Furnace wall structure capable of tolerating high heat load for use in electric arc furnace

Disclosed is a furnace wall structure which is placed in opposed relation with electrodes of a electric arc furnace and is made of copper or copper alloy in order to ensure a long service life, improved safety and a minimum thermal loss and in which a front plate which defines a heat-receiving surface exposed within the furnace is cooled by the forced circulation of cooling water.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to furnace wall structures which may be used 
as wall components of a furnace shell of an ultra-high-power (UHP), 
super-ultra-high-power (SUHP) are furnace or an arc furnace of the type 
wherein finely divided materials such as sponge iron are continuously 
charged and which may be placed in opposed relation with electrodes or at 
any other places subjected to high heat loads. 
Water-jackets and cast blocks including water cooling pipes which are by 
far superior than water-jacket have long been used as furnace wall 
components placed at the so-called hot spots in opposed relation with 
electrodes. Meanwhile, in order to attain high productivity, electric 
power of arc furnace has been increased so that heat loads to the furnace 
wall have been increased accordingly. Furthermore, with the increase use 
of arc furnaces of the type wherein finely divided materials such as 
sponge iron are continuously charged, the furnace walls are subjected to 
high heat loads for an increased time. 
As a result, with the prior art water jackets, a danger of explosion due to 
water leakage is increased. The cast blocks with an indirect cooling 
construction cannot be used with heat load in excess of a certain level. 
More specifically, when firebricks are used to construct a furnace wall 
which is in opposed relation with an electrode and receives high heat 
load, the increase in electric power imposes a limit to the improvement of 
a service life of the firebricks only by the improvement of qualities 
thereof. To overcome this problem, steel water-jackets have come to be 
used instead of the firebricks. They are placed between the firebricks and 
steel shells so as to increase the service life of the former, but there 
exists a gap between the firebricks and the water-jacket so that the 
effective cooling of the firebricks cannot be attained. As a result, they 
are easily consumed so that the water-jacket is exposed directly to the 
heat inside the furnace. Therefore the prior art water-jackets have come 
to be designed in such a way that their heat-receiving surfaces may be 
directly exposed inside the furnace. 
The prior art water-jackets are assembled from steel plates by welding, and 
because of their construction the flow rate of cooling water is limited to 
the order of 0.01 to 0.5 m (meter)/s (second). The water-jackets with a 
flow rate exceeding 1 m/s have not been available. In general, a steel 
plate has a thermal conductivity .lambda. = 40 Kcal/m h .degree. C 
(Kilocaloric/meter.hour.degree) and its thickness is limited to 10 to 25 
mm due to the construction of water-jackets. (A minimum thickness is 
dependent upon the pressure of cooling water whereas a maximum thickness 
is dependent upon the temperature difference between the heat-receiving 
and cooling surfaces thereof.) As a result, a thermal resistance which is 
defined as l/.lambda. ranges from (2.5 to 6.0) .times. (0.sup.-4 m.h. 
.degree. C/Kcal (square meter.hour.degree/kilocaloric) so that with the 
decrease in heat load the heat-receiving surface may not be satisfactorily 
cooled. That is, the prior art steel water-jackets cannot withstand high 
heat loads. In addition, the steel water-jackets have the following 
problems: 
(a) Variation in heat load results in the variation in temperature of steel 
plates of the water-jacket so that cracks may occur along welded lines. 
(b) Because of a small heat capacity and a small heat conductivity, sparks 
tend to cause the leakage of cooling water from the water-jacket. 
(c) The water-jackets are easily adversely affected by the fuel and oxygen 
burners. 
(d) They are also easily adversely affected by the misblowing of oxygen. 
(e) They are also easily adversely affected by the contact with slag and 
(f) With the little amount of molten steel. The adverse effects (c), (d), 
(e) and (f) result in the leakage of cooling water and burnout. In 
addition, no one can predict when and where such leakage and burnout occur 
so that the safe operation is adversely affected. In the prior art 
water-jackets, slag receiving shelves or the like are formed on the 
heat-receiving surface so that the adhesion to and accumulation on the 
heat-receiving surface of slag and the like may be facilitated and their 
falling-off may be prevented, whereby the thermal loss may be minimized 
and the safety in operation may be assured. However, the problems 
described above have not been essentially solved yet. 
