Process for operating a press section for forming a web

A process for operating a press section for producing a fibrous material web, with a nip that is elongated in the direction of web travel that includes an inlet zone, a main pressing zone, and an outlet zone, and where a pressure gradient of pressure p exerted in the main pressing zone of the nip is selected as a function of whether the fibrous material web entering in the nip has an average or high dry matter content, on the one hand, or a low dry matter content, on the other hand.

BACKGROUND INFORMATION 
1. Field of the Invention 
The invention relates to a press section of a machine for producing a 
fibrous material web, such as a paper and/or cardboard web, with at least 
one nip that is elongated in the direction of web travel and includes an 
inlet zone, a main pressing zone, and an outlet zone, where the fibrous 
material web is guided through the nip. 
2. Discussion of Background Information 
In press sections of this type, particularly where the fibrous material web 
to be treated has an average to high incoming dry matter content, 
compaction of the web surface due to excessive pressures frequently 
occurs. Steep pressure increases are particularly damaging in this regard. 
This results, among other things, in a lower pressure output in the drier 
section, and in a relatively low bond strength. Furthermore, this reduces 
the effectiveness of subsequent press processes. 
When the fibrous web to be processed has a low dry matter content, it also 
has low strength. Moreover, with an excessive pressure increase in the 
direction of web travel, undesirable compression of the web in the nip may 
result. A danger of this kind of compression exists particularly for 
fibrous web material having a surface weight measured in g/m.sup.2 in the 
high range. 
SUMMARY OF THE INVENTION 
To avoid these problems, the preset invention provides a press section with 
optimal de-watering capacity and which at the same time produces a fibrous 
web of as high a quality as possible. 
The present invention is premised on the fact that the pressure gradient 
dp/dl of the pressure p produced in the main zone of a nip extending a 
length l in a direction of web travel (where the pressure gradient occurs 
in the direction of web travel and therefore in the direction of the nip 
length l), is selected as a function of whether the fibrous material web 
entering the nip has an average to high dry matter content or a low dry 
matter content. The calculation for selecting the pressure gradient is 
performed once per type of web material. 
For an incoming fibrous web having average to high dry matter content, the 
pressure gradient dp/dl lies in the range of approximately 
0.01.cndot.p.sub.max /cm to approximately 0.08.cndot.p.sub.max /cm, and 
preferably has a value of approximately 0.04.cndot.p.sub.max /cm, where 
p.sub.max is a predeterminable maximum pressure at an end section of the 
main pressing zone. Predeterminable maximum pressure p.sub.max may have a 
value within the range of approximately 20 bar to 100 bar, where a value 
for p.sub.max of approximately 380 bar may be typical. 
For an incoming web having low dry matter content, the maximum pressure 
gradient (dp/dl).sub.max of pressure p to be produced in the inlet one 
and/or the main pressing zone (where the pressure gradient occurs in the 
direction of travel) is preferably selected so that it satisfies the 
relation: 
##EQU1## 
where p is the pressure of the main pressing zone, and FG is surface 
weight (in g/m.sup.2) of the incoming fibrous material web. Pressure p to 
be produced in the main zone may have a value of approximately 5 bar to 30 
bar. 
Particularly with the incoming fibrous web having low dry matter content, 
the maximum pressure gradient is advantageously selected as a function of 
the surface weight of the incoming fibrous material web. As a result, the 
maximum permissible pressure gradient is a function of both the dry matter 
content of the incoming fibrous material web, and the surface weight of 
the web. The dry matter content of the fibrous web is considered low when 
it is approximately 8% to 25%. 
According to the present invention, it is therefore possible to cover 
de-watering regions of the web in the press section that were previously 
reserved for the screen section. 
