Electrostatic precipitator

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Not to be confused with Electrofiltration.

An electrostatic precipitator (ESP) is a filterless

device that removes fine particles, such as dust and
smoke, from a flowing gas using the force of an
induced electrostatic charge minimally impeding the
flow of gases through the unit.[1]

In contrast to wet scrubbers, which apply energy

directly to the flowing fluid medium, an ESP applies
energy only to the particulate matter being collected
and therefore is very efficient in its consumption of Electrodes inside electrostatic
precipitator
energy (in the form of electricity).

Collection electrode of an
electrostatic precipitator in a waste
incineration plant

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 Insulator assembly with housing
and high voltage bus removed for
maintenance and inspection.
Insulators are typically used to hold
up the electrode fields between the
grounded collection plates.

Invention
The first use of corona discharge to remove particles from an aerosol was by Hohlfeld in
1824.[2] However, it was not commercialized until almost a century later.

In 1907 Frederick Gardner Cottrell, a professor of chemistry at the University of

California, Berkeley, applied for a patent on a device for charging particles and then
collecting them through electrostatic attraction—the first electrostatic precipitator. Cottrell
first applied the device to the collection of sulphuric acid mist and lead oxide fumes
emitted from various acid-making and smelting activities.[3] Wine-producing vineyards in
northern California were being adversely affected by the lead emissions.

At the time of Cottrell's invention, the theoretical basis for operation was not understood.
The operational theory was developed later in Germany, with the work of Walter Deutsch
and the formation of the Lurgi company.[4]

Cottrell used proceeds from his invention to fund scientific research through the creation
of a foundation called Research Corporation in 1912, to which he assigned the patents.
The intent of the organization was to bring inventions made by educators (such as
Cottrell) into the commercial world for the benefit of society at large. The operation of
Research Corporation is funded by royalties paid by commercial firms after
commercialization occurs. Research Corporation has provided vital funding to many
scientific projects: Goddard's rocketry experiments, Lawrence's cyclotron, production
methods for vitamins A and B1, among many others.

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Research Corporation set territories for manufacturers of this technology, which included
Western Precipitation (Los Angeles), Lodge-Cottrell (England), Lurgi Apparatebau-
Gesellschaft (Germany), and Japanese Cottrell Corp. (Japan), and was a clearinghouse
for any process improvements. However, anti-trust concerns forced Research
Corporation to eliminate territory restrictions in 1946.[5]

Electrophoresis is the term used for migration of gas-suspended charged particles in a

direct-current electrostatic field. Traditional CRT television sets tend to accumulate dust
on the screen because of this phenomenon (a CRT is a direct-current machine operating
at about 15 kilovolts).

Types
There are two main types of precipitators:

High-voltage, single-stage - Single-stage precipitators combine an ionization and a

collection step. They are commonly referred to as Cottrell precipitators.
Low-voltage, two-stage - Two-stage precipitators use a similar principle; however,
the ionizing section is followed by collection plates.

Described below is the high-voltage, single-stage precipitator, which is widely used in

minerals processing operations. The low-voltage, two-stage precipitator is generally used
for filtration in air-conditioning systems.

Plate and bar

The majority of electrostatic precipitators installed are the plate type. Particles are
collected on flat, parallel surfaces that are 8 to 12 in. (20 to 30 cm) apart, with a series of
discharge electrodes spaced along the centerline of two adjacent plates. The
contaminated gases pass through the passage between the plates, and the particles
become charged and adhere to the collection plates. Collected particles are usually
removed by rapping the plates and deposited in bins or hoppers at the base of the
precipitator.

The most basic precipitator contains a row of thin

vertical wires, and followed by a stack of large flat
metal plates oriented vertically, with the plates typically
spaced about 1 cm to 18 cm apart, depending on the
application. The air stream flows horizontally through
the spaces between the wires, and then passes
through the stack of plates.

A negative voltage of several thousand volts is applied

between wire and plate. If the applied voltage is high Conceptual diagram of a plate and
bar electrostatic precipitator
enough, an electric corona discharge ionizes the air
around the electrodes, which then ionizes the particles
in the air stream.

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The ionized particles, due to the electrostatic force, are diverted towards the grounded
plates. Particles build up on the collection plates and are removed from the air stream.