In the cast blocks, cooling water tubes or pipes are casted in the block so 
that a heat capacity may be increased and consequently the accidents 
encountered in the prior art water-jackets may be prevented. However, the 
cast blocks have a thermal resistance considerably higher than the 
water-jackets so that more-soft-cooling results. As a result, they are 
consumed at higher rates under high heat loads. 
Because of the fundamental safety problems of the prior art furnace wall 
structures, when they are used in the SUHP arc furnaces and arc furnaces 
of the type wherein sponge iron is continuously charged, they cannot 
satisfy the conditions required for the furnace walls under high heat 
loads; that is, (1) safety, (2) long service life and (3) decrease in 
thermal loss, alone or in combination of (1) + (2) as well as (1) + (2) + 
(3). 
Therefore there has long been a demand for the furnace wall structures for 
use in the SUHP arc furnaces and arc furnaces of the type wherein the main 
charge mainly consisting of sponge iron is continuously loaded, the 
furnace wall structures being satisfactorily withstanding not only the 
high heat loads due to the thermal radiation from strong arc plasma and 
the thermal convection from the arc flares but also the adverse thermal 
effects due to the above-mentioned causes; that is, due to auxiliary 
burners, the misblowing of oxygen by carelessness of operators to the 
furnace walls, the sparks caused by arcs, the reladle, the contact with 
slags and a small quantity of molten steel. In short, there has long been 
a strong demand for the furnace wall structures whose long service life 
and safety under any adverse thermal effects due to the increase in heat 
load may be satisfactorily assured. To satisfy the above-mentioned 
conditions, those skilled in the art have been so far considered that 
cooling effects on the furnace wall structures must be considerably 
increased and the resultant increase in thermal loss is unavoidable. 
In view of the above, one of the objects of the present invention is to 
provide a safe furnace wall structure having a longer service life. 
Another object of the present invention is to provide a furnace wall 
structure with a minimum thermal loss.

Same reference numerals are used to designate similar parts in FIGS. 7 
through 12. 
To attain the present invention, the inventors made extensive studies and 
experiments on the furnace structures, the results of which will be 
described prior to the description of the preferred embodiments of the 
present invention. 
It is when the furnace wall is directly exposed to the heat source that it 
is subjected to a large quantity of heat loads which may be classified, in 
general, as follows: 
(1) the thermal radiation from molten steel (including slags), the furnace 
walls and other walls after the melt-down stage and when the electric 
supply is suspended, 
(2) the heat load from hot spots after the melt-down stage and when the 
electric current is being supply; that is, the sum of the heat load (1), 
radiation mainly from the arc plasmas and convection mainly due to arc 
flares, 
(3) the heat loads which are increased due to the oxygen blowing and may be 
divided into 
(3 -- 1) the heat load due to the cutting of scraps by oxygen, and 
(3 -- 2) the heat load due to the oxygen refining of molten steel, 
(4) the heat load from fuel and oxygen burners, 
(5) the heat load due to the oxethermic reaction produced when the 
additional or auxiliary charging (CaO and so on) is made, 
(6) the heat load due to the sparks between the scraps and the electrodes, 
(7) the load due to the radiation and depositions of splashes during 
re-ladle stage, 
(8) the heat load due to the direct contact with the slag, and 
(9) the heat load due to the direct contact with molten steel. 
In the experiments conducted by the inventors, the said furnace wall 
structure was made of a well known material such as copper having an 
excellent thermal conductivity and were disposed to cool hot spots on the 
walls of an arc furnace, and temperature measurements were made at least 
two points along the flow of heat between the heat receiving surface and 
the heat dissipating or cooling surface to determine a temperature 
gradient .DELTA.T so that the heat flux defined as q = (Kcal/m.sup.2 h) of 
each load may be obtained by the following relation: 
EQU q = .DELTA.T/(l/.lambda.) 
where 
l = a distance between the two measuring points, and 
.lambda. = a thermal conductivity of a metal plate placed between the heat 
receiving surface and the cooling surface for permitting the 
above-mentioned temperature measurements. 
