The invention concerns a grouping of the cylinders in the drying section of a paper machine, according to the preamble of claim 1.
In the prior-art constructions of drying sections in paper machines, see WO-A-90 12 150 on which the preamble of claim 1 is based, the drying section consists of drying groups. Each drying group comprises heated drying cylinders and cold leading cylinders placed in a level different from the level of the drying cylinders. Being supported by the wire, the paper web is passed meandering from a drying cylinder onto a leading cylinder and further from the leading cylinder onto the next drying cylinder. The drying cylinders are placed in two levels in relation to one another, those levels being different from the level of the leading cylinders.
In such a prior-art construction of drying sections, the overall length of the drying section becomes remarkably long. On the other hand, the overall length of the drying section affects the requirement of the building area of the paper mill. If it is possible to reduce the length of the drying section, it is also possible to lower the building cost required by the paper machine in the same proportion.
Thus, the object of the invention is to provide a drying section of a novel type for a paper machine, which is considerably shorter than a prior-art drying section with an equivalent drying capacity and with an equal number of cylinders.
The object of the invention has been achieved by the features of the characterizing clause of claim 1. As a result of the novel grouping, the overall length required by the drying section can be reduced considerably. However, the drying section in accordance with the invention has a drying capacity equal to that of a prior-art drying section with the same number of cylinders.
Thus, according to the invention, the drying section has been formed so that it comprises heated, in particular steam-heated, drying cylinders of two sizes. The drying cylinders of larger diameter are fitted to be placed at a height level of their own, and, in a corresponding way, the drying cylinders of smaller diameter are fitted to be placed at a height level of their own. Further, the drying section comprises cold leading cylinders, which are placed at two different height levels, i.e. at a height level Y3 and at a height level Y2', the latter height level being substantially the same as that of the drying cylinders of smaller diameter. The paper web and the wire are passed meandering from a drying cylinder of larger diameter onto a leading cylinder and from the leading cylinder onto a drying cylinder of smaller diameter. From the drying cylinder of smaller diameter the paper web and the wire are passed onto a second leading cylinder and from the leading cylinder onto a second heated drying cylinder of larger diameter. By means of the above grouping of cylinders and choice of diameters of drying cylinders, it is possible to form, for example, a drying section whose overall length is, compared with a prior-art 71-cylinder Sym-Run drying section, 18 metres shorter.
The invention will be described in the following with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying drawings, the invention being, yet, not supposed to be confined to said embodiments alone.
Figure 1 shows a prior-art drying section.
Figure 2 shows a drying section in accordance with the invention.
Figure 3 is a more detailed illustration of the grouping of the cylinders in a drying section in accordance with the invention.
As is shown in Fig. 1, a prior-art drying section consists of groups R1,R2,...Rn of drying cylinders. Each group R1,R2,R3,R4,...Rn of drying cylinders comprises hot, preferably steam-heated drying cylinders of larger diameter and smaller cold, so-called leading cylinders placed at a level different from the level of the drying cylinders. In each group R1,R2,..., the paper web is passed, being supported by the wire H, from a drying cylinder K onto a leading cylinder V placed at a different level and further from the leading cylinder V back to the upper level onto a heated drying cylinder placed at said level.
The leading cylinders are preferably perforated Uno-Vac cylinders, into which an internal negative pressure is introduced and which negative pressure is allowed to act upon the entire space in the interior of the drying cylinder, whereby the web is kept in contact with the wire also in the web draws in which the web W is not supported by a wire. Fig. 1 shows a construction of a drying section that comprises seven groups of drying cylinders, of which the second group from the end is an inverted group, in which the steam-heated drying cylinders K are placed at a lower level and the cold leading cylinders V are placed at a higher level above them.
Fig. 2 shows an improvement over the prior art illustrated in Fig. 1. In the construction, it has been possible to reduce the length of the drying section by forming the drying groups by means of a novel grouping of the drying cylinders and leading cylinders. In the solution shown in Fig. 2, there are six drying groups R1...R6, the second group R5 from the end being a so-called inverted group, in which the steam-heated drying cylinders K are placed at the lowest levels and the leading cylinders V at a higher level above them.
