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NSCETE-LEARNING
PRESENTATIONLISTEN … LEARN… LEAD…
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DEPARTMENT OF CIVIL ENGINEERING
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Mr.K.Kirubakaran.,M.E.,
Assistant Professor, Civil,
Nadar Saraswathi College of Engineering & Technology,
Vadapudupatti, Annanji (po), Theni – 625531.
CE8005 - AIR POLLUTION AND CONTROL ENGINEERING
Branch/Year/Sem: B.E. Civil Engineering/III/VI
OBJECTIVE
To impart knowledge on the principle
and design of control of Indoor/
particulate/ gaseous air pollutant and its
emerging trends.
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UNIT III
CONTROL OF PARTICULATE CONTAMINANTS
Factors affecting selection of control
equipment – gas particle interaction –
working principle, design and
performance equations of gravity
separators, centrifugal separators fabric
filters, particulate scrubbers,
electrostatic precipitators – operational
considerations.
In this presentation
• Working principle, Design and performance
equations of Gravity Separators, Centrifugal
separators, Fabric filters
• Particulate Scrubbers
• Electrostatic Precipitators
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The available mechanism are
Gravitational Settling chambers
Centrifugal impaction
Inertial impaction
Direct interception
Diffusion
Electrostatic attraction
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Department of Civil Engineering, NSCET, Theni
Inertial impaction
Inertial (அசைவற்ற)- it is the deposition of large aerosol particles on the walls of an
airway conduit (குழாய்). The impaction (இறுகிய பிசைப்பு) tends to occur where
the airway direction changes. Small particles have less inertia and are more likely to be carried
around corners and continue in the path of the airflow.
Direct interception (இசைமறிப்பு)
Direct interception occurs if the gas streamlines carrying the particles are close to the filter
elements for contact. Inertial impaction occurs when the particles have sufficient momentum
and cannot follow the gas stream, when the stream is diverted by the filter element and the
particles strike the filter.
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Direct interception
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Department of Civil Engineering, NSCET, Theni
Centrifugal collectors
Separate particulate matter (of 0.1 to 1000 m) using centrifugal force. Particles
of size (5 m to 25 m) are removed by this instrument.
Some Centrifugal collectors are
1. Cyclone collector or (Cyclone separator)
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In a cyclone separator, dirty flue gas is fed into a chamber.
The inside of the chamber creates a spiral vortex (சுழல்).
The lighter components of this gas have less inertia, so it is easier for them to be influenced by
the vortex and travel up it.
Contrarily, larger components of particulate matter have more inertia and are not as easily
influenced by the vortex.
Since these larger particles have difficulty following the high-speed spiral motion of the gas and
the vortex, the particles hit the inside walls of the container and drop down into a collection
hopper.
The cleaned flue gas escapes out the top of the chamber.
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Most cyclones are built to control and remove particulate matter that is larger than 10 micrometers
in diameter.
However, there do exist high efficiency cyclones that are designed to be effective on particles as
small as 2.5 micrometers.
As well, these separators are not effective on extremely large particulate matter. For particulates
around 200 micrometers in size, gravity settling chambers or momentum separators are a better
option.
Out of all of the particulate-control devices, cyclone separators are among the least expensive.
They are often used as a pre-treatment before the flue gas enters more effective pollution control
devices.
Therefore, cyclone separators can be seen as "rough separators" before the flue gas reaches the
fine filtration stages.
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The operating problems are
Erosion
Corrosion
Build-up
Applications
Saw mills- to remove saw dust from air
Oil refineries- to separate oil and gases
Cement industry
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Dynamic precipitator
Particles of size (5 m to 25 m) are removed by this instrument.
7 to 10 times higher than Cyclone separator.
Suited for
Ceramic, food, wood working and pharmaceutical industries.
Not suited for
Wet fibrous material
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Fabric filters
Dust collection of Particulate matter of size (up to 1 m)
The air pollution control equipment using fabric filters are known as bag houses.
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Air-to-cloth ratio (or filter velocity) is simply the amount of air going through each square foot of
filter media each minute. It is calculated by dividing the amount of airflow (CFM) by the amount
of filter media in the dust collector.
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Department of Civil Engineering, NSCET, Theni
Fabric Filters or baghouses, are air pollution control devices that work by
removing particles from the gas stream of industrial or electrical production
processes by collecting them on the surface of filter bags. Since the late 1970’s,
with the introduction of high-temperature fabrics (> 350 degrees Fahrenheit), the
baghouse has become increasingly popular, especially in the utility market. In
contrast to ESPs, the baghouse is a highly efficient particulate collection device
regardless of the incoming dust loading or particle size. Also, with stricter EPA
regulations for the removal of gases and heavy metals, the baghouse offers
adaptability as a dry collection device using absorbents.
