Unusually fine particles present a problem under the following circumstances:
1. When the ESP is not designed to handle them
2. When a process change or modification shifts the particle size distribution into the
range where ESP performance is poorest.
A shift in particle size distribution tends to alter electrical characteristics and increase the
number of particles emitted in the light-scattering size ranges (opacity).
There are two principal charging mechanisms: field charging and
diffusion charging. Although field charging tends to dominate in the ESP and acts on particles
greater than 1 micrometer in diameter, it cannot charge and capture smaller particles.
Diffusion charging, on the other hand, works well for particles smaller than 0.1 micrometer
in diameter. ESP performance diminishes for particulates in the range of 0.2 - 0.9
micrometer because neither charging mechanism is very effective for particles in this
range. These particles are more difficult to charge and once charged, they are easily
bumped around by the gas stream, making them difficult to collect. Depending upon the
type of source being controlled, the collection efficiency of an ESP can drop from as high
as 99.9% on particles sized above 1.0 micrometer or below 0.1 micrometer, to only 85 to
90% on particles in the 0.2 - 0.9 micrometer diameter range. If a significant quantity of
particles fall into this size range, the ESP design must be altered to accommodate the fine
particles.
When heavy loadings of fine particles enter the ESP, two significant electrical effects can
occur: space charge and corona quenching. At moderate resistivities, the space-charge
effects normally occur in the inlet or perhaps the second field of ESPs. Because it takes a longer time to charge fine particles and to force them to migrate to the plate, a cloud of
negatively charged particles forms in the gas stream. This cloud of charged particles is
called a space charge. It interferes with the corona generation process and impedes the
flow of negatively charged gas ions from the wire to the collection plate. The interference
of the space charge with corona generation is called corona quenching.When this occurs,
the T-R controller responds by increasing the operating voltage to maintain current flow
and corona generation. The increase in voltage usually causes increased spark rates, which
may in turn signal the controller to reduce the voltage and current in an attempt to maintain
a reasonable spark rate. Under moderate resistivity conditions, the fine dust particles
are usually collected by the time they reach the third field of the ESP which explains the
disappearance of the space charge in these later fields. The T-R controller responds to the
cleaner gas in these later fields by decreasing the voltage level, but the current levels will
increase markedly. When quantities of fine particles being processed by the ESP increase,
the space charging effect may progress further into the ESP.
1. When the ESP is not designed to handle them
2. When a process change or modification shifts the particle size distribution into the
range where ESP performance is poorest.
A shift in particle size distribution tends to alter electrical characteristics and increase the
number of particles emitted in the light-scattering size ranges (opacity).
There are two principal charging mechanisms: field charging and
diffusion charging. Although field charging tends to dominate in the ESP and acts on particles
greater than 1 micrometer in diameter, it cannot charge and capture smaller particles.
Diffusion charging, on the other hand, works well for particles smaller than 0.1 micrometer
in diameter. ESP performance diminishes for particulates in the range of 0.2 - 0.9
micrometer because neither charging mechanism is very effective for particles in this
range. These particles are more difficult to charge and once charged, they are easily
bumped around by the gas stream, making them difficult to collect. Depending upon the
type of source being controlled, the collection efficiency of an ESP can drop from as high
as 99.9% on particles sized above 1.0 micrometer or below 0.1 micrometer, to only 85 to
90% on particles in the 0.2 - 0.9 micrometer diameter range. If a significant quantity of
particles fall into this size range, the ESP design must be altered to accommodate the fine
particles.
When heavy loadings of fine particles enter the ESP, two significant electrical effects can
occur: space charge and corona quenching. At moderate resistivities, the space-charge
effects normally occur in the inlet or perhaps the second field of ESPs. Because it takes a longer time to charge fine particles and to force them to migrate to the plate, a cloud of
negatively charged particles forms in the gas stream. This cloud of charged particles is
called a space charge. It interferes with the corona generation process and impedes the
flow of negatively charged gas ions from the wire to the collection plate. The interference
of the space charge with corona generation is called corona quenching.When this occurs,
the T-R controller responds by increasing the operating voltage to maintain current flow
and corona generation. The increase in voltage usually causes increased spark rates, which
may in turn signal the controller to reduce the voltage and current in an attempt to maintain
a reasonable spark rate. Under moderate resistivity conditions, the fine dust particles
are usually collected by the time they reach the third field of the ESP which explains the
disappearance of the space charge in these later fields. The T-R controller responds to the
cleaner gas in these later fields by decreasing the voltage level, but the current levels will
increase markedly. When quantities of fine particles being processed by the ESP increase,
the space charging effect may progress further into the ESP.
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