WO2000024965A1

WO2000024965A1 - Method and device for impulse dewatering

Info

Publication number: WO2000024965A1
Application number: PCT/FI1999/000871
Authority: WO
Inventors: Antti Ilmarinen
Original assignee: Metso Paper, Inc.
Priority date: 1998-10-23
Filing date: 1999-10-20
Publication date: 2000-05-04
Also published as: FI982293A0; FI105050B; FI982293A

Abstract

The invention concerns a method and a device for impulse dewatering, in which method a paper or board web (10) is pressed in an extended press zone (I...IV) formed between a heatable backup roll (20) and a press shoe (18). Seen in the direction of progress of the web (10), the press zone (I...IV) first includes a first hydrodynamic area (I), which is followed by a hydrostatic area (II) and by a second, short hydrodynamic area (III). These three high-pressure areas (I...III) are followed in the press zone by a third hydrodynamic area (IV), which forms the final part of the press shoe (18). The length of this last area (IV) is at least one third of the total length of the press zone (I...IV), and its shape and/or loading is such that, in this area (IV) in the press zone, the pressure is first lowered to a level (p2) which is lower than the nip pressure (p1) proper but higher than the pressure in the environment, an the pressure remains then at this level substantially invariable substantially to the end of the area (IV).

Description

Method and device for impulse dewatering

The invention concerns a method for impulse dewatering of a paper or board web, in which method the web to be dewatered is passed on support of a press fabric through an extended press zone formed between a heatable backup roll and a press shoe, in which press zone pressing is applied to the web in a number of component stages directly following one after the other so that, in the first stage, the pressure is increased evenly in the first hydrodynamic area in the initial part of the press zone from the starting level to the nip pressure proper, in the second stage the web is pressed with a uniform pressure in the area of effect of the hydrostatic pressure chamber in the press shoe, and in the third stage the pressing is continued in the second, short hydrodynamic area following after the hydrostatic area by using a compression pressure which is at least equally high as the compression pressure used in the preceding second stage.

Further, the invention comprises a device for impulse dewatering, which comprises a backup roll and means for heating the face of the backup roll, a press fabric for supporting the web, and a press shoe device, which comprises a press shoe and a glide belt, which belt has been fitted to glide over the lubricated front face of the press shoe, and the press shoe has been fitted to be pressed against the face of the heatable backup roll by means of at least two successive sets of loading members, by whose means at least two separate loading forces are produced, and between the heatable backup roll and the press shoe an extended press zone has been formed, which comprises, in the direction of progress of the web, first a first hydrodynamic area, then an area of effect of a hydrostatic pressure chamber, and after that a second short hydrodynamic area.

Impulse dewatering is employed in the press section of a paper machine to intensify the draining of water out of the web. As an example of prior-art solutions, reference is made to the US Patent 4,324, 613, in which the paper web is passed on support of a press fabric between two press rolls, of which rolls the face of one roll has been heated to a high temperature. In the nip in the press, heat is transferred from the heated roll face to the paper web, in which connection the water contained in the web starts vaporizing. The pressure of the vapour that is formed forces water, which has been pressed into the intermediate spaces in the fibre mesh, into the press fabric placed at the opposite side of the web. At the same time, the high temperature lowers the viscosity and the surface tension of the water and alters the elastic properties of the fibre mesh and also thereby enhances the mechanical pressing of the water out of the web into the press fabric.

In the solution described above, the time of dwell in the roll nip remains short, and the effect of the high temperature cannot be utilized fully. This is why, frequently, as one of the press members, in stead of a press roll an extended-nip shoe is employed, by whose means the time of effect of the temperature and of the pressure upon the web can be extended. As an example of a typical shoe of an extended-nip press, reference is made to the applicant's FI Patent Application 925943, in which a press shoe has been described, whose face has a curve radius substantially equal to the curve form of the backup roll. By means of this geometry of the shoe, a triangular pressure curve is aimed at, in which the pressure is lowered rapidly after the nip. Such a shoe is poorly suited for use in impulse drying, in which a risk is delamination of the web as it departs from the press nip.

