Equipment and method for preheating a continuously moving steel strip

Abstract

Equipment and a method for preheating a continuously moving steel strip, in particular before feeding the same into a continuous annealing or hot-dip galvanizing furnace, involves the continuous movement of the steel strip in a preheating chamber including a preheating circuit having at least one preheating tube, the inner surface of which is in contact with externally-recovered burnt gases (e.g. from the furnace). A portion of the outer surface of the preheating tube is disposed directly opposite a surface of the strip in order to provide a first preheating mode by irradiating heat onto the strip and the walls of the chamber, and a second preheating mode, mainly by convection, of a gas constituting a controlled atmosphere in the preheating chamber.

Claims

1. Equipment for preheating a continuously moving steel strip, the equipment comprising: a preheating chamber for the steel strip, said preheating chamber including walls and a preheating circuit having at least one preheating tube; and said at least one preheating tube having an inner surface in contact with externally recovered burnt gases and an outer surface with at least one external radiant fin configured to be disposed directly opposite a surface of the strip, to provide a first preheating mode by thermal radiation onto the strip and said walls of said preheating chamber and a second preheating mode, mainly by convection, of a gas constituting a controlled atmosphere in said preheating chamber, wherein there is no direct contact between the strip and the burnt gases.

2. The preheating equipment according to claim 1, which further comprises: a continuous annealing or galvanizing furnace for heating or maintaining a temperature of the strip in the furnace; ducting conveying burnt gases recovered from said furnace to said preheating circuit in said preheating chamber under a controlled atmosphere; and an evacuation device for extracting the gases from said preheating chamber.

3. The preheating equipment according to claim 2, wherein said evacuation device includes an exhauster.

4. The preheating equipment according to claim 1, wherein said at least one preheating tube includes a plurality of preheating tubes disposed side by side and forming at least one layer substantially parallel to the strip and directly opposite at least one of the faces of the strip.

5. The preheating equipment according to claim 1, which further comprises: rollers disposed above and below said preheating chamber and configured to impart a vertical movement to the strip in at least one pass between said rollers; said at least one preheating tube including at least one layer of preheating tubes directly opposite a face of each rising or falling pass of the strip.

6. The preheating equipment according to claim 4, wherein said at least one layer of preheating tubes includes at least two layers of preheating tubes each disposed directly opposite a respective one of two faces of the strip and configured to concomitantly provide for heating the two faces of each rising or falling pass of the strip.

7. The preheating equipment according to claim 4, wherein said layers of preheating tubes include a plurality of preheating tubes having adjacent ends, and manifolds are configured to be integrated with said adjacent ends for interconnecting said preheating tubes.

8. The preheating equipment according to claim 2, wherein: said at least one preheating tube includes a plurality of preheating tubes disposed side by side and forming layers; and bodies support each of said layers of preheating tubes in said preheating chamber.

9. The preheating equipment according to claim 1, wherein said at least one preheating tube includes at least two preheating tubes positioned with between a zero interstice therebetween and an interstice therebetween of 1/40 of a distance between said preheating tubes and the strip.

10. The preheating equipment according to claim 1, wherein said at least one preheating tube includes at least two preheating tubes positioned with an interstice between two successive preheating tubes having an interstice width, and a ratio between a distance between said preheating tubes and the strip and said interstice width is between 4 and 40.

11. The preheating equipment according to claim 10, wherein said at least two preheating tubes are disposed successively and form layers, said successively disposed preheating tubes have a spacing therebetween, and a ratio of said spacing in said layers and said distance between said preheating tubes and the strip is between 1 and 5.

12. The preheating equipment according to claim 10, wherein the gas in said preheating chamber is forced to circulate between at least one recovery point in said preheating chamber and at least one space situated between adjacent groups of preheating tubes directly opposite one another.

13. The preheating equipment according to claim 10, wherein the gas in said preheating chamber is heated between adjacent groups of preheating tubes directly opposite one another and blown orthogonally over the surface of the strip through interstices between successive preheating tubes.

14. The preheating equipment according to claim 1, wherein said at least one preheating tube includes a plurality of preheating tubes in adjacent groups, and the gas in said preheating chamber is forced to circulate between at least two of said adjacent groups of said preheating tubes directly opposite one another and at least one point at which the gas is blowing over the surface of the strip.

15. The preheating equipment according to claim 14, wherein the gas in said preheating chamber is blown over the surface of the strip counter to a direction of movement of the strip.

16. The preheating equipment according to claim 15, wherein the gas is blown through blowing buses.

17. The preheating equipment according to claim 1, wherein each said at least one preheating tube is fitted with at least one respective internal recovery fin.

18. The preheating equipment according to claim 1, further comprising a plurality of preheating tubes, each of said plurality of preheating tubes being fitted with said at least one external radiant fin.

