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Dry ESP's are employed on hot process exhausts (250 - 850
deg. F) that operate above the dew point of the gas stream.
They typically collect dry dust particles such as wood ash, incinerator ash,
or coal ash from boiler or incinerator applications. Additional applications
include carbon anode ovens, cement kilns, and petroleum cat crackers. Dry
units are attractive due to their ability to collect and transport the dust
in a dry condition. This eliminates the use of water and the concerns of pollution,
corrosion and dewatering efforts associated with scrubbers. If the dust particles
can be collected and handled in a dry condition it is always more advantageous
to employ a Dry ESP.
Wet ESP's are employed on exhausts that contain wet, sticky, tarry,
tacky or oily particulates.
Wet ESP's are an old technology originally designed in the 1920's to collect
sulfuric acid mist using lead collection tubes. Today, they are employed
on gas streams that include oily and sticky particulates or gas streams that
must be cooled to saturation in order to condense aerosols that were formerly
in the gas phase. Due to thedifferent characteristics of the collected precipitate,
the mechanical removal systems (rappers and vibrators) in a Dry ESP are not
effective. Consequently, the Wet ESP uses a water flushing system to remove
the particles from the collecting surface. The gas stream is either saturated
before entering the collection area or the collecting surface is continually
wetted to prevent agglomerations from forming. Some mist aerosols simply
gravity flow down the collecting surfaces. Wet ESP's are effective on acid
mist, oil and tar based condensed aerosols, or applications where dry dust
particles combine with condensables to form paste like residues. Due to the
wet environment, they are typically fabricated out of corrosion resistant
materials such as stainless steel or special alloys.
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