Note: Descriptions are shown in the official language in which they were submitted.
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13ACKGROUND OF TNE__NVENTION
The invention concerns an arrangement of nozzles with
negative pressure intended for contact-free treatment of mobile
material webs, comprising a nozzle box provided with a foil face
placed facing the web, in connection with which face there are two
nozzle slots, and the construction of which said nozzle arrangement
is asymmetric relative to the perpendicular center plane of its
negatively pressurized carrier face.
The nozzle arrangement subject of the invention is intended
for contact-free supporting and treatment~ such as, e.g., drying,
heating or cooling, of paper webs etc. continuous webs.
The prior-art apparatuses based on contact-free treatment of
the web consist of a number of nozzle boxes, out of whose nozzles a
gas flow that s~pports and dries the web is directed at the web.
The nozzles in said apparatuses can be divided into two
groups: nozzles with positive pressure (float nozzles) and nozzles
with negative pressure (foil nozzles). The operation of the nozzles
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with positive pressure is based on the air-cushion principle. The
nozzles with negative pressure attract the web and stabilize the run
of the web, in which case the attractive force applied to the web is,
as is well known, based on a gas flow field parallel to the web, said
field forming a static negative pressure between the web and the
carrying face of the nozzle.
Both in nozzles with positive pressure and in those with
negative pressure, the so-called Coanda effect is commonly used to -
guide air in the desired direction.
With respect to the prior art related to the present
invention, reference is made to the Applicant's FI Patent No. 68,723
(equivalent of U.S. Patent 4,247,993), wherein a nozzle with negative
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pressure is described which is mainly characterized in that, in the
direction of flow of the gas, the nozzle slot of the nozzle with
negative pressure is placed before the plane of the inlet edge of the
curved guide face and that the ratio of the width of the nozzle slot
to the curved radius of said guide face is, with the gas flow rates
occurring in practice, chosen so that the gas flow is separated from
the curved guide face substantially before this trailing edge.
~ he prior art most closely related to the present invention
is represented by the arranqement of nozzles with positive pressure
described in the Applicant's FI Pat. Appl. No. 874255, in connection
with which there are two nozzle slots that blow substantially against
each other. In the last-mentioned FI patent application it has been -
considexed novel that the arrangement of nozzles has a first nozzle
slot, which is placed relative to the carrying face in such a way
that the gas jet blown out of the nozzle slot follows in contact the
curved carrying face up to the area between the nozzle slots and that
the arrangement of nozzles includes a second nozzle slot, which is
placed within the area of the edge of the curved guide face placed in
its connection or, in the gas flow direction, before said edge, the
ratio of the width of said second nozzle slot to the curved radius of
said guide face being chosen such, with the flow rates of the second
nozzle occurring in practice, that the gas flow is separated from the
curved guide face of the second nozzle preferably before the
carrying-face area proper between the first and the second nozzle
slot.
O~JECTB AND 8UMMARY OF THE INVENTION
The object of the present invention is further development
of the nozzle arrangements described in the Applicant's patents and
applications mentioned above while making ~se of the practical
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experience obtained by the Applicant with different nozzles and with
combinations of the latter.
A particular object of the invention is to provide a nozzle
with negative pressure, i.e. a foil nozzle, that has a good
web-aligning ability so that in a contact-free air-support apparatus
consisting of nozzles in accordance with the invention, formation of
folds in the web can be reduced. This object is also associated with
the object of providing a nozzle arrangement by whose means the web
edge supported by air runs calmly, which is achieved partly in that
the difference in air pressures between opposite sides of the web is
made relatively small.
A further object of the invention is to provide a nozzle
arrangement that has a wide range of blow rates, even as wide as -~
15...85 m/s, which means flexibility of operation.
A further object of the invention is to provide a foil
nozzle that has an evaporating capacity higher than in the prior art.
Another object of the invention is to provide a noxzle that
can be used both with upstream blowing and with downstream blowing.
In view of achieving the ob;ections stated above and those
that will come out laterr the invention is mainly characterized in
that the nozzle box has a substantially plane carrier face, at one of
whose edges there is a curved Coanda air-guide face, in connection
with which there is the first nozzle of the nozzle arrangement, at
which the initial direction of its blowing is substantially
perpendicular to the plane of the web to be carried, that said Coanda
air-guide is arranged to turn the air blow that comes from the first
nozzle towards the direction of the carrier face and of the plane of
the web running over the carrier face, that in the lateral area of
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said plane carrier face opposite to the first nozzle, there is a
substantially planar guide face or guide wing, which extends said
planar carrier face continuously, that said guide face is parallel to
the plane of the carrier face or, when a guide wing is used, it is,
starting from the plane of the carrier face, to some extent inclined
towards the web, and that in connection with, or at the proximity of,
the outer edge of said guide face or guide wing there is a second
blow nozzle, whose direction is parallel to the guide wing of said
guide face.
In the nozzle in accordance with the invention, no field of
positive pressure is formed, which would cause folds in the web,
thereby deteriorating the running quality. By dimensioning the
nozzles in accordance with the invention and the carrier faces as
well as the air velocities it is possible by means of the invention
to accomplish both good transfer of heat and improved running ~uality
of the web.
In an attempt to achieve a maximal transfer of heat, in the
nozzles a blowing in a direction contrary to the direction of running
of the web is used.
