1) The document discusses different types of steam power plants, including their basic layout and components. It focuses on coal-fired steam power plants. 2) The modern steam power plant is divided into four main circuits: coal and ash, air and gas, feed water and steam flow, and cooling water. 3) Coal handling and firing systems are described in detail, including various equipment for transporting, storing, and feeding coal into the boiler furnace. The key types of stokers - overfeed and underfeed - are explained.

1. SNITS (JNTUH) – B.Tech/M.Tech (Thermal)
UNIT-I:
STEAM POWER PLANT
THERMAL ANDNUCLEARPOWERPLANT
Dr. S. VIJAYA BHASKAR
M.Tech (Mech), Ph.D (Mech), Ph.D (Mgmt)
Professor in Mechanical Engineering

2. Unit-I: Syllabus
Steam power plant:
Introduction
General layout of steam power
plant
Modern coal fired steam
power plant
Power plant cycle
Fuel Handling, Combustion 2

3. CLASSIFICATION OF POWER PLANTS
3
Based on the form of energy converted into
electrical energy, the power plants are classified as
1) Steam Power Plants
2) Diesel Power Plants
3) Gas Power Plants
4) Hydro-electric Power Plants
5) Nuclear Power Plants

4. STEAM POWER PLANT
4
 A Steam Power Plant converts the chemical energy of the fossil fuels
(coal, oil, gas) or fissile fuels (Uranium, Thorium) into electrical energy.
 Steam Power Plant basically works on Rankine cycle. Steam is produced
in the boiler by utilizing the heat of fuel combustion; thus steam is
expanded through the steam turbines. The Steam turbine drives the
generator which converts mechanical energy of the turbine into electrical
energy.

5. STEAM POWER PLANT
5

6. 6

7. Modern Steam Power Plant

8. 8
1.Boiler 2. Super heater
3. Reheater 4. Air preheater
5. Economizer 6. Steam Turbine
7. Condenser 8. Cooling Tower
9. Circulating water pump
10. Boiler Feed Pump
11. Generator
12. Water Treatment Section
13.Chimney and Control Room
Major Components of Steam Power Plant

9. Steam Power Plant Layout
9
The Modern Steam Power Plant Mainly Divided Into
Four Circuits.
1) Coal And Ash Circuit.
2) Air And Gas Circuit.
3) Feed Water And Steam Flow Circuit.
4) Cooling Water Circuit.

10. 10

11. Coal And Ash Circuit
11
 It comprises of coal handling
equipment and ash handling
equipment.
 Coal is supplied to the boiler
from the storage point by coal
handling equipment and
 Steam is generated in the boiler.
 Ash resulting from the
combustion of coal is removed to
the ash storage yard through ash
handling equipment.

12. Air And Gas Circuit
12
 Air is blown to the
combustion chamber by
induced draught fan or
forced draught fan or
both.
 The dust present in the
air is removed by dust
catching device or
precipitator.

13. 13
AIR AND GAS CIRCUIT
 The Induced Draught (ID) fan helps in sucking
the exhaust gases and exhausting them through
the chimney into the atmosphere.
 The heat in the exhaust gases is partly extracted
by passing
 (i) through the Economiser to preheat the water
and then
 (ii) through the Air Preheater for heating up the
incoming air.

14. ID and FD Fan
14

15. Feed Water And Steam Flow Circuit
15
The high pressure steam produced in the boiler is used to
operate the turbine. The exhaust steam is condensed in
the condenser.
The condensate is heated in the feed heaters using the
steam trapped from different points of turbine and finally
the condensate is
pumped into the
boiler.

16. 16
FEED WATER AND STEAM FLOW CIRCUIT
 During this process a part of steam and water is
lost by passing through different components and
this is compensated by supplying additional feed
water
 This feed water
should be purified
before supply to
prevent scaling of
the tubes of the
boiler.