The groups R1...R4 and R6 are provided with a grouping of the heated drying cylinders K and of the leading cylinders V in accordance with the invention. The whole drying section comprises 28 steam-heated drying cylinders K and 29 cold cylinders, i.e. leading cylinders V. In the way shown in Fig. 2, the heated drying cylinders K in a drying group R are placed in such a way in relation to one another that a drying cylinder K of smaller diameter is placed at a height level Y2, which is a height level different from the height level Y1 of the drying cylinders K of larger diameter. The leading cylinders V are placed at the height level Y2' and at the height level Y3. The paper web W is passed, e.g. in the drying group R1, from the leading cylinder V1 placed at the height level Y2' onto the drying cylinder K2 placed at the height level Y1 and from the drying cylinder K2 onto the leading cylinder V2 placed at the height level Y3, and from said leading cylinder onto the heated drying cylinder K3 of smaller diameter placed at the height level Y2. From the heating cylinder K3 the web R is passed onto the leading cylinder V3 placed at the height level Y3 and from the leading cylinder V3 onto the drying cylinder K4 placed at the height level Y1. From the drying cylinder K4 the web R is again passed onto the leading cylinder V4 placed at the height level Y2'.
In a corresponding way, the wire H1 is passed so that in connection with the heated drying cylinders K2 and K4 the paper web W runs between the wire H1 and the face of the drying cylinder K2 and K4, and in connection with the leading cylinders V2 and V3 the web W runs outermost. Correspondingly, in connection with the drying cylinder K3 with smaller diameter the paper web W runs between the drying cylinder K3 and the wire H1.
Thus, in the groups R1,R2,R3,R4 and in the group R6 the hot drying cylinders K and the leading cylinders V are placed at four different height levels Y1,Y2,Y2',Y3 so that the hot drying cylinders K are placed at two different levels Y1,Y2 and the leading cylinders V at the level Y2' and at the level Y3. The height levels Y2 and Y2' are levels substantially equal to one another. The level Y2' is placed slightly higher than the level Y2. The diameter dimensions of the leading cylinders V placed at the levels Y2' and Y3 are just slightly different from one another. The leading cylinders V at the level Y2' have a slightly larger diameter than the leading cylinders V at the level Y3. The diameters of both of the leading cylinders V placed at the levels Y3 and Y2' are slightly smaller than the diameters of the heated drying cylinders K placed at the height level Y2. The diameter of the drying cylinder K placed at the middle height position Y2 is smaller than the diameter of the drying cylinders K placed at the first height level Y1. In this way, between the drying cylinders K and the leading cylinders V, a novel grouping has been provided which results in considerable economies of space in the machine direction. The drying capacity is good and corresponds to that of a prior-art drying section of equal number of cylinders and larger length.
Fig. 3 is a more detailed illustration of the grouping in accordance with the invention between the heated drying cylinders and the cold cylinders, i.e. the leading rolls. In the way shown in Fig. 3, each drying group R1,R2... consists of cylinder groupings in which the heated drying cylinders are placed at two height levels, the levels Y1 and Y2. The leading cylinders are placed at the height level Y2' and at the height level Y3. Moreover, the diameter of the heated drying cylinder placed at the middle height level Y2 is smaller than the diameter of the drying cylinder placed at the level Y1.
Together with the wire H, the paper web W is passed from the leading cylinder Vn-2 of the height level Y2' onto the drying cylinder Kn-2, of the level Y1. From the drying cylinder Kn-2, the web W is passed onto the leading cylinder Vn-1, and from the leading cylinder Vn-1 further, with the wire H, onto the middle heated drying cylinder Kn-1. From the heated drying cylinder Kn-1 the paper web W and the wire H are passed onto the second leading cylinder Vn of the level Y3. From the leading cylinder Vn the paper web W is passed onto the second heated drying cylinder Kn of larger diameter, placed at the height level Y1. From the drying cylinder Kn the web W is passed onto the leading cylinder Vn+1 of the height level Y2'. In a corresponding way, the wire H is passed from one cylinder onto the other. The paper web W runs in contact with the faces of the drying cylinders on the heated drying cylinders K. On the leading cylinders V the paper web W runs outermost, and the wire H is placed in contact with the cylinder V face. The leading cylinders Vn-1, and Vn are placed symmetrically at the same distance from the vertical plane X drawn through the centre point of the heated drying cylinder Kn-1, of smaller diameter. The heated drying cylinders Kn-2,Kn of larger diameter are also placed symmetrically at equal distances from the vertical plane. In a corresponding way, the leading cylinders Vn-2,Vn+1 of the height level Y2' are placed symmetrically to the centre plane X and at equal distances from same.