FI105050B - Pulse drying method and apparatus - Google Patents

Description

5 105050

Pulse drying method and apparatus Förfarande och anordning för impulstorkning

The present invention relates to a method for pulse drying a paper or board web, wherein the web to be dried is passed through an extended compression zone formed between the backing roll to be heated by the press fabric and the press shoe, wherein the compression in the first hydrodynamic region from the starting level to the actual nip pressure, in a second step the web is compressed at a constant pressure in the area of action of the hydrostatic pressure chamber of the press shoe and in a third step the compression is continued at the second short hydrodynamic region following

The invention further relates to an impulse drying device comprising a backing roll and means 20 for heating its surface, a press fabric for supporting a web, and a press shoe device comprising a press shoe and a conveyor belt adapted to slide over the lubricated front surface of the press shoe. loading members providing at least two separate loading forces, and between the heated counter roll and the press shoe, are provided with an extended compression zone comprising first a hydrodynamic region, then a hydrostatic pressure chamber region, and then a second short hydrodynamic region. 1 30 Pulse drying is used on the press section of a papermaking machine to enhance dewatering from the web. As an example of known solutions, reference is made to U.S. Patent No. 4,324,613, in which a paper web is guided, supported by a press fabric, between two press rolls, one of which is heated to a high temperature. In the press nip, heat is transferred from the heated roll surface to the paper web, whereupon the water contained in the web begins to evaporate. The pressure of the vapor produced forces water pressed into the interstices of the fiber into the press fabric on the opposite side of the web. At the same time, the high temperature reduces the viscosity and surface tension of the water and alters the elastic properties of the fiber, thereby further enhancing the mechanical compression of the water from the dough into the press fabric.

In the solution described above, the residence time in the roll nip remains short and the effect of the high temperature 10 lane cannot be fully utilized. As a result, a long nip shoe is often used as a second compression member instead of a press roll to extend the action time of the temperature and pressure to the web. As an example of a typical long nip press shoe, reference is made to Applicant FI Patent Application 925943, which discloses a press shoe having a radius of curvature of a surface substantially corresponding to the curvature of a counter roll. This shoe geometry 15 aims at a triangular pressure curve where the pressure after the nip decreases rapidly. Such a shoe is unsuitable for use in impulse drying, where there is a risk of delamination of the web as it leaves the press nip.

For pulse drying, a temperature range of 150 to 300 ° C is typically used and the pressure is generally in the range of 0.5 to 8 MPa. The retention time of the web in the nip is 15 ... 100 ms. The heat transferred from the heated back roll to the paper web raises the temperature of the web, whereby the water tends to evaporate, especially in the surface layers of the web. However, the high pressure in the nipple limits water evaporation. As the web leaves the press nip, the pressure quickly drops to ambient pressure, water above 100 degrees is evaporated abruptly, and the vapor generated expands explosively. This easily results in web delamination, i.e., breakage of the structure when the fiber is uncontrollably expanded.

To prevent delamination, it has been proposed that the press shoe be arranged so that the pressure in the nip drops towards the end of the compression zone, so that the web is not subjected to too rapid a drop in pressure at the end of the compression. For example, U.S. Patent No. 5,047,122 discloses an impulse drying method which attempts to prevent delamination of the pressed web 3 105050 by adjusting the position of the press shoe relative to the heated back roll and thereby affecting the shape of the pressure curve in the press zone. However, the possibilities for controlling the shape of the pressure curve are limited e.g. because the press shoe is loaded with only one load member.

U.S. Patent No. 5,071,513 discloses an impulse drying method wherein the compression zone comprises at least two hydrostatic compression steps. In the first step, high compression pressure is used, whereby the pores of the web are filled with water. In the last step, a pressure is used which is lower than the saturated vapor pressure 10 at that temperature but higher than the ambient pressure. The hot water in the pores of the web is then evaporated in a controlled manner while the compression zone limits the expansion of the steam and the web. This purely hydrostatic long nipple shoe has complex adjustment of the actual nip pressure profile due to a number of separate shoe rows, each with a combined loading piston that provides both shoe loading pressure and surface lubrication pressure.

