Boiler - Efficiency

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) c_p dT         (2)

where

q = heat transfer (kJ/s, kW)

m / t = mass flow (kg/s)

m = mass (kg)

t = time (s)

c_p = 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) h_e         (3)

where

m = mass flow of evaporated water (kg)

t = time (s)

h_e = 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.

• Gross Calorific Values of some common Fuels

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)]

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

• 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