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Boiler - Efficiency

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Boiler efficiency may be indicated by

  • Combustion Efficiency - indicates a burners ability to burn fuel measured by unburned fuel and excess air in the exhaust
  • Thermal Efficiency - indicates the heat exchangers effectiveness to transfer heat from the combustion process to the water or steam in the boiler, exclusive radiation and convection losses
  • Fuel to Fluid Efficiency - indicates the overall efficiency of the boiler inclusive thermal efficiency of the heat exchanger, radiation and convection losses - output divided by input.

Boiler Efficiency is in general indicated by either Thermal Efficiency or Fuel to Fluid Efficiency depending the context.

Boiler Efficiency

Boiler Efficiency related to the boilers energy output to the boilers energy input can be expressed as:

Boiler efficiency (%) = 100 (heat exported by the fluid (water, steam ..) / heat provided by the fuel)                (1)

Heat Exported from the Boiler to the Fluid

If a fluid like water is used to transfer heat from the boiler - the heat transfer can be expressed as:

q = (m / t) cp dT         (2)


q = heat transfer (kJ/s, kW)

m / t = mass flow (kg/s)

m = mass (kg)

t = time (s)

cp = specific heat (kJ/kg oC)

dT = temperature difference between inlet and outlet of the boiler (oC)

For a steam boiler the heat exported as evaporated water at saturation temperature can be expressed as:

q = (m / t) he         (3)


m = mass flow of evaporated water (kg)

t = time (s)

he = evaporation energy in the steam at the saturation pressure the boiler is running (kJ/kg)


Heat Provided by Fuel

The energy provided by a fuel may be expressed in two ways - 'Gross' or 'Net' Calorific Value.

Gross Calorific Value

This is the theoretical total of the energy in the fuel. The gross calorific value of the fuel includes the energy used for evaporating the water in the combustion process. The flue gases from boilers are in general not condensed. The actual amount of heat available to the boiler plant is therefore reduced.

An accurate control of the air supply is essential to boilers efficiency.

  • to much air cools the furnace and carries away useful heat
  • too little air and the combustion will be incomplete. Unburned fuel will be carried over and smoke produced

Net calorific value

Net calorific value excludes the energy in the water vapor discharged to the stack in the combustion process. The combustion process can be expressed as:

[C + H (fuel)] + [O2 + N2 (Air)]  ->  (Combustion Process)  ->  [CO2 + H2O + N2 (Heat)]


C = Carbon

H = Hydrogen

O = Oxygen

N = Nitrogen

In general it is possible to use the approximation:

net calorific value = gross calorific value - 10%

Btu Content of Fuel Oil

Fuel Oil - Energy Content
No. 1 Oil Gallon 137400
No. 2 Oil Gallon 139600
No. 3 Oil Gallon 141800
No. 4 Oil Gallon 145100
No. 5 Oil Gallon 148800
No. 6 Oil Gallon 152400
Natural Gas cu. ft. 950 - 1050
Propane cu. ft. 2550
Butane cu. ft. 3200
  • 1 Btu (British thermal unit) = 1055.06 J
  • 1 Gallon (U.S.) = 3.785x10-3 m3 = 3.785 dm3 (liter)
  • 1 ft3 = 0.02832 m3
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Related Topics


Combustion processes and their efficiency. Boiler house and chimney topics. Properties of fuels like oil, gas, coal and wood and more. Safety valves and tanks.

Related Documents

ASME - International Boiler and Pressure Vessel Code

The International Boiler and Pressure Vessel Code safety rules governing design, fabrication, and inspection of boilers and pressure vessels, and nuclear power plant components during construction.

Boiler Blowdown

Suspended solids in the feed water will remain in the boiler when steam is generated.

Boiler Capacities

Steam boilers output can be expressed in Boiler Horsepower, MBTU or in Pounds of Steam delivered per hour.

Boiler Feed Water - Chemistry and Impurity Limits

ABMA recommended feed water chemistry limits for steam boilers.

Boiler Horsepower

Boiler horsepower vs. heat transfer area.

Boiler Rooms - Sizing

Minimum area in a boiler room.

Boilers - Classification

Classification of boilers according the ASME Boiler and Pressure Vessel Code.

Combustion Efficiency and Excess Air

Optimizing boilers efficiency is important to minimize fuel consumption and unwanted excess to the environment.

Combustion Processes and Combustion Efficiency

Typical furnace combustion efficiencies in fireplaces, space heaters, boilers and more.

Combustion Testing

Combusting testing of fuel oil and gas burners.


The measure of usefulness.


Efficiency is the ratio useful energy output to energy input.

Intermittent Combustion and Boiler Efficiency

Efficiency reduction due to intermittent boiler operation.

Optimal Combustion Processes - Fuel vs. Excess Air

Stable and efficient combustion requires correct mixture of fuels and oxygen.

Steam Boiler Rating vs. Feed Water Temperature

Boiler output vs. feed water temperature.

Steam Boiler Shells - Stress vs. Boiler Pressure

Calculate the stress in steam boiler shells caused by steam pressure.

Stoichiometric Combustion

Stoichiometric combustion and excess air.

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