A two-stage design (separate charging section ahead of the collecting section) has the
benefit of minimizing ozone production,[6] which would adversely affect the health of
personnel working in enclosed spaces. For shipboard engine rooms where gearboxes
generate an oil mist, two-stage ESP's are used to clean the air, improving the operating
environment and preventing buildup of flammable oil fog accumulations. Collected oil is
returned to the gear lubricating system.

Tubular
Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes
located on the axis of the cylinder. The contaminated gases flow around the discharge
electrode and up through the inside of the cylinders. The charged particles are collected
on the grounded walls of the cylinder. The collected dust is removed from the bottom of
the cylinder.

Tubular precipitators are often used for mist or fog collection or for adhesive, sticky,
radioactive, or extremely toxic materials.

Components
The four main components of all electrostatic precipitators are:

Power supply unit, to provide high-voltage DC power

Ionizing section, to impart a charge to particulates in the gas stream
A means of removing the collected particulates
A housing to enclose the precipitator zone

The collected material on the electrodes is removed by rapping or vibrating the collecting
electrodes either continuously or at a predetermined interval. Cleaning a precipitator can
usually be done without interrupting the airflow.

Collection efficiency (R)

The following factors affect the efficiency of electrostatic precipitators:

Larger collection-surface areas and lower gas-flow rates increase efficiency

because of the increased time available for electrical activity to treat the dust
particles.
An increase in the dust-particle migration velocity to the collecting electrodes
increases efficiency. The migration velocity can be increased by:
Decreasing the gas viscosity
Increasing the gas temperature
Increasing the voltage field

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Precipitator performance is very sensitive to two particulate properties: 1) electrical
resistivity; and 2) particle size distribution. These properties can be measured
economically and accurately in the laboratory, using standard tests. Resistivity can be
determined as a function of temperature in accordance with IEEE Standard 548. This test
is conducted in an air environment containing a specified moisture concentration. The test
is run as a function of ascending or descending temperature, or both. Data is acquired
using an average ash layer electric field of 4 kV/cm. Since relatively low applied voltage is
used and no sulfuric acid vapor is present in the test environment, the values obtained
indicate the maximum ash resistivity.

In an ESP, where particle charging and discharging are key functions, resistivity is an
important factor that significantly affects collection efficiency. While resistivity is an
important phenomenon in the inter-electrode region where most particle charging takes
place, it has a particularly important effect on the dust layer at the collection electrode
where discharging occurs. Particles that exhibit high resistivity are difficult to charge. But
once charged, they do not readily give up their acquired charge on arrival at the collection
electrode. On the other hand, particles with low resistivity easily become charged and
readily release their charge to the grounded collection plate. Both extremes in resistivity
impede the efficient functioning of ESPs. ESPs work best under normal resistivity
conditions.

Resistivity, which is a characteristic of particles in an electric field, is a measure of a

particle's resistance to transferring charge (both accepting and giving up charges).
Resistivity is a function of a particle's chemical composition as well as flue gas operating
conditions such as temperature and moisture. Particles can have high, moderate
(normal), or low resistivity.

Bulk resistivity is defined using a more general version of Ohm’s Law, as given in
Equation (1) below:

(1)

Where:
E is the Electric field strength.Unit:-(V/cm);
j is the Current density.Unit:-(A/cm2); and
ρ is the Resistivity.Unit:-(Ohm-cm)

A better way of displaying this would be to solve for resistivity as a function of applied
voltage and current, as given in Equation (2) below:

(2)

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 Where:
ρ = Resistivity.Unit:-(Ohm-cm)
V = The applied DC potential.Unit:-(Volts);
I = The measured current.Unit:-(Amperes);
l = The ash layer thickness.Unit:-(cm); and
A = The current measuring electrode face area.Unit:-(cm2).

Resistivity is the electrical resistance of a dust sample 1.0 cm2 in cross-sectional area,
1.0 cm thick, and is recorded in units of ohm-cm. A method for measuring resistivity will
be described in this article. The table below, gives value ranges for low, normal, and high
resistivity.

Resistivity Range of Measurement

Low between 104 and 107 ohm-cm

Normal between 107 and 2 × 1010 ohm-cm

High above 2 × 1010 ohm-cm

Dust layer resistance

Resistance affects electrical conditions in the dust layer by a potential electric field
(voltage drop) being formed across the layer as negatively charged particles arrive at its
surface and leak their electrical charges to the collection plate. At the metal surface of the
electrically grounded collection plate, the voltage is zero, whereas at the outer surface of
the dust layer, where new particles and ions are arriving, the electrostatic voltage caused
by the gas ions can be quite high. The strength of this electric field depends on the
resistance and thickness of the dust layer.