U.S. Patent No. 5,302,252 discloses a solution in which the actual press nip is followed by a long region of low pressure pressure, during which the vapor pressure in the web slowly decreases. This low pressure region is achieved by a permeable metal band between the web and the press fabric 20, which presses the web against the back roll. The amount of pressure and the length of the pressure range can be influenced by the tightening of the metal strip and the separate loading roller. The press has only one row of load elements, so that the shape of the nip pressure curve cannot be changed. In practice, only very low pressures can be achieved with the tension of the metal strip. In addition, the multi-roll metal strip 25 with the metal strip is an expensive solution.

· '- It is an object of the present invention to provide an im-. a method and apparatus suitable for pulse drying which avoids delamination of the web while effectively controlling the properties of the web.

30 4 105050

In the impulse drying process of the invention, the web is subjected to compression in a plurality of immediately subsequent sub-steps, the first and third stages being hydrodynamic and the second stage being hydrostatic. The invention is characterized in that in the fourth 5 compression steps following the three compression sub-steps mentioned above, the compression pressure is lowered in the hydrodynamic region of the rest of the press shoe to a level lower than the actual nip pressure but higher than the duration of the final compression step is at least one third of the total duration of the total compression time.

The impulse drying apparatus of the invention comprises a compression shoe having a compression zone comprising a hydrostatic region preceded and followed by short hydrodynamic regions. The invention is characterized in that the aforementioned three compression zone regions 15 are followed by a third hydrodynamic region forming the remainder of the compression shoe, which is at least one third of the total compression zone length and is shaped and / or loaded so that pressure to a higher level and then remain at this level substantially constant until the end of region 20.

*

The impulse drying method and apparatus of the invention can effectively remove water from the web without the risk of web delamination as it exits the compression zone. The relatively long low pressure region after the actual compression area 25 ensures that the web expands in a controlled manner when released from the compression.

· '- The device according to the invention is also able to influence pulse properties of the dried web in various ways by adjusting the loading of the press shoe. The initial portion of the shoe is loaded with two successive loading forces, each of which is mutually resized to affect the heel of the shoe (Tilt) and thereby maximize the crushing pressure affecting the hydrostatic region in the next hydrodynamic region. the rest of the area.

In the following, the invention will be described in detail with reference to certain embodiments of the invention shown in the accompanying drawings, but the details are not intended to be limited thereto.

Figure 1 is a general view of an impulse drying apparatus according to the invention.

Figure 2 illustrates, on a larger scale, a section of the compression zone of an impulse drying apparatus.

Figure 2A shows an alternative embodiment of the press shoe geometry.

Fig. 3 shows the pressure distribution in the compression zone of the compression shoe of Fig. 2.

Figure 4 shows an alternative embodiment of the invention in which the rest of the press shoe is pivotally connected to the front of the shoe.

20

Figure 5 shows an alternative embodiment of the invention, with the rest of the press shoe

V

is flexible.

Figure 6 shows an alternative embodiment of the invention in which the position of the press shoe 25 is adjusted by a screw member or the like.

Figure 7 shows an alternative embodiment of a press shoe comprising two successive separate parts of a shoe.

Figure 8 shows an alternative embodiment of the invention in which the design of the rest of the press shoe differs from that shown in Figure 2.

6 105050

According to Figure 1, the impulse drying apparatus comprises a heated counter roll 20 and a press shoe device 16 between which an elongated compression zone is formed. The web 10 is passed through a compression zone supported by a press fabric 12, which also receives and conveys water pressed in a nip. The surface 5 of the counter roll 20 is heated by the induction heater 22 to a temperature above 150 ° C. Other heating methods known per se can also be used to heat the roll.

The clamping shoe device 16 comprises a clamping shoe, i.e. a long nipple shoe 18, the curved end face 25 of which is pressed against the heated surface 20 of the counter roll 10 by means of loading members. In this case, two successive rows of hydraulic cylinders 23a, 23b are used as loading members to provide forces on the press shoe 18 and Fg. The distribution of the compression pressure between the different parts of the shoe can be controlled by adjusting the heel. It is also possible to adjust the pressure profile in the transverse direction of the web 15 by adjusting the load on the individual cylinders of the cylinder rows 23a, 23b.

