Air Cleaning Devices
Is An Air Cleaner Needed ?
 Toxicity of materials discharged
 Amount of material to be discharged
 Value of material to be discharged
 Local government regulations
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Selection Of Dust Collection Equipment
WILL MAINLY DEPEND ON
Contaminant characteristics
Efficiency required
Gas stream characteristics
Energy considerations
Dust disposal
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Air Cleaning Devices
Dust collectors
Electrostatic precipitators
Fabric collectors
Wet collectors
Dry centrifugal collectors
Air filters
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Dust Collector Types
Electrostatic precipitators (ESP)
Mechanism:
 Ionizing the gas
 Charging the dust particles
 Transporting the particles to the collecting surface
Iv) neutralizing the dust particles
V) removing the dust from the collecting surface
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Electrostatic Precipitators
TYPES :
i ) Cottrell or Single-stage - Combines ionization and collection
in a single stage
ii ) Penny or Two-stage - Ionization and collection in different
stages
ADVANTAGES :
Large gas volumes & high collection efficiency for small
particles needed
PROBLEMS :
Not suited where exhaust gas is flammable or explosive
Lose of efficiency between cleaning cycles if not maintained
properly
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Dust Collector Types
Fabric Collectors
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Collection mechanism :
I ) impaction
Ii ) interception
Iii) diffusion
Design features :
I ) type of fabric
Ii ) fabric configuration
Iii) intermittent or continuous service
Iv) type of reconditioning
V) housing configuration
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FABRIC COLLECTORS ( Contd.)
Amount of filter area depends on:
Release characteristics of dust
Porosity of dust cake
Concentration of dust in carrier gas stream
Type of fabric and surface finish if any
Type of reconditioning
Reconditioning interval
Air flow pattern within the collector
Temperature and humidity within the gas
stream
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Fabric Collectors ( Contd.)
Advantages:
Can handle varying exhaust gas flow rates &
particle loading
Problems :
Bag failure
Abrasion
Chemical degradation of fabric
Exceeding temperature limit of fabric
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Dust Collector Types
Wet Collectors
Collection mechanism :
– Contact particles with water or another liquid
– Collect the droplets
Types :
I ) chamber or spray tower
Ii) packed towers
Iii) wet centrifugal collectors
Iv) wet dynamic precipitator
V) orifice type
Vi) Venturi
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Wet Collectors ( contd.)
ADVANTAGES:
 Can handle high-temperature and moisture-laden gases
 Dust which represent explosion or fire hazards in the
dry form can be wetted and collected
PROBLEMS :
 Use of water may introduce corrosive conditions.
 Freeze protection needed if the collectors are used
outdoors in cold climates.
 Plugging of nozzles due to chemical imbalances in
scrubbing liquids
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Dust Collector Types
Dry centrifugal collectors
Collection efficiency is influenced by
I ) size, shape and weight of the particles
Ii) size and design of the collector
Iii) inlet velocity
Iv) concentration of dust
Types :
I ) gravity separators
Ii) inertial separators
Iii) cyclone collectors
Iv) high efficiency centrifugals
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Dust Collector Types
V) GASEOUS CONTAMINANT COLLECTORS
 TYPES :
i ) Absorbers
ii) Adsorbers
iii) Thermal oxidizers
iv) Direct combustors
v) Catalytic oxidizers
VI) UNIT COLLECTORS :
 Small fabric collectors having capacities in the range 200 2000cfm
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Gaseous Contaminant Collectors
 Absorbers: these remove soluble or chemically reactive gases
from an air stream by contact with a suitable liquid. Water is the
most frequently used absorbent. Packed towers are typical
absorbers.
 Adsorbers: Adsorbers remove contaminants by collection on a
solid. Activated carbon or molecular sieves are popular
adsorbents.
 Thermal oxidizers: thermal oxidizers, or afterburners, may be
used where the contaminant is combustible. Most combustible
contaminants can be oxidized at temperatures between 1000
and 1500 F.
