WO2005017433A2

WO2005017433A2 - Method and device for drying a non-metallic coating on a steel band

Info

Publication number: WO2005017433A2
Application number: PCT/FR2004/001701
Authority: WO
Inventors: Fabrice Dronsart; Philippe Duhamel
Original assignee: Stein Heurtey
Priority date: 2003-07-16
Filing date: 2004-07-01
Publication date: 2005-02-24
Also published as: AU2004265466B2; FR2857734B1; WO2005017433A3; ATE513174T1; EP1644680A2; FR2857734A1; AU2004265466A1; EP1644680B1; CN1823251A; CN100462656C

Abstract

The invention relates to a method of drying a non-metallic coating on a steel band (1), whereby the steel band (1) is conveyed into a vertical drying furnace (3) with relatively cold walls. According to the invention, hot air is then injected into the upper part (10a) and the lower part (10b) of the furnace in order to prevent the condensation of vapours on the walls of the furnace, and the injected air is subsequently drawn off (4). A dynamic brake is produced by injecting (13a, 13b) hot air between the lower part and the upper part of the furnace at a distance from the inlet and the outlet thereof and by creating an overpressure condition in the tunnel of the drying furnace, such as to limit the entry of cold dead air and the chimney effect in the vertical furnace.

Description

The invention relates to a method for drying a non-metallic coating on a steel strip, according to which the strip is passed through in a steel oven. The invention relates to a method for drying a non-metallic coating on a steel strip. vertical drying with relatively cold walls, hot air is injected at the bottom and at the top of the oven to avoid condensation of vapors on the walls of the oven, and the injected air is drawn off. The invention relates more particularly to a method of drying a coating consisting of a layer of paint.

The non-metallic coatings of steel strips are produced with production facilities or lines generally comprising a section for preparing the surface of the strip, followed by a section for applying a liquid coating film, in particular by coating. by rollers, then a drying section of this film.

Drying is obtained by bringing the support and the film to a required temperature allowing the coating to dry and to cure. During drying, the solvents in the coating are removed by evaporation and drawn off by a fan. If necessary, the solvents are incinerated, which is the case for organic solvents.

For safety reasons, the organic solvents evaporated in the drying oven must be diluted, generally with air, to keep their solvent concentration below a certain limit and eliminate any risk of explosion due to the solvents. In addition, the drying oven is maintained in a slight vacuum to prevent any backflow outside the enclosure of generally toxic or dangerous solvents.

The walls of the drying oven, especially in the case where the strip drying means consist of electromagnetic induction coils, are relatively cold and condensation must be avoided, in particular on cold walls of evaporated solvents. For that hot air is injected into the oven, for example as described in FR 2 734 501. In a vertical oven, hot air is injected at the bottom and at the top of the oven, with a racking injected air.

Horizontal drying ovens are limited in length by controlling the position of the strip, which cannot be supported until its coating is sufficiently dry to withstand the stresses of contact with this support. To increase the capacity of the coating lines or for special materials, the drying ovens are installed vertically to avoid problems of belt deflection. Vertical drying ovens, by the difference between their internal temperature and the ambient temperature, behave like a chimney and a circulation of air from the bottom to the top is established spontaneously. Significant quantities of fresh air, in particular from the lower opening of the oven, are removed, mixed with the hot air injected and drawn off. These parasitic air inlets dilute the hot air injected intended to avoid the phenomena of condensation of the solvents, so that it is necessary to increase the temperature of injection of hot air so that the mixture with the cold air sucked is maintained at a sufficient temperature. This results in a loss of thermal efficiency, poor control of the concentration of solvents and a malfunction of the withdrawal injection circuit which must be oversized. Likewise, the gas outlet for the upper opening of the oven poses a risk of explosion, degrades the working conditions of the operators and produces solvent condensation on the equipment located nearby. A vertical drying oven has at its lower entrance a passage window for the strip. We could have thought of reducing the dimensions of this window to slow down the entry of parasitic air. Such a reduction in the dimensions of the passage window is however not possible since this would increase the risk of scratches on the coating of the strip, in particular for painted surfaces. It is the same for the passage provided at the outlet of the oven. It would have been possible to provide devices of the “air curtain” type at the inlet and at the outlet of the drying oven, but they are not suitable: indeed to be effective, they require a high air speed, which can cause appearance defects due to the impact on the fresh paint film. Furthermore, these air curtains should operate in hot air to avoid cooling the atmosphere of the tunnel, which would cause the solvent to start to evaporate, part of which would inevitably be forced back outside the tunnel. This would pose safety (risk of burns and explosion) and toxicity problems. The invention aims, above all, to limit and control the chimney effect in a vertical drying oven in a simple manner and which does not involve any risk of degradation of the coating to be dried.

