Dryer section

A web guiding structure in a drying section downstream from a coating device, and having suspension and infrared dryers and with a directly preceding coating device for paper or board. A rigid beam-like chest-like web bend device with blowing elements is adapted to produce a pressurized gas cushion for the web for contact-free bending of the web in its direction through an angle between 50.degree. and 120.degree., so that with an increase in its distance from the bending device, the web also moves away from the coating device. Following the bend device a heating device or a suspension device having beam-like, chest-like blowing air guide devices in each case produces a course of the web undulating in the direction of movement of the web (web valleys and crests extending in the transverse direction) on the upper and lower sides of the web. Further dryers are included, with radiation and/or a hot gas action, and are alternately arranged with drying devices in the form of infrared dryers and suspension dryers, alternating at least twice.

BACKGROUND OF THE INVENTION 
The invention relates to a web guiding structure in a drying section, 
downstream from a coating device and having suspension and infrared 
dryers. Such a device is described in the German patent publication 
3,910,898 A, so that the combination of infrared and suspension dryers is 
known. 
One object of the invention is connected with the problem of so guiding the 
web of paper or board after coating thereof, while it still has a moisture 
content of approximately 30%, that the web is dried and the web is 
subjected to minimum stresses and only has a minimum tendency to form 
folds, more particularly in the longitudinal direction. There has been a 
proposal in the European patent 236,819 to bend a web relatively sharply 
along an arc in a contact-free manner by a rigid, beam-like air chest with 
air blowing elements in order to prevent the wet material of the coating 
composition sticking to a roll as is otherwise utilized for bending and 
guiding the web. 
The object of substantially reducing web corrugations in the transverse 
direction and of preventing the development of folds is to be attained 
using the features of the present invention. Web corrugation in the 
transverse direction means that the corrugations (crests and troughs) 
follow each other in the transverse direction of the web. 
In order to better stabilize the course of the web it is possible to 
provide a beam-like air plenum or chest with jets upstream from the 
deflecting device, and directed against the outer side of the web arc. 
Preferably this beam-like device is curved, there then additionally being a 
more powerful blowing action at the center of the web than at its edges so 
that there is a more pronounced curvature or bending in the center than at 
the edges and therefore a tendency to stretch it in the breadth direction. 
Furthermore, it is appropriate as well if the drying is substantially 
performed to completion, there being an alternating succession of infrared 
and suspension dryers, an optimum maximum drying being achieved in the 
shortest possible distance. 
Although there has been a proposal in the European patent publication 0 291 
832 to have an alternating succession of infrared and suspension dryers, 
this was not for the same purpose as in the present invention, that is to 
say smoothing the web by guiding at along an undulating path. Accordingly, 
the suspension dryers in this case have such a design that the web is not 
caused to undulate but is rather more guided along a straight path at a 
very regular distance from the infrared dryers in order to make best use 
of their heating effect. After each infrared dryer a suspension dryer is 
arranged for the removal of the water vapor, which is formed under each 
infrared dryer. The dryers are arranged on only one side of the web. 
SUMMARY OF THE INVENTION 
In the invention the longitudinal undulation of the web (i.e. waviness in 
the longitudinal direction of the web) causes stiffness in the transverse 
direction of the web in the initial part of drying, that is to say prior 
to the subsequent dryer group designed with drying cylinders, so that 
waviness or corrugation in the transverse direction of the web is reduced 
and the formation of folds is prevented.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1 the web B is moved through a coating device which for 
instance comprises two pressing rolls 12 and 13 and each of which has an 
application device 51, 52, respectively, for a coating composition. The 
vertical direction is in this case denoted by V and it extends essentially 
parallel to the longitudinal edges of the plane of the drawing. At an 
angle of between 25.degree. and 45.degree. the web is drawn downwards 
following the coating device and firstly passes to a web stabilizing 
device 6, and thereafter a web bending or deflecting device 2 using air 
and having blowing jets 21 arranged along an arc in a row side by side. 
Due to the tension of the web and the air pressure of the blown air from 
the web bending device 2, the web is guided onwards in an arc without 
making contact and then arrives in a suspension drying device 3, which has 
blowing devices 23 as well. On the other side there is also a similar 
suspension drying device 3'. The blowing devices 23 of the two suspension 
drying devices ar arranged opposite to each other with an offset so that 
the web moves along a sinusoidal path in a known manner. The temperature 
of the blown air is preferably between 150.degree. and 300.degree. C., 
although it may be at room temperature, in which case it is no longer a 
question of a suspended web drying device but of a simple suspended web 
guiding device. Preferably, however, air at a temperature substantially 
above normal is used so that the web is dried as well. 
