Gas flow rate determines most of the key design and operating parameters such as specific
collection area (ft2/1000 acfm), gas velocity (ft/sec) and treatment time within the
ESP, and specific corona power (watts/1000 acfm). The operator should calculate the flue
gas flow rate if the ESP is not operating efficiently. For example, significant variations in
oxygen may indicate large swings in the gas flow rate that may decrease ESP performance
and indicate the need to routinely determine ESP gas volume. Low SCA values, high
velocities, short gas treatment times (5 seconds or less), and much higher oxygen levels at
nearly full load conditions are indicators that excess flue gas flow rate may be causing
decreased ESP performance.
Presently, most sources do not continuously measure gas velocities or flow rates. Gas
velocities are generally only measured during emission compliance testing or when there
is a perceived problem. Manual pitot tube traverses are normally used to measure gas
velocity (EPA Reference Methods 1 and 2). Because of new technologies and regulations,
some of the larger sources are beginning to install continuous flow measurement systems.
Multi-point pitot devices, ultrasonic devices, and temperature-based flow devices can be
used to continuously measure gas velocity. These devices must be calibrated to the individual
stack where they are installed. Most existing facilities currently use indirect indicators
to estimate gas flow rate; these include fan operating parameters, production rates or
oxygen/carbon dioxide gas concentration levels. However, EPA is now requiring large
coal-fired utility boilers to install and certify flow monitors (EPA Acid Rain Program, Part
75 Regulations).
Another important parameter is gas flow distribution through the ESP. Ideally, the gas
flow should be uniformly distributed throughout the ESP (top to bottom, side to side).
Actually, however, gas flow through the ESP is not evenly distributed, and ESP manufacturers
settle for what they consider an acceptable variation. Standards recommended by the Industrial Gas Cleaning Institute have been set for gas flow distribution. Based on a
velocity sampling routine, 85% of the points should be within 15% of the average velocity
and 99% should be within 1.4 times the average velocity. Generally, uneven gas flow
through the ESP results in reduced performance because the reduction in collection efficiency
in areas of high gas flow is not compensated for by the improved performance in
areas of lower flow. Also, improper gas distribution can also affect gas sneakage through
the ESP. As stated earlier, good gas distribution can be accomplished by using perforated
plates in the inlet plenum and turning vanes in the ductwork.
Measurement of gas flow distribution through the ESP is even less common than measuring
flue gas flow rate. Because the flow measurements are obtained in the ESP rather than
the ductwork (where total gas volumetric flow rates are usually measured), more sensitive
instrumentation is needed for measuring the low gas velocities. The instrument typically
specified is a calibrated hot-wire anemometer. The anemometer test is usually performed
at some mid-point between the inlet and outlet (usually between two fields). Care must be
taken to assure that internal ESP structural members do not interfere with the sampling
points.
Gas flow distribution tests are conducted when the process is inoperative, and the ESP and
ductwork are relatively cool. This often limits the amount of gas volume that can be drawn
through the ESP to less than 50% of the normal operating flow; however, the relative
velocities at each point are assumed to remain the same throughout the normal operating
range of the ESP. A large number of points are sampled by this technique. The actual number
depends upon the ESP design, but 200 to 500 individual readings per ESP are not
unusual. By using a good sampling protocol, any severe variations should become readily
apparent
collection area (ft2/1000 acfm), gas velocity (ft/sec) and treatment time within the
ESP, and specific corona power (watts/1000 acfm). The operator should calculate the flue
gas flow rate if the ESP is not operating efficiently. For example, significant variations in
oxygen may indicate large swings in the gas flow rate that may decrease ESP performance
and indicate the need to routinely determine ESP gas volume. Low SCA values, high
velocities, short gas treatment times (5 seconds or less), and much higher oxygen levels at
nearly full load conditions are indicators that excess flue gas flow rate may be causing
decreased ESP performance.
Presently, most sources do not continuously measure gas velocities or flow rates. Gas
velocities are generally only measured during emission compliance testing or when there
is a perceived problem. Manual pitot tube traverses are normally used to measure gas
velocity (EPA Reference Methods 1 and 2). Because of new technologies and regulations,
some of the larger sources are beginning to install continuous flow measurement systems.
Multi-point pitot devices, ultrasonic devices, and temperature-based flow devices can be
used to continuously measure gas velocity. These devices must be calibrated to the individual
stack where they are installed. Most existing facilities currently use indirect indicators
to estimate gas flow rate; these include fan operating parameters, production rates or
oxygen/carbon dioxide gas concentration levels. However, EPA is now requiring large
coal-fired utility boilers to install and certify flow monitors (EPA Acid Rain Program, Part
75 Regulations).
Another important parameter is gas flow distribution through the ESP. Ideally, the gas
flow should be uniformly distributed throughout the ESP (top to bottom, side to side).
Actually, however, gas flow through the ESP is not evenly distributed, and ESP manufacturers
settle for what they consider an acceptable variation. Standards recommended by the Industrial Gas Cleaning Institute have been set for gas flow distribution. Based on a
velocity sampling routine, 85% of the points should be within 15% of the average velocity
and 99% should be within 1.4 times the average velocity. Generally, uneven gas flow
through the ESP results in reduced performance because the reduction in collection efficiency
in areas of high gas flow is not compensated for by the improved performance in
areas of lower flow. Also, improper gas distribution can also affect gas sneakage through
the ESP. As stated earlier, good gas distribution can be accomplished by using perforated
plates in the inlet plenum and turning vanes in the ductwork.
Measurement of gas flow distribution through the ESP is even less common than measuring
flue gas flow rate. Because the flow measurements are obtained in the ESP rather than
the ductwork (where total gas volumetric flow rates are usually measured), more sensitive
instrumentation is needed for measuring the low gas velocities. The instrument typically
specified is a calibrated hot-wire anemometer. The anemometer test is usually performed
at some mid-point between the inlet and outlet (usually between two fields). Care must be
taken to assure that internal ESP structural members do not interfere with the sampling
points.
Gas flow distribution tests are conducted when the process is inoperative, and the ESP and
ductwork are relatively cool. This often limits the amount of gas volume that can be drawn
through the ESP to less than 50% of the normal operating flow; however, the relative
velocities at each point are assumed to remain the same throughout the normal operating
range of the ESP. A large number of points are sampled by this technique. The actual number
depends upon the ESP design, but 200 to 500 individual readings per ESP are not
unusual. By using a good sampling protocol, any severe variations should become readily
apparent
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