Shell: The shell of the boiler is the main container usually of cylindrical shape which contains water and steam.
Furnace: A furnace is another important part of the boiler. This may be a grate to burn coal or a burner to atomize and burn liquid fuel. Suitable area and volume should be provided of efficient combustion.
Water flow path: The water flow path is the path followed of absorption of heat from hot gases and conversion into steam. The water should be free from dissolved material in order to reduce the scaling of the heating surface.
Gas flow path: The hot gas path either in fire tube or in water tube should be arranged in such a way that maximum heat of hot gases should be transferred to the water for steam generation. The boiler efficiency mainly depends upon the gas flow path.
Steam path: In most of the boiler, the steam is taken out preferable at the top of the shell to avoid water partials being carried with the steam. To reduce the water particles carried by the steam, it mountings, water level indicator.
Fittings: The valves and gauges which are necessary for the safety of the boiler are blow-off-cock and fusible plug are some of the mountings.
Accessories: Some equipment like economizer, air pre heater and super heater are attached to the boiler to improve its overall efficiency. The economizer and air pre heater are used to extract maximum heat from the flue gases and superheater is used to increase the temperature of steam above saturation temperature.