In many situations, ESP operation is evaluated in terms of the opacity monitored by a
transmissometer (opacity monitor) on a real-time basis. Under optimum conditions the
ESP should be able to operate at some base-level opacity with a minimum of opacity spiking
from rapper reentrainment. A facility can have one or more monitors that indicate
opacity from various ESP outlet ducts and from the stack.
An opacity monitor compares the amount of light generated and transmitted by the instrument
on one side of the gas stream with the quantity measured by the receiver on the other
side of the gas stream. The difference, which is caused by absorption, reflection, refraction,
and light scattering by the particles in the gas stream, is the opacity of the gas stream.
Opacity is expressed as a percent from 0 to 100% and is a function of particle size, concentration,
and path length.
Most of the opacity monitors being installed today are double-pass monitors; that is, the
light beam is passed through the gas stream and reflected back across to a transceiver. This
arrangement is advantageous for several reasons:
1. Automatic checking of the zero and span of the monitor is possible when the process is operational.
2. The monitor is more sensitive to slight variations in opacity because the path length is longer.
3. The entire electronics package is located on one side of the stack as a transceiver.
Although single-pass transmissometers are available at a lower cost (and sensitivity), the
single-pass monitors cannot meet the requirements in EPA Performance Specification 1,
Appendix B, 40 CFR 60.
For many sources, dust concentration and opacity correlations can be developed to provide
a relative indication of ESP performance. These correlations are very site-specific,
but can provide plant and agency personnel with an indication of relative performance for
given opacity levels. In addition, site-specific opacity charts can be used to predict deterioration
of ESP performance that requires attention by plant personnel. Readings from
opacity monitors can also be used to optimize spark rate, voltage/current levels, and rapping
cycles, even though the conditions within the ESP are not static. In high-efficiency
ESPs, reentrainment may account for 50 to 70% of the total outlet emissions. Therefore,
optimization of the rapping pattern may prove more beneficial than trying to optimize the
voltage, current and sparking levels. Dust reentrainment from rapping must be observed
by using the opacity monitor operating in a real-time or nonintegrating mode because rapping
spikes tend to get smoothed out in integrated averages such as the 6-minute average
commonly in use. However, the integrated average does provide a good indication of average
opacity and emissions.
When parallel ESPs or chambers are used, an opacity monitor is often placed in each outlet
duct, as well as on the stack, to measure the opacity of the combined emissions.
Although the stack monitor is commonly used to indicate stack opacity (averaging opacities
from different ducts can be difficult), the individual duct monitors can be useful in
indicating the performance of each ESP or chamber and in troubleshooting. Although this
option is often not required and represents an additional expense, it can be very useful,
particularly on relatively large ESPs.
New systems, such as the digital microprocessor design, are available in which the opacity
monitor data can be used as input for the T-R controller. In this case, the data are used to
control power input throughout the ESP to maintain an opacity level preselected by the
source. If the opacity increases, the controller increases power input accordingly until the
opacity limit, spark limit, current limit, or voltage limit is reached. This system (often sold
as an energy saver because it uses only the power required) can save a substantial quantity
of energy:
1. On large, high-efficiency ESPs
2. For processes operating at reduced gas loads.
In many cases, however, reduction of ESP power does not significantly alter ESP performance
because dust reentrainment and gas sneakage constitute the largest sources of emissions;
additional power often does not reduce these emissions significantly. In some
observed cases, reducing power by one-half did not change the performance. For units
typically operated at 1000 to 1500 watts/1000 acfm, operating the ESPs at power levels of
500 to 750 watts/1000 afcm still provide acceptable collection efficiencies.
transmissometer (opacity monitor) on a real-time basis. Under optimum conditions the
ESP should be able to operate at some base-level opacity with a minimum of opacity spiking
from rapper reentrainment. A facility can have one or more monitors that indicate
opacity from various ESP outlet ducts and from the stack.
An opacity monitor compares the amount of light generated and transmitted by the instrument
on one side of the gas stream with the quantity measured by the receiver on the other
side of the gas stream. The difference, which is caused by absorption, reflection, refraction,
and light scattering by the particles in the gas stream, is the opacity of the gas stream.
Opacity is expressed as a percent from 0 to 100% and is a function of particle size, concentration,
and path length.
Most of the opacity monitors being installed today are double-pass monitors; that is, the
light beam is passed through the gas stream and reflected back across to a transceiver. This
arrangement is advantageous for several reasons:
1. Automatic checking of the zero and span of the monitor is possible when the process is operational.
2. The monitor is more sensitive to slight variations in opacity because the path length is longer.
3. The entire electronics package is located on one side of the stack as a transceiver.
Although single-pass transmissometers are available at a lower cost (and sensitivity), the
single-pass monitors cannot meet the requirements in EPA Performance Specification 1,
Appendix B, 40 CFR 60.
For many sources, dust concentration and opacity correlations can be developed to provide
a relative indication of ESP performance. These correlations are very site-specific,
but can provide plant and agency personnel with an indication of relative performance for
given opacity levels. In addition, site-specific opacity charts can be used to predict deterioration
of ESP performance that requires attention by plant personnel. Readings from
opacity monitors can also be used to optimize spark rate, voltage/current levels, and rapping
cycles, even though the conditions within the ESP are not static. In high-efficiency
ESPs, reentrainment may account for 50 to 70% of the total outlet emissions. Therefore,
optimization of the rapping pattern may prove more beneficial than trying to optimize the
voltage, current and sparking levels. Dust reentrainment from rapping must be observed
by using the opacity monitor operating in a real-time or nonintegrating mode because rapping
spikes tend to get smoothed out in integrated averages such as the 6-minute average
commonly in use. However, the integrated average does provide a good indication of average
opacity and emissions.
When parallel ESPs or chambers are used, an opacity monitor is often placed in each outlet
duct, as well as on the stack, to measure the opacity of the combined emissions.
Although the stack monitor is commonly used to indicate stack opacity (averaging opacities
from different ducts can be difficult), the individual duct monitors can be useful in
indicating the performance of each ESP or chamber and in troubleshooting. Although this
option is often not required and represents an additional expense, it can be very useful,
particularly on relatively large ESPs.
New systems, such as the digital microprocessor design, are available in which the opacity
monitor data can be used as input for the T-R controller. In this case, the data are used to
control power input throughout the ESP to maintain an opacity level preselected by the
source. If the opacity increases, the controller increases power input accordingly until the
opacity limit, spark limit, current limit, or voltage limit is reached. This system (often sold
as an energy saver because it uses only the power required) can save a substantial quantity
of energy:
1. On large, high-efficiency ESPs
2. For processes operating at reduced gas loads.
In many cases, however, reduction of ESP power does not significantly alter ESP performance
because dust reentrainment and gas sneakage constitute the largest sources of emissions;
additional power often does not reduce these emissions significantly. In some
observed cases, reducing power by one-half did not change the performance. For units
typically operated at 1000 to 1500 watts/1000 acfm, operating the ESPs at power levels of
500 to 750 watts/1000 afcm still provide acceptable collection efficiencies.
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