The experiments were conducted under the condition that nothing was 
deposited on the heat receiving surface. 
From the experiments maximum heat loads exerted to the walls of various arc 
furnaces were determined, and it was found that the prior art 
water-jackets have some problems, which may be solved by the hard cooling 
as will be described in detail below 
1. Heat loads described in (1) and (2) above: 
The operating conditions in the furnace are as shown in FIG. 1 during the 
flat bath period. In FIG. 1, reference numeral 1 denotes a hot spot; 2, 
electrodes; 3, molten steel; 4, arc flares; 5, arc plasma; and 6, slag. 
The thermal conduction through the hot spot 1 under the normal conditions 
is effected as shown in FIG. 2. The heat flux q.sub.T or heat load per 
unit area of the hot spot 1 is given by 
EQU q.sub.T = q.sub.PC + q.sub.K + q.sub.HC + q.sub.EC + q.sub.SC + q.sub.RC 
(Kcal/m.sup.2.h) 
where 
q.sub.PC = heat flux from the arc plasma 5, 
q.sub.K = heat flux due to the convection from the arc flare 4, 
q.sub.HC = heat flux due to the radiation from the molten steel 3, 
q.sub.EC = heat flux due to the radiation from the arc spot on the 
electrode 2, 
q.sub.SC = heat flux due to the radiation from the arc spot in the molten 
steel 3, and 
q.sub.RC = heat flux due to the radiation from the surrounding linings. 
These fluxes vary over a wide range depending upon the operating conditions 
such as the profile and construction of the furnace, rating of equipments 
used such as the capacity of a transformer used, power supply, operation 
power factor, the thickness of slag and so on. 
As the measure of the head load exerted to the hot spot on the wall of the 
furnace, the effective refractory erosion index defined as 
##EQU1## 
is generally used, where 
p.sub.P = arc plasma power (MW), 
V.sub.P = voltage drop (V) of arc plasma, and 
L = minimum distance (m) from the side surface of the electrode to the wall 
of the furnace. 
In order to determine the relationship between R.sub.EP and the heat flux 
q.sub.T at the spot on the wall of the furnace, the 
temperature-gradient-measuring water-jackets of the type described were 
embedded in the walls of various arc furnaces and the measurements were 
made under the condition that the heating surface of the jacket was 
covered with nothing. The results are shown in FIG. 3, wherein the 
characteristic curve A indicates the maximum thermal flux at the hot spot 
whereas the curve B, the heat flux at the hot spot due to q.sub.PC + 
q.sub.K + q.sub.HC + q.sub.EC. In case of quick melting, R.sub.EP is 
inevitably increased and has been limited to a value not exceeding 
R.sub.EP = 500 (MW.multidot.V/m.sup.2) in the conventional arc furnaces in 
order to protect the walls. However, according to the present invention 
the upper limit is set to R.sub.EP = 1,300 (MW.multidot.V/m.sup.2) under 
the assumptions that in the future medium- and large-sized SUHP arc 
furnaces, a maximum allowable transformer capacity be 10,000 
k.multidot.VA.multidot./t (ton)(for instance, for a 100 -ton arc furnace, 
a transformer capacity is 100 MVA) and that the high-power operation 
(long-arc operation) be effected at a power factor of the order of 88% 
which is the upper practical safety limit in the arc furnace and which 
causes the most adverse heat load to be exerted on the hot spot. The 
inventors found out that this upper limit of R.sub.EP is sufficient even 
with a future SUHP arc furnace and even if the operation mistakes should 
happen. From FIG. 3 it is seen that the upper limit of the thermal flux at 
the hot spot does not exceed one million Kcal/m.sup.2 .multidot.h even 
when the hot spot is not deposited with slag and so on. From the 
experimental results, the inventors found out that the heat flux when 
electric current flows is between (50 and 150) .times. 10.sup.3 
Kcal/m.sup.2 h and does not exceed 200 .times. 10.sup.3 KCM/m.sup.2 h. 
