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Flue Gas systemAir pre Heater
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Presentation Plan Air heaters
Types of air heaters
Materials Used
Sealing arrangement for air heaters
Air heater Performance
Performance tests
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APH is the last heatexchanger in theboiler flue gascircuit. To achievemaximum boilerefficiencymaximumpossible usefulheat must beremoved from thegas before itleaves the APH.However certainminimumtemperature hasto be maintainedin the flue gas toprevent cold endcorrosion
AIR PRE HEATERRADIAL SEAL
AXIAL
SEAL
BYPASS SEAL
COLD END
HOT END
HOT INTERMEDIATE
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Air Pre-Heater-functions An air pre-heater heats the combustion air where it is
economically feasible.
The pre-heating helps the following:
Igniting the fuel.
Improving combustion.
Drying the pulverized coal in pulverizer.
Reducing the stack gas temperature and increasing theboiler efficiency.
There are three types of air heaters:Recuperative
Rotary regenerative
Heat pipe
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Advantages by use of APH
Stability of Combustion is improved by use of hot air. Intensified and improved combustion.
Permitting to burn poor quality coal.
High heat transfer rate in the furnace and hence lesser heattransfer area requirement.
Less un-burnt fuel particle in flue gas thus combustion andefficiency is improved.
Intensified combustion permits faster load variation andfluctuation.
In the case of pulverized coal combustion, hot air can beused for heating the coal as well as for transporting thepulverized coal to burners.
This being a non-pressure part will not warrant shut-down ofunit due to corrosion of heat transfer surface which isinherent with lowering of flue gas temperature.
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Types Of Air Preheater Recuperative
Regenerative
Plate type Airheater
Steam Air Preheater Langsdorm type
Rothemuhle type
Tri sector Air Heater
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Tubular Air Heaters(Recuperative)
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Tubular Air Pre-Heater
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Design Parameters Tubes are generally arranged in staggered pattern.
Steel tubes of Dia: 37 63 mm.
Transverse pitch: S1/d = 1.5 1.9
Longitudinal pitch: S2/d = 1.0
1.2 The height of air chamber:1.4 4.5 m.
Gas and Air flow velocity : 10 16 m/s.
Plate Recuperators:
Instead of tube, parallel plates are used. The gas passage is 12 16 mm wide.
The air passage is 12 mm wide.
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Regenerative air pre heaterRADIAL SEAL
AXIAL
SEAL
BYPASS SEAL
COLD END
HOT END
HOT INTERMEDIATE
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Rotary Plate (Regenerative)type Pre-Heater Rotates with a low speed : 0.75 rpm.
Weight : 500 tons.
This consists of : rotor, sealing apparatus, shelletc.
Rotor is divided into 12 or 24 sections and 12 or 24
radial divisions.
Each sector is divided into several trapezoidalsections with transverse division plates.
Heat storage pales are placed in these sections.
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The Material used in APH forheat storage
Material used Cold end in the basket is a special type ofsteel (corten steel (trade name)) which has high resistanceto the low temperature sulphur corrosion, thus prolongingoperational life.
In the hot end mild steels are used
The optimal geometric shape is usually corrugated andsizes are determined based on design modelling andexperimental data. The turbulence of air and gas flowthrough the package increases the heat transfer rate.
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Stationary-Plate Type Air Pre-Heater
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Stationary-Plate Type Air Pre-Heater The heat storage elements are static but the air/gas flow
section rotates.
The storage plates are placed in the stator.
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Trisector air Heater
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Heating Elements
Hot End Baskets
Hot Intermediate Baskets
Cold End Baskets
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TYPES OF SEALS
RADIAL SEAL (HE & CE)
AXIAL SEAL
CIRCUMFERENTIAL SEAL
ROTOR POST SEAL
SECTOR PLATE STATIC SEAL
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1. HE Radial seal leakage - 62.21%
2. CE Radial seal leakage - 11.98%
3. Axial seal leakage - 08.78%
4. By pass or circumferential seal leakage - 0.87%
5. Center post seal leakage - 3.17%
_______________________________________________
Total percentage = 87.01%
Entrapped leakage = 12.99%
TOTAL = 100%
PERCENTAGE AIR LEAKAGE
OF TOATAL LEAKAGES
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RADAIAL SEALS &SECTOR PLATE
RADIAL SEALS AND SECTOR PLATES ARE
LOCATED AT THE HOT AND COLD ENDS OF THE AIR
PREHEATER. THE RADIAL SEALS ARE ATTACHED
TO THE DIAPHRAGMS, WHICH SEPARATE THEINDIVIDUAL ROTOR COMPARTMENT.