According to the invention, the method for drying a non-metallic coating on a steel strip, of the kind defined above, is characterized in that a dynamic brake is produced by injecting hot air between the lower part and the upper part of the oven and away from the inlet and outlet of the oven, and by creating an overpressure in the tunnel of the drying oven so as to limit the entry of parasitic cold air and the effect chimney in the vertical oven. Preferably when the withdrawal of the injected air is carried out at at least one point, the dynamic brake is produced below one of these withdrawal points, close to the latter.

Advantageously, in the method according to which hot air is blown in order to produce a dynamic barrier, this hot blown air is used in order to constitute a point for holding the strip to limit its oscillations over the length of the oven.

The method may provide for the implementation of two superimposed sets of hot air blowing orifices of the dynamic barrier. The air used to create the dynamic brake can be recycled. The recycled air used to create the dynamic brake can be drawn in the vicinity of the blow holes of the dynamic brake.

The method advantageously comprises the use of adjustment members intended to adjust the flow rate or the pressure of the hot air of the dynamic brake according to the production parameters, the nature of the product to be treated or the characteristics of the solvents released.

The control members can be controlled by an automatic oven control system depending on the operating conditions of the oven and the nature of the product and its coating.

The invention also relates to a device for drying a coating, in particular a layer of paint on a steel strip, comprising a vertical drying oven with relatively cold walls and comprising heating means, in particular by electromagnetic induction, of the strip which crosses this furnace, means for injecting hot air in the lower part and upper part of the furnace to prevent condensation of vapors on the walls of the furnace and a means for withdrawing the injected air.

Such a device according to the invention is characterized in that it comprises, between the bottom part and the top part of the oven, means for injecting hot air into the oven and for producing a dynamic brake limiting the entry of parasitic cold air by creating a local overpressure in the drying oven.

In a device where the withdrawal of the injected air takes place at a single point, the injection of air from the dynamic barrier comprises a box making it possible to blow the hot air into the oven through one or more orifices, on one or more two sides of the strip.

In such a device where hot air is blown in order to produce a dynamic barrier, the blowing orifices can be arranged over a large part of the width of the strip. The hot air blowing orifices can be produced in the form of holes or nozzles so as to hold the strip in line with these orifices.

The air injection means can comprise boxes making it possible to blow hot air into the oven. The box can be equipped with means for adjusting the flow rates of the injected air as well as the shape of the injected air jets.

The air injected by the box can advantageously be recirculated by a fan.

Two or more blowing assemblies can be implemented in the box in order to reinforce the maintenance of the strip and the efficiency of the dynamic barrier.

Adjusting members can be implemented on the hot air ducts supplying each set of blowing holes in order to allow independent adjustment of the pressures in the oven enclosures located below and above the dynamic barrier.

Several dynamic barriers by blowing hot air and installed at several heights on the oven, at a distance from the oven's inlets and outlets, can be implemented to limit the circulation of parasitic air in the oven and ensure the maintenance of the strip over the entire height of the oven. The invention consists, apart from the above arrangements, of a certain number of other arrangements which will be more explicitly discussed below, with regard to examples of embodiments described with reference to the appended drawings, but which are not in no way limiting. In these drawings:

Fig. 1 is a diagram in elevation of a drying device according to the state of the art.

Fig. 2 is a diagram in elevation of a drying device according to the invention.

Fig. 3, 4 and 5 are diagrams in elevation of alternative embodiments of the device of FIG. 2. Referring to Fig. 1 of the drawings, one can see a vertical drying oven according to the state of the art. In general, a non-metallic coating line, in particular a line for applying a layer of paint to a steel strip comprises a section for preparing the surface of the strip not shown in FIG. 1, followed by a machine 6 for applying a liquid film of the coating, in particular paint, to the strip 1. The application is generally provided by coating using rollers.