An endless conveyor belt 14 made of a resilient and impermeable material is arranged to pass over the press shoe 18, and lubrication is provided in the space between the belt 14 and the press shoe 18 to minimize slip friction. In the solution 20 of Figure 1, the press shoe device acts as a belt sheath roll 16 in which the tubular conveyor belt sheath 14 is rotated about a stationary roll axis. Other solutions known per se for supporting a press shoe and a conveyor belt can also be used as a press shoe device.

Figure 2 shows in greater detail the structure of the press shoe 18 and the press zone I-IV. As stated above, the press shoe is loaded with two load factors. in the row of cylinders 23a, 23b. On the back side of the shoe is a support member 27 which receives the horizontal forces caused by the rotation of the counter roll 20 by the press shoe 18. The curvature of the shoe front surface 25 is close to that of the counter roll 20, but the radii of curvature 30 are not the same. On the surface of the press shoe 18 there is a recess 24, preferably a row of partitions separated by a partition wall, which act as a hydrostatic chamber. At the bottom of each chamber 26 is a capillary channel 26 through which hydraulic oil is supplied to the chamber.

The compression zone of the impulse drying apparatus according to the invention can be divided into four functionally different zones I-IV. In the following, these areas will be described with reference to the racket in Figures 2 and 3, the latter showing the distribution of the compression pressure in the various operating zones of the shoe.

At the inlet side of the press nip there is first a hydrodynamic region I where the press pressure rises 10 quite linearly from the starting level to the actual nip pressure p j. In this region of the press shoe, the web 10 is compressed by pressure, its pores are filled with water, and water begins to flow from the web to the press fabric 12. At the same time, the web 10 contacts the heated surface of the counter roll 20 and heat transfer from the counter roll begins.

15

The actual nip region is formed by the hydrostatic region II of the press shoe 18, which contains a hydrostatic pressure chamber 24. The supply of pressurized hydraulic oil to the chamber 24 and the shoe loading forces FA and Fg together provide a substantially constant hydrostatic pressure p 1 throughout. Thanks to the thick oil film, area II is 20 frictionless.

Following the hydrostatic zone II is followed by a short hydrodynamic zone III of up to 30 mm in length, whereby the shape of the pressure curve can be further influenced, if necessary, by changing the relationship of the shoe forces, F 2 and FB. As the force Fg is increased relative to the force F1, a pressure peak pt is formed in the hydrodynamic region III, thereby improving the compression efficiency.

In regions II and III of high compression pressure, heat transfer from the heated counter roll 20 to the web 10 is efficient, whereupon the water in the web fiber is heated and evaporated. 30 Evaporation continues until a state of equilibrium humidity is reached. Due to the high pressure, not all of the water contained in the web is evaporated, but some of it remains in a liquid state and at a supercritical temperature.

Wet compression of the web generally involves the use of a long nip shoe, the compression zone 5 of which ends with a sharp drop in pressure at the end of zone II or III. In this case, the pressure should be released as quickly as possible to prevent the web from wetting again on the back of the shoe. However, the problem with impulse drying is not the wetting of the web but delamination resulting from too sudden pressure drop at the end of the compression zone.

10

In the impulse drying apparatus of the invention, the risk of web delamination is reduced by providing a relatively long hydrodynamic region IV after the actual high pressure compression zone I-III where the compression pressure is initially allowed to drop to the actual nip pressure p1 and then remain at this level Thus, immediately after the second hydrodynamic region III, the pressure is not allowed to drop to ambient pressure, but at the end of the compression zone is a portion which maintains a low compression pressure. The pressure P2 is slightly higher than the ambient pressure but still low enough to easily evaporate the water above 100 degrees Celsius. The expansion of the web as a result of sudden steam formation 20 is limited by the nearly parallel arcuate surfaces of the counter roll 20 and the press shoe 18.

