Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~701~
90--311
PRI~S~ 8E:CTION OF A PAP13R OR
PAPE:RBOARD l~aRING MaC~IN~
R7~CRI~O~ND OF THI~ lN V~ lON
The invention concerns a press section of a paper or
paperboard making machine, comprising a number of rolls which form
press nips that dewater the web, the web being arranged to run
through these nips, and of which rolls at least one roll is a
smooth-faced so-called center roll, which forms a press nip with at
least one other press roll, over which a press felt, which has been
formed as an endless loop, is passed to absorb water from the web.
In a paper making machine, out of fiber pulp, a web is
formed in the former of the paper machine, whereupon the formed web
is passed, being supported and carried by one or more felts in the
paper making machine, into the press section of the paper making
machine, wherein both the web and the felts that support it are
passed through nips formed by the rolls in the press section to
absorb water from the web into the felts. From the press section the
web is passed into the drying section of the paper machine. A
conventional construction in a press section comprises a large and
massive center roll as well as wire or felt loops grouped around it,
the rolls placed inside these felt loops forming press nips either
with one another or together with the center roll, and when the web
runs through said press nips, water is drained out of the web by the
effect of compression, this water being absorbed into the felts. In
the drying section, water is removed from the web by means of
evaporation, which is highly energy-consuming and therefore expensive
and uneconomical. This is why attempts are made to remove a maximal
proportion of water out of the web before it reaches the drying
section, in the press section, by mechanical means.
` 2~70~0
It is known from the prior art that water is removed out of
a web considerably more readily at an elevated temperature, because
the viscosity of water and the springback coefficient of the web are
thereby lowered together with the surface tension. Owing to this
phenomenon, it has been previously found desirable to raise the
temperature of the web in the press section. Based on this earlier
experience, it can be established that, e.g., an increase in the
temperature by 6...10C in the press section produces an increase of
an order of 1% or more in the dry solids content of the web. An
increased dry solids content in the press section produces
considerable cost economies. For example, in paper making machines a
rule of thumb is that, if the moisture content in the web in the
press section can be lowered by 1%, the consumption of steam in the
drying section is lowered by about 5%.
One drawback in the press sections which have been
heretofore commonly used relates to the center roll in the press
section. Generally, some suitable rock, such as granite, is used for
the center roll. As is well known, rock rolls are quite sensitive to
large and sudden changes in temperature, and the effects of such
changes may be so severe as to crack the roll. This is why attempts
have been made to develop suitable substitutes for granite rolls. As
substitutes for rock rolls, e.g., rolls have been used which are
coated, e.g., with a mixture of polyurethane and rock dust to make
the surface properties of the roll similar to those of a rock roll.
Advantages of metal rolls compared with rock rolls include their
considerably better ability to tolerate variations in temperature.
Moreover, owing to this phenomenon, they can be run at considerably
higher temperatures than rock rolls. Moreover, a metal roll can be
run at considerably higher running speeds than rock rolls.
3 ~170~0
A conventional construction of the press section of a paper
machine wherein a center roll and a plurality of press rolls grouped
around it are employed constitutes three press nips. In such a
construction, the first press nip is formed between a grooved roll
and a press-suction roll. In this construction, the second press nip
is formed between a press-suction roll and the center roll, and a
third press nip is formed between the center roll and a second
grooved roll. Since, in the nips in the press section, it must be
possible to make the linear loads as uniform as possible, in such a
structure, as a rule, the grooved rolls are variable-crown rolls,
preferably rolls whose crowns are variable in zones thereof. Thus,
owing to the crown variation of the grooved rolls, in the first nip
and in the third nip in the press section, a uniform linear load is
achieved. In order that a linear load as uniform as possible can
also be obtained for the second press nip, the mantles of the
press-section roll and the center roll, which form the second press
nip, are generally cambered. Because of the camber, a uniform linear
load is never obtained for a nip and it is a further drawback of the
cambering that the camber is always "fixed". If the camber has to be
changed, the roll must be subjected to a grinding operation. This is
a costly and laborious procedure. Also, cambering alone does not
render the linear load profile subject to full control. For example,
a problem with the metallic center rolls presently in use has been
uneven heating. This has caused distortions in the linear load
profile. Since the grooved rolls are provided with crown-variation
means, the grooved rolls have been highly expensive. In this
respect, the high cost has also been contributed to by the fact that
it has been difficult to fit the crown-variation means inside a
grooved roll, because the diameters of the grooved rolls are
'l 2~7010
relatively small. When a fourth, separate press nip has been added
to such a press section, a variable-crown grooved roll has also been
used to achieve the fourth press nip.
