AU2004265466B2

AU2004265466B2 - Method and device for drying a non-metallic coating on a steel band

Info

Publication number: AU2004265466B2
Application number: AU2004265466A
Authority: AU
Inventors: Fabrice Dronsart; Philippe Duhamel
Original assignee: Fives Stein SA
Current assignee: Fives Stein SA
Priority date: 2003-07-16
Filing date: 2004-07-01
Publication date: 2010-04-29
Anticipated expiration: 2024-07-01
Also published as: WO2005017433A3; FR2857734B1; ATE513174T1; CN1823251A; EP1644680B1; EP1644680A2; CN100462656C; AU2004265466A1; FR2857734A1; WO2005017433A2

Abstract

The drying procedure, in which a coated steel strip (1) is passed vertically through a drying oven (3) with hot air injected at the top and bottom to prevent vapours condensing on the relatively cool walls of the oven, includes the formation of a dynamic brake by injecting hot air (13a, 13b) between the top and bottom of the oven at a distance between its outlet and inlet and creating a pressure above atmospheric inside the oven to limit the entry of parasite cold air by a stack effect. The injected air is drawn out of the oven at one or more points (4), with the dynamic brake created below and close to one of the points, and the air drawn from the oven is recycled.

Description

WO 2005/017433 - 1 - PCT/FR2004/001701 METHOD AND DEVICE FOR DRYING A NONMETALLIC COATING ON A STEEL STRIP The invention relates to a method for drying a 5 nonmetallic coating on a steel strip, whereby the strip is conveyed into a vertical drying oven with relatively cold walls, hot air is injected into the lower part and the upper part of the oven to prevent vapors from condensing on the oven walls, and the injected air is 10 drawn off. The invention relates in particular to a method for drying a coating consisting of a paint film. 15 Nonmetallic coatings of steel strips are produced with installations or production lines generally comprising a strip surface preparation section, followed by a section for application of a liquid film, particularly by roll coating, followed by 20 a section for drying this film. The drying is obtained by heating the support and the film to a requisite temperature for drying and baking the coating. During the drying, the solvents of 25 the coating are removed by evaporation and drawn off by a blower. If necessary, the solvents are incinerated, which is the case for organic solvents. For safety reasons, it is vital to ensure that 30 the organic solvents evaporated in the drying oven are diluted, generally by air, to keep the solvent concentration lower than a predetermined limit and eliminate any risk of explosion due to the solvents. Moreover, the drying oven is maintained under slight 35 negative pressure to prevent the discharge of generally toxic or hazardous solvents outside the vessel. The walls of the drying oven, particularly if the strip drying means consist of electromagnetic - 2 induction coils, are relatively cold, and any condensation of evaporated solvents on the cold walls must be prevented. For this purpose, hot air is injected into the oven, for example as described in FR 5 2 734 501. In a vertical oven, the hot air is injected in the lower part and the upper part of the oven, and the injected air is drawn off. Horizontal drying ovens are limited in length 10 by the need to control 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 coating lines or for special materials, drying ovens are installed 15 vertically to avoid strip sagging problems. Owing to the difference between their internal temperature and the ambient temperature, vertical drying ovens behave like a chimney, and an upward air 20 flow is spontaneously established. Large quantities of fresh air originating in particular from the bottom opening of the oven are swept up, mixed with the injected hot air, and drawn off. This undesirable inward leakage of air dilutes the injected hot air 25 intended to prevent solvent condensation, and the hot air injection temperature must therefore be increased so that the mixture with the sucked in cold air is maintained at a sufficient temperature. This results in a loss of thermal efficiency, poor control of the 30 solvent concentration, and poor operation of the injection/drawoff circuit, which must be oversized. Similarly, the exit of the gases through the upper opening of the oven represents an explosion hazard, degrades the working conditions of the operators, and 35 causes solvents to condense on equipment located nearby. A vertical drying oven comprises a strip admission window at its bottom inlet. The size of this C :\RPonbl\DCC\ELU782127 1.DOC-19/03/2010 -3 window could conceivably be reduced to limit the ingress of undesirable air. However, such a size reduction of the passage window is impossible because this would increase the risks of scratching the coating on the 5 strip, particularly with painted surfaces. The same applies to the window provide at the oven outlet. "Air curtain" devices could conceivably be used at the inlet and outlet of the drying oven, but they are unsuitable for the purpose: in fact, to be effective, 10 they require a high air speed, which can cause appearance defects due to impact on the wet paint film. Moreover, these air curtains should operate with hot air to avoid cooling the tunnel atmosphere, which would cause incipient evaporation of the solvents, part of 15 which would inevitably be discharged outside the tunnel. This would raise problems of safety (risk of burns and explosion) and toxicity. Examples of the invention seek to limit and control the chimney effect in a vertical drying oven in a simple 20 manner and without incurring any risk of degradation of the coating to be dried. In accordance with one aspect of the present invention, there is provided a method for drying a nonmetallic coating on a steel strip whereby a steel 25 strip is conveyed into a vertical drying oven with relatively cold walls, in which hot air is injected into the upper part and lower part of the oven to prevent vapors from condensing on the oven walls and the injected air is drawn off, wherein a dynamic brake is 30 produced by injecting hot air between the lower part and the upper part of the oven and at a distance from the C:W4Rotb\DCCEJL\72272 LDOC-19/03/2010 -4 oven inlet and outlet, and by creating an overpressure in the tunnel of the drying oven in order to limit the ingress of undesirable cold air and the chimney effect in the vertical oven. 5 Preferably, when the injected air is drawn off at at least one point, the dynamic brake is produced below one of the drawoff points, and close thereto. Advantageously, in the method whereby hot air is blown to create a dynamic barrier, this blown hot air is 10 used to form a strip steadying point in order to limit its oscillations along the length of the oven. The method may provide for the use of two superimposed sets of hot air blowing orifices for the dynamic barrier. 15 The air used to create the dynamic brake can be recycled. The recycled air used to create the dynamic brake can be extracted from the neighborhood of the dynamic brake blowing orifices. The method advantageously comprises the use of 20 regulating members for adjusting the flow rate or pressure of the hot air of the dynamic brake according to the production parameters, the type of product to be treated or the properties of the solvents liberated. The control of the regulating members can be 25 provided by an automatic oven control system according to the oven operating conditions and the type of product and its coating. The invention also relates to a device for drying a C:WRPorbrlDCCtEJL\7227 .DOC-1913/2010 -4A coating, particularly a paint film, on a steel strip, comprising a vertical drying oven with relatively cold walls, and having means for heating, particularly by electromagnetic induction, the strip passing through the 5 oven, means for injecting hot air into the lower part and into the upper part of the oven to prevent vapors from condensing on the cold walls of the oven, and a means for drawing off the injected air. In accordance with another aspect of the present 10 invention, there is provided a device for drying a coating, particularly a paint film, on a steel strip, comprising a vertical drying oven with relatively cold walls, means for heating, particularly by electromagnetic induction, the strip passing through the 15 oven, means for injecting hot air into the lower part and into the upper part of the oven to prevent vapors from condensing on the cold walls of the oven, and a means for drawing off the injected air, wherein it comprises, between the lower part and the upper part of 20 the oven, means for injecting hot air into the oven to create a dynamic brake limiting the ingress of undesirable cold air, by creating a local overpressure in the drying oven.

