Wiki article on this topic: A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtight cavity which can be expanded and contracted by operating the handles, and fitted with a valve allowing air to fill the cavity when expanded, and with a tube through which the air is forced out in a stream when the cavity is compressed. It has many applications, in particular blowing on a fire to supply it with air.
The term "bellows" is used by extension for a flexible bag whose volume can be changed by compression or expansion, but not used to deliver air. For example, the light-tight bag allowing the distance between the lens and film of a folding photographic camera to be varied is called a bellows.
"Bellows" is only used in plural. The Old English name for 'bellows' was blǽstbęlg, blást-bęlg 'blast-bag, blowing-bag'; the prefix was dropped and by the eleventh century the simple bęlg, bylg, bylig was used. The word is cognate with "belly". There are similar words in Old Norse, Swedish, and Danish, but the derivation is not certain. 'Bellows' appears not to be cognate with the apparently similar Latin follis.
Several processes, such as metallurgical iron smelting and welding, require so much heat that they could only be developed after the invention, in antiquity, of the bellows. The bellows are used to deliver additional air to the fuel, raising the rate of combustion and therefore the heat output.
Various kinds of bellows are used in metallurgy:
The manufacturing of different alloys in China often required a continuous stream of air that was able to be blown over through molten metals. The use of double-action piston bellows was described by Chinese philosopher Lao Tzu around 500 BC to produce a continuous stream of air. The Han Dynasty Chinese mechanical engineer Du Shi is credited with being the first to use double-action piston pumps to apply hydraulic power, through a waterwheel, to operate bellows in metallurgy. His invention was used to operate piston bellows of blast furnaces in order to forge cast iron. The ancient Greeks, ancient Romans, and other civilizations used bellows in bloomery furnaces producing wrought iron. Bellows are also used to send pressurized air in a controlled manner in a fired heater.