Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Air doctor
The present invention relateQ to an ai:r doctor in accordance
with the preamble of claim 1.
Air doctors are used for doctoring away exce~s coating mix
from the web surface in the co~ting process of paper web~.
During coating~ the mix is generally applied onto the web
with an excess of approx. 20...50 %, and after application,
the excess is removed by means of an air doctor. Extending
over the entire length of the web, the air doctor has an air
channel which exits into a narrow, slitted nozzle directed
toward the web. When the channel of the air doctor is
pressurized with a compressor, a narrow air knife jet is ~-
discharged from the nozzle toward the web. The air lcnife is
generally oriented contradirectionally to the machine
direction o the web. Air doctors can as well be used for
issuing against the web other types of gases than air such
as different kinds of vapours. These substances can be
availed to control the propertie~ of the coated web
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including, e.g., its moisture.
The nozzle of the air doctor must be relatively narrow,
since the air knife jet exiting from the nozzle has to be
thin and sharply delineated. Because the air doctor is ;
placed close to the web, the air jet discharging from the
nozzle splashes the coating mix also on the nozzle, thereby
soiling it. With the contamination o~ the nozzle, the air
knife jet becomes weaker and less defined, whereby the air
knife cannot anymore accurately doctor away the surplus
coating mix from the web surface. Thence, at the soiling of
the nozzle, the coater must be run down and the nozzle
cleaned, which results in production halts.
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In order to make it pos~ible to en~ure a continuous
operation at the coater, rotatable air doctors with two
nozzles have been developed. In such an air doctor the
nozzles are placed to the opposite sides of the longitudinal
axis of the air doctor, and at the soiling of the nozzle,
the air f,eed is cut off and the air doctor iY rotated by
180 so as to aim a clean nozzle toward the web. The
contaminated nozzle can now be cleaned, and after cleaning,
be correspondingly rotated toward the web when the other
nozzle in its turn becomes soiled.
A prior art construction has two air channels extending over
the entire width of the web, each channel exiting via a
separate nozzle. The air channelq have cut-off means, with
which the air feed to the nozzle in the cleaning position is
shut off, while the air feed channel to the other nozzle,
which is in the air knife position, is opened. This air
doctor construction has additionally narrow air feed ducts,
through which air i9 blasted onto the nozzle being in the
cleaning position. These small ducts are connected to a sep-
arate auxiliary compressor. The purpose of these ducts is to
prevent the nozzle being in the cleaning position from
becoming clogged by any possible mix residues in the nozzle.
The above-described air doctor with its plurality of air
ducts and compressors is a complicated and expensive design.
The air flowing in the air channel is heated at the
restrictions of the channel, since a channel of relatively
small cross section must be used. Because the air is fed to
the nozzle being used via a dedicated channel of the nozzle,
the air doctor becomes heated from side of the air channel
being used, and the heating results in an asymmetric tension
in the air doctor body that distorts the body.
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~nother construction of dual-channel air doctor is comprised
of a round pipe forming the air duct, and of a body adapted
about the pipe. The air channel has a 31it exte~ding over
the entire width of the web and aiming toward the web.
Correspondingly, the body has two oppoaingly aligned
openings both of which exit into their respective nozzles. `~
puring the operation of the air brush, the air flows via the ~ ;~
air channel and the openings of the body to the nozzle~ and
therefrom Eurther toward the web. With the contamination of
the nozzle, the body is rotated by 180 about the pipe
forming the air channel, whereby the one of the openings of ~ ;
the body and the nozzle associated with it are rotated to
coincide with the ~lit of the air channel. This construction ~`
is simple and practicable. Such an air doctor has, however,
relatively high production costq because of the tight, yet
rotatable joint that must be fabricated between the air
channel pipe and the body. For air doctors used in paper
machines of particularly large width, the fabrication o
such a joint is complicated.
It is an object of the present invention to achieve such an
air doctor of simple and reliable construction that provides
for an easy control of the air knife jet.
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The invention is ba ed on designing the air doctor to have a
body enclosing an air channel leading to nozzle ducts for
i~quing air knife jets onto the web, said ductq being
provided with cut-off means. Such cut-off meanq can be u~ed
for shutting off air flow to the nozzle when the nozzle is
in the cleaning position.
More specifically, the air brush in accordance with inven-
tion is characterized by what is stated in the characteriz-
ing part of claim 1.
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The invention provides 3ignificant benefits.
The air doctor in accordance with the present invention has
a particularly simple construction. ThuiY, it can be produced
at a low cost and provides an extremely reliable operation
even in coaters of large web widths. The air flow rate
through the nozzles is easy to control by choking as
necessary according to run conditions and coating mix used. ~,
A construction in accordance with one of the embodiments of
the invention makes it also possible to achieve an air knife
jet with a vortex, complemented with the poiYsibility of
altering the direction of the vortex. By virtue of a single
large-diameter air channel in the air doctor, an even heat
distribution over the entire cross section of the air doctor
is achieved. Air flows in this channel at a low rate,
thereby avoiding the generation of heat streY~e3 from
restriction of the air flow and isubsequent heating. Since
the distortion of the air brush body by heat i3tresses is
circumvented, the nozzle tip can be held at a constant
distance from the web. Due to the single-channel construc-
tion of the air doctor, only a single compresisor is needed.