Thus, to summarize the drawbacks of the prior art, they
include high cost of construction, the aforementioned problems of
uneven temperature related to metallic rolls, as well as the
difficulties in providing uniform and, if necessary, adjustable
profiles of linear loads in the press nips.
8UMMARY OF T~ lNvKN~lON
The principal object of the present invention is to provide
a press section by means of which the drawbacks described above are
minimized and by means of which an essential improvement is obtained
with respect to increased dry solids content and increased running
speed of a web passing through a paper making machine. A further
object is to provide a structure to substitute for the rock rolls
employed in press sections. With a view to achieving these
ob~ectives, the solution in accordance with the invention is mainly
characterized in that the center roll is a variable-crown roll, which
comprises a metallic roll mantle arranged so as to revolve around a
stationary roll axle as well as at least one set of crown-variation
means, which are arranged to load the roll mantle in the direction of
the nip plane of the center roll and the roll that forms a press nip
with the center roll so as to regulate the linear load profile in the
nip.
As compared with the prior art, by means of the invention, a
number of advantages are obtained, some of which have been heretofore
discussed. With regard to the invention's advantages, the following
can also be mentioned. First, in accordance with the invention, if
necessary or desirable, two variable-crown rolls can be replaced by
5 2g~7~
one variable-crown roll. Use of this option results in considerable
economical savings. Since the crown-variation means are used
exclusively in connection with the center roll, it is considerably
easier to construct these members in the interior of the roll,
because the available space is larger. With the use of the present
invention, if desirable, it is also possible to control the profile
of linear loads in the nip between a press-section roll and the
center roll.
Moreover, by means of the structure in accordance with the
invention, problems related to the uneven heating of the prior-art
metallic center rolls are avoided. When the structure of the
invention is used, the mantle of the center roll can be made thinner
than in the prior art, whereby it has an improved thermal
conductivity. If desired, the roll can also be used for heating the
web. Nor is it necessary to grind any camber on a roll to achieve
the press section structure in accordance with the invention.
An additional advantage obtainable by means of the invention
is related to the construction of the grooved rolls that are
presently used. Currently, the grooved rolls are manufactured such
that they are provided with a suitable coating, e.g., of
polyurethane. This results in their having a limited ability to
tolerate heat. Variable-crown rolls, and in particular rolls whose
crowns are adjustable in zones thereof, however, develop a
considerable amount of heat. Since, when a structure in accordance
with the invention is used, the grooved rolls therein do not have to
be variable-crown rolls, they can be provided with cooling in a
simple manner. An additional advantage is that the invention can be
utilized as an addition to existing press sections. Other advantages
of the invention are explained hereinafter in the detailed
description of the invention.
( 2~17010
BRI~F D~CRIPTION OF T~ DRAWING~
In the following, the invention will be described in detail
with reference to the Figures in the accompanying drawings.
Figure 1 is a schematical side view of the press section of
a paper or paperboard making machine wherein a preferred embodiment
of the invention is applied.
Figure 2 is a corresponding view of an alternative preferred
embodiment to the structure shown in Fig. 1.
Figure 3 is a corresponding view of a further preferred
alternative embodiment of the structures shown in Figs 1 and 2.
Figure 4 shows yet another preferred alternative embodiment
of a structure in accordance with the invention.
D~SCRIPTION OF TH~ PREF~RRED ENBODIM~NTS
As is shown in Figs., 1, 2 and 3, the web W is formed on the
wire 50, which is either a fourdrinier wire or the carrying wire of a
twin-wire former. On the downwardly inclined run of the wire 50
between the wire suction roll 51 and the wire draw roll 52, the web W
is transferred on the detaching line P on the suction zone 14 of the
pick-up roll 13 onto the first press felt 10, which, thus, also acts
as the pick-up felt. The first press felt 10 carries the web W on
its lower face into the first press nip Nl, which is formed between
two press rolls, i.e. a press-section roll 11 and a grooved roll 22
or an equivalent roll provided with a hollow face 23. The first
press felt 10 forms an endless loop by means of guide and alignment
rolls (not shown), which also keep the first press felt 10
appropriately tensioned. The first nip Nl is provided with two
press felts, i.e. the first press felt 10 and a second press felt 20,
the latter felt also forming an endless loop by means of guide and
alignment rolls 21, which keep the second press felt 20 appropriately
2017~iO
tensioned. The nip plane of the first nip Nl is denoted with the
reference K1 notation.
After the first nip Nl the web W is sucked by means of the
suction zone 12 of the press-section roll 11 out of contact with the
second press felt 20 onto the face of the first press felt 10 and,
being guided by the press-suction roll 11, into the second nip N2.
