DRYING BOX

Technical field

The present invention relates to a drying box for drying a cellulose pulp web, the drying box comprising a plurality of fans for generating a positive pressure in the interior of the drying box, each fan being driven in a first rotational direction by means of a motor. The invention further relates to a cooling disc and a break pin suitable for use in such a drying box.

Technical background

Drying boxes of the initially mentioned kind are well known since a few decades. Such a drying box usually includes hundreds of fans for generating a positive pressure in its interior. Traditionally, a back stream operated damper has been placed downstream of each fan. In case for instance a failure occurs in one motor/fan combination, the damper is closed and keeps the pressure in the the drying box interior up to some extent. Additionally, the damper prevents that the fan is driven in its reverse direction by the pressure in the drying box such that the maintenance of the motor/fan combination is made possible.

One problem associated with the above-mentioned known drying boxes is that the back stream operated damper entails losses in the forward direction. This, of course, reduces the efficiency of the motor/fan combination.

Summary of the invention

An object of the present invention is to provide a drying box with improved efficiency. This object may be achieved by means of a drying box as claimed in claim 1. Additionally, a cooling disc as claimed in claim 13 and a break pin as claimed in claim 16 may be useful to achieve the above-mentioned object.

More specifically, there is provided a drying box for drying a cellulose pulp web, the drying box comprising a plurality of fans for generating a positive pressure in the interior of the drying box, each fan being driven in a first rotational direction by means of a motor. At least one of the fans is connected to a motor via a shaft, wherein the shaft is provided with a reverse blocking arrangement, which prevents the at least one fan from being driven,

by the positive pressure, in a second rotational direction, opposite to said first rotational direction.

The reverse blocking arrangement thus stops the fan from rotating in the reverse direction. The fact that the fan is prevented from rotating in the reverse direction serves also to keep the pressure in the drying box up in case e.g. of a motor failure. The back stream operated damper can therefore be omitted. Additionally, maintenance work can still easily be carried out on the motor as it is not moving. Moreover, if during start-up of the drying box all fans are not started simultaneously, it is prevented that a motor rotates backwards when started which reduces the winding currents of the motor during its start-up.

The reverse blocking arrangement may comprise at least one blocking member, which is pivotably mounted to the shaft at a pivot point and a stopping member, which is fixedly mounted in relation to the drying box, wherein the blocking member is spring loaded by a spring to pivot in a first pivotal direction, such that, in a blocking mode, the blocking member protrudes in the radial direction from the shaft in a projecting position and interacts with the stopping member, such that the shaft is prevented from rotating in the second rotational direction. This provides a simple but yet reliable reverse blocking arrangement. The mass centre of the blocking member may be offset with respect to the pivot point, such that, in a free mode, when the shaft rotates with a speed exceeding a threshold speed, the blocking member is pivoted in a second pivotal direction, such that the blocking member is retracted from the projecting position. This may provide the advantage that the blocking member does not interact at all with the stopping member at higher speeds, which reduces noise and losses.

In the blocking mode, the blocking member may be prevented from pivoting further in the first pivotal direction.

The blocking member may be mounted on a cooling disc, which is mounted on and extends radially from the shaft. The cooling disc provides the additional advantage of protecting to some extent the motor from heat conveyed by the shaft.

Two blocking members may then be symmetrically mounted on the cooling disc. This reduces the forces generated when the shaft is stopped in the reverse direction and also serves to balance the cooling disc.

The stopping member may comprise a break pin, which is configured to be mounted in a radial direction with respect to the shaft, the break pin

having an indication of fracture, which ensures that the pin is deformed should the motor accidentally be connected to be driven in the second direction. This protects the motor from excessive currents that would otherwise be generated when the motor is driven against the reverse blocking arrangement.

The break pin may further comprise a fin, which is attached to the break pin at the end proximate to the shaft, extends in the second rotational direction from the break pin and presents a cam surface, wherein the distance between the cam surface and the shaft decreases gradually in said first rotational direction. This serves to smoothly fold the blocking member in to pass the stopping member e.g. when the shaft begins to rotate in the permitted direction. The fin may be made of a plastic material, which reduces emitted acoustic noise.

Alternatively, the reverse blocking arrangement may comprise a brake, such as a disc brake.

The motor may be releasably connected to the fan by means of connection means on the shaft. The reverse blocking arrangement may then be placed between the connection means and the fan. This allows the motor to be replaced while the drying box is operative and without releasing the fan such that it begins to rotate in the reverse direction.

Additionally, a cooling disc suitable for use in such a drying box comprises means for attaching the cooling disc on a shaft between a motor and a fan. Further, the cooling disc comprises at least one blocking member, which is pivotably mounted on the cooling disc at a pivot point, wherein the blocking member is spring loaded by a spring to pivot in a first pivotal direction, such that, in a blocking mode, the blocking member protrudes in the radial direction from the shaft in a projecting position, and wherein the mass centre of the blocking member is offset with respect to the pivot point, such that, in a free mode, when the shaft rotates with a speed exceeding a threshold speed, the blocking member is pivoted in a second pivotal direction such that the blocking member is retracted from the projecting position. Two blocking members may be symmetrically mounted on the cooling disc. The cooling disc may comprise a hub portion against which each blocking member abuts in the blocking mode, such that the blocking member is prevented from pivoting further in the first pivotal direction.