The blowing devices have lateral slots for the emergence of the air, which 
extend adjacent to the pressure surface 31 for the air cushion. Between 
the blowing device the air is removed again (see U.S. Pat. No. 4,833,794). 
The purpose of the undulating path of the web in this portion is to prevent 
the waves produced due to transverse shrinkage of the web, which is 
relatively moist after leaving the coating device, from becoming larger or 
turning into folds. Furthermore the action of the web stabilizer 6 and of 
the contact-free web bending device 2 is to considerably reduce this 
waviness in the transverse direction which is due to the extension of the 
web. Then following the suspension dryers 3 and 3' there is an infrared 
dryer 4', which in turn is followed by suspension dryers 3" and 3'" with 
blowing devices offset in relation to each other in the direction of 
movement of the web. Such blowing devices are essentially in the form of 
chests of beams and are of known design, see for instance the U.S. Pat. 
No. 4,833,794. The contact-free web bend device 2 allowing for the "air 
turn" may be, for instance, as described in the European patent 
publication 0 236 819 A1. 
The waviness of the web in the suspension dryers 3, 3' and respectively, 
3'" or the sine wave form thereof should have a wavelength of between 20 
and 60 cm and preferably between 20 and 40 cm. 
FIG. 2 furthermore indicates that suspension drying units 3 and infrared 
units 4 are arranged in alternate succession, the last infrared unit being 
reference I.sub.2 and the last suspension drying unit being reference 
S.sub.2. After this, the web is diverted onto a breadth stretching or 
tentering roll 16 and passed to the first drying cylinder 15, which is 
followed by further drying cylinders in a known manner. The angle to the 
vertical V at which the web is moved upwards after bending preferably 
amounts to between 40.degree. and 65.degree.. The web bands as it passes 
through web bend device 2 at a bend angle of between 50.degree. and 
120.degree.. 
The first blowing device 23 of the suspension device on the outside of the 
arc in this case directs air radially inwards against the convex side of 
the web B so that the web, or respectively the course thereof, is 
completely regular at the end of the arc as well. 
FIG. 3 illustrates that the contact-free web bending device 2 is made in 
the form of a curve so that in the middle of the deflection arc the web is 
more deflected than towards the margin thereof. Furthermore the pressure 
of blowing is made higher in the middle than towards the edge so that the 
web assumes the form shown in broken lines in an exaggerated manner. This 
in addition enhances the effect of reducing the formation of waves 
transversely in relation to the course of the web and preventing the 
formation of folds. The curvature is in this case naturally shown with 
some exaggeration. 
It is furthermore possible to adopt only the one or the other measure. The 
difference in the blowing pressure between the middle and the edge should 
amount to approximately 20% and the curvature to between 5 and 10 times 
the width of the web. 
The bend device 2" with blowing nozzles in accordance with FIG. 4 
furthermore has blowing device 21' radially on the outside of the arc of 
the web; in this case air is utilized and drying of the web may commence 
here on both sides. 
The effect described in the above is also produced by a stretching of the 
web in the width direction in this case, and a further purpose of the 
arrangement is to cause an even greater stabilization of the web. 
FIG. 5 shows the overall arrangement in a practical form thereof, the 
preferred arrangement being so designed with regard to the removal of 
moisture that the last heating dryer 68 arranged upstream from the drying 
roll 15 of the following drying roll group is constituted by a suspension 
dryer. 
The suspension dryers are denoted 66 through 68 and 66' through 68', the 
infrared dryers 69 and 70, the bearing units for the coating rolls 12, 13 
are shown as 61 and 62, respectively, and the floor of the machine as 63. 
The contact-free bend device 2 is mounted by means of bearings on the 
foundation of the machine. 
While this invention has been described as having a preferred design, the 
present invention can be further modified within the spirit and scope of 
this disclosure. This application is therefore intended to cover any 
variations, uses, or adaptations of the invention using its general 
principles. Further, this application is intended to cover such departures 
from the present disclosure as come within known or customary practice in 
the art to which this invention pertains and which fall within the limits 
of the appended claims.