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Electrostatic Precipitator
Electrostatic Charging of Dust Particles
Cutaway of Electrostatic Precipitator
Electrostatic precipitators
• Works on the principle of electrical charging of
particulate Matter (-ve) and collecting it in a
(+ve) charged surface.
• 99% efficiency.
• Can remove particle size range of 0.1 μm to 1 μm.
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Electrostatic Precipitator
Principle
 The particles in a polluted gas stream are charged by passing them through
an electric field.
 The charged particles are led through collector plates
 The collector plates carry charges opposite to that on the particles
 The particles are attracted to these collector plates and are thus removed
from the gas steam
Construction and Operation of Electrostatic Precipitator
 Charging Electrodes in the form of thin wires are placed in the path of the
influent gas.
 The charging electrodes generate a strong electric field, which charges the
particles as they flow through it.
 The collector plates get deposited with the particles. the particles are
occasionally removed either by rapping or by washing the collector plates.
Six major components
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A source of high voltage
Discharge electrodes and collecting electrodes
Inlet and outlet for gas
A hopper for disposal of collected material
An electronic cleaning system
An outer casing to form an enclosure around
electrodes
Electrodes :
Based on DC current flow terminals elctrodes
can be divided as below:-
Discharge electrode :Electrodes wire which carries negatively
charged high voltage (between 20 to 80KV)
act as discharge or emitting electrodes.
Collector electrode :Electrode wire which carries positively
charged high voltage act as
Collecting electrodes.
Collector
electrodes
Discharge
electrode
WORKING OF ELECTROSTATIC PRECIPITATOR
Stage - 1
Several things happen very rapidly (in a matter of a millisecond) in the small area
around the discharge electrode. Electric field is emerged due to dc terminal
arrangement. The applied (-) voltage in discharge electrode is increased until it
produces a corona discharge, which can be seen as a luminous blue glow around
the discharge Electrode.
Due to the formation of corona discharge,
free electrons are emitted with high
velocity from discharge electrode.
This fast moving free electrons strikes the
gas molecule thus emission of free
electron from gas molecules takes place.
The positive ion molecule move towards
discharge electrode by electrostatic
attraction
As a result using gas molecule more
free electrons are emitted near the
discharge electrode.
Stage - 2
As the electrons leave the strong
electrical field area around the
discharge electrode, they start slowing
down. This free electron again strikes
the gas molecule but this time they
are captured by gas molecule and
became negatively charged ion.
As the gas molecule are negatively
ionized they move towards the (+)
electrode (i.e., collector electrode).
This negative gas ion fills the space of
Dust particle and becoming
negatively charged particle.
This particle are captured by collector
electrode using electrostatic
attraction.
CORONA FORMATION
Corona discharge
The flow of electrons and gaseous ions from the discharge electrode toward the
collecting plates. Corona discharge occurs after the discharge electrode has achieved
high enough secondary voltages.
Principles
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Gas stream passed two electrodes.
High potential difference is maintained.
Out of two electrodes, one is discharging other collecting.
Potentials of 100 kv are used.
Ionization creates active glow (‫ )تخلیه‬zone called “corona”.
Gas ionization is dissociation of gas molecules into free
ions.
• As particulates pass through field, they get charged and
migrate to oppositely charged electrode.
• Particles deposited on collecting electrodes, lose charge
and removed mechanically by rapping., vibration or
washing to a hopper.
Single stage and two stage precipitators
• Single stage gas ionization and particulate
collection in a single stage.
• Two stage, particle ionized in first chamber
and collected in second chamber.
• Two stage used for lightly loaded gases.
• Single stage for more heavily loaded gas
streams.
Wet electrostatic precipitator
• A wet electrostatic precipitator (WESP or wet ESP) operates with
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, have
particles that are sticky in nature.
• The preferred and most modern type of WESP is a downflow
tubular design. This design allows the collected moisture and
particulate to form a slurry that helps to keep the collection
surfaces clean.
• Plate style and upflow design WESPs are very unreliable and
should not be used in applications where particulate is sticky in
nature.
Design of Electrostatic Precipitators
• The efficiency of removal of particles by
an Electrostatic Precipitator is given by
η = fractional collection efficiency
w = drift (‫ )رانش‬velocity, m/min.
A = available collection area, m2
Q = volumetric flow rate m3/min
Migration velocity
Where,
q = charge (Columbus)
Ep = collection field intensity (volts/m)
r = particle radius (m)
μ = dynamic viscosity of gas (Pa-S)
c = Cunningham correction factor
• Cunningham correction factor
where,
T = absolute temperature (°k)
dp = diameter of particle (μm)
Problem
• An ESP is designed to treat 50,000
m3/min with 97 % efficiency. Assuming an
effective drift velocity of 2.5 m/min,
calculate the required plate area and the
number of plates. The plate size is 10 m
by 5 m (height by length).
Solution
• Step 1:
Efficiency of an Electrostatic Precipitator is
given by
A =-[ (Q/w)*ln(1- η)]
A = 70,000 m2
• Step 2:
Number of plates = total area/plate area
= 1400
Efficiency
• General collection efficiency is high, nearly 100%
• Acid mist and catalyst recovery efficiencies in excess
of 99%.
• Carbon black, because of agglomeration tendency
collection efficiency less than 35%.
• The efficiency is usually at a minimum in the range of
0.1 to 0.5 micrometers.
Design parameter
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Volumetric flow rate
Composition
Temperature
Dew point
Dust particle conc.
Size of particle
Bulk density
Tendency of allgomoration
Resistivity of dust
1. When dust resistivity drops below this range, the dust
releases its charge readily to the collecting surface. As a
result, the dust migrates to the collecting plates where it
immediately loses its charge. The charge in conjunction
with the cohesive nature of the dust keeps the dust on the
collecting plates. If the charge is lost, the dust is likely to
be re-entrained back into the gas stream.
• high resistivity dust retains charge for extended periods.
the high resistivity dust deposits on the collecting plates,
charge does not dissipate. As a result, high resistivity dust
is very difficult to remove from the collecting plates. It is
not uncommon for high resistivity dust applications to
require periodic manual cleaning to restore precipitator
performance.
Advantages of Electrostatic Precipitators
 Electrostatic precipitators are capable very high efficiency,
generally of the order of 99.5-99.9%.
 Since the electrostatic precipitators act on the particles and not
on the air, they can handle higher loads with lower pressure
drops.
 They can operate at higher temperatures.
 The operating costs are generally low.
 High collection efficiency.
 Particles may be collected dry or wet.
 Can be operated at high temp. (300-450˚c).
 Maintenance is normal.
 Few moving parts.
Disadvantages of Electrostatic Precipitators
 The initial capital costs are high.
 Although they can be designed for a variety of operating
conditions, they are not very flexible to changes in the operating
conditions, once installed.
 Particulate with high resistivity may go uncollected.
 Require high voltage.
 Collection efficiency reduce with time.
 Space requirement is more.
 Possible of explosion during collection of combustible gases or
particulates.
Application
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Cement factories
Pulp and paper mills
Steel plants
Non- ferrous metal industry
Chemical industry
Petroleum industry
Carbon black industry
Electric power industry