At an average to high dry matter content of the incoming fibrous web, the 
pressure gradient dp/dl of the pressure p to be produced in the inlet zone 
of the nip is selected so that it satisfies the relation: 
##EQU2## 
where p.sub.1 is the predeterminable pressure at the beginning of the main 
pressing zone, or the predeterminable pressure at the end of the inlet 
zone, and FG is the surface weight (in g/m.sup.2) of the incoming fibrous 
web. Pressure p to be produced in the inlet zone may have a value within 
the range of approximately 0.1 bar to 5 bar. Further, predeterminable 
pressure p.sub.1 at the end of the inlet zone may have a value within the 
range of approximately 1 bar to 5 bar, and predeterminable pressure 
p.sub.1 at the beginning of the main pressing zone may have a value of 
approximately 10 bar. 
It is preferred, with the incoming fibrous web having average to high dry 
matter content, that the pressure gradient dp/dl of the falling pressure p 
to be produced in the outlet zone of the (where the pressure gradient is 
produced in the direction of web travel) be selected so that it satisfies 
the relation: 
##EQU3## 
where p.sub.max is the predeterminable maximum pressure at the end of the 
main pressing zone, and FG is the surface weight (in g/m.sup.2) of the 
incoming fibrous web. Decreasing pressure p to be produced in the outlet 
zone may have a value within the range of about 80 bar to 0 bar. 
According to another embodiment of the invention, the predeterminable 
pressure p.sub.1 at the beginning of the main pressing zone is selected so 
that it satisfies the relation: 
##EQU4## 
where T is the dry matter content of the incoming fibrous web. Together 
with a pressure gradient selected in the manner described above, a 
pressure p.sub.1 that satisfies this relation leads on the whole to an 
optimal pressure profile. It takes into account that with a corresponding 
pressure gradient, the dry matter content of the fibrous material web 
along the length of the nip will increase due to de-watering, and that the 
chronological increase in pressure is determined by means of the pressure 
gradient. In particular, the desired increase in press pressure can be 
predetermined based on a corresponding shoe parameter (e.g., its length, 
shape, etc.). 
The elongated (or extended) nip is preferably positioned between a 
rotating, flexible press belt, which is guided via at least one press 
shoe, and a counter roll. In particular, the elongated nip can be embodied 
as a shoe press. 
The counter roll is preferably provided with holes, grooves, and/or 
suction. Consequently, a suitable lubricant an flow out of the perforated 
and/or grooved counter roll, and/or it can be suctioned out of the roll 
with a suction device. 
Particularly at low levels of force in the longitudinal nip press, for 
example in the region of 10 kN/m, and where the lubricant film thickness 
is relatively high, the region between the press shoe and the flexible 
press belt can be lubricated with water. As a result, it is possible 
according to the present invention to use the press shoe in a continuous 
loop that is one at its end faces, and to correspondingly use an open 
system in which the shoe length is relatively large, for example, markedly 
larger than 350 mm. 
According to another embodiment of the present invention, it is preferred 
to use a continuous loop formed by the rotating, flexible press belt at 
its two open end faces. 
The present invention provides a process for operating a press section for 
producing a fibrous web, including forming a nip that is elongated in a 
web travel direction, the nip including an inlet zone having an inlet 
beginning and an inlet end, a main pressing zone having a main beginning 
and a main end, and an outlet zone; determining a pressure to be applied 
in the main pressing zone; selecting a pressure gradient in the main 
pressing zone as a function of a dry matter content of the fibrous web 
entering the elongated nip, wherein the pressure gradient is related to 
the selected pressure to be applied in the main pressing zone, guiding the 
fibrous web in a web travel direction through the elongated nip; and 
forming the selected pressure gradient. 
According to another aspect of the invention, the selecting of the pressure 
gradient, where the dry matter content of the web entering the nip is in 
an average condition or a high condition, by determining a maximum 
pressure to be applied at the main end of the nip and selecting the 
pressure gradient from a range of approximately 0.04.cndot.p.sub.max /cm. 
A dry mater content of the web as it enters the elongated nip of 
approximately 20% to approximately 40% constitutes an average condition, 
and a dry mater content of approximately 30% to approximately 60% 
constitutes a high condition. 
Another aspect of the invention involves selecting an inlet pressure 
gradient to be produced in the inlet zone according to the relation 
##EQU5## 
where p.sub.1 is a pressure to be exerted at tone of the main beginning 
and the inlet end, and where FG is a surface weight (in g/m.sup.2) of the 
web entering the tip. 
