A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.

The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Electric steam boilers use resistance- or immersion-type heating elements. Nuclear fission is also used as a heat source for generating steam, either directly (BWR) or, in most cases, in specialised heat exchangers called “steam generators” (PWR). Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as turbine.

By means of automatic combustion control it becomes easy to maintain a constant steam pressure
and uniform furnace draught and supply of air or fuel can be regulated to meet the changes in steam demand. The boiler operation becomes more flexible and better efficiency of combustion is
achieved. This saves manual labour also.
Hagan system of automatic combustion control. Master relay R1, is sensitive
to small vanations in steam pressure and is connected to steam pressure gauge.

A fall in pressure operates the master relay R1 which in turn operates the servomotor coupled to
the vanes of the induced draught (LD) fan to open them slightly and simultaneously the secondary air
fan damper gets opened proportionately. By this readjustment of induced draught takes place and stabilized
conditions in the combustion chamber get changed. These changes operate relay R2 to alter the
position of forced draught fan servo-motor to adjust the position of forced draught fan vanes so that
stable conditions in combustion chamber are maintained. This change causes more air to flow through
passage which in turn operates relay R3. This causes stoker motor to supply extra fuel into the furnace.
In case of an increase of pressure of steam the above process is reversed. Hand regulators are provided
to servo motors and master relay for manual control of system.