Boiler Performance

Efficiency in Boilers

Efficiency in boilers, is calculated similar to other machines.
Efficiency = Useful output/ Input

In this case, useful output is the heat content in the steam.


Therefore

The heat content in the fuel can be calculated by knowing the calorific value of the fuel. 

The heat content of the steam is measured by calculating factors such as the steam flow rate, steam pressure and the temperature of the feed water.  Using these values, the heat content of the steam can be obtained from the steam tables.

Net Calorific Value and Gross Calorific Value

Calorific value is the total energy contained in fuel.  It is indicated in kilojoules per litre or cubic metre.

All fuels, such as coal, oil or wood contain a small amount of water in the form of moisture.  When fuel is consumed in the furnace, some of the energy is used to evaporate the water contained in the fuel.  This water escapes as steam in the flue gases.

Gross Calorific Value

This includes the total energy in the fuel which  includes the energy used in heating the water.  The Gross calorific value is also known as the higher heating value.

Net Calorific Value

Net Calorific Energy is calculated after subtracting the energy used to evaporate the moisture in the fuel.  Net Calorific Energy is used for boiler efficiency calculation.  The Net calorific value is also known as the lower heating value.
The Net Calorific Value is generally 10 % less than the Gross calorific value.