A boiler is a closed vessel where drinking water or other liquid is heated. The fluid does not boil. (In North America, the term "furnace" is normally used if the reason is not to boil the fluid.) The heated or vaporized liquid exits the boiler for use in a variety of procedures or heating applications,[1 - [2 - including water heating, central heating, boiler-based power generation, food preparation, and sanitation.
The pressure vessel of the boiler is usually made of steel (or alloy steel), or historically of wrought iron. Stainless steel, of the austenitic types especially, is not found in wetted parts of boilers due to corrosion and stress corrosion cracking.[3 - However, ferritic stainless steel is often found in superheater sections that will not be exposed to boiling drinking water, and electrically heated stainless shell boilers are allowed under the European "Pressure Equipment Directive" for creation of steam for sterilizers and disinfectors.[4 -
https://en.wikipedia.org/wiki/Boiler - https://en.wikipedia.org/wiki/Boiler
In live steam models, copper or brass is often used since it is more fabricated in smaller size boilers easily. Historically, copper was often used for fireboxes (particularly for vapor locomotives), because of its better formability and higher thermal conductivity; however, in more recent times, the high price of copper often makes this an uneconomic choice and cheaper substitutes (such as metal) are used instead.
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