Department of

Chemical
Engineering

Electrostatic
Precipitator
(ESP)
 Comparison Department of
CYCLONES versus ELECTROSTATIC PRECIPITATORS Chemical
Engineering
Cyclones and electrostatic precipitators are two different types of equipment, each
capable of removing particles from an air stream. When the decision arises
regarding which type to adopt in a specific situation, one needs to know the
advantages and disadvantages of each type of equipment.

CYCLONES: ELECTROSTATIC PRECIPITATORS:

Advantages: Advantages:
❖ Low operating cost
❖ Low capital cost (= relatively cheap to buy and (except at very high efficiencies)
install) ❖ Very high efficiency, even for smaller particles Ability to
❖ Ability to operate at high temperatures handle very large gas flow rates with low pressure losses
❖ Low maintenance requirements
(absence of moving parts) ❖ Ability to remove dry as well as wet particles (= mist ok)
❖ Temperature flexibility in design
Disadvantages: Disadvantages:
❖ Relatively low efficiency (especially for the ❖ High capital cost (= expensive to purchase and
smaller particles) install) Taking a lot of space
❖ Limited to dry particles (= not operating well on
❖ Not flexible once installed
❖ mist)
❖ Failure to operate on particles with high electrical
❖ High operating cost (= expensive to run,
because of pressure loss) resistivity
 Department of
Chemical
Efficiency of ESP Engineering

Typical efficiency of an
electrostatic precipitator is
obtained by calculating
(Watts) consumed divided
by the airflow in cubic
feet per minute (cfm).
 Department of
Corona Discharge Chemical
Engineering

❖ Corona Discharge (also known as the Corona Effect) is an electrical

discharge caused by the ionization of a fluid such as air surrounding a
conductor that is electrically charged.
 Department of
Chemical
Corona Effect Engineering

❖ The corona effect occurs naturally due to the fact that air is not a perfect
insulator – containing many free electrons and ions under normal conditions.
When an electric field is established in the air between two conductors, the
free ions and electrons in the air will experience a force.
❖ Due to this effect, the ions and free electrons get accelerated and moved in
the opposite direction.

Two factors are important for corona discharge to occur:

❖ Alternating electrical potential difference must be supplied across
the line.
❖ The spacing of the conductors, must be large enough compared to
the line diameter.
 Department of
At the core of the apparatus Chemical
Engineering

we
drift speed

Principle: Electrodes at high voltage create a corona effect (ionized

atmosphere) surrounding them. This charges the passing particles. Once
charged, particles are subject to a transverse electrostatic force that pulls
them toward the collecting plates.
Plates are periodically rapped (vibrated) to make the collected particles fall
down into a receiver basket.
 Department of
Chemical
Electrical Field Generation Engineering
 Department of
Various types of charging Chemical
Engineering
electrodes and collecting plates
❖ The baffles along the
collecting plates are
there to catch better
the drifting particles.

❖ Note the indentations and

sharp corners on some of the
electrodes. These are to
enhance the corona effect.
 Department of
Components of an Electrostatic Chemical
Precipitator Engineering
 Department of
Components of an Electrostatic Chemical
Precipitator Engineering
 Department of
BASIC DIAGRAM OF AN Chemical
ELECTROSTATIC PRECITATOR Engineering
 Department of
Components of an Electrostatic Chemical
Precipitator Engineering
 Department of
Components of an Electrostatic Chemical
Precipitator Engineering
 Department of
WORKING OF ELECTROSTATIC Chemical
PRECIPITATOR Engineering
 Department of
WORKING OF ELECTROSTATIC Chemical
PRECIPITATOR Engineering
 Department of
Components of an Electrostatic Chemical
Precipitator Engineering
 Department of
Chemical
Engineering
Gas Distribution Screen
 Department of
Chemical
Support and Shaft Insulator Engineering
 Department of
Chemical
Engineering
Discharge Electrodes
 Department of
Chemical
Engineering
Discharge Electrodes
 Department of
Chemical
Collecting Electrodes Engineering
 Department of
Chemical
Principles of ESP Operation Engineering
 Department of
Chemical
Principles of ESP Operation Engineering
 Department of
Chemical
Principles of ESP Operation Engineering
 Department of
Chemical
Principles of ESP Operation Engineering
 Department of
Chemical
Principles of ESP Operation Engineering
 Department of
Chemical
Principles of ESP Operation Engineering
 Department of
ESPs: Design Factors Chemical
Engineering

Affecting Performance
 Department of
ESPs: Design Factors Chemical
Engineering
Affecting Performance
❑ In all three kind of devices, the viscous (Stokes’ law)
resistance of the particle to be driven to the wall is
proportional to the particle diameter (Fd = 3πµDv).

❑ For gravity and centrifugal separators, the force that

can be exerted is proportional to the mass of the
particle, which, for constant density, is proportional to
the diameter cubed.
❑ Thus the ratio of driving force to resisting force is
proportional to (diameter cubed/diameter) or to
diameter squared.

❑ As the diameter decreases, this ratio falls rapidly.

