Boiler Furnace Design and Duty

Boiler Furnace Design and Duty
The design and duty of modern furnaces; i.e. basically the size and the circulation
system, are a function of:
a)
b)
complete combustion of the fuel and

preservation of satisfactory furnace wall metal temperatures.
The modern furnace is designed to have the following characteristics:

1.
A surface area sufficient to reduce the temperature of the furnace gases to a level
acceptable to superheater requirements.
2.
Adequate water circulation in the furnace tubes. This means a tube diameter which
has a friction pressure drop which allows sufficient fluid flow to avoid overheating.
Natural circulation boilers have larger diameter tubes (with smaller friction drops)
than pump-assisted circulated boilers because of the reduced driving force.
3.
A dimension in the path of the burner sufficient to avoid flame impingement on the
opposite wall, but not so excessive that the heat transfer is reduced.
4.
A width sufficient to accommodate all burners on acceptable pitchings to avoid

flame impingement on side and division walls, and interference of one flame with
another.
5.
Overall dimensions and shape sufficient to ensure a gas path which will fill the
furnace, provide optimum absorption to all parts and a fuel ash particle residence
time sufficient to ensure burn out.
6.
For a coal-fired boiler furnace together with the platen must have sufficient area to
cool the gases below the ash softening temperature, this is to ensure that the
deposits outside the furnace envelope are solidified.
Furnace for burning coal have to be designed to prevent the formation of objectionable
slag deposits, which can increase the furnace outlet gas temperature above the design
value. Generally, the larger furnaces and the wall deposits associated with coal firing
result in a relatively low furnace wall absorption rates, so that the tube wall temperature
does not influence furnace size.
When firing oil, the larger amount of fuel reaction surface available for combustion,
produces complete combustion in a smaller furnace volume. The higher consequent heat
release rate per unit area of furnace wall surface can cause high localized heat
absorption rates. Furnace sizing for oil burning then, must account for the effect of high
absorption rates.
Gas fired furnace design is similar to oil fired except for the fact that gas firing
inherently results in lower localized heat absorption rates. This allows designing
somewhat smaller furnaces with even higher heat release rates than used for oil firing.
Boiler Furnace Ratings
The dimensions of a furnace vary to accommodate the factors described above.
Typical furnace ratings for coal and oil firing boiler furnaces are given in the
Table below.
The heat release rating can only be regarded as a guide.

Projected area rating is indicative of the furnace gas temperature at the furnace
nose (or throat), whilst volumetric rating is indicative of the residence time
allowed for the fuel particles burn out.
Side View of a Typical Coal-firing Power Plant Boiler
Plan area together with burner belt height is indicative of the burner belt heat
release rates.
These factors are only useful as a comparison between new design and existing
boilers to ensure that operation is within existing experience. A much more
detailed examination of the heat transfer and combustion processes must be
carried out before a design is considered acceptable.
Comparison Between Coal and Oil Boiler Furnaces

It can be seen from the rating table above that oil-fired boiler furnaces are more
highly rated than coal-fired boiler furnaces. A number of factors lead to this
differences:
a. The average oil droplets burn out time is roughly half that of an average
pulverized coal particle.
b. In coal-firing, unlike oil-firing, there is a small percentage of oversize
particles which require higher residence times than average particles.
c. Heat transfer to the boiler furnace walls is dependent on the amount of
ash adhering to the walls and, because a coal-fired furnace has much
more ash on the furnace walls than oil-fired furnaces, there is higher
resistance to heat transfer in the coal-fired furnace.
The result is that simply to accommodate the combustion process the coal-fired
boiler furnace has to be much larger than an oil-fired boiler furnace. The lower
heat transfer rates and the requirement to reduce gas temperature below ash
deformation temperature in the coal-fired boiler furnace further increases the
size and/or surface requirement and, instead of having to provide surface for
evaporation duty alone, coal-fired furnaces for high pressure boilers have also
to incorporate superheater surface. A size comparison between a typical coal
and oil boiler furnace is as shown.
For coal-firing, special provision has to be made for ash collection at the bottom
of the furnace. The furnace hopper slopes at around 60o to the horizontal to
ensure free flow of ash, whilst for oil-firing this is not necessary and the slope
can be nearer to the horizontal.
The diagram shown below gives a relative comparison of furnace sizes for the
above three different fuels; i.e. coal, oil and gas fuels.

Boiler Furnace Design and Duty

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Boiler Furnace Design and Duty

complete combustion of the fuel and

The modern furnace is designed to have the following characteristics:

A width sufficient to accommodate all burners on acceptable pitchings to avoid

The heat release rating can only be regarded as a guide.

Side View of a Typical Coal-firing Power Plant Boiler

Comparison Between Coal and Oil Boiler Furnaces

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