Corona Power

As stated previously, a strong electric field is needed for achieving high collection efficiency
of dust particles. The strength of the field is based on the rating of the T-R set. The
corona power is the power that energizes the discharge electrodes and thus creates the
strong electric field. The corona power used for precipitation is calculated by multiplying
the secondary current by the secondary voltage and is expressed in units of watts. In ESP
design specifications, the corona power is usually given in units of watts per 1000 m3/h
(watts per 1000 acfm). Corona power expressed in units of watts/1000 acfm is also called
the specific corona power. Corona power for any bus section of an ESP can be calculated
by the following approximate relation:

As you can see, corona power increases as the voltage and/or current increases. The total
corona power of the ESP is the sum of the corona power for all of the individual T-R sets.
In an ESP, the collection efficiency is proportional to the amount of corona power supplied
to the unit, assuming the corona power is applied effectively (maintains a good sparking
rate).

From above equation, you can see that for a given exhaust flow rate, the collection efficiency
will increase as the corona power is increased. This efficiency will depend on the operating
conditions of the ESP and on whether the amount of power has been applied effectively.
For high collection efficiency, corona power is usually between 59 and 295 watts
per 1000 m3/h (100 and 500 watts per 1000 acfm). Recent ESP installations have been
designed to use as much as 470 to 530 watts per 1000 m3/h (800 to 900 watts per 1000
acfm).
The terms current density and power density are also used to characterize the design of the
ESP. Current density is the secondary current supplied by the T-R set for the given plate
area and expressed in units of mA/ft2 of plate area. Power density is the corona power
supplied to the plate area and is expressed in units of watts per ft2 of plate area.
The size of the individual power sets (T-R sets) in the ESP will vary depending on their
specific location and the conditions of the flue gas such as particle size, dust concentration,
dust resistivity, and flue gas temperature. In an ESP, the T-R sets are selected to provide
lower current density at the inlet sections, where the dust concentration will tend to
suppress the corona current, and to provide higher current density at the outlet sections,
where there is a greater percentage of fine particles.

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Gas Flow Distribution

Gas flow through the ESP chamber should be slow and evenly distributed through the
unit. Gas velocity is reduced by the expansion, or diverging, section of the inlet plenum
(Figure). The gas velocities in the duct leading into the ESP are generally between 12
and 24 m/s (40 and 80 ft/sec). The gas velocity into the ESP must be reduced to
0.6-2.4 m/s (2-8 ft/sec) for adequate particle collection. With aspect ratios of 1.5, the optimum
gas velocity is generally between 1.5 and 1.8 m/s (5 and 6 ft/sec).

Gas inlet with perforated diffuser plates

In order to use all of the discharge and collection electrodes across the entire width of the
ESP, the flue gas must be evenly distributed. The inlet plenum contains perforated openings,
called diffuser plate openings to evenly distribute the gas flow into the chambers
formed by the plates in the precipitator.

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Aspect Ratio

The aspect ratio, which relates the length of an ESP to its height, is an important factor in
reducing rapping loss (dust reentrainment).When particles are rapped from the electrodes,
the gas flow carries the collected dust forward through the ESP until the dust reaches the
hopper. Although the amount of time it takes for rapped particles to settle in the hoppers is
short (a matter of seconds), a large amount of "collected dust" can be reentrained in the gas
flow and carried out of the ESP if the total effective length of the plates in the ESP is small
compared to their effective height. For example, the time required for dust to fall from the
top of a 9.1-m plate (30-ft plate) is several seconds. Effective plate lengths must be at least
10.7 to 12.2 m (35 to 40 ft) to prevent a large amount of "collected dust" from being carried
out of the ESP before reaching the hopper.
The aspect ratio is the ratio of the effective length to the effective height of the collector
surface. The aspect ratio can be calculated using Equation

The effective length of the collection surface is the sum of the plate lengths in each consecutive
field and the effective height is the height of the plates. For example, if an ESP
has four fields, each containing plates that are 10 feet long, the effective length is 40 feet.
If the height of each plate is 30 feet, the aspect ratio is 1.33 as shown below:

Aspect ratios for ESPs range from 0.5 to 2.0. However, for high-efficiency ESPs (those
having collection efficiencies of > 99%), the aspect ratio should be greater than 1.0 (usually
1.0 to 1.5) and in some installations may approach 2.0.

