Cooled roof for electric arc furnaces and for ladle furnaces

Abstract

 Cooled roof for electric arc furnaces and ladle furnaces, of the type comprising a plurality of radial, contiguous panels (11), each panel (11) including at least a cooling conduit (12), through which the cooling water flows, arranged radially with respect to the roof (10) and including a transverse transit section which progressively narrows from the outer periphery towards the centre of the roof (10).

Description

[0001] This invention concerns a cooled roof for electric arc furnaces and for ladle furnaces as set forth in the main claim.

[0002] The cooled roof according to the invention is applied as a cover in electric arc furnaces and in ladle furnaces used to melt metallic alloys, both ferrous and non-ferrous.

[0003] Electric arc furnaces are composed of a sheet cylinder lined on the inside at the lower part with refractory material and closed at the upper part by a roof.

[0004] In order to prevent excessive heating of the roof, and to remove the heat generated by the melting of the metal, the roof includes a water cooling system.

[0005] It is well known that the heat flow which has to be removed is not constant, but substantially increases from the periphery towards the centre of the roof into which the electrodes are inserted.

[0006] In cooling systems known to the state of the art, the coefficient of heat exchange of the roof is substantially constant along the radial generatrix of the roof, and consequently the whole roof has to be designed in such a way as to ensure a coefficient of heat exchange relative to the most critical, central area.

[0007] Consequently, for about two thirds of the surface, corrresponding to the outermost annular area, the cooling system is oversized.

[0008] US-A-4.815.096 teaches a cooling system including sprays which cooperate with the outer face of the roof, the sprayed water flowing on the outer face and being collected in the outer peripheral area of the roof.

[0009] This system has the disadvantage that the greatest flow of water is in the outer peripheral area, where the least heat flow is required.

[0010] Moreover, the quantity of water used to ensure the removal of the desired heat flow is very high.

[0011] Other cooling systems have been proposed where the cooling water flows under pressure inside cooling conduits which affect the whole roof.

[0012] These conduits under pressure may be circular or helical rings, or of another shape.

[0013] These systems however have not given satisfactory results as, on the one hand they cannot in any case satisfy the requirements of a uniform heat exchange, and on the other hand, in the event of a breakage in the conduits where the cooling water flows, the quantity of water which penetrates inside the furnace, because of the high feeding pressure, is very high and requires the melting process in the furnace to be stopped.

[0014] There are also roofs known to the state of the art which are composed of autonomous panels in contiguous segments, each panel including its own autonomous cooling system.

[0015] In order to optimise the heat exchange, the system of cooling conduits associated with each panel is shaped like a coil, and comprises a high number of curves which considerably increase losses in the water load.

[0016] In order to ensure the desired water flow, it is therefore necessary to use high pressures inside the system of cooling conduits, generally more than 6 bar, and an outlet pressure of about 3 bar or similar.

[0017] In these systems, the speed at which the water flows is constant, and therefore the coefficient of heat exchange is also constant over all the panel.

[0018] In systems known to the state of the art, any overloads of the heat flows stress the panels dangerously, and consequently cracks can easily be caused especially in correspondence with the elbows from which, in the event of breakages, the cooling water comes out and enters the furnace.

[0019] Given the high pressure inside the conduits where the water flows, the amount of water which enters the furnace is, in this case, very great, and as soon as it comes into contact with the molten metal, it immediately evaporates with a consequent sudden rise in pressure which can also cause an inner explosion.
This requires the melting process inside the furnace to be stopped immediately, with all the technical and economic problems which that involves.

[0020] The present applicants have designed, tested and embodied this invention to overcome the disadvantages of the state of the art, and to obtain further advantages.

[0021] This invention is set forth and characterised in the main claim, while the dependent claims describe variants of the idea of the main embodiment.

[0022] The purpose of the invention is to provide a cooled roof for electric arc furnaces and ladle furnaces which has a low water consumption and a low operating pressure of the cooling water, and yet at the same time ensures a high level of operational safety.

[0023] The cooled roof according to the invention guarantees at all times and at all points the substantial removal of the desired heat flow, preventing local over-heating which could lead to the cooling conduits breaking.

[0024] The roof according to the invention does not require the melting process to be stopped even in the event that, for reasons such as localised heat overload or localised heat strain, there is a breakage in one of the conduits for the circulation of the cooling water.