Additionally, a break pin suitable for use as a stopping member in a drying box of the above-indicated kind has a mounting portion, a head

portion, an indication of fracture between the mounting portion and the head portion, and a fin, which is attached to the break pin at the head portion, the fin extending substantially perpendicularly in a protruding direction from the break pin. The fin presents a cam surface facing away from the mounting portion, and the distance between the cam surface and a point, situated at a prolongation of the break pin in the head portion direction, increases gradually in said protruding direction. The fin may be made of a plastic material. ,

Brief description of the drawings Fig 1 illustrates schematically a section of a drying box.

Fig 2 shows a perspective view of a cooling disc. Fig 3 shows a cross section of the cooling disc of fig 2 as applied on a shaft, when the cooling disc is in a blocking position.

Fig 4 shows the cooling disc of fig 3 having begun a rotation in the permitted direction.

Fig 5 shows the cooling disc of fig 3 rotating at a speed exceeding a threshold speed.

Detailed description Fig 1 illustrates schematically a section of a drying box 1. Such a drying box is used for drying a cellulose pulp web. The drying box comprises plurality of fans 3, 5 for generating a positive pressure, i.e. a pressure higher than the normal atmospheric pressure, in its interior 7.

Typically, a drying box comprises hundreds of fans, only two of which are illustrated in fig 1 , in order to simplify the description. As drying boxes in general are well known, no overall description of a drying box is given.

Each fan 3, 5 is driven by a motor, 9, 11 , respectively, to which it is connected via a shaft 13, 15. Optionally, the motor may be releasably connected to the fan by means of a shaft coupling 17 or another connection means on the shaft 13. This allows the motor to be replaced without removing the fan. The reverse blocking arrangement to be described is however also useful in cases where the motor is fixedly mounted to the fan as illustrated in the lower portion of fig 1.

Each fan is driven in a first, forward rotational direction by its motor, in order to generate the positive pressure in the drying box interior. The motor is typically an electrical motor.

If a motor failure occurs in one of the motors 9, the corresponding fan 3 will quickly slow down, and will then be driven at high speed in a second, reverse rotational direction by the positive pressure in the drying box interior 7. This makes maintenance work on the defect fan/motor combination difficult and potentially dangerous.

A similar problem occurs if, during start-up of the drying box, all fans are not started simultaneously. The fans started first will then generate a positive pressure in the drying box interior which drives the fans still to be started in the reverse direction. When starting the latter fans' motors, their windings will be subjected to very high currents which may be detrimental.

As initially mentioned, these problems may be obviated by means of a back stream operated damper placed downstreams in relation to the fan in the drying box, which, as mentioned, entails some disadvantages.

In the illustrated drying box 1 , there is instead provided a reverse blocking arrangement 19 on the shaft 13 between the motor 9 and the fan 3. The reverse blocking arrangement 19 prevents the fan 3 from being driven in the reverse direction. This obviates the above indicated problem and additionally serves to keep the pressure in the drying box interior up.

In cases where the shaft 13 comprises a shaft coupling 17, the reverse blocking arrangement 19 is preferably placed between the shaft coupling 17 and the fan, such that the motor 9 may be disconnected without the fan beginning to rotate in the reversed direction.

There will be described in detail, with reference to figs 2-5, a reverse blocking arrangement comprising a blocking member, which is mounted on a cooling disc.

However, it should initially be mentioned that the reverse blocking arrangement may also be provided as a brake, e.g. a disc brake. The motor speed should then be monitored, and in case the motor speed approaches zero or begins to be negative, the brake should be activated. Fig 2 shows a perspective view of a cooling disc 21 which is devised to operate also in a reverse blocking arrangement. The operation of the reverse blocking arrangement will be described in more detail with reference to figs 3- 5. The cooling disc 21 comprises two halves 23, 25, which may be more or less identical. Each half comprises a disc portion 27 and a hub portion 29. The halves 23, 25 may be fitted around a shaft and may be attached to each other by means of bolt joints 31. The disc portions 27 then extend radially from the shaft, and serve to radiate heat that is conveyed by the shaft. As the

above mentioned fans are placed in a hot region, the cooling disc 21 therefore serves to protect the motor and bearings associated therewith from excessive heat. Additionally, on the disc portion at least one blocking member 33 is pivotably mounted, as will now be described. The blocking member 33 is mounted between the disc portion 27 and a lid portion 35, which is attached to the disc portion.

Fig 3 shows a cross section of the cooling disc of fig 2 as applied on a shaft and in a reverse blocking arrangement, when the cooling disc is in a blocking mode, the shaft being urged in the reverse direction (counter- clockwise). The cross section is made in a plane perpendicular to the axial direction of the shaft, such that the lid portion of fig 2 is not shown and the blocking member 33 is exposed. The reverse blocking arrangement further comprises a stopping member in the form of a break pin 37, which will be described in more detail later. The blocking member 33 is pivotably mounted to the shaft via a pivot point 39 on the cooling disc. The blocking member is attached to the cooling disc by means of a bolt, penetrating the blocking members as well as holes in the disc portion and in the lid portion as illustrated in fig 2.

