Electrostatic Precipitation (ESP) Factors
PARTICULATE & OPACITY CONTROL
Factors in ESP
There are many factors that can affect the correct sizing of a precipitator.
The gas stream moisture content affects the flow of electrons and the subsequent collection of particulates in that gas stream. The graph to the right shows the effects of moisture upon a gas stream containing cement dust. Normal dust resistivity ranges between 107 ohm-cm and 1010 ohm-cm. Precipitators whose gas streams exhibit high dust resistivity can improve their collection efficiency by adjusting temperature or introducing additional water through evaporation or steam addition or by adding other gas stream conditioning agents.
Temperature alone can affect the resistivity of the dust particles. The graph below shows how resistivity changes as temperature changes for a variety of dusts. Precipitators whose gas streams exhibit high resistivity can improve their collection efficiency by adjusting temperature or introducing additional water through evaporation or steam addition or by adding other gas stream conditioning agents.
The gas stream velocity through the precipitator can affect performance in three fundamental ways.
- The gas velocity can be so fast that the electrostatic attraction forces cannot overcome the velocity head or turbulence created by the flow of the gas stream
- High gas velocity can strip collected particles off of the collecting surfaces, reducing overall collection efficiency.
- Increased gas velocity means decreased residence time resulting in the particles spending less time in the active collection areas.
Large amounts of particulate can create excessive sparking inside the precipitator. Sparking is a normal phenomena where the current of the high voltage areas rushes to a grounded surface. Normal spark rates are less than 100 sparks per minute. Examining the equation for drift velocity presented above one can see that the charging voltage has a direct impact upon the calculation of w. The graph from "An Electrostatic Precipitator Performance Model (EPA-650/2-74-132)” shows that the higher the grain loading, the higher the reduction in charge intensity.
The larger the particle the more readily it can be collected in a precipitator. The graphs below provided in the EPA publication EPA-600/8-77-020b dated January 1978 shows the relationship between the particle diameter and collection efficiency as well as particle size and migration velocity.
The equation presented above (w=dpEoEp/(4πµ) gives an indication how the collecting and discharge voltages affect drift velocity; the higher the collecting and discharge voltage the higher the value of migration (drift) velocity will be. Examining the Deutsch-Anderson Equation, also presented above, ( A= -[ Q / w ] x [ ln ( 1 - Eff ) ] reveals that as "w" (the denominator) increases in size the required collection plate area decreases. Simply put, for a given efficiency, as voltage increases the collection plate area required decreases. Therefore, for a given fixed amount of collection plate area, as voltage increases the efficiency increases.
The potential voltage that can be generated is based upon two primary limitations 1) the physical capabilities of the transformer and 2) the dielectric value of the gap between the high voltage electrode and collection electrode. All things remaining the same, the larger the gap the larger the dielectric value and the greater the potential voltage; larger gap = higher potential voltage.
Years ago, Japan had changed the particulate emission requirements for power plants with the result that many of their existing precipitators would have to be replaced or if room permitted added on to. Research and experimentation proved that the internal components could be removed and replaced with those with a larger gap between the high voltage electrodes and the collection plates while at the same time they installed transformers that could generate much higher voltages. The increasing of the gap resulted in less actual collection plate area because the external shell of the precipitators was a fixed sized. The result was an increase in efficiency with less collection plate area because of the increase migration velocity.
The Environmental Protection Agency (EPA) has recognized this feature, that voltage directly impacts collection efficiency, and has established criteria for the monitoring the power of consumed by precipitators as an indicator of performance. See EPA excerpt table.
The physical design of the precipitator can impact collection efficiencies in three primary ways: Airflow, Voltage Potential, and Corona Generation.
1) Airflow, as already discussed, can be too fast through the unit causing the re-entrainment of particles or the prevention of collection due to velocity head. Improper inlet duct design and improper air distribution can create situations where portions of the precipitator are receiving more flow per square foot of cross sectional area than others leading to reduces residence time, increased velocity head and re-entrainment in those areas with the net overall effect of the precipitator being less efficient.
2) Voltage potential, efficiencies as shown in equations presented above, has a direct impact upon collection. The physical design of the precipitator, specifically the clear space distance between the high voltage components and the grounded (or collection) surfaces dictate the maximum achievable voltage. When voltage reaches a point where its potential is greater than that of the resistance created by the distance between high voltage and collection surface the system will discharge this potential rapidly creating a spark or arc. Take for instance a high voltage electrode that is separated from ground by one quarter of an inch, the voltage required to spark over (jump the gap) is much less than if that gap was five inches. The closer the gap the higher the current and the greater the gap the higher the voltage. Two extremes of this concept can be visualized by connecting the high voltage electrode directly to ground, a short circuit and separating the high voltage from ground by ten feet, an open circuit. In a short circuit there is no voltage between the points and very high current, in the open circuit there is full voltage but no current. Precipitators operate at point between these two extremes, at a point where there is high voltage, enough to generate corona, and relatively low current.
3) Corona generation, also called ion generation, is directly dependent upon the high voltage electrode (discharge electrode) design. Sharp points and fine wires generate corona at much lower voltages than rounded or smooth corners. The collection of efficiency and current consumption rapidly increases once corona starts to occur. Corona can be seen as a visible blue or violet glow around the generation points.