19. The preheating equipment according to claim 1, wherein said at least two preheating tubes form layers.

20. A method for preheating a continuously moving steel strip, the method comprising the following steps: removing burnt gases having thermal energy from an annealing or galvanizing furnace; ducting the burnt gases into preheating tubes in the preheating chamber of the preheating equipment according to claim 1 for preheating the steel strip prior to feeding the steel strip into the furnace; giving up part of the thermal energy of the burnt gases to the preheating tubes through contact with inner walls of the preheating tubes; and transferring part of the thermal energy of the burnt gases by conduction to an outer surface of the preheating tubes for: heating the strip by radiation and heating the gas and walls of the preheating chamber by convection and radiation.

21. The preheating method according to claim 20, which further comprises: fitting each of the preheating tubes of a preheating unit with at least one internal recovery fin and at least one external radiant fin; giving up part of the thermal energy of the burnt gases to the preheating tubes with the internal recovery fins as well as through the contact with the inner walls; transferring part of the thermal energy through the external radiant fins as well as by the conduction to the outer surface of the preheating tubes for: heating the strip by radiation and heating the gas and the walls of the preheating chamber by convection and radiation; and exchanging another part of the thermal energy given up to the preheating tubes through contact between the burnt gases and the inner walls and the internal recovery fins through radiation between the inner walls of the preheating tubes and the internal recovery fins.

22. The preheating method according to claim 20, which further comprises: forcing circulation of the gas in the preheating chamber between at least one recovery point and at least one space situated between adjacent groups of the preheating tubes disposed opposite one another; and blowing the heated gas orthogonally over the surface of the strip through interstices disposed between the preheating tubes or the external radiant fins.

23. The preheating method according to claim 20, which further comprises circulating the gas in the preheating chamber between adjacent groups of the preheating tubes disposed opposite one another and buses blowing the gas over the surface of the strip counter to a direction of movement of the strip.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIGS. 1 to 6 provide a better understanding of the invention.

DESCRIPTION OF THE INVENTION

(2) FIG. 1 describes the general principle of the preheating equipment according to the invention: burnt gases are taken from a furnace (1) by means of collection ducts (2). These ducts feed preheating units (3) for a moving strip (B) in an insulated preheating chamber (4) under a neutral gas atmosphere (=additional gas, distinct from the burnt gases). Extraction ducts (5) recover the cooled burnt gases after they have passed into the preheating units (3) and an exhauster (6) ducts said gases towards an evacuation circuit (7) outside the preheating units.

(3) FIG. 2 describes the principle of preheating a strip (B) by means of a preheating tube (31) in a preheating unit or circuit (3). When tubes (31) are mentioned in the following description, it refers to a plurality of tubes each being designated with reference numeral 31. The burnt gases emerging from a continuous annealing or galvanizing furnace circulate inside each of the preheating tubes, which include an inner wall (311) in contact with the hot gases and an outer wall (312) situated in the immediate proximity of the strip. On this figure, the tube presents, as an example, a circular cross-section and has two internal recovery fins (313) and two external radiant fins (314).

(4) The burnt gases give up part of their heat to the preheating tubes (31) through contact with their inner walls (311) and with their internal recovery fins (313). Most of this heat is transferred by conduction to the outer wall (312) and to the external radiant fins (314) which provide both for heating the strip (B) and the chamber walls by radiation, and heating the neutral gas in said chamber by convection. Part of this heat is exchanged by radiation between the inner wall (311) and the internal recovery fins (313).

(5) FIG. 3 describes an example of an arrangement of two preheating units (3a, 3b). Each heating unit has two layers made up of a plurality of tubes (31), here, as an example, tubes with fins, each of the layers being positioned as close as possible to each of the two faces of the strip, which moves on rollers (41) in at least two vertical passes. Each preheating unit thus provides for the heating of a rising pass (for 3a) or a falling pass (for 3b) of the strip (B) moving in the preheating chamber (4).

(6) In this example, therefore, the following are found successively in the direction of progression of the strip: A first layer (3a1) belonging to a first preheating unit (3a) with one of its faces turned towards the strip. A rising pass of the strip. A second layer (3a2) belonging to the first preheating unit (3a) with one of its faces turned towards the strip. A third layer (3b1) belonging to a second preheating unit (3b) adjacent to the first unit (3a) with one of its faces turned towards the strip. A falling pass of the strip. A fourth layer (3b2) belonging to the second preheating unit (3b) with one of its faces turned towards the strip.

(7) Between layers (3a1) and (3a2) and also between layers (3b1) and (3b2), the moving strip is subjected respectively to radiation from the two preheating units (3a-3b).

(8) Between layers (3a2) and (3b1), a volume of neutral gas is subjected to heating by the adjacent faces of the first and second preheating units.