«

Conventional long nipple shoes comprise only functional areas I-III which provide the main compression effect using relatively high compression pressures. In the solution of the invention, a considerable part of the water vaporization occurs in the final compression step IV, which is 1/3 to 2/3 of the entire length of the shoe, where the pressure is lower than the water vaporization pressure at that temperature but higher than ambient pressure.

Ψ-

In the arrangement of Fig. 2, the press shoe 18 is shaped such that its radius of curvature R1i is greater than the sum of the curvature radius Ry of the counter roll 20, the thickness of the press fabric 12 and the thickness of the conveyor belt 14. Thus, the distance between the 105050 face 25 of the press shoe 9 and the face of the counter roll 20 is at its smallest in the region of the shoe center, and the gap expands slightly toward the edges of the shoe. As the web advances in the region IV of the remainder of the compression zone, the compressed web 10 gradually expands while the water contained therein is evaporated. As the pressure drops in the region of 5 IV below the saturation pressure of the steam, the evaporation of water from the web increases, which increases the partial pressure of the vapor in the web and in the press fabric.

The operation of the press shoe in area IV is based on hydrodynamic lubrication. In the hydrodynamic lubrication system, the shape and relative motion of the sliding surfaces provides 10 liquid films with sufficient pressure to keep the moving surfaces apart. As the distance between the sliding surfaces decreases in the direction of movement, the hydrodynamic pressure increases wedge-like. As the spacing between the sliding surfaces increases in the direction of travel, the hydrodynamic pressure decreases and eventually becomes a vacuum. When the thickness of the oil well and the shape of the shoe, i.e. its radius of curvature relative to the curvature of the counter roll, are set, the desired pressure curve in region IV is obtained. The thickness of the oil film on the hydrodynamically lubricated surface is affected not only by the shape of the shoe, but also by the speed of movement of the surface and the viscosity of the oil.

For example, the thickness of the oil film in the hydrodynamic region I may be 0.3 to 0.6 mm and in the III region 0.1 to 0.3 mm, respectively. For example, at the origin of area IV, the thickness of the oil film may be 0.2 mm, and when the shoe radius R1 is greater than the radius of the reel Ry, the film thickness at the end of area IV is greater than 0.2 mm, preferably e.g. 0.4 mm. is the oil film between the press surface of the press shoe 25 and the conveyor belt 14, the greater pressure exerted through the conveyor belt 14 on the press fabric 12 and web 10. As the hydrodynamic pressure decreases, the vapor pressure IV as shown in Figure 3.

Fig. 2A shows an alternative solution for the geometry of the press surface of the press shoe. In this press shoe, the radius of curvature R 1 j of the shoe's front surface in regions I-III of the shoe is smaller than the corresponding radius of curvature Rj q of the rest of the shoe in region 10 105050 IV. Both the radii of curvature Rjq, Rjq are still larger than the radius of curvature Ry of the counter roll. The differently curved surfaces 25a, 25b of the front and the end of the shoe have a common tangent t at the point P where area III changes to area IV.

Figures 4-8 show other alternative solutions for providing the low pressure zone IV according to the invention and for adjusting the compression pressure in this zone. The pictures have been simplified by omitting the web, dryer fabric and conveyor belt.

In Figure 4, the clamping shoe 18 is comprised of two articulated parts 18a and 18b, the first portion 18a of which contains high pressure regions I-III and the second portion 18b of low pressure region IV. The pivot point 28 between the parts 18a and 18b may be accomplished not only by the actual pivot but also by suitable shoe design (e.g., thinning) or by the use of a resilient material in the shoe. The front portion 18a 15 of the shoe is subjected to forces and Fg and a separate loading force F1 is applied to the rest of the shoe 18b. The force F ^ is exerted, for example, by means of hydraulic cylinders (not shown), but other loading means may also be used as a screw (cf. Fig. 6). By adjusting the loading of the rest of the press shoe 18b, the distance between the press shoe 18b and the surface of the counter roll 20 can be affected, and thus the press pressure 20 in region IV.

