Cause-Effect Analysis of Steam Generator

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Cause-Effect Analysis of Steam Generator Rule
Based Design

• BY
• P M V Subbarao
• Associate Professor
• Mechanical Engineering Department
• I I T Delhi

Observation Experience based methods for
design of complex systems ..
2
Cause Effect Analysis

• Combustion is a cause
• Steam Generation is an effect
• Heat transfer is a mediation.
• Combustion caused generation of flame heat in
  side furnace volume and finally produces high
  temperature gases.
• These high temperature gases will initiate
  Radiation and convection heat transfer.
• Heat Transfer carries heat to furnace wall.
• Furnace wall transfer heat to steam tubes.
• Steam tubes transfers the same to steam by means
  of Heat Conduction.
• A relatively cold exhaust leaves the furnace.
• This is final effect in the furnace!!!!!!!

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Analysis of Primary Cause

• Combustion caused generation of flame heat in
  side furnace volume and finally produces high
  temperature gases.
• This cause can be defined as combustion in an
  adiabatic furnace.

CXHYSZOK e 4.76 (XY/4Z-K/2) AIR Moisture
in Air Ash Moisture in fuel ? P CO2 Q H2O R
SO2 T N2 U O2 V CO W C Ash
Adiabatic furnace to increase the enthalpy of
gas
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Temperature of gasses coming out of an adiabatic
furnace is called as Adiabatic (Flame)
Temperature. Adiabatic Flame Temperature is A
primary Cause.
Furnace with absorbing walls or boiler tubes.
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Furnace Exit Gas Temperature

• The temperature of products of combustion at the
  exit of the furnace is called FEGT.
• An important design parameter.
• Defines the ratio of furnace heat absorption to
  outside heat absorption.
• High FEGT Compact furnace Large secondary
  section
• FEGT lt Ash Deformation Temperature.
• Generally FEGT Ash Softening Temperature 100.
• General design conditions.
• FEGT lt 1100 0C Strong slag (Molten Ash).
• FEGT lt 1200 0C Moderate slag

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General Rules for Rule based Design

• The furnace should provide the required physical
  environment and the time to complete the
  combustion of fuel.
• The furnace should have adequate radiative
  heating surfaces to cool the flue gas
  sufficiently to ensure safe operation of the
  downstream convective heating surface.
• Aerodynamics in the furnace should prevent
  impingement of flames on the water wall and
  ensure uniform distribution of heat flux on the
  water wall.
• The furnace should provide conditions favoring
  reliable natural circulation of water through
  water wall tubes.
• The configuration of the furnace should be
  compact enough to minimize the amount of steel
  and other construction material.

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Basic Geometry of A Furnace
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Determination of Furnace Size

• What is the boundary of a furnace?
• The boundary of a furnace is defined by
• Central plane of water wall and roof tubes
• Central lines of the first row super heater
  tubes.
• ? 30 to 50O
• ? gt 30O
• ? 50 to 55O
• E 0.8 to 1.6 m
• d 0.25 b to 0.33 b

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Design ConstrainsHeat Release Rate

• Heat Release Rate per Unit Volume, qv, kW/m3
• Heat Release Rate per Unit Cross Sectional
  Area,qa, kW/m2
• Heat Release Rate per Unit Wall Area of the
  Burner Region, qb, kW/m2

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Heat Release Rate per Unit Volume, qv

• The amount of heat generated by combustion of
  fuel in a unit effective volume of the furnace.
• Where, mc Design fuel consumption rate, kg/s.
• V Furnace volume, Cu. m.
• LHV Lower heating value of fuel kJ/kg.
• A proper choice of volumetric heat release rate
  ensures the critical fuel residence time.
• Fuel particles are burnt completely.
• The flue gas is cooled to the required safe
  temperature.

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Heat Release Rate per Unit Cross Sectional Area,qa

• The amount of heat released per unit cross
  section of the furnace.
• Also called as Grate heat release rate.
• Agrate is the cross sectional area or grate
  area of the furnace, Sq. m.
• This indicates the temperature levels in the
  furnace.
• An increase in qa, leads to a rise in temperature
  in burner region.
• This helps in the stability of flame
• Increases the possibility of slagging.

