Here’s a typical dilemma: The perforated plate provides laminar gas flow to the box. The diagram at the left shows ash buildup in the entry plenum, reducing the effective size and efficiency of the precipitator. During startup, ash is picked up, creating a brief excursion. It’s a nuisance for a plant crew to clean the duct at every maintenance outage.
Now look at the diagram at the right. In an attempt to eliminate ash accumulation, the bottom of the perforated plate has been cut off, creating a gap. This keeps the duct swept clean, but creates a zone of high-velocity gas just prior to entry to the hopper. As ash is rapped from the collector plates, it must pass through this zone and much of it is re-entrained into the gas stream rather than captured by the ash handling system. The modification also causes hopper sneakage and distorts gas flow through the box. It actually creates more problems than had originally existed. Some newer precipitators are designed with such a gap. One investigation revealed that a gas-flow model study did not test below that line. The solution is restoration of the perforated plate and an air horn or puff-blower system to clean the plenum at frequent intervals during operation.