17. Cooling Water Circuit
17
 The exhaust steam entering the condenser is
cooled by circulating the cooling water.
 The cooling water supply to the condenser helps
in maintaining a low pressure in it.
 The cooling water may be taken from any source
like river, lake or alternatively,
same water may be cooled and
circulated again. Based on these, this
circuit is divided
1) Open system
2) Closed system

18. 18
Open System:
Water from any natural source (river, sea)
is used to for condensation of steam in this
open system. Water from the upstream of
the river is taken and circulated in the
condenser. The heated water is let down
into the stream of river in this system.

19. 19
Closed System:
When enough quantity of water is not
available the same water will be used in the
condenser. The heated water from the
condenser is cooled by cooling tower or
spray pond and recirculated. Make up water
is added periodically to cover the water/
steam loss.

20. Depends on
20

21. 21

22. 22
Effect of Steam Temperature on Cost and Efficiency

23. 23

24. 24

25. 25
Factors affecting the Selection of Equipment
 1. Plant Fuel Rate
 2. Plant Location in respect of Fuel Shipping
 3. Storage area Available
These aspects become important in view of
the high operating costs (50 -60% for the fuel
purchase and handling)

26. Fuels and Coal handling Equipments
(contd.)
Means of Coal Shipment
 1. Transportation by sea or river
 2. Transportation by rail
 3. Transportation by ropeways
 4. Transportation by road, and
 5. Transportation of coal by pipe line
Advantages of Transport by Pipeline
i. Simplicity in installation and increased safety in
operation
ii. More economical when large volumes need to be
transported over long distances
iii. Continuous operation and not affected by climate
and weather
iv. High degree of reliability v. loss due to theft and
pilferage eliminated
v. Man power requirement is low
26

27. Fuels and Coal handling Equipments
(contd..)
Requirement of Good Coal Handling Plant
 1. Should operate with minimum maintenance.
 2. Should be reliable
 3. Simple in design and to operate
 4. Require minimum of operatives
 5. Should be able to deliver requisite quantity of
coal at destination during peak demand
 6. There should be minimum wear in running the
equipment due to abrasive action of the coal
particles
27

28. 28
Advantages of Mechanical Handling:
1. Higher Reliability
2. Less Labour required
3. Economical for medium and Large capacity
plants
4. Operation is Easy and Smooth
5. Can be easily started and economically adjusted
as per need
6. Minimized labour and hence easier management
and control of the plant
7. Reduced health hazards as less labour involved
8. Losses in transport – minimized.
Mechanical Handling vs Manual handling

29. 29
Disadvantages of Mechanical Handling:
1. Needs continuous and timely attention for
maintenance and repairs (Preventive vis-
à-vis Breakdown Maintenance)
2. Higher Capital Cost
3. A part of Power is used up for running the
equipment.
 Reduced net power output.
Mechanical Handling vs Manual handling

30. 30
Coal Delivery  Unloading  Preparation
Transfer  Out door Storage  Covered
Storage  In-plant Handling  Weighing
and Measuring  Furnace Firing
1. Coal Delivery 2. Unloading 3. Preparation
6.Covered Storage 5.Out door Storage 4. Transfer
7. In-plant Handling
8. Weighing and
Measuring 9.Furnace Firing
 

31. 31
Various Stages of
Coal handling
1. Coal Delivery
2. Unloading
3. Preparation
4. Transfer
5. Outdoor Storage
6. Covered Storage
7. Inplant Handling.
8. Weighing and
measuring
9. Furnace Firing

32. 32

33. 33
Coal delivery
The coal from supply points is delivered by
ships or boats to power stations situated near
to sea or river whereas coal is supplied by
rail or trucks to the power stations which
are situated away from sea or river.
The transportation of coal by trucks is
used if the railway facilities are not available.

34.  The type of equipment to be used for unloading
the coal received at the power station depends
on how coal is received at the power station.
 If coal delivered by trucks, there is no need of
unloading device as the trucks may dump the
coal to the outdoor storage.
 Coal is easily handled if the lift trucks with
scoop are used.
34

35.  In case the coal is brought by railways
wagons, ships or boats, the unloading may be
done by car shakes, rotary car dumpers,
cranes, grab buckets and coal accelerators.
 Rotary car dumpers although costly are quite
efficient for unloading closed wagons.
35

