1. Cause-Effect Analysis of Steam Generator & Rule Based DesignBY
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!!!!!!!

3. 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.
n fuel
n air
n fluegas
Q=0
Wfans
CXHYSZOK +e 4.76 (X+Y/4+Z-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…

4. Adiabatic Flame Temperature is A primary Cause.
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.
n fuel
n air
n fluegas
Qstem
Qloss
Wfans

5. 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 < Ash Deformation Temperature.
Generally FEGT = Ash Softening Temperature – 100.
General design conditions.
FEGT < C – Strong slag (Molten Ash).
FEGT < C – Moderate slag

6. 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.

7. Basic Geometry of A Furnace

8. 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
 > 30O
 = 50 to 55O
E = 0.8 to 1.6 m
d = 0.25 b to 0.33 b

9. Design Constrains:Heat 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

10. 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.

12. 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.

13. A

14. 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.

16. 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.

18. 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 = P+Q+R+T+U+V
100 X CV of fuel = Snex. Gashf,gas
Calculate Adiabatic Flame Temperature.
Calculate total heat transfer area of furnace, Afur

19. 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

20. Furnace Exit Gas Temperature
FEGT = AST – 100
FEGT < C – Strong slag
FEGT < C – Moderate slag
FEGT < C -- Weak slag
Any design procedure can be used but it should satisfy the requirements of FEGT.

21. Effect of Coal Quality on Furnace Size

22. Role of SG in Rankine Cycle
Using Natural resources of energy.

23. Steam Generator Super Heating SurfacesBY
Dr. P M V Subbarao
Mechanical Engineering Department
I I T Delhi
A highly sensitive zone to recover the energy from hot gases……..

24. 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. > 17 MPa
< 280 kJ/kg for medium pressure. 7 Mpa – 17 MPa
< 170 kJ/kg for low pressure. < 7 MPa

25. Thermal Balance Equation for SH
Steam in
Steam out
Energy given out by flue gas:
Energy absorption for a SH:
Gas in
Gas out

26. Mechanism of Heat Transfer : Generalized Newton’s 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.
Thot gas,in
Tcold steam,in
Thot gas,out
Tcold steam,out

27. Thot gas,in
Thot gas,out
Tcold steam,out
Tcold steam,in
Thot gas,in
Thot gas,out
Tcold steam,out
Tcold steam,in

28. Log Mean Temperature Difference
Rate of Heat Transfer
U Overall Heat Transfer Coefficient, kW/m2.K

29. Thermal Structure of A Boiler Furnace
DPNL SH
Platen SHTR R H T
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCW
pump
Bottom ash
stack
screen
tubes

30. 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

31. 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.

33. 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 > 4.5
Longitudinal pitch : S2/d > 3.5.
The outside tube diameter : 32 – 51mm
Tube thickness : 3 – 7mm S1 S2

34. 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 S1 S2

36. 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)
Convective SH, U (W/m2 K)
60 – 80

37. 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.

38. 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

39. Tube Bank Arrangement

41. 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.

42. Thermal Balance in Economizer.
The energy absorbed by steam
The convective heat lost by flue gas
Overall Coefficient of Heat Transfer, U

43. 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

44. Tubular Air Pre-Heater

45. 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.

46. Rotary or Regenerative Air Pre-Heater

47. 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.

48. Stationary-Plate Type Air Pre-Heater

49. 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.

50. Design Considerations

51. Thermal Balance in Air Pre-Heater.
The energy absorbed by air
The convective heat lost by flue gas
Overall Coefficient of Heat Transfer, U

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