Class III

includes a great variety of constructions. generally called retort furnaces. The best known are ordinary crucible furnaces or melting holes, and the Belgian and Silesian zinc furnaces. When using gas as fuel, the regenerative system is used. The hot gases leaving the furnace pa through chambers filled with brickwork, called regenerators, in which they give up their heat, which is returned to the furnace by being used to heat up the gas and air. Such a furnace is the Siemens, in which large quantities of steel can be melted to complete fluidity.