Guide roll and suction box for twin-wire forming system

A procedure for dewatering and detaching a paper web from the second former wire. The composite of first and second wire and interposed web is contacted with a first cover part of a special suction box. A roll guiding the second wire, pressing from the side of the latter deflects its run. This pressure plus centrifugal force, and the differential pressure arising from the suction box, dewater the web. The second wire is separated to follow along with the guide roll, while web and first wire are acted on by the suction box vacuum. The first wire is then contacted with a second cover part of the suction box. A twin-wire former is also disclosed, featuring within its carrying wire loop, on its twin-wire run, a rotating forming roll deflecting the composite of wires and web as it laps a given sector of said forming roll. A special suction box at the ultimate end of the wires has a bipartite cover its parts angulated against each other and preferably planar.

The present invention concerns a procedure in the wire section of a paper 
machine, wherein the web formation at least partially takes place between 
two wires, for dewatering the web and for detaching the web from one of 
the two wires. 
The present invention moreover concerns a twin-wire former comprising a 
loop of the carrying wire, in the initial part of the run of which the web 
is formed with dewatering taking place in one direction therethrough, and 
a loop of a covering wire, which is at a suitable web forming stage 
conducted to cover the web and the carrying wire loop, in such manner that 
dewatering of the web is achieved through the covering wire loop as well, 
and in support of which after the twin-wire section the web is transferred 
for further conduction to the press section of the paper machine. 
The present invention concerns twin-wire formers which are generally of the 
type disclosed in applicant's Finnish Pat. No. 50648, which corresponds to 
U.S. Pat. No. 3,846,233. Such former comprises a lower wire loop, the web 
being in actual fact formed on the substantially horizontal run or on a 
run thereof deviating from horizontal to some degree, after the headbox, 
with dewatering taking place in one direction downwardly through this 
wire, and an upper wire loop, which at a given web forming stage is 
conducted to cover the lower wire loop and the wet web being supported by 
the lower wire loop, in such manner that the dewatering takes place also 
through the upper wire. In this former the web being produced rests on 
termination of the forming and dewatering process supported on the lower 
wire, from which the web is thereafter detached for further conduction to 
the press section of the paper machine. 
In a modification of the known former, the upper wire loop, i.e. the 
covering wire, also operates as a web forming wire, but that modification 
is concerned with the production of a two-ply or multi-ply web. The 
present invention may in certain substantial parts be applied in such 
formers as well in which a two-ply or multi-ply web is produced. 
In said Finnish Pat. No. 50648 corresponding to U.S. Pat. No. 3,846,233, a 
special stationary forming shoe is used after a single-wire forming part, 
this forming shoe being provided with a suction zone and/or a blowing 
zone, but in the use of the stationary forming shoe a problem has been 
encountered that as it is operating against the carrying wire it often 
causes excessive attrition of the wire. Similarly, the surface of the 
forming shoe facing the wire is subject to wear at the same time. A 
problem additional to the wear problem just mentioned is the fact that the 
friction arising from the stationary shoe gives rise to extra energy 
consumption. In the said Finnish Patent has been disclosed, as an 
alternative, the use of a rotating dewatering roll, but this affords only 
a partial solution to the problems. 
Regardless of whether formers of the described above are used to produce a 
single-ply or two-ply web, a problem frequently encountered is that after 
the dewatering stage between two wires the web fails to follow reliably 
along with the lower, carrying wire, but tends instead, from time to time 
at least, to follow along with the upper, covering wire. This may result 
in a web break and shutdown of the machine. In addition, even if no actual 
break should occur, the web may partially adhere to the upper wire, which 
may have the consequence for instance of poor upper surface finish of a 
single-ply web or of a splitting tendency of the web in the case where a 
two-ply web is being produced. 
The object of the twin-wire former of the invention, considered as a whole, 
is to provide a construction which causes only a minimum of wire attrition 
and which reduces the drive energy consumption of the wire section, while 
at the same time the consumption of suction energy required for the actual 
dewatering is reduced. In general, structural components causing wear of 
the wire are naturally all those stationary members which guide the run of 
the wire and exert an influence on the dewatering of the web. In the 
present invention, the major part of dewatering takes place in a manner 
known in itself in the art on the first, single-wire part of the wire 
section, where stationary so-called foils are used. Considering the large 
water quantity escaping in the initial part of the wire section, which 
substantially reduces the friction of the wire against the dewatering 
members, the foils have rather minor significance as members causing wear 
of the wire and their use is to be recommended. Naturally, the foils may 
in some instances, as need arises, be totally or partially replaced with 
rotating dewatering structural components, such as table rolls or 
equivalent. 