Yet another aspect of the invention entails selecting an outlet pressure 
gradient to be produced in the outlet zone according to the relation 
##EQU6## 
where p.sub.max is a maximum pressure to be applied at the main end, and 
where FG is a surface weight (in g/m.sup.2) of the web entering the nip. 
Still another aspect of the invention involves determining a main beginning 
pressure (p.sub.1) in accordance with the relation 
##EQU7## 
where T is the dry matter content of the web entering the nip. Further, 
the forming of the nip may include positioning a rotating, flexible press 
belt, which is guided via a press shoe, against a counter roll. This may 
also involve suctioning the counter roll, and the counter roll also may 
have holes and/or grooves. Further, the present invention may involve 
lubricating a region between the press section and the flexible press belt 
with water. Moreover, the rotating, flexible belt comprises two ends 
faces, and wherein the belt is open at the end faces. 
According to another aspect of the invention, the selecting of the pressure 
gradient, when the dry content of the web entering the nip is in a low 
condition, by selecting a maximum main zone pressure gradient according to 
the relation 
##EQU8## 
where FG is a surface weight (in g/m.sup.2) of the web entering the nip. 
Moreover, the forming of the nip may comprise guiding a rotating, flexible 
press belt via a press show against a counter roll. This may also involve 
suctioning the counter roll, and the counter roll may have holes and/or 
grooves. Further, the present invention may involve lubricating a region 
between the press section and the flexible press belt with water. 
Moreover, the rotating, flexible belt comprises two ends faces, and 
wherein the belt is open at the end faces. A dry mater content of the web 
as it enters the elongated nip of approximately 8% to approximately 25% 
constitutes the low condition. 
According to another aspect of the invention, the selecting of the pressure 
gradient, when the dry content of the web entering the nip is in a low 
condition, by selecting a maximum inlet zone pressure gradient according 
to the relation 
##EQU9## 
where FG is a surface weight (in g/m.sup.2) of the web entering the nip. 
Moreover, the forming of the nip may comprise guiding a rotating, flexible 
press belt via a press shoe against a counter roll. This may also involve 
suctioning the counter roll, and the counter roll may have holes and/or 
grooves. Further, the present invention may involve lubricating a region 
between the press section and the flexible press belt with water. 
Moreover, the rotating, flexible belt comprises two ends faces, and 
wherein the belt is open at the end faces. A dry mater content of the web 
as it enters the elongated nip of approximately 8% to approximately 25% 
constitutes the low condition. 
The present invention also provides a press section of a machine for 
producing a fibrous material web, which includes: a nip that is elongated 
in a web travel direction that includes an inlet zone having an inlet 
beginning and an inlet end, a main pressing zone having a main beginning 
an a main end, and an outlet zone, where a pressure p is applied in the 
main pressing zone, where a pressure gradient in the main pressing zone is 
selected as a function of a dry matter content of the fibrous material web 
entering the elongated nip, where the pressure gradient is related to the 
pressure p applied in the main pressing zone, and where the fibrous web is 
guided in a web travel direction through the elongated nip to form the 
pressure gradient. Further, where the dry content of the web entering the 
nip is determined to be average or high, the maximum pressure to be 
applied at the main end of the nip is determined such that the pressure 
gradient has a value within a range of approximately 0.01.cndot.p.sub.max 
/cm to approximately 0.08.cndot.p.sub.max /cm, where p.sub.max represents 
the determined maximum pressure to be applied. It is preferred if the 
selected pressure gradient has a value of approximately 
0.04.cndot.p.sub.max /cm. 
According to another aspect of the invention, for an incoming web having 
average to high dry matter content, the inlet pressure gradient of this 
press section is selected for pressure produced in the inlet zone 
according to the relation 
##EQU10## 
where p.sub.1 is a pressure exerted at one of the main beginning and inlet 
end, and where FG is the surface weight (in g/m.sup.2) of the web entering 
the nip. Further, for an incoming web having an average to high dry matter 
content, an outlet pressure gradient is selected for pressure produced in 
the outlet zone according to the relation 
##EQU11## 
where p.sub.max is a maximum pressure applied at a main end and where FG 
is the surface weight (in g/m.sup.2) of the web entering the nip. 