In impulse drying, typically a temperature range of 150...300 °C is used, and the pressure is, as a rule, in a range 0.5...8 MPa. The time of dwell of the web in the press nip is 15...100 ms. The heat that is transferred from the heated backup roll to the paper web raises the temperature of the web, whereat water attempts to vaporize in particular in the surface layers of the web. The high pressure present in the nip, however, restricts the vaporization of the water. When the web departs from the press nip, the pressure is lowered rapidly to the level of the ambient pressure, the water of a temperature higher than 100 degrees vaporizes abruptly, and the vapour that is formed expands explosively. This tends to result in delamination of the web, i.e. in disruption of the structure as the fibre mesh expands without control.

In order to prevent delamination, it has been suggested that the shoe in the press be arranged so that the compression pressure in the nip is lowered towards the end of the press zone, in which case the web is not subjected to an excessively rapid lowering of the pressure at the end of the pressing. For example, in the US Patent 5,047,122, a method for impulse drying is described in which attempts are made to prevent delamination of the pressed web so that the position of the press shoe in relation to the heated backup roll is regulated and that the form of the pressure curve in the press zone is thereby affected. The possibilities to control the form of the pressure curve are, however, limited, among other things, because the press shoe is loaded by means of one set of loading members only.

From the US Patent 5,071,513, a method for impulse drying is known in which the press zone comprises at least two hydrostatic press stages. In the first stage, a high compression pressure is used, in which connection the pores in the web are filled with water. In the last stage, a pressure is used that is lower than the pressure of saturated vapour at the temperature concerned but higher than the ambient tempera- ture. In such a case, the hot water contained in the pores in the web vaporizes in a controlled way, at the same time as the press zone restricts the expansion of the vapour and of the web. In this purely hydrostatic extended-nip shoe, the regulation of the nip pressure profile proper is complicated because of the number of separate rows of shoes, all of which have a combined loading piston which produces both the pressure that loads the shoe and the pressure necessary for lubrication of the face.

In the US Patent 5,302,252, a solution is described in which the press nip proper is followed by a long area of low compression pressure, in which the vapour pressure in the web is lowered slowly. This low-pressure area is produced by means of a penetrable metal band fitted between the web and the press fabric, which band presses the web against the backup roll. The magnitude of the pressure and the length of the pressure area can be affected by means of tensioning of the metal band and by means of a separate loading roll. The press comprises just one row of loading elements, for which reason the form of the nip pressure curve cannot be altered. By means of the tension of the metal band, in practice, just very low pressures can be produced. Further, a metal band circulation with a number of rolls and with the metal band is a solution of high cost.

The object of the present invention is to provide a method and a device suited for impulse dewatering of a paper or board web by whose means it is possible to avoid delamination of the web and, at the same time, to control the properties of the web efficiently.

In the method of impulse dewatering in accordance with the present invention, compression is applied to the web in a number of component stages directly following each other, of which stages the first and the third stage are hydrodynamic, and the second stage is hydrostatic. The invention is characterized in that in the fourth component stage of pressing, which follows after the three component stages of pressing mentioned above, in the hydrodynamic area in the final part of the press shoe, the compression pressure is lowered to a level, which is lower than the nip pressure proper but higher than the pressure in the environment, and the com- pression pressure is maintained at this level and substantially invariable to the end of the third hydrodynamic area, the duration in time of said last pressing stage being at least one third of the total duration in time of the whole pressing.

The device for impulse dewatering in accordance with the present invention com- prises a press shoe whose press zone includes a hydrostatic area which is preceded and followed by short hydrodynamic areas. The invention is characterized in that the three areas of the press zone mentioned above are followed by a third hydrodynamic area, which forms the final part of the press shoe and whose length is at least one third of the total length of the press zone and whose shape and/or loading has/have been fitted such that, in this last area of the press zone, the pressure is initially lowered to a level that is lower than the nip pressure proper but higher than the pressure in the environment, and the pressure is then maintained at this level substantially invariable substantially to the end of the area.

By means of the method and the device for impulse dewatering in accordance with the present invention, water can be drained out of the web efficiently without risk of delamination of the web as the web departs from the press zone. The relatively long area of low pressure after the press zone proper secures that the web expands in a controlled way when it becomes free from compression.