In order to achieve the aims stated above and others which will become 
apparent later on, the procedure of the invention is mainly characterized 
in that the procedure comprises the following steps: 
conduction of a sandwich structure constituted by the first wire, the web 
fibre web and the second wire to the dewatering and detaching area, and 
conducting the first wire over the first cover part of a special suction 
box; 
producing a directional change in the run of said sandwich structure with 
the aid of a guide roll of the second wire, which presses on the sandwich 
structure from the side of the second wire; 
causing a web dewatering effect mainly in region of said directional change 
or adjacent thereto, partly by the aid of the pressure of said wires 
directed against the web and of centrifugal force, and partly with the aid 
of the differential pressure created by the special suction box; 
separating the second wire from said sandwich structure and causing the 
second wire to follow along with said guide roll, and simultaneous 
subjecting of the web, together with the first wire, to the action of the 
subatmospheric pressure prevailing in said special suction box; and 
contacting the first wire with the second cover part of said special 
suction box. 
In a favorable embodiment of the invention a rotating forming roll is used, 
which replaces the stationary dewatering shoe employed e.g. in the Finnish 
Pat. No. 50648 corresponding to U.S. Pat. No. 3,846,233 and which has a 
positive effect on the total energy consumption of the structure according 
to the present invention and in that connection, the wire attrition is 
reduced. One avoids with the aid of the above-mentioned forming roll the 
drawbacks introduced by the stationary forming shoes of the prior art; 
above all one achieves a reduction of the wire attrition and energy 
losses. These objects are obtained, as taught by the present invention, in 
a manner enabling the advantages sought in the Finnish Pat. No. 50648 
corresponding to U.S. Pat. No. 3,846,233 to be achieved at the same time. 
Of these advantages may be mentioned, above all, efficient dewatering and 
good retention of fillers and other fines, and their uniform distribution 
across the web thickness. 
The web formation may in principle take place, according to the present 
invention, in the same way as in the Finnish Patent mentioned above, that 
is, so that the first dewatering step takes place on a single-wire run, 
whereafter the direction of dewatering is inverted on the twin-wire 
section. However, one substantial difference in the respective variant of 
the invention is the replacement of the forming shoe with a rotating 
forming roll. This forming roll may, in equivalence with the Finnish Pat. 
No. 50648, have a foraminous surface and be provided with a suction or 
blow box. In some instances, a grooved surface design may be contemplated, 
as will be presented later on. It has been found, however, that a smooth 
surface roll is usable in most instances. It should moreover be taken into 
account that such a simple roll is remarkably lower in price than a 
foraminous surface roll. The radius of this forming roll usually cannot be 
as large as the radius of curvature of a stationary shoe, with the 
consequence the that distance which the web travels in the region of this 
dewatering element in contact therewith is reduced. Likewise, the 
dewatering capacity may be reduced, of course depending on the types of 
pulp stock used. 
In the present invention, compensation for the reduced dewatering capacity 
should be provided by means of dewatering elements following after the 
forming roll in a manner, however, so that these dewatering elements will 
not cause any excessive wire attrition effect. If, expressly, the amount 
of upwardly directed dewatering turns out to be inadequate, owing to the 
brief wire contact of the forming roll, it is possible in the present 
invention, with a view to enhancing the dewatering, to use a grooved 
surface roll e.g. equivalent to that shown in the Finnish patent 
application No. 772129 corresponding to U.S. Pat. No. 4,172,759, but which 
roll is provided with a pressure chamber encircling it on that part of 
which has no wire contact, with the over-pressure present in the chamber 
being allowed to communicate through the grooves in the roll surface to 
influence the dewatering on that sector which is lapped by the wire. 
When the invention is being applied, a smooth surface roll is one of the 
most economical embodiments for the forming roll. The potentially lowered 
dewatering capacity incurred in using such a design must thus be made up 
for by other expedients. 
With respect to the object of the invention of providing a means and an 
arrangement by the aid of which the above-presented drawbacks impairing 
the reliability in operation of the former and affecting the quality of 
the web that is manufactured, the invention relates to a special detaching 
suction box construction and to an upper wire guide roll substantially 
cooperating therewith. The said detaching suction box has as one of its 
important tasks to accomplish dewatering of the web, this being achieved 
in part due to suction and to the suction box cover design, and in part by 
utilizing the directional change taking place expressly in the run of the 
lower wire and the inertial, or centrifugal, force resulting therefrom. 
This change of direction of the wire is achieved by the combined effect of 
the suction box cover design and of the upper wire guide roll adjacent 
thereto. The detaching suction box mentioned in the invention constitutes 
the sole element causing any significant wire attrition, and this 
attrition can be fairly well minimized. 
Another important function imposed on the detaching suction box under 
discussion is to detach the formed web, reliably and positively, from the 
upper wire and to transfer it to be supported by the lower wire for 
further conduction to the press section. 
In a favorable embodiment of the invention, the detaching suction box has 
been disposed within the lower wire loop, and it operates expressly in 
cooperation with the upper wire guide roll. The detaching suction box is 
equivalent in its basic design and mode of operation to conventional 
so-called flat suction boxes used on a Fourdrinier wire section. A 
substantial difference from these flat suction boxes of the prior art is 
that the cover of the suction box used in the invention is composed of two 
substantially planar parts which are disposed at an angle to each other 
and between which runs a free gap extending across the machine breadth. 