36. 36
Lift Truck with Scoop
iv. Unloading Bridges
v. Self unloading Boats

37.  When the coal delivered is in the form of big
lumps and it is not of proper size, the
preparation (sizing) of coal can be achieved
by crushers, breakers, sizers, driers and
magnetic separators.
37

38. Fuel/Coal Handling Equipment
After preparation coal is transferred to the dead
storage by means of the following systems.
 1. Belt conveyors
 2. Screw conveyors
 3. Bucket elevators
 4. Grab bucket elevators
 5. Skip hoists
 6. Flight conveyor
38

39. 39
1. Belt Conveyor
i. Average Belt Speed: 60 – 100 m / minute
ii. Load Carrying Capacity of Belt: 50 – 100 t / h
iii. Distance Covered (Typical): 400 m
iv. Inclination of Drive: 20deg.
Note: Used in Medium and Large Power Plants.

40. 40

41. 41
2. Bucket Conveyor

42.  Bucket Conveyor/elevators move bulk
material vertically much like the conveyor
belt.
 Buckets are attached to a rotating belt and fill
with the material at the bottom of the
elevator then move it to a designated point.
 When the bucket reaches this point, it
discharges the contents, returns to the start
point, and begins the process again. Buckets
prevent spillage with their upright position
design.
42

43. 43
3. Screw Conveyor

44. 44
4. Grab Bucket Conveyor
Capacity
~ 50 tonnes/ h

45. 45
5. Flight Conveyor
Capacity: 10 – 100 tonnes/h
Inclination with horizl; 0 to 35 deg
Max speed~ 30 m/min

46. 46
Skip Hoist

47. 47

48. 48
Lay out of a Fuel handling Equipment

49. 49

50. 50
Outline of Coal Handling Equipment

51.  AFuel is basically a source of heat. The usual method of
producing heat from fuel is by the process of
Combustion.
 It is a chemical reaction between the Fuel and the
Oxidant.

52. The combustion efficiency depends on:
 Freely ignited Fresh charge of fuel
 Steady combustion for obtaining the desired amount of heat
release.
 Adequate combustion space
 Quantity of air supply for complete combustion
 The method of air supply
Combustion equipment includes heaters, ovens, stoves,
furnaces, fireplaces, dryers, stokers, burners, and many
more.

54. 54
FIRING SYSTEM
The firing system is mainly classified into two types
1) Hand firing 2) Stoker firing.
Hand firing:
• This is the simplest method of fuel firing. The
combustion efficiency is very low, when compared to
others.
• Due to lower combustion efficiency it cannot be
used in Modern power plants.

55.  This is a simple method of firing coal into the furnace.
It requires no capital investment. It is used for smaller
plants.
 Adjustments are to be made every time for the supply
of air when fresh coal is fed into furnace.
 Hand fired grates: used to support the fuel bed and
admit air for combustion.
 While burning coal the total area of air openings
varies from 30 to 50% of the total grate area.
 The grate area required for an installation depends
upon various factors such as its heating surface, the
rating at which it is to be operated and the type of fuel
burnt by it.
 The width of air openings varies from 3 to 12 mm.
 The construction of the grate should be such that it is
kept uniformly cool by incoming air. It should allow
ash to pass freely.
 Hand fired grates are made up of cast iron.

56. 56
Stoker Firing
• A stoker is a power- operated fuel feeding
mechanism using a grate.
• This method of firing is used for burning solid coal
on a grate. Stoker are classified as follows:
1) overfeed stokers 2) underfeed stokers

57. 57
Stokers
Overfeed
Stoker
Travelling Grate
Stoker
Chain Grate
Stoker
Bar
Grate Stoker
Spreader
Stoker
Underfeed
Stoker
Single Retort
Stoker
Multi Retort
Stoker

58. 58
Over feed stokers
• In the overfeed mechanism a forced draught fan slightly
pressurizes the atmosphere air before it enters under
the bottom of the grate.
• The fuel bed receives fresh coal on top surface
• The ignition zone lies between green coal and
incandescent coke.
• The air gets heated up as it follows through the grate
openings where as the grate gets cooled.
• This warm air gets additional heat energy by further
passing through a layer of hot ashes.