(9) In this example of FIG. 3, the preheating tubes are positioned so that an interstice 46 of a size between and 1/40 of their distance from the strip is placed between their respective external radiant fins and an exhauster (42) provides for forced circulation of the neutral gas between an extraction duct (43) and a blowing duct (44) feeding the space situated between layers (3a2-3b1) of the two adjacent preheating units (3a-3b). The gas is heated by convection between these two layers and is blown over the surface of the strip through the interstices placed between the radiant fins (314). The blowing duct (44) can be split into as many branches as necessary to ensure that the neutral gas is blown into as many spaces situated between the set of adjacent preheating units as are included in the preheating chamber.

(10) In order further to increase the exchanges by convection and radiation with the strip, the walls and the chamber gas, one or more layers may be added between the layers represented in FIG. 3.

(11) FIG. 4 describes another example of an arrangement of two preheating units (3a, 3b). Each heating unit has two layers made up of a plurality of tubes (31), each of the layers being positioned as close as possible to each of the two faces of the strip, which moves on rollers (41) in at least two vertical passes. Each preheating unit thus provides for the heating of a rising pass (for 3a) or a falling pass (for 3b) of the strip (B) moving in the preheating chamber (4).

(12) In this example, therefore, the following are found successively in the direction of progression of the strip: A first layer (3a1) belonging to a first preheating unit (3a) with one of its faces turned towards the strip. A rising pass of the strip. A second layer (3a2) belonging to the first preheating unit (3a) with one of its faces turned towards the strip. A third layer (3b1) belonging to a second preheating unit (3b) adjacent to the first unit (3a) with one of its faces turned towards the strip. A falling pass of the strip. A fourth layer (3b2) belonging to the second preheating unit (3b) with one of its faces turned towards the strip.

(13) Between layers (3a1) and (3a2) and also between layers (3b1) and (3b2), the moving strip is subjected respectively to radiation from the two preheating units (3a-3b).

(14) Between layers (3a2) and (3b1), a volume of neutral gas is subjected to heating by the adjacent faces of the first and second preheating units.

(15) In this example of FIG. 4, the preheating tubes are positioned so that there is no notable interstice between them. An exhauster (42) provides for forced circulation of the neutral gas between an extraction duct (43) and two blowing ducts (44a-44b) feeding blowing buses (441) blowing the heated neutral gas over the surface of the strip with an almost tangent incidence and in a direction opposite to its direction of movement. The extraction duct (43) collects the heated neutral gas between layers (3a2) and (3b1). The blowing duct (44) can be split into as many branches as necessary to ensure that the neutral gas is blown into as many rising or falling passes of the strip as are included in the preheating chamber.

(16) In order further to increase the exchanges by convection and radiation with the strip, the walls and the chamber gas, one or more layers may be added between the layers represented in FIG. 4.

(17) FIG. 5 describes two design examples for preheating tubes (31). In 5a, the length of the internal recovery fins (313) has been increased in order to improve the exchange with the burnt gases circulating inside the tube. In 5b, the addition of a tube (315) concentric with tube (31) also improves the exchange with the burnt gases and, at an identical flow rate, increases their speed of circulation.

(18) FIG. 6 describes an option for modular design of the layers of preheating tubes, facilitating replacement of them. The number of tubes constituting a layer element may vary depending on needs and the example of the figure representing four tubes is not restrictive. Each layer element is made up of a plurality of tubes (31), each being fitted with two fins (314). The burnt gases circulate between an inlet and an outlet inside the tubes (31) connected together by manifolds (316). The layer is supported by a fixing plate (318) detachably fixed to the wall (45a) of the chamber (4) and by at least one lug (317) resting on the other wall (45b). Support bodies (319) integrating the different elements make it possible to give the layer the rigidity needed for usage and handling. Such an arrangement can be implemented with tubes with or without fins as represented in the figure.

(19) This arrangement provides for total interchangeability of the layers of heating tubes with traditional radiant tubes fitted with burners. Thus, the method according to the invention can substitute economically for preheating using radiant tubes on existing equipment.

(20) With respect to the prior art, the invention has numerous advantages: No need to use exchangers external to the equipment. No need to dilute the burnt gases prior to feeding into the heating circuits. Heating by convection with no direct contact between the strip and the burnt gases. Great strip heating effect by radiation. Very simple structure of the equipment, based on tubes with fins, the manufacture of which is easy and can easily be automated. Flexibility of arrangement of the tube positions, distance between tubes and between tube and surface of the strip by means of a simple base plate. Layers of heating tubes easily interchangeable from outside the preheating chamber. Tubes may be detachable. Easy inter-tube or inter-layer nesting of manifolds. Layers of heating tubes interchangeable with radiant tubes. Greatly cooled gases evacuated, not requiring exhausters capable of withstanding high temperatures (advantage identical to equipment with external exchanger but without the drawbacks).