Open hearth furnace

Engineering portal

Siemens furnace from 1895

Open hearth furnaces are one of a number of kinds of furnace where excess carbon and other impurities are burnt out of the pig iron to produce steel. Since steel is difficult to manufacture due to its high melting point, normal fuels and furnaces were insufficient and the open hearth furnace was developed to overcome this difficulty. Most open hearth furnaces were closed by the early 1990s, not least because of their slow operation, being replaced by the basic oxygen furnace or electric arc furnace.

Technically perhaps, the first primitive open hearth furnace was the Catalan forge, invented in Spain in the eighth century, but it is usual to confine the term to certain nineteenth century and later steelmaking processes, thus excluding bloomeries (including the Catalan forge), finery forges, and puddling furnaces from its application.

The Siemens regenerative furnace

Sir Carl Wilhelm Siemens developed the Siemens regenerative furnace in the 1850s, and claimed in 1857 to be recovering enough heat to save 70–80% of the fuel. This furnace operates at a high temperature by using regenerative preheating of fuel and air for combustion. In regenerative preheating, the exhaust gases from the furnace are pumped into a chamber containing bricks, where heat is transferred from the gases to the bricks. The flow of the furnace is then reversed so that fuel and air pass through the chamber and are heated by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough to melt steel, but Siemens did not initially use it for that.

The regenerators are the distinctive feature of the furnace and consist of fire-brick flues filled with bricks set on edge and arranged in such a way as to have a great number of small passages between them. The bricks absorb most of the heat from the outgoing waste gases and return it later to the incoming cold gases for combustion.

Open hearth steelmaking

Edgar Thomson Steel Works at Braddock, Pennsylvania, circa 1915

In 1865, the French engineer Pierre-Émile Martin took out a license from Siemens and first applied his furnace for making steel. Their process was known as the Siemens-Martin process, and the furnace as an "open-hearth" furnace. The most appealing characteristic of the Siemens regenerative furnace is the rapid production of large quantities of basic steel, used for example to construct high-rise buildings. The usual size of furnaces is 50 to 100 tons, but for some special processes they may have a capacity of 250 or even 500 tons. The Siemens-Martin process complemented rather than replaced the Bessemer process. It is slower and thus easier to control. It also permits the melting and refining of large amounts of scrap steel, further lowering steel production costs and recycling an otherwise troublesome waste material. Its worst drawback is the fact that melting and refining a charge takes several hours. This was an advantage in the early 20th C.,as it gave plant chemists time to analyze the steel and decide how much longer to refine it. But by about 1975, electronic instruments such as atomic absorption spectrophotometers had made analysis of the steel much easier and faster. The work environment around an open hearth furnace is said to be extremely dangerous, although that well be even more true of the environment around a basic oxygen or electric arc furnace.

The basic oxygen steelmaking eventually replaced the open hearth furnace. It rapidly superseded both the Bessemer process and Siemens-Martin process in the Western Europe by the 1950s^{[clarification needed]} and in the Eastern Europe by the 1980s, the last European open hearth furnace being stopped in 1993. In the US, steel production using the open hearth furnaces had stopped by 1992. The last open hearth shop in China was shut down in 2001. The nation with the highest share of steel produced with open hearth furnaces (almost 50%) remains Ukraine.^[1] The process is still in use also in India and Russia.

A. gas and air enter
B. pre-heated chamber
C. molten pig iron
D. hearth
E. heating chamber (cold)
F. gas and air exit.

Notes

1. ^ http://www.energystar.gov/ia/business/industry/41724.pdf

Further reading

• K. Barraclough, Steelmaking 1850–1900 (Institute of Metals, London 1990), 137–203.
• W. K. V. Gale, Iron and Steel (Longmans, London 1969), 74–77.

External links

• Precursors to the Blast Furnace
• "Administering Doses of Liquid Iron to Steel Furnaces", Popular Science, February 1919, page 64, scanned by Google Books.