BACKGROUND In plants that produce pulp bales, a long pulp web is created, which passes a drying unit in order to be dried with warm air. The pulp web continues from the drying unit to a scissors device where the pulp web is cut to sheets, see, for example, the document WO, A1, 96/22419.

Warm air in the drying unit is led in a circuit from a first steam battery over the pulp web to a second steam battery and then over the pulp web back to the first steam battery. The warm air streams over the surface of the pulp web whereby loose fibres are captured and follow with the air stream. Thus the warm air becomes laden with fibres when it passes over the pulp web between the steam batteries.

In association with, and in front of, each steam battery, a filter is placed that filters out the loose fibres from the drying-air. The warm air passes a filter, is warmed in the particular