PURPOSE: - THE PURPOSE OF RADIAL SEALS IS
TO REDUCE THE AREA AVAILABLE FOR LEAKAGE
FROM THE AIR TO THE GAS SIDE BETWEEN THEDIAPHRAGM AND THE SECTOR PLATE
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AXIAL SEALS ANDSEALING PLATES AXIAL SEALS MINIMIZE LEAKAGE PASSING
RADIALLY AROUND THE ROTOR SHELL. THE AXIALSEALS ARE MOUNTED ON THE OUT SIDE OF THE
ROTOR SHELL AND SEAL AGAINST THE AXIAL SEAL
PLATES MOUNTED ON THE AIR PREHEATER
HOUSING.
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AXIAL SEAL DIAPHRAGM
COG RIM PIN
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AXIALSEALPLATE
JACK BOLT
AXIAL SEAL INSPECTION DOOR
ADJUSTABLEBOLT
TURN-BUCKLE
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Axial Seal Arrangement Curved axial sector plate adjustable from
outside
Seal strips are attached to the rotor.
The thickness of seal strips :
6 MM straight strips in Russian.
2.5 mm thick and bend backward in
BHEL.
BHEL APH has better accessibility of axialseal adjustment as compared to Russian
design
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CIRCUMFERENTIALSEALS THE CIRCUMFERENTIAL SEALS PREVENT
AIR AND GAS FROM BYPASSING THE
HEATING SURFACE THROUGH THE SPACE
BETWEEN THE ROTOR AND THE HOUSING
SHELL. THEY ALSO PREVENT AIR AND GAS
FROM FLOWING AXIALLY AROUND THE
ROTOR.
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CIRCUMFERENTIAL SEAL-
RUSSIANCIRCUMFERENTIAL SEAL
H.E. ROTOR FLANGE2.5MM RADIAL SEALH.E. DIAPHRAGM
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ADJUSTABLE BOLTCIRCUMFERENTIAL SEAL
ROTOR FLANGE AXIAL SEAL
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CIRCUMFERENTIAL SEALS
ACTUATING MECHANISM-
RUSSIANActuating Bolt
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ROTOR POST SEALS ROTOR POST SEALS PREVENT LEAKAGE
BETWEEN THE ENDS OF THE ROTOR POST AND
THE AIR PREHEATER HOUSING.
THE STATIC SEALS PREVENT LEAKAGE BETWEENTHE HOT & COLD END SECTOR PLATES AND THE
HOT AND COLD END CENTER SECTIONS.
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SECTOR PLATE STATIC SEAL
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DIAPHRAGM
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AIR SEAL HOUSING
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ROTOR POST SEAL
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THICKNESS OF RADIAL SEALSTRIPS
RUSSIAN MODEL : 6 MM
BHEL DESIGN : 2.5 MM
SOFT SEAL : 0.1 MM
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FLEXIBLE / SOFTSEALS THE FLEXIBLE SEALS WAS DEVELOPED TO
REDUCE NORMAL LEAKAGE CAUSED BY THE
THERMAL EXPANSION OF THE ROTOR WHILE THE
UNIT IS OPERTAING. THE ROTOR EXPANSION
OPENS UP AREAS OF DIRECT AIR TO GASLEAKAGE THAT CAN BE GREATLY REDUCED BY
INSTALLING FLEXIBLE SEALS.
MERIT : - SOFT SEAL IS SET TO A NEGATIVE
CLEARANCE IN COLD CONDITION, AND WHICHWILL EXTEND IN THE HOT CONDITION TO
OPERATE AS A STANDARD PROXIMITY SEAL.
DEMERIT : - SINCE THIS IS AN INTERFERENCE OR
CONTACT SEAL, THE WEAR LIFE IS VERY LOW.
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COST OF SOFTSEALS RADIAL SOFT SEAL HE/ SET :- 1 LAC
RADIAL SOFT SEAL CE/ SET :- 0.95 LAC
AXIAL SOFT SEAL/ SET :- 0.4 LAC
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MODULAR APH - BHEL
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RADIAL SEAL
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BY PASS SEAL RUSSIAN
BY PASS SEALRUSSIAN
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APH PERFORMANCE Boiler efficiency decreases generally on account of APH performance
degradation. This also affects ESP, ID & FD fan loadings & at timesunit capability
Factors affecting APH performance
Excess air level / No of Mills in service
Primary Air to Secondary Air ratio
Moisture in coal/ Air ingress level
Performance of upstream ash evacuation system
Procedure for cleaning, soot blowing & regular maintenance etc.
