|Fig. 1: Heat Transfer within Steam Boiler|
A steam boiler is designed to absorb the maximum amount of heat released from the process of combustion. Heat transfer within steam boiler is accomplished by three methods: radiation, convection, and conduction. The heating surface in the furnace area receives heat primarily by radiation. The remaining heating surface in the steam boiler receives heat by convection from the hot flue gases. Heat received by the heating surface travels through the metal by conduction Heat is then transferred from the metal to the water by convection.
The relative percentage of each heat transfer within steam boiler is dependent on the type of steam boiler, the designed transfer surface, and fuels. Each of these methods is discussed in more detail below:
Radiation is the process of continuous interchange of energy by means of electromagnetic waves without a change in the temperature of the medium between two bodies involved. Radiation is the most important method of heat transfer in the furnace of steam boiler. The amount of heat transfer depends on the area of the heating surfaces and hot surfaces in the furnace. The physical laws governing transmission of heat transfer by radiation is: Heat is transmitted in the straight line, heat can be reflected and refracted, and heat is radiated in all direction.
The steam boiler tubes absorb the radiant heat from flame and radiate a small portion of the heat back to the furnace. Radiation is present in all steam boilers.
Conduction within steam boiler is the transfer of heat from one part of a body to the other or from one body to another in physical contact. It is the flow of energy that occurs in a substance without appreciable movement of molecules due to lattice vibrations and flow of free electrons. Heat flow from flue gases to water/steam across the metal wall in a steam boiler, loss of furnace heat to the surrounding atmosphere through insulation, and heat loss from hot steam/water pipes through insulation are all examples of conduction.
Convection is the process of transferring heat by movement energy through a material. When any fluid in a steam boiler is heated, its density is decreases and the fluid become lighter. Then the heavier or cooler fluid will flow and replace the heated portion of the fluid. In turn the cooler portion becomes heated. The flow of the cooler fluid to heated area is called convection. Heat transfer by convection normally occurs from a lower to higher elevation of steam boiler.