Effects Of Operating Conditions On Dust Cake Behavior In Filters With High Surface To Volume Ratios
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Effects of Operating Conditions on Dust Cake Behavior in Filters with High Surface-to-volume Ratios
Generally, new filter elements with large S/V ratios typically have inlet passages with a small cross sectional area to reach a large filter surface area internal to the filter element. To prevent plugging of the inlet passages, or to prevent the filter passage inlet velocity from becoming dangerously high causing cake disturbance, the high S/V filter can only accommodate a very thin cake. The conventional backpulse cleaning technique may not be able to dislodge this thin cake. The multi-tier design using ceramic candles can tolerate a thick dust cake. The conventional back-pulse cleaning technique could then be used to remove the dust cake. However, due to the proximity of the neighboring tiers, the large-volume flow of the back-pulse mixing with a huge quantity of dislodged dust cake could potentially perturb the local flow distribution from its initially well-mixed uniform flow and dust concentration. As soon as one spot or patch of dust cake forms on the neighboring candle surface due to this redistributed nonuniform flow and dust concentration, patching dust redeposition will occur on the neighboring tiers, leading eventually to ash bridging. This is because the rigid ceramic candle, unlike the fabric filter, does not flex during back-pulse cleaning; therefore, it cannot tolerate the uneven flow distribution. The back-pulse flow universally seeks the least resistance path, resulting in an uneven or patchy cleaning. Ash bridging is known to cause ceramic candle breakage due to the pulse ratchet problem. To understand the back-pulse cleaning problem caused by the use of high S/V rigid filter elements or the implementation of a multi-tier design, a bench-scale, two-dimensional (2-D) cold flow filter test facility is being constructed, and is scheduled for shakedown and testing in July 1993.