-5 In a device in which the injected air is drawn off at a single point, the injection of dynamic barrier air comprises a tank for blowing hot air into the oven 5 via one or more orifices, on one or two faces of the strip. In such a device in which hot air is blown in to produce a dynamic barrier, the blowing orifices can 10 be arranged over a large part of the width of the strip. The hot air blowing orifices can be made in the form of holes or nozzles in order to effect a steadying 15 of the strip opposite these orifices. The air injection means may comprise tanks for blowing hot air into the oven. The tank may be equipped with means for adjusting the injected air flow 20 rates and the shape of the injected air jets. The air injected by the tank can advantageously be recirculated by a blower. Two or more blowing units can be used in the 25 tank to enhance the steadying of the strip and the effectiveness of the dynamic barrier. Regulating members can be used on the hot air ducts supplying each set of blowing holes, for 30 independently adjusting the pressures in the chambers of the oven located below and above the dynamic barrier. A plurality of dynamic hot air blowing barriers 35 installed at several heights on the oven, at a distance from the oven inlets and outlets, can be used to limit the circulation of undesirable air in the oven and to steady the strip from the bottom of the oven to the top.

-6 Apart from the above arrangements, the invention comprises a number of other arrangements which are explicitly discussed below, concerning 5 illustrative embodiments described with reference to the drawings appended hereto, but which are nonlimiting. In these drawings: Fig. 1 is an elevation view of a drying device 10 of the prior art. Fig. 2 is an elevation view of a drying device according to the invention. 15 Figs. 3, 4 and 5 are elevation views of variants of the device of Fig. 2. Fig. 1 of the drawings shows a vertical drying oven of the prior art. In general, a nonmetallic 20 coating line, particularly a line for applying a paint film on a steel strip, comprises a strip surface preparation section not shown in Fig. 1, followed by a machine 6 for applying a liquid coating film, particularly of paint, on the strip 1. The coating is 25 generally applied by the use of rolls. The strip 1 with its coating then passes into the drying section consisting of the oven forming a tunnel 3. The strip travels vertically through this 30 oven as shown in Fig. 1. The drying is obtained by heating the steel strip 1 and the coating film to the requisite temperature for drying and baking the coating. The strip 1 can be advantageously heated using an electromagnetic induction system not shown in 35 the figure. During the drying, the solvents of the coating applied to the strip 1 are removed by evaporation and drawn off by a blower 7 arranged on a drawoff duct -7 connected to the internal space of the oven at 4 located at a height between the inlet 5 in the lower part and the outlet 2 in the upper part. 5 To prevent solvents from condensing on the inside walls of the chamber 3, hot air is injected into the lower part and the upper part of the oven, respectively at 10a and lob as described, for example, in FR-A-2 734 501. This hot air is injected into the 10 oven chamber using a blower 8, and the injected hot air flow rates can be adjusted by manual or automatic valves such as 9a and 9b. Large quantities of fresh air originating in 15 particular from the bottom opening 5 dilute the hot air injected at 10a and lob, requiring an increase in the temperature of this injected air. Fig. 2 shows an elevation section through a 20 drying oven equipped with the invention. According to the invention, to limit and efficiently control the chimney effect, a dynamic brake is produced in the oven to create an overpressure inside the oven and limit the ingress of undesirable cold air. 25 Preferably, the dynamic brake created by the air injection is provided below the drawoff point 4 or close to this point. 30 The injection means may comprise a tank 12 made in one or more parts as shown in Fig 2 by 12a and 12b with blowing orifices 13a and 13b and their hot air feeds by means of the blower 8. 