Next the temperature gradient .DELTA.T was measured from the relation 
described below under the conditions that the upper limit of heat flux be 
1 .times. 10.sup.6 Kcal/m.sup.2 .multidot.h and that an allowable limit of 
thermal stress caused by the temperature difference between the inner and 
outer surfaces of a steel plate (a steel disk whose periphery being 
securely held or tied stationary) of a steel water jacket be 4500 
Kg/cm.sup.2. The relation is 
##EQU2## 
where 
.alpha. = coefficient of thermal expansion, 
E = Young's modulus, and 
.UPSILON. = Poisson's ratio. 
Then 
##EQU3## 
With a thermal conductivity = 40 Kcal/m.multidot.h.multidot..degree. C, an 
allowable thickness l.sub.st is given by 
##EQU4## 
When the water-jacket is made of copper plates, 
##EQU5## 
With a thermal conductivity .lambda. = 300 
Kcal/m.multidot.h.multidot..degree. C. of copper plate, an allowable 
thickness l cu is given by 
##EQU6## 
It is seen that when the copper plates are used, the allowable thickness is 
four times as thick as the allowable thickness of steel plates. This 
suggests that a heat capacity may be also increased four times as much as 
when steel plates are used. The steel water-jackets are subjected to 
crackings along the welded lines due to the high heat load, but this 
phenomenon is not observed with the said furnace wall structure. Thus it 
is apparent that the said furnace wall structure is by far superior to the 
steel water-jackets. 
2. Heat loads defined in (3) and (5): 
The excessive increase in heat load (3 -- 1) to the walls of the furnace 
due to the cutting of scraps with oxygen is caused by carelessness on the 
part of the operators, but cannot be completely eliminated and rather can 
happen very frequently. It is difficult to quantitatively define the above 
excessive increase in heat load due to carelessness. According to the 
experiments, because of its nature the heat load (3 -- 1) does not overlap 
with the maximum heat load due to the arcs and does not exceed 1 .times. 
10.sup.6 Kcal/m.sup.2 .multidot.h even at a local spot. 
Experiences show that holes are formed in the steel water-jackets because 
of the slow diffusion of heat and rapid oxidation due to the misblow of 
oxygen, but this accident may be completely prevented in case of the said 
furnace wall structure. 
As with the auxiliary material charging (See (5) above) which results in 
the exothemic reaction, oxygen blowing (3 -- 2) results, in the exothermic 
reaction which in turn results the rapid increase in temperature of molten 
steel, slag and gas in the furnace. However the walls are not subjected to 
locally high heat loads. According to the experiments, the heat load (3 -- 
2) will not exceed 500 .times. 10.sup.3 Kcal/m.sup.2 .multidot.h. 
Thus it is seen that the problems encountered when the prior art steel 
water-jackets may be substantially overcomed by the use of the said 
furnace wall structure. 
3. Heat load (4): 
In general, the fuel and oxygen burners flame are not directed toward 
walls, but it frequently happens that the high-temperature combustion 
gases from the burners flow through the space between the walls and scraps 
when pressed or large scraps are charged just in front of the burners so 
that the walls are subjected to excessive heat loads. However, the heat 
load (4) is completely independent of the heat load from the arcs, and 
according to the experiments the thermal flux will not exceed 500 .times. 
10.sup.3 Kcal/m.sup.2 h. 
4. Heat load (6): 
Experiences show that sparks with a large electric current tend to occur 
when an electrode is broken and made into contact with the wall or between 
the remaining scrap and the water-jacket, causing the water leakage of the 
steel water-jackets. However, the said furnace wall structure has a high 
electrical conductivity and a high thermal conductivity so that the rapid 
diffusion of electric current and heat can be made through the said 
furnace wall structure properly, and consequently the safe operation may 
be assured. 
5. Heat load (7): 
The heat load (7) to the walls due to the radiation when the molten steel 
is returned to the furnace, does not produce simultaneously with the heat 
loads from the arcs and the radiation from the walls so that the heat load 
(7) is almost equal to the heat load (1) and will not exceed 200 .times. 
10.sup.3 Kcal/m.sup.2 h in practice, which was confirmed from the 
experiments. However, due to the depositions of molten steel splashed, the 
walls are locally subjected to the high heat loads, but it can not be 
considered that a large quantity of molten steel is continuously kept in 
contact with one spot of the said furnace wall structure. In this case, 
the said furnace wall structure is more advantageous in view of low 
thermal resistance, high cooling efficiency and high heat capacity. 
