Note: Descriptions are shown in the official language in which they were submitted.
~ethod and apparatus for the levelling ~f th~ humidity
profile o~ a ~ontinuous web by dielectric drying
The present invention relates tG a method for tne improve-
ment of the moisture profile of a continuous web by means of
dielectric drying in accordance witn the preamble of claim 1.
The invention also involves an apparatus for the implementa-
tion of the method.
The product quality of several continuous web processing
machines including, for instance, paper and textile
web drying sections, is impaired by the uneven final
moisture proflle of the web.
The problem is most recognized and quantitatlvely most
important in the paper industry. Users require a product
with an even final moisture content in order to ensure
non-problematic runnability in printing, as well as
to achieve a high dimensional stability of the web for
accurate alignment of colours.
The unevenness of the web moisture is caused by different
defective functions at the wet end either before the
drying section or in the dryer itself. The web defects may
be generated by incorrect crowning of the press rolls,
unevenness of the pressing felts or their cleaning,
or by temperature differentials of the web. Persistent
or random defects on the final humidity profile of the
web on the dryer are caused by the edge effect related
to the low humidity of the drying atmosphere and higher
heat transfer factors at the edge of the web as well
as by the surface of the drying cylinder extending over
the web edge the nonsymmetrical air circulations, uneven
action of the drying felts together with, for instance,
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the defects of condensate removal from the cylinders
and asymmetrical flows of the condensate.
In order to compensate for these defects, several different
auxiliary apparatuses have been developed. Their operation
is based on the principle of intentionally providing a
functional defect w'nich compensates for a defect of t'ne
machine, often originating from an unrecognized cause.
Rather typical examples of these means are different
kinds of air blowing units, in which the atmospheres
in the drying pockets are sectionally adjusted in a
compensatory manner. The effect of one air blowing
unit is minimal and, consequently, several units are
needed for each macnine; generally, as many as possible
within the space limitations.
When high-velocity hoods are used, relatively few will
suffice because the method of blowing air via nozzles
on the paper achieves high characteristic evaporation
rates and differential rates. Due to their clumsiness
and high powe- consumption, these solutions are not
very popular.
In contrast, infrared radiant heaters heated by gas
or electricit~ with a sectionally controlled effect
are very effective and popular.
In some cases, a high-frequency electromagnetic field
has been used for profile improvement by locating bar
electrodes cross-directionally to the web over a sufficiently
large area. The dielectric constant and loss factor
that determine the power absorbed as heat by the web
are higher at the wetter parts of the web. Consequently,
a dielectric dryer drives moisture more effectively
out from the wetter parts of the web than from the dry
parts when the unit is located at the dry end of the
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drying section. However, the disadvantage of
crosswise located bar electrodes is the increased
power consumption of the dryer due to the unnecessary
simultaneous drying of the dry streaks. This leads
to -the fact that the dryer cannot produce an even
final moisture but only reduces the highest absolute
and relative humidity differentials. In orde~ to
achieve any effective differential drying at all,
this kind of moisture profile correcting dryer must
be located at the relatively dry section of the web
with a correspondingly low dielectric constant and
loss factor. Naturally, this is disadvantageous with
consideration to the size of the drying section.
Compared to the other methods described above,
however, this solution provides the advantage of
disposing of any control means in the crosswise
direction of the web.
The present invention relates to a method for
reducing differentials of moisture profile on a web,
especially a paper web, by means of high-frequency
electromagnetic energy.
The invention is characterized by the use of bar
electrodes, energized by a high-frequency electro-
magnetic power source and aligned approximately in
the machine direction, for drying the web.
In accordance with a particular embodiment of the
invention, there is provided a method for reducing
moisture differentials in a moving web containing wet
streaks extending in the processing direction of the
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web, by the application of high-frequency electro-
magnetic energy fed to bar-formed electrodes ~,Jhich
are located close to the surface of the web, wherein:
the longitudinal axes of the electrodes are
aligned approximately parallel to the processing
direction of the web; and
the electrodes can be moved transverse to the
~rocessing direction of the web and rotated at an
angle relative to the processing direction of the web
so as to be positioned only over each wet streak of
the web in order to provide a uniform moisture
profile in the web.
From a different aspect, and in accordance with the
invention, there is provided an apparatus for
reducing moisture differentials in a moving web
containing wet streaks therein which comprises:
means for conveying the web of material;
bar-formed electrodes operatively associated
with the surface of the moving web for applying
high-frequency electromagnetic energy thereto,
wherein the longitudinal axes of the electrodes are
aligned substantially parallel to the processing
direction of the moving web;
means for moving the electrodes transverse to
the processing direction of the web; and
means for rotating the electrodes at an angle
relative to the processing direction of the web,
whereby the electrodes can be selectively positioned
over each of the wet streaks in the web in order to
provide a substantially uniform moisture profile in
the web.