[0025] The cooled roof according to the invention includes a coefficient of heat exchange which can be varied in a radial direction in such a way as to guarantee an optimum flow of heat exchange which is coherent with the requirements of the roof.

[0026] In the cooled roof according to the invention, in each transverse section, the coefficient of heat exchange is at least double that of standard panels known to the state of the art.

[0027] The roof according to the invention is composed of a plurality of adjacent and autonomous panels, of a reduced size, each of which has at least a longitudinal conduit through which the cooling water flows, and a coefficient of heat exchange which increases radially as it goes from the outer periphery of the roof towards the central area of the roof where the electrodes are inserted.

[0028] In the roof according to the invention, the development of the heat coefficient is a function of the development of the heat flow, generated by the melting furnace, which is to be removed, in such a way as to ensure optimum conditions of heat exchange at every point of the furnace.

[0029] According to the invention, the cooling water flows lengthwise in the conduits associated with each panel, therefore in a radial direction with respect to the furnace, and at a speed which increases as it passes from the peripheral area towards the central area of the roof.

[0030] According to a variant, the cooling water flows in a radial direction in each panel and at a speed which decreases as it goes from the centre towards the periphery.

[0031] In the cooling system for the roof according to the invention, the transverse transit section for the cooling water inside the conduits associated with the panels decreases as it goes from the periphery towards the central area of the roof.

[0032] This reduction in the section of the conduits from the periphery to the centre of the roof can be from 25% to 50%.

[0033] In the roof according to the invention, the increase in the speed of the water, and consequently in the coefficient of heat exchange, going from the outer diameter of the roof to the inner diameter near the electrodes, can reach 250% or even more of the speed and the coefficient of heat exchange at the outer diameter of the roof.

[0034] In the roof according to the invention, the cooling water works at low pressure, with a reduced overpressure, so that, even in the event of a breakage in the conduit, the quantity of water entering the furnace is slight, and it evaporates before it comes into contact with the molten metal without causing sudden increases in pressure, and thus the melting process does not need to be stopped.

[0035] To be more exact, the pressure inside the conduits is between 1 and 5 bar, and the fall in pressure of the water inside each panel is less than 1.5 bar.

[0036] According to a variant, the conduits through which the water passes include inner partition means which define at least a lower section for the water to pass through, facing towards the inside of the furnace.

[0037] According to this embodiment, these partition means define two sections inside the conduits, respectively a lower section, facing towards the inside of the furnace, and an upper section, facing towards the outside.

[0038] These partition means include a variable section which increases according to the increase in the section of the conduits with which they are associated.

[0039] At least the area of the lower section through which the water flows and which faces towards the inside of the furnace is reduced as it goes from the periphery towards the centre of the roof, in such a way as to increase the speed of flow of the water passing through.

[0040] Whereas in the lower section the water flows at high speed, in the upper section the water is still, or flows at a reduced speed.

[0041] According to a variant, in the upper section facing towards the outside of the furnace there is no cooling water, which leads to a considerable overall saving in terms of water consumed.

[0042] The section of the conduits can be circular, elliptical, oval, sickle-shaped or any other shape suitable for the purpose, the sections always increasing, in cross section, as they go from the centre towards the periphery of the roof.

[0043] In order to obtain these sections, it is possible to use circular conduits equipped with suitably shaped partition means, or conduits made of two convex walls with differing convexities appropriately paired, or also with one convex wall and the other substantially plane.

[0044] With the system according to the invention, during the design stage it is possible to obtain a high safety coefficient, even more than three, without having to make the cooling system excessively big, which therefore makes the system safer.

[0045] The panels which make up the cooled roof according to the invention have at their lower part means to anchor the layers of waste which is progressively formed; these anchor means are made advantageously of copper, or of copper lined with iron, so as to ensure the efficient and correct anchoring of a desired layer of waste on the face of the panels which faces towards the inside of the furnace.

[0046] This layer of waste protects the cooling panels from heat stresses and mechanical stresses which may occur inside the furnace.

[0047] The anchor means may be associated directly with the panels, in an intermediate position or at the point where contiguous panels join, or they may be inserted directly between adjacent panels.

[0048] According to a variant, the cooled roof according to the invention is equipped with one or more burners, for example of the type using oxymethane, suitable to create a protective screen of natural gas in the area immediately underneath the roof itself.