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Specific Collection Area

The specific collection area (SCA) is defined as the ratio of collection surface area to the
gas flow rate into the collector. This ratio represents the A/Q relationship in the Deutsch-
Anderson equation and consequently is an important determinant of collection efficiency.
The SCA is given in Equation

For example, if the total collection area of an ESP is 600,000 ft2 and the gas flow rate
through the ESP is 1,000,000 ft3/min (acfm), the SCA is 600 ft2 per 1000 acfm as calculated
below.

Increases in the SCA of a precipitator design will, in most cases, increase the collection

efficiency of the precipitator. Most conservative designs call for an SCA of 20 to 25 m2

per 1000 m3/h (350 to 400 ft2 per 1000 acfm) to achieve collection efficiency of more than

99.5%. The general range of SCA is between 11 and 45 m2 per 1000 m3/hr (200 and 800

ft2 per 1000 acfm), depending on precipitator design conditions and desired collection

efficiency.

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Electrical Sectionalization

Field Sectionalization
An electrostatic precipitator is divided into a series of independently energized bus
sections or fields (also called stages) in the direction of the gas flow. Precipitator performance
depends on the number of individual bus sections, or fields, installed. Figure
 shows an ESP consisting of four fields, each of which acts as an independent precipitator.

Field sectionalization

Each field has individual transformer-rectifier sets, voltage-stabilization controls, and

high-voltage conductors that energize the discharge electrodes within the field. This

design feature, called field electrical sectionalization, allows greater flexibility for

energizing individual fields to accommodate different conditions within the precipitator.

This is an important factor in promoting higher precipitator collection efficiency.

Most ESP vendors recommend that there be at least three or more fields in the precipitator.

However, to attain a collection efficiency of more than 99%, some ESPs have

been designed with as many as seven or more fields. Previous experience with a particular

industry is the best factor for determining how many fields are necessary to

meet the required emission limits.

The need for separate fields arises mainly because power input requirements differ at

various locations within a precipitator. The maximum voltage at which a given field

can be maintained depends on the properties of the gas and dust being collected. The

particulate matter concentration is generally high at the inlet fields of the precipitator.

High dust concentrations tend to suppress corona current, requiring a great deal of

power to generate corona discharge for optimum particle charging. In the downstream

fields of a precipitator, the dust loading is usually lighter, because most of the dust is

collected in the inlet fields. Consequently, corona current flows more freely in downstream

fields. Particle charging will more likely be limited by excessive sparking in

the downstream than in the inlet fields. If the precipitator had only one power set, the

excessive sparking would limit the power input to the entire precipitator, thus reducing

the overall collection efficiency. The rating of each power set in the ESP will vary

depending on the specific design of the ESP.

Modern precipitators have voltage control devices that automatically limit precipitator

power input. A well-designed automatic control system keeps the voltage level at

approximately the value needed for optimum particle charging by the discharge electrodes.

The voltage control device increases the primary voltage applied to the T-R set

to the maximum level. As the primary voltage applied to the transformer increases, the

secondary voltage applied to the discharge electrodes increases. As the secondary

voltage is increased, the intensity and number of corona discharges increase. The voltage

is increased until any of the set limits (primary voltage, primary current, secondary

voltage, secondary current, or spark rate limits) is reached. Occurrence of a spark

counteracts high ESP performance because it causes an immediate, short-term collapse

of the precipitator electric field. Consequently, power that is applied to capture particles is used less efficiently. There is, however, an optimum sparking rate where

the gains in particle charging are just offset by corona-current losses from sparkover.

Measurements on commercial precipitators have determined that the optimum sparking

rate is between 50 and 150 sparks per minute per electrical section. The objective

in power control is to maintain corona power input at this optimum sparking rate by

momentarily reducing precipitator power whenever excessive sparking occurs.

Besides allowing for independent voltage control, another major reason for having a

number of fields in an ESP is that electrical failure may occur in one or more fields.

Electrical failure may occur as a result of a number of events, such as over-filling hoppers,

discharge-wire breakage, or power supply failure. These failures are discussed in

more detail later in this course. ESPs having a greater number of fields are less dependent

on the operation of all fields to achieve a high collection efficiency.