The nozzle of the air doctor in accordance with the inven-
tion can be provided with a baffle, whereby a vortex with a
desired direction can be imposed on the air knife jet.
The invention iq next examined in detail with the help of
the attached drawings.
.
Figure 1 shows a cross-sectional view of an air doctor
construction in accordance with the present invention.
Figure 2 shows a partially cross-isectional view of another
air doctor construction in accordance with the present
invention.
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2~3~74
Figure 3 shows a partially cro~s-sectional view of a third
air doctor construction in accordance with the present
invention.
The preferred embodiment of the invention is illu~trated in
Fig. 1. The air doctor shown in Fig. 1 comprises a body 1
forming a wide air channel 2. Exiting from the opposite
sides of the air channel 2, there are nozzle ductq 4 routed
to nozzles 3. Flexible pressure hoses 5 and 6 are fitted
onto grooves fabricated on the nozzle ducts 4, said hoses
having a ridged-tip profile 7 on their sides facing the
nozzle duct 4.
The body 1 with its enclosed air channel 2, the nozzle ducts
4 and the nozzles 3 are extended over the entire width of
the web. When the nozzle 3 being in the operating position
becomes soiled, its associated nozzle d~lct 4 is Qhut o~f,
and the air doctor is rotated about its longitudinal axis by
180, whereby the nozzle 3 on the opposite side is rotated
to be oriented toward the web. Then, the nozzle duct 4
leading to the nozzle 3 is opened, allowing the air to
discharge via the nozzle 3 onto the web.
In the air doctor illustrated in Fig. 1, the nozzle 3 being
in the cleaning position is shut off by releasing compressed
air into the pressure hose 6. The pressurized hose 6 then
expands outwardly from its groove, and the tip part 7 of the
hose 6 is pressed tightly against the opposite wall of the
nozzle d~ct 4. During the time the nozzle duct 4 of the
nozzle 3 being cleaned is shut off, air is discharged from
- the air channel 2 onto the web to be coated via the nozzle 3
being used as indicated by the arrows in Fig. 1. The
pressure hose 5 in the air duct 4 of the nozzle 3 being in
the operating position is depressurized, whereby the hose 5
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contracts to the bottom of the groove, thuq allowing an
unobstructed air flow in the nozzle duct 4.
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One of the advantage~ of the thiq embodiment i9 that, when
desired, a small air flow can be arranged to pas~ via the
nozzle 3 being in the cleaning position as a controlled
leakage from the air channel 2 by appropriately notching the
tip part 7 of the pressure hoses 5 and 6.
In an advantageous e~bodiment of the invention illustrated
in Fig. 2, the pressure hose with a ridged-tip profile is
replaced by a conventional pressure hose 8 and a flap 9
adapted over the groove of the nozzle duct 4. The flap 9 is
attached at its edge on the side of the air channel 2 to the
edge of the groove, allowing its rotation about this
attachment. In this embodiment the nozzle duct 4 is shut off
by inflating the pressure hose 8 in the same manner as
described above. The expansion of the hose rotates the other
edge of the flap 9 against the opposite wall of the nozzle
duct 4, whereby the flap 9 obstructs the nozzle duct 4. The
other side of the air doctor, not shown, is the mirror image
of the described diagram. The illustrated construction makes
both walls of the nozzle duct 4 smooth when the duct 4 is
controlled to open.
The advantageous embodiment illustrated in Fig. 3 has a
baffle 10 adapted to the middle of the nozzle ducts 4. Both
sides of the duct 4 have grooves filled with the cut-off
means 8, 9 shown in Fig. 2. The baffle 10 divides the nozzle
duct 4 into two parts, making the independent cut-off of
both parts possible. Closing one of the ducts generates a
vortex into the air jet exiting the nozzle 3, and further,
allows the selection of the rotational direction of the
vortex according to which one of the ducts is cut off. When
the nozzle 3 has been rotated to the cleaning position, the
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ducts on both sides of the baffle are shut off. ~ven in thiq
construction, tha other side with its nozzles 3, ducts 4,
baffles 10 and cut-off means 8, 9 is a mirror image of the
construction shown in this diagram.
The disclosed invention is not limited to the alternative
embodiments described above. Thus, the flap 9 used in the
embodiments illustrated in Figs. 2 and 3 can be replaced by
a flexible membrane. Further, the medium used in the
pressure hose 5 can alternatively be either a gas or a
liquid. A controlled air leakage to the no7zle 3 being in
the cleaning position can also be achieved by a reduction of
the pressure in the hose. This arrangement dispense~ with ~--
the need for use of a notched hose 5, 6. Moreover, the shape
of the air channel 2 need not necessarily be square, but
in~tead, can be freely selected to have, e.g., a circular or
ellipsoidal cross section. The shape of the nozzles 3 and
nozzle ducts ~ must, of course, be determined with the
design methods o flow dynamics separately for each case to
obtain a desired form of the air knife jet. The slitted gap
of the nozzle 3 can be either fixed, or alternatively,
variable by conventional means such as open/close screws.
Finally, the cut-off means 5 can be located within the
nozzle duct 4 in a manner different from that described for
the exemplifying embodiments.
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