[0049] This natural gas reacts with the atmosphere of the furnace to produce a layer of carbon-based powders which are deposited on and attach themselves to the lower part of the roof.

[0050] During those steps of the process when the roof is removed, the carbon powders come into contact with oxygen, react and burn, thus maintaining the temperature of the whole inner surface of the roof and therefore making it possible to have a roof which is already prepared, so that it does not require a transitional step or reconditioning step when the melting cycle is restarted.

[0051] The attached figures are given as a non-restrictive example and show some preferred embodiments of the invention as follows:

Fig.1

shows a plane view of a roof according to the invention;

Fig.2

shows a large scale plane view of a panel of the roof shown in Fig.1;

Fig.3

shows the panel shown in Fig.2 in a lengthwise section B-B;

Fig.4

shows the panel shown in Fig.2 in a cross section A-A;

Fig.5

shows a three dimensional part section of an embodiment of the panel according to the invention which illustrates the means to anchor the waste;

Fig.6

shows a variant of the panel shown in Fig.5;

Fig.6a

shows a view from below of the anchor means illustrated in Fig.6;

Figs. 7a to 7f

show in cross section other forms of embodiment of the panel according to the invention;

Fig.8

shows the cross section of a possible variant to the anchor means shown in Fig.6a;

Figs. 9 and 10

show in diagram form two possible embodiments of a panel according to the invention;

Fig.11

shows a variant of the cooled roof shown in Fig.1;

Fig.12

shows the distribution of the gas at the lower part of the roof shown in Fig.11;

Fig.13

shows a cross section of a detail from the roof shown in Fig.11;

Fig.14

shows a variant of Fig.13.

[0052] The cooled roof 10, applied to an electric arc furnace or ladle furnace, is composed of a plurality of panels 11 side by side and autonomous.

[0053] Each of these panels 11 includes at least a lengthwise conduit 12 through which the cooling water passes, where the water is made to circulate lengthwise in a radial direction.

[0054] In the case shown in Fig.2, each panel 11 includes three conduits 12 associated at one end to the same inlet water collector 13 and, at the other end, to the same outlet water collector 14.

[0055] In Fig.2, the water flows from the periphery towards the centre of the roof 10 at an increasing speed.

[0056] According to a variant, not shown here, the water flows from the centre towards the periphery of the roof 10 at a decreasing speed.

[0057] In the roof 10 according to the invention the transverse transit section of each conduit 12 varies along the longitudinal axis of the panel 11, from the periphery to the centre of the roof 10, narrowing in a ratio, advantageously but not necessarily progressively, which may be even more than 2.5.

[0058] Consequently, the speed of flow of the water inside the conduit 12, and also the coefficient of heat exchange which is proportional to the speed of flow, increase in a desired manner towards the centre of the roof 10.

[0059] The variable section of these conduits 12 may be achieved with conduits which have cross sections of a different shape.

[0060] Fig.4 shows the cross section of a conduit 12 with a sickle-shaped section which widens as it goes towards the periphery of the cooled roof 10.

[0061] In one advantageous embodiment of the conduits 12, substantially conventional pipes are used which have inside them partition means 17 which extend lengthwise so as to define at least a lower section 12a for the passage of the water.

[0062] According to a first embodiment of the invention, the partition means 17 are composed of an intermediate plate 11), suitably shaped, which defines a lower section 12a and an upper section 12b through which the water passes.

[0063] At least the lower section 12a narrows in section as it goes towards the central area of the roof 10.

[0064] In this particular embodiment, the water in the lower section 12a is made to flow at a speed of between 0.8 and 1.5 metres per second in correspondence with the outer diameter of the roof 10, and at a speed of between 2 and 4 metres per second in correspondence with the inner diameter of the roof 10, while the water in the upper section 12b is still, or flows at a reduced speed, for example less than 1 metre per second.

[0065] According to a variant, the upper section 12b is empty and the water flows only in the lower section 12a which faces inside the furnace.

[0066] According to other variants, the partition means 17 are composed of a solid body 217, of a shape which mates with the section of the conduit 12 and associated solidly to its upper inner face in such a way as to define only one lower section 12a where the cooling water circulates.

[0067] The conduits 12 may have a cross section of any desired shape, for example elliptical or oval (Figs. 7a, 7b, 7c), or substantially circular (Figs. 27d, 7e), provided the cross section narrows as it goes towards the centre of the roof 10.