 Department of
ESPs: Design Factors Chemical
Engineering
Affecting Performance
◼ In ESPs the force moving the particle toward the
wall is electrostatic.
◼ This force is practically proportional to the
particle diameter squared, and thus the ratio of
driving force to resisting force is proportional to
(diameter squared/diameter) or to the diameter.
◼ The basic idea of all ESPs is to give the particles an
electrostatic charge and then put them in an
electrostatic field that drives them to a collecting
wall.
 Department of
ESPs: Design Factors Chemical
Engineering
Affecting Performance
◼ This is an inherently two-step process.
◼ In one type of ESP, called a two-stage precipitator,
charging and collecting are carried out in separate
parts of the ESP (used in building air conditioners).
◼ For most industrial applications, the two separate
steps are carried out simultaneously in the same
part of the ESP.
◼ The charging function is done much more quickly
than the collecting function, and the size of the
ESP is largely determined by the collecting
function.
 Department of
Chemical
Engineering

◼ The gas passes between the plates, which are electrically

grounded (i.e., voltage = 0).
◼ Between the plats are rows of wires, held at a voltage of
typically -40,000 volts.
◼ The power is obtained by transforming ordinary alternating
current to a high voltage and then rectifying it to direct
current through some kind of solid-state rectifier.
◼ This combination of charged wires and grounded plates
produces both the free electrons to charge the particles and
the field to drive them against the plates.
 Department of
Chemical
Engineering

◼ On the plates the particles lose their charge and adhere to

each other and the plate, forming a “cake”.
◼ Solid cakes are removed by rapping the plates at regular
time intervals with a mechanical or electromagnetic
rapper that strikes a vertical or horizontal blow on the
edge of the plate.
◼ For liquid droplets the plate is often replaced by a circular
pipe with the wire down its center.
◼ Some ESPs (mostly the circular pipe variety) have a film of
water flowing down the collecting surface, to carry the
collected particles to the bottom without rapping
 Department of
Chemical
Engineering

❑ When the charge grows with time, reaching a steady

state value of

  
q = 3  
 o Eo
D 2
(13)
 +2

❑ here q is the charge on the particle, and ε is the dielectric constant of the
particle –a dimensionless number that is 1.0 for a vacuum, 1.0006 for air, and
4 to 8 for typical solid particles.

❑ The permittivity of free space εo is a dimensionless constant whose value in

the SI system of units is 8.85 x 10-12 C/(V • m).

❑ D is the particle diameter, and Eo is the local field strength.

 Department of
Chemical
Engineering

The electrostatic force on a particle is

F = qE p
Here Ep is the local electric field strength causing the force.
If we substitute for q from Eq. (13), we find

  
F = 3   2
 o Eo E p
D (14)
 +2

The two subscripts on the Es remind us that one represents

the field strength at the time of charging, the other the
instantaneous (local) field strength.
 Department of
Chemical
Engineering

◼ For all practical purposes we use an average

E; and in the rest of this chapter we will use
Eo = Ep = E.
◼ If the particle’s resistance to being driven to
the wall by electrostatic forces is given by the
Stokes drag force, we can set the resistance
force equal to the electrostatic force in Eq.
(14) and solve for the resulting velocity,
finding:
 Department of
Chemical
Engineering

Drift Velocity

D o E 2 (  + 2 )
t = = (15)


This velocity is called the drift velocity in the

ESP literature, and is given the symbol ω.
 Department of
Chemical
Engineering

If we now consider the section between the

row of wires and one plate on Fig. 9.7, we see
its collecting area is
A=Lh
The volumetric flow through the section is
Q = Hhvavg
 Department of
Chemical
Engineering

◼ Making these substitutions in Eqs. (1) and (4), we

find
A
◼ block = (16)
Q
and

 A 
mixed = 1 − exp  −  (17)
 Q 

◼ Eq. (17) is the Deutsch-Anderson equation, the most

widely used simple equation for design, analysis, and
comparison of ESPs.
 Department of
Chemical
Engineering
Drift speed
❖ Drift speed (we) results from a balance between the electrostatic
force due to the charge (Fe) and the resisting drag force (Fd) exerted
by the air due the relative motion between air and particle.

❖ For the drag force, we assume that the particles are very small.
(The purpose of an ESP is precisely to catch very small particles!).
So, we use Stokes’ Law with the Cunningham Slip factor correction
(refer to slide in lecture on Transport Phenomena).

❖ Fe = electrostatic force = charge electric field = qE

 Department of
Chemical
Efficiency of ESP Engineering

 The efficiency of removal of particles by an Electrostatic

Precipitator is given by

η = fractional collection efficiency

w = drift velocity, m/min.
A = available collection area, m2
Q = volumetric flow rate m3/min
 Department of
Chemical
Engineering
Migration velocity

Where,
q = charge (columbos)
Ep = collection field intensity (volts/m)
r = particle radius (m)
μ = dynamic viscosity of gas (Pa-S)
c = cunningham correction factor
 Department of
Chemical
Engineering

 Cunningham correction factor

where,
T = absolute temperature (°k)
dp = diameter of particle (μm)