In high-resistance dust layers, the dust is not sufficiently conductive, so electrical charges
have difficulty moving through the dust layer. Consequently, electrical charges
accumulate on and beneath the dust layer surface, creating a strong electric field.

Voltages can be greater than 10,000 volts. Dust particles with high resistance are held too
strongly to the plate, making them difficult to remove and causing trapping problems.

In low resistance dust layers, the corona current is readily passed to the grounded
collection electrode. Therefore, a relatively weak electric field, of several thousand volts,
is maintained across the dust layer. Collected dust particles with low resistance do not
adhere strongly enough to the collection plate. They are easily dislodged and become
retained in the gas stream.

The electrical conductivity of a bulk layer of particles depends on both surface and
volume factors. Volume conduction, or the motions of electrical charges through the
interiors of particles, depends mainly on the composition and temperature of the particles.
In the higher temperature regions, above 500 °F (260 °C), volume conduction controls the
conduction mechanism. Volume conduction also involves ancillary factors, such as
compression of the particle layer, particle size and shape, and surface properties.

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Volume conduction is represented in the figures as a straight-line at temperatures above
500 °F (260 °C). At temperatures below about 450 °F (230 °C), electrical charges begin to
flow across surface moisture and chemical films adsorbed onto the particles. Surface
conduction begins to lower the resistivity values and bend the curve downward at
temperatures below 500 °F (260 °C).

These films usually differ both physically and chemically from the interiors of the particles
owing to adsorption phenomena. Theoretical calculations indicate that moisture films only
a few molecules thick are adequate to provide the desired surface conductivity. Surface
conduction on particles is closely related to surface-leakage currents occurring on
electrical insulators, which have been extensively studied.[7] An interesting practical
application of surface-leakage is the determination of dew point by measurement of the
current between adjacent electrodes mounted on a glass surface. A sharp rise in current
signals the formation of a moisture film on the glass. This method has been used
effectively for determining the marked rise in dew point, which occurs when small
amounts of sulfuric acid vapor are added to an atmosphere (commercial Dewpoint Meters
are available on the market).

The following discussion of normal, high, and low resistance applies to ESPs operated in
a dry state; resistance is not a problem in the operation of wet ESPs because of the
moisture concentration in the ESP. The relationship between moisture content and
resistance is explained later in this work.

Normal resistivity
As stated above, ESPs work best under normal resistivity conditions. Particles with
normal resistivity do not rapidly lose their charge on arrival at the collection electrode.
These particles slowly leak their charge to grounded plates and are retained on the
collection plates by intermolecular adhesive and cohesive forces. This allows a particulate
layer to be built up and then dislodged from the plates by rapping. Within the range of
normal dust resistivity (between 107 and 2 × 1010 ohm-cm), fly ash is collected more
easily than dust having either low or high resistivity.

High resistivity

If the voltage drop across the dust layer becomes too high, several adverse effects can
occur. First, the high voltage drop reduces the voltage difference between the discharge
electrode and collection electrode, and thereby reduces the electrostatic field strength
used to drive the gas ion-charged particles over to the collected dust layer. As the dust
layer builds up, and the electrical charges accumulate on the surface of the dust layer, the
voltage difference between the discharge and collection electrodes decreases. The
migration velocities of small particles are especially affected by the reduced electric field
strength.

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Another problem that occurs with high resistivity dust layers is called back corona. This
occurs when the potential drop across the dust layer is so great that corona discharges
begin to appear in the gas that is trapped within the dust layer. The dust layer breaks
down electrically, producing small holes or craters from which back corona discharges
occur. Positive gas ions are generated within the dust layer and are accelerated toward
the "negatively charged" discharge electrode. The positive ions reduce some of the
negative charges on the dust layer and neutralize some of the negative ions on the
"charged particles" heading toward the collection electrode. Disruptions of the normal
corona process greatly reduce the ESP's collection efficiency, which in severe cases, may
fall below 50% . When back corona is present, the dust particles build up on the
electrodes forming a layer of insulation. Often this can not be repaired without bringing
the unit offline.