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A
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Heat Release Rate per Unit Wall Area of the
Burner Region

• The burner region of the furnace is the most
  intense heat zone.
• The amount of heat released per unit water wall
  area in the burner region.
• a and b are width and depth of furnace, and Hb is
  the height of burner region.
• This represents the temperature level and heat
  flux in the burner region.
• Used to judge the general condition of the burner
  region.
• Its value depends on Fuel ignition
  characteristics, ash characteristics, firing
  method and arrangement of the burners.

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Furnace Depth Height

• Depth (a) to breadth (b)ratio is an important
  parameter from both combustion and heat
  absorption standpoint.
• Following factors influence the minimum value of
  breadth.
• Capacity of the boiler
• Type of fuel
• Arrangement of burners
• Heat release rate per unit furnace area
• Capacity of each burner
• The furnace should be sufficiently high so that
  the flame does not hit the super heater tubes.
• The minimum height depends on type of coal and
  capacity of burner.
• Lower the value of height the worse the natural
  circulation.

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Performance of Analysis of Furnace

• Get Fuel Ultimate Analysis.
• Compute Equivalent Chemical Formula.
• Select recommended Exhaust Gas composition.
• Carry out first law analysis to calculate
  Adiabatic Combustion Temperature.
• Total number of moles of wet exhaust gas for 100
  kg of fuel nex.gas PQRTUV
• 100 X CV of fuel Snex. Gashf,gas
• Calculate Adiabatic Flame Temperature.
• Calculate total heat transfer area of furnace,
  Afur

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Furnace Characterization Criteria

• G Furnace quality factor
• M Temperature Field Coefficient
• Tad Theoretical combustion temperature
• Tout Furnace Exit Gas Temperature
• Afur Total surface area of furnace
• mf Flow rate of fuel

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Furnace Exit Gas Temperature

• FEGT AST 100
• FEGT lt 1100 0C Strong slag
• FEGT lt 1200 0C Moderate slag
• FEGT lt 1250 0C -- Weak slag
• Any design procedure can be used but it should
  satisfy the requirements of FEGT.

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Effect of Coal Quality on Furnace Size
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Role of SG in Rankine Cycle
Using Natural resources of energy.
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Steam Generator Super Heating Surfaces

• BY
• Dr. P M V Subbarao
• Mechanical Engineering Department
• I I T Delhi

A highly sensitive zone to recover the energy
from hot gases..
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Super heaters

• Super heater heats the high-pressure steam from
  its saturation temperature to a higher specified
  temperature.
• Super heaters are often divided into more than
  one stage.
• Divisional Panel Super Heater.
• Platen Super Heater.
• Pendent Super Heater.
• Horizontal Super Heater.
• The enthalpy rise of steam in a given section
  should not exceed
• 250 420 kJ/kg for High pressure. gt 17 MPa
• lt 280 kJ/kg for medium pressure. 7 Mpa 17 MPa
• lt 170 kJ/kg for low pressure. lt 7 MPa

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Thermal Balance Equation for SH
Steam in
Steam out

• Energy given out by flue gas
• Energy absorption for a SH

Gas in
Gas out
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Mechanism of Heat Transfer Generalized Newtons
Law of Cooling

• Rate of heat transfer from hot gas to cold steam
  is proportional to
• Surface area of heat transfer
• Mean Temperature difference between Hot Gas and
  Cold Steam.