Parallel Sectionalization

In field sectionalization, the precipitator is designed with a single series of independent

fields following one another consecutively. In parallel sectionalization, the

series of fields is electrically divided into two or more sections so that each field has

parallel components. Such divisions are referred to as chambers and each individual

unit is called a cell. A precipitator such as the one shown in Figure  has two parallel

sections (chambers), four fields, and eight cells. Each cell can be independently

energized by a bus line from its own separate transformer-rectifier set.

Parallel sectionalization (with two parallel

sections, eight cells, and four fields)

One important reason for providing sectionalization across the width of the ESP is to

provide a means of handling varying levels of flue gas temperature, dust concentration,

and problems with gas flow distribution.When treating flue gas from a boiler, an

ESP may experience gas temperatures that vary from one side of the ESP to the other,

especially if a rotary air preheater is used in the system. Since fly ash resistivity is a

function of the flue gas temperature, this temperature gradient may cause variations in

the electrical characteristics of the dust from one side of the ESP to the other. The gas

flow into the ESP may also be stratified, causing varying gas velocities and dust concentrations

that can also affect the electrical characteristics of the dust. Building

numerous fields and cells into an ESP design can provide a means of coping with variations in the flue gas. In addition, the more cells provided in an ESP, the greater the

chance that the unit will operate at its designed collection efficiency.

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Measuring Resistivity

Particle resistivity is determined by measuring the leakage current through a dust layer
to which a high voltage is applied using conductivity cells. A number of conductivity
cells have been used in particle-resistivity measurements. For a good review of the
different kinds of cells employed, see White (1974). Resistivity can be measured by a
number of methods in either the laboratory or the field. In the lab method, dust samples
are first extracted from the flue gas leaving the industrial process and collected on
a filter as described in EPA Reference Method 5. The samples are then taken back to
the laboratory and analyzed.
Resistivity measurements are made in the field using an in-situ resistivity probe. The
probe is inserted into the duct leaving the industrial process and a dust sample is
extracted into the probe. High voltage is applied across a point and plate electrode system
inside the probe. Particles are charged and then collected on the plate. After a sufficiently
thick layer of dust has collected on the plate, the power to the point is turned
off and a disc is lowered onto the collected dust sample. The thickness of the dust
layer is first measured. Increasing voltages are then applied to the disc, and the corresponding
current is recorded until the dust layer breaks down and sparkover occurs.
The resistivity is calculated from the last set of voltage and current readings obtained
before sparkover occurs. Since these resistivity measurements are made at the industrial
process conditions, these data are generally more useful than data obtained from
the laboratory methods. A good review of in-situ resistivity measuring techniques is
given by White (1974) and Gallaer (1983).

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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 reentrained in the gas stream. Thus, attractive and repulsive electrical
forces that are normally at work at 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. Baffles are often installed on the collection
plates to help eliminate this precipitation-repulsion phenomenon.
The addition of liquid ammonia (NH3) into the gas stream as a conditioning agent has
found wide use in recent years. It is theorized that ammonia reacts with H2SO4 contained
in the flue gas to form an ammonium sulfate compound that increases the resistivity
of the dust. Ammonia vapor is injected into the duct leading to the precipitator at
concentrations of 15 to 40 ppm by volume. The injection of NH3 has improved the
resistivity of fly ash from coal-fired boilers with low flue gas temperatures (Katz
1979).
Table summarized the characteristics associated with low, normal and high resistivity
dusts.

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Dust Layer Resistivity

Let’s take a closer look at the way resistivity affects electrical conditions in the dust
layer. A potential electric field (voltage drop) is formed across the dust layer as negatively
charged particles arrive at the dust layer 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 resistivity and thickness
of the dust layer.
In high resistivity 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 resistivities
are held too strongly to the plate, making them difficult to remove and causing rapping
problems.
In low resistivity 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 resistivity do not
adhere strongly enough to the collection plate. They are easily dislodged and become
reentrained in the gas stream.
The following discussion of normal, high, and low resistivity applies to ESPs operated
in a dry state; resistivity is not a problem in the operation of wet ESPs because of the
moisture concentration in the ESP.

