Steam boiler is a closed vessel into which water is heated until the water is converted into steam at required pressure. Water is stored inside the vessel and fuel (coal) is bunt in hot furnace to produce hot gas. These hot gas then comes in contact with the vessel. The heat from the hot gas is transferred to the water in the vessel and steam is produced in the boiler. Then this steam is piped to the turbine of thermal power plant.
The efficiency of Steam Boiler is the percentage of total heat exported by outlet steam to the total heat supplied by the fuel. Typical efficiency of steam boiler is 80% to 88%. The efficiency depends upon the size of the boiler used.
Some of the important term of a steam boiler are Boiler Shell, Combustion Chamber, Furnace, Grate,Heating Surface Mounting and Accessories. Boiler shell is made up of steel plates. It should have sufficient capacity to contain water and steam. Below the boiler shell is the Combustion Chamber, meant for burning the fuel. Grate is a platform in the combustion chamber, upon which fuel (coal or wood) is burnt. Above the grate and below the boiler shell is the furnace in which fuel is actually burned.
Heating Surface is the part of boiler surface, which is exposed to fire. Fittings which are mounted on the boiler for the proper functioning are called mountings. Boiler cannot function safely without the mountings. Accessories are devices which are integral part of a boiler, but are not mounted on it. Accessories help in controlling and running the boiler efficiently.
Fire Tube Boiler and Water Tube Boiler are two main types of boiler. Fire tube boiler consists of numbers of tubes through which hot gasses are passed. These hot gas tubes are immersed into water, in a closed vessel. The water in the vessel is heated by the hot gas tubes and is converted into steam. The steam remains in the same vessel. Therefore this boiler cannot produce steam at very high pressure. It can produce maximum of 17.5 kg/cm2 and with a capacity of 9 Metric Ton of steam per hour. Water Tube Boilers are opposite of Fire Tube Boilers where water is heated inside tubes and the hot gas surrounds them.