59. 59

60. Primary Air
Secondary A
Flame
Green Coal
Incandescent coke
Grate
CO+CO2+N2+
VM+CO+CO2
O2+CO2+N2+H
ASH

61. 61
Different types of over feed stokers:
These type of stokers are used for large capacity
boiler installations where the coal is burnt
without pulverization.
1)Travelling grate stoker
a) Chain grate type
b) Bar grate type
2) Spreader stoker

62. 62
 A chain grate stoker consists of an endless
chain which forms a support for the fuel bed.
 The chain surface is made of a series of cast
iron links connected by pins.


63. 63
 The chain travels over two sprocket wheels
located at the front and at the rear of the
furnace.
 The front sprocket is connected to a variable
speed drive mechanism
 Speed range 15 cm/ min to 50 cm/ min

64.  The chain travels over two sprocket wheels, one at the front and one at the rear of furnace. The
traveling chain receives coal at its front end through a hopper and carries it into the furnace.
 The speed of grate (chain) can be adjusted to suit the firing condition.
 The air required for combustion enters through the air inlets situated below the grate.
 The stokers are suitable for low ratings because the fuel must be burnt before it reaches the rear of
the furnace.

65. 65
ADVANTAGES:
1) Simple in construction
2) Low initial cost.
3) Self-cleaning stoker.
4) The rate of heat release can be controlled just by
controlling chain speed.
5) Rate of heat release is high per unit volume of furnace.
DISADVANTAGES:
1) cannot be used for high capacity boilers.
2) temperature of pre-heated air limited to 180 0 C
3) clinker troubles are very common.
4) loss of coal in the form of fine particles - carried away
with the ash.

66. 66
 Spreader stoker mechanism involves throwing
(spreading) the coal uniformly on the grate.
 The grate may be of stationary or moving type, with
air openings for admitting the air.
 The selection of coal size is very important for a
spreader stoker
-- the coal size should be in between 6 cm to 36 cm.
 The spreader stoker is mostly used for steam
capacities of 9.5 to 50 kg/sec. (34 to 180 tonnes/hr)
 It can burn a wide variety of coals from high ranking
bituminous to lignite.

67. 67

68. 68

69. 69
Advantages:
1) A wide variety of coal can be burnt.
2) The clinkering problems can be reduced by spreading action.
3) High temperature preheated air can be used.
4) Volatile matter is removed by burning coal in suspension.
5) Good response to load fluctuation.
6) Low running cost.
Disadvantage:
 Difficult to operate with varying sizes of coal with
varying moisture content.
 Fly ash is a major problem.
 Fuel loss due to suspension and exhaust gases.

70. 70
1) This underfeed mechanism is best situated for
bituminous and semi bituminous coals.
2) In this underfeed mechanism the fuel is fed
from underneath the fire and moves upwards
gradually.
3) The air entering through the grate opening
comes in contact with raw fuel and mixes with
the volatile matter released from raw fuel and
enters into the combustion chamber.

71. 71
Principle of Underfeed Stokers

72. 72

73. 73
Different types of under feed stokers.
1) single retort stoker
2) multi-retort stoker.
Single retort stoker
 The single retort stoker consists of a trough
shaped retort to which the fuel (coal) is fed by a
reciprocating ram or screw conveyor.
 The capacity of this stoker ranges from 100 to
2000 kg of coal burned per hour.

74. 74
 The coal falling from the hopper is pushed forward
during the inward stroke of stoker ram. The distributing
rams (pushers) then slowly move the entire coal bed
down the length of stoker.
 The slope of stroke helps in moving the fuel bed
downwards.
 The primary air enters the fuel bed from main wind box
situated below the stoker. Partly burnt coal moves on to
the extension grate.
 The air entering from the main wind box into the
extension grate wind box is regulated by an air damper.

75. 75
The number of retorts may vary from 2 to 20 with
coal burning capacity of 300 to 2000 kg/hr.

76. 76
1) High thermal efficiency when compared with
chain grate stoker.
2) The grate is self cleaning.
3) Part load efficiency is high with multiple retort
system.
4) high combustion rate.
5) wide variety of coals can be used.
6) best suitable for non-clinkering, high volatile
and low ash content coals.