By means of the device in accordance with the invention, it is also possible to affect the properties of the impulse-dewatered web in a versatile way by means of regulation of the loading of the press shoe. The initial part of the shoe is loaded with two successive loading forces, and by means of alteration of the relative magnitudes of these forces, it is possible to affect the tilt of the shoe and, thereby, the maximal compression pressure which is effective in the hydrodynamic area following after the hydrostatic area. Further, in a number of embodiments of the invention, it is also possible to regulate the magnitude of the pressure in the area of the final part of the shoe separately.

In the following, the invention will be described in detail with reference to some embodiments of the invention illustrated in the accompanying drawings, the invention being, however, not supposed to be confined to the details of said embodiments.

Figure 1 is a general view of an impulse dewatering device in accordance with the invention.

Figure 2A is a sectional view in an enlarged scale of the press zone of the impulse dewatering device.

Figure 2B illustrates an alternative embodiment of the geometry of the press shoe. Figure 3 illustrates the distribution of the pressure in the press zone of a press shoe as shown in Fig. 2A.

Figure 4 shows an alternative embodiment of the invention, in which the final part of the press shoe is connected with the initial part of the shoe by means of an articulated zone.

Figure 5 shows an alternative embodiment, in which the final part of the press shoe is resilient.

Figure 6 shows an alternative embodiment of the invention, in which the position of the press shoe is regulated by means of a screw member or equivalent.

Figure 7 shows an alternative embodiment of the invention in which the shoe consists of two separate parts fitted one after the other.

Figure 8 shows an alternative embodiment of the invention, in which the shape of the final part of the press shoe differs from that shown in Fig. 2A.

As is shown in Fig. 1, the impulse dewatering device comprises a heatable backup roll 20 and a press shoe device 16, between which an extended press zone is formed. The web 10 is passed through the press zone on support of a press fabric 12, which press fabric also receives and carries away the water that was pressed out of the web in the nip. The face of the backup roll 20 is heated by means of an induction heater 22 to a temperature higher than 150 °C. For heating the roll, it is also possible to use other heating methods in themselves known.

The press shoe device 16 comprises a press shoe, i.e. an extended-nip shoe 18, whose curved front face 25 is pressed, in the way illustrated in Fig. 2A, by means of loading members against the heated face of the backup roll 20. In this case, the loading members that are used are two successive rows of hydraulic cylinders 23a, 23b, by whose means forces F_A and F_B that load the press shoe 18 are pro- duced. The distribution of the compression pressure between different parts of the shoe can be affected by regulating the tilt of the shoe, i.e. the relative magnitude of the forces F_A and F_B. Also, regulation of the pressure profile in the cross direction of the web is possible so that the loading of the individual cylinders in the rows of cylinders 23a, 23b is regulated.

An endless glide belt 14 made of a resilient and non-permeable material has been fitted to run over the press shoe 18. In the space between the belt 14 and the press shoe 18, lubrication has been arranged in order to minimize the glide friction. In the solution as shown in Fig. 1, the press shoe device consists of a belt mantle roll 16, in which a hose-like glide belt mantle 14 has been fitted to revolve around a stationary roll axle. As the press shoe device, it is also possible to use other solutions in themselves known so as to support the press shoe and the glide belt.

Fig. 2 A is a more detailed illustration of the construction of the press shoe 18 and of the press zone I... IV. As was already stated above, the press shoe is loaded by means of two rows of loading cylinders 23a,23b. At the outlet side of the shoe, there is a support member 27, which receives the horizontal forces applied by the rotation of the backup roll 20 to the press shoe 18. The curve form of the front face 25 of the shoe is close to the curve form of the face of the backup roll 20, but the curve radii are, however, not equal. On the face of the press shoe 18, in the initial half of the shoe, there is a recess 24, favourably a row of recesses 24 separated from each other by means of a partition wall, which recesses operate as a hydrostatic chamber. In the bottom of each chamber 26, there is a capillary duct 26, through which hydraulic fluid is passed into the chamber.

