Electrostatic Precipitation (ESP) Fundamentals

How Does ESP Work?

In the simplest terms, a precipitator is a device that utilizes electrostatic forces for the collection of particulate. The particulate laden gases enter one area of the vessel and exit another area clean. Inside, high voltage electrodes impart a charge to the particles. Most modern precipitators utilize negative voltage electrodes though positive voltage electrodes do exist. These charged particles are then attracted to a collecting surface (grounded, neutral or oppositely charged). The particles build up on the collection surfaces over time and are then periodically removed, typically at set intervals.

What are the Different Types of ESP’s?

The vast majority of Industrial Electrostatic Precipitators are of the dry type, though wet precipitators are gaining popularity. Dry Electrostatic Precipitators are often referred to as Dry ESP's or DESP's while the Wet Electrostatic Precipitators are referred to as Wet ESP's or WESP's. Regarding particulate removal, the surfaces in a Dry ESP are rapped or vibrated, and the tubes in a Wet ESP are flushed , causing the particles to separate from the collection surface. In a Dry ESP the particles are usually evacuated from the hopper by rotary screws or drag chain conveyors . In a Wet ESP the hoppers are designed to gravity drain or pump the flush water for designated treatment or disposal.

Wide Range of Collectible Particles

Wide Range of Collectible Particles

Precipitators can be used to collect many different kinds of particles with a wide range of physical properties. Some of the compounds that can be collected include:

  • Aluminum
  • Ammonium Sulfates
  • Arsenic
  • Asphalt
  • Bagasse Ash
  • Bentonite
  • Catalyst
  • Cement
  • Coal Ash
  • Coke
  • Copper
  • Diesel Smoke
  • Fluorspar
  • Fly Ash
  • Glass
  • Gold
  • Grease
  • Gypsum
  • Hydrochloric Acid
  • Hydrofluoric Acid
  • Incinerator Ash
  • Iron Oxide
  • Lead
  • Oil
  • Pigments
  • Plastic
  • Salt Cake
  • Smoke
  • Sulfuric Acid Mist
  • Tar
  • Wood Ash
  • Wood Acids
  • Wood Tar

Processes Employing Precipitators

Processes Employing Precipitators

Precipitators have been employed on many separate processes and pieces of process equipment such as:

  • Biofuel Production
  • Biomass to Energy
  • Ethanol Production
  • Food Processing
  • Glass Making
  • Oil Refineries
  • Paint Processing
  • Pellet Mills
  • Plastic Processing
  • Pulp and Paper
  • Semiconductors
  • Sugar Mills
  • Waste Disposal
  • Waste to Energy
  • Wood Products

Determining Size

Determining Size

The basic size of a precipitator can be determined through the Deutsch-Anderson Equation.

  • A = -( Q / w ) x [ ln ( 1 - Eff ) ]
  • Where:
    • A = collection area required in square feet.
    • w = Drift or Migration velocity of the particles to be collected.
    • Q = Gas Flow Rate in cubic feet per second.
    • Eff = Fractional collection efficiency desired.
    • Drift velocity or migration velocity can be specifically defined as:
    • w = dp x Eo x Ep / (4πµ)
  • Where:
    • dp =diameter of particle, microns
    • Eo =strength of field in which particles are charged, V/m (V/ft)
    • Ep = strength of field in which particles are collected, V/m (V/ft)
    • µ = Gas viscosity, Pa *s (cp)
    • π = 3.1415

Each type of particulate or process gas stream has a different migration or drift velocity associated with it. Once the process or particulate source is known a range of drift velocities can be selected. It is important to note that the drift velocities shown below are highly dependent upon not only the upstream process but the configuration and design of the precipitator as well. For example two identical processes each with their own precipitator made by a different manufacturer will often have different apparent drift velocities. This is because each manufacturer has a different physical design in terms of gas flow, electrode design and power supplies.

The calculator below can be used to determine the rough size of a precipitator.