The suction box used in the invention is preferably so placed with respect 
to the upper wire guide roll that the free gap between the cover parts 
substantially corresponds to a given sector of the upper wire guide roll 
in a manner which will be described below. In principle, the central angle 
of the cross section corresponding to this sector is substantially equal 
to the angle between the planar parts of the detaching suction box cover. 
The said cover parts may comprise, for instance, ceramic material with a 
low coefficient of friction or of plastic and they may be similar in 
construction to those conventionally used on the flat suction boxes of 
Fourdrinier wire sections. The cover parts may both be provided with 
piercing apertures, which may be round, elongated, or in the form of slits 
extending over the entire breadth of the cover transversely to the 
direction of travel of the wire. In certain instances, the first cover 
part may be a solid strip. The detaching suction box is in a conventional 
manner, mostly through the end portion, connected to the vacuum system of 
the paper machine.

Although in the following the invention is described, referring to FIG. 1 
in regard to a web former primarily applicable in manufacturing printing 
papers, it is emphasized that the invention is also appropriate for use in 
multi-ply web formers with a similar basic structure. 
The web former depicted in FIG. 1 comprises two wires, namely, the lower 
wire 12 and the upper wire 13, wherein the lower wire 12 serves as a 
carrying wire for the web W and the upper wire 13 serves as a wire 
covering the web. The lower wire 12 constitutes after the headbox 10, a 
single-wire planar wire section 121, in connection with which are provided 
a suitable number of dewatering elements known in themselves in the art, 
such as a forming board 14, a wet suction box 141, and foils 142. The 
guide rolls 17 and 18 guide the run of the upper wire 13 to conjoin with 
that of the lower wire 12. In FIG. 1 a headbox 16 is illustrated by dash 
lines, which may supply pulp stock e.g. at the roll 17 into the interspace 
of wires 12 and 13 for the forming of a surface layer upon the web W which 
has already been formed. In the region of the joined run of wires 12 and 
13 there is, to begin with, a fairly large diameter forming roll 15, which 
may be a smooth roll, a grooved roll or a perforated roll. Within the 
lower wire loop have furthermore been placed a plurality of wire guide 
rolls 19, a breast roll 190 and a return roll 191. 
Adjacent to the upper wire guide roll 23, within the loop of the lower wire 
12, there is a special detaching suction box 30, by the aid of which is, 
among other things, ensured that the web W formed on the wire 12 will be 
detached from the upper wire 13 and will conducted, supported by the lower 
wire 12, further to the suction roll 24, whereafter the web W is 
transferred with the aid of a pick-up roll 25 onto the felt 26, which 
carries the web W forward to the press section. The suction zone of the 
suction roll is indicated by the reference numeral 240. The conventional 
drain chutes and save-alls, which are commonly employed on wire sections, 
being component forming no part of the invention, have been omitted. 
The forming roll 15 mentioned above is advantageously one with a smooth and 
solid shell, but it may also have a foraminous surface and be provided 
with an internal suction zone. In some cases a recessed surface forming 
roll may be used which has been provided with an external suction box, as 
has been shown in FIG. 6, to which reference will be made hereinbelow. One 
recessed surface roll of this kind has been disclosed in the applicant's 
U.S. Pat. No. 4,172,759. 
The roll 15 has a rather large diameter. The diameter of the roll is, for 
instance, between 1000 and 1500 mm, depending on the working breadth of 
the wire section. It is one substantial feature of the invention that a 
rotating forming roll 15 is used, and that subsequent to this roll within 
the carrying wire 12 has been disposed a special detaching suction box 30. 
Most essential in this suction box 30, from the viewpoint of the 
invention, is the circumstance that as a whole it may be considered to 
have a concave surface and that it acts in cooperation with the wire guide 
roll 23, which has been placed at a corresponding location within the loop 
of the covering wire 13. 
In FIG. 1, the run of the wire 12 after the detaching suction box 30 has 
been shown as being substantially horizontal, since the cover of athe 
suction box 30 is at the level of the top point of the suction roll 24. 
The run of the wire after the suction box 30 may also be clearly 
ascending, as depicted in FIG. 2, or descending as shown in FIG. 4, which 
figures shall be described in greater detail below. 
In FIG. 2, the guide roll 23 has been shown as placed between the forming 
roll 15 and the suction roll 24 so that the center of the roll 23 is 
substantially at the same level as the center of the suction roll 24. The 
consequence of this construction is that the run of the wire 12 after the 
detaching suction box 30 is clearly ascending and, consistent herewith, 
the directional change achieved with the aid of suction box 30 and roll 23 
in the run of the wire 12 has a remarkable magnitude, for example about 60 
degrees. In all different structural alternatives of the invention, this 
direction change of the wire 12 equals that central angle .beta. of the 
roll 23 which is defined by the points at which the lower wire 12 is 
tangent to the guide roll 23. 