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Gaseous Contaminant Collectors
 Direct Combustors: Direct Combustors differ from thermal
oxidizers by introducing the contaminated gases and auxiliary
air directly into the burner as fuel.
Auxiliary fuel, usually natural gas or oil, is generally required for
ignition.
 Catalytic Oxidizers: These may be used where the contaminant
is combustible. The contaminated gas stream is preheated and
then passed through a catalyst bed which promotes oxidation of
the combustibles to carbon dioxide and water vapor.
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Dust Collecting Equipment Cost
The following factors should be evaluated while estimating
the equipment cost
Price versus capacity
Accessories
Installation cost
Special construction
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Dust Collecting Equipment Cost
 Price versus capacity: prices per cfm of gas will vary
with the gas flow rate for all the dust collectors. The
smaller the flow rate, the higher the cost per cfm.
 Accessories included: meticulous analysis of
components of equipment included is essential.
 Some of the collector designs include exhaust fan,
motor, drive, and starter. Dust connections between
elements may be included or omitted. Recirculating
water pumps or setting tanks may be required but
not included in the equipment price.
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Dust Collecting Equipment Cost
 Installation cost: installation cost can equal or exceed the cost
of the collector. Factory installed media will reduce the
installation cost. The cost can also be greatly influenced by the
need for water and drain connections, special or extensive
electrical work, and expensive material handling equipment for
collection material disposal.
 Special construction: prices shown in any tabulation must
necessarily assume standard or basic construction. The increase
in cost for corrosion resisting material, special hightemperature fabrics, insulation or weather protection for
outdoor installations can introduce a multiplier of one to four
times the standard cost.
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Selection Of Air Filtration Equipment
The selection of air filtration equipment is based on
 Efficiency
 Dust holding capacity
 Pressure drop
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Methods Of Air Filtration
Following are the five basic methods of air filtration
 Straining
 Impingement
 Interception
 Diffusion
 Electrostatic
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Methods Of Air Filtration
 Straining: straining occurs when a particle is larger than the
opening between fibers and cannot pass through. It is a very
ineffective method of filtration because the vast majority of
particles are far smaller than the spaces between fibers.
 Impingement: when air flows through a filter, it changes
direction as it passes around each fiber. Larger dust particles,
however, cannot follow the abrupt changes in direction because
of their inertia. As a result, they do not follow the air stream
and collide with a fiber. Filters using this method are often
coated with an adhesive to help fibers retain the dust particles
that impinge on them.
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Methods Of Air Filtration
 Interception: interception is a special case of impingement
where a particle is small enough to move with the air stream
but, because its size is very small in relation to the fiber, makes
contact with a fiber while following the tortuous air flow path of
the fiber.
 Diffusion: diffusion takes place on particles so small that their
direction and velocity are influenced by molecular collisions.
These particles do not follow the air stream, but behave more
like gases than particulate. Diffusion is the primary mechanism
used by most extremely efficient filters.
 Electrostatic: A charged dust particle will be attracted to a
surface of opposite electrical polarity. Most dust particles are not
electrically neutral, therefore, electrostatic attraction between
dust particle and filter fiber aids the collection of efficiency of all
barrier type air filters.
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Radioactive And High Toxicity
Operations:
There are three major requirements for air cleaning equipment to
be utilized for radioactive or high toxicity applications:
1. High efficiency
2. Low maintenance
3. Safe disposal
High efficiency is essential because of extremely low tolerances
for the quantity and concentration of stack effluent and the
high cost of the materials handled
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Radioactive And High Toxicity
Operations
 The need for low maintenance is of special importance when
exhausting any hazardous material.
 For many radioactive processes, the changing of bags in a
conventional fabric collector may expend the daily radiation
tolerances of 20 or more persons, so infrequent, simple and
rapid maintenance requirements are vital.
 Disposal of radioactive or toxic materials by air, water or land is
a serious and very difficult problem.
 For example, scalping filters loaded with radioactive dust are
usually incinerated to reduce the quantity of material that must
be disposed of in special burial grounds.
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