EP2803765A1

EP2803765A1 - Method and system for the stabilization of paper applicable in paper drying machines and processes

Info

Publication number: EP2803765A1
Application number: EP13380019.3A
Authority: EP
Inventors: Luis Oscar López Mesas; Iraide Larrea Uribiarte
Original assignee: BRUNNSCHWEILER SA
Current assignee: BRUNNSCHWEILER SA
Priority date: 2013-05-17
Filing date: 2013-05-17
Publication date: 2014-11-19

Abstract

Method and system for the stabilization of paper applicable in a dryer section of a sheet of paper (1) that is conveyed along with a fabric (5), where there is at least one critical area (7) in which they (1, 5) separate from each other. The method comprises at least the steps of generating a high circulation vortex (12) in the critical area (7) and confining the high circulation vortex (12) in the critical area (7). This way, a high vacuum is in turn confined in the critical area (7) that prevents the separation of the sheet of paper (1) and the fabric (5), without having to use confinement elements that are aggressive to the fabric (5), and with reasonable energy consumption.

Description

Technical field

The invention refers to a method and system for the stabilization of paper applicable in paper drying machines and processes, widely used in industrial plants for the production of paper.

Prior art

In paper making processes it is common to use a variety of machines that have to operate at high speeds. This requirement for speed usually results in the generation of unwanted interferences in the immediate area of the paper sheet that come in the form of induction currents or external airflows. These external air currents cause creases and/or wrinkles to the paper leading to a loss of the paper's quality, a risk of deterioration and even potential tears in the sheet of paper. These problems often cause unwanted interruptions in the paper making process with a negative effect on production. Thus, paper-making processes require solutions to ensure the stabilization of paper, stabilization being understood as the elimination of the aforementioned interferences that impact on the paper causing it to move, wrinkle or become damaged. The requirements for stabilizing the paper increase when machines are to be operated at higher speeds.
One of the main processes comprised in paper manufacturing is the paper dryer section. In said dryer section, a continuous sheet of paper is conveyed via a horizontal arrangement or line of metallic drying cylinders, which are used for reducing the degree of humidity of the sheet of paper that was formed in previous stages of the process. The drying cylinders are internally fed with steam, which condenses inside the drying cylinder, thereby releasing heat towards the surface of the drying cylinder and thus creating the temperature in said surface to reach values in the order of 80 - 150 ºC. The continuous sheet of paper that is conveyed along the exterior of the cylinders gradually loses its humidity due to the heat absorbed from the external surface of each cylinder. In order to assist the dehydration of the paper, a woven mesh or dryer fabric is commonly used that presses the sheet of paper against the drying cylinder, in a sandwich-like arrangement, thus improving the contact between the continuous sheet of paper and the external surface of the drying cylinder in order to enhance the heat transfer from the surface of the drying cylinder to the paper.
A first problem that arises in paper drying processes is that the high level of humidity of the paper sheet makes it prone to adhere to the surface of the drying cylinder due to the high temperature of said surface. This adhesion phenomenon is more pronounced in the first dryers of the plant, which is the phase in the process where paper is less resistant because of its high level of initial humidity, so that higher temperatures are required on the surfaces of these first drying cylinders for various reasons of operability or paper quality requirements. At the same time, it is not possible to increase the tension of the paper in order to compensate this increased tendency of adhesion due, on the one hand, to the lower resistance of the paper on account of its high level of humidity, and on the other hand, because the paper would undergo a the significant transverse shrinkage as a result of said increased tension of the paper. In conclusion, the aforementioned adhesion effect is disadvantageous to the objective of stabilizing paper, as it strongly conditions the operability of the machines used in the dryer section.
A second problem that occurs in paper drying processes is that the external air currents induced by the machines' speed are capable of passing through the woven mesh or dryer fabric, as those are porous and permeable. This results in the formation of air pockets in certain points that cause the paper sheet (not permeable) to separate or detach from the dryer fabric (permeable). These points can be considered as critical areas, as it is in these areas where the paper loses stability and may even tear up. This problem caused by external air currents further aggravates the negative impact of the adhesion phenomenon mentioned above.
