A furnace of a reverberatory type fired by producer gas and used air preheated using a pair of regenerative heaters. The use of open hearth furnaces was first patented for the melting of glass and metals by Frederick Siemens in 1856, based on the principles set out in Stirling’s patent of 1812, and Nielson’s use of hot blast in the coke fire blast furnace.
The 1856 patent outlined the use of two sets of checkerwork firebrick heat exchangers through which the air flowing into, or out of, the furnace passed. One set was heated by the waste gas from the furnace, while the other (hot from a previous heating phase) was used to heat the air going into the furnace. Periodically, the control valves were changed so that the air flow through the furnace and heat exchangers was reversed, the checkerwork that had been cooled by the incoming air now being heated by the exhaust gas, and vice versa.
Initially the furnaces were fuelled by coke, or other solid fuel, but the ash carried over by the air eventually blocked the checkerwork. This problem was eliminated if producer gas was used as the fuel instead of a solid fuel. A further patent in 1861 by both William and Frederick improved the furnace temperature by having a separate pair of checkerworks to preheat the gas as well as the air. Such furnaces were capable of reaching temperatures as high as 1600°C, and thus steel could be melted in them.