The height of the detaching suction box 30 and of the guide roll 23 of the 
upper wire 13 can be changed in greater or smaller amount as required, and 
this obviously affects the directional change of the lower wire 12 even 
quite remarkably. The observation can be made by comparing FIGS. 1 and 2 
that in actual fact by a comparatively minor change in height of the 
suction box 30 the directional change of the lower wire 12 can be 
influenced to a great extent, and thereby the length of that sector in the 
region of which the upper and lower wires travel together, exerting strong 
pressure on the wet web W interposed between them, can be similarly 
varied. Increase of the said sector implies a longer time available for 
dewatering and, correspondingly, enhanced dewatering. 
In FIG. 2 has also been shown the principle design, in cross section, of 
the detaching suction box 30. The ends of detaching suction box 30 are 
naturally solid. The design of the detaching suction box 30 may vary 
regarding its internal and body structures on one hand and the cover 
design on the other. Certain structural variants have been additionally 
depicted in FIGS. 3A,3B and 4. The frame structure of the suction box 30 
is equivalent to the frame of a conventional wire section flat suction box 
and it may have a cross sectional shape e.g. of a quadrangle or of a 
letter U or V. The connecting of the suction box 30 to the vacuum system 
40 of the paper machine and the fittings, connectors, control valves and 
water traps therein employed are known in themselves in the art and they 
are represented by a collective symbol 32. They have in themselves no 
bearing on the understanding and application of this invention. 
The cover design of the detaching suction box 30 is peculiar and different 
from conventional Fourdrinier wire section suction boxes in the respect 
that the cover comprises two parts 33 and 34 inclined with respect to each 
other, which are most commonly planar and which are separated by a 
relatively wide free gap F, which is illustrated in developed form in FIG. 
5. This free gap communicates directly with the interior space of the 
suction box 30 and allows the suction in the suction box 30 to act on the 
web in this region. 
This free gap is free only upwardly towards the wire 12 covering the 
suction box. The ends of the suction box 30 are naturally closed over the 
whole cover structure, and especially at the said free gap. Seals (not 
depicted) have been provided on the ends of the suction box 30 which act 
against the wire 12. In the area of the said free gap the directional 
change of the wires 13 and 12 takes place so that no friction causing wear 
of the wire occurs because the said directional change is accomplished 
with the aid of the roll 23 and there is then no absolute need to use any 
stationary members. 
The free gap F separating the cover parts 33 and 34 of the suction box 30 
is in normal operation of the wire section covered on its substantial part 
by the upper and lower wires 12,13 and by the interposed fibre web W, the 
sandwich structure 12/W/13 being urged by the upper wire guide roll 23 
against the cover structure of the suction box 30. 
The significance of said free gap as, in effect, a part of the suction box 
of the invention has to be stressed. In this regard reference is made to 
the general considerations mentioned above which relate to the operation 
of the suction box 30 of the invention as a structural component of 
twin-wire formers, and to the embodiments of the cover structure of the 
suction box 30 of the invention to be described below. 
As shown in FIG. 2, there is on either side of the sector .beta. in the run 
of the wires 12 and 13 a comparatively open zone between the cover parts 
33 and 34 and the sector .beta.. At these zones, which extend over the 
entire breadth of the machine, "leakage air" is admitted into the suction 
box 30 in some amount through the sandwich structure 12/W/13. The detailed 
geometry of the zones is illustrated in FIGS. 4 and 5, where they have 
been denoted D' and D" in FIG. 5, corresponding to the central angles 
.DELTA.' and .DELTA." in FIG. 4. 
The first leakage air zone D' is formed in the throat between the upper 
wire 13 and the roll 23, where owing to the rotation of the roll 23 an 
overpressure is created, which partly discharges through the zone D', 
contributing to promotion of the dewatering of the web and to the web's 
attachment to the lower wire 12. This leakage air zone also causes the 
pressing of the web W on the sector .beta. between the wires 12 and 13 to 
begin gently, without destroying the structure of the web W. 
A leakage air zone which is useful regarding the operation of the suction 
box of the present invention may also be created with the aid of the 
special cover strip design shown in FIG. 7. It is also possible in certain 
cases that a perforation, mentioned below, of the first cover part can be 
used in lieu of the leakage air zone. 
The second leakage air zone D" is formed on the trailing side of the roll 
23, where the leakage air flow through the web W on the lower wire 12 and 
into the suction box 30 has an important significance in the attachment of 
the web to the lower wire 12. 
In the circumstances which prevailed on the applicant's experimetal paper 
machine on which the extension of the sector .beta. was 29 degrees and the 
corresponding peripheral lengths of the surface roll 23 (diameter of roll 
23 being 800 mm) was about 200 mm, the appropriate leakage air zone width, 
which was adjustable, was found to be D',D"=30 to 80 mm. A favorable 
starting value for the designing and practical application of the 
apparatus of the invention within the apparatus geometry presented in FIG. 
