Electrostatic precipitators (ESPs) or electrostatic air cleaners are particulate collection devices which use electrostatic forces to separate particulate matter (PM) from exhaust gases. Electrostatic precipitators (ESP) are highly-efficient and can remove dust and smoke from the airstream by minimally interfering with the flow of gases through the device. The collection efficiency of electrostatic precipitators is largely dependent on the electrical properties of the particulate being collected.

ESP Operation

Electrostatic precipitators use electrostatic charges to separate particles from a dirty gas stream. High voltage, direct current electrodes are used to establish a strong electric field. This field (known as a corona) delivers a (usually) negative charge to particles as they pass through the device. This charge forces the particles onto the walls of collection plates or tubes. These collection surfaces (or collection electrodes) are then rapped, vibrated, or washed with water to dislodge the particles, which fall into a hopper to be disposed.

Two-stage ESP - Image Credit: BPA Air Quality Solutions LLC.

In summary, the major components of a precipitator include:

• Collection electrodes
• Discharge electrodes
• High voltage power supply
• Precipitator controls
• Rapping or spray washing systems
• Purge air systems

Efficiency losses occur in ESPs due to reentrainment, sneakage, and back corona. Reentrainment occurs due to rapping, causing a small percentage (10-15 percent) to be projected back into the gas stream. Sneakage is the small part of gas flow that moves around the charging zones untreated due to practical design constraints. Back corona occurs when an electric field becomes large enough to cause an electrical breakdown which reduces the charge on particles. 

ESP Design

Although the actual design of ESP systems is typically done by the manufacturer, a basic understanding of the design process is helpful for making an informed selection.

To begin, a manufacturer will ask for a number of process variables which describe the conditions and requirements of the system. These variables include:

• Gas flow rate - how fast the gas is moving through the system. At higher flow rates, particle reentrainment increases rapidly, but insufficient flow will result in poor gas distribution or particle dropout.
• Particle size and size distribution - the average size and size distribution of PM in the gas flow. Larger particles pickup charge more easily, while an abundance of small particles may suppress the generation of the corona.
• Particle resistivity - measure of the particles' resistance to electrical conductance. Particles with high resistivity have difficulty acquiring charge, while particles with low resistivity may lose their charge too easily and not stick to the collection plate. Resistivity is influenced by the particulate's chemistry and the gas temperature.
  • Gas temperature - the temperature of the gas flow in the system.
  • Particle chemistry - the chemical makeup of the particulate matter in the gas flow.

The manufacturer will then use these variables to construct the ESP. Design factors that determine an ESP's performance include:

• Precipitator size - the size of the precipitator affects its collection efficiency, footprint, and gas flow capacity. The sizing process is complex and often involves the use of computer models to aid in accounting for the numerous associated variables.
• Power input - the power supplied to the system to induce the electric field. Increasing power input improves collection efficiency under normal conditions.

Types

ESPs are classified based on a number of different factors, including the collector design, the number of stages, and whether the process is dry or wet.

Plate Precipitators

Plate ESPs primarily collect dry particles and are used more often than tubular precipitators. They can have wire-plate or flat-plate electrodes.

• Plate-Wire Precipitators

In a plate-wire ESP, gas flows between parallel plates of sheet metal and high-voltage long metal wires. It allows many flow lanes to operate in parallel, making it suitable for handling large volumes of gas.

Plate-wire precipitators are among the most common types of ESPs. In industry, they are used in cement kilns, incinerators, boilers, cracking units, sinter plants, furnaces, coke oven batteries, and a variety of other applications.

• Flat Plate Precipitators

Plate ESP - Image Credit: EPA.gov

Smaller precipitators use flat plates instead of wires for high-voltage electrodes. The flat plates increase the average electric field used to collect particles and provide additional surface area for particle collection. They are less susceptible to back corona than conventional plate-wire precipitators but also have higher rapping losses.

Flat plate ESPs can be used in applications with high-resistivity particles with small (1 to 2 µm) diameters. Fly ash can be captured using flat plate ESPs, but typically requires low velocities to prevent significant rapping losses.

Tubular Precipitators

Tubular ESPs consist of parallel arrangements of tubes with high-voltage electrodes running on their axis. The tubes may be arranged as a circular, square, or hexagonal honeycomb with gas flowing upwards or downwards. They are designed as one-stage units in which all the gas passes through the tube, eliminating sneakage. They are still susceptible to inefficiencies from corona non-uniformities.