In Fig. 5, the rest of the press shoe 18c is formed as a resilient structure by thinning it below the entire length of area IV. Also in this case, a loading force F 1 is applied to the rest of the shoe 18c which causes the remainder of the shoe 18c to deflect at a desired distance from the surface of the counter roll 20.

Also, in the solution of Fig. 6, the distance between the end of the press shoe 18 and the surface of the roll 20 is adjusted by the force F ^ applied to the end of the shoe. Since the structure of the shoe in this case is rigid, the loading force F ^ also affects the loading of the front part of the shoe, and thus the pressure in the areas of high compression pressure I-III is dependent on the resultant of all the loading forces F? In the case of Fig. 6, the load member is a screw 30 secured to an internal threaded hole in the bracket 29 of the support member 27 and whose clamping force can be adjusted from the outside of the belt shroud. When the structure of the shoe is not springy, it is of particular importance to achieve the desired pressure profile that the ratio of the radius of curvature of the press shoe 5 to the radius of curvature Ry of the press roll 5 is suitably selected such that R

In the solution of Fig. 7, the press shoe comprises two separate consecutive portions 18d 'and 18d', the first portion 18d 'of which contains the high pressure regions I-III and the second 10 portion 18d' contains the low pressure region IV. As usual, the first part 18d 'is loaded with forces and Fg and the second part 18d' is loaded independently with the forces Fqj and F ^ independently of the first part. This solution provides versatile possibilities for adjusting the pressure profile in the rest of the shoe 18d '.

Figure 8 shows an alternative solution in which the shape of the front surface 25 'in the region of the rest of the press shoe is different from that of the area of the front of the shoe (25'). This circumferential structure also achieves the object of the invention, that is, a long low and relatively uniform pressure zone IV after the high pressure pressure zones I-III. The clamping shoe is shaped such that at the beginning of the rest of the low pressure region IV, the distance between the counter roll 20 20 and the clamping shoe 18 expands stepwise from that spacing at the end of the region III. Towards the end of Area IV, the spacing slightly decreases and at the same time * the thickness of the oil film between the sliding surfaces decreases.

As in the other press shoes previously shown, the shoe of Figure 8 also supplies pressurized lubricating oil to the hydrostatic pressure chamber 24 through channel 26 to provide hydrostatic pressure in the pressure zone II and to lubricate the hydrodynamic sliding surfaces. In addition, and to ensure lubrication, low-pressure auxiliary oil is fed to the top of area IV via channel 32. As usual, the shoe is loaded with the forces F ^ and Fg.

30 105050 12

In the solution of Fig. 8, the thickness of the oil film between the press shoe 18 and the conveyor belt 14 may vary, for example, as follows: in region I, i.e., compression at the beginning of the zone 0.5 mm; hydrostatic area II following a hydrodynamic range ΠΙ 0.15 mm, low pressure area IV at the beginning 0.6 ... 2.0 mm and same area 5 IV at the end 0.3 ... 0.5 mm.

ψ yrs

Claims (12)

