The contents of this report cover the benefits of a gas distribution analysis in evaluating and improving electrostatic precipitator efficiency. Establishing uniform gas flow through an electrostatic precipitator is essential in maintaining its maximum operating efficiency. Losses in precipitator efficiency can account for increases in particulate emissions, and corrections to poorly distributed flow patterns can significantly reduce emissions. This report describes the testing procedure, the calculation of the results, the measures taken to improve flow, and the impact on precipitator efficiency. The gas distribution analysis conducted on June 29 through July 1, 1992 at Georgia Power Company, Plant Mitchell in Albany, Georgia is used to illustrate this strategy in reducing particulate emissions.

   Even though the flow regime is considered turbulent throughout the precipitator, electrostatic precipitators operate under the premise of relative uniform flow. The precipitator as a pollution control device is designed to treat the gases that flow through its plates within certain velocity criteria in every portion of the treatment cross section. If velocities exceed this criteria range in any one area, the precipitator will not have a chance to effectively treat the gas through that area. The result is an increase in particulate emissions.

   This report demonstrates the use of a gas distribution analysis to determine and correct the flow regime in an electrostatic precipitator. The example used is the test conducted on June 29 through July 1, 1992 at Georgia Power Company, Plant Mitchell in Albany, Georgia. The result of the work on the precipitator showed an increase to a total efficiency of 96%, which is considered high for a small precipitator.

Figure 1:    view of collection plates

   The effectiveness of the treatment is directly related to two factors: the charge induced drift velocity and the reduction of the velocity of the gas flowing through the precipitator box. The charge induced drift velocity is the speed perpendicular to the flow at which the particles are charged by the electrical field corona and attracted to the plates. The precipitator is responsible for charging and capturing the suspended particles before the gas traveling through the collection plates leaves the precipitator. If the gas velocity is not reduced enough for adequate capture time, the uncollected suspended particles are untreated and are collected as particulate emissions during compliance testing. The particles in the gas are charged by the wires and are drawn to the collection plates. Figure 2 illustrates the perpendicular relationship between the vector associated with the flow velocity (Vf) and the vector associated with the charge induced velocity (Vc).

Figure 2: Relationship between charge
induced velocity and flow velocity

   Reduction in gas velocity is accomplished by enlarging the cross section of the flow. The treatment velocity required may dictate that the precipitator box cross section be as much as ten to twenty times the size of the entrance duct cross section. Excessively large box sizes are undesirable; therefore box sizes are minimized by designing around minimum treatment retention time with the assumption of relative uniform gas flow through the precipitator.

   Unfortunately, the assumption of uniform gas flow is rarely taken into account in the ductwork immediately before and after the precipitator box. The box in all respects may meet design criteria, but the excessively turbulent non-uniform flow entering the box will frequently cause the box to fall below expectations.  It is not unusual for large boxes to perform at 98% efficiency when flow strata entering or leaving the box is optimal, but it is also not unusual for poorly placed ductwork to bring efficiencies down to the 60-70% range of performance.  This will efffectively raise particulate emissions to eight or nine times their desired limits.

   The IGCI standard dictates that the flow cannot exceed a 15% deviation for more than 15% of the points sampled, and a 40% deviation for more than 1% of the points. This generally allows for 3 to 4 points that exceed 40% and 45 to 60 points that can exceed 15%, depending on the number of points sampled.

   Another commonly used standard is the Root Mean Square Deviation (RMS) standard. It follows virtually the same lower criteria of the IGCI standard of 15% but uses the statistical root mean square deviation from the mean of the matrix of numbers collected. The IGCI standard counts the points that exceed the IGCI criteria, while the RMS deviation uses a more complex formula of statistical deviation from the mean. An RMS deviation of 15% is generally considered marginal flow strata in a precipitator box.

   Another more visible standard of judging the flow is the full face presentation of the flow variations. The IGCI standard is utilized, but graphically depicted in hot and cold colors on a contour map of the cross section. This lends itself to flow correction more than the other measurement standards and aids in the flow adjustment because flow variations can be directly correlated to duct geometry. In the example presented in this report, all of the above methods were used to determine the flow corrections needed for effective gas treatment.

