1. Air pollution and control VIII
CONTROL OF AIR POLLUTION BY
EQUIPMENTS

2. Atmospheric cleansing processes….
• Dispersion
• Gravitational settling
• Flocculation
• Absorption-washout or scavenging
• Rainout (within cloud)
• Adsorption (within friction layer)
Approaches to contaminant control
• Dilution
• Control at source

3. Control devices for particulate
contaminants
• Gravitational settler
• Centrifugal collector
• Wet collector
Spray tower
Cyclonic wet collector
Impingement Spray tower
Ventury Spray tower
• Electrostatic precipitator
• Fabric filter (bag house filter)

4. Gravitational settler
• Minimum particle size removed >50µm
• Efficiency (mass basis) <50%
• Advantages….
Low pressure loss
Simplicity of design and easy construction
Easy to maintain.
• Disadvantages…..
Low efficiency
High space requirement

6. Modified Gravitational settler

7. Assuming that, Stock’s law is applicable…..
• The minimum dia. of particle collected at
100% efficiency in a chamber of length L can
be calculated using eqn…
• or
Where ……
• Vt …terminal settling velocity

8. Where….
• G…gravitational constant (m/s2)
• ρp….. Density of particle (kg/m3)
• ρa…. Density of air (@1.2 kg/m3)
• dp….dia. Of particle(m)
• μ…..viscosity of particle(kg/m.s)
• H….height of settling chamber(m)
• L…. Length of settling chamber(m)
• vh… horizontal flow through velocity(m/s)
Finally….

9. Centrifugal collector
• Minimum particle size removed =5 to25µm
• Efficiency (mass basis) =50-90%
• Advantages….
Little floor space required
Simplicity of design and easy construction
Easy to maintain.
Low to moderate pressure loss
Handle high dust loading
Temperature independent
• Disadvantages…..
Sensitive to variable dust load and flow rate
High headroom requirement

10. Particular Size (in
terms of D)
L1 2D
L2 2D
De 0.5D
h 0.5D
b 0.25D
Dd 0.25D
L3 0.125D

11. • Separation efficiency of cyclone depends upon
the magnitude of centrifugal force exerted on
particles. That can be calculated as…
• Where…
• Fc…centrifugal force(N)
• Mp….particulate mass (kg)
• vi…particulate velocity(m/s)
• R….radius of cyclone(m)

12. • Particle size that can be removed with 50%
efficiency can be calculated by…..
• Where….
• d50…dia of particle that can be removed with
50%efficiency
• μ…gas viscosity(kg/m.s)
• b…width of cyclone inlet (m)
• Ne….no of effective turns within the cyclone
• vi….inlet gas velocity(m/s)
• ρp…density of particulate matter(kg/m3)

13. • Cyclone in series…
• The efficiency of two cyclone dust collector
operating in series is given by:
• η= ηp+ηs(100-ηp)
• η….overall efficiency,
• ηp… efficiency of primary cyclone,
• ηs… efficiency of secondary cyclone.

14. Wet collectors/scrubbers
• Wet collectors remove particulate matter from
gas streams by incorporating the particles into
liquid droplets directly on contact. either inertial
impingement or interception during settling may
be contact mechanism.
• Collection efficiency for well designed wet
collector is a function of energy consumed in air
to water contact process. The energy consumed
is directly proportional to the pressure drop.

15. • Average pressure drops from well designed
collectors of several different types…
Wet collector Pressure drops in cm
(w.g)
Chamber 1-4
Centrifugal 5-15
Dynamic 15
Atomizing……
Orifice type 8-15
Ventury type 12-250

16. Types of wet collector
1. spray tower

17. 2.wet cyclone scrubber

18. 3.Ventury scrubber(pg.533)

19. Fabric filter (Bag house filter)
• The particulate laden gas passes through a woven
or felted fabric that filters out the particulate
matter and allows the gas to pass through.
• Small particles are initially retained by direct
interception, inertial impaction, diffusion,
electrostatic attraction and gravitational settling.
After the formation of dust mat more efficient
collection of fine particles is accomplished.

23. Factors affecting efficiency of bag house filter
• Excessive filter ratio,
• Improper selection of filter media
Operating problems….
• Cleaning
• Rupture of cloths
• Temperature
• Bleeding
• Humidity
• Chemical attack.

24. Applications….
• Metallurgical industries
• Foundries
• Cement industries
• Chalk and lime plants
• Brick works
• Ceramic industries
• Flour mills

25. Electrostatic precipitator
Major components of system
• A source of high voltage
• Discharge electrodes and collecting electrodes
• Inlet and outlet of gas
• A mean for disposal of the collected material
• An electronic cleaning system
• An outer casing to form an encloser

26. Electrostatic precipitator

29. • Efficiency….99% or more.
• Particle size collected….almost any
• Volume handling capacity…. 25 to 1000m3/s.
• Low pressure drop.
• Very little maintenance.
• Can be used to collect acid and tar mist.
• Can not handle explosive material.
• Initial installation cost is high.
• Needs high space.

30. Choice of equipments
• Particulate size, shape and density.
• Particulate loading.
• Required efficiency.
• Properties of carrier gas.
• Flow characteristics of carrier gas.
• Specific properties of the contaminants.
• Allowable pressure drop.
• Ease of maintenance and reliability.
• Capital and operating cost.

31. References
Environmental Engineering…..
• By ….H S Peavy, D R Rowe and G Tchobanoglous
• Published by……McGraw Hill Publication