For this purpose, the press-suction roll is provided with a suction
zone 12 of appropriate length, which ensures the detaching of the web
W from the second press felt 20 onto the face of the first press felt
10. The second nip N2 is formed between the press-suction roll 11
and the center roll 1 in the press section. After the second nip
N2, the first press felt 10 is moved away from the center roll 1.
Owing to the surface properties of the smooth-faced center roll 1,
the web W is detached after the second nip N2 from the first press
felt 10 and adheres to the face of said center roll, under whose
guidance the web W is thereupon transferred into the third nip N3
in the press section. The third nip N3 is formed between the
center roll 1 and an oposite grooved roll 32 or an equivalent roll
provided with a hollow face 33. Over this grooved roll 32, a third
press felt 30 is passed, which is formed as an endless loop by means
of guide and alignment rolls 31.
In the embodiments shown in Figs. 1, 2 and 3, the press
section further includes a fourth, separate press nip, in which case
the fourth nip N4 in the press section is formed between a second
center roll 70, corresponding to the center roll, and an adjacent
grooved roll 44 or an equivalent roll provided with a hollow face
45. After the third nip N3 the web W is detached from the third
press felt 30 onto the face of the center roll 1, from which it is
detached by means of a transfer-suction roll 63. Thus, an open draw
20170~0
is formed between the center roll 1 and the transfer-suction roll 63.
From the transfer-suction roll 63 the web W is transferred onto the
fourth press felt 40 on the suction zone 42 of the suction roll 41,
which acts as a pick-up roll. Between the transfer-suction roll 63
and the suction roll 41, there is also an open draw. The fourth
press felt 40 is also a pick-up felt. The fourth press felt 40 is
formed as an endless loop by means of guide and alignment rolls 43.
The fourth press felt 40 transfers the web W into the fourth press
nip N4, after which, owing to the surface properties of the second
center roll 70, the web W is detached from the fourth press felt 40
onto the face of the second center roll 70. The nip plane of the
fourth nip N4 is denoted with the reference notation K4. From
the second center roll 70, the web W is transferred onto the drying
wire 60 by means of transfer-suction rolls 61 and 62, while the
drying wire 60 transfers the web W further into the drying section
(not shown).
Fig. 1 shows a first preferred embodiment of a press section
in accordance with the invention. As is shown in Fig. 1, the center
roll 1 in the press section is a variable-crown roll, which comprises
a metallic roll mantle 2 which is arranged to revolve around a
stationary roll axle 3. In the embodiment shown in Fig. 1, inside
the center roll 1, crown-variation means 4 and 5 are fitted which act
in the nip plane K2 of the second nip N2 as well as in the nip
plane K3 of the third nip N3, by means of which crown-variation
means the roll mantle 2 is loaded in the nip planes K2 and K3 to
produce the desired linear load profile. In the embodiment of Fig.
1, the crown-variation means comprise loading shoes supported on the
roll axle 3 and acting upon the inner face of the roll mantle 2.
Thus, in the embodiment of Fig. 1, the center roll 1 is a roll
9 ~017~10
adjustable in zones thereof, wherein the crown variation is provided
in two directions, which form an angle ~ with each other. Thus, in
the embodiment shown in Fig. 1, regulation of the respective linear
load profiles is accomplished both in the second nip N2 and in the
third nip N3 in the press section. Since, in the center roll in
accordance with the embodiment of Fig. 1, there are two directions of
crown variation, it is necessary to employ precisely the
above-mentioned loading-shoe constructions for the crown variation
relay. If a crown variation were provided in the center roll 1 in
one direction only for example, either in the nip plane K2 of the
second nip N2 or in the nip plane K3 of the third nip N3, in
such a structure it would be possible to employ any crown-variation
means used in the prior art.
In the embodiment of Fig. 1, if necessary or desirable, it
would also be possible to provide crown-variation means in the
grooved roll 22, in which case the linear loads profile could also be
controlled in the first part Nl of the press section.
As has been heretofore mentioned, the press section shown in
Fig. 1 also includes a fourth press nip N4. Thus, in the
embodiment of Fig. 1, the second center roll 70 is arranged as a
variable-crown roll, so that the second center roll 70 comprises a
hollow, metallic roll mantle 71, which is arranged to be revolving
around a stationary roll axle 72. In the interior of the roll mantle
71, crown-variation mans 73 are provided, by which means the fourth
nip N4 is loaded in the direction of the nip plane K4 to produce
the desired linear load profile. In Fig. 1 it is shown that in the
second center roll 70, as the crown-variation means 73, a
loading-shoe construction similar to that described above in relation
to the center roll 1 is used. However, since in the fourth, separate
I()
2017010
press it is necessary to control one regulation direction only, as
the crown-variation means 73 it is possible to use any structure that
has been used for this purpose in the prior-art. Thus, if necessary,
or desirable, in the embodiment shown in Fig. 1, it is possible to
control linear load profiles in all the press nips in the press
section.