The usual solution for preventing the formation of air pockets is to generate vacuum zones in the critical areas. A vacuum zone refers to an area where air is displaced towards one direction in order to flatten the paper sheet against the dryer fabric. The generation of vacuum zones is necessary for avoiding the formation of wrinkles in the paper sheet and for enabling the containment of unwanted external air currents, which would disrupt the generated vacuum. In this way, an additional supplementary tension is generated that acts against the tendency of the paper to adhere to the hot surface of the dryer.
Traditional stabilization systems or equipment that create vacuum zones by means of a permeable fabric can be classified into two categories: positive pressure systems and negative pressure systems. In positive pressure systems, based on the Coanda and Bernoulli effects, the required vacuum is generated by insufflating, injecting or blowing a high velocity jet or airflow towards the immediate surroundings of the fabric. In negative pressure systems, vacuum is created by means of direct aspiration or suction in certain areas between the fabric and the stabilization equipment. The blowing or suction means commonly used may be located in the external stabilization equipment or in one of the turning rolls placed between two consecutive drying cylinders for guiding the sheet of paper. Turning rolls are usually perforated or slotted for enabling suction. The two stabilization systems described, but especially the positive pressure systems, are more effective when the vacuum generating devices and the fabric are in close proximity. It is precisely in the critical areas where the paper becomes detached that there is a need for higher vacuums, which requires a closer proximity of the blowing jets or suction elements. In addition, once the high vacuum is generated, it is inevitably necessary for the stabilization system to be provided with sealing or closing elements that allow for the concentration or confinement of the high vacuums generated in said critical areas, where the adhesion phenomena are at their maximum.
An important constraint to the known solutions is that high vacuums generate a suction of the fabric, which, being a deformable element, has the tendency to bend and approach or even come in contact with the vacuum generating devices and/or the sealing or closing elements. This approaching and/or coming in contact causes a progressive deterioration of the fabric, complicates certain tasks of maintaining and cleaning the machine and provokes interruptions in production in order to replace the dryer fabric and reload the sheet of paper. In addition to the problems caused by these production downtimes and to the increased need to substitute the dryer fabric, one must also consider the high consumption of mechanical power by the equipment necessary for generating high vacuums.
One example of a stabilization system based on the principles explained above is the one described in patent US8117765B2 . This solution incorporates a closing element for sealing the critical area and inhibiting the entry of external air currents through the dryer fabric. Said closing element is based on the use of a Coanda surface combined with a blowing jet. The positioning of both the closing element as well as the exit pipe of the blowing jet is adjustable for achieving the sealing. Other solutions, such as the ones described in patents US6848195 and US6247247 , include mechanisms that allow closing elements to draw back by including various mechanical elements, springs or hinges that are actuated by actuators and limiters. However, when it comes to critical situations in the solutions described above, as for example in the event of certain interferences or discontinuities of the paper sheet, air leakages or approximations between the fabric and the closing elements occur that make it difficult to control the generated high vacuums in terms of maintaining the sealing of the critical area and the safety distance between the fabric and the stabilizing equipment, without increasing energy consumption.
The objective of the present invention is to offer a method and system of stabilization that succeeds in avoiding the use of sealing or closing elements or other elements aggressive to the fabric that could damage the same. Another objective of the invention is a stabilization system that manages to control the generated high vacuums and to maintain the sealing of the critical area and the safety distance between the fabric and the stabilizing equipment without entailing an increase of the energy consumption.