The third, and generally most common problem with high resistivity dust is increased
electrical sparking. When the sparking rate exceeds the "set spark rate limit," the
automatic controllers limit the operating voltage of the field. This causes reduced particle
charging and reduced migration velocities toward the collection electrode. High resistivity
can generally be reduced by doing the following:

Adjusting the temperature;

Increasing moisture content;
Adding conditioning agents to the gas stream;
Increasing the collection surface area; and
Using hot-side precipitators (occasionally and with foreknowledge of sodium
depletion).

Thin dust layers and high-resistivity dust especially favor the formation of back corona
craters. Severe back corona has been observed with dust layers as thin as 0.1 mm, but a
dust layer just over one particle thick can reduce the sparking voltage by 50%. The most
marked effects of back corona on the current-voltage characteristics are:

1. Reduction of the spark over voltage by as much as 50% or more;

2. Current jumps or discontinuities caused by the formation of stable back-corona
craters; and
3. Large increase in maximum corona current, which just below spark over corona gap
may be several times the normal current.

The Figure below and to the left shows the variation in resistivity with changing gas
temperature for six different industrial dusts along with three coal-fired fly ashes. The
Figure on the right illustrates resistivity values measured for various chemical compounds
that were prepared in the laboratory.

Results for Fly Ash A (in the figure to the left) were acquired in the ascending temperature
mode. These data are typical for a moderate to high combustibles content ash. Data for
Fly Ash B are from the same sample, acquired during the descending temperature mode.

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The differences between the ascending
and descending temperature modes are
due to the presence of unburned
combustibles in the sample. Between the
two test modes, the samples are
equilibrated in dry air for 14 hours
(overnight) at 850 °F (450 °C). This
overnight annealing process typically
removes between 60% and 90% of any
unburned combustibles present in the
samples. Exactly how carbon works as a
charge carrier is not fully understood, but it
is known to significantly reduce the
resistivity of a dust.

Carbon can act, at first, like a high

resistivity dust in the precipitator. Higher Resistivity Values of Representative Dusts and
voltages can be required in order for Fumes From Industrial Plants
corona generation to begin. These higher
voltages can be problematic for the TR-Set
controls. The problem lies in onset of
corona causing large amounts of current to
surge through the (low resistivity) dust
layer. The controls sense this surge as a
spark. As precipitators are operated in
spark-limiting mode, power is terminated
and the corona generation cycle re-
initiates. Thus, lower power (current)
readings are noted with relatively high
voltage readings.

The same thing is believed to occur in

laboratory measurements. Parallel plate
geometry is used in laboratory
measurements without corona generation.
A stainless steel cup holds the sample.
Resistivity Values of Various Chemicals and
Another stainless steel electrode weight
Reagents as a Function of Temperature
sits on top of the sample (direct contact
with the dust layer). As voltage is increased
from small amounts (e.g. 20 V), no current is measured. Then, a threshold voltage level is
reached. At this level, current surges through the sample... so much so that the voltage
supply unit can trip off. After removal of the unburned combustibles during the above-
mentioned annealing procedure, the descending temperature mode curve shows the
typical inverted “V” shape one might expect.

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 Resistivity Measured as a Function of
Temperature in Varying Moisture Concentrations
(Humidity)

Low resistivity
Particles that have low resistivity are difficult to collect because they are easily charged
(very conductive) and rapidly lose their charge on arrival at the collection electrode. The
particles take on the charge of the collection electrode, bounce off the plates, and
become re-entrained in the gas stream. Thus, attractive and repulsive electrical forces
that are normally at work at normal and higher resistivities are lacking, and the binding
forces to the plate are considerably lessened. Examples of low-resistivity dusts are
unburned carbon in fly ash and carbon black.

If these conductive particles are coarse, they can be removed upstream of the precipitator
by using a device such as a cyclone mechanical collector.

The addition of liquid ammonia (NH

3) into the gas stream as a conditioning agent has found wide use in recent years. It is
theorized that ammonia reacts with H
2SO
4 contained in the flue gas to form an ammonium sulfate compound that increases the
cohesivity of the dust. This additional cohesivity makes up for the loss of electrical
attraction forces.

The table below summarizes the characteristics associated with low, normal and high
resistivity dusts.