The press zone in the impulse dewatering device in accordance with the invention can be divided into four operationally different areas I... IV. In the following, these areas will be described with reference to Figs. 2A and 3, the distribution of the compression pressure in the different operational zones of the shoe being illustrated in the latter one of said figures. At the inlet side of the press nip, there is first a hydrodynamic area I, in which the compression pressure rises along a rather linear path from the starting level to the nip pressure pi proper. In this area of the press shoe, the web 10 is compressed by the effect of the pressure, its pores are filled with water, and water starts flowing out of the web into the press fabric 12. At the same time, the web 10 reaches contact with the heated face of the backup roll 20, and transfer of heat from the backup, roll to the web starts.

The nip area proper is formed in the hydrostatic area II of the press shoe 18, which area includes a hydrostatic pressure chamber 24. The feed of pressurized hydraulic fluid into the chamber 24 and the loading forces F_A and F_B of the shoe together produce a hydrostatic pressure p_{l 5} which remains substantially invariable in the whole area II. Owing to the thick fluid film, the area II is free from friction.

The hydrostatic area II is followed by a short hydrodynamic area III, whose length is at the maximum 30 mm and in which the shape of the pressure curve can, if necessary, still be affected by varying the tilt of the shoe, i.e. the relative proportion of the loading forces F_A and F_B. When the force F_B is increased in relation to the force F_A, a pressure peak p_t can be produced in the hydrodynamic area III, in which connection the pressing efficiency is increased.

In the areas II and III of high compression pressure, the transfer of heat from the heated backup roll 20 to the web 10 is efficient, in which connection the water present in the fibre mesh in the web is heated and vaporizes. The vaporization goes on until a state is reached in which a moisture of equilibrium is present. Owing to the high pressure, all the water contained in the web does not vaporize, but part of it remains in liquid form and at a supercritical temperature.

In wet pressing of a web, as a rule, an extended-nip shoe is employed whose press zone ends with an abrupt pressure drop at the end of the area II or III. In such a case, the pressure must be lowered as rapidly as possible in order that rewetting of the web could be avoided at the outlet side of the shoe. However, in impulse dewatering, rewetting of the web is not a problem, but delamination of the web, which follows from an excessively rapid loss of pressure at the end of the press zone.

In the impulse dewatering device in accordance with the invention, the risk of delamination of the web is reduced by, after the high-pressure press area I... Ill proper, providing a further, relatively long hydrodynamic area IV, in which the compression pressure is initially allowed to go down to a level p₂ lower than the nip pressure pi proper, and the pressure is then kept at this level substantially invariable up to the end of the area IV. Thus, right after the second hydrodynamic area III, the pressure is not allowed to go down to the level of the pressure in the environment, but in the end of the press zone there is a part in which a low compression pressure is maintained. The pressure p₂ is somewhat higher than the pressure in the environment, but, however, to such an extent low that the water at a temperature higher than 100 degrees vaporizes readily. Expansion of the web as a result of sudden formation of vapour is restricted by the almost parallel arcuate faces of the backup roll 20 and of the press shoe 18.

A conventional extended-nip shoe comprises the operational areas I... Ill only, by whose means the principal compression effect is produced while employing relatively high compression pressures. In the solution in accordance with the present invention, a considerable proportion of the evaporation of water takes place in the last press stage IV, whose length is 1/3...2/3 of the entire length of the shoe and in which the pressure is lower than the vaporizing pressure of water at the temperature concerned but higher than the pressure in the environment.

In the solution as shown in Fig. 2A, the press shoe 18 is shaped so that the curve radius R_κ of its front face 25 is larger than the sum of the curve radius Ry of the backup roll 20, of the thickness of the press fabric 12 and of the thickness of the glide belt 14. Thus, the distance between the front face 25 of the press shoe and the face of the backup roll 20 is shortest in the area of the middle portion of the shoe, and the gap becomes just a little wider towards the edges of the shoe. When the web proceeds in the area IV of the final part of the press zone, the web 10 compressed in the nip expands gradually, at the same time as the water contained in the web vaporizes. When the pressure becomes lower in the area IV to a level lower than the saturation pressure of the vapour, vaporization of the water out of the web is intensified, which increases the component pressure of vapour in the web and in the press fabric.

