Open hearth furnace

Open hearth furnaces are one of a number of kinds of furnace where excess carbon and other impurities are burnt out of 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 fuel inefficiency, being replaced by basic oxygen furnace or electric arc furnace.

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Technically perhaps, the first primitive open hearth furnace was the Catalan forge, invented in Spain in the 8th century, but it is usual to confine the term to certain 19th 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

  In 1865, Emile Martin and Pierre Martin took out a licence 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.

Basic oxygen steelmaking or LD process replaced the open hearth furnace. In the US, steel production using the inefficient open hearth furnaces had stopped by 1992. The highest share of steel produced with open hearth furnaces (almost 50%) still retained in Ukraine. (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.