Evaporation in Steam Boiler

Evaporation is the process changing water into steam. In this evaporation process requires heat transfer that be obtained from combustion process in the furnace steam boiler. Evaporation occurs in the water wall tubes of steam boiler. Water from steam drum is flown through downcomer into lower header, and the proceed to water wall tubes either in the left side, right side, front side and rear side steam boiler.

Fig. 1: Evaporation Process in Steam Boiler

Surface water wall tubes get heat transfer by radiation, and then heat is transferred in entire of surface water wall tubes of steam boiler by conduction. When water in the water wall tubes is heated, reduction of density occurs, so water becomes lighter and tends to move upward. Then water which has heavy density will flow downward and replace the heated portion of the water. So there is heat transfer between particle of water which has density lighter and heavier. This process is called convection.

The quantity of water that be evaporated into steam is known as Evaporation Rate and usually expressed in pounds of steam per hour, or pounds of steam per hour per square feet heating surface, or pounds of steam per hour per cubic volume of furnace steam boiler. Enthalpy of evaporation or laten heat evaporation is the amount of heat energy required to convert water into steam.

Steam which be generated in the water wall tubes of steam boiler is saturated steam (still mixture between steam and water where temperature of steam is same with the water from which it was converted). The mixture goes to steam drum, where steam is separated from water. The steam goes from the drum to superheater to be heated again, so steam will be superheated steam. The water is recirculated through downcomer, lower header, and water wall tubes again. 

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