Arrangement of Boiler Auxiliaries
Coal bunker: Stores crushed coal from CHP Made up of welded steel plates Kept at the top of the mills, so as to assist
induced gravity flow Normally 6 in number Coal feeder: Drag link mechanism used for the
transport of coal from bunker
Boiler Auxiliaries
Mills: For every 200 MW unit, there are 6 mills (25%
capacity) which powder the coal to the required fineness
Located at the same elevation as that of furnace Pulverization is done in the mill and crushing in
the bunker Primary Air fans (PA fans): 2 fans per 200 MW unit operating at 50%
capacity Heating air up to 50C
Boiler Auxiliaries
Air pre-heaters: It is used to preheat primary air and
secondary air using regenerative heat exchangers utilizing the flu gases
2 such heaters for each 200MW unit 16m elevation from datum• Burners: - to burn the pulverised coal
with oil
A short note on various heat exchangers
1) Shell and tube heat exchanger: used in suction heaters of oil fired TPPs
2) Plate heat exchangers 3) Adiabatic wheel heat exchangers 4) Plate fin heat exchangers 5) Regenerative heat exchangers
Boiler Auxiliaries
FD fans: Forced draft fans , 2 per 200MW unit Used to handle the secondary air 0 metres elevation, at the same level as of
the PA fan Wind box: To supply the excess air, placed on either
sides of the furnace facing chimney Distributing medium Scanner fan: To cool the scanner
Boiler Auxiliaries
Soot blowers: There are 3 types of soot blowers, viz
Long retractable soot blowers, air heater blowers, Wall blowers
• Pressure reducing valves
Why we need a fan
The fan is required for 2 purposes: -A) To provide sufficient oxygen to assist the
combustionB) To remove the products of combustionC) Evacuation of flue gases
Selection criterion for fan
Air or gas flow-mass flow rate (kg/hr) Density of gas (function of state variables
like temperature and pressure) System resistance
What is draft?
The term draft refers to the pressure difference between the atmosphere and that existing in the furnace
It is required to ensure that required amount of oxygen is available for combustion, removal of products of combustion, losses due to flow is to be minimized
Types of draft
Various types of drafts are A) Natural draft: Vacuum is created in
furnace by making very tall chimneys, this pressure difference draws air to furnace
B)Induced draft: The air is sucked into the furnace by the height of chimney and flue gases are pushed out by the ID fans. Negative pressure is created
C) Forced draft: It has positive pressure and air is pushed out by the pressurization by FD fan
Requirement of TPP
Mechanical drafts (ID and FD) are more advantageous than natural draft because proper combustion is ensured, easy flow of flue gases and more efficiency
Air is required to carry coal from mill to furnace
To remove the flue gases To ensure proper combustion
FURNACE
PRIMERY AIR HEATER
SECONDRY AIR HEATERMILLS
FD FAN A
PA FAN BPA FAN A
FD FAN B
ESP
ID FAN A
ID FAN B
CHIMNEY
Atmospheric air
Atmospheric air
Primary air system
The primary air is driven by axial flow reaction fan
Air discharging from each fan is divided into two parts, one passes first through a air pre-heater then through a gate into the P.A bus duct. The second goes to the cold air duct. The mix of both is used to carry the pulverized coal to the boiler.
Ignitor fan
These fans, again two per boiler, are used to supply air for cooling ignitors and combustion of ignitor air fuel mixture. Fan makes the suction from atmosphere directly and supplies air to the wind boxes of individual igniters at a fixed constant uncontrolled rate at ambient temperature.
Burner Box Dampers
Coal/Air Dampers
Secondary Air Dampers
Oil/Secondary Air Dampers
Bottom Tier Secondary Air Damper
Over Fire Damper
Secondary air Damper control
Fuel Air Dampers: which regulate air surrounding a fuel compartment. Fuel air dampers are modulated in proportion to the rate of fuel fired in that elevation.
Auxiliary Air Dampers: which regulate air
adjacent to a fuel compartment are termed. Auxiliary air dampers are modulated to maintain a fixed differential pressure between the wind box and the furnace.
Variation of Pressure in Boiler
Furnace Pressure At Various Points in Boiler
-300
-250
-200
-150
-100
-50
0
50
100
150
200
250
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Points in Boiler
Furn
ace
Pre
ss
ure
1FD Fan Inlet
2FD Fan Outlet
3Airheater Inlet
4Airheater Outlet
5Windbox Pressure
6 Furnace
7
Superheater Platen Inlet
8Reheater Inlet
9 LTSH Inlet
10 Economiser Inlet
11Airheater Inlet
12E.P. Inlet
13I.D. Fan Inlet
14I.D. Fan Outlet
Hoisting machines used in ASHA
Chains: 2 types. Viz coil chains and stud link chains
Coil chains are used in hoists, cranes and dredges
Made of open hearth steel for smaller ones and high grade wrought iron for larger ones
Load capacity equals 1.5 times proof load or load at the elastic limit whichever may be the least
FOS=2 for normal applications, 3 for
3 for intermittent applications, 3.5 for continuous machine operations
Stud link chains are used for anchors and moorings
Can carry loads which are 20-25% more than open link chains
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