Brief description of the invention

The object of the present invention is a method and system for the stabilization of paper applicable in a dryer section of a sheet of paper that is conveyed in a direction H, where said dryer section comprises a first drying cylinder, at least a second drying cylinder adjacent to the first drying cylinder and a fabric that keeps the sheet of paper in position while the fabric and the sheet of paper are conveyed along the surface of the drying cylinders. An intermediate space is delimited between the drying cylinders and external to the fabric; in this intermediate space there is at least one critical area where the sheet of paper and the fabric tend to separate due to an external air current or due an adhesion phenomenon of the sheet of paper against the surface of the drying cylinders.
The method according to the invention has the particularity of comprising the following steps: procuring a stabilizer box positioned in the intermediate space, where the stabilizer box comprises at least one blowing element capable of generating an airflow against the traveling direction H of the sheet of paper; generating a high circulation vortex in the critical area; and confining the high circulation vortex within the critical area.
To this end, the stabilizer box in the stabilization system according to the invention comprises a blowing outlet that is adapted to inject an airflow, from inside the stabilizer box towards the critical area and against the travelling direction H, that generates a high circulation vortex in the critical area. The stabilizer box also comprises a superior confinement element, an inferior confinement element and a concave surface that is located between the two confinement elements. Said confinement elements and concave surface surround the critical area in order to confine the high circulation vortex within the critical area. The confinement of the high circulation vortex within the critical area prevents it from being distorted and become dispersed due to the adverse effect of the fabric's velocity.
The thus generated and confined high circulation vortex interacts directly on the one hand with the fabric and on the other hand with the stabilizer box, while also delimiting a high vacuum zone inside the high circulation vortex. This high vacuum zone confined inside the high circulation vortex prevents the extreme proximity between the confinement elements of the stabilizer box and the fabric, thus preventing the contact between the fabric and said confinement elements that are susceptible to wear out or damage the fabric. By eliminating the need to provide for sealing or closing elements that are too close, and therefore aggressive, to the fabric and which would gradually deteriorate the same, routine operations for maintaining the components of the stabilizer box in the appropriate conditions of use, cleanliness and operability are simplified or altogether eliminated. In addition, the usual maneuvers for loading and stretching the sheet of paper that normally require programming complex automatisms or performing complex manual sequences are also simplified. Moreover, the mounting of dryer fabrics is made easier as there are no physical elements in the immediate proximity of the fabric that would have to be removed in order to proceed to mount said fabric.
Another inherent advantage of the mechanism of generating a high vacuum according to the invention, as an alternative to the traditional methods based solely on direct blowing or suction mechanisms, is the generation of vacuums of a similar magnitude with a lower consumption of mechanical power.
In one especially advantageous embodiment of the system according to the invention, a second vortex is generated, which improves the sealing of the critical area, contributes in preventing the penetration of any external air current induced by the movement of the fabric and enhances the high vacuum in the critical area.
In the present document, the high vacuum zone inside the high circulation vortex is referred to as high vacuum because its magnitude is higher in comparison with other zones of medium vacuum that are present in the intermediate space between the drying cylinders.

Brief description of the drawings

Details of the invention are depicted in the accompanying figures, which are intended to be illustrative and non-limiting:

Figure 1 shows an embodiment of a stabilization system according to the invention detailing its arrangement in the space comprised between two consecutive drying cylinders of a dryer section.
Figure 2 shows the stabilizing system depicted in Figure 1 with more detail.
Figure 3 shows a diagram of the airflow and the formation of the high circulation vortex in one embodiment according to the invention. The formation of a second external vortex, which acts as a booster to the high vacuum in the critical area, is also represented.