10/17
The moisture content of the flue gas stream also affects particle resistivity. Increasing the
moisture content of the gas stream by spraying water or injecting steam into the duct
work preceding the ESP lowers the resistivity. In both temperature adjustment and
moisture conditioning, one must maintain gas conditions above the dew point to prevent
corrosion problems in the ESP or downstream equipment. The figure to the right shows
the effect of temperature and moisture on the resistivity of a cement dust. As the
percentage of moisture in the gas stream increases from 6 to 20%, the resistivity of the
dust dramatically decreases. Also, raising or lowering the temperature can decrease
cement dust resistivity for all the moisture percentages represented.

The presence of SO
3 in the gas stream has been shown to favor the electrostatic precipitation process when
problems with high resistivity occur. Most of the sulfur content in the coal burned for
combustion sources converts to SO
2. However, approximately 1% of the sulfur converts to SO
3. The amount of SO
3 in the flue gas normally increases with increasing sulfur content of the coal. The
resistivity of the particles decreases as the sulfur content of the coal increases.

Range of
Resistivity measurement Precipitator characteristics

Low between 104 1. Normal operating voltage and current levels unless
and 107 ohm- dust layer is thick enough to reduce plate
cm clearances and cause higher current levels.
2. Reduced electrical force component retaining
collected dust, vulnerable to high reentrainment
losses.
3. Negligible voltage drop across dust layer.
4. Reduced collection performance due to (2)

Normal between 107 1. Normal operating voltage and current levels.

and 2 × 1010 2. Negligible voltage drop across dust layer.
ohm-cm 3. Sufficient electrical force component retaining
collected dust.
4. High collection performance due to (1), (2) and (3)

Marginal to between 2 × 1. Reduced operating voltage and current levels with

high 1010 and 1012 high spark rates.
ohm-cm 2. Significant voltage loss across dust layer.
3. Moderate electrical force component retaining
collected dust.
4. Reduced collection performance due to (1) and (2)

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 High above 1012 1. Reduced operating voltage levels; high operating
ohm-cm current levels if power supply controller is not
operating properly.
2. Very significant voltage loss across dust layer.
3. High electrical force component retaining collected
dust.
4. Seriously reduced collection performance due to
(1), (2) and probably back corona.

Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and soda
ash (sometimes as raw trona), have also been used to reduce particle resistivity.
Therefore, the chemical composition of the flue gas stream is important with regard to the
resistivity of the particles to be collected in the ESP. The table below lists various
conditioning agents and their mechanisms of operation.

Conditioning agent Mechanism(s) of action

Sulfur trioxide and/or 1. Condensation and adsorption on fly ash surfaces.

sulfuric acid 2. May also increase cohesiveness of fly ash.
3. Reduces resistivity.

Ammonia Mechanism is not clear, various ones proposed;

1. Modifies resistivity.
2. Increases ash cohesiveness.
3. Enhances space charge effect.

Ammonium sulfate Little is known about the mechanism; claims are made
for the following:
1. Modifies resistivity (depends upon injection
temperature).
2. Increases ash cohesiveness.
3. Enhances space charge effect.
4. Experimental data lacking to substantiate which of
these is predominant.

Triethylamine Particle agglomeration claimed; no supporting data.

Sodium compounds 1. Natural conditioner if added with coal.

2. Resistivity modifier if injected into gas stream.

Compounds of transition Postulated that they catalyze oxidation of SO

metals 2 to SO
3; no definitive tests with fly ash to verify this postulation.

Potassium sulfate and In cement and lime kiln ESPs:

sodium chloride 1. Resistivity modifiers in the gas stream.
2. NaCl - natural conditioner when mixed with coal.

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If injection of ammonium sulfate occurs at a temperature greater than about 600 °F
(320 °C), dissociation into ammonia and sulfur trioxide results. Depending on the ash, SO
2 may preferentially interact with fly ash as SO
3 conditioning. The remainder recombines with ammonia to add to the space charge as
well as increase cohesiveness of the ash.

More recently, it has been recognized that a major reason for loss of efficiency of the
electrostatic precipitator is due to particle buildup on the charging wires in addition to the
collection plates (Davidson and McKinney, 1998). This is easily remedied by making sure
that the wires themselves are cleaned at the same time that the collecting plates are
cleaned.[8]

Sulfuric acid vapor (SO

3) enhances the effects of water vapor on surface conduction. It is physically adsorbed
within the layer of moisture on the particle surfaces. The effects of relatively small
amounts of acid vapor can be seen in the figure below and to the right.

