BACKGROUND OF THE INVENTION [0002] The drying section of a paper machine utilizes drying wires with which a paper web to be dried is conveyed through the drying section.

The fabric structure of the drying wire is formed of high-temperature and moisture resistant yarns by using weaves that are appropriate for the purpose.

In order for the contact between the web and the wire to be even and smooth, spun cross-machine direction yarns are arranged, according to the prior art, on the web-side surface. Present smooth-surface drying wires are multilayer structures having a high cross-machine direction yarn density. Manufacturing such drying wires is expensive. The cost of the present wires is further increased because the structures have a lot of expensive spun yarn. In addition, such drying wires are relatively thick, which causes draw between the web and the wire.

BRIEF DESCRIPTION OF THE INVENTION [0003] An object of the present invention is to achieve a novel and an improved drying wire.

[0004] The drying wire according to the invention is characterized in that the wire is a 1. 5-layer structure; that the web-side surface is provided, upon weaving, with additional cross-machine direction yarns, which have an easily shapable structure; and that the additional yarns are arranged to cover the spaces formed between machine-direction yarns on the web-side surface substantially completely.

[0005] An essential idea of the invention is that the drying wire is a 1. 5-layer structure where the monofilament cross-machine direction yarns are arranged to run on the backside surface of the wire and where there are additional cross-machine direction yarns that differ from the rest of the yarns running on the web side surface of the wire. The additional cross-machine direction yarns are easily shapable, being shaped during the weaving, in a heat treatment and/or during use in such a way that they can become wider in the direction of the wire surface, whereby they fill the spaces between machine-direction yarns on the web-side surface substantially completely.

[0006] An advantage of the invention is that owing to the shapable additional yarns, the web-side surface of the wire has an extremely large contact area. This is because the additional yarns fill the spaces on the web- side surface of the wire. In addition, the monofilament machine-direction yarns form, relative to the area, a large number of contact points on the web-side surface. Thus, the heat transfers well from the wire to the web, and the drying is efficient. Further, an advantage of the invention is that a 1. 5-layer fabric has significantly fewer yarns than a multilayer fabric, and therefore it is less expensive to manufacture. In addition, fewer expensive additional yarns than previously are needed for the structure according to the invention. Yet another advantage is that the wire according to the invention has the desired surface properties immediately after the weaving. Thus, the wire needs not necessarily be ground smooth, calendered or given an intensive shrinking treatment. In this way, the manufacturing may be quicker and the manufacturing costs smaller. Furthermore, no shrinkable or other special yarns need to be used in the basic weave.

[0007] The essential idea of an embodiment of the invention is that the starting point for the basic structure is a 3-shed weave, in which case each machine-direction yarn runs in the basic weave over two basic cross-machine direction yarns and further under one basic cross-machine direction yarn. In the final structure, the machine-direction yarn runs over three cross-machine direction yarns and under three cross-machine direction yarns.

[0008] The essential idea of an embodiment of the invention is that the dimensions of the yarns relative to each other are arranged in such a way that the structure is asymmetric.

[0009] The essential idea of an embodiment of the invention is that the additional cross-machine direction yarns running on the web side are of spun yarn. Spun yarn is flexible and shapes up well on the wire surface both during weaving and when it is subjected to pressing forces during use. Due to the effect of pressing forces, the structure of spun yarns can expand in the direction of the wire surface. Thus, the contact area of the wire surface may be extremely large, up to nearly 100%, depending on the pressing force. Despite the large contact area, the number of contact points is also large.

[0010] The essential idea of an embodiment of the invention is that the proportion of the additional yarns on the web-side surface is less than 60% of the area. When the number of expensive additional yarns is smaller in the structure, the wire is less expensive to manufacture. Further, when there are a limited number of additional yarns on the web-side surface, the wire remains clean for a long time.

[0011] The essential idea of an embodiment of the invention is that the proportion of the additional yarns is preferably between 30 and 60%.

[0012] The essential idea of an embodiment of the invention is that the cross-machine direction yarns of the backside surface of the wire are flat monofilaments. Flat yarns support the wire structure in the direction of its surface. Further, the surface of the backside surface may be smoother when flat yarns are used, compared with the use of round yarns, and therefore the wire carries less air with it. Thus, the wire has good aerodynamic properties and good runnability in a paper machine.

[0013] The essential idea of an embodiment of the invention is that the thickness of the wire is less than 1.5 mm. Owing to the wire being thin and having an asymmetric structure, the draw between the wire and the web in a paper machine can be small. Thus, detrimental stretching of the web and/or wearing of the wire can be avoided.

[0014] The essential idea of an embodiment of the invention is that the contact area of the monofilaments on the web side in the wire is 40% or more, and at the same time, the number of contact points of the monofilaments is 30 points/cm2 or more. Such a wire has a good heat transfer capacity.