Detailed description of the invention

The invention defines a method for the stabilization of paper applicable in a paper dryer section, where a continuous sheet of paper (1) is conveyed via an arrangement or line that usually comprises various drying cylinders. The invention also refers to a stabilizing system, a specific embodiment of which is shown in Figure 1. Figure 2 shows a more detailed representation of the embodiment of Figure 1 and its arrangement between two consecutive drying cylinders. As can be seen in Figure 1, the sheet of paper (1) in the partially represented dryer section is conveyed in a direction H along the surface of a first drying cylinder (2), along the surface of at least a second drying cylinder (3) adjacent to the first drying cylinder (1) and along the surface of a turning roll (4) placed between two drying cylinders (2, 3). The turning roll (4) guides the sheet of paper (1) and is usually placed between each pair of two consecutive drying cylinders of the dryer section. The dryer section also comprises a fabric (5) that is conveyed along with the sheet of paper (1) and keeps the sheet of paper (1) in position, in such a way that both are conveyed along the surface of the drying cylinders (2, 3) and the turning roll (4). An intermediate space (6) is delimited between the drying cylinders (2, 3) and external to the fabric (5). In said intermediate space (6) there is at least one critical area (7) where the sheet of paper (1) and the fabric (5) tend to separate due to an external air current (8) or due to the phenomenon of the adhesion of the sheet of paper (1) against the surface of the drying cylinders (2, 3). The critical area (7) represented in Figure 1 is specifically located around a detachment area (9) where the sheet of paper (1) detaches from the first drying cylinder (2) and continues its way towards the turning roll (4). The critical area (7) around the detachment area (9) is among the most problematic, but there may also be other critical areas in the intermediate space (6), not represented in Figure 1, that may be affected by the adhesion phenomena and/or external air currents.
The stabilization method according to the invention comprises the following steps: procuring a stabilizer box (10) positioned in the intermediate space (6), where said stabilizer box (10) comprises at least one blowing element that is capable of generating an airflow (11) against the traveling direction H of the sheet of paper (1), as can be observed in Figure 2; generating a high circulation vortex (12) in the critical area (7) and confining the high circulation vortex (12) within the critical area (7). A vortex is a circular or rotary flow that possesses vorticity; vorticity is the circulation per unit area at a determined point of the flow and it is a vector magnitude, the direction of which is the axis of the vortex or eddy. Figure 3 shows a detailed enlargement of the critical area (7) and a schematic representation of the formation of the high circulation vortex.
The fact of placing a stabilizer box (10) inside the intermediate space (6) allows reducing the total space in which vacuum zones have to be generated for controlling the various critical zones. In addition, the stabilizer box (10) also serves as a support for positioning the necessary media that generate a high vacuum zone (13) in the critical area (7), in particular, for positioning the blowing element that generates the airflow (11).
Thus, the method according to the invention is based on generating a high speed rotary flow or high circulation vortex (12) right at the critical area (7), and preventing the high circulation vortex (12) from being dispersed outside the critical area (7). On account of this confinement, the high circulation vortex (12) interacts simultaneously with both the fabric (5) and the wall of the stabilizer box (10) in the critical area (7). This way, the high vacuum zone (13) is confined inside the high circulation vortex (12) without the need of additional elements and with optimal energy consumption. In addition, the stabilizer box (10) may be kept at an adequate distance from the fabric (5) and be free of closing, sealing or confining elements that are aggressive to the fabric (5), aggressiveness being understood as said elements being placed at too close of a distance to the fabric (5).
The embodiment of the stabilization system represented in the figures allows performing the steps of the method according to the invention. The stabilization system according to the invention is applicable in a dryer section of a sheet of paper (1) conveyed in a direction H. The dryer section comprises a fist drying cylinder (2), at least a second drying cylinder (3) adjacent to the first drying cylinder (1) and a fabric (5) that keeps the sheet of paper (1) in position while both are conveyed along the surface of the drying cylinders (2, 3). A turning roll (4) is placed between the drying cylinders (2, 3) for guiding the sheet of paper (1). An intermediate space (6) is delimited between the drying cylinders (2, 3) and the turning roll (4), and external to the fabric (5). In said intermediate space (6) there is at least one critical area (7) where the sheet of paper (1) and the fabric (5) tend to separate due to an external air current (8) or due to the phenomenon of the adhesion of the sheet of paper (1) against the surface of the drying cylinders (2, 3). The stabilization system according to the invention has the particularity of comprising a stabilizer box (10) that is placed in the intermediate space (6). The stabilizer box (10) comprises a blowing outlet (14) that is adapted to inject an airflow (11), which generates a high circulation vortex (12) in the critical area (7). As can be observed in Figure 3, the airflow (11) is injected from the blowing opening (14), from inside the stabilizer box (10) towards the critical area (7) and against the travelling direction H of the sheet of paper (1). In addition, the stabilizer box (10) also comprises a superior confinement element (15), an inferior confinement element (16) and a concave surface (17) located between the two confinement elements (15, 16). The confinement elements (15, 16) are projections, sealing elements or other elements capable of redirecting the airflow (11) in order to form and confine the high circulation vortex (12). The confinement elements (15, 16) and the concave surface (17) surround the critical area (7), as can be seen in the embodiment represented in the figures, in order to confine the high circulation vortex (12) within the critical area (7).