Tubular precipitators are less common than plate types. They are used in applications involving wet or sticky particulate, and are typically cleaned with water for lower reentrainment losses than typical ESPs. They also can be tightly sealed to prevent leakage of material, an important consideration for valuable or hazardous substances. 

Tubular ESP - Image Credit: EPA.gov

Single-Stage Precipitators

Most industrial scale ESPs are single stage. They use very high voltages to charge particles and incorporate charging and collection together in the same stage. Sets of electrodes and collector surfaces (plates or tubes) operate in parallel to each other.

Two-Stage Precipitators

Two-stage ESPs operate in series rather than parallel configuration. Instead of using a side by side design, they incorporate separate particle charging and collection stages. This allows more time for particle charging, less susceptibility to back corona, and economical construction for smaller sizes.

Two-stage precipitators are separate and distinct from other ESPs, originally designed for air purification in conjunction with air conditioning systems. They are typically used for smaller, lower-volume applications. They are usually applied to submicron sources emitting oil mists, smokes, fumes, or other liquid aerosols. Many are sold as pre-engineered, package systems.

Two stage precipitator - Image Credit: EPA.gov

Selection Tip: Many small ESPs that do not have a self-cleaning mechanism are best used for ambient capture of light dusts and mists. Under heavy particulate loads, ESP collector plates will fill up much more quickly than fabric bags or other filter media because there is much less surface area. Heavy dust collection for commercial ESPs requires storage for large volumes of solids.

Dry ESPs

Dry electrostatic precipitators are used to capture particles in dry product streams. They use periodic rapping to separate the accumulated dust from the collector plates and discharge electrodes. The dust layer (released by rapping) is collected in a hopper and then removed by an ash handling system. Typically, rapping will also project some of these particles (around 10-15 percent) back into the gas stream (known as reentrainment). Dry electrostatic precipitators are often not suitable for submicron particulate applications because of particle size, resistivity, and other issues.

Wet ESPs (WESPs)

Wet electrostatic precipitators are used to strip wet (saturated) gas streams of particles. They use water sprays to condition/trap particles for collection and also to clean the particles off collection surfaces. WESPs collect particulate matter not suitable for dry ESPs, including sticky, moist, flammable, explosive, or high resistivity solids. WESPs can also remove very fine (submicron) particulate that dry ESPs cannot capture effectively. The use of water also gives these devices gas scrubbing capabilities. Most wet precipitators are tubular designs.

However, WESPs are more costly than dry ESPs. Because they incorporate water and corrosive gases, they must be designed from more expensive corrosion-resistant materials. Another disadvantage of WESPs is that the PM is collected as a slurry instead of a dry solid. This form is unsuitable for high value or recyclable materials and is more expensive to handle and dispose. If the water is being recycled and reused, the system also must incorporate a water purification step.

Performance Specifications

The most important performance specifications to consider when selecting an ESP are the airflow rating and the minimum particle size.

• Airflow or volumetric flow rate is the acceptable flow rate or range of flow rates of the gas stream through the ESP, measured in cubic feet per minute (cfm). It describes the acceptable flow rate(s) that the ESP is designed to support.
• Minimum particle size indicates the minimum diameter of particulate matter that the ESP is capable of capturing, measured in micrometers (µm). This rating effectively defines the range of capability of the precipitator.

Applications

ESPs may be specifically designed to meet the needs of certain industries or applications. Some applications and media types include:

• Abrasives - baghouse fabrics are designed to withstand and capture abrasive particles.
• Coolant and oil mists - unit is capable of filtering coolant smoke and mist from metal finishing and forming processes, and machining oil mists.
• Explosive media - unit is capable of filtering explosive dusts, mists, and/or fumes.
• Fine powders - unit is capable of filtering fine powders such as carbon black, talc, pigments, oxides, and plastic compounding dusts.
• Metalworking chips & fluids - unit can capture aerosols and fumes emitted by metalworking fluids, including oils, lubricants, and coolants.
• Toxic media - unit is capable of filtering toxic materials such as dust, mist, fume, or smoke from the air.
• Welding fumes - unit is designed specifically for the collection of welding fumes or dust; these may include flux recovery systems.

References

Infohouse - Electrostatic Precipitators (pdf)

Neundorfer - Electrostatic Precipitator KnowledgeBase

Image credit:

Losma, Inc.