4 can be considered D'=D"=50 mm. The structure must naturally be arranged 
in a suitable way to be adjustable. 
As shown in FIGS. 3A,4 and 5, the first cover part 33 in the direction of 
travel of the wires 12,13 is rather narrow. This cover part 33 is in most 
instances merely a sealing strip of the suction box 30 acting against the 
wire 12 and which may also operate as a kind of water doctor. The cross 
section of the strip may be selected with comparative freedom, and it may 
even be circular for instance. At the first cover part 33 the web has a 
comparatively high water content, whereby at this point there may still 
escape from the web a comparatively large quantity of water, which acts as 
an efficient lubricant and remarkably reduces the risk of wear of the wire 
12 on this cover part 33. 
The first cover part 33 may also be fairly broad, in which case it is 
advantageous to provide it, as planar suction box covers known in 
themselves in the art ane provided, with through holes or slits 
communicating with the interior of the suction box 30, whereby in the 
region of this cover part 33 there is exerted on the web W between the 
wires 12 and 13, substantially the effect of the vacuum prevailing in the 
detaching suction box 30. 
It is of significance for the operation of the suction box 30 of the 
invention that it has the said free or open gap between the cover parts, 
in the area of which the change in direction of web travel takes place 
while as a result hereof a fairly powerful compression acts on the web W 
between the upper wire 13 and covering wire 12. As a result, a great 
amount of water can be expelled from the web W, this water being flung by 
centrifugal force effect into the suction box 30. 
As regards the design of the second cover part 34 of the detaching suction 
box 30, several alternatives are available as compared with the first 
cover part, and these are in fact needed, since in the last run this 
second cover part determines how reliably the web W will be detached from 
the covering wire 13 and affixed to the carrying wire 12. Some of these 
alternatives are shown in FIGS. 3A,4 and 9. 
The second cover strip 341, as shown in FIG. 3A, is in principle a 
conventional flat suction box cover known in itself in the art, which is 
perforated with through holes. The cover part shown in FIG. 4 comprises a 
perforated first, fairly wide strip 348, in addition to which the cover 
part comprises two narrow strips 349, by the aid of which in the cover 
part are established two suction slits 350 extending transversally across 
the entire wire breadth. The strips 348 and 349 may also be mounted in 
reverse order to that the suction slits precede the pierced-through strip 
348 in the direction of web travel. The strip 348 may have a similar 
profile as the strip 33. The cover part illustrated in FIG. 9 has been 
separately described. It is clear that due to the task of the second cover 
part its total width must substantially exceed that of the first. The 
total width of the second cover part may be 2-3 times that of the first. 
In the cover design of FIG. 3B, however, the free gap separating the cover 
parts 331 and 342 is equally large as the arc of the corresponding central 
angle .beta.. It is advantageous in this case to use for cover parts of 
the suction box 30 such strip constructions 331,342 as have been depicted 
in FIGS. 7,8 and 9. 
The body part of the suction box 30 of FIG. 3B has a partition 301 provided 
with an adjustable air flow aperture 321 and dividing the suction box 30 
into two compartments, one of these corresponding to the first cover part 
331 and to the sector .beta., while the other is delimited by the latter 
cover part 342. Since the vacuum system 40 is immediately connected to the 
compartment corresponding to the first cover part 331, in this compartment 
as a rule prevails a greater vacuum than in the compartment of the suction 
box corresponding to the latter cover part 342. Regarding the second cover 
part 342 it is essential that one be able to direct on the web sufficient 
suction so that the web W will after its detachment from the upper wire 13 
follow reliably along with the lower wire 12. It should be noted that 
although the upper wire 13 has already become detached from the web W, 
there may at high speeds in the throat between the roll 23 and the lower 
wire 12 be created a suction effect which in certain cases tends to wrench 
the web W towards the upper wire 13 from the surface of the lower wire 12 
unless there is a sufficient "supporting suction" at the second cover part 
342. 
The detaching suction box 30 may in certain instances have several 
compartments by the aid of which the suction is adjustable within the 
desired limits. 
The providing of the detaching suction box 30 with a minimum of two parts 
also enables the suction to a be adjusted to desired level at various 
points of the suction box while the web W interposed between the wires 
12,13 is passing over the cover part of the suction box 30. By the aid of 
such suction control, the desired dewatering effect is achieved, as well 
as the desired detachment of the web W from the upper covering wire 13, 
and its transfer to be carried onward by the lower carrying wire 12 alone. 
The suction control facility in this connection implies optimum use of 
vacuum energy, that is, avoidance of an unnecessarily high vacuum energy 
consumption. Whether stepwise control of suction is necessary in the first 
place, is however a question dependent on several factors, such as the 
paper machine speed, the type of pulp stock, etc. 
