[0001] The present invention concerns devices for checking paper quality control parameters
during the production cycle, and in particular it concerns a new system for adjusting
and controlling the water content in paper in the transverse direction with respect
to the forming direction during the production cycle.
[0002] The paper production cycle, first of all, includes the preparation of the liquid
compound for the production of the paper sheet, said compound consisting of approximately
99 % of water and of cellulose fibre and technological aids for the remaining 0,4-1
%, said fibre and additive percentage varying according to the weight and type of
paper to be produced.
[0003] Said liquid compound is introduced in the system and spread homogeneously on a surface,
commonly defined forming wire, having first of all the function to guarantee the distribution
of the fibre and the natural drainage of at least part of the water.
[0004] Said formation wire slides so that the liquid compound spread on it passes through
various processing stations, where it is progressively dehydrated due to the effect
of gravity only and/or due to the action of vacuum sucking devices.
[0005] At the end of the formation station the paper web has a content of dry material that
ranges from 15 to 20%.
[0006] In the successive station, or pressing station, the material is pressed and further
dehydrated until reaching a fibre content included between 40 and 50%, according to
the technology adopted and the type of product.
[0007] In particular, said dehydration is obtained by pressing, through the use of pressing
cylinders and a simultaneous sucking action.
[0008] Considering the special chemical structure of cellulose, drying the sheet further
means using considerable energy. At present heat energy is employed, in the form of
vapour or infrared radiation.
[0009] For example, systems are known in which the paper web, moving out of said pressing
station, passes through drying cylinders whose contact surface is heated to a temperature
of approximately 130° C and thus dries the paper web.
[0010] The paper web obtained in this way, before being wound in a coil, passes through
a continuous computerized monitoring station that carries out nondestructive measurements
mainly focused on two parameters, that is, grammage and humidity.
[0011] Usually the grammage parameter, that is, the weight expressed in grams per square
meter of paper, is determined using a scanner installed downstream of the production
line, before the final winder.
[0012] Said scanner employs radiation emitters and, by means of an apposite correlation
algorithm, determines said grammage starting from the evaluation of the intensity
extinction of said radiations.
[0013] In the known systems, the grammage control system thus works retroactively and communicates
with the devices installed upstream of the system and responsible for adjusting the
quantity of liquid compound introduced in the production line.
[0014] Said devices, therefore, modify said quantity of liquid compound according to the
grammage measured downstream of the system.
[0015] In this way it is possible to guarantee a constant grammage of the paper web with
good approximation, at least in the longitudinal direction, that is, parallel to the
advance direction of the paper web in the production line. Sensors are also known
that are positioned downstream of the production line, too, and that are suited to
determine the variations in the grammage value along different sections positioned
crosswise with respect to the direction of the production line.
[0016] Said sensors thus make it possible to retroactively correct the grammage values,
thus guaranteeing rather limited variability, in the order of 0.1 - 0.2%.
[0017] Regarding the other parameter, that is, the humidity contained in the paper web,
in the systems known at present said value is determined by means of a measuring head
installed on said scanner for measuring the grammage, that is, downstream of the production
line.
[0018] The presence of some more or less humid areas can be due for example to the non-homogeneous
distribution of the compound during the formation step, or to particular conditions
in the setting up of the system.
[0019] Also in this case the humidity control system works retroactively, causing a variation
in the heat energy introduced upstream of the system in order to obtain the evaporation
of water.
[0020] In particular, systems are known in which said variation in the heat energy supplied
to the system is obtained by varying the quantity of overheated vapour that feeds
said drying cylinders.
[0021] This system is rather effective in the control of the water content in the longitudinal
direction with respect to the production line.
[0022] Vice versa, at present the operation for controlling the water content crosswise
with respect to the direction of the production line is extremely complicated and
articulated, and its results are not satisfying.
[0023] In particular, according to the current state of the art, in order to intervene on
the inevitable oscillations of the humidity content in the transverse direction, water
is introduced using mixed air/vapour systems, in the driest areas where the water
content is lower, thus levelling the peak values below the desired average value.
[0024] On the contrary, it is much more complicated to properly dry some bands of the paper
web in order to level the peak values above the average value or the desired reference
value.
