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2. Boiler Efficiency
• Boiler efficiency is a combined result of efficiencies of
different components of a boiler. A boiler has many sub
systems whose efficiency affects the overall boiler efficiency.
Couple of efficiencies which finally decide the boiler efficiency
are-
1. Combustion efficiency
2. Thermal efficiency
• Apart from these efficiencies, there are some other losses
which also play a role while deciding the boiler efficiency

3. Combustion Efficiency
• The combustion efficiency of a boiler is the indication of
burner’s ability to burn fuel.
• The two parameters which determine the burner efficiency
are unburnt fuel quantities in exhaust and excess oxygen
levels in the exhaust.
• As the amount of excess air is increased, the quantity of
unburnt fuel in the exhaust decreases. This results in lowering
the unburnt fuel losses but elevating the enthalpy losses.
Hence, it is quite important to maintain a balance between
enthalpy losses and un-burnt losses.
• Combustion efficiency also varies with the fuel being burnt.
• Combustion efficiency is higher for liquid and gaseous fuels
than for solid fuels.

4. Thermal Efficiency
• The thermal efficiency of a boiler specifies the
effectiveness of the heat exchanger of the
boiler which actually transfers the heat energy
from fireside to water side.
• Thermal efficiency is badly affected by scale
formation/soot formation on the boiler tubes.

5. Direct and Indirect Boiler Efficiency
• The overall boiler efficiency depends on many
more parameters apart from combustion and
thermal efficiencies.
• These other parameters include ON-OFF
losses, radiation losses, convection losses,
blow down losses etc.
• In actual practice, two methods are commonly
used to find out boiler efficiency, namely
direct method and indirect method of
efficiency calculation.

6. Direct efficiency
• This method calculates boiler efficiency by using the
basic efficiency formula-
• η=(Energy output)/(Energy input) X 100
• Calculation of direct efficiency-
• η = [Q (H-h)/q*GCV]*100
• Where,
 Q= Quantity of steam generated (kg/hr)
 H= Enthalpy of steam (Kcal/kg)
 h= Enthalpy of water (kcal/kg)
 GCV= Gross Calorific Value of the fuel.

8. Performance evaluation of Boilers

15. Tips for Energy Efficiency in Boilers
• Preheat combustion air with waste heat.
• (22°C reduction in flue gas temperature increases boiler efficiency by 1%)
• Use variable speed drives on large boiler combustion air fans with variable flows.
• Burn wastes if permitted.
• Insulate exposed heated oil tanks.
• Clean burners, nozzles, strainers, etc.
• Inspect oil heaters for proper oil temperature.
• Close burner air and/or stack dampers when the burner is off to minimize heat loss
up the stack.
• Improve oxygen trim control (e.g. -- limit excess air to less than 10% on clean
fuels). (5% reduction in excess air increases boiler efficiency by 1% or: 1% reduction
of residual oxygen in stack gas increases boiler efficiency by 1%)
• Automate/optimize boiler blowdown. Recover boiler blowdown heat.
• Use boiler blowdown to help warm the back-up boiler.

16. • Optimize deaerator venting.
• Inspect door gaskets.
• Inspect for scale and sediment on the water side.
• (A 1 mm thick scale (deposit) on the water side could increase fuel consumption by 5
to 8%.)
• Inspect for soot, flyash, and slag on the fire side.
• (A 3 mm thick soot deposition on the heat transfer surface can cause an increase in
fuel consumption to the tune of 2.5%)
• Optimize boiler water treatment.
• Add an economizer to preheat boiler feedwater using exhaust heat.
• Recycle steam condensate.
• Study part-load characteristics and cycling costs to determine the most-efficient
mode for operating multiple boilers.
• Consider multiple or modular boiler units instead of one or two large boilers.
• Establish a boiler efficiency-maintenance program. Start with an energy audit and
follow- up, then make a boiler efficiency-maintenance program a part of your
continuous energy management program.