6. Heat load (8): 
The direct contact of the slag with the water-jackets occurs very often as 
the water-jackets are set up at lower positions adjacent to the molten 
steel surface in order to increase the service life of refractories 
adjacent to the slag line. Especially the use of sponge iron results in 
increase in quantity of slag and enhanced bubbling so that the chance of 
direct contact is extremely high. Meanwhile, because of insufficient 
cooling capacity of the prior art water-jackets they are so arranged as to 
avoid the direct contact with the slag as less as possible. The heat 
fluxes due to the contact with the slag vary over a wide range depending 
upon the temperature, qualities and movement of slag, and are in general 
(600 to 1,000) .times. 10.sup.3 Kcal/m.sup.2 .multidot.h and will not 
exceed 2,000 .times. 10.sup.3 Kcal/m.sup.2 .multidot.h even when iron 
oxides are large in quantity or when the slag with molten steel moves and 
is made in continuous contact with the water-jackets. As shown in FIG. 4, 
with the said furnace wall structure with a thickness of 40mm, the surface 
temperature is maintained less than 400.degree. C. This means that the use 
of the said furnace wall structure ensures a higher degree of safety as 
compared with the prior art steel water-jackets. In FIG. 4, the 
characteristic curves A, B, C and D indicate the temperature of the 
heating surface; that is, the surface temperatures of the said furnace 
wall structure 10mm, 30mm, 40mm and 50mm, respectively, in thickness. The 
characteristic curves A', B', C' and D' indicate those of the steel 
water-jackets 10mm, 20mm, 30mm and 50mm, respectively in thickness. 
Melting points of copper and steel are indicated by CM and SM, 
respectively. 
7. Load heat (9): 
In case of the said furnace wall structure, the direct contact with molten 
steel will not cause the excessive thermal fluxes if cooling water is 
flowing at sufficiently high flow rates regardless of the quantity of 
molten steel made into contact with the water-jackets. However, in an 
extreme case which hardly occurs, molten steel is caused to be made into 
continuous contact with the same surface of the water-jacket so that the 
cooling water changes from nuclerate boiling to film boiling with the 
resultant temperature increase of the surface to a burnout temperature. In 
the electric furnaces, the direct and continuous contact of molten steel 
with the walls may avoided under the normal operations, but in order to 
ensure the safety, the direct and continuous contact must be taken into 
consideration and consequently a high value of burnout thermal flux 
q.sub.BO must be used in design. 
The burnout thermal flux which may be obtained by the dropping tests of 
molten steel varies over a wide range depending upon a sub-cool 
temperature .DELTA.T sub and a flow rate v of cooling water as shown in 
FIG. 5 wherein the experimental data which were obtained with the use of 
the said furnace wall structure 20mm in thickness are plotted with the 
sub-cool temperature .DELTA.T sub as parameters. With the prior art water 
jackets, q.sub.BO was (4 to 8) .times. 10.sup.6 Kcal/m.sup.2 .multidot.h, 
because the flow rate v is less than 1 m/s, but it may be increased to 12 
.times. 10.sup.6 Kcal/m.sup.2 .multidot.h when the flow rate may be 
increased in excess of 4 m/s so that the safety may be considerably 
increased, which was confirmed by the actual furnace tests conducted by 
the inventors. It was also found out that when the flow rate is in excess 
of 4 m/s the deposition on the cooling surfaces may be minimized. 
So far the experimental data or results have been described under the 
assumption that the heat receiving surfaces of the water jackets are 
completely exposed within the furnace. In practice, however, if the heat 
receiving surface is sufficiently cooled with cooling water, the thermal 
balance is attained when slag is deposited on the heat-receiving surface 
in such a thickness that the temperature at the surface of the slag 
deposited is equal to a melting point of the slag. Under this condition, 
the heat flux is balanced at the order of (30 to 80) .times. 
103Kcal/m.sup.2 .multidot.h. As an example, shown in FIG. 6 are Kcal 
fluxes at hot spots of a 60-ton arc furnace during operation. The 
characteristic curve X indicates when the prior art water-jackets were 
used, whereas the curve Y, when the furnace wall structure in accordance 
with the present invention were used. 
