Fixation device

Fixation device for permeable web materials, such as felts and wires for paper machines, where air at controlled temperature and pressure is pressed through the web from jets in at least one pressure chamber (12) on one side of the web, to at least one corresponding suction chamber (15) on the other side of the web, the device thereby comprising an upper structure (9) and a lower structure (10) between which the web is running during operation where a pressure chamber (16) with a corresponding suction chamber (13) being arranged in front of, and a pressure chamber (14) with a corresponding suction chamber (11) being arranged behind, at least one middle pressure chamber (12) with a corresponding suction chamber (15), each pressure chamber (12, 14, 16) being provided with two jets arranged in an acute angle to the web and towards each other, with one front jet (17, 19, 21) and one rear jet (18, 20, 22), thereby establishing and maintaining an overpressure between the jets and the web, jets arranged in opposed pressure chambers and adjacent each other thereby directing air passed each other in such a way that air from one pressure chamber substantially is prevented in penetrating into the adjacent suction chamber, and the upper and lower structures (9, 10) being provided with ducts (30, 31) for an air flow having a controlled temperature, thereby avoiding deformations of the upper structure (9) due to temperature influences from the heated air in the fixation process.

The present invention is related to a fixation device for felts and wires 
for paper machines, according to the preamble of the claims. 
Felts and wires for paper machines today are produced in widths up to 10 
meter and constructed to withstand large strains of different types, such 
as high moisture content, large temperature, large pressure, tension 
forces in the longitudinal direction and influence from chemicals. 
The felts and the wires, shaped as endless bands, are running at very hight 
speeds in the paper machines, in extreme cases up to 2000 meters per 
minute. It therefore is substantial that they are homogeneous both in the 
longitudinal as well as in the thermally stable direction to ensure 
operation without problems. 
One of the processes involved in the production of all types of felts and 
wires today is the heat treatment at a temperature substantially higher 
than the highest temperature by which the products are used in the paper 
maschines. This heat treatment, the fixation, provides the felts and the 
wires with thermical stabile properties and desired stretching properties. 
Additionally the felts and the wires also are treated with chemical 
solutions hardening at specified temperatures. 
To provide the felts and the wires with the desired properties it is 
important that the same maximum temperatures is achieved across the whole 
width during the fixation process. Furthermore it is desired to achieve 
the same temperature throughout the thickness of the felt and the wire, in 
other words in the upper and lower surfaces as well as in the middle. 
During fixation, the felt and the wires, as endless bands, are stretched 
between two parallel rolls of which at least one is driven in such a way 
that the felts can be moved through the fixation zone which is arranged 
parallel to and between the axis of the rolls. 
A normal heating method in the fixation process is blowing heated air 
against both sides at the felt. As stated above, it is very important, and 
necessary, to maintain the same air temperature across the whole width of 
the felt or the wire. It is, however, not sufficient that only the 
temperature of the blown air is the same across the width as pressure 
differences may occur in the fixation zone due to small differences in the 
high air velocity which is necessary to achieve a good heat transfer to 
the felt. Surrounding air very easily will be drawn into the process by 
induction, which means that the environmental air is drawn into the 
fixation zone itself and here mixed with the air having controlled 
temperature. 
The main parameters influencing the result of the fixation process are the 
temperature of the air penetrating the felt, the amount of air 
penetrating, the velocity of the air through the felt and strongly 
influencing the afore-mentioned, the distance between the jets and the 
felt is of major importance, e.g. the distance must be uniform along the 
entire length of the jets. Influencing the distance between the jets and 
the felt during operation is the behaviour of the upper structure due to 
the heat influence, which across a length of 10 meters or even more, can 
lead to a not acceptable distance difference between the middle of the 
structure and the ends. 
Any leakage of surrounding air into the fixation zone will lead to a 
temperature reduction, and especially local reductions thereby increasing 
the danger of undesired temperature variations across the surface of the 
felt. Such air leakage is reduced to a certain extent by the use of 
sealings at apertures where the felt and the wire is transferred into or 
out of the heating zone, but a complete elimination so far has been 
difficult to achieve in practical work. By the fixation device according 
to the present invention, however, this problem in the fixation process is 
avoided in such a way that leakage air from the environment is not 
effecting the conditions in the fixation zone. 
This advantage is achieved by the fixation device according to the 
invention as defined by the features stated in the characterizing clauses 
of the claims.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 discloses the principal structure of a fixation device according to 
the invention, comprising an upper structure 9 and a lower structure 10 
extending across the total width of the felt or the wire 27, 
perpendicularly to direction of movement of the felt. The felt as such 
creates an endless band, whereas only a short segment is disclosed in the 
Figure. 
The upper structure 9 comprises a middle pressure chamber 12 supplied with 
pressurized air through a slit 28 in a duct 29, arranged in such a way 
that the air conditions as to temperature and pressure in the lower end of 
the pressure chamber 12 and especially across the full length of jets 5 
and 17 arranged in the chamber, are substantially uniform. 
The upper structure 9 is arranged in a distance from the lower structure 
10, whereby the felt or the wire 27 is transfered between the structures. 
