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open-hearth furnace[′ōpən ¦härth ′fər·nəs]
an open-flame regenerative furnace for making steel of a given chemical composition and quality from cast iron and steel scrap.
An open-hearth furnace has the following major components: a melting chamber, which consists of a hearth, front and back walls, and a crown in which smelting takes place; right and left heads, which consist of the heads themselves and vertical channels for supplying fuel and air to the melting chamber and removal of products of combustion; air and gas slag chambers for the precipitation and accumulation of dust and slag particles from the products of combustion passing through them; air and gas regenerators for heating the air and gas entering the furnace by means of the heat of the products of combustion leaving the melting chamber; horizontal flues (channels) for air, gas, and products of combustion; a system of butterfly valves for changing the direction of fuel and air input into the furnace and elimination of products of combustion from the melting chamber; a waste-heat boiler; and a furnace flue.
The melting chamber and furnace heads are located higher than the working area of the shop and are conventionally called the superstructure of the furnace. The other parts of the furnace are below the working area and are called the lower structure.
An open-hearth furnace is a symmetrical unit: its right and left sides are identical in construction, relative to the vertical axis. Fuel and air for combustion enter the melting chamber alternately from the right and left sides. Products of combustion are removed from the melting chamber on the opposite side. A change in the direction of fuel and air input—that is, a change in the direction of the fuel spray in the melting chamber—is accomplished using a system of valves and gates and is called valve transfer. The products of combustion enter the regenerator from above, through the slag chamber, at a temperature of 1500°-1600°C and, as they pass over the checkerwork, transmit a significant portion of their heat to it. Subsequent passage of cold air or gas over the heated checkerwork raises its temperature to 1100°-1200°C.
All parts of an open-hearth furnace are lined with refractory material. Open-hearth furnaces are called acid or basic, depending on the nature of the refractory materials lining the melting chamber. Magnesite, magnesite-chromite, and chrome-magnesite bricks are used to line basic open-hearth furnaces, and magnesite powder is used to fettle the hearth. Dinas brick and quartz sand are used for lining acid open-hearth furnaces. Forsterite or high-alumina, magnesite, and fireclay bricks are used in the lower structure of the furnace. The lining is reinforced with a metal framework to strengthen the overall structure of the furnace. Furnace units and parts operating at high temperatures are cooled constantly.
Open-hearth furnaces may be of the stationary or tilting types. Most open-hearth furnaces are stationary. Tilting open-hearth furnaces are usually used for treatment of phosphoric pig iron, since the phosphorus-rich slag must be drawn off repeatedly. This is easier in tilting furnaces. Open-hearth furnaces may be fired by liquid fuel (fuel oil) or gaseous fuel (natural, mixed, or producer gas). Mixed gas (coke-oven and blast-furnace gas) and producer gas, which have an insufficient heat of combustion, are heated in the regenerators to about 1150°C before passing into the melting chamber. Natural gas and fuel oil are used without prior heating. Oxygen used to enhance fuel combustion is introduced through tuyeres located in the furnace heads; oxygen for blowing the bath enters through tuyeres lowered through openings in the furnace crown. A certain amount of the fuel may be introduced together with oxygen into the melting chamber using fuel-oxygen burners, which are also lowered through inlets in the furnace crown.
Furnaces heated by low-grade gas have two pairs of slag chambers and two pairs of regenerators (for heating the gas and air), located in pairs under each head. Furnaces heated by fuel oil or natural gas have one slag chamber and one regenerator under each head (for heating of air only). In spite of the presence of regenerators, the exhaust gases have a temperature of 400° -800°C at the flue. Waste-heat boilers are installed to use this heat. Furnaces are equipped with process instrumentation, which makes possible not only monitoring of the operation of the furnace but also automatic maintenance of a given heating mode in various smelting periods.
The use of oxygen to intensify the operation of open-hearth furnaces has led to a steady decrease in the role of regenerators. In the 1960’s, tandem furnaces without regenerators were put into operation in a number of metallurgical plants.
The major indexes characterizing the operation of open-hearth furnaces are yearly and hourly output, output of steel per square meter of hearth area per day, and fuel consumption. The yearly output most fully characterizes the operation of the furnace, since it takes into account all shutdowns, including idle time with and without fuel shutoff, and makes possible the objective comparison of the operation of similar types of furnaces. The output of large open-hearth furnaces exceeds 0.5 million metric tons of steel per year. Steel output per square meter of hearth area allows comparison of the operation of furnaces of varying capacities under different conditions. It is normally 12-13 tons/m2. The highest technical and economic indexes for the operation of open-hearth furnaces have been achieved in the USSR.
I. B. POLIAK