The furnace wall structures for high heat load in accord with the present 
invention are based upon the above experimental results, and one preferred 
embodiment thereof will be described in detail with particular reference 
to FIGS. 7, 8 and 9. 
As best shown in FIG. 9, a furnace wall structure I in accordance with the 
present invention has a main body 11 with a front plate 12 and a cooling 
water passage 13. The front plate 12 is made of copper or copper alloy 
with the thermal resistance l/.lambda.= 0.5 to 1.5 .times. 10.sup.-4 
m.sup.2 .multidot.h.multidot..degree. C./Kcal, the thermal conductivity 
.lambda. Kcal/m.multidot.h .degree. C. and the thickness in m, and the 
rear surface of the front plate 12 is sufficiently smoothed so that the 
deposition from cooling water may be prevented and the cooling water may 
flow at a higher flow rate through the cooling water passage 13. The 
furnace wall structure I is further provided with a cooling water inlet 14 
and a cooling water outlet 15. The front surface of the front plate 12 is 
used as a heat receiving surface 16 while the rear surface, as a cooling 
surface 17, and the heat receiving surface 16 is provided with a slag 
receiving shelves 18 which may prevent the falling off of layers 19 of 
slags and the like deposited and cooled on the heat-receiving surface 16 
due to the mechanical external forces exerted to the layers as when a 
charge is loaded. Cooling water is forced into the cooling water passage 
13 through the inlet 14 for cooling the cooling surface 17 of the front 
plate 12 and is discharged through the outlet 15. The furnace wall 
structure I with the above construction is set up mainly at a hot spot of 
the walls of a furnace. That is, the structure I is mounted on a furnace 
shell plate 20 in such a way that the lower end may be located adjacent to 
the slag line 21 and the heat-receiving surface 16 of the front plate 12 
may be directed toward the center of the furnace as best shown in FIGS. 7 
and 8, and refractories 22 are filled between the shell plate 20 and the 
furnace wall structure I. In this embodiments, the cooling water passage 
13 is defined by copper plates which are joined together by electron beam 
welding in order to improve the dimensional accuracies. 
In operation, cooling water is circulated at a flow rate higher than 4.0 
m/s. Since the cooling water passage 13 is defined by the smooth surfaces 
and the cooling water is circulated at a high flow rate, the deposition 
from cooling water on the cooling surface 17 may be prevented. During the 
operation, the slag and the like are deposited and solidified upon the 
heat-receiving surface 16, but they may be sufficiently cooled because the 
cooling water is circulated at high speeds. As described above, the front 
plate 12 is made of copper or copper alloy and has a sufficient thickness 
within the limit that the thermal resistance is (0.5 to 1.5) .times. 
10.sup.-4 m.sup.2 .multidot.h.multidot..degree. C./Kcal, so that it may 
have a sufficient heat capacity to encounter the heat load due to the 
contact with the slag and or molten steel and to the sparks. In addition, 
the front plate 12 may sufficiently withstand the pressure exerted from 
the cooling water and the water leakage problem may be eliminated. 
The reason why the thermal resistance l/.lambda. must be within the range 
from (0.5 to 1.5) .times. 10.sup.-4 m.sup.2 
.multidot.h.multidot..degree.C./Kcal will be described below. 
The lower limit = 0.5 .times. 10.sup.-4 (m.sup.2 .multidot.h.multidot. 
.degree. C./kcal): 
In order to withstand the pressure exerted from the cooling water and the 
impact of the charged material such as scraps and to provide a sufficient 
heat capacity to encounter the heat load due to the contact with molten 
steel and sparks, the minimum allowable thickness is determined as 15mm. 
From this thickness and the thermal conductivity .lambda. = 260 to 300 
(kcal/m.multidot.h.multidot..degree. C.), l/.lambda. = 0.015/300 = 0.5 
.times. 10.sup.-4 (m.sup.2 .multidot.h.multidot..degree. C./kcal) is 
determined. 