The inherent resistivity of the sample at 300 °F (150 °C) is 5 × 1012 ohm-cm. An
equilibrium concentration of just 1.9 ppm sulfuric acid vapor lowers that value to about 7 ×
109 ohm-cm.

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 Resistivity Modeled As A Function of Environmental Conditions - Especially Sulfuric Acid
Vapor

Modern industrial electrostatic precipitators

ESPs continue to be excellent devices for control of
many industrial particulate emissions, including smoke
from electricity-generating utilities (coal and oil fired),
salt cake collection from black liquor boilers in pulp
mills, and catalyst collection from fluidized bed
catalytic cracker units in oil refineries to name a few.
These devices treat gas volumes from several hundred
thousand ACFM to 2.5 million ACFM (1,180 m³/s) in
A smokestack at coal-fired
the largest coal-fired boiler applications. For a coal-
Hazelwood Power Station in
fired boiler the collection is usually performed Victoria, Australia emits brown
downstream of the air preheater at about 160 °C smoke when its ESP is shut down

14/17
(320 °F) which provides optimal resistivity of the coal-ash particles. For some difficult
applications with low-sulfur fuel hot-end units have been built operating above 370 °C
(698 °F).

The original parallel plate–weighted wire design (see figure of Plate and Bar precipitator
above) has evolved as more efficient (and robust) discharge electrode designs were
developed, today focusing on rigid (pipe-frame) discharge electrodes to which many
sharpened spikes are attached (barbed wire), maximizing corona production.
Transformer-rectifier systems apply voltages of 50–100 kV at relatively high current
densities. Modern controls, such as an automatic voltage control, minimize electric
sparking and prevent arcing (sparks are quenched within 1/2 cycle of the TR set),
avoiding damage to the components. Automatic plate-rapping systems and hopper-
evacuation systems remove the collected particulate matter while on line, theoretically
allowing ESPs to stay in continuous operation for years at a time.

Electrostatic sampling for bioaerosols

Electrostatic precipitators can be used to sample biological airborne particles or aerosol
for analysis. Sampling for bioaerosols requires precipitator designs optimised with a liquid
counter electrode, which can be used to sample biological particles, e.g. viruses, directly
into a small liquid volume to reduce unnecessary sample dilution.[9][10] See Bioaerosols
for more details.

Wet electrostatic precipitator

A wet electrostatic precipitator (WESP or wet ESP) operates with water vapor saturated
air streams (100% relative humidity). WESPs are commonly used to remove liquid
droplets such as sulfuric acid mist from industrial process gas streams. The WESP is also
commonly used where the gases are high in moisture content, contain combustible
particulate, or have particles that are sticky in nature.[11]

15/17
 Example of "dirty" process gas with
100% opacity entering a WESP in a
metallurgical sulfuric acid plant. A
backlight is used to illuminate the
process gas.

Household electrostatic air cleaners

Plate precipitators are commonly marketed to the public as air purifier devices or as a
permanent replacement for furnace filters, but all have the undesirable attribute of being
somewhat messy to clean. A negative side-effect of electrostatic precipitation devices is
the potential production of toxic ozone[12] and NO
[13] However, electrostatic precipitators offer benefits over other air purifications
x.
technologies, such as HEPA filtration, which require expensive filters and can become
"production sinks" for many harmful forms of bacteria.[14][15]

With electrostatic precipitators, if the collection plates are allowed to accumulate large
amounts of particulate matter, the particles can sometimes bond so tightly to the metal
plates that vigorous washing and scrubbing may be required to completely clean the
collection plates. The close spacing of the plates can make thorough cleaning difficult,
and the stack of plates often cannot be easily disassembled for cleaning. One solution,
suggested by several manufacturers, is to wash the collector plates in a dishwasher.

Some consumer precipitation filters are sold with special soak-off cleaners, where the
entire plate array is removed from the precipitator and soaked in a large container
overnight, to help loosen the tightly bonded particulates.

A study by the Canada Mortgage and Housing Corporation testing a variety of forced-air
furnace filters found that ESP filters provided the best, and most cost-effective means of
cleaning air using a forced-air system.[16]

The first portable electrostatic air filter systems for homes was marketed in 1954 by
Raytheon.[17]

16/17
See also

Heating, ventilation, and air conditioning

17/17