Figure 3 also shows the rotational airflow of the high circulation vortex (12) of the embodiment according to the invention shown in Figures 1 and 2. As can be seen in Figure 3, the high circulation vortex (12) is generated and is centered, localized or confined in the critical area (7) by the combined action of the airflow (11), the movement of the sheet of paper (1) in the direction H, the concave surface (17) and the confinement elements (15, 16).
The high circulation vortex (12) makes it possible to generate and confine the desired high vacuum in the critical area (7) by means of a method that is not exclusively based on the Bernoulli and Coanda effects, as is the case in other, conventional solutions. The Coanda effect is known in Fluid Mechanics as the physical phenomenon whereby a fluid current tends to be attracted to a surface adjacent to its trajectory under the right conditions with respect to the fluid's angle of incidence and the curvature of the surface. The result of this attraction that the fluid experiences is the creation of an imbalance of pressures in the affected fluid due to the Coanda effect, which leads to the appearance of a vacuum in the proximity of the blowing outlet that insufflates said stream or fluid. The Bernoulli effect, which derives from the Bernoulli equation on conservation of energy in a fluid, implies that an increase of a fluid's velocity results in a decrease of the fluid's pressure while the potential energy is maintained constant. The known stabilization systems that generate high vacuums based on the combination of these two principles and/or direct suction entail, as explained above, a high energy consumption and an excessive approximation between the devices generating the vacuum and the fabric. In the method and system according to the invention, the vacuum is generated by the Bernoulli effect through the airflow (11), but the vacuum is confined in the critical area (7) by the high circulation vortex (12) and using non-aggressive confinement elements, with the system presenting an acceptable consumption. The presence of the high circulation vortex (12) makes it possible not to have to bring the confinement elements (15, 16) too close to the fabric (5), thus achieving a safety distance d or d', e.g. within the range of 10 - 20 mm, which is sufficient for avoiding damages to the fabric (5).
The geometry of the stabilizer box (10) according to the invention is especially designed to maximize the circulation of the high circulation vortex (12), in a way that the vacuum in the high vacuum zone (13) is as high as possible, and contributes in avoiding its distortion. By way of example, the concave surface (17) presents a substantially circular curvature that distorts the confined high circulation vortex (12) as less as possible. The curvature presents a preferable diameter of about 40 - 70 mm.
The inferior confinement element (16) prevents the high circulation vortex (12) from being dispersed towards other areas of the intermediate space (6) that are below the critical area (7). Preferably, said inferior confinement element (16) is free of sharp edges that are aggressive or damaging to the fabric (5) and also complicate maintenance tasks.
Preferably, the method according to the invention comprises the additional step of generating a second vortex (18) outside the critical area (7) that prevents the external air current (8) from penetrating inside the critical area (7). The presence of this second vortex (18) entails a great advantage: it assists and boosts the action of the high circulation vortex (12) by acting as a physical stopper and preventing the entry of external air currents induced by the fabric's (5) velocity. The joint action of the two vortices (12, 18) optimizes the formation of the high vacuum zone (13) in the critical area (7).
Optionally, it is the airflow (11) itself of the stabilization system according to the invention that generates the second vortex (18) outside the critical area (7) for preventing the external air current (8) from penetrating inside the critical area (7). The formation of this second vortex (18) in an area above the critical area (7) can be observed in the embodiment shown in Figure 3. Thus, the formation of both vortices (12, 18) is achieved due to a certain symmetry of the airflow (11), symmetry being understood as the dispersion of the airflow (11) in two directions, as shown in the figure. The formation of the high circulation vortex (12) has already been explained above. The second vortex (18) is generated and maintained outside the stabilizer box (10) by the combined action of the airflow (11), the external air current (8) and the geometry of the stabilizer box (19). The stabilizer box (10) in the embodiment shown in Figure 3 presents an external wall (19) that favours the confinement of the second vortex (18) and prevents its distortion.
As the case may be, the exact form of the external wall (19), the velocity of the airflow (11) and the geometry in general of the stabilizer box (19) in the system according to the invention may be variable in order to adapt to the variety of machines present in dryer sections, as long as the formation of the high circulation vortex (12) is achieved. The operability of these machines ranges between their minimum and maximum working speed, and those speed limits will define the exact geometry of the stabilizer box (10) in order to adapt to each critical area.
Optionally, the stabilization method comprises the additional step of generating a direct aspiration over the critical area (7) for enhancing the confinement of the high circulation vortex (12) in the critical area (7). In the embodiment of the stabilization system represented in the figures, said direct aspiration on the critical area (7) is carried out by an aspiration element (not shown in the figures) located inside the concave surface (17), between the blowing outlet (14) and the inferior confinement element (16).
In addition, the stabilizer box (10) of the system according to the invention comprises at least one additional blowing outlet (20) that allows for the control of the vacuum generated in other areas of the intermediate space (6) that are located in the lower part of the critical area (7). In the embodiment of Figure 2, the additional blowing outlet (20) is located at the lower part of the stabilizer box (10), generating a stream against the moving direction of the turning roll (4), which contributes to the generation of a zone of medium vacuum (21) in the intermediate space (6).
Thus, the method and system of the invention makes it possible to combine the generation of the vortices (12, 18) with other, additional elements of suction or blowing that assist or boost the vacuums in the intermediate space (6). These additional suction or blowing elements may be placed in various locations: in the stabilizer box (10), in the turning roll (4) -which may have openings, slots or perforations designed to this end-, or others.