The blocking member 33 is spring loaded by a spring 41 to pivot in a first pivotal direction (clockwise) to reach the blocking mode, which is illustrated in fig 3 and where the blocking member protrudes in the radial direction from the shaft in a projecting position. In the projecting position the blocking member reaches beyond the outer periphery of the disc portion, even though it should be noted that this is not essential. In the projecting position, a shoulder surface 43 of the blocking member 33 interacts with the stopping member 37, which is fixedly mounted in relation to the drying box, such that the shaft 13 is prevented from rotating in the reverse direction.

In this position, the blocking member 33 is prevented from pivoting further in the first pivotal direction. This may, as illustrated, be accomplished by letting a part of the blocking member 33, which is situated on the other side of the pivot point 39 as seen from the shoulder surface 43 abut the hub portion 29 of the cooling disc. In this case a small bolt 45, having a resilient head and being fastened in the blocking member 33, abuts the hub portion 29.

The illustrated cooling disc has two blocking members, which are symmetrically mounted with respect to the shaft on the cooling disc. The

advantage with this configuration, apart from the improved balance, is that the shaft will not rotate more than 180° before being blocked. The shaft will not accelerate as much as if only one blocking member was used, and therefore the blocking system need not absorb as much kinetic energy when the blocking takes place. In an application where this is not important one of the blocking members could be replaced with a dummy weight.

More blocking members than two may of course be used at the price of increased complexity. However, it may be useful to keep some free space on the disc portion 27, as this space can be used to attach small balancing weights in order to balance the cooling disc. This may typically be done with different numbers of washers that are attached with bolts through holes (not shown) in the disc portion 27.

The break pin 37 is configured to be mounted in a radial direction with respect to the shaft 13. The break pin has a mounting portion 47, which as illustrated can be attached to the drying box frame. The break pin further includes a head portion 49, having a shoulder surface 51 , which is intended to interact with the shoulder surface 43 of the blocking member 33.

Additionally, the break pin has an indication of fracture 53 between the mounting portion 47 and the head portion 49. The indication of fracture 53 ensures that the pin is deformed should the motor accidentally be driven in the second direction, which protects the motor from excessive winding currents.

Fig 4 shows the cooling disc of fig 3 having begun a rotation in the permitted forward direction (as illustrated: clockwise). In this state, the break pin 37 does not stop the shaft 13 from rotating, instead the blocking member 33 is pivoted in the second pivoting direction (counter-clockwise), such that the blocking member 33 leaves the protruding position and slips past the break pin 37. A smoothly curved surface 55 on the blocking member, at the side facing away from the shoulder surface 43, facilitates this movement. Additionally, the break pin 37 may have a fin 57, which is attached at the head portion 49 and extends substantially perpendicularly from the break pin in the reverse rotational direction. The fin presents a cam surface 59 facing away from the mounting portion 47. The distance between the cam surface and the shaft decreases gradually in the forward rotational direction. The blocking member 33 interacts with the cam surface 59 when the shaft rotates in the forward direction, such that the blocking member is smoothly pushed away from the protruding position. This reduces the acoustic noise

emitted when the shaft begins to rotate in the forward direction, and even more prominently when the shaft slows down to stop, as this usually takes more time. The effect may be improved if the fin is provided in a plastic material, such as PC (polycarbonate). A wide range of other plastic materials are of course possible. When the blocking member has past the break pin, it snaps out to the protruding position again due to the spring loading. The emitted noise at this instant is reduced by the resilient material of the head of the bolt 45.

Thus, the reverse blocking arrangement allows the shaft to begin its rotation in the permitted forward direction.

Fig 5 shows the cooling disc of fig 4 when rotating in the permitted direction at a speed exceeding a threshold speed. At such speeds, the cooling disc enters a free mode where the blocking member does not at all interact with the break pin. This is accomplished by letting the mass centre 61 of the blocking member 33 be offset with respect to the pivot point where it is attached to the cooling disc. The mass centre 61 is offset in such a way that the centrifugal force caused by the rotation counteracts the force applied by the spring and eventually, at the threshold speed, exceeds the latter force enough to retract the blocking member fully from its projecting position. Therefore, in the free mode illustrated in fig 5, the blocking member runs free from the stopping member. In most cases, the fans will operate at speeds around 1500-1800 rpm, depending on the frequency of the supply grid. The threshold speed may be set substantially lower.

As the blocking member runs free, the acoustic noise emission and the losses caused by the blocking member-break pin interaction in the state in fig 4 are eliminated.

It is possible to lock the blocking member in the retracted position by sticking a bolt through a hole 63 in the blocking member 33 and through corresponding holes 65, 67 in the disc portion and the lid portion (fig 2). This allows, optionally, the cooling disc to be run without an operative reverse blocking arrangement, e.g. during installation and maintenance.

The invention is not restricted by the described embodiments, and may be varied and altered within the scope of the appended claims.