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 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.
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% (White 1974).

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
• Using hot-side precipitators (occasionally)
Figure  shows the variation in resistivity with changing gas temperature for six different
industrial dusts (U.S. EPA 1985). For most dusts, resistivity will decrease as the
flue gas temperature increases. However, as can be seen from Figure  the resistivity
also decreases for some dusts (cement and ZnO) at low flue gas temperatures.

Resistivity of six different dusts at various

temperatures

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. Figure 3-2 shows

the effect of temperature and moisture on the resistivity of cement dust. As the percentage

of moisture in the dust increases from 1 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.

Effect of temperature and moisture on the

resistivity of cement dust

The presence of SO3 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 SO2. However,

approximately 1% of the sulfur converts to SO3. The amount of SO3 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

Fly ash resistivity versus coal sulfur content

for several flue gas temperature bands

The use of low-sulfur western coal for boiler operations has caused fly ash resistivity

problems for ESP operators. For coal fly ash dusts, the resistivity can be lowered

below the critical level by the injection of as little as 10 to 30 ppm SO3 into the gas

stream. The SO3 is injected into the duct work preceding the precipitator. Figure 3-4

shows the flow diagram of a sulfur-burning flue gas conditioning system used to

lower resistivity at a coal-fired boiler.

Flow diagram of sulfur-burning flue gas conditioning system

Courtesy of Wahlco, Inc.

Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and soda

ash, have also been used to reduce particle resistivity (White 1974). 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. Table 3-5 lists various conditioning agents

and their mechanisms of operation (U.S. EPA 1985).

Two other methods that reduce particle resistivity include increasing the collection

surface area and handling the flue gas at higher temperatures. Increasing the collection

area of the precipitator will increase the overall cost of the ESP, which may not be

desirable. Hot-side precipitators, which are usually located in front of the combustion

air preheater section of the boiler, are also used to combat resistivity problems. However,

the use of conditioning agents has been more successful and very few hot-side

ESPs have been installed since the 1980s.

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Resistivity

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.
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 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 later in this lesson. Table

 gives value ranges for low, normal, and high resistivity.

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Design Parameters

Once the basis of the ESP design has been set, the vendor will complete the design by incorporating
a number of parameters that can be adjusted for each specific industrial application.
However, before starting this design phase, the vendor must take into account the effect that
particle resistivity can have on the actual collection efficiency.

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Using Computer Programs and Models

Engineers can also use mathematical models or computer programs to design precipitators.
A mathematical model that relates collection efficiency to precipitator size and various
operating parameters has been developed by Southern Research Institute (SoRI) for
EPA. The (SoRI/EPA) model is used to do the following:
• Design a full-scale ESP from fundamental principles or in conjunction with a pilotplant
study·
• Evaluate ESP bids submitted by various manufacturers
• Troubleshoot and diagnose operating problems for existing ESPs
• Evaluate the effectiveness of new ESP developments and technology, such as flue gas
conditioning and pulse energizing.

Table lists the input data used in the SoRI/EPA Model. Assuming that accurate input
data are available for use, the model usually can estimate emissions within ± 20 percent of
measured values (U.S. EPA 1985). The computer model goes through an iterative computational
process to refine its predictions of emission levels for a particular ESP. First, the
model uses secondary voltage and current levels (corona power) to predict emission levels
leaving the ESP. Then, actual emission levels are measured and compared to the predicted
emission levels. Empirical factors are then adjusted and the process repeats itself until the
predicted emission levels of the model agree with the actual, measured levels. This model
can be used to obtain reasonable estimates of emission levels for other ESP operating conditions
(U.S. EPA 1985). For example, once you create a good, working computer model
for a particular ESP design under one set of operating conditions, you can run the model
for different scenarios by altering one or more of the parameters (precipitator length, number
of fields, etc.) to obtain reasonably accurate emission level predictions.

Another model, the EPA/RTI model, has been developed by the Research Triangle Institute

(RTI) for EPA (Lawless 1992). The EPA/RTI model is based on the localized electric

field strengths and current densities prevailing throughout the precipitator. These data can

be input based on actual readings from operating units, or can be calculated based on electrode

spacing and resistivity. The data are used to estimate the combined electrical charging

on each particle size range due to field-dependent charging and diffusional charging.