In the following the invention will be described in detail
with reference to an advantageously exemplifying embodiment of the
invention illustrated in the figure in the accompanying drawing, the
invention being not strictly confined to the details of said
embodiment. -~
BRIEF DE8CRIPTION OF THE DRAWING
The figure is a vertical sectional view of a preferred
exemplifying embodiment of the geometry of a nozzle in accordance
with the invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENT8
The nozzle arrangement in accordance with the figure
comprises a nozzle box 10, out of whose interior the gas blown
through the openings 11 is passed into the spaces 12 and 13 in the
nozzle box, said spaces being defined between the inner walls 14, 15
and the outer walls 21 and 26 of the nozzle. The inner wall 14 is
curved at its upper edge towards the carrier face 16, being
substantially shaped as an arc of a circle (radius Rl). The nozzle
box 10 has a planar carrier face 16, over which the web W runs as
supported free of contact. From one side, the first nozzle 17 is
defined b~ the curved wall 16R (curve radius R1), which qoes on
continuously as a planar carrier face 16. In a preferred embodiment
of the invention, the curve radius of the curved guide face of the
first nozzle 17 is within the range of Rl = 10...35 mm. From the
other side the first nozzle is defined by the edge plate 24. The
walls 14 and 21 of the lateral ducts 12 are interconnected by
intermediate parts 23.
The direction blowing SO of the first nozzle 17 is
perpendicular to the plane of the web W to be supported. The blowing
out of the first nozzle 17 is turned on the basis of the Coanda
effect, being guided by the curved guide fàce 16R, maximally about
70-, whereupon th~e blowing,~which is illustrated in the figure by the
vector v1 is directed to some extent towards the web W so that the
blowing vl has a larger speed component vl~ parallel to the plane
of the carrier face 16 of the web W as compared with the
perpendicular speed component vlh.
In the connection described above, the novelty of the
invention is the arrangement and direction of the second nozzle 18.
The second nozzle 18 is opened into the space 13, into which the air -
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i8 passed from the interior of the nozzle box 10 through the slot 11. ~ ~ ~
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From the side of the web W, the second nozzle 18 is defined by the
plane end 16a of the carrier face, or by a guide wing 16T
(illustrated with dashed lines) bent out of it. The guide wing 16T,
if used, is a continuous and direct extension of the carrier face
16. The guide wing 16T "rises" ~rom the carrier face 16 so that the
"difference in height" between the guide wing 16T and the plane
carrier face 16 is hl ~ 0...3 mm, most appropriately h ~ 1.0 mm.
The length L of the wing 16T in the direction of running of the web W
is L ~ 2...20 mm, most appropriately L 2Y 5 mm.
In some special cases the guide wing 16T may be slightly
convex or concave when seen from the side of the web W, which is,
however, as a rule, not advantageous. The guide wing 16T directs the
second blowing v2 towards the web W in a direction substantially
parallel to its plane.
From the second nozzle 18, which is placed underneath the
edge 20 of the guide wing 16T and which extends across the entire
width of the web W, the blowing v2 is directed substantially in the
direction of the plane of the web W. The direction of the blowing
V2 forms advantageously a small angle a in relation to the web W
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plane facing it. Under these circumstances, the blowing v~ always
has a considerably large speed component v2h parallel to the plane
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of the web W and a speed component v2h, which is perpendicular to
the web W and directed towards the web and which is equal to zero or
more appropriately quite small. The angle a is, as a rule, chosen
within the range of a = 0... 90-, most appropriately a = 0... 15-. The ~;
plane carrier face 16 and the guide wing 16T, if any, rising towards
the plane of the web W together form an air-support face in whose
area a negatively pressurized foil zone K is formed that guides the ~ -
web W.
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The magnitude s1 of the first nozzle slot 17 is most
appropriately within the range of S1 = 1.5...3.0 mm. In a
corresponding way, the magnitude S2 of the second nozzle slot 18 is
S2 = 1.5...2.5 mm, most appropriately s2 ~ 2 mm. The air
velocities v1 and v2 in the first and the second nozzle 17, 18
are, as a rule, within the range of 15...85 m/s. These velocities
can be adjusted to be of different magnitudes compared with one
another (vl ~ v2) with a view to optimizing the operation o~ the
nozzle arrangement.
Even though, above, nozzle slots have been spoken of, it is
understood that, in some special cases it is possible to use a
corresponding series of nozzle holes or series of nozzle slots.
By means of the speeds components vlv and v2v parallel
to the plane of the web W of the blowings vl and v2, a stable
control of the web W is achieved, whereas by means of the speed
component v1h or by means of the speed components v1h and v2h
directed towards the web breaking of the boundary layer and
improvement of the coefficient of heat transfer are achieved.
Depending on whether, in the field of application of the
invention the main emphasis is on stable running of the web or on
maximization of the heat transfer coefficient (efficiency of drying),
a blowing applied`to the web either downstream or upstream is
applied. In some special cases, it is also possible to apply the ~;
invention such that in subsequent nozzle boxes or groups of nozzle
boxes directions of blowing contrary to one another are used in
ad~acent boxes. ~ -
The apparatuses of drying and supporting in practice
comprise several nozzle boxes 10 similar to that illustrated in the
figure, extending across the entire width of the web W and fitted one
after
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the other, said boxes 10 comprising intermediate spaceR through which
air is sucked as exhaust air or for circulation. The subsequent
nozzle boxes 10 are attached, e.g., to a common support plate 30, the
blow air being introduced through openings 31 provided in said plate
30 and through related air ducts, in the direction of the arrows A,
into the nozzle boxes 10, and the air being distributed out of said
boxes through the openings 11 into the spaces 12 and 13 and from
these further into the nozzle slots 17 and 1~ which extend across the
entire width of the web W.
In the following, the patent claims will be given, whereby
the various details of the invention may show variation within the
scope of the inventive idea defined in said claims and may differ
from the details given above.
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