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A Cause – Effect Analysis of Furnace Heat Transfer. BY P M V Subbarao Associate Professor Mechanical Engineering Department I I T Delhi. Closed form solutions for performance analysis of complex heat transfer devices…. Cause – Effect Analysis. Combustion is a primary cause

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A Cause – Effect Analysis of Furnace Heat Transfer


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a cause effect analysis of furnace heat transfer

A Cause – Effect Analysis of Furnace Heat Transfer

BY

P M V Subbarao

Associate Professor

Mechanical Engineering Department

I I T Delhi

Closed form solutions for performance analysis of complex heat transfer devices…..

cause effect analysis
Cause – Effect Analysis
  • Combustion is a primary cause
  • Steam Generation is an ultimate effect.
  • Heat transfer is a mediation.
  • Combustion causes the generation of heat in side furnace volume.
  • Heat generation causes the production of high temperature gases.
  • These high temperature gases cause the Radiation and convection heat transfer processes.
  • Heat Transfer processes carry the thermal energy to furnace wall & steam tubes.
  • Conduction through the tubes and walls causes the convection inside the tubes.
  • Convection Causes the generation of steam.
  • A cause effect analysis can simplify the design analysis of a furnace.
analysis of the primary cause
Analysis of the Primary Cause
  • Chemical Energy ↔ Thermal (Sensible) Energy
  • Reactants ↔ Products
  • At a given temperature the Gibbs free energy of products is less than Reactants.
  • Depending on the effectiveness of heat release rate, the sensible energy of products will be higher than sensible energy of reactants.
  • Hence, the temperature of products of combustion is very high.

The temperature of the gases in an adiabatic furnace attain a maximum temperature called adiabatic flame temperature.

general design principles
General Design Principles
  • The effective heat release rate is depends on the size of furnace.
  • 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.
determination of furnace size
Determination of Furnace Size
  • What is the boundary of a furnace?
  • The boundary of a furnace is defined by
    • Central horizontal plane of water wall and roof tubes
    • Central horizontal lines of the first set of super heater panels.
  •  = 30 to 50O
  •  > 30O
  •  = 50 to 55O
  • E = 0.8 to 1.6 m
  • d = 0.25 b to 0.33 b
design constrains heat release rate
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
  • The maximum allowable heat flux of the water wall is restricted by its water-side burnout (dryout) heat flux.
heat release rate per unit volume q v
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(coal) 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 substantially
  • The flue gas is cooled to the required safe temperature.
heat release rate per unit cross sectional area q a
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.
heat release rate per unit wall area of the burner region
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.
furnace depth height
Furnace Depth & Height
  • Depth to breadth 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.
analysis of the secondary cause
Analysis of the Secondary Cause
  • Emissive power of flame:
  • Where eflame is the emissivity of flame.

How to find the area of a Flame ?

analysis of the tertiary cause
Analysis of the Tertiary Cause
  • Radiation heat transfer
  • Where eeff is the emissivity of flame and water wall system.
  • Heat flux is non uniform.
  • Wall temperature is non uniform.
  • This effect is another cause for further analysis.
analysis of the last but one effect
Analysis of the Last but One Effect

Tfe

  • Final effect : Tfl gets changed to Furnace Exit GasTemperature.
  • Due to energy lost by hot gases.
    • Loss due to Environment
    • Energy absorbed by water walls
  • Energy lost by hot gasses from flame to exit.

Tflame