The upper limit = 1.5 .times. 10.sup.-4 (m.sup.2 
.multidot.h.multidot..degree. C./kcal): 
As stated hereinbefore, under the conditions that the upper limit of heat 
flux be 1 .times. 10.sup.6 kcal/m.sup.2 .multidot.h an allowable thickness 
of copper plate is 40mm which is determined by the upper limit of heat 
stress produced due to the temperature difference between the front and 
rear surfaces of the copper plate. From this thickness and the thermal 
conductivity .lambda. = 260 to 300 (kcal/m.multidot.h.multidot..degree. 
C.), l/.lambda.= 0.04/260 .apprxeq. 1.5 .times. 10.sup.-4 (m.sup.2 
.multidot.h.multidot..degree. C./kcal) is determined. In the case of the 
contact with the molten steel, under the conditions that upper limit of 
heat flux be q = 5 .times. 10.sup.6 (kcal/m.sup.2 .multidot.h), the 
temperature difference between the front and rear surfaces is .DELTA.t = 5 
.times. 10.sup.6 .times. 1.5 .times. 10.sup.-4 = 750.degree. C. The 
temperature of the rear or cooling surface is 
EQU t.sub.c = t.sub.s + .DELTA.t sat 
where 
t.sub.s = a saturation temperature of cooling water, and 
.DELTA.t.sub.sat = degree of superheat on the cooling surface. 
With the pressure of cooling water P = 2 kg/cm.sup.2, t.sub.s .apprxeq. 
120.degree. C. 
##EQU7## 
Therefore, 
EQU t.sub.c = 120 + 39 = 159.degree. C. 
then, the temperature of the front or heat-receiving surface is t.sub.h = 
159 + 750 = 909.degree. C. 
This temperature is lower than the melting point 1080.degree. C. of native 
copper, and therefore the meltdown of the main body will not occur. 
FIG. 10 shows the relationship between the thermal resistance which is 
dependent upon both the thickness l and thermal conductivity .lambda. of 
the front plate 12 and the temperature drop across the front plate 12 
which is dependent upon the thermal resistance and heat flux q. In other 
words, FIG. 10 shows the heat transmission characteristics of the furnace 
wall structures in accordance with the present invention. In FIG. 10, the 
present invention uses the thermal resistance within a hatched area L. The 
corresponding range of the prior art steel water jackets is indicated by 
L' and is from (2.5 to 6.0) .times. 10.sup.-4 m.sup.2 
.multidot.h.multidot..degree. C./Kcal, which is by far greater than the 
range of the present invention. 
In FIG. 11 there is shown another preferred embodiment of a furnace wall 
structure in accordance with the present invention which is substantially 
similar in construction to that shown in FIG. 9 except that pole pieces 23 
and an electromagnet 24 are provided. More specifically, the pole pieces 
23 each made of a suitable magnetic material and having a sufficiently 
large area are disposed within the cooling water passage 13, so that the 
gap between the electromagnet 24 and the iron material or the 
iron-containing slag attracted on the front surface of the front plate 12 
may be compensated to decrease the magnetic resistance and thus obtain 
stronger magnetic force and the electromagnet 24 is disposed on the rear 
surface of a plate which defines together with the front plate 12 the 
cooling water passage 13 so that the iron-containing slag and steel may be 
easily trapped on the heat-receiving surface 16 of the front plate 12. 
Because of the pole pieces 23, and the electromagnet 24 the slag and main 
charges are more densely and strongly accumulated over the heat-receiving 
surface 16 of the front plate 12 so that as compared with the embodiment 
shown in FIG. 9, the slag and the like may be deposited in greater 
thickness. In addition, the thermal efficiency may be increased and the 
more positive protection of the walls of the furnace may be ensured when 
the furnace wall structures of the type shown in FIG. 11 are used in an 
arc furnace of the type wherein iron-containing metal particles such as 
reducing iron particles are continuously charged. 
