A boiler is a closed vessel in which water, under pressure, is transformed into steam by the application of heat. Open vessels and those generating steam at atmospheric pressure are not considered to be boilers. The furnace converts the chemical energy of the fuel into heat. The function of the boiler is to transfer this heat to the water in the efficient manner.
Progress in steam-boiler development has been rapid. The first boilers were very crude affairs, as contrasted with our present-day standards. The greatest number of contributions have been made in the last half century. The field of application is diversified. Boilers are used for heating, supplying steam for processes, furnishing steam to operate engines, etc.
Maintaining the correct boiler water level is the most important duty of the boiler operator. It is of the utmost importance that the manufacturer supply suitable and reliable devices for indicating the water level. Coal as well as liquid and gaseous fuels are used for boiler firing. The ideal boiler must be of correct design, sufficient steam and water space, and good water circulation.