In a prior-art drying plant of this type, 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 in 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.

If there should be a break of the web between the press plant and the drying plant, great problems arise.

By a so-called rubber band effect, an accumulation of the material of the web occurs adjacent to the turning rolls located in the upper portion of the housing, and un-

controlled lateral oscillations often occur in the part of the travelling web that is positioned closest to the break and result in the breaking of the web into small pieces. Substantial parts of the web located in the housing are then damaged, and time-consuming cleaning work is necessary in order to remove these damaged parts from the housing.

The object of the present invention is to provide a drying plant, which is so constructed that the cleaning time is considerably reduced when a break of the web occurs between the press plant and the drying plant.

According to the invention, this object is achieved by a plant of the type mentioned by way of introduction and characterised in that at least one brake means is arranged to brake, when activated, at least one of those of said undriven rolls which are arranged closest to the entry of the housing, seen along the web, that a regu- lating unit is connected to the brake means to control its activation, and that at least one web break detector is arranged upstream of the housing and connected to the regulating unit to make the regulating unit, when a break of the web is detected, activate the brake means to brake the associated roll.

The regulating unit is preferably arranged to in- activate the brake means after activation of the same after a predetermined time.

Below, the invention will be described in more detail with reference to the accompanying drawings.

Fig. 1 is a schematic side view of a drying plant according to the invention.

Fig. 2 is a block diagram illustrating the control of a brake arranged in the drying plant.

The drying plant 1 shown in Fig. 1 is intended for the drying of a web 2 consisting of papermaking pulp. The web 2 is fed 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 receive a 50-2000 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 group of lower blow boxes 9, which discharge heated air at their upper side to dry the web 2. Each group of lower blow boxes 9 is associated with a group of upper blow boxes 10, which discharge heated air at their underside to dry the web 2.

The housing 4 also contains a plurality of rotatable but undriven turning rolls 11,11', 11", which are ar- ranged at the ends of the drying decks, i. e. adjacent to the front wall 5 and the rear wall 7 of the housing 4. The web is led over the turning rolls 11,11', 11"to be con- veyed in zigzag across the drying decks, i. e. the groups of lower blow boxes 9. In this connection, the web 2 obtains a horizontal run over each drying deck or group of lower blow boxes 9, which run is fed between the aroup of lower blow boxes 9 and the associated group 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, respectively.

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 undriven 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 an undriven pressure roll 19.

A first web break detector 20 is arranged between the two pairs of press rolls 12 and 13, and a second web break detector 21 is arranged between the supporting roll 15 and the input roll 16. The detectors 20 and 21, which can, for instance, consist of photocell units, are arranged to detect a break in the web 2.

As shown in Fig. 2, the detectors 20 and 21 are con- nected to a regulating unit 22, which comprises a control means 23, to which the detectors 20 and 21 are connected, a valve 24 and a compressed-air unit 25. The control means 23 comprises time relays (not shown) and is con- nected to the valve 24 to control the same. The com- pressed-air unit 25 is connected via the valve 24 to a pneumatic brake means 26, which is arranged, when acti- vated, to brake the turning roll 11', which, seen along the web 2, is located closest to the entry 6.

When one of the detectors 20 and 21 detects a web break, it emits a signal to the control means 23, which then immediately opens the valve 24 in order to open the connection between the compressed-air unit 25 and the brake means 26 and thus activate the same. The brake means 26 then brakes the turning roll 11'in a sliding manner, so that it is rotated with greater inertia in its friction engagement with the web 2. The time relays of the control means 23 are so adjusted that after a pre- determined time, e. g. 30 s, the control means 23 closes the valve 24 again, which then interrupts the connection between the compressed-air unit 25 and the brake means 26 and inactivates the same. Thus, the braking of the turning roll 11'ceases. The time relays can also be so adjusted that the control means 23 can only open the valve 24 when the detectors 20 and 21 have not detected a web break during a predetermined time, e. g. 6 min, that is indirectly detected that a web 2 passes the detectors.

The brake means 26 can be arranged to brake one of the other turning rolls, other than the turning roll 11', which are arranged, seen along the web 2, closest to the entry 6, e. g. the turning roll 11"which is second closest to the entry 6. The brake means 26 can also be arranged to brake more than one of these turning rolls.

Alternatively, a brake means can be arranged for each turning roll which is to be braked. The input rolls 16 and 17 are also part of the undriven rolls that can be braked by the brake means 26 or a brake means of their own.