77. 77
1) Initial cost is high.
2) Large building area is required.
3) Clinker troubles are usually
present.
4) Low grade fuels with high ash
content
cannot be burnt economically.

78. 78
• A pulverized coal-fired boiler is an industrial or utility
boiler that generates thermal energy by burning pulverized
coal (also known as powdered coal or coal dust since it is
as fine as face powder in cosmetic makeup) that is blown
into the firebox.
• The basic idea of a firing system using pulverised fuel is to
use the whole volume of the furnace for the combustion of
solid fuels.
• Coal is ground to the size of a fine grain, mixed with air and
burned in the flue gas flow. Biomass and other materials
can also be added to the mixture.

79. 79
• Coal contains mineral matter which is converted to ash
during combustion. The ash is removed as bottom ash
and fly ash.
• The bottom ash is removed at the furnace bottom.
• This type of boiler dominates the electric power industry,
providing steam to drive large turbines.
• Pulverized coal provides the thermal energy which
produces about 50% of the world's electric supply.

80. 80
Pulverized coal power plants are broken down into three
categories; subcritical pulverized coal (SubCPC) plants,
supercritical pulverized coal (SCPC) plants, and ultra-
supercritical pulverized coal (USCPC) plants.
The primary difference between the three types of pulverized
coal boilers are the operating temperatures and pressures.
• Subcritical plants operate below the critical point of
water (647.096 K and 22.064 MPa).
• Supercritical and ultra-supercritical plants operate above
the critical point. As the pressures and temperatures
increase, so does the operating efficiency.

81. 81
There are two requirements which are must for
Pulverised Coal to burn successfully in a furnace.
1) Presence of large quantities of Fine particles
of coal usually that would pass enough through
a 200-mesh to ensure Spontaneous Ignition
because of their large surface to volume ratio.
2) Presence of minimum quantity of Coarser
particles to ensure High Combustion Efficiency.

82. 82
ELEMENTS OF PULVERIZED COAL SYSTEM

83. 83

84. 84
1) Any grade ofcoal can be used efficiently
because it is powdered before use.
2) Higher boiler efficiency due to complete
combustion.
3) Flexible method and can respond well for
sudden change in demand.
4) Fan power required is low.
5) Free from clinker problem.
6) Volume of the furnace required is less.
7) No major problem in ash handling.

85. 85
8) The system works successfully with or in
combination with gas and oil.
9) No moving parts in the furnace are subjected
to high temperatures.
10) Much smaller quantity of air is required as
compared to the stoker firing.
11) The external heating surfaces are free from
corrosion.
12) It is possible to use highly preheated
secondary air (350 deg. C) which helps in rapid
flame propagation.

86. 86
1) Additional investment for coal preparation
unit/ plant.
2) Extra power is needed for pulverising coal.
3) Maintenance cost is more which depends on
quality of coal.
4) Due to very high temperature, maintenance of
furnace walls is difficult.
5) More space is required.
6) Special equipment is required to start the
system.

87.  Coal is pulverized (powdered) to increase its surface area (and
therefore exposure) thus permitting rapid combustion.
 The pulverized coal is obtained by grinding the raw coal in
pulverizing mills. Various types of pulverizing mills are:
 Ball mill
 Ball and race mill
 Hammer mill
 Bowl mill
Essential functions of pulverizing mills are:
 Drying of the coal
 Grinding
 Separation of particles of a desired size.
 Coal pulverizing mills reduce coal to powder by any (or all) action
such as
 Impact, Abrasion and Crushing

88.  This is a low speed
unit in which
grinding pressure is
maintained by
adjustable springs.
 The coal passes
between the two
rotating elements
again and again until
it has been
pulverized to desired
degree of fineness.

90. 90

92. Basically, pulverized fuel plants
may be divided into two systems
based on the method used for
firing the coal:
 Unit System or Direct System
 Bin System or Central System
 Unit or Direct System: This
system works as follows:
 Coal from bunker drops on to
the feeder.
 Coal is dried in the feeder by
passage of hot air.
 The coal then moves to a mill
for pulverizing.
 A fan supplies primary air to
the pulverizing mill.
 Pulverized coal and primary air
are mixed and sent to a burner
where secondary air is added.