Inlet Ducts and Nozzles

Inlet ducts and nozzles should be designed with air flow dynamics and particle fallout or sticking in mind. The duct work leading to the precipitator has to have sufficient velocity to keep the particles from falling out of suspension and accumulating on the bottom of the duct. When enough particles collect on surfaces it acts as insulation that can allow the surface of the metal to cool and condense water that can cause corrosion of the duct. When the velocity is too high the particles can wear away the duct at bend locations. Bends or turns in the ductwork can create unbalanced flow in that ductwork and upon entering the precipitator result in more flow in certain areas than others. Turning vanes should be used to reduce or eliminate non-uniform flow. The inlet nozzle itself should incorporate flow distribution devices or screens to equalize non-uniform flow and spread the gas stream across the cross sectional area of the precipitator. The increase in cross sectional area of the inlet nozzle compared to the inlet duct work will reduce the velocity of the gases dramatically allowing the larger suspended particles to fall out therefore, the inlet nozzle must be designed in a way that facilitates the removal of these particles.

Outlet Ducts and Nozzles

Many of the same design considerations for inlet ducts and nozzles apply to the outlet ducts and nozzles but in reverse order. Flow distribution devices in the outlet nozzle are often utilized to create uniformity of flow as the gases enter the outlet duct or stack. The outlet duct or stack is designed at velocities to eliminate corrosion or particle fallout. Regardless of whether the outlet nozzle is connected to a duct or stack if testing is to be performed then uniformity of flow is desired. Care should be taken, meaning turning vanes and flow uniformity devices should be utilized. In the case of stacks, velocity is vitally important because of rain and the fact that stacks are often not insulated. Two potential problems immediately come to mind if the velocity of the stack is insufficient. The first is that rain will be able to enter the stack and the velocity head of the flue gas will push that rain to the outer edges of the gas stream which would be the inside diameter of the stack. Most stacks on dry precipitators are constructed of carbon steel and the repetitive wetting of the stack will accelerate corrosion (rust). The second is that eddy currents can form along the side walls cooling the stack material that allows condensation to form with the same result.

Maintenance Required

The most important maintenance item in any precipitator are the insulators for if they fail then that field will cease to function. Many manufacturers utilize a purge air or heated air system to clean the insulators as clean as possible. Other areas of maintenance can include conveyors, ash removal systems and rotary seals.

Dry Electrostatic Precipitator ESP Inspection Checklist

Dry electrostatic precipitator ( ESP ) devices are employed on hot process exhausts(250 - 850 deg. F) that operate above the dew point of the gas stream. Dry electrostatic precipitator devices typically collect dry dust particles such as wood ash, incinerator ash, or coal ash from boiler or incinerator applications. Additional dry electrostatic precipitator applications include carbon anode ovens, cement kilns, and petroleum cat crackers. Dry electrostatic precipitator devices are attractive due to their ability to collect and transport the dust in a dry condition. This eliminates the use of water and the concerns of pollution, corrosion and dewatering efforts associated with scrubbers. If the dust particles can be collected and handled in a dry condition it is always more advantageous to employ a Dry ESP.

Wet electrostatic precipitator ( WESP ) devices are employed on exhausts that contain wet, sticky, tar like, tacky or oily particulates. Wet electrostatic precipitator ( WESP ) devices are an old technology originally designed in the 1920's to collect sulfuric acid mist using lead collection tubes. Today, WESP devices are employed on gas streams that include oily and sticky particulates or gas streams that must be cooled to saturation in order to condense aerosols that were formerly in the gas phase. Due to the different characteristics of the collected precipitate, the mechanical removal systems (rappers and vibrators) in Dry electrostatic precipitator devices are not effective. Consequently, the Wet electrostatic precipitator uses a water flushing system to remove the particles from the collecting surface. The gas stream is either saturated before entering the collection area or the collecting surface is continually wetted to prevent agglomerations from forming. Some mist aerosols simply gravity flow down the collecting surfaces. Wet electrostatic precipitator ( WESP ) devices are effective on acid mist, oil and tar based condensed aerosols or applications where dry dust particles combine with condensables to form paste like residues. Due to the wet environment of wet electrostatic precipitator devices, they are typically fabricated out of corrosion resistant materials such as stainless steel or special alloys.