The operation of the press shoe in the area IV is based on hydrodynamic lubrication. In a hydrodynamic system of lubrication, the shape and the relative movement of the glide faces produces a fluid film, whose pressure is sufficient to keep the moving faces apart from one another. When the gap between the glide faces becomes smaller in the direction of progress of the movement, the hydrodynamic pressure becomes higher in wedge shape. When the gap between the glide faces becomes larger in the direction of movement, the hydrodynamic pressure becomes lower and is finally converted to a vacuum. When the thickness of the fluid film and the shape of the shoe, i.e. its curve radius in relation to the curve form of the backup roll, have been set appropriately, the desired pressure curve is obtained in the area IV. Besides by the shape of the shoe, the thickness of the fluid film on a face with hydrodynamic lubrication is also affected by the speed of movement of the face and by the viscosity of the fluid.

The thickness of the fluid film in the hydrodynamic area I can be, for example, 0.3...0.6 mm, and in the area III similarly 0.1...0.3 mm. At the initial point of the area IV, the thickness of the fluid film can be, for example, 0.2 mm and, when the radius R_κ of the shoe is larger than the radius of the backup roll Ry, the thickness of the film at the final point of the area IV is larger than 0.2 mm, favourably, for example, 0.4 mm. Generally speaking, the thinner the fluid film is between the front face 25 of the press shoe and the glide belt 14, the higher is the pressure that is applied through the glide belt 14 to the press fabric 12 and to the web 10. When the hydrodynamic pressure becomes lower, the vapour pressure is, at the same time, increased at the opposite side of the glide belt 14 in the web 10 and in the press fabric 12, in which case the compression pressure p₂ as a whole remains substantially invariable in the area IV, as is illustrated in Fig. 3.

Fig. 2B shows an alternative solution for the geometry of the front face of the press shoe. In this press shoe, the curve radius R_K1 of the front face of the shoe in the initial part of the shoe in the areas I... Ill is smaller than the corresponding curve radius R_K2 in the final part of the shoe in the area IV. Both curve radii Rκι_>Rκ2 ^are still larger than the curve radius R_v of the backup roll. The faces 25a, 25b with different curve forms in the initial part and final part of the shoe have a common tangent t at the point P, at which the area III is converted to the area IV.

Figs. 4...8 illustrate further alternative solutions for formation of the low-pressure zone IV in accordance with the invention and for regulation of the compression pressure in this zone. The illustrations have been simplified by omitting the web, the drying fabric, and the glide belt in the illustrations.

In Fig. 4, the press shoe 18 is composed of two parts 18a and 18b interconnected by means of an articulated zone, of which parts the first part 18a comprises the areas I... Ill with high compression pressure, and the second part 18b comprises the area IV with low pressure. The articulation point 28 between the parts 18a and 18b can be accomplished, besides by means of an articulated joint proper, also by means of appropriate design of the shoe (e.g. a thinner portion) or by using a resilient material in the shoe. The initial part 18a of the shoe is loaded with the forces F_A and F_B, and a separate loading force F is applied to the final part 18b of the shoe. The force F_c is produced, for example, by means of hydraulic cylinders (not shown), but other loading members can also be used, such as a screw (cf. Fig. 6). When the loading of the final part 18b of the press shoe is regulated, it is possible to affect the distance between the press shoe 18b and the face of the backup roll 20 and, thereby, the compression pressure in the area IV.

In Fig. 5, the final part 18c of the press shoe has been made into a resilient construction by making it thinner at the bottom side over the entire length of the area IV. Also in this case, a loading force F_c is applied to the final part 18c of the shoe, which force makes the final part 18c of the shoe to be bent to the desired distance from the face of the backup roll 20.