[0025] In fact, the devices currently known are able to operate on areas or bands whose
width exceeds a dozen centimetres, while the humid areas, that is, the areas containing
more water than desired, are often narrower, so that the dehumidifying process involves
also portions of the paper web that are already in optimal humidity conditions and
that therefore do not need a higher energy supply.
[0026] A further drawback posed by the known technique lies in that the heat is always supplied
to the paper web from the outside towards the inside, so that a positive humidity
gradient is inevitably obtained from the surface layers of the paper web towards the
inner layers of the same, and consequently the surface layers contain less water than
the inner ones.
[0027] This gradient is a function of the total thickness of the paper web and of the possible
use of technological-functional aids employed in the production cycle.
[0028] In fact, the thicker the paper web, the more evident the gradient, for example in
paper webs with high grammage values and above all in paperboard sheets made up of
two or more overlapping layers.
[0029] Furthermore, since cellulose fibre varies its dimensions depending on the water content,
this gradient causes lengthening and/or shortening stresses that become evident with
the appearance of deformations on the paper web at the end of the production cycle,
that is, waves, curves, etc.
[0030] In order to overcome all the drawbacks described above, a new type of system for
adjusting and controlling at least the water content in paper during the production
cycle has been studied and implemented.
[0031] The main object of the present invention is to guarantee minimum variability of the
humidity content in paper both in the longitudinal direction with respect to the production
line and crosswise.
[0032] Another object of the present invention is to be able to treat in a focused manner
and with maximum precision only the areas or paper portions that require such a treatment,
according to the humidity profile measured along one or more cross sections of the
paper web.
[0033] Another object of the present invention is to minimize the humidity gradient between
the surface layers and the inner layers of the paper web, by means of microwave dehumidifying
devices that effectively act on the innermost part of the web thickness, differently
from the systems currently used.
[0034] Another object of the present invention is to ensure reduced treatment times, particularly
in relation to the feeding speed of the paper web in the production line.
[0035] These and other direct and complementary objects are achieved by the new system for
adjusting and controlling at least the water content in paper during the production
cycle, comprising one or more dehumidifying devices that emit microwaves at controlled
frequency, connected to at least one device suited to measure and determine the paper
humidity profile along at least one given cross section, said cross section being
preferably located downstream of the production line and of said microwave emitting
devices. Said measuring device thus acts retroactively, determining the humidity content
profile of a cross section downstream of the production line.
[0036] The data referring to said humidity profile are processed by an electronic unit that
communicates with said microwave emitting devices positioned upstream, activating
or deactivating them and/or adjusting their power so that they have effect only on
the areas with humidity peaks above the reference value or outside a reference interval.
[0037] Said microwave emitting devices are preferably installed on a head and are suited
to act on all the points of a cross section located upstream of said cross section
where the measurement has been carried out.
[0038] In particular, according to the invention said head can be a mobile head, travelling
crosswise with respect to the direction of the production line, in such a way as to
position said microwave emitting devices exactly at the height of the paper portion
to be dehumidified, according to the humidity profile given by said measuring device.
[0039] In this way, during the paper feeding movement, an entire longitudinal area is treated
at the level of the portion of paper web to be dried.
[0040] Alternatively, according to the invention, said head may occupy an entire cross section,
and said microwave emitting devices can be distributed in such a way as to affect
the whole cross section and to be activated alternately or simultaneously in order
to act on some specific areas, according to the humidity profile given by said measuring
device.
[0041] The main advantage offered by the new control system lies in that it makes use of
said dehumidifying devices that emit microwaves and are suited to make the water contained
in the paper evaporate starting from the innermost layers, thus obtaining also the
normalization of the humidity gradient through the thickness of the paper web.
[0042] Thus the surface layers of paper will not be drier than the inner layers, as is the
case when the known drying systems, for example the drying cylinders, are used.
[0043] Furthermore, the use of microwave emitters guarantees higher precision of the paper
dehumidifying treatment, since even very narrow or very wide areas can be treated.
[0044] The action of the microwave emitting devices, furthermore, is very rapid, in fact
said devices can be activated and deactivated instantaneously and operate rather quickly,
differently from the known dehumidifying devices that on the contrary do not have
the same effects in the required time as dictated by the paper feeding speed, considering
also the heat insulation effect exerted by the paper surface layers.