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Baghouses have the capacity to remove 99.99% of even stubborn sub-micron
particles. As dust-laden gas enters the baghouse compartment(s), larger particles
drop out into the hopper, and smaller particles collect on the filter bags/media,
forming a layer or cake; clean gas passes through the media. When the dust layer
get thick enough to substantially restrict flow through the media (referred to as
pressure drop), cleaning is started. Cleaning can be done while a baghouse
compartment is still filtering (online) or with the compartment isolated and not
filtering (offline), depending on baghouse type and cleaning strategy.
Department of Civil Engineering, NSCET, Theni
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Department of Civil Engineering, NSCET, Theni
Gravity settling chambers
Gravity settling chambers are the oldest and simplest means of removing
suspended particles from a gas
In principle, pollutants are removed by reducing the velocity of the gas stream
sufficiently to allow particles to settle out under the influence of gravity
The simplest chamber is merely a horizontal duct in which large particles settle
out on the floor.
High-efficiency settling chambers are often fitted with baffles or deflectors to
change the gas flow direction.
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24Advantages
• Low capital cost
• Very low energy cost
• No moving parts, therefore, few maintenance requirements and low operating
costs
• Excellent reliability
• Low pressure drop through device
• Device not subject to abrasion due to low gas velocity Provide incidental
cooling of gas stream; Temperature and pressure limitations are only
dependent on the materials of construction
• Dry collection and disposal.Department of Civil Engineering, NSCET, Theni
25DisAdvantages
• Relatively low PM collection efficiencies, particularly for PM less than 50
micrometer in size
• Unable to handle sticky or tacky materials
• Trays in multiple-tray settling chamber may warp during high-temperature
operations
• The most common failure mode of settling chambers is plugging of the chamber
with collected dust. In expansion settling chambers the plugging can result from
hopper bridging or hopper discharge seal failure
• Multiple-tray settling chambers may experience plugging of the individual gas
passages. Department of Civil Engineering, NSCET, Theni
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Department of Civil Engineering, NSCET, Theni
Wet scrubbers
Wet scrubbers can remove particulate matter by capturing them in liquid
droplets. The droplets are then collected, with the liquid dissolving or absorbing the
pollutant gases. Any droplets that are in the scrubber inlet gas must be separated
from the outlet gas stream using a mist eliminator.
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Spray chamber
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A spray tower (or spray column or spray chamber) is gas-liquid contactor
used to achieve mass and heat transfer between a continuous gas phase (that
can contain dispersed solid particles) and a dispersed liquid phase.
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venturi scrubbers
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A Venturi Scrubber is a type of pollution control equipment that is used to
remove contaminated particles from gaseous exhaust streams. A type of
Wet Scrubber, the Venturi Scrubber uses Venturi technology, which has
been used for over 100 years.
Clean 400 liters of gas/min
Mist can remove by this
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venturi scrubbers
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cyclonic scrubbers
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• Cyclonic spray scrubbers are an air pollution control technology. They use
the features of both the dry cyclone and the spray chamber to
remove pollutants from gas streams.
• Cyclonic scrubbers are generally low- to medium-energy devices, with
pressure drops of 4 to 25 cm (1.5 to 10 in) of water.
• Clean 200 liters of gas/min
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The “packed tower,” an air pollution treatment system that resembles the wet scrubber system used for removal
of particulates and discussed earlier, is the most common technology used for removal of gaseous (and
aerosol) pollutants.
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Packed towers35
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Department of Civil Engineering, NSCET, Theni
Originally designed for recovery of valuable industrial-process
materials, electrostatic precipitators are used for air pollution control,
particularly for removing particles from waste gases at industrial facilities
and power-generating stations.
37PRINCIPLES OF ELECTROSTATIC PRECIPITATOR
Electrostatic precipitation is a method of dust collection that uses electrostatic
forces, and consists of discharge wires and collecting plates. A high voltage is
applied to the discharge wires to form an electrical field between the wires and
the collecting plates, and also ionizes the gas around the discharge wires to
supply ions. When gas that contains an aerosol (dust, mist) flows between the
collecting plates and the discharge wires, the aerosol particles in the gas are
charged by the ions. The Coulomb force caused by the electric field causes the
charged particles to be collected on the collecting plates, and the gas is
purified.
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Department of Civil Engineering, NSCET, Theni
Particle collection: The –vely charged particles get attracted towards the +vely
charged collecting electrodes and form a layer on the surface of the electrode.
Similarly the +vely charged particles are deposited on the discharging electrode. But
this is merely 10% of the total deposited material.
Removal of particles: The deposited particles forms a layer on the electrodes, and
after certain time interval they are removed from the electrodes by mechanisms
depending upon the type of ESP. Ash particles are collected in the hopper , and from
there they are transported to somewhere else.
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