According to another aspect of the invention, a main beginning pressure 
p.sub.1 is determined according to the relation 
##EQU12## 
where T is the dry matter content of the web entering the nip. Further, 
the press section may include a rotating, flexible press belt, which is 
guided via at least one press shoe, and a counter roll, where the 
elongated nip is positioned between the rotating, flexible press belt and 
the counter roll. Additionally, the counter may include holes and/or 
grooves, and a suction device may be associated with the counter roll. 
Moreover, a region between the press shoe and the flexible press belt that 
is lubricated with water. Further, the rotating, flexible belt further 
comprising two ends faces, and where in the belt is open at its end faces. 
According to another aspect of the invention, where the where the dry 
content of the web entering the nip is determined to be low, a pressure p 
is exerted in the inlet zone and/or the main zone, and the maximum 
pressure gradient is selected according to the relation 
##EQU13## 
where FG is the surface weight (in g/m.sup.2) of the web as it enters the 
nip. Further, this press section may also include a rotating, flexible 
press belt, which is guided via at least one press shoe, and a counter 
roll, wherein the elongated nip is positioned between the rotating, 
flexible press belt and the counter roll. the counter roll may include 
holes and/or grooves, and a suction device may be associated with the 
counter roll. A region between the press shoe and the flexible press belt 
may be lubricated with water. Moreover, the rotating, flexible belt may 
include two ends faces, where the rotating belt is open at its end faces. 
Further, the aforementioned and following characteristic features of the 
present invention can be used not only in the described combinations, but 
also in other combinations or alone, without departing from the scope of 
the invention. Further embodiments and advantages can be seen from the 
detailed description and the accompanying Figures.

DETAILED DESCRIPTION OF THE INVENTION 
The particulars shown herein are by way of example and for purposes of 
illustrative discussion of the preferred embodiments of the present 
invention only and are presented in the cause of providing what is 
believed to be the most useful and readily understood description of the 
principles and conceptual aspects of the invention. In this regard, no 
attempt is made to show structural details of the invention in more detail 
than is necessary for the fundamental understanding of the invention, the 
description taken with the drawing making apparent to those skilled in the 
art how the several forms of the invention may be embodied in practice. 
FIG. 1 depicts a graphical representation of a profile of pressure p over a 
length l of the nip. According to FIG. 1, the elongated nip includes an 
inlet zone a, a main pressing zone b, and an outlet zone c, and the 
pressure profile in each zone is graphically represented with profile 
section a, profile section b and profile section c. While the pressure p 
continuously increases in inlet zone a (profile section a) and in main 
pressing zone b (profile section b), it drops in outlet zone c (profile 
section c). 
A predeterminable pressure to be applied at the beginning of main pressing 
zone b is indicated as p.sub.l. At substantially the end of the main 
pressing zone b, a predeterminable maximum pressure p.sub.max is applied. 
The pressure gradient dp/dl of pressure p occurring in the main zone b of 
the nip (where the pressure gradient is produced in direction of web 
travel and therefore in the direction of length l) lies, according to the 
invention, in a range of approximately 0.01.cndot.p.sub.max /cm to 
approximately 0.08.cndot.p.sub.max /cm for a fibrous material web having 
an average to high dry matter content when entering the nip, and it 
preferably has a value of approximately 0.04.cndot.p.sub.max /cm. 
In contrast, for a fibrous web having low dry matter content when it enters 
the nip, the maximum pressure gradient (dp/dl).sub.max of pressure p 
produced in the inlet zone a and/or main processing zone b of the nip 
(where the pressure gradient occurs in the web travel direction) is 
selected so that it satisfies the relation: 
##EQU14## 
where p once again is the pressure in the main pressing zone b, and FG is 
the surface weight (in g/m.sup.2) of the incoming fibrous web. According 
to the present invention, the dry matter content of the fibrous material 
web is considered low when it is approximately 8% to approximately 25%. 