[0045] The characteristics of the new system will be highlighted in greater detail in the
following description with reference to the drawing shown in Figure 1, provided as
a non-limiting example.
[0046] The new system for adjusting and controlling the water content can be installed in
paper (F) production plants.
[0047] The paper (F), produced in webs by pouring on a surface a given quantity of liquid
compound consisting of water and cellulose fibre, slides in a direction (X), or direction
of the production line, and passes through various stations, where it is progressively
dehydrated so that the dry material passes from the initial value of 0.4-1% to an
optimal value of 40-50%.
[0048] In order to further dehydrate the paper web (F), the new system for adjusting and
controlling the water content comprises one or more dehumidifying devices (M) that
emit microwaves at controlled frequency, suited to treat at least one area of the
paper web (F).
[0049] Said dehumidifying devices (M) are arranged at the level of at least one part or
section (F1) of the paper web (F), crosswise (Y) with respect to the direction (X)
of the production line and therefore, while the paper slides in said direction (X),
said dehumidifying devices (M) treat a longitudinal area (Fm), that is, an area that
is parallel to said direction (X) of the production line.
[0050] In particular, said dehumidifying devices (M) are installed on a head (T) in such
a way as to allow the paper (F) to be treated alternately or simultaneously on all
the points of said section (F1).
[0051] According to a possible solution, said head (T) can move and travel crosswise (Y)
with respect to the direction (X) of the production line, in such a way as to position
one or more of said microwave emitting devices (M) along said section (F1) only at
the level of the portion of paper (F) to be treated.
[0052] In this way, it is possible to treat only some parts of said section, while for the
parts that do not require further dehydration the treatment will be interrupted or
will not be carried out at all.
[0053] In the preferred embodiment of the invention, as an alternative to or in combination
with said travelling head (T), said head (T) may occupy the whole section (F1) of
the paper web (F) and said microwave emitting devices (M) may be distributed in such
a way as to occupy the whole section (F1), too. Said microwave emitting devices (M)
can thus be activated alternately or simultaneously so that they act only on some
given portions of said section (F1) of the paper web (F).
[0054] The new adjusting and controlling system also comprises at least one measuring device
(C) suited to determine the humidity profile (P) of the paper web (F) along a measurement
section (F2) preferably located downstream of the section (F1) to be treated by said
microwave emitting devices (M).
[0055] Said measuring device (C), in particular, determines the humidity content at every
point of said measurement section (F2), thus tracing a profile (P) as schematically
shown in Figure 1, wherein peak values (P1) are taken that, for example, are higher
than a reference value (Pr), for example an average or optimal value, or than the
maximum value of a reference interval.
[0056] Said profile (P) also shows peak values (P2) that are lower than the reference value
(Pr) or than the minimum value of a reference interval.
[0057] Said measuring device (C) thus operates retroactively, determining the humidity profile
(P) of a measurement section (F2) located downstream of the section (F1) where the
dehumidifying treatment is carried out with respect to the direction (X) of the production
line.
[0058] The new adjusting and controlling system also comprises at least one electronic unit
(S) that receives from said measuring device (C) the data concerning said humidity
profile (P), processes them and, through a PLC control unit (R) of said microwave
emitting devices (M), activates or deactivates one or more of said microwave emitting
devices (M) so that they operate in a focused manner only on some portions (Fx) of
the paper web (F).
[0059] In particular, once said peak values (P1) of said humidity profile (P) and the corresponding
portion (F2x) of said measurement section (F2) have been identified, one or more of
said microwave emitting devices (Mx) are activated, said microwave emitting devices
(Mx) being positioned and operating only on the corresponding section (F1x) of paper
(F) that is aligned, along said direction (X) of the production line, with said section
(F2x) presenting said peak humidity values (P1).
[0060] In combination with the feeding motion of the paper (F), said activated microwave
emitting devices (Mx) thus generate a continuous band (Fx) of dehumidified paper.
[0061] Said measuring device (C), being positioned downstream of said microwave emitting
devices (Mx) and therefore of said dehumidified band (Fx), continues measuring said
humidity profile (P) and, according to the measured values, deactivates said microwave
emitting devices (Mx) or modifies their power so that the paper (F) is correctly dehumidified.