The strip 1 with its coating then passes into the drying section formed by the oven forming a tunnel 3. The strip runs vertically in this oven as illustrated in FIG. 1. Drying is obtained by bringing the steel strip 1 and the coating film to the required temperature allowing the coating to be dried and cured. The heating of the strip 1 can advantageously be carried out using an electromagnetic induction system not shown in the figure. During drying, the solvents of the coating applied to the strip 1 are removed by evaporation and drawn off by a fan 7 placed on a drawing duct connected to the internal space of the oven at 4 situated in height between the entry 5 at the bottom. and exit 2 located in the upper part.

To avoid condensation of the solvents on the internal walls of the enclosure 3, hot air is injected at the bottom and at the top of the oven, respectively at 10a and 10b as described, for example, in FR-A-2 734 501. This hot air is injected into the oven enclosure by means of a fan 8, the injected hot air flows can be adapted by manual or automatic valves such as 9a and 9b. Large quantities of fresh air coming in particular from the lower opening 5 dilute the hot air injected at 10a and 10b, which requires increasing the temperature of this injected air.

Fig. 2 shows a section in elevation of a drying oven equipped with the invention. According to the invention, to effectively limit and control the effect chimney, a dynamic brake is produced in the oven to create an overpressure inside the oven and limit the entry of parasitic cold air.

Preferably, the dynamic brake created by the injection of air is provided under the draw point 4 or close to this point. The injection means can comprise a box 12 made in one or more parts as shown diagrammatically in FIG. 2 by 12a and 12b with blowing orifices 13a and 13b and their hot air supplies by means of the fan 8.

The parts 12a, 12b of the box project inside the furnace in the direction of the strip 1 so that the distance between this strip 1 and the blowing orifices 13a, 13b is relatively reduced, and less than the distance separating the strip 1 of the vertical walls of the oven. The direction of the width of the strip 1 is perpendicular to the plane of FIG. 2.

An adjusting member 11, with manual or automatic control, makes it possible to adjust the flow and / or pressure characteristics of the hot air injected into the box 12 through the orifices 13 as a function of the setting of the oven, of the type of production carried out or characteristics of the coating deposited on the strip and of the solvents produced. The adjusting member 11 can be manual or automatic operating under the control of the oven control device. The orifices 13a and 13b can be positioned and dimensioned so as to carry out an injection of hot air concentrated in a reduced number of points or produced according to a series of holes distributed over the entire width of the strip on each side thereof. or even produced in the form of a continuous opening in the form of a blowing nozzle over the entire width of the strip. In general, the blowing is carried out in a direction orthogonal to the plane of the strip 1.

The orifices 13a and 13b can be produced in the known form of blowing nozzles intended, in addition to controlling the chimney effect in the oven, to ensure that the strip is held in place for, for example, limiting its beating during its passage through the oven . According to a second possibility illustrated in FIG. 3, the blowing box is separated horizontally by a partition so as to obtain two boxes 15 (15a, 15b) and 16 (16a, 16b) superimposed and each comprising a set of blowing orifices. Fig. 3 shows an example of this embodiment with, from the fan 8, two separate hot air supplies for the upper and lower box, each of these supplies being equipped with an adjustment member 14a and 14b. By this means, it is possible to adjust the hot air flow rates blown by the assemblies of the upper and lower orifices in order to obtain the adjustment of the pressures in the lower and upper parts of the oven to eliminate the chimney effect in this enclosure. .

According to an alternative embodiment of FIG. 3, the blowing orifices of the boxes 15a and 15b as well as 16a and 16b can be respectively oriented upwards and downwards in order to facilitate the adjustment of the pressures in the upper and lower parts of the oven and limit the chimney effect.

Fig. 4 shows a variant of the device of FIG. 2 for which the hot air blown by the box 12 is blown by an independent fan 17 in order to allow an adjustment of the pressure level of the air blown in the box 12 of the dynamic brake independently of the pressure level delivered by the fan d air injection 8. The fan 17 supplying the box 12 can be supplied with hot air at the outlet of the fan 8 as shown in the figure, at the inlet of the fan 8 or supplied by any other network at pressure and temperature different from those used for injecting air into the oven at 10a and 10b. Fig. 5 shows a variant of the device of FIG. 4. The fan 17 for supplying hot air to the box 12 recirculates this air by means of suction 19a and 19b as well as 20a and 20b. These suction points may be located near the blowing points of the box 12 of the dynamic brake in order to improve the efficiency of the device or be simply connected to the enclosure 3. The variants of Figs. 4 and 5 can be produced with a dynamic brake box produced according to FIG. 3. The withdrawal sheath 4 can be located substantially halfway up the oven. The box 12 is located below the withdrawal point 4. The temperature of the hot air injected into the oven can be of the order of 250 ° C.