93.  Bin or Central System:
 Coal from bunker is fed by gravity to a dryer where hot air is admitted to dry
the coal.
 Dry coal is then transferred to the pulverizing mill.
 Pulverized coal then moves to a cyclone separator where transporting air is
separated from coal.
 Primary air is mixed with coal at the feeder and supplied to the burner.
 Secondary air is supplied separately to complete the combustion

94. 94
1) Simple layout and easy operation.
2) It requires less space.
3) It is cheaper when compared with central
system.
4) Less maintenance.
5) Simple coal transportation system.
6) Direct control of combustion from the
pulveriser is possible.
7) Better control over fuel feed rate.

95. 95
1) Less flexible when compared to central system.
2) With the load factor in common practice, the
total capacity of all the mills must be higher than
for the control system.
3) Any fault in the coal preparation unit may stop
the entire steam generating system.
4) Excessive wear and tear of the fan blades as it
handles air and coal particles.

96. 96

97. 97
1) More flexible because the quantity of fuel and
air can be controlled separately.
2) More reliable.
3) No problem of excessive wear of fan blades.
4) Less labour is required.
5) Low power consumption per tonne of coal
handled.

98. 98
1) High initial cost.
2) It requires large space area.
3) Possibility of fire and explosion hazards.
4) Driers are necessary.
5) Operation and maintenance costs are high
when compared to unit system of same
capacity.
6) More number of auxiliaries.

99. Various types of burners are used
for combustion of pulverized coal.
99

100. Contd.

101. 10
1

102. Various types of burners are used for
combustion of pulverized coal.
10
2

103. 10
3

104.  Long Flame (U-Flame) Burner: In this burner, air and coal
mixture travels a considerable distance thus providing sufficient
time for complete combustion

105. 10
5

106. 10
6

107. 10
7
 In this system, the coal and air will be
mixed due to cyclonic whirling action
 The coal is crushed into tiny powder in
addition to pulverized coal
The ash is easily collected due to cyclonic
action

108. 10
8

109. 10
9

110.  Cyclone Burner: In this system, the cyclonic action whirls coal
and air against the wall of the furnace to facilitate thorough
mixing of coal and air.
 Advantage of this burner is that it can also use crushed coal
in addition to pulverized coal thus providing an option.
 When crushed coal is used, ash is collected in molten form for
easy disposal.


111. 11
1
1) Costly pulverisers are not required. Instead,
simple coal crushing equipment can be used.
2) By using forced draught fan it can be operated
with small quantities of excess air.
3) It can burn low grades of coal effectively.
4) High temperatures are obtained.
5) Boiler fouling (sticking) problems can be
reduced as all the incombustibles are retained in
the cyclone burner.
6) Boiler efficiency is increased.

112. 11
2

113. 11
3
Ash Disposal Equipment.
General Layout of Ash handling and Dust Collection System

114. 11
4

115. 11
5
Hydraulic System

116. 11
6
Ash Disposal Equipment contd.
Low Pressure System

117. 11
7
Below the boiler nozzles(top and sides), are fixed

118. 11
8

119. 11
9
Pneumatic or Vacuum Extraction Ash Handling
System

120. 12
0

121. 12
1

122. Ash handling Equipment-Desired Characteristics
12
2
i. Enough capacity to cope up with the volume of ash
being produced
ii. Able to handle large clinkers (stony residue),
boiler refuse, soot etc. with little personal attention of
the workmen.
iii. Able to handle hot and wet ash effectively and with
good speed.
iv. Possible to action of the ashes while minimize
corrosive or abrasive
v. economical in erection and operation.
vi.noiseless
vii. Should be possible to add additional units

123. Equipment commonly used for Ash Handling
12
3
i. Bucket Elevator
ii. Bucket Conveyor
iii. Belt Conveyor
iv. Pneumatic Conveyor
v. Hydraulic Sluicing Equipment
vi. Trollies or Rail cars etc.