[0062] Said electronic unit (S), in fact, processes said data concerning said humidity profile
(P), identifying also peak values (P2) that are lower than a reference value or value
interval and, through said PLC control unit (R) of said microwave emitting devices
(M), deactivates and/or modulates the power of one or more of said microwave emitting
devices (Mx) that act on said portions of paper (F) aligned, along said direction
(X) of the production line, with the portions of said measurement section (F2) corresponding
to said peak values (P2).
[0063] The humidity ratio of the paper (F) is thus controlled continuously in the different
areas, both in the longitudinal direction, that is, parallel to the direction (X)
of the production line, and crosswise (Y).
[0064] Said microwave dehumidifying devices (M) cause the evaporation of the water contained
in the paper (F) starting from the innermost layers, thus obtaining also the normalization
of the humidity gradient through the thickness of the paper (F), that is, also in
the direction (Z) orthogonal to the sliding plane (XY) of the paper (F).
[0065] Therefore, with reference to the above description and the attached drawing, the
following claims are expressed.
1. System for adjusting and controlling the humidity content of paper (F) during the
production cycle in paper production plants, comprising one or more devices suited
to dehumidify said paper (F), characterized in that said dehumidifying devices comprise one or more devices (M) that emit microwaves
at controlled frequency and are suited to treat at least one area of the paper (F)
while said paper (F) slides.
2. System for adjusting and controlling at least the humidity content of paper (F) during
the production cycle according to claim 1, characterized in that it comprises one or more measuring devices (C) suited to determine the humidity profile
(P) of said paper (F) along at least one measurement section (F2), and wherein said
measurement section (F2) is located downstream of the area to be dehumidified and
of said microwave emitting devices (M) with respect to said direction (X) of the production
line.
3. System for adjusting and controlling at least the humidity content of paper (F) during
the production cycle according to claims 1, 2, characterized in that it also comprises at least one electronic unit (S) suited to receive data from one
or more of said measuring devices (C) in order to determine said humidity profile
(P), processes said data identifying the peak values (P1) higher than a reference
value or value interval and, through at least one PLC control unit (R) of said microwave
emitting devices (M), activates/deactivates and/or modifies the power of one or more
of said microwave emitting devices (Mx) that act on portions (F1x) of paper aligned,
along said direction (X) of the production line, with the portions (F2x) of said measurement
section (F2) corresponding to said peak values (P1), and wherein, in combination with
the feeding movement of the paper in said direction (X) of the production line, said
microwave emitting devices (Mx) generate a dehumidified band (Fx), said band (Fx)
being parallel to said direction (X) of the production line and being substantially
as wide as said portion (F1x).
4. System for adjusting and controlling at least the humidity content of paper (F) during
the production cycle according to claim 3, characterized in that said electronic unit (S) processes said data concerning said humidity profile (P),
identifying the peak values (P2) that are lower than a reference value or value interval
and, through said PLC control unit (R) of said microwave emitting devices (M), activates/deactivates
and/or modifies the power of one or more of said microwave emitting devices (M) that
act on portions of paper (F) aligned, along said direction (X) of the production line,
with the portions of said measurement section (F2) corresponding to said peak values
(P2).
5. System for adjusting and controlling at least the humidity content of paper (F) during
the production cycle according to claims 1, 2, 3, 4, characterized in that said dehumidifying devices (M) are suited to treat said paper (F) at the level of
at least one section (F1) or portion of section (F1x) to be treated arranged crosswise
(Y) with respect to the sliding direction (X) of said paper (F).
6. System for adjusting and controlling at least the humidity content of paper (F) during
the production cycle according to claim 5, characterized in that one or more of said dehumidifying devices (M) travel crosswise (Y) with respect to
said direction (X) of the production line, in such a way as to be positioned at the
level of one or more points or portions (F1x) of said section (F1) to be treated.
7. System for adjusting and controlling at least the humidity content in paper (F) during
the production cycle according to claim 5, characterized in that said microwave emitting devices (M) are distributed in such a way as to occupy the
entirety or part of said section (F1), and wherein two or more of said microwave emitting
devices (M) are activated alternately or simultaneously so that they act only on some
portions (F1x) of said section (F1) to be treated.