In FIG. 12 there is shown a further preferred embodiment of the present 
invention which is substantially similar in construction to those shown in 
FIGS. 9 and 11 except that means is provided for increasing a melting 
point of the heat-receiving surface of the front plate 12. More 
specifically with the front plate 12 made of copper or copper alloy a 
local meltdown of the heat-receiving surface tends to occur when it is 
subjected to an extremely high heat load in excess of its melting point 
about 1,080.degree. C. caused by the continuous contact with slag or 
molten metal in large quantity. Furthermore, because of a greater thermal 
conductivity .lambda. of copper, greater thermal loss results (A high 
thermal conductivity is one of the features of the furnace wall structures 
in accordance with the present invention, but it is of course preferable 
to minimize the thermal loss caused by this fact). In addition, the front 
plate 12 is exposed within the furnace so that it tends to be damaged by 
the impact of materials like a falling scrap harder than copper. In order 
to solve these and other problems, in this embodiment the slag holding 
ledges 18 are eliminated, and instead the heat-receiving surface 16 of the 
front plate 12 is formed with alternate ledges and valleys which in turn 
are coated to a desired thickness with a layer 25 of a metal (for example, 
Ti, Zr, Cr, Mo, W and their carbonated or nitric materials), cermet (which 
are compounds of metal and ceramic) or ceramic having a hardness and a 
melting point both higher than those of copper. For this purpose, any 
suitable means such as plating, vapor-metal plating, metal spraying and so 
on may be employed. The layer 25 thus formed serves to increase the 
mechanical strength of the heat-receiving surface of the front plate 12 so 
that the latter may be prevented from being damaged even when it is struck 
by with solid materials harder than copper. Furthermore the melting point 
of the heat-receiving surface of the front plate 12 may be increased so 
that meltdown due to the continuous contact with molten steel in large 
quantity may be prevented. Moreover, the thermal resistance may be 
increased with the resultant decrease in thermal losses. Because of the 
ledges formed in the heat-receiving surface of the front plate 12, the 
slag and the like may be more positively and strongly adhered to and 
accumulated on the surface. It is to be understood that the ridges and 
valleys may be eliminated and instead the layer 25 may be directly formed 
on the flat heat-receiving surface. 
The features and advantages of the furnace wall structures in accordance 
with the present invention may be summarized as follows: 
(i) Since the cooling water may be circulated at a higher flow rate, the 
thermal conductivity between the cooling surface of the front plate and 
the cooling water may be considerably increased; that is, the heat may be 
rapidly dissipated from the cooling surface to the cooling water, and 
since the heat-receiving surface of the front plate exhibits a low thermal 
resistance, the temperature of the outer wall of the furnace shell may be 
maintained satisfactorily at low temperatures even against high heat loads 
in a SUHP arc furnace. 
(ii) The furnace wall structures exclude refractories which are exposed 
within the arc furnace so that consumption may be minimized. 
(iii) Because the furnace wall structures are made of copper or copper 
alloy, they may readily and safely dissipate heat and current applied 
thereto due to sparks. 
(iv) The front plate has an increased thickness so that it may safely 
withstand against the direct contact with the combustion gases discharged 
from the auxiliary oxygen-fuel burners, misblown oxygen gas, slags and 
molten steel under the normal conditions. 
(v) No cooling water leakage occurs at all. 
(vi) A longer service life and a minimum thermal loss may be ensured. 
(vii) Because of the provision of the pole pieces and the electromagnet and 
because of the exclusion of any refractories within the main body, strong 
forces for attracting iron-containing compounds may be provided. 
(viii) Because of the rough surface or provision of ledges and valleys in 
the heat-receiving surface of the front plate, the deposition and 
accumulation of slag and the like may be much facilitated and their 
falling-off from the heat-receiving surface may be prevented. 
(iv) Because of the coating of the heat-receiving surface of the front 
plate with a material having a high melting point, the melting point of 
the heat-receiving surface itself may be increased so that damage due to 
the heat load in excess of a melting point of copper may be prevented. 
Furthermore the thermal resistance may be increased so that the thermal 
loss may be minimized. In addition, the mechanical strength of the 
heat-receiving surface may be increased and consequently may be prevented 
from being damaged. 
(x) Because of the above-mentioned features and advantages, a long service 
life of the furnace wall structures may be ensured. 
Water-cooled furnace wall structures according to the present invention 
have been now in practical use by several companies in U.S.A., Mexico and 
Japan. The one used by TOKYO KOTESTU Co., LTD has been working over 5,000 
heats without trouble.