The second embodiment of the invention shown in Fig. 2
differs from that shown in Fig. 1 in the respect that, in the center
roll 1, crown-variation means 4 are provided in the nip plane K2 f
the second nip N2 only. Owing to this, in the embodiment shown in
Fig. 2, as crown-variation means 4 it is possible to use any
structure that has been used. In Fig. 2, the linear load profile is
also regulated in the first nip Nl in the direction of the nip
plane K1 so that the grooved roll 22 is formed as a variable-crown
roll. Thus, the grooved roll 22 comprises a tubular roll mantle 24,
which is mounted to revolve around the stationary roll axle 25.
Inside the roll mantle 24, the necessary crown-variation means 26 are
provided to regulate the linear load profile in the nip Nl. Since
herein one direction of regulation only is concerned, as a
crown-variation means it is possible to use any structure that has
been used for this purpose in the prior art.
With a view to regulating the linear load profile in the
third nip N3 in the press section, in the embodiment shown in Fig.
2, the grooved roll 32 in said third nip N3 is formed as a
variable-crown roll, whose construction corresponds to that of the
grooved roll 22 in the first nip Nl described above. Thus, the
grooved roll 32 comprises a tubular roll mantle 34, which is arranged
to revolve around the roll axle 35. Further, inside the roll mantle
34, crown-variation means 36 is provided, which acts in the nip plane
I 1 20~ 7010
K3 of the third nip N3 to regulate the profile of linear loads.
Since, in this case as well, regulation is required in one direction
only, the crown-variation means 36 can be embodied in several known
structures. As the crown-variation means 36, it is possible to
employ, for example, the loading shoes shown in Fig. 2, but in their
place it is also possible to use, for example, a pressure fluid
chamber or a series of such chambers provided between the roll axle
35 and the roll mantle 34. In the fourth, separate nip N4 in the
press section, in the embodiment shown in Fig. 2, the regulation of
the load profile is arranged in a manner differing from that of Fig.
1. In the embodiment shown in Fig. 2, the grooved roll 44 in the
fourth nip N4 is arranged as a variable-crown roll so that the roll
44 comprises a tubular roll mantle 46, which is arranged to revolve
around the axle 47 of the roll. Inside the roll mantle 46,
crown-variation means 48 similar to the crown-variation means 26 and
36 of the grooved rolls 22 and 32 in the first nip N1 and in the
third nip N3 are provided. Thus, in this embodiment, for the
fourth nip N4, it is possible to employ a conventional metal roll
or even a rock roll as the center roll 70.
The third embodiment of the invention, shown in Fig. 3,
combines some of the structural elements of the embodiments shown in
Figs. 1 and 2. In the structure shown in Fig. 3, control of the
linear load profiles in the first, second and third nips N1, N2,
N3 in the press section is achieved in the same respective manners
as in the structure shown in Fig. 2, and in the fourth nip N4 in
the same manner as is shown in Fig. 1.
The embodiment shown in Fig. 4 differs from the embodiments
of Figs 1-3 as follows. With respect to the first nip N1 in the
press section, the structure is similar to that described in the
Ia 2~ 10
above mentioned embodiments. However, in Fig. 4, the second nip N2
in the press section is not formed between the press-suction roll 11
and the center roll l, but in this embodiment, on the run of the
first press felt 10 after the press-suction roll 11, a grooved roll
15 or an equivalent roll provided with a hollow face 16 is provided,
which forms the second nip N2 in the press section with the center
roll 1. The first press felt 10 is formed as an endless loop, as is
the case in the other embodiments, by means of guide and alignment
rolls 17. In the embodiment of Fig. 4, the construction and the
operation of the center roll 1 are identical with those of the center
roll shown in Fig. 1. The grooved roll 32 in the third nip N3 and
the related constructions are also, in this embodiment, identical
with those shown in Fig. 1. In the solution of Fig. 4, a fourth,
separate press nip is not shown but in this structure the web W can
be transferred by means of the transfer-suction roll 63 directly onto
the drying wire (not shown). It is clear, however, that this
embodiment can also be provided with a fourth, separate press nip
similar to that described in relation to the embodiments described
above. In a corresponding way, it is fully clear that the
embodiments of Figs. 1, 2 and 3 may also be accomplished without a
fourth, separate press.
Details of the present invention may easily vary within the
scope of the inventive concepts set forth above, which have been
presented by way of example only. Therefore, the preceding
description of the present invention is merely exemplary, and is not
intended to limit the scope thereof in any way.