[0015] The essential idea of an embodiment of the invention is that the additional yarns are hydrophilic. When the web-side surface of the wire is appropriately hydrophilic and smooth, the web is well pressed adhered to the wire surface, owing to which the runnability of the wire is good.

BRIEF DESCRIPTION OF THE FIGURES [0016] The invention will now be described in greater detail with reference to the attached drawing, of which Figure 1 shows schematically a cross-section of a drying wire according to the invention from the cross-machine direction; and Figure 2 shows schematically a part of the web-side surface of the drying wire of Figure 1.

[0017] For the sake of clarity, the figures show the invention in a simplified manner. Similar parts are denoted with the same reference numerals.

DETAILED DESCRIPTION OF THE INVENTION [0018] Figure 1 shows a cross-section of a wire according to the invention. The wire comprises a backside surface A which is against the paper machine and a web-side surface B which is against the web to be dried. The wire is run in the running direction MD of the paper machine. Further, the cross-direction of the paper machine is indicated in the figure as CMD. The wire is a 1. 5-layer structure formed by weaving from machine-direction yarns 1 and cross-machine direction yarns 2. The weave has a 3-shed structure. The machine-direction yarns 1 run over two cross-machine direction yarns 2 and further under one cross-machine direction yarn 2, continuing then according to the same pattern. All machine-direction yarns 1 in the fabric run according to the same weave pattern. However, adjacent machine-direction yarns 1 always have a shift of one cross-machine direction yarn in the same direction or in a different direction. Further, the web-side surface B is provided, upon weaving with additional cross-machine direction yarns 3, which may be of spun yarn.

Spun yarn shapes up well in the space between machine-direction yarns 1 on the surface of the wire. The material of spun yarns may be, for instance, polyacrylonitrile (PAN) and/or aramid. Alternatively, the additional yarns 3 may be of meltable fibre, in which case the heat treatment after weaving causes the additional yarns 3 to shape up on the surface of the wire. Further, suitable twisted yarn, multifilament and/or multi-component yarn may be used as additional yarns 3. One option is to use polypropylene (PP) as the additional yarn material. What is essential in the selection of additional yarns 3 is that the yarns 3 are relatively easily shapable yarns which shape up due to weaving forces and/or pressing forces during the use of the paper machine in such a way that they fill the spaces between machine-direction yarns 1 on the web- side surface substantially completely, whereby a large contact area is achieved. The original cross-section of the additional yarns 3 may be flat or round. The machine-direction yarns 1 may be monofilaments and their material may be polyethylene terephthalate (PET), polyamide (PA), polyphenylene sulphide (PPS), polyether ether ketone (PEEK), polycyclohexylenedimethylene terephthalate (PCTA) or polyethylene naphthalene (PEN). The cross-section of the machine-direction yarns 1 may be flat, in which case a large contact surface is achieved. The cross-machine direction yarns 2 running on the backside surface A of the wire may also have a flat cross-section. The cross- section of flat yarns may be oval or rectangular with rounded edges. Flat yarns are, as known, rigid in one direction and very flexible in another direction.

Thus, a wire woven of flat yarns is very firm in the direction of the wire surface.

Further, owing to the flat cross-machine direction yarns 2, the backside surface A may be smooth, in which case it carries little air with it.

[0019] Since the basic weave of the drying wire is a one-layer structure, the wire may be thin. In empirical tests it has been observed that preferably the thickness of the wire is less than 1.5 mm, particularly preferably less than 1.3 mm. Further, it has been observed that the air permeance of the wire may be even less than 1 000 m3/m2h. However, the air permeance is preferably in the range of 1 000 to 3 000 m3/m2h.

[0020] Figure 2 shows the web-side surface of the drying wire according to the invention. Between machine-direction yarns 1, there are empty spaces 4, which are filled by additional yarns 3. Since the additional yarns 3 are easily shapable, they fill the empty spaces 4 substantially completely. As seen from the figure, the wire surface has a large contact area.

On the other hand, the machine-direction yarns 1 form a large number of contact points on the web-side surface.

[0021] It has been observed in the use that if the proportion of additional yarns on the web-side surface is too high, the wire surface may become too hydrophilic. If the wire surface is too wet, dirt sticks easily to it and the wire may be clogged prematurely. In addition, the wire surface is rougher, which also contributes to the sticking of dirt. In the invention, the proportion of additional yarns on the surface is less than 60% of the area, and no such problems have been observed.

[0022] Further, the drying wire according to the invention may, in some cases, have a two-shed, four-shed or six-shed structure.

[0023] The drawings and the related description are only intended to illustrate the idea of the invention. The details of the invention may vary within the scope of the claims.