Also in the solution as shown in Fig. 6, the distance between the final part of the press shoe 18 and the face of the backup roll 20 is regulated by means of a force F_c applied to the final part of the shoe. Since the construction of the shoe is rigid in this case, the loading force F also acts upon the loading of the initial part of the shoe at the same time, and, thus, the pressure in the areas I... Ill with high compression pressure depends on the resultant of all the loading forces F_A, F_B and F_c applied to the shoe. In the case of Fig. 6, a screw 30 operates as a loading member, which screw has been attached to a hole fitted in a bracket 29 of the support member 27 and provided with inside threading, and the tightening force of the screw can be adjusted from outside the belt mantle roll. Since the construction of the shoe is not resilient, in view of obtaining the desired pressure profile it is particularly important that the ratio of the curve radius R_κ of the press shoe 18 to the curve radius R_v of the backup roll has been chosen appropriately so that R_κ > R_v.

In the solution as shown in Fig. 7, the press shoe comprises two separate parts 18d' and 18d" fitted one after the other, of which parts the first part 18d' comprises the areas I... Ill with high pressure, and the second part 18d" comprises the area IV of low pressure. The first part 18d' is loaded in the customary way with the forces F_A and F_B, and the second part 18d" is again loaded, independently from the first part, with the forces F_C1 and F_C2. This solution provides versatile possibilities for regula- tion of the pressure profile in the area of the final part 18d" of the shoe.

Fig. 8 shows an alternative solution in which, in the area of the final part of the press shoe, the shape of the front face 25" is different from that in the area (25') of the initial part of the shoe. Also by means of this shoe construction, it is possible to achieve the objective of the invention, i.e. a long area IV of low and relatively invariable pressure after the areas I... Ill with high compression pressure. The press shoe has been shaped so that in its final part, in the beginning of the area IV of low pressure, the distance between the backup roll 20 and the press shoe 18 becomes stepwise wider from what this gap is in the end of the area III. Towards the end of the area IV, the gap becomes slightly narrower and, at the same time, the thickness of the fluid film between the glide faces becomes thinner.

As is the case in the press shoes illustrated above, also in the case of the shoe shown in Fig. 8 pressurized lubrication fluid is fed through a duct 26 into the hydrostatic pressure chamber 24 so as to produce a hydrostatic pressure in the area II of the press zone and in order to lubricate the hydrodynamic glide faces. In addition to this, and in order to secure the lubrication, to the beginning of the area IV, low- pressure additional fluid is fed through the duct 32. The shoe is loaded in the customary way with the forces F_A and F_B.

In the solution as shown in Fig. 8, the thickness of the fluid film present between the press shoe 18 and the glide belt 14 can vary, for example, as follows: in the area I, i.e. in the beginning of the press zone, 0.5 mm; in the hydrodynamic area III following after the hydrostatic area II, 0.15 mm, in the beginning of the area IV of low pressure, 0.6...2.0 mm, and in the end of the same area IV, 0.3...0.5 mm.

Claims

1. A method for impulse dewatering of a paper or board web, in which method the web (10) to be dewatered is passed on support of a press fabric (12) through an extended press zone (I...IV) formed between a heatable backup roll (20) and a press shoe (18), in which press zone (I... IV) pressing is applied to the web (10) in a number of component stages directly following one after the other so that, in the first stage, the pressure is increased evenly in the first hydrodynamic area (I) in the initial part of the press zone (I... IN) from the starting level to the nip pressure (p_t) proper, in the second stage the web (10) is pressed with a uniform pressure (p_j) in the area of effect (II) of the hydrostatic pressure chamber (24) in the press shoe (18), and in the third stage the pressing is continued in the second, short hydrodynamic area (III) following after the hydrostatic area (II) by using a compression pressure (p_t) which is at least equally high as the compression pressure (p ) used in the preceding second stage (II), characterized in that in the fourth component stage of pressing, which follows after the three component stages (I... Ill) of pressing mentioned above, in the third hydrodynamic area (IV) in the final part of the press shoe (18), the compression pressure is lowered to a level (p₂), which is lower than the nip pressure (p^ proper but higher than the pressure in the environment, and the com- pression pressure is maintained at this level and substantially invariable to the end of the third hydrodynamic area (IV), the duration in time of said last pressing stage (IV) being at least one third of the total duration in time of the whole pressing (I... IV).

2. A method as claimed in claim 1, characterized in that, in the third component stage (III) of pressing, a pressure pulse (p_t) is applied to the web, whose amplitude is higher than the nip pressure (pi) proper.

