Heat Transfer Modes in Boiler

As we all know that there are three modes of heat transfer (i.e conduction,convection and radiation), during steam generation process in a boiler all the three modes of heat transfer takes place as explained below

Conduction:

Conduction is the heat transfer process from one part of the material to the other through the molecules. In boilers, conduction heat transfer takes place from outer surface of the boiler tubes through with water and steam flow takes place to the inner surface of the tubes.

Convection:

Convection heat transfer takes place when the heated medium displaces. In boilers, when water is heated, its density becomes less hence it is displaced by the high density cold water. This movement takes place continuously. If the convection flow is caused naturally because of the density difference it is called natural circulation. If this movement is assisted by a fan or pump it is called forced convection. In a boiler mostly the tubes receive the heat from the convectional flow of hot gases from combustion chamber (furnace).?Also within the tubes the water or steam picks up the heat from inner surface of the tubes by convection.

Radiation:

Radiation heat transfer is a common phenomenon that takes place in all the heater bodies as they radiate the heat energy. There is no requirement for any medium for the radiation to take place. The amount of radiation energy depends on the size and the quantum of heat energy a body have. In boilers, radiation heat transfer takes place in the combustion chamber from the hot gases to the boiler tubes surrounding the combustion chamber.