Suction control in the region of the suction box 30 may take place 
similarly as in the case of conventional flat suction boxes by providing 
the suction box or each compartment thereof with its own connector 
communicating with the main vacuum line and fitted with a control valse 
32. As mentioned hereinabove, the system may also be adapted so that one 
of the compartments of the suction box 30 is a so-called main suction 
compartment which communicates directly with the suction system of the 
paper machine, with the highest suction present therein. The suction in 
the rest of the compartments may then be controlled with the aid of a 
controllable throttling aperture 321 or equivalent placed in the partition 
separating the main compartment and the compartment in question. 
FIGS. 4 and 5 display the geometry of the apparatus of the invention on the 
basis of a tried-out construction. In FIG. 5, the curved run of the wires 
12 and 13 has been depicted as developed, i.e., as linear, and 
correspondingly the cover parts of the detachment suction box have been 
presented as lying in one plane. This enables the mutual magnitude 
proportions of the details belonging to the structure of the invention to 
be simply described. 
The following correspondences are valid in FIGS. 4 and 5: A.about..alpha.; 
D'.about..DELTA.'; B.about..beta.; D".about..DELTA."; C.about..gamma.. The 
free or open gap of the cover of the suction box 30 is F=D'+D"+B. 
At a speed of 1000 m/min, the dewatering of the apparatus was found to be 
quite efficient, replacing 3 to 5 of the suction boxes which are 
conventionally used between the forming roll 15 and the couch roll 24. The 
free gap F was about 300 mm, thereof the contribution of the directional 
change sector, 200 mm. 
FIG. 7 displays in axonometric projection, a structural variant of the 
first cover part 331 of the suction box 30. The cover part 331 comprises 
one single strip, its plane face 332 which operates against the wire 12 
being provided with special grooves 333, which parallel the run of the 
wire 12. The grooves 333 separated by lands 334 have a wedge-shaped 
longitudinal cross section so that they start, in the direction of travel 
of the wire, on the surface 332 of the cover part and increase in depth 
towards the rear margin of the strip so that a kind of "serration" is 
created on this rear margin. In FIG. 7, the reference symbols a,a',b,c and 
d indicate the dimensioning of the grooves. The principal dimensions of 
the cover strip, again, have been indicated with e and L. 
It should be understood that in FIG. 7 only one possible groove arrangement 
has been merely schematically and in principle shown. The ratio a/b 
between the breadth a of the grooves 333 and the breadth b of the lands 
334 thereinbetween may vary within fairly wide limits, depending on the 
operating conditions of the paper machine. The type of the wire fabric 12 
used in the paper machine and which acts against the cover strip 331 in 
question, also has a great influence on the selection of the grooving. The 
ratio may be, for instance, a/b=1.5 to 0.2, the lower value representing a 
case wherein e.g. a=2 mm and b=10 mm, while the upper limit value 
corresponds to a grooving with grooves 50% wider than the lands. It is not 
possible to state any exact value because the grooving is dependent, as 
mentioned already, on the quality of the wire fabric 12 used and in 
particular, on the stiffness of the fabric 12 in its transverse direction. 
The type of the grooving is also described by the ratio between the depth 
c of the grooves 333 and the breadth a of the groove at the rear margin of 
the strip 331, and which ratio may vary within the range c/a=5 to 1, for 
instance. 
As shown in FIG. 7 and as stated above, the grooves have been so machined 
or made that their walls are perpendicular to the plane of the strip. 
Likewise, it is shown in the figure that the grooves have uniform breadth, 
that is a=a'. One advantageous groove structure which partly solves the 
problems arising e.g. from insufficient stiffness of the wire fabric 12 is 
where the grooves 333 taper down significantly in the plane of the cover 
strip's surface towards the rear margin of the strip, for instance so that 
the ratio a'/a=5 to 3. The resultant reduction of the grooves' 333 
effective volume is made up for by slanting the walls of the grooves 333 
so that the breadth of the groove 333 in the plane of the cover strip 
surface is considerably less than on the bottom of the groove. Such a 
structural detail variant has been shown in FIG. 8; the ratio of the 
dimensions indicated therein may be e.g. a"/a=5 to 2 and c/a"=1 to 3. 
The cover strips 33,34 of the suction box 30 may be completely smooth, that 
is without any grooving whatsoever. In such a case, there is at the 
starting point of the first cover part's rear margin and the directional 
change zone of the wires, preferably, a gap .DELTA.' (FIG. 5) such that it 
renders possible a free air flow from the atmosphere into the suction box. 
The length of this gap .DELTA.' is advantageously 30 to 70 mm, depending 
on the operating conditions of the machine. Satisfactory operation has 
been achieved with the said gap .DELTA.' set at 50 mm. 
The fact that the first cover strip 331 is grooved, e.g., as shown FIGS. 7 
and 8, enables the rear margin of the strip 331 to be placed substantially 
at the same point where the directional change sector of the wires 12,13 
commences. The grooving 333 in the strip 331 will then at least partially 
replace the said leakage air sector D' so that an air flow is achieved in 
the area in question through its sandwich structure 12/W/13 into the 
suction box 30, which is important for the operation of the suction box 30 
both from the point of view of dewatering and with a view to the 
detachment of the web W from the upper wire 13 and its attachment to the 
lower wire 12 at this stage already. 