[0068] Figs. 4, 5 and 6 show conduits 12 with a sickle-shaped section, composed of two convex walls, respectively the lower wall 15a and the upper wall 15b, welded lengthwise at the sides so as to define a transit conduit 12 which narrows longitudinally in section as it goes from the outer periphery towards the centre of the roof 10.

[0069] In the case of Fig.7a, the conduit 12 has an elliptical or oval section, and the partition means 17 are composed of a convex metal plate 117 which is associated at its ends with the inner wall of the conduit 12.

[0070] In the case of Fig.7b, the partition means 17 are composed of a hollow body 317, in this case elliptical in section, which is associated with the inner upper wall of the conduit 12.

[0071] The inner part of the hollow body 317 defines, in this case, the upper section 12b filled with air or water passing through at low speed.

[0072] In the case of Fig.7c, the partition means 17 are composed of a solid body 217, elliptical in section, which is associated with the inner upper wall of the conduit 12.

[0073] The oval or elliptical section shown in Figs.7a to 7c has optimum characteristics of heat exchange, due to a substantially homogenous heat field over the whole wall where heat exchange takes place.

[0074] In the form of embodiment shown in Fig.3, the lower wall 15a is thicker than the upper wall 15b so as to prevent localised over-heating caused by any possible sprays of liquid metal.

[0075] Figs.7d and 7e show conduits 12 with a circular section which narrows as it goes towards the centre of the roof 10, and where the partition means 17 are composed respectively of a hollow cylindrical body 317 and a solid cylindrical body 217.

[0076] In the case of Fig. 7f, the partition means 17 are composed of a wall 117, substantially U-shaped so as to define an upper section 12b and a lower section 12a through which the cooling water passes.

[0077] In all the cases shown, the lower section 12a through which the cooling water flows is substantially shaped like a circular sickle, and has given excellent results in terms of heat exchange.

[0078] According to a variant not shown here, the lower section 12a through which the cooling water flows is substantially shaped like a circular segment.

[0079] The cooled roof 10 according to the invention has at its lower part anchor means 16 onto which a layer of waste 18 is progressively anchored.

[0080] Figs.5, 6 and 6a show anchor means 16 composed of a plurality of fins 19 arranged offset and facing sideways alternatively from opposite sides.

[0081] In the case of Fig.5, the fins 19 are directly associated along the joint line of contiguous conduits 12.

[0082] In the case of Fig.6, the fins 19 are inserted between one conduit 12 and the contiguous one.

[0083] In the case of Fig.8, the anchor means 16 are composed of a plurality of T-shaped fins 119.

[0084] These fins 119 can be made completely of copper, or of copper lined with iron as shown on the right of Fig.8.

[0085] The lower wall 15a of the panels 11, which faces towards the inner part of the furnace, may be undulated, and the upper wall 15b, which faces towards the outer part of the furnace, substantially plane (Fig.9).

[0086] According to a variant shown in Fig.10, the lower wall 15a is substantially plane while the upper wall 15b is undulated.

[0087] The upper wall 15b is normally spot welded to the lower wall 15a, for example by means of flash welding or some other type of welding, at the points referenced as 25 in Fig.10.

[0088] Figs.11 to 14 show a variant where the roof 10 is equipped with a plurality of burners 20, in this case substantially distributed on a circumference outside the circumference which circumscribes the electrode 21.

[0089] The seatings for the burners 20 are obtained between the conduits 12 where the cooling water for the roof 10 circulates.

[0090] The burners 20 are fed, in this case, by an oxygen feed conduit 22 and a methane feed conduit 23.

[0091] The burners 20 are suitable to deliver a flow of natural gas which, during the working cycle of the furnace, leads to the formation of a protective shield, made of particles of carbon dust and of waste, for the lower part of the roof 10.

[0092] This protective shield, referenced by 24 in Fig.12, keeps the roof 10 from over-heating and prevents even partial cooling of the atmosphere inside the furnace.

[0093] In order to form this protective shield on the inner surface of the roof 10, the gas is made to flow from the burners 20 when the temperature of the scrap has reached between about 850°C and 1100°C with a delivery flow of about 45-65 Nm³ per hour.