35 The parts 12a, 12b of the tank project into the oven toward the strip 1 so that the distance between this strip 1 and the blowing orifices 13a, 13b is relatively small, and shorter than the distance between the strip 1 and the vertical walls of the oven. The -8 direction of the width of the strip 1 is perpendicular to the plane of Fig. 2. A regulating member 11, with manual or 5 automatic control, is used to adjust the flow rate and/or pressure characteristics of the hot air injected into the tank 12 via the orifices 13 according to the settings of the oven, the type of production, or the properties of the coating deposited on the strip and of 10 the solvents produced. The regulating member 11 may be manual or automatic, operating under the control of the oven control device. The orifices 13a and 13b may be positioned and 15 constructed so as to concentrate the injection of hot air at a small number of points or created through a series of holes distributed over the entire width of the strip on each side thereof, or even created in the form of a continuous opening in the form of a blowing .20 nozzle over the entire width of the strip. In general, the blowing is effected in a direction perpendicular to the plane of the strip 1. The orifices 13a and 13b can be provided in the 25 known form of blowing nozzles which, in addition to controlling the chimney effect in the oven, are suitable for steadying the strip in order, for example, to limit its oscillations during its passage through the oven. 30 According to a second possibility shown in Fig. 3, the blowing tank is divided horizontally by a partition in order to obtain two superposed tanks 15 (15a, 15b) and 16 (16a, 16b) each comprising a set of 35 blowing orifices. Fig. 3 shows an example of this execution with, from the blower 8, two separate hot air feeds for the upper and lower tank, each of these feeds being equipped with a regulating member 14a and 14b. It is thereby possible to adjust the flow rates of the -9 hot air blown in via the sets of upper and lower orifices in order to adjust the pressures in the lower and upper parts of the oven in order to eliminate the chimney effect in this chamber. 5 According to a variant of Fig. 3, the blowing orifices of the tanks 15a and 15b, and 16a and 16b, can be oriented respectively upward and downward to facilitate the adjustment of the pressures in the upper 10 and lower parts of the oven and limit the chimney effect. Fig. 4 shows a variant of the device of Fig. 2 in which the air blown by the tank 12 is blown by an 15 independent blower 17 for adjusting the pressure of the air blown into the tank 12 of the dynamic brake independently of the pressure delivered by the air injection blower 8. The feed blower 17 of the tank 12 can be supplied with hot air at the outlet of the 20 blower 8 as shown in the figure, at the inlet of the blower 8, or supplied by any other network at a pressure and temperature different from those used for injecting air into the oven at 10a and 10b. 25 Fig. 5 shows a variant of the device of Fig. 4. The hot air feed blower 17 of the tank 12 recirculates this air via extraction points 19a and 19b, and 20a and 20b. These extraction points may be located close to the blowing points of the tank 12 of the dynamic brake 30 in order to improve the efficiency of the device, or may simply be connected to the chamber 3. The variants of Figs. 4 and 5 can be produced with a dynamic brake tank produced according to Fig. 3. 35 The drawoff duct 4 may be located substantially at the mid-height of the oven. The tank 12 is located below the drawoff point 4. The temperature of the hot air injected into the oven may be about 250 0 C. The C:\NRPortbl\DCC\EJL\2782127_1.DOC-19/03/2010 10 strip travel speed may typically range between 60 and 120 m/min. All combinations of the devices described with 5 their regulating members are suitable for creating a dynamic barrier in the oven with hot air so as to brake the flow of undesirable air, particularly the upflow created by the chimney effect, and thereby limit the ingress of undesirable air. 10 While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. It will be apparent to a person skilled in the relevant art that 15 various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the present invention should not be limited by any of the above described exemplary embodiments. 20 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information 25 derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. Throughout this specification and the claims which follow, unless the context requires otherwise, the 30 word "comprise", and variations such as "comprises" and C:\NRPortbl\DCC\EJL\2782127 I.DOC-19/03/2010 11 "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