As regards further the other dimensions L and e of the strip 331 presented 
in FIG. 7, L is exclusively dependent on the working width of the paper 
machine. The breadth of the strip e in the direction of travel of the wire 
12 depends on several secondary structural factors, and the said breadth 
has no substantial effect on the operation of the suction box 30. It 
should be noted, however, that the breadth e must be substantially greater 
than the length d of the grooves, which is advantageously about 30 to 80 
mm. The ratio d/e may be e.g. 0.6 to 0.8. 
FIG. 9 presents, likewise in axonometric projection, a design of the latter 
cover part of the suction box 30 according to the invention, comprising a 
strip 342 provided with a planar top surface. The groovings 345 of the 
strip 342 start from the front margin of the strip 342, in the direction 
of travel of the wire. The various possible constructions of the grooving 
of this cover part are obvious from the descriptions referring to FIGS. 7 
and 8. It is advantageous if the front side 347 of this strip 342 defines 
an acute angle with the surface 344 so that the strip 342 may in this part 
produce a kind of doctor effect. The cover strip 342 shown in FIG. 9 has 
been provided, on the side of the grooves 345 separated by lands 346, with 
holes 343, through which the cover part allows the subatmospheric pressure 
prevailing in the suction box 30 to act on the wire 12 lapping the cover 
part and on the fibre web W thereupon. 
It has to be noted further as regards FIG. 9 that although the grooves 345 
of the latter cover strip 342 have been presented with comparatively great 
width for the sake of clarity, the preferred construction in practice is 
one in which the grooves 345 are comparatively narrow so that the return 
of water removed from the web as it changes direction over the 
direction-changing sector of wires 12, 13 into the web W is prevented. 
With this in mind, a favorable groove geometry is one wherein a.about.2 to 
3 mm; c/a.about.5 to 10 and a'/a.about.5 to 10. 
In the following is described, with reference to FIG. 6, an alternative 
design of the forming roll. As has been stated, the forming roll 15 may be 
a smooth surface roll or a grooved roll, or another kind of roll as well. 
As shown in FIG. 1, the forming roll 15 is one with a smooth surface, 
whereas according to FIG. 6 the forming roll 151 has been provided with a 
grooving 152 encircling its surface. 
The forming roll 151 is lapped on a given sector by the lower wire 12, upon 
which the web W resides. Adjacent to the roll 151, the upper wire 13 is 
conducted upon the web W, and the sandwich structure thus achieved, 
12/W/13, changes direction on the roll 151 and continues on its way to the 
detaching suction box 30. Between the roll 151 and the detachment suction 
box 30 there are preferably no members which would cause any substantial 
dewatering or any wire attrition. 
As shown in FIG. 6, the forming roll is encircled by a chamber 153a except 
for the sector .beta..sub.F, in the region of which the wire 12 is 
substantially adjacent to and in contact with the forming roll. In the 
area of this sector, the upper wire 13 is conducted to cover the web W. In 
the region of this sector a gradually increasing pressure acts on the web 
W, caused by the wire 13, and owing thereto water tends to escape from the 
web through both wires 12 and 13. The chamber 153a has been connected to 
an overpressure source 41 through a valve connector 154. The overpressure 
(+) is admitted to act through the grooves 151 at the sector .beta..sub.F 
on the web upon the lower wire 12 so that the dewatering through the wire 
12 can be at least partially inhibited if this is necessary. At all events 
it is possible with the aid of the pressure in the chamber 153a to control 
the dewatering in the region of this roll 151. The chamber 153a has been 
provided with a cover part 157 operating against the wire 12, and which 
may be formed with through holes. 
In certain special cases a subatmospheric pressure may be arranged in the 
chamber 153a as well. 
In immediate conjunction with the chamber 153a and before it, as viewed in 
the direction of travel of the wire, has been disposed another chamber 
153b, connected through a connector 155 to a vacuum surce 40. The chamber 
153b has been provided with a cover part 156 which partly extends to cover 
the chamber 153a. The cover part 156 forms a small angle with the wire 12 
and it is, partially at least, provided with through holes in its portion 
overlying the chamber 153b. 
With the aid of the chambers 153a and 153b, the direction and amount of 
dewatering in the region of the roll 151 are easily controllable. 
Considering generally the operation of the detachment suction box 30 and of 
the guide roll 23 of the upper wire 13 operating in its connection, the 
following factors may be observed to have their significance. The rotation 
of the wire guide roll 23 induces an "air cushion" which develops in the 
throat defined by the wire guide roll 23 and the wires 12,13 and gives 
rise to a pressure urging the web W interposed between the wires against 
the lower wire 12. Acting in the same direction is the vacuum prevailing 
in the suction box 30 and in its different compartments, if any. 
