A boiler is a closed vessel in which water is heated, steam is generated or superheated, or any combination thereof, under pressure or vacuum by the application of heat resulting from the combustion of fuel (such as in a natural gas boiler), electrical resistance heating or the recovery and conversion of normally unused energy. There are two basic types of boilers - watertube, a boiler in which the tubes contain water and steam, the heat being applied to the outside surface; and firetube, a boiler with straight tubes, which are surrounded by water and steam and through which the products of combustion pass.
The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). The make-up water is usually natural water either in its raw state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial outlay. The tendency is to reduce this percentage to a very small figure.
Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of the boiler. Dissolved gases such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment. For more information check the boiler water treatment web page.
Boilers date back at least to steam production undertaken by Greek scientist/mathematician, Hero, in 200 BC. Out of the late 1700's grew a manufacturing base in the United States that became the envy of the world—steam boilers were the spark to a successful Industrial Revolution. In 1888, to combat early poor construction practices that led to unsafe operational conditions, the founders of the American Boiler Manufacturers Association initiated and promoted construction and safety-related boiler codes and standards. These, along with many innovative technological advances, have stimulated the use of safe, clean and efficient boilers and boiler-related systems. The boiler industry provides the products that drive the engine of U.S. industrialization, yet also heats and cools the hospitals, schools, churches, offices, gathering places, and homes of America. It is no exaggeration to say that few technologies devised by man have produced so much to advance mankind as has the safe and dependable generation of steam made possible by the boiler.
American Boiler Manufacturers Association