- Electrostatic Precipitator Definition: An electrostatic precipitator is a device used in industries to remove particles from flue gases to control air pollution.
- Corona Power Ratio: Higher corona power ratios lead to higher efficiency in electrostatic precipitators by indicating more energy used per cubic foot of air filtered.
- Dust Resistivity: The electrical resistivity of dust affects the efficiency of collection, with normal resistivity being ideal for higher efficiency.
- Particle Size Impact: Larger particles are collected more efficiently than smaller ones in an electrostatic precipitator.
- ESP Efficiency Formula: The Deutsch-Anderson equation is used to calculate the efficiency of an electrostatic precipitator, considering terminal drift velocity, collection area, and volumetric air flow rate.
The efficiency of Electrostatic precipitators in a thermal power plant or other power plants is the fraction of inlet dust mass collected. It depends on corona power, dust resistivity, particle size and the Deutsch-Anderson terms w, A and Q. Plants set a design efficiency from the emission limit; it is not one worldwide legal number.
The following factors affect the efficiency of an electrostatic precipitator in service.
Corona Power Ratio
Corona power ratio is not the same as corona discharge. This ratio is the power consumed (in watts) divided by the airflow (in cubic feet per minute). It indicates the energy used to filter one cubic foot of air per minute. Raising corona power usually raises collection until spark-over. The image below shows how efficiency varies with the corona power ratio in that historical plot.
The Resistivity of Dust Collected
The efficiency of an electrostatic precipitator depends on its ability to collect dust from flue gases, which is influenced by the dust’s electrical resistivity. Particles with mid-range resistivity are collected well by electrostatic precipitators. Low-resistivity particles lose charge on the plates and bounce back into the gas, a process called re-entrainment. High-resistivity dust can cause back corona, which also cuts collection. Resistivity is therefore a process variable, not a single “normal” number for every ash.
The Particle Size
Collection also depends on aerosol size (dust, mist). Coarse particles are often collected more readily than the mid-size band around 0.1 to 1 μm. Very small particles can still collect well by diffusion charging, so smaller is not always worse.
The formula to calculate the efficiency
The Deutsch-Anderson equation gives the efficiency of an electrostatic precipitator, and the equation is as follows:
η = fractional collection efficiency
W = particle migration velocity in m/s
A = total collection area in m2
Q = volumetric air flow rate in m3/s
The derivation is omitted here. The useful reading is how w, A and Q move η.
Migration velocity w is the speed the charged particle moves toward the plate under the electric field, not the speed of a stone falling in still air. Total collection area here denotes the entire area of the collecting plates. Volumetric air flow rate is the volume of gas which passes per unit time. Using the above equation, we can find out the fractional collection efficiency of an electrostatic precipitator.