The running speed of the strip can typically be between 60 and 120 m / min. All the combinations of the devices described with their adjustment members make it possible to create a dynamic barrier in the oven with hot air so as to slow down the parasitic air flow, in particular the ascending flow created by the chimney effect, and therefore limit the entry of parasitic air.

Claims

1. Method for drying a non-metallic coating on a steel strip (1) according to which a steel strip (1) is passed through in a vertical drying oven (3) with relatively cold walls into which one injects hot air in the upper and lower part of the oven to avoid condensation of vapors on the walls of the oven and a drawing of the injected air is carried out, characterized in that a dynamic brake is produced by injection of hot air (13a, 13b; 15a, 15b, 16a, 16b) between the lower part and the upper part of the oven and away from the inlet and outlet of the oven, and by creating an overpressure in the tunnel drying oven so as to limit the entry of parasitic cold air and the chimney effect in the vertical oven.

2. Method according to claim 1, according to which the withdrawal of the injected air is carried out at at least one point (4), characterized in that the dynamic brake is produced below one of these withdrawal points ( 4), close to it.

3. Method according to claim 1 or 2, according to which hot air is blown in order to produce a dynamic barrier, characterized by the use of this hot blown air in order to constitute a point of retention of the strip to limit its oscillations. along the length of the oven.

4. Method according to one of claims 1 to 3, characterized by the implementation of two superimposed sets (15a, 15b; 16a, 16b) of hot air blowing orifices of the dynamic dam.

5. Method according to one of claims 1 to 4, characterized in that the air used to create the dynamic brake is recycled.

6. Method according to claim 5 characterized in that the recycled air used to create the dynamic brake is sucked (19a, 19b; 20a, 20b) in the vicinity of the blowing orifices of the dynamic brake.

7. Method according to one of the preceding claims, characterized by the use of adjustment members (11; 14a, 14b; 18) intended to adjust the flow or pressure of the hot air of the dynamic brake depending on the production parameters, the nature of the product to be treated or the characteristics of the solvents released.

8. Method according to claim 7 characterized by the control of the adjustment members by an automatic oven control system according to the operating conditions of the oven and the nature of the product and its coating.

9. device for drying a coating, in particular a layer of paint, on a steel strip, comprising a vertical drying oven (3) with relatively cold walls, heating means, in particular by electromagnetic induction, of the strip passing through the oven, means for injecting hot air at the bottom (10b) and at the top (10a) of the oven to prevent condensation of vapors on the cold walls of the oven, and a means (4 , 7) for drawing off the injected air, characterized in that it comprises, between the lower part (5) and the upper part (2) of the furnace, means (13a, 13b; 15a, 15b; 16a, 16b) to inject hot air into the oven and to achieve a dynamic brake limiting the entry of parasitic cold air, by creating a local overpressure in the drying oven.

10. Device according to claim 9, according to which the withdrawal of the injected air takes place at a single point (4), characterized in that the injection of air from the dynamic barrier comprises a box (12) for blowing hot air in the oven in one or more orifices, on one or two sides of the strip.

11. Device according to one of claims 9 or 10, according to which hot air is blown in order to produce a dynamic barrier, characterized by the arrangement of the blowing orifices (13a, 13b; 15a, 15b; 16a, 16b) over a large part of the width of the strip.

12. Device according to one of claims 9 to 11, in which the orifices (13a, 13b; 15a, 15b; 16a, 16b) for blowing hot air are made in the form of holes or nozzles so as to maintain the strip in line with these orifices.

13. Device according to one of claims 9 to 12, characterized by the implementation of two sets (15a, 15b; 16a, 16b) or more blowing in the box in order to strengthen the maintenance of the strip and the efficiency of the dynamic dam.

14. Device according to claim 13 characterized by the use of adjustment members (14a, 14b) on the hot air ducts supplying each set of blowing holes in order to allow independent adjustment of the pressures in the oven enclosures located below and above the dynamic dam.

15. Device according to one of claims 9 to 14, characterized by the implementation of several dynamic dams by blowing hot air and located at several heights on the oven, remote from the inlets and outlets of the oven (2 and 5 ) to limit the circulation of parasitic air in the oven and maintain the strip over the entire height of the oven.