Claims

Method for the stabilization of paper applicable in a dryer section of a sheet of paper (1) that is conveyed in a direction H, where said dryer section comprises a first drying cylinder (2), at least one second drying cylinder (3) adjacent to the first drying cylinder (2) and a fabric (5) that keeps the sheet of paper (1) in position while both are conveyed along the surface of the drying cylinders (2, 3), where an intermediate space (6) is delimited between the drying cylinders (2, 3) and external to the fabric (5), and in this intermediate space (6) there is at least one critical area (7) where the sheet of paper (1) and the fabric (5) tend to separate due to an external air current (8) or due to the phenomenon of the adhesion of the sheet of paper (1) against the surface of the drying cylinders (2, 3), where said method is characterized in that it comprises the steps of:
- procuring a stabilizer box (10) that is placed in the intermediate space (6), where said stabilizer box (10) comprises at least one blowing element that is capable of generating an airflow (11) against the traveling direction H of the sheet of paper (1),

- generating a high circulation vortex (12) in the critical area (7),

- confining the high circulation vortex (12) in the critical area (7).
Method, according to claim 1, characterized in that it comprises the additional step of generating a second vortex (18) outside the critical area (7) that prevents the external air current (8) from penetrating inside the critical area (7).
Method, according to claim 1, characterized in that it comprises the additional step of generating a direct suction on the critical area (7) in order to enhance the confinement of the high circulation vortex (12) in the critical area (7).
System for the stabilization of paper applicable in a dryer section of a sheet of paper (1) that is conveyed in a direction H, where said dryer section comprises a first drying cylinder (2), at least one second drying cylinder (3) adjacent to the first drying cylinder (2) and a fabric (5) that keeps the sheet of paper (1) in position while both are conveyed along the surface of the drying cylinders (2, 3), where an intermediate space (6) is delimited between the drying cylinders (2, 3) and external to the fabric (5), and in this intermediate space (6) there is at least one critical area (7) where the sheet of paper (1) and the fabric (5) tend to separate due to an external air current (8) or due to the phenomenon of the adhesion of the sheet of paper (1) against the surface of the drying cylinders (2, 3), where said system is characterized in that it comprises:
- a stabilizer box (10) that is placed in the intermediate space (6), where said stabilizer box (10) comprises a blowing outlet (14) adapted to inject an airflow (11) from inside the stabilizer box (10) towards the critical area (7) and against the travelling direction H of the sheet of paper (1), generating a high circulation vortex (12) in said critical area (7); where

- the stabilizer box (10) further comprises a superior confinement element (15), an inferior confinement element (16) and a concave surface (17) located between the two confinement elements (15, 16), where said confinement elements (15, 16) and concave surface (17) surround the critical area (7) in order to confine the high circulation vortex (12) in the critical area (7).
System, according to claim 4, characterized in that the inferior confinement element (16) is free of sharp edges.
System, according to claim 4, characterized in that the blowing outlet (14) is adapted so that the airflow (11) also generates a second vortex (18) outside the critical area (7) that prevents the external air current (8) from penetrating inside the critical area (7).
System, according to claim 4, characterized in that it comprises at least one aspiration element for generating a direct aspiration over the critical area (7) and enhancing the confinement of the high circulation vortex (12) in the critical area (7), where said aspiration element is located inside the concave surface (17), between the blowing outlet (14) and the inferior confinement element (16).
System, according to claim 4, characterized in that the stabilizer box (10) comprises at least one additional blowing outlet (20) that allows for the control of the vacuum generated in other areas of the intermediate space (6) located in the lower part of the critical area (7).