In the following, the invention will be examined in
more detail by means of exemplifying embodiments
described in the attached figures. For the sake of
presentation
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simplicity and clarity, the figures omit all equipment
unessential to the invention, such as components associated
with the generation and feed of the electromagnetic energy;
equipment necessary for environmental protection from
radiated energy, possible automatic control devices,
actuators and their details required for movement, ventila-
tion equipment necessary for humidity removal and advancement
of drying, and the like.
Figure 1 shows an apparatus in accordance with the invention
in a side elevation view aligned with a plane of a processed
web 1.
Figure 2 shows the same apparatus viewed from the direction
of arrow B in Figure l.
Figure 3 shows a preferred location for the apparatus,
especially in a high-speed paper web drying section.
The processing of a wet streak in the machine direction
of the web requires a drying unit consisting of at least
two electrodes ~, which are located parallel to the
processed web 1, and approximately aligned in the mashine
direction of the web (arrow A). Figures 1 and 2 show
the mounting of the electrodes in a carriage 4, which
can move along a beam 3 by automatic, remote-controlled
or manually controlled means with the help of the apparatus
not described in the figures, onto a selected wet streak
of the web. These kind of units can be located lengthwise
to the béam 3, that is, in the cross direction of the
web in a number corresponding to the number of the wet
streaks.
When a mounting base 7 of the electrodes is mounted
pivotable with an actuator 6 around a pin, an angle L
can be provided between the direction of the electrodes
and the machine direction of the web. In addition,
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the mounting base 7 can be complemented with required
fixtures and possible actuators that are not shown in
the figures, and with w'nich a mutual distance a of the
electrodes can be adjusted automatically, by remote-
control or manually in accordance with the correction
requirements of the web moisture. Thus by varying the
oxientation of the electrodes, a local energy densitv
P/A imposed on the weh can be ~rovided in the direction
of the crosswise coordinate x of the web yielding an
energy distribution that conforms as accurately as possible
with the moisture distribution of the wet streak to
he corrected. The lower part of Figure 2 shows the
variabLe P/A as a functlon of coordinate x for the electrode
orientation shown in Figure 2.
rhe invention offers appreciable benefits.
The power consumption required is decreased and the
e~uipment necessary for generating the high-frequency
energy may be smaller. This is produced by the disposal
of unnecessary drying which avoids consuming power for
processing intermediate areas between the wet streaXs,
which also reduces power consumption at the wet streaks
because less evaporation is required for correcting
the moisture profile.
In principle, an evenly distributed moisture content
can be achieved. This is not possible with crosswise
electrodes extending over the entire weh, whose selective
drying effect for drying a wet part of the web faster
than a dryer part of the web is based on the differentials
of the electrical characteristics of the dried substance
at different moisture levels. For this reason, although
the crosswise electrodes are capable of reducing the
moisture differential, they are not capable of fully
levelling them off.
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In addition to achieving a reduced number of electrodes
by disposing of the electrodes at the dry parts of the
web, reduction in electrode numbers also achie~ed '~
advantageously locating the moisture profile correction
units over the wet streaks at the wet end of the machine
where the dielectric constant of the web is most suitable
for the method.
The described moisture profile correction units are
located in a paper machine for drying the we',~ at the
web transfer when the web is conveyed from one drying
cylinder to anot'ner. The suggested location is especially
advantageous in modern high-speed machines t'nat tend
to employ as a design of the art the single wire transf~r
at the wet end of the machine immediately after the
press section. This means that the web is conveyed,
supported by the drying wire, from one cylinder to another,
a method which reduces the frequency of paper breaks
in a high-speed drying section. The disadvantage of
the method is that on every other cylinder, generally
on the lower cylinders, the drying wire passes between
the paper web and the cylinder.
special advantage in paper machines is achieved by
locating a moisture profile correction unit 12 in a
slngle wire group as shown in Figure 3. ~nen the correction
unit 12 is located on the side of a web 9 supported
by wire 8, a ventilation equipment 10, e.g. nozzle blowing
equipment for drying and profile correction, can be
located on the pape. side and adapted to blow immediately
against the paper. This provides an especially safe
arrangement against breaXs and an effective location
for drying. In addition, Figure 3 shows that the cylinder
is preferably positioned lower than normal for easier
location and higher efficiency of the correction unit,
which also lengthens the free passages of web transfer.
