In a prior-art drying plant of this kind, a web of pulp leaving a press plant is fed into the housing via an input roll arranged outside the entry of the housing. The web is drawn through the housing by means of a draw roll arranged outside the exit of the housing. The entry is arranged in the upper portion of one of the walls of the housing, and the exit is arranged in the lower portion of the opposite wall of the housing. The web is conveyed zigzag across the superposed drying decks, which are defined by lower blow boxes, which discharge heated drying air at their upper side. Upper blow boxes, which discharge heated drying air at their underside, are arranged above the lower blow boxes. At each drying deck level, the web is fed in a floating manner between the upper and the lower blow boxes.

In this prior-art plant, the blow boxes are provided with drying air from fan units, which each consist of a

plurality of fans, which are driven at a substantially constant speed and have fixed blades, i. e. fans in which the emanating air flow cannot be adjusted or controlled.

These fans contribute to a great extent to the power con- sumption of the drying plant. The fans which are used to introduce air into and discharge air from the housing also contribute to this power consumption.

The object of the present invention is to provide a method of reducing the power consumption in a drying plant of this kind.

According to the invention, this object is achieved by a method of the kind mentioned by way of introduction and characterised in that at least some fan units are used, in which each fan is provided with an adjustment means for adjusting the air flow emanating from the fan, and that the air flow emanating from each of these fan units is adjusted with the aid of said adjustment means at a level, which is only slightly higher than the lowest air flow level which is required to keep the web float- ing, via the group of blow boxes which is supplied by the fan unit in question, at that blow box in the group which, seen along the web, is located closest to the entry of the housing.

Preferably, only such fan units as consist of one fan are used. Furthermore, preferably only such fans as are provided with adjustment means are used.

The groups of blow boxes are suitably arranged in such a manner that the blow boxes in one and the same group are located relatively close to each other, seen along the web.

The invention will now be described in detail with reference to the accompanying drawings, in which Fig. 1 is a schematic side view of a drying plant, in which the method according to the invention is used, and

Fig. 2 corresponds to Fig. 1, but it also shows schematically a plurality of circulation fans, which are arranged at one side of the drying plant.

The drying plant 1 shown in Fig. 1 is intended for drying a web 2 consisting of papermaking pulp. The web 2 is discharged from a press plant 3, which is arranged before the drying plant 1.

The drying plant 1 has a housing 4, whose height can be 2-15 m and whose length can be 10-100 m, which implies that the housing 4, in a manner described more specifi- cally below, can accommodate a 50 to 2000-m-long section of the web 2. The housing 4 has an entry 6 in the upper portion of its front wall 5 facing the press plant 3 and an exit 8 in the lower portion of its opposite rear wall 7. The web 2 is fed through the housing 4 from the entry 6 to the exit 8.

The housing 4 contains a plurality of superposed, horizontal drying decks, each defined by a row of juxta- posed lower blow boxes 9, which at their upper side dis- charge heated air for drying the web 2. Each row of lower blow boxes 9 is associated with a row of juxtaposed upper blow boxes 10, which at their underside discharge heated air for drying the web 2.

The housing 4 also contains a plurality of rotatable but nondriven turning rolls 11, which are arranged at the ends of the drying decks, i. e. in the vicinity of the front wall 5 and rear wall 7 of the housing 4. The web is led over the turning rolls 11 to be conveyed zigzag across the drying decks, i. e. the rows of lower blow boxes 9. Thus, the web 2 obtains over each drying deck or row of lower blow boxes 9 a horizontal run, which is fed between the row of lower blow boxes 9 and the associated row of upper blow boxes 10 to be dried by means of the hot air discharged by the blow boxes 9 and 10. At each drying deck level, the web 2 is fed in a floating manner between the lower and the upper blow boxes 9 and 10, re- spectively.

The press plant 3 has, in the example shown in Fig.

1, two pairs of press rolls 12 and 13, in which the web 2 is pressed. In the example shown in Fig. 1, the web 2 fed from the press plant 3 is led over two supporting rolls 14 and 15 and two nondriven input rolls 16 and 17, which are arranged immediately outside the entry 6 of the housing 4, and is introduced into the housing 4 via the entry 6. The web 2 is drawn through the housing 4 by a driven draw roll 18, which is arranged outside the exit of the housing 4 and cooperates with a nondriven pressure roll 19. The web 2 is drawn through the housing 4 in parallel with the side walls thereof.

The housing 4 is considerably wider than the web 2 and has side compartments beside the same, in which fan units 20 are arranged. The fan units 20 are schematically shown in Fig. 2 by means of"squares". The fan units 20 which are arranged in the vicinity of one of the side walls of the housing 4, i. e. in one of the side compart- ments, are shown in full lines in Fig. 2, and the fan units 20 which are arranged in the vicinity of the oppo- site wall of the housing 4, i. e. in the second side com- partment, are shown in dashed lines.

The fan units 20 supply a group of blow boxes 9,10 each with hot air. In the example shown, these groups of blow boxes 9,10 can consist of the blow boxes which are contained in the"square"which in Fig. 2 defines the re- spective fan units 20.

It should be noted that fans (not shown) are used to introduce air into and discharge air from the housing 4 and that heating batteries (not shown), which are sup- plied with water vapour, are arranged in the side com- partments of the housing 4 to heat the air which the fan units 20 blow into the blow boxes 9,10.

In the shown embodiment, each fan unit 20 consists of a fan 21, which is provided with an adjustment means (not shown) for adjusting the air flow from the fan.

To reduce the power consumption, the air flow ema- nating from at least some of the fan units 20, and pre- ferably from all the fan units 20, is adjusted with the aid of the adjustment means at a level, which is only slightly higher than the lowest air flow level which is required to keep the web 2 floating, via the group of blow boxes 9,10 which is supplied by the respective fan units 20, at that pair of blow boxes 9,10 which is located closest to the entry 6 of the housing 4, seen along the web 2. At that pair of blow boxes 9,10 in each group which is located closest to the entry 6 of the housing 4, seen along the web 2, the web 2 is at its heaviest since it has then passed the drying process during a shorter time than at the other pairs of blow boxes 9,10 in the group in question. To reduce the power consumption as efficiently as possible, the groups of blow boxes 9,10 should be arranged in such manner that the blow boxes in one and the same group are located relatively close to each other, seen along the web 2.

Generally speaking, this implies geometrically (cf.

Fig. 2) that the"squares" (20) should have a vertical extent which is as small as is practically possible. The greater height of a"square", i. e. the more drying decks the"square"covers, the greater is the difference in moisture content and thus weight of the web 2 in its portion (in the"square") located closest to the entry 6 of the housing 4 and its portion (in the"square") located furthest away from the entry 6, seen along the web.