Boiler Efficiency

Boiler efficiency - combustion gross and net calorific value

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 (%)

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

Heat Exported from the Boiler to the Fluid

If a fluid like water is used to export heat from the boiler, exported heat can be expressed as:

q = (m / t) cp dT         (2)

where

q = heat exported (kJ/s, kW)

m / t = mass flow (kg/s)

m = mass (kg)

t = time (s)

cp = specific heat capacity (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 the saturation temperature can be expressed as:

q = (m / t) he         (3)

where

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 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 the 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

This is the calorific value of the fuel, excluding 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)]

where

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

TypeUnitBtu
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) = 1,055.06 J
  • 1 Gallon (U.S.) = 3.785x10-3 m3 = 3.785 dm3 (liter)
  • 1 ft3 = 0.02832 m3

Related Topics

  • Combustion - Boiler house topics - fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency

Related Documents

Tag Search

  • en: combustion calorific efficiency

Search the Engineering ToolBox

- "Search is the most efficient way to navigate the Engineering ToolBox!"

Engineering ToolBox - SketchUp Edition - Online 3D modeling!

3D Engineering ToolBox - draw and model technical applications

Engineering ToolBox - SketchUp Edition - add standard and customized parametric components - like flange beams, lumbers, piping and more - to your SketchUp model - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro. Add from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!
About the Engineering ToolBox!

close