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APH PERFORMANCE Higher than expected leakage would decrease the flue
gas exit temperature, resulting in false sense ofimproved working.
Higher inlet flue gas temperature is rather rare, but thiscould be one reason for high exit temperature.
Optimum flue gas temperature is required for effectiveESP performance
Unequal temperature at air heater exit should beinvestigated.
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FLUE GAS EXIT TEMP ATAPH OUTLET FLUE GAS TEMP AT AH OUTLET IS INDICATIVE OF HEAT
LEAVING THE UNIT .THIS IS LOWERED ON ACCOUNT OFAH LEAKAGES.
FGET TO BE MEASURED AT A LOCATION SLIGHTLY
AWAY FROM AIR HEATERS.
NO OF TEMPERATURE SENSOR PROVIDED SHOULDCOVER THE DUCT ADEQUATELY.
CORRECTED TEMP SHOULD BE USED FORCOMPARISION.
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Oxygen in Flue Gas at AH A Inlet / Outlet
0
2
4
6
8
10
A B C D E F
Probe
InletO2%
0
2
4
6
8
10
12
OutletO2
%
Inlet O2 Outlet O2
Oxygen in Flue Gas at AH B Inlet / Outlet
0
2
4
6
8
10
A B C D E F
Probe
InletO2
0
2
4
6
8
10
OutletO2
Inlet O2 Outlet O2
Typical Oxygen Levels at
APH Inlet / Outlet
CO t i f d d t hi h
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Air Leakage Weight of air passing from air side to gas side; This
leakage is assumed to occur entirely between air inlet and
gas outlet
Hot End / Cold End / Entrained Leakage
Calculation Empirical relationship using the change in
concentration of O2 or CO2 in the flue gas
= CO2in - CO2out * 0.9 * 100
CO2out
= O2out - O2in * 0.9 * 100 = 5.72.8 * 90
(21- O2out) (21-5.7)
= 17.1 %
CO2 measurement is preferred due to high
absolute values; In case of any measurement
errors, the resultant influence on leakage
calculation is small.
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PERFORMANCEDEGRADATION OF APH
Seal Leakage Erosion
Corrosion
High Press Drop Across APH APH Fire
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APH Performance Test APH Leakage
Gas Side Efficiency
X-Ratio
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GAS SIDE EFFICIENCY
= (Temp drop / Temperature head) * 100
X- RATIO = T (gas in) T (gas out) (no lkg)/ T(air out)
T (air in)
Air Leakage = CO2in - CO2out * 0.9 * 100
CO2out
= O2out - O2in * 0.9 * 100
(21- O2out)
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FG TEMP (Corr.) Vs BLR EFF & GAS SIDE EFF
85
85.2
85.4
85.6
85.8
86
86.2
150 155 160 165 170 175 180
CORRECTED FLUE GAS TEMP (Deg C)
BOILEREFFICI
ENCY(%)
58
59
60
61
62
63
64
65
66
G
ASSIDEEFFIC
IENCY(%)
Boiler Efficinecy
Gas Side Efficiency
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XRatioRatio of heat capacity of air passing through the air heater
to the heat capacity of flue gas passing through the air
heater.
= Wair out * Cpa
Wgas in * Cpg
= Tgas in - Tgas out (no leakage)Tair out - Tair in
Say AH leakage17.1%, Gas I n Temp333.5 C, Gas Out
Temp133.8 C , Air I n Temp36.1 C, Air Out Temp288 C
X ratio = (333.5150.5) / (28836.1) = 0.73
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X-Ratiodepends on
moisture in coal, air infiltration, air & gas mass flow rates leakage from the setting
specific heats of air & flue gas
X-ratio does not provide a measure of thermal
performance of the air heater, but is a measure of the
operating conditions.
A low X-ratio indicates either excessive gas weight
through the air heater or that air flow is bypassing the air
heater.
A lower than design X-ratio leads to a higher thandesign gas outlet temperature & can be used as an
indication of excessive tempering air to the mills or
excessive boiler setting infiltration.
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Flue Gas Exit Temperature
Flue Gas Exit Temperature is corrected for inlet air temperature
X1 = Reference Air Temp * (Gas Temp In Gas Temp Out) +
Gas Temp In * (Gas Temp Out - Air Temp In)
X2 = Gas Temp In - Air Temp In
EGTcor. = X1/ X2= 35 * (345-143.9) + 345 (143.941.6) / (34541.6)
= 139.5 C
EGT Corrected for inlet air temp and for AH leakage
= AL * Cpa * (EGTcor - Tair in) + EGTcorCpg * 100
= [13.7* (139.541.6)] / 100 + 139.5 = 152.9
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