Industrial electric heating furnaces – Arc furnace device – Furnace body detail
Reexamination Certificate
2000-10-02
2002-06-11
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Arc furnace device
Furnace body detail
C373S076000, C373S074000
Reexamination Certificate
active
06404799
ABSTRACT:
TECHNICAL FIELD
This invention relates to a water-cooled wall and roof panel for installation in an electric-arc furnace used for melting metal material and refining molten metal.
BACKGROUND TECHNOLOGY
In electric-arc furnaces used to melt metal materials and refine molten metal up to the mid-1970s, the furnace body was lined with refractory at the inside of shell and the roof used arch-like refractory at the inside of a metal frame called a furnace roof frame. From around the latter half of the 1970s, however, the need to increase productivity led to a rapid increase in electric-arc furnace size. Since electric-arc furnace power consumption rose in proportion, wear and tear on the refractory used in the furnace body and roof increased markedly. The result was higher refractory-related costs and more downtime for refractory repair.
One attempt made to overcome these problems was aimed at extending the refractory service life while minimizing a decrease in electric-arc furnace heat efficiency by using a furnace body cooler constituted as an structure of one or more cooling water pipes and bricks embedded within cast iron or a copper casting (Japanese Unexamined Published Utility Model Application 49-118635). However, in the cooling structure using cast iron, the iron casting of the cooler proper reaches a temperature of 1000° C. at the surface on the furnace interior side. During use for several hundred to one thousand charges, therefore, the casting experiences cracking caused by thermal stress and becomes brittle owing to change in texture. As the cracking and embrittlement proceed, the casting undergoes wear and the bricks within the casting drop out. When the cracks occurring in the casting surface propagate as far as the cooling water pipe(s), water leakage occurs. In the cooling structure using cast copper, wear and cracking do not arise quickly because the copper casting has higher ductility than the iron casting and does not experience texture change. Still, since the bricks are embedded in the panel on the furnace interior side and the pipe or pipes are present behind the bricks, the ends of the bricks on the furnace interior side reach a high temperature that causes them to wear rapidly. The panel proper also has greater weight owing to its larger thickness. The material cost is therefore higher than when cast iron is used, especially in the case of a very large copper casting.
On the other hand, Japanese Unexamined Published Utility Model Application 56-29798 teaches a method for overcoming the foregoing problems by casting a low-melting-point metal such as copper or aluminum around a cooling water pipe so as to branch radially, thereby enhancing cooling capability and preventing propagation of cracks occurring at the casting surface. Calculations show that this method should hold the temperature of the casting of the cooler proper on the furnace interior side to around 500° C. In fact, however, when the cooler is installed at a high thermal load location where slag does not adhere or barely adheres to the casting surface, the surface temperature reaches 1000° C. or higher. Because of this, the problem of texture change and cracking of the casting cannot be overcome. This method also increases cost because complicated fabrication steps are required in order to cast the low-melting-point metal, which has different properties from the cooler proper, around the cooling water pipe.
Because of the texture change and cracking of the casting, along with other problems, the cast cooler of this structure has not come into general use. The most commonly used structure used today is the water-cooled panel used in a furnace having no refractory at its inner surface and constituted as a cooler of welded steel plate structure, steel pipework structure, copper casting structure or welded copper plate structure. The water-cooled panel is helping to reduce refractory wear also in large-size, high-power electric-arc furnaces. (See, for example Japanese Unexamined Published Patent Applications 51-97506, 56-66680 and 56-45800.)
Various ways have also been suggested for increasing the durability of the electric-arc furnace roof. A vertical sectional view of a conventional electric-arc furnace is shown in FIG.
13
. The top of the shell
21
of the electric-arc furnace is closed by an openable roof
23
made of refractory and formed with electrode insertion holes
16
for passage of electrodes
22
. During operation, the refractory roof
23
incurs fusion damage under high-temperature heating and has to be replaced. This increases cost. In response to this, Japanese Unexamined Published Patent Application 53-107729 teaches the furnace roof shown in vertical section in FIG.
14
. All of the furnace roof, except for the inverted cone portion formed with the electrode insertion holes
16
for passage of the electrodes
22
, is made of steel plate and the interior of this portion is formed with helical passages
24
to constitute a water-jacket roof
25
. The inner surface of water-jacket roof
25
is formed with a metal film
26
of high thermal conductivity and capable of reflecting radiant heat. This structure prolongs the service life of the furnace roof.
Still, owing to the occurrence of cracks with continuing operation of the electric-arc furnace, this water-jacket type furnace roof made of steel plate frequently experiences water leakage from the water jacket. Moreover, in an electric-arc furnace whose wall and roof are formed with water jackets made of steel plate, the amount of heat lost to water cooling accounts for about 10% of the total energy required by the electric-arc furnace. About half of the lost heat is carried away by the roof cooling water. Also in the electric-arc furnace roof, therefore, there is a need to reduce the amount of heat lost to the cooling water without increasing wear of the refractory.
Japanese Unexamined Published Patent Application 50-142709 teaches a roof for an electric-arc furnace that uses an appropriate number of coolers composed of one or more cooling water pipes and bricks embedded in cast iron, cast copper or other such casting. This furnace roof reduces the amount of heat lost to the cooling water. However, the furnace roof of this structure has the same problems as pointed out regarding the furnace body cooler describe earlier. Specifically, the casting of the cooler proper reaches a temperature of 1,000° C. at the surface on the furnace interior side. During use for several hundred to one thousand charges, therefore, the casting experiences cracking caused by thermal stress and becomes brittle owing to change in texture. As the cracking and embrittlement proceed, the casting undergoes wear and the bricks within the casting wear and drop out. When the cracks occurring in the casting surface propagate as far as the cooling water pipe(s), water leakage occurs.
Therefore, like the furnace body cooler, the furnace roof cooler is also susceptible to cracking of the steel plate and the steel pipework portion as well as to the water leakage this causes. Despite such shortcomings, coolers of the welded plate structure and steel pipework structure, known as water-cooled panels, are in general use.
The technologies described in the foregoing attempt to reduce wear of the electric-arc furnace refractory, lower cost and decease downtime for refractory repair by equipping the furnace interior with water-cooled panels which, being of welded steel plate structure, steel pipework structure, copper casting structure or welded copper plate structure, have no refractory on the furnace interior side. Owing to the absence of refractory on the furnace interior side, however, the water-cooled panel must be supplied with a large amount of cooling water to protect the panel proper. Problems therefore persist regarding heat loss to the cooling water and the need for a high-power pump for supplying the cooling water. Against the backdrop of intensifying calls for more efficient energy utilization in order to reduce emission of carbon dioxide—a greenhouse gas that promo
Kirishiki Koichi
Mori Tadashi
Uchida Shinjiro
Hoang Tu Ba
Kenyon & Kenyon
Nippon Steel Corporation
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