[0094] When the carbon powders come into contact with oxygen at the time when the roof 10 is removed, they react by burning, and thus maintain the temperature of the lower face of the roof 10 at least for part of the time it is not being used.

[0095] Figs. 13 and 14 show two possible uses where, respectively, the conduits 12 are circular and include inner partition means 17 (Fig.13), and have a circular sickle section (Fig.14).

Claims

1. Cooled roof for electric arc furnaces and ladle furnaces, of the type comprising a plurality of radial, contiguous panels (11), the roof being characterised in that each panel (11) includes at least a cooling conduit (12), through which the cooling water flows, arranged radially with respect to the roof (10) and including a transverse transit section which progressively narrows from the outer periphery towards the centre of the roof (10).

2. Cooled roof as in Claim 1, in which each panel (11) has several contiguous conduits (12) associated at one end with a common inlet water collector (13) and at the other end with a common outlet water collector (14).

3. Cooled roof as in Claim 1 or 2, in which the inlet water collector (13) is arranged on the periphery of the roof (10).

4. Cooled roof as in Claim 1 or 2, in which the inlet water collector (13) is arranged on the central area of the roof (10).

5. Cooled roof as in any claim hereinbefore, in which the section of the conduits (12) from the periphery to the centre of the roof (10) is reduced by a value of between 25% and 50%.

6. Cooled roof as in any claim hereinbefore, in which the speed of the cooling water in the conduits (12) is between 0.8 and 1.5 metres per second in correspondence with the outer periphery of the roof (10), and between 2 and 4 metres per second in correspondence with the centre of the roof (10.

7. Cooled roof as in any claim hereinbefore, in which the water pressure inside the conduits (12) is between 1 and 5 bar.

8. Cooled roof as in any claim hereinbefore, in which the fall in water pressure in each panel (11) is less than 1.5 bar.

9. Cooled roof as in any claim hereinbefore, in which the conduits (12) include inside partition means (17) which extend lengthwise to define at least a lower section (12a) through which the water passes and at least an upper section (12b).

10. Cooled roof as in Claim 9, in which the lower section (12a) through which the water passes is shaped like a circular sickle.

11. Cooled roof as in Claim 9, in which the speed of the cooling water in the upper section (12b) is between 0 and 1 metres per second.

12. Cooled roof as in Claim 9, in which no cooling water circulates in the upper section (12b).

13. Cooled roof as in Claim 9, in which the partition means (17) are composed of a partition plate (117).

14. Cooled roof as in Claim 9, in which the partition means (17) are composed of a hollow body (317).

15. Cooled roof as in Claim 9, in which the partition means (17) are composed of a solid body (217).

16. Cooled roof as in any claim from 1 to 15 inclusive, in which the cross section of the conduit (12) is circular.

17. Cooled roof as in any claim from 1 to 15 inclusive, in which the cross section of the conduit (12) is elliptical or oval.

18. Cooled roof as in any claim from 1 to 15 inclusive, in which the cross section of the conduit (12) is a circular sickle.

19. Cooled roof as in any claim from 1 to 15 inclusive, in which the cross section of the conduit (12) is a circular segment.

20. Cooled roof as in any claim hereinbefore, in which the conduit (12) includes at its lower part means (16) to anchor the waste (18).

21. Cooled roof as in Claim 20, in which the anchor means (16) comprise a plurality of fins (19) offset and facing alternatively in opposite side directions.

22. Cooled roof as in Claim 20, in which the anchor means (16) include a plurality of T-shaped fins (119).

23. Cooled roof as in any claim hereinbefore, in which the anchor means (16) are made of copper.

24. Cooled roof as in any claim hereinbefore, in which the anchor means (16) are made of lined copper.

25. Cooled roof as in any claim from 1 to 24 inclusive, in which the conduit (12) is defined by an undulated lower wall (15a) and a plane upper wall (15b).

26. Cooled roof as in any claim from 1 to 24 inclusive, in which the conduit (12) is defined by a plane lower wall (15a) and an undulated upper wall (15b).

27. Cooled roof as in any claim hereinbefore, in which the lower wall (15a) of the conduit (12) is greater in section than the upper wall (15b).

28. Cooled roof as in any claim hereinbefore, which includes a plurality of burners (20) able to emit a flow of natural gas towards the inner part of the furnace at a delivery rate of at least 45 Nm³ per hour.

Drawing