A middle suction chamber being arranged in the lower structure 10 is 
adapted to receive the pressurized air from the middle pressure chamber 12 
in the upper structure after the air having passed through the felt 27. 
From the suction chamber 15 the air is brought to the devices adapted for 
conditioning of the air before reentrance into the pressure chamber 12, 
when using a closed circuit system. 
In the lower structure 10, one pressure chamber is arranged on each side of 
the middle suction chamber 15, a front pressure chambers 16 and a rear 
pressure chamber 14. The pressure chambers 14 and 16 are supplied with 
pressurized air principally in the same manner as the pressure chamber 12, 
in such a way that the pressure conditions in the upper parts of the 
pressure chambers 14 and 16 being substantially uniform when entering 
first and second jets 19 and 22 in the pressure chamber 14 and first and 
second jets 21 and 20 in the pressure chamber 16. 
Front and rear suction chambers 13 and 11 are arranged in the upper 
structure 9 opposed to the lower pressure chambers on each side of the 
middle pressure chamber 12. From the suction chambers 11 and 13 the air is 
guided to devices for reintroduction into the pressure chambers 14 and 16, 
separately or independently when using a closed circuit system. 
The front jet 17 in the middle pressure chamber 12 is arranged on the 
inside, e.g., towards the middle of the fixation device as such, of the 
second jet 20 in the lower pressure chamber 16 in such a way that 
substantially all air from the jet 17 pressed through the felt and into 
the middle suction chamber 15 whereas the air from the jet 20 in the lower 
pressure chamber 16 passed into the front suction chamber 13. 
All jets of the pressure chambers are arranged substantially in the front 
or rear part of the chambers and directed in an acute angle in relation to 
each other and to the web in such a way that the air from the jets is 
building up an overpressure between the jets and the felt and pressed 
through the felt into the opposed suction chamber. In this way the jets 20 
and 21 of the front pressure chamber 16 in the lower structure are 
directing the air into the opposed suction chamber 13 in the upper 
structure, the jets 17 and 18 in the middle pressure chamber 12 are 
directing the air into the opposed middle suction chamber 15 and the jets 
19 and 22 of the rear pressure chamber 14 are directing the air into the 
opposed suction chamber 11 in the upper structure 9. 
For furthermore to avoid the air coming from the rear jet 20 of the front 
pressure chamber 16 and from the front jet 19 of the rear pressure chamber 
14 in penetrating through the felt into the middle suction chamber 15, a 
bar 23 is extending across the entire length of the structure arranged in 
front of the jet 17 thereby substantially closing the gap between the felt 
and the structure without touching the felt to thereby avoid destruction 
of the felt surface. The air from the jet 20 thereby is guided on the 
outside of the bar 23, respectively the air from the jet 19 being guided 
on the outside of a bar 26 arranged behind the jet 18. Correspondingly, by 
means of a bar 24 inside the jet 20, and a bar 25 inside the jet 19, air 
from jets 17 and 18 is guided on the inside of the bars 24 and 25, thereby 
additionally contributing to avoid air from the front and rear pressure 
chambers to be brought into the middle air system. The bars 23-26 have 
such a form and are such arranged that they do not or just even touch the 
felt and substantially close the distance between the felt and the upper, 
respectively the lower structure 9 and 10. 
During operation the environmental air necessarily is sucked in together 
with the felt 27 through the apertures between upper and lower structures 
9 and 10, at both their front and rear portions. This environmental air 
will be taken up by the suction chambers 11 and 13 together with air from 
the suction chambers 11 and 13 and therefore will not influence the heat 
treatment of the felt or the wire. 
As disclosed in FIG. 1, the upper structure 9 is provided with a support 
structure having closed channels 30 in the upper portion and corresponding 
closed channels 31 in the lower portion. The upper structure naturally 
also can be provided with further channels for the same purpose. The 
channels 30, 31 are used for air flow having a controlled temperature 
thereby securing that the upper structure 9 is not being deformed by 
influence of the heat from the fixation operation. This feature is 
essential especially for upper structures 9 having a relatively small 
height and bridging a rather long distance across the felt. The air 
through the channels 30 and 31 may be taken from one of the pressure 
chambers or from an auxiliary device. 
By providing a separate circuit for the air between the middle chambers and 
a further circuit for the air in the outer chambers, possibly separate 
circuits even for the two outer chambers, a uniform treatment of the felt 
is ensured during fixation and thereby a homogeneous felt. Control of such 
circuits may be computerized by a computer device. 
Tests have been performed with measurement of the temperature in the 
internal parts of the felt and on the surface of the felt at measuring 
points 1-8 as disclosed in FIG. 1. The measured values from those tests 
are disclosed in diagrams FIG. 2 and 3. Nominal temperature of the air was 
150.degree. C. and the resting time of the felt in the treatment zone was 
approximately 2 minutes and 15 seconds. The weight of the felt used was 
1.03 kilo per m.sup.2 and the permeability of the felt was 40 meter per 
minute at 250 Pa. 
As disclosed in the diagrams 2 and 3, a very homogeneous temperature curve 
was achieved between the measuring points 4 and 6 and the temperature 
curves in this area approximately are the same on the upper surface of the 
felt as in the middle of the felt.