Particle size-dependent migration velocities are then used in a Deutsch-Anderson type

equation to estimate particle collection in each field of the precipitator. This model takes

into account a number of the site specific factors including gas flow maldistribution, particle

size distribution, and rapping reentrainment.

These performance models require detailed information concerning the anticipated configuration

of the precipitator and the gas stream characteristics. Information needed to operate

the EPA/RTI model is provided below. It is readily apparent that all of these parameters

are not needed in each case, since some can be calculated from the others. The following

data is data utilized in the EPA/RTI computerized performance model for electrostatic precipitators.

ESP Design

• Specific collection area

• Collection plate area

• Collection height and length

• Gas velocity

• Number of fields in series

• Number of discharge electrodes

• Type of discharge electrodes

• Discharge electrode-to-collection plate spacing

Particulate Matter and Gas Stream Data

• Resistivity

• Particle size mass median diameter

• Particle size distribution standard deviation

• Gas flow rate distribution standard deviation

• Actual gas flow rate

• Gas stream temperature

• Gas stream pressure

• Gas stream composition

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Using Pilot Plants

Probably the most reliable method for designing ESPs is to construct and operate a pilot
plant. However, time limitations and the expense of construction may make this impossible;
a pilot plant can easily cost one million dollars or more. A pilot ESP project can be
constructed on an existing industrial process. In this case, a side stream of flue gas is sent
to the small pilot ESP. Flue gas sampling gives valuable information such as gas temperature,
moisture content, and dust resistivity. Relating these parameters to the measured collection
efficiency of the pilot project will help the design engineers plan for scale-up to a
full-sized ESP.

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Matts-Ohnfeldt Equation

Another modification to the Deutsch-Anderson equation that accounts for non-ideal
effects was devised by Sigvard Matts and Per-Olaf Ohnfeldt of Sweden (Svenska
Flaktfabriken) in 1964. The Matts-Ohnfeldt equation is

The term, wk, the average migration velocity in equation is determined from

information obtained from similar installations. The terms wk and weare similar in that both are average migration velocities. 

The constant, k, in the equation is usually between 0.4 and 0.6, depending on the standard

deviation of the particle size distribution and other dust properties affecting collection

efficiency. However, most people who have used this equation report that a value of k

equal to 0.5 gives satisfactory results (Gallaer 1983 and U.S. EPA 1985). In an Electric

Power Research Institute (EPRI) study, a table was constructed to show the relationship

of predicting collection efficiency using the Deutsch-Anderson and Matts-

Ohnfeldt equations. This information is given in Table

When k = 1.0, the Matts-Ohnfeldt equation is the same as the Deutsch-Anderson

equation. To predict the collection efficiency of an existing ESP when the collection

area or gas flow rate is varied, using lower values for k gives more conservative

results. From Table you can see that the efficiency estimates calculated using the

Matts-Ohnfeldt equation are more conservative than those estimated using the

Deutsch-Anderson equation, and may more likely predict how efficiently the ESP will

actually operate.

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Modified Deutsch-Anderson Equation Using the Effective-Precipitation Rate

To make the Deutsch-Anderson equation more accurate in cases where all particles
are not uniform in size, a parameter called the effective precipitation rate (we) can be
substituted for the migration velocity in the equation. Therefore, Dr. Harry White proposed
modifying the Deutsch-Anderson equation by using the term we instead of w in
the Deutsch-Anderson equation (White 1982).

In contrast to the migration velocity (w), which refers to the speed at which an individual
charged particle migrates to the collection electrode, the effective precipitation
rate (we) refers to the average speed at which all particles in the entire dust mass move
toward the collection electrode. The variable, we, is calculated from field experience
rather than from theory; values for we are usually determined using data banks accumulated
from ESP installations in similar industries or from pilot-plant studies. In
summary, the effective precipitation rate represents a semi-empirical parameter that
can be used to determine the total collection area necessary for an ESP to achieve a
specified collection efficiency required to meet an emission limit.
Using the Deutsch-Anderson equation in this manner could be particularly useful
when trying to determine the amount of additional collection area needed to upgrade
an existing ESP to meet more stringent regulations or to improve the performance of
the unit. However, other operating parameters besides collection area play a major
role in determining the efficiency of an ESP.

