BACKGROUND OF THE INVENTION [0002] A dryer section of a paper machine employs dryer wires, by means of which a paper web to be dried is guided through the dryer section.

The weave of a dryer wire is formed from yarns that sustain high temperatures and moisture using suitable weave structures. The dryer wires also carry air with them, which may cause problems during the run in high-speed paper ma- chines, in particular. The aerodynamic properties of existing wires have proven inadequate in some situations.

BRIEF DESCRIPTION OF THE INVENTION [0003] The object of the invention is to provide a new and improved dryer wire.

[0004] The dryer wire of the invention is characterized in that at least one surface of the wire is provided with a plurality of machine direction grooves and that at least some of the longitudinal yarns travelling in the first layer are arranged to partly overlap the longitudinal yarns travelling in the sec- ond layer, the total width of the partly overlapping yarns being greater than that of a single longitudinal yarn.

[0005] The invention is based on the idea that at least one surface of the wire is provided with a plurality of longitudinal grooves. Furthermore, longitudinal yarns of the overlapping layers are arranged to partly overlap each other, which makes the structure denser.

[0006] An advantage of the invention is that the air carried in the wire can be controlled well since the grooves guide airflows in the longitudinal direction of the wire. Thus the wire has good aerodynamic properties, and con- sequently, the runability of the wire is good. The wire runability is further im- proved by the fact that the longitudinal yarns arranged to overlap each other make the wire structure denser, and thus the wire may have a relatively low air permeability. High-speed paper machines, in particular, require a dense struc- ture. Thanks to the grooves, the drying capacity of the wire may be better than earlier. A further advantage is that the cleaning of the wire becomes easier since at least some of the water sprays of a typical injection device also hit the grooves, and thus the water sprays can also act on the dirt stuck onto the wire surface from below. In addition, an advantage is that the grooves are empty and have a shape which prevents dirt from sticking to them easily. Thanks to the basic structure of the wire according to the invention, the wire is also wear resistant and stable in use.

[0007] The basic idea of an embodiment of the invention is that the depth of the grooves is substantially equal at least to the thickness of the longi- tudinal yarns that travel on the surface provided with grooves. In that case, the grooves are distinct and sufficiently large for guiding airflows.

[0008] The basic idea of an embodiment of the invention is that in the cross machine direction of the wire, the cross section of the grooves has the shape of a substantially sharp-angled letter U. It is easy to remove dirt from a groove of this shape. In addition, the groove is free from yarns that could hinder the airflow, in which case the airflow passing in the groove is not sub- jected to significant turbulence.

[0009] The basic idea of an embodiment according to the invention is that the grooves have been formed during weaving.

[0010] The basic idea of an embodiment of the invention is that at least the transverse yarns are made of a material that shrinks considerably in heat and that the wire has been treated thermally after weaving to provide the wire surface with machine direction grooves.

[0011] The basic idea of an embodiment of the invention is that the surface of the transverse yarns has been treated with a friction reducing sub- stance before weaving.

[0012] The basic idea of an embodiment according to the invention is that longitudinal yarns are provided in two layers and that the cover ratio formed by the longitudinal yarns is less than 80%.

[0013] The basic idea of an embodiment according to the invention is that the air permeability of the dryer wire is less than 5000 m3/m2h.

[0014] The basic idea of an embodiment of the invention is that the wire comprises longitudinal grooves both on the back surface and on the sur- face facing the web to be dried.

BRIEF DESCRIPTION OF THE FIGURES [0015] The invention will be described in greater detail in the ac- companying drawing, in which Figure 1 illustrates a schematic perspective view of a dryer wire ac- cording to the invention from the machine direction, and Figure 2 schematically illustrates part of a dryer wire according to the invention from the machine direction.

For the sake of clarity, the figures illustrate the invention in a simpli- fied manner. Like reference numbers refer to like parts in the figures.

DETAILED DESCRIPTION OF THE INVENTION [0016] Figure 1 illustrates a dryer wire of the invention, which has been woven from a plurality of machine direction MD longitudinal yarns 1 and a plu- rality of cross machine direction CMD transverse yarns 2, 3 and 4. The wire comprises a back surface (A) and a surface (B) facing the web to be dried. The longitudinal yarns 1 may have the cross section of a flat monofilament, having the shape of a rectangle with rounded edges or an oval shape, for example.

The flat yarn refers to a yarn with a greater dimension in the width direction than in the height direction. The transverse yarns 2 to 4 may have the cross section of round monofilaments. Alternatively, flat transverse yarns may be used or the transverse yarns may be made of'soft yarn', which may shape up during the manufacture. The longitudinal yarns may be made of polyethylene terephthalate (PET), polyamide (PA), polyphenylenesuplhide (PPS), polyether etherketone (PEEK), polymethylene cyclohexylene terephthalate (PCTA) or polyethylene naphthalate (PEN). The transverse yarns 2 to 4 may be made of one of the above-mentioned materials.

