Other furnace types

The protective gas is supplied through a manifold with several outlets. The positions of the outlets are chosen to give protection to all areas of the metal surface. Areas closer to hatches that will be opened frequently during operation need a higher flow of gas than areas where interaction with the surrounding atmosphere is small. 

In order to optimise safety and minimise gas consumption, totally encapsulated melting and casting processes are applied. 

If replacement is not feasible, the following technical measures allow a reduction of SFg consumption and emissions improved sealing of furnaces fully automatic cover gas dosage electronic control of both gas mix and flowrate reduction of overdosing. 

The replacement of SFg avoids using this greenhouse gas, which has a GWP of 22200 over a 100 years time horizon. 

SO2 is a toxic gas and exposure limit values for workers should be taken into account. The occupational exposure limit in most countries is 2 ppm (5 mg/m ) over 8 hours. Sulphur- and oxygen-containing deposits may form on the furnace wall. Under unfavourable conditions these deposits can be immersed into the molten metal where they cause reactions leading to metal eruptions from the surface. Frequent removal of scaling can prevent this from happening. 

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Batch Furnaces, Other Furnace Types, and Kilns. 12-45... 

Depending on the installed power density and the melting practice the thermal efficiency can exceed 80 %, but usually ranges from 60 to 70 %. If the efficiency of the electric power generation is taken into account, an overall efficiency of 15 - 20 % results, which is rather low in comparison with other furnace types. 

Limiting the slag is very important for good operation of the coreless induction furnace. The operation is more affected by the scrap cleanliness than the other furnace types. 

For the other furnace types, BAT mainly focuses on the efficient collection of furnace off-gas and/or the reduction of fugitive emissions. 

Ferrous foundries consist of two types steel foundries in which electric furnaces (EAF and induction) are used, and iron foundries in which hot-blast cupolas and/or electric furnaces are used. Electric furnaces use virtually 100% scrap charges. Cupolas are shaft furnaces which use preheated air, coke, fluxes, and metallic charges. Scrap is over 90% of the metallic charge. Cupolas accounted for about 64% of total iron foundry scrap consumption in 1994 and electric furnaces accounted for about 34%. The balance was consumed by other furnaces, such as air furnaces. Iron foundry products have a high carbon content and the scrap charge usually contains a high percentage of cast iron or is used in combination with pig iron. 

Table 1 shows the average percentages of scrap and pig iron used in the metallic charges for each of the three principal furnace types. DRI consumption averaged about 2% in electric furnaces and only a fraction of 1% in BOFs and cupolas. These percentages do not include the scrap consumed in blast furnaces and certain other special furnaces which amounted to 1.9 million t in 1994. DRI consumption in blast furnaces totaled 490,000 t in 1994. 

Multiple-Hea.rth Roasters. The circular types consist of a series of hearths arranged vertically in such a way that the ore entering the top is rabbled and dropped down from hearth to hearth, until it is completely oxidized. The hearths are usually stationary and the plows revolve, such as in the Wedge, Herreshoff, Ord, Skinner, and other roasters (21). In other furnaces, the hearths revolve and the rabbles are fixed, eg, the deSpirlet and its modification, the Barrier. 

The other major type of pulverized-coal-fired boiler is the wall-fired boiler. In wall-fired units, instead of being mounted in the comers, the burners are mounted on the walls of the furnace. They may be mounted on only one wall, or they may be mounted on opposing walls. The burners typically are mounted in a grid pattern. 

The selection of a melting fiimace is an important aspect in the setting up of a foundry process. Each furnace type has its own properties concerning feed requirements and alloying possibilities, which in turn will have repercussions on the full foundry process. On the other hand, the t5q)e of metal to be melted determines which furnace may or may not be used. The applicability of the various furnace types is given in Table 2.5. 

These techniques are applied in other industrial sectors, such as steel and non-ferrous metal production and waste incineration. Judging on a technical basis, they may be transposed to foundry furnace types that show a risk of dioxin formation cupola, rotary and electric arc furnaces melting iron and steel (Section 3.8.2). For new and existing installations primary dioxin reduction measmes, such as efficient combustion, furnace design modifications and scrap quality control have to be taken into consideration on a case-by-case basis, before turning to secondary measures. 

Heat transfer in batch-type furnaces is limited by the same variable factors as in all other furnaces (e.g., furnace temperature, refractory radiation, gas radiation, convection, scale on the load, hearth heat loss, and location of the control temperature measurement). See also the list of improvements that can help furnace productivity in sections 4.6.1,4.6.1.2, and 4.6.1.3. Tables B.3 and B.4 in reference 52 give heat requirements for drying. 

Higher furnace capacity is necessary to keep pace with other mill improvements. Recommendations 1 to 8 below suggest ways to match the furnace capacity to the production line equipment in series with it. Furnace types such as rotary hearth, walking beam, walking hearth, pushers, and some other high-temperature continuous furnaces can benefit from one or more of these recommendations. 

The most common types of sintering furnaces are electrical resistance furnaces in which a current carrying resistor, commonly called the furnace element or winding, provides the source of heat. In addition to size and cost, important considerations in the selection of a furnace are the maximum temperature capability and the atmosphere in which it can be operated for extended periods. Table 12.1 provides a selected list of common furnace elements. Several metal alloys (e.g., nichrome) can be used as furnace elements for temperatures up to 1200°C in both oxidizing and reducing atmospheres. For extended use in air, other furnace elements can deliver higher temperatures e.g., Pt (up to 1400°C) SiC (1450°C) ... 

Submerged-Arc Furnace. Furnaces used for smelting and for certain electrochemical operations are similar in general design to the open-arc furnace in that they are usually three-phase, have three vertical electrode columns and a shell to contain the charge, but dkect current may also be utilised They are used in the production of phosphoms, calcium carbide, ferroalloys, siUcon, other metals and compounds (17), and numerous types of high temperature refractories. 

Furnaces of this type, such as the steam locomotive furnace—boHet design, had the obvious disadvantage that pressure was limited to ca 1 MPa (150 psi). The development of seamless, thick-waH tubing for stationary power plants (ie, water-tube furnaces) and other engines for motive power, such as diesel—electric, has in many cases ecHpsed the fire-tube boHet. For appHcations calling for moderate amounts of lower pressure steam, however, the modern fire-tube boHet continues to be the indicated choice (5). 

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