1. A method for drying a nonmetallic coating on a steel strip whereby a steel strip is conveyed into a vertical drying oven with relatively cold walls, in 5 which hot air is injected into the upper part and lower part of the oven to prevent vapors from condensing on the oven walls and the injected air is drawn off, wherein a dynamic brake is produced by injecting hot air between the lower part and the upper part of the oven 10 and at a distance from the oven inlet and outlet, and by creating an overpressure in the tunnel of the drying oven in order to limit the ingress of undesirable cold air and the chimney effect in the vertical oven.

2. The method as claimed in claim 1, whereby the 15 injected air is drawn off at at least one point, wherein the dynamic brake is produced below one of the drawoff points, and close thereto.

3. The method as claimed in either of claims 1 and 2, whereby hot air is blown to create a dynamic barrier, 20 this blown hot air being used to form a strip steadying point in order to limit its oscillations along the length of the oven.

4. The method as claimed in one of claims 1 to 3, wherein two superimposed sets of hot air blowing 25 orifices are used for the dynamic barrier.

5. The method as claimed in one of claims 1 to 4, wherein the air used to create the dynamic brake is recycled.

6. The method as claimed in claim 5, wherein the C.\NRPOrtbl\DCC\EJL\2782127_1.DOC-19/03/2010 13 recycled air used to create the dynamic brake is extracted from the neighborhood of the dynamic brake blowing orifices.

7. The method as claimed in one of the preceding 5 claims, wherein regulating members are used for adjusting the flow rate or pressure of the hot air of the dynamic brake according to the production parameters, the type of product to be treated or the properties of the solvents liberated. 10

8. The method as claimed in claim 7, wherein the regulating members are controlled by an automatic oven control system according to the oven operating conditions and the type of product and its coating.

9. A device for drying a coating, particularly a 15 paint film, on a steel strip, comprising a vertical drying oven with relatively cold walls, means for heating, particularly by electromagnetic induction, the strip passing through the oven, means for injecting hot air into the lower part and into the upper part of the 20 oven to prevent vapors from condensing on the cold walls of the oven, and a means for drawing off the injected air, wherein it comprises, between the lower part and the upper part of the oven, means for injecting hot air into the oven to create a dynamic brake limiting the 25 ingress of undesirable cold air, by creating a local overpressure in the drying oven.

10. The device as claimed in claim 9, whereby the injected air is drawn off at a single point, wherein the injection of dynamic barrier air comprises a tank for 30 blowing hot air into the oven via one or more orifices, C:\NRPortbl\DCC\EJL\2782127_1.DOC-19103/2010 14 onto one or two faces of the strip.

11. The device as claimed in either of claims 9 and 10, whereby hot air is blown in to produce a dynamic barrier, wherein the blowing orifices are arranged over 5 a large part of the width of the strip.

12. The device as claimed in one of claims 9 to 11, in which the hot air blowing orifices are made in the form of holes or nozzles in order to effect a steadying of the strip opposite these orifices. 10

13. The device as claimed in one of claims 9 to 12, wherein two or more blowing units are used in the tank to enhance the steadying of the strip and the effectiveness of the dynamic barrier.

14. The device as claimed in claim 13, wherein 15 regulating members are used on the hot air ducts supplying each set of blowing holes, for independently adjusting the pressures in the chambers of the oven located below and above the dynamic barrier.

15. The device as claimed in one of claims 9 to 14, 20 including a plurality of dynamic hot air blowing barriers installed at several heights on the oven, at a distance from the oven inlets and outlets to limit the flow of undesirable air in the oven and to steady the strip from the bottom of the oven to the top. 25

16. A method for drying a nonmetallic coating on a steel strip substantially as hereinbefore described with reference to the drawings and/or Examples.

17. A device for drying a coating substantially as C:\NRPortbl\DCC\EJL\2782127_1.DOC-19/03/2010 15 hereinbefore described with reference to the drawings and/or Examples.