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Thot gas,in
Thot gas,out
Tcold steam,out
Tcold steam,in
Thot gas,in
Thot gas,out
Tcold steam,out
Tcold steam,in
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Log Mean Temperature Difference

• Rate of Heat Transfer
• U Overall Heat Transfer Coefficient, kW/m2.K

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Thermal Structure of A Boiler Furnace
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Platen Superheater

• Platen Superheater Flat panels of tubes located
  in the upper part of the furnace, where the gas
  temperature is high.
• The tubes of the platen SH receive very high
  radiation as well as a heavy dust burden.
• Mechanism of HT High Radiation Low convection
• Thermal Structure
• No. of platens
• No. of tubes in a platen
• Dia of a tube
• Length of a tube

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Geometry of Thermal Structure Platen SH

• The outer diameter of platen SH is in the range
  of 32 42 mm.
• The platens are usually widely spaced, S1 500
  900 mm.
• The tubes within a platen are closely spaced,
  S2/d 1.1.
• The number of parallel tubes in a platen is in
  the range of 15 35.
• Design Problem To find out
• Length of tubes.
• Number of PSHs.
• Design Constraints Max. allowable steam flow
  rates.

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Convective Superheater (Pendant)

• Convective super heaters are vertical type
  (Pendant ) or horizontal types.
• The Pendant SH is always arranged in the
  horizontal crossover duct.
• Pendant SH tubes are widely spaced due to high
  temperature and ash is soft.
• Transverse pitch S1/d gt 4.5
• Longitudinal pitch S2/d gt 3.5.
• The outside tube diameter 32 51mm
• Tube thickness 3 7mm

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Convective Superheater (Horizontal)

• The horizontal SH are located in the back pass.
• The tubes are arranged in the in-line
  configuration.
• The outer diameter of the tube is 32 51 mm.
• The tube thickness of the tube is 3 7 mm.
• The transverse pitch S1/d 2 3.
• The longitudinal pitch S2/d 1.6 2.5.
• The tubes are arranged in multiple parallel sets.
• The desired velocity depends on the type of SH
  and operating steam pressures.
• The outside tube diameter 32 51mm
• Tube thickness 3 7mm

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Thermal Balance in Super Heater.

• The energy absorbed by steam
• The convective heat lost by flue gas
• Overall Coefficient of Heat Transfer, U

Platen SH, U (W/m2 K) 120 140
Pendent SH, U (W/m2 K) 120 140
Convective SH, U (W/m2 K) 60 80
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Reheater

• The pressure drop inside reheater tubes has an
  important adverse effect on the efficiency of
  turbine.
• Pressure drop through the reheater should be kept
  as low as possible.
• The tube diameter 42 60mm.
• The design is similar to convective superheaters.
• Overall Heat Transfer Coefficient 90 110 W/m2
  K.

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Economizer

• The economizer preheats the feed water by
  utilizing the residual heat of the flue gas.
• It reduces the exhaust gas temperature and saves
  the fuel.
• Modern power plants use steel-tube-type
  economizers.
• Design Configuration divided into several
  sections 0.6 0.8 m gap

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Tube Bank Arrangement
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Thermal Structure of Economizer

• Out side diameter 25 38 mm.
• Tube thinckness 3 5 mm
• Transverse spacing 2.5 3.0
• Longitudinal spacing 1.5 2.0
• The water flow velocity 600 800 kg/m2 s
• The waterside resistance should not exceed 5 8
  . Of drum pressure.
• Flue gas velocity 7 13 m/s.

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Thermal Balance in Economizer.

• The energy absorbed by steam
• The convective heat lost by flue gas
• Overall Coefficient of Heat Transfer, U

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Air Pre-Heater

• 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
  the boiler efficiency.
• There are three types of air heaters
• Recuperative
• Rotary regenerative
• Heat pipe

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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 chamber1.4 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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Rotary or Regenerative Air Pre-Heater
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Rotary Plate type Pre-Heater

• Rotates with a low speed 0.75 rpm.
• Weight 500 tons.
• This consists of rotor, sealing apparatus,
  shell etc.
• Rotor is divided into 12 or 24 sections and 12
  or 24 radial divisions.
• Each sector is divided into several trapezoidal
  sections with transverse division plates.
• Heat storage pales are placed in these sections.

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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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Design Considerations
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Thermal Balance in Air Pre-Heater.

• The energy absorbed by air
• The convective heat lost by flue gas
• Overall Coefficient of Heat Transfer, U

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Combustion Losses
C R losses
Hot Exhaust Gas losses
APH
Economizer
CSH
Pendent SH
Reheater
Platen SH
Furnace absorption