Figure 3:    Test team member situated
over flow cross section

   Testing crews were positioned in the precipitator while the induction fans were given adequate time to balance the flow. Test traverses averaged forty minutes to an hour. After the data was collected, the teams returned the information for calculation of adjustments.

Figure 4:    Overhead view of precipitator box

   Initial adjustments to the inlet turning vanes were based on the analysis of the contour graphs of the baseline flow. The result was a significant improvement in the flow pattern. The second adjustment further balanced the flow, but the third adjustment cast the balance of the flow off by 0.4%. This was easily corrected after the fourth test results were compared to the vane adjustments. An over corrected turning vane was the source of the problem and was fixed in the final adjustment.

   The flow corrections were performed on a newly re-constructed precipitator. Our flow adjustments were the first tests conducted on the new design, and three weeks after the corrections were performed, an efficiency test was conducted on the precipitator. The precipitator tested at 96% efficiency, which is high for a small, two field precipitator. Time was taken to allow for particulate equilibrium in the ductwork and precipitator to get a true picture of operating efficiency. This was the baseline test for precipitator efficiency.

   In many precipitators, there is a distinct and recognizable need for flow uniformity. It is an essential operation and design criteria. Most precipitators are designed around the concept of uniformity of flow and are minimized in their precipitator box size based on this assumption. Given properly constructed ductwork that reduces turbulence before and after the precipitator box, most of the precipitators that are design for an application will perform as expected.

   Losses associated with poor efficiencies can be directly related to poor flow regimes. In addition, losses due to operating parameters such as rapping losses are increased with the amount of flow imbalance.- The presence of excessive flow velocity causes loss of treatment time, but it also causes losses in terms of reentrainment of captured particulate. Reentrainment is a term that denotes the recirculation of particulate that is captured by the electrostatic process by excessive turbulence. Particulate that is captured is pulled away from hoppers and plates and exits the precipitator box with the high velocity gas.

   The last page (Figure 7) illustrates gas distribution analysis conducted on units that have had particulate emissions problems. The correlation between poor flow regime and poor capture efficiency is obvious in these examples. CONCLUSION The effects of precipitator flow corrections coupled with a gas distribution analysis proved extremely beneficial in the case of the precipitator at Georgia Power Company, Plant Mitchell. Despite extreme inlet and exit duct geometry, the designed precipitator operating efficiency was realized and flow turbulence was cut in half.

Figure 5:    (above)  The graphs depict the improvements made to inlet turning vanes of a small, two field, steam plant precipitator.  The baseline test of the inlet cross section revealed excessive turbulence with an RMS deviation of 21.8%.  The first correction to the truning vanes reduced turbulence with a deviation of 18.8%.  The second correction brought the RMS deviation down to 12.3%, which is within the 15% criteria.  The third correction increased the deviation to 12.7%, but the final deviation decreased the deviation to 10.4%.

The end result was a decrease of 11.4% over a four day period.

   The flow corrections were performed on a newly re-constructed precipitator.  Our flow adjustments were the first tests conducted on the new design, and three weeks after the corrections were performed, an efficiency test was conducted on the precipitator.  The precipitator tested at 96% efficiency, which is high for a small, two filed precipitator.  Time was taken to allow for particulate equilibrium in the ductwork and precipitator to get a true picture of the operating efficiency.   This was the baseline test for precipitator efficiency.

   Losses associated with poor efficiencies can be directly related to poor flow regimes.  In addition, losses due to operating parameters such as rapping losses are increased with the amount of flow imbalance.  the presence of excess flow velocity causes loss of treatment time, but it also causes loss in terms of reentrainment of captured particulate.  Reentrainment is a term that denotes the recirculation of particulate that is captured by the electrostatic process by the excessive turbulence.   Particulate that is captured is pulled away from the hoppers and plates and exits the precipitator box with the high velocity gas.

   The last Figure (Figure 7) illustrates gas distribution analysis conducted on units that have had particulate emissions problems.  The correlation between poor flow regime and poor capture efficiency is obvious in these examples.