A method of impulse drying a paper or cardboard web, wherein the web (10) to be dried is supported by a press fabric (12) through an extended press zone (I-IV) formed between a heated counter roll (20) and a press shoe (18), in which press zone (I-IV) web (10) is pressed in several immediately sub-sequential stages in such a way that in the first stage the pressure is evenly raised in a first hydrodynamic region (I) at the beginning of the press zone (I). -IV) from the output level to the actual nip pressure (pj), in the second stage (II), the web (10) is evenly pressured (p ^) in the area of operation (II) by the hydrostatic pressure chambers (24) of the press cone (18) and in in the third stage, pressing is continued in a second short hydrodynamic region (III), which follows the hydrostatic region (II), using a compression pressure (pt) that is at least equal to the compression pressure (pj) used in the preceding second the stage (II), characterized in that in one the fourth sub-stage, following the three first-mentioned sub-stages 15 (I-III), the compression pressure in a third hydrodynamic region (IV) at the end of the press cone (18) is lowered to a level (P2) which is lower than the actual nip pressure (pj) but higher than the ambient pressure, and the compressive pressure is poured at this level and substantially unchanged to the end of the third hydrodynamic region (IV), the temporal extension of the last pressing stage (IV) constituting at least one third of the total temporal extent. of the entire press procedure (I-IV). Method according to claim 1, characterized in that in the third part stage (III) of the pressing process, the web is subjected to a pressure pulse (pt), the amplitude of which is greater than the actual nip pressure (pj). 25 3. A pulse drying device comprising a counter roll (20) and means (22) for heating its surface, a press fabric (12) for supporting the web (10), and a press shoe device (16) comprising a press shoe (18) and a a sliding belt (14) arranged to slide over the lubricated front surface (25) of the lubricating cone (18), and the lubricating cone (18) being arranged to be pressed against the surface of the heated counter roll (20) by means of at least two consecutive load means (23a, 23b), with which at least two separate loading forces (FA, Fg) are provided, and between the heatable counter roll (20) and the press cone (18), an extended press zone (I-IV) is formed which comprises in the running direction (10) first a first hydrodynamic region (I), then a region of operation (II) of a hydrostatic press chamber (24) and then a second short hydrodynamic region (III), characterized in that after the three first mentioned press zone regions (Ι-ΙΠ) follow a third hydrodynamic direction on (IV) constituting the latter part of the press cone (18), the length of this third hydrodynamic region (IV) being at least one third of the total length of the press zone (I-IV) and the shape and / or loading thereof being such that the pressure at this last region (IV) in the press zone first drops to a level (p £) 10 which is lower than the actual nip pressure (pj) but higher than the ambient pressure and is then kept at this level fairly unchanged substantially to the end of the region (IV). Pulse drying device according to claim 3, characterized in that the region (IV) with low pressure in the latter part of the press zone (I-IV) constitutes about 15 1/3 ... 2/3 of the total length of the press zone (I-4). IV). Impulse drying device according to claim 3 or 4, characterized in that the radius of curvature (R R; Rk1, Rjq) of the front surface (25) of the press cone (18) is greater than the sum of the radius of curvature (R And the thickness of the slip belt (14). Pulse drying device according to claim 5, characterized in that the radius of curvature (Rjq) of the front face of the press cone at the last region (IV) of the press zone is greater than the corresponding radius of curvature (Rjj) of the first three regions (I-III) and that the frontal surfaces (25a, 25b) of the first and second portions of the press cone have a common key (t) at the intersection point (P) of the regions (I-III, IV). . > 7. Impulse drying device according to any of claims 3-6, characterized in that an additional load force (F F) adjustable to its size is arranged to be directed to the later part of the press cone (18). 18 105050 Impulse drying device according to claim 7, characterized in that the press cone (18) is rigid in its construction, the load force directed at the latter part of the press cone (Fq> also influencing the total load of the previous part of the press cone. Pulse drying device according to claim 7, characterized in that the later part (18b) of the press cone is connected to the previous part (18a) of a press cone through a joint or elastic part (28), the distance between the counter roll (20) and the front surface (25) of the the area of the latter part of the shoe (18b) can be changed by controlling the load (Fq) of the latter part of the shoe. Impulse drying device according to claim 7, characterized in that the latter part (18c) of the press cone is elastic, whereby the distance between the counter roll (20) and the front surface (25) of the shoe in the region of the latter part (18b) can be changed by controlling the load ( F ^) of the latter part of the shoe. Impulse drying device according to any of claims 3-6, characterized in that the pressing shoe consists of two separate shoe parts (18d \ 18d '), of which the first shoe (18d') is loaded with first loading forces 20 (F Fg) and the second shoe (18d ”) are loaded with other loading forces (FC1> FC2> · • < Pulse drying device according to claim 3, characterized in that the lateral part of the press cone is designed such that the distance between its front surface (25 ") and the surface of the counter roll (20) is smaller at the end of the region (IV) at low pressure than at the beginning of it. and to this region (IV) on the front surface (25 ") of the shoe is measured at low pressure in addition to the oil which is customarily fed through the hydrostatic chamber (24) to the front of the press cone.