The pressure gradient dp/dl of the pressure p produced in inlet zone a of 
the nip, for an incoming fibrous web of average to high dry matter content 
(where the pressure gradient is produced in the direction of web travel) 
is selected so that it satisfies the relation: 
##EQU15## 
where p.sub.1 is the predeterminable pressure at the beginning of main 
pressing zone b, or the predeterminable pressure at the end of inlet zone 
a, and FG is the surface weight (in g/m.sup.2) of the incoming fibrous 
material web. 
For an incoming fibrous web having average to high dry matter content, the 
pressure gradient dp/dl of the decreasing pressure p in outlet zone c of 
the nip (where, again, the pressure gradient occurs in the direction of 
web travel) is selected so that it essentially satisfies the relation: 
##EQU16## 
where p.sub.max represents the predeterminable maximum pressure at the end 
of the main pressing zone b, and FG is the surface weight of the incoming 
fibrous web. Furthermore, the predeterminable pressure p.sub.1 at the 
beginning of main pressing zone b is preferably selected so that is 
satisfies the relation: 
##EQU17## 
where T is the dry matter content of the incoming fibrous material web. 
FIG. 2 shows, a rough approximation of the increase in the pressure profile 
where at least two press shoes I, II are successively positioned one after 
the other in the direction of web travel. The first press show (press shoe 
I) is designed for treating a fibrous web having an average or high 
incoming dry matter content, in the range of, for example, approximately 
25% to approximately 30%. In turn, press shoe II is designed to increase 
the dry matter content from approximately 35% to approximately 50%. The 
pressure gradient dp/dl of pressure p occurring in main pressing zone b of 
the nip (see also FIG. 1) in this example has a value of approximately 
0.04.cndot.p.sub.max /cm. In FIG. 2, the increase in the pressure profile 
in inlet region a also is indicated in a purely graphical form (see also 
FIG. 1). 
FIG. 3 shows a rough approximation of the increase of the pressure profile 
where three press shoes I, II, III are successively positioned one after 
the other in the direction of web travel, and where the first press shoe I 
is designed for treating a fibrous web with a low dry matter content of, 
for example, less than about 20%. In turn, press shoe II is designed to 
increase the dry matter content from approximately 32% to approximately 
40%, and press shoe III is designed to increase the dry matter content 
from approximately 40% to approximately 52%. The pressure gradient dp/dl 
of the pressure p produced in main pressing zone b of the nip (see also 
FIG. 1) is not permitted to exceed the previously indicated maximum 
pressure gradient (dp/dl).sub.max which has been selected as a function of 
pressure p.sub.max at the end of the main pressing zone, and the surface 
weight (in g/m.sup.2) of the incoming fibrous material web. In this 
example, the value of the pressure gradient dp/dl is 0.08.cndot.p.sub.max 
/cm where p.sub.max once again indicates the predeterminable pressure at 
the end of main pressing zone b. In the Figure, the increase in the 
pressure profile in inlet region a once again is indicated in a purely 
schematic form. 
FIGS. 2 and 3 indicate the length l of the nip (in cm) plotted on the 
abscissa and the pressure p (in bar) plotted on the ordinate. In 
particular, the respectively desirable increase in pressure can be 
predetermined based on a corresponding shoe parameter (e.g., its length, 
shape, etc.). 
It is noted that the foregoing examples have been provided merely for the 
purpose of explanation and are in no way to be construed as limiting of 
the present invention. While the invention has been described with 
reference to a preferred embodiment, it is understood that the words which 
have been used herein are words of description and illustration, rather 
than words of limitation. Changes may be made, within the purview of the 
appended claims, as presently stated and as amended, without departing 
from the scope and spirit of the invention in its aspects. Although the 
invention has been described herein with reference to particular materials 
and embodiments, the invention is not intended to be limited to the 
particulars disclosed herein; rather, the invention extends to a 
functionally equivalent structures, methods and uses, such as are within 
the scope of the appended claims.