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Deutsch-Anderson Equation

Deutsch-Anderson Equation
Probably the best way to gain insight into the process of electrostatic precipitation is
to study the relationship known as the Deutsch-Anderson equation. This equation is
used to determine the collection efficiency of the precipitator under ideal conditions.
The simplest form of the equation is given below.

This equation has been used extensively for many years to calculate theoretical collection

efficiencies. Unfortunately, while the equation is scientifically valid, a number of

operating parameters can cause the results to be in error by a factor of 2 or more. The

Deutsch-Anderson equation neglects three significant process variables. First, it completely

ignores the fact that dust reentrainment may occur during the rapping process.

Second, it assumes that the particle size and, consequently, the migration velocity are

uniform for all particles in the gas stream. As stated previously, this is not true; larger

particles generally have higher migration velocity rates than smaller particles do.

Third, it assumes that the gas flow rate is uniform everywhere across the precipitator

and that particle sneakage (particles escape capture) through the hopper section does

not occur. Particle sneakage can occur when the flue gas flows down through the hopper

section instead of through the ESP chambers, thus preventing particles from being

subjected to the electric field. Therefore, this equation should be used only for making

preliminary estimates of precipitator collection efficiency.

More accurate estimates of collection efficiency can be obtained by modifying the

Deutsch-Anderson equation. This is accomplished either by substituting the effective

precipitation rate, we, in place of the migration velocity, w, or by decreasing the calculation

of collection efficiency by a factor of k, which is constant (Matts-Ohnfeldt

equation). These calculations are used in establishing preliminary design parameters

of ESPs.

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INFORMATION REQUIRED FOR DESIGNING OF ESP

The efficiency of an ESP depends upon two factors
 The size of the unit i.e. total square ft. of the collecting plate area
 Amount of independent electrical energisation
In addition following details are required for designing an ESP
1. Source of the emission : Properties of the process by which the pollutants are produced
2. Particle size distribution
3. Chemical analysis of dust in relation to particle size
4. Specific eclectic resistivity of dust
5. Dust concentration of clean gas
6. Required dust concentration of clean gas(efficiency)
7. Properties of gas: composition, temperature, pressure.
8. Corrosive properties of gas
9. Gas flow rate
Apart from these variables the design of ESP also include the determination of ancillary
factors such as rappers to shake the dust loose from the plates, automatic control system,
measures for ensuring high-quality gas flow, dust removal system, provisions for structural and
heat insulation and performance monitoring system

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Using Estimates of Collection Efficiency

Collection efficiency is the primary consideration of ESP design. The collection efficiency and/or the collection area of an ESP can be estimated using several equations. These equations give a theoretical estimate of the overall collection efficiency of the unit operating under ideal conditions. Unfortunately, a number of operating parameters can adversely affect the collection efficiency of the precipitator. A discussion of collectionefficiency equations and operating parameters affecting collection-efficiency equations follows.

Particle-Migration Velocity
Before determining the collection area and the collection efficiency, the designer must estimate or measure (if possible) the particle-migration velocity. This is the speed at which a particle, once charged, migrates toward the grounded collection electrode. Variables affecting particle velocity are particle size, the strength of the electric field, and the viscosity of the gas. How readily the charged particles move to the collection electrode is denoted by the symbol, w, called the particle-migration velocity, or drift velocity. The migration-velocity parameter represents the collectability of the particle within the confines of a specific ESP. The migration velocity is expressed in Equation

                                                                                 equation 1

migration velocity depends on the voltage strength of both the charging and collection fields. Therefore, the precipitator must be designed using the maximum electric field voltage for maximum collection efficiency. The migration velocity also depends on particle size; larger particles are collected more easily than smaller ones.

 Particle-migration velocity can also be determined by Equation

 

                                                                   equation 2

 jThe particle-migration velocity can be calculated using either Equations 1 or 2, depending on the information available on the particle size and electric field strength. However, most ESPs are designed using a particle-migration velocity based on field experience rather than theory. Typical particle migration velocity rates, such as those listed in Table



              Typical effective particle-migration velocity rates for various applications

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