discussion it is clear that small cyclones are more efficient than large cyclones. Small cyclones, however, have a higher pressure drop and are limited with respect to volumetric flow rates. Another option is arrange smaller cyclones in series and/or in parallel to substantially increase efficiency at lower pressure drops. These gains are somewhat compensated, however, by the increased cost and maintenance problems. Also these types of arrangements tend to plug more easily. When common hoppers are used in such arrangements, different flows through cyclones can lead to reentrainment problems. A typical series arrangement is shown in figure 5.3. In such arrangements large particle can be arrested in the first cyclone and a smaller, more efficient cyclone can collect smaller particles. Due to that it reduces dust loading in the second cyclone and avoids problems of abrasion and plugging. Also, if the first cyclone is plugged, still there will be some collection occurring in the second cyclone. The additional pressure drop produced by the second cyclone adds to the overall pressure drop of the system and higher pressure can be a disadvantage in such series system design. Cyclone efficiency can also be improved if a portion of the flue gas is drawn through the hopper. An additional vane or lower pressure duct can provide this flow. However, it may then become necessary to recirculate or otherwise treat this as purge exhaust to remove uncollected particulate matter.

Figure 5.3: Typical series arrangement