[0017] As appears from Figure 1, the longitudinal yarns 1 travelling on the wire surface A may be arranged in a plurality of groups formed by at least two longitudinal yarns 1. Seen in the cross machine direction CMD of the wire, the longitudinal yarns 1 are substantially against each other in the groups. In that case, there are machine direction MD grooves 5 between these yarn groups. Grooves 5 may be provided only on one surface of the wire or alternatively on its both surfaces. The grooves 5 can be formed during weav- ing. The groove 5 formation can be influenced by selecting a certain weave structure for the wire, certain materials for the yarns or certain settings for the weaving machine. In the weave structure of the wire according to the invention, the number of longitudinal yarns 1 may be relatively scarce in relation to the width of the longitudinal yarns 1, which facilitates the formation of grooves 5. In addition, the formation of grooves 5 may be influenced by applying an abnor- mally high tension to the longitudinal yarns 1 during weaving. This generates cross forces in the wire to be woven, pulling the longitudinal yarns 1 so that they will overlap. The tension on longitudinal yarns 1 may be over 10 kN/m, preferably over 15 kN/m.

[0018] The surface of the transverse yarns 2 to 4 may have been made slippery so that the longitudinal yarns 1 move easier due to the influence of the weaving forces, enabling the formation of the grooves 5. Thus a material with a small friction coefficient can be selected as the material for the trans- verse yarns 2 to 4 or the transverse yarns 2 to 4 may be alternatively treated with a friction reducing material before weaving, such as polytetrafluoroethene (PTFE). The outer surface of the transverse yarns 2 to 4 may be coated with a friction reducing material or it may be treated by spraying the transverse yarns with a friction reducing material, for example.

[0019] In addition, the groove 5 formation can be influenced by se- lecting a material that shrinks considerably in heat for the transverse yarns 2 to 4. After weaving, the wire can be treated thermally, in which case the shrinking of the transverse yarns 2 to 4 may intensify the groove 5 formation. In addition, the thermal treatment may stabilize the wire structure since the yarns attach better to each other and form yarn floats.

[0020] It can be seen from Figure 2 that the wire comprises longitu- dinal yarns 1 in two overlapping and adjacent layers. Longitudinal yarns 1a travelling on the back surface A of the wire and longitudinal yarns 1 b travelling on the surface B facing the web partly overlap by a predetermined distance M.

The distance M is clearly shorter than the width P of the longitudinal yarns 1a and 1b. In that case, the total width N of the longitudinal yarns 1a and 1b is always greater than the width of a single longitudinal yarn 1 a or 1 b. When the longitudinal yarns 1a and 1b of different layers are arranged to partly overlap, the resulting structure is dense. Thus the air permeability of the wire can be rendered sufficiently small so that the wire is also suitable for use in high- speed paper machines. The air permeability of a dryer wire may be less than 5000 m3/m2h.

[0021] The weave structure of the wire can be selected so that the transverse yarns 2,3 and 4 keep the longitudinal yarns 1 firmly in the partly overlapping position described above. In addition, the weave structure may be selected so that the wire will be stable in a paper machine.

[0022] Figure 2 further shows that the cross section of the grooves 5 may have the shape of a sharp-angled letter U. This shape is illustrated with line 6 in the figure. In grooves with this shape, airflows are not subjected to significant turbulence. Furthermore, dirt does not easily stick to this kind of groove. In addition, the grooves on the wire surface intensify drying.

[0023] When the cross section of the longitudinal yarns 1 is oval, for example, the cross section of the grooves 5 differs from that shown in Figure 2.

Nevertheless, the grooves 5 may be deep and the bottom of the grooves 5 flat.

Thus airflows can easily pass along the grooves 5 in this case, too.

[0024] Preferably, the width L of the grooves 5 is at least 0.4 mm and their depth S at least 0.2 mm.

Example 1: A second feasible embodiment is as follows : There are longitudinal yarns, i. e. warps, in two layers o The cross section of the longitudinal yarns is flat and their width is 0.60 mm and height 0.3 mm @ The density of the longitudinal yarns in the wire, i. e. warps, is set to 200 yarns/10 cm in the weaving machine The wire has shrunk about 20% from its original width during the manufacture. The shrinkage results from weave technical factors and thermal treatment after weaving, for example After the manufacture, the warp density of the wire is 240 yarns/10 cm The width of the grooves is 0.6 mm and their depth 0.3 mm The number of grooves in the width direction is 6 per cm The warp coverage of the wire is calculated to be 72%. The warp coverage can be calculated by multiplying the warp density by the width of the warps and by further dividing the product by the number of warp layers. The resulting number can then be converted into percentage in the conventional manner. In this case, the warp coverage is calculated using the formula : [ (240 * 0. 6)/2] * 100% = 72% Example 2: A third feasible embodiment is as follows : There are longitudinal yarns, i. e. warps, in two layers The cross section of the longitudinal yarns is flat and their width 0.70 mm The density of the longitudinal yarns of the wire, i. e. warps, is set to 180 yarns/10 cm in the weaving machine The wire has shrunk about 20% from its original width during the manufacture. The shrinkage results from weave technical factors and thermal treatment after weaving, for example After the manufacture, the warp density of the wire is 220 yarns/10 cm The warp coverage of the wire is calculated to be 77%. The warp coverage is calculated using the formula : [(220 * 0. 7)/2] * 100% [0025] Furthermore, in some cases the weave structure of the wire may differ from those illustrated in Figures 1 and 2. Thus longitudinal yarns may be arranged into groups of three longitudinal yarns instead of two yarns, for instance. In addition, longitudinal and transverse yarns may be in arranged in several layers.

[0026] The drawing and the related description are only intended to illustrate the inventive concept. The details of the invention may vary within the scope of the claims.