Furthermore, the centrifugal force in connection with the directional 
change of the travel of wires 12,13 naturally has an effect tending to 
detach the web W from the upper wire 13. 
All those factors notwithstanding which promote the detachment of the web W 
from the upper wire 13, the detachment of the web W is not fully 
ascertained without the latter cover part of the detachment suction box 
30, by the aid of which a "safeguarding suction" is made to act on the web 
W. Among those factors which render such "safety" suction necessary is, 
for instance, the excessively strong adhesion, resulting from one reason 
or another, of the upper wire 13 to the web W, and the suction effect 
produced at the point of separation of the upper and lower wires 12,13. It 
depends on the paper grade or fibre web quality and on the machine speed 
how high and how prolonged a suction must be directed against the web W 
through the lower wire 12 in order to reach the desired result. The 
duration of suction is obviously proportional to the breadth, in the 
direction of travel of the wires, of the latter cover part of the suction 
box 30. 
The combination apparatus formed together by the detachment suction box 30 
and the wire guide roll 23 guiding the run of the wires 12 and 13 renders 
possible a dewatering process in the region of the suction box which is 
adjustable and controllable within wide limits. The task of this apparatus 
is dual, and it has reference both to the dewatering of the web W and to 
the detaching of the wire W from the upper wire 13 and its attachment to 
the lower wire 12, that is to the carrying wire. Which one of the two 
tasks has to be given main weight depend on several factors, e.g. on the 
type of pulp stock being handled and its dewatering properties; on whether 
plain or multi-ply web is being made; and on the properties of adherence 
of the web material to the upper wire. Process parameters affecting the 
operation of the apparatus in question, particularly from the viewpoint of 
dewatering action, are the tension of the lower wire 12 against the upper 
wire 13 and the roll 23, the magnitude of the directional change angle of 
the lower wire 12, the length of the free gap F of the suction box 30, the 
vacuum prevailing in the suction box 30, the length B of the arc 
corresponding to the directional change angle, which with a fixed 
directional change angle is proportional to the diameter of the guide roll 
23, the centrifugal force acting on the sector .beta., which is inversely 
proportional to the diameter of the guide roll 23, and the total time 
which the web W spends in the region A+F+ C of the suction box 30. 
Process variables which in their turn exert an influence on the detaching 
of the web W from the upper wire 13 and its transfer to the lower wire 12 
are the material, "mesh", fabric structure etc. of the wires used, the 
design of the second cover part 34 of the suction box 30 after the 
directional change sector .beta., the time which the web W is subject to 
the effect of suction in the leakage air zone D" immediately following the 
directional change sector, on the second cover part 34 of the suction box, 
and the magnitude of vacuum prevailing in the suction box 30, expressly in 
the region of the second cover part 34. 
In the miniature paper machine of the applicant's pilot plant, the 
inclination of the lower wire 12 arriving at the suction box 30 with 
reference to the horizontal plane is about 37 degrees. Since the 
inclination of the wire 12 as it leaves the suction box 30 is 8 degrees 
downward, the directional change of the lower wire 12 is thus 29 degrees, 
which is the extent of the sector on which the upper and lower wire 12,13 
travel together, separated by the web W, at the guide roll 23 (with a 
diameter of 800 mm). The location, shown in FIGS. 1 and 4, of the rolls 
15,23 and 24 and of the suction box 30 is roughly consistent with 
conditions as they were in the trial paper machine. 
The guide roll 23 is preferably one with a smooth surface, and it is then 
most advantageous to use the above-described leakage air zones D' and D" 
on the either side of the directional sector .beta. of the web W and the 
wire 12. In some special cases, the invention may also be implemented so 
that there are no actual leakage air zones of the kind mentioned at all. 
In that case an effect corresponding to the operation of the leakage air 
zones, if employed at all, can be obtained by arranging the shell of the 
guide roll 23 to be grooved so that the air is able to move via this 
grooving through the sandwich structure 12/W/13. This design may however 
present the drawback that the said grooves may owing to effect of the 
compression between the wires 12 and 13 accumulate water, which is 
splashed back into the web after the directional change sector .beta. and 
as the web W separates from the wire 13. The said grooving of the guide 
roll 23 is preferably one running around the roll shell and achieved e.g. 
by fitting the roll 23 with a covering made of profiled strip material by 
winding. 
Although in the foregoing the talk has been of wires 12,13, this wording is 
also meant to refer to various fabrics used in wire sections, e.g. to 
felts of a special design. 
In the foregoing, the detachment suction box 30 of the invention and the 
procedure in the wire section of a paper machine described in connection 
therewith have been described as being applied in connection with a 
certain type of wire section. It should be noted, however that the 
detachment suction box of the invention is well applicable in many 
different kinds of wire section, which differ even altogether 
substantially e.g. from the wire section total design presented in FIG. 1. 
In the following are stated the claims within the scope of the inventive 
idea defined by which various details of the invention may vary.