Post on 08-Aug-2018
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Design Steps : Furnace Of A Steam Generator
P M V Subbarao
Professor
Mechanical Engineering Department
Selection of Geometric Parameters….
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Heat available to the furnace
• Incomplete combustion loss
• Unburned Carbon loss
• Loss due to slag
• Energy brought in by preheated air & fuel.
• A part of this total heat should be absorbed in furnace.
• The designer should provide an environment for the same.
ai slag C COc fuQQQQ LHV mQ
COQ
slag Q
C Q
aiQ
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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.
3/mkW
V
LHV mq
c
v
burning r
r
cv t t
Vt LHV mq
*
• 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 thecritical 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.
2/mkW A
LHV m
q grate
c
A
• 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.
2
/2 mkW H ba
LHV m
qb
b
• a and b are width and depth of furnace, and H b 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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Selection of Furnace Design Parameters
• A suitable value for ;
• the rate of heat generated by combustion of fuel in a unit
effective volume of the furnace, qv
.
• the rate of heat released per unit cross section of the
furnace, qA.
• The rate of heat released per unit wall area of burner
section of the furnace, q b
.
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General Guide Lines for 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.
• Furnace should proved an exit and path for free fall of ash, without
major heat loss.
• 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
v
c
q
LHV m
V
A
c
grate
q
LHV mba A
b
cb
q
LHV m H ba
2
Any limit on minimum height of furnace?
Any limit on depth of furnace C.S.?
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Furnace Depth & Height
•Depth (a) to breadth (b)ratio is an important parameter from bothcombustion 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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Modfications in Geometry of A Furnace
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Boiling process in Tubular Geometries
Water
Heat Input
H e a t
I n p u tWater
Water
Steam
Steam
Partial Steam Generation Complete or Once-through
Generation
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Further Geometrical Details 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
• > 30O
• = 50 to 55O
• E = 0.8 to 1.6 m
• d = 0.25 b to 0.33 b
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Heat Transfer in A Furnace
• The flame transfers its heat energy to the water walls in the furnace by Radiation.
• Convective Heat Transfer < 5%.
• Only Radiation Heat Transfer is Considered!
• Complexities:
• Non uniform temperature of tubes.
• Fouling of surfaces of tubes.
• Variation of furnace temperature along its
– Height
– Width – Depth.
Non uniform Heat Flux !!!!!
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Simplified Approach
• Emitted Radiation heat flux of flames:
• Emitted Radiation = Available Heat
• Heat flux absorbed by walls :
• Thermal efficiency factor, y.
•The rate of heat absorption
24/ mkwT J fl fl fl
fl abs J q y
kW T AQ fl fl abs 4
y
444
fl fl wa fl eff rad abs T AkW T T AQQ y
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Coal fired furnace
Structure of
water walls*Hot Exhaust gases
Burner
Flame
Furnace Exit
Heat Radiation
& Convection
*www.directindustry.com
• Two functions of coal fired
furnace:
Release of chemical energy by
combustion of fuel
Transfer of heat from flame to
water walls
• Combustion space
surrounded by water walls