3. A device for impulse dewatering, which comprises a backup roll (20) and means (22) for heating the face of the backup roll, a press fabric (12) for supporting the web (10), and a press shoe device (16), which comprises a press shoe (18) and a glide belt (14), which belt has been fitted to glide over the lubricated front face (25) of the press shoe (18), and the press shoe (18) has been fitted to be pressed against the face of the heatable backup roll (20) by means of at least two successive sets of loading members (23a,23b), by whose means at least two separate loading forces (F_A,F_B) are produced, and between the heatable backup roll (20) and the press shoe (18) an extended press zone (I... IN) has been formed, which comprises, in the direction of progress of the web (10), first a first hydrodynamic area (I), then an area of effect (II) of a hydrostatic pressure chamber (24), and after that a second short hydrodynamic area (III), characterized in that the three areas (I... Ill) of the press zone mentioned above are followed by a third hydrodynamic area (IV), which forms the final part of the press shoe (18) and whose length is at least one third of the total length of the press zone (I... IN) and whose shape and/or loading has/have been fitted such that, in this last area (IN) of the press zone, the pressure is initially lowered to a level (p₂) that is lower than the nip pressure (p_j) proper but higher than the pressure in the environment, and the pressure (p₂) is then maintained at this level substantially invariable substantially to the end of the area (IN).

4. A device for impulse dewatering as claimed in claim 3, characterized in that the area (IN) of low pressure placed towards the end of the press zone (I... IN) constitutes about 1/3...2/3 of the total length of the press zone (I... IN).

5. A device for impulse dewatering as claimed in claim 3 or 4, characterized in that the curve radius (Rκ>Rκi_'^K₂) °f ^me fr°^nt f^ace (25) of the press shoe (18) is larger than the sum of the curve radius (R_v) of the heatable backup roll (20), of the thickness of the press fabric (12) and of the thickness of the glide belt (14).

6. A device for impulse dewatering as claimed in claim 5, characterized in that the curve radius

^{or me} front face of the press shoe in the last area (IN) of the press zone is larger than the corresponding curve radius (RJ ) in the area (I... Ill) of the three first parts of the press zones, and that the curved front faces (25a,25b) of the initial part and the final part of the shoe have a common tangent (t) at the boundary point (P) between the areas (I... Ill, IN).

7. A device for impulse dewatering as claimed in any of the claims 3 to 6, characterized in that an additional loading force (F_c) of adjustable magnitude has been fitted to be applied to the area of the final part of the press shoe (18).

8. A device for impulse dewatering as claimed in claim 7, characterized in that the construction of the press shoe (18) is rigid, in which case the loading force. (F_c) applied to the final part of the shoe also affects the overall loading of the initial part of the shoe.

9. A device for impulse dewatering as claimed in claim 7, characterized in that the final part (18b) of the press shoe is connected with the initial part (18a) of the shoe by the intermediate of an articulated joint or of a resilient portion (28), in which connection the distance between the backup roll (20) and the front face (25) of the shoe in the area of the final part (18b) of the shoe can be altered by regulating the loading (F_c) of the final part of the shoe.

10. A device for impulse dewatering as claimed in claim 7, characterized in that the final part (18c) of the press shoe has been formed resilient, in which connection the distance between the backup roll (20) and the front face (25) of the shoe in the area of the final part of the shoe can be altered by regulating the loading (F_c) applied to the final part (18c) of the shoe.

11. A device for impulse dewatering as claimed in any of the claims 3 to 6, characterized in that the press shoe consists of two separate press shoes (18d\ 18d") placed one after the other, of which shoes the first shoe (18d') is loaded with first loading forces (F_A,F_B) and the second shoe (18d") is loaded with second loading forces (F ~i ,F_C2).

12. A device for impulse dewatering as claimed in claim 3, characterized in that the final part of the press shoe has been shaped so that the distance of its front face

(25") from the face of the backup roll (20) is smaller in the end of the area (IN) of low pressure than in the beginning of said area, and that in said area (IN) additional fluid of low pressure is fed to the front face (25") of the shoe, in addition to the fluid that is fed customarily through the hydrostatic pressure chamber (24) to the area of the initial part of the press shoe.