124. 12
4
 i. in the production of Cement
 ii. Production of concrete (20% Fly Ash And
30% Bottom Ash)
 iii. Treating acidic soils
 iv. Extraction (Recovery) of metals such as
Al, Fe, Si, and Titanium from the ash

125. 12
5
Ash Disposal Equipment

126. General layout of ash handling system
126

127. Mechanical ash handling system
127

128. Hydraulic ash handling System
128
• Low velocity

129.  High velocity
129

130. Pneumatic ash handling system
130

131. DUST COLLECTORS
131
 Dust collectors are grouped into two types
1.Mechanical Dust Collectors
i) Wet type dust collectors
ii) Dry type dust collectors
Gravitational separators
Cyclone separators
2.Electrical Dust Collectors
i) Rod type
ii) Plate type

132. Basic Principles of Mechanical Dust
Collectors
132
 Enlarging the duct cross sectional area to slow down the gas gives
the heavier particles a chance to settle out.
 When a gas makes a sharp change in flow direction, the heavier
particles tend to keep going in the original direction and so settle
out.
 Impingement baffles have more effect on the solid particles than the
gas, helping them to settle.

133. Mechanical Dust Collectors
133
i) Wet type dust collectors:
 Wet types called scrubbers operate with water sprays to wash dust
from the air.
 Large quantities of wash water are needed for central station gas
washing that this system is seldom is used.
 It also produces a waste water that may require chemical
neutralization before it can be discharged into the central bodies of
water.
ii) Dry type dust collectors:
These are commonly used dust collectors.
 a) Gravitational Separators: These collectors act by slowing down
gas flow so that particles remain in a chamber long enough to settle
in the bottom. They are not very suitable because of large chamber
volume needed.

134. Cyclone (or) Centrifugal
separator
134
• The cyclone is a separating chamber
where in high-speed gas rotation is
generated for the purpose of centrifuging
the particles from the carrying gases.
• There is an outer downward flowing
vortex which turns into an inward flowing
vortex.
• Involute inlets and sufficient velocity head
pressures are used to produce the
vortices.
• The factors which affect the performance
are gas volume, particles loading, inlet
velocity, temperature, diameter- to- height
ratio of cyclone and dust characteristics.

135. Advantages and Disadvantages of Cyclone Separator
135
ADVANTAGES:
 Rugged in construction
 Maintenance costs are relatively low
 Efficiency increases with increase in load
 Easy to remove bigger size particles
DISADVANTAGES:
 Requires more power than other collectors
 Incapable to remove dust and ash particles which
remain in suspension with still air
 Less flexible
 High pressure loss
 Requires considerable head room and must be placed
outside the boiler room

136. Electrical Dust Collector
136
The main elements of an electrostatic precipitator are:
 Source of high voltage
 Ionizing and collecting electrodes
 Dust removal mechanism
 Shell to house the elements

137. Electrical Dust Collector
137
 The precipitator has two sets of electrodes, insulated
from each other, that maintain an electrostatic field
between them at high voltage
 The field ionizes dust particles that pass through it,
attracting them to the electrode of opposite charge.
 The high voltage system maintains the negative potential
of 30,000 to 60,000 volts with the collecting electrodes
grounded.
 Accumulated dust falls of the electrode when it is rapped
mechanically
 Wet type of unit removes dust by a water film flowing
down on the inner side of the collecting electrode.
 These units have collection efficiency around 90%

138. Electrical Dust Collector
138
ADVANTAGES:
 Can effectively remove very small particles like smoke, mist and
fly-ash
 Easy operation
 Draught loss is quite less
 Most effective for high dust loaded gas
 Maintenance charges are minimum
 The dust collected in dry form and can be removed either dry or
wet
DISADVANTAGES:
 Space requirement is more
 Need to protect the collector from sparking
 Running charges are high
 Capital cost of equipment is high

139. Efficiency of Dust
Collectors
139
 The collection efficiency of a dust separator is the amount of dust
removed per unit weight of dust.
 Though dust collectors remove contaminants, they increase draught
losses and hence the fan power.
 The absolute efficiency of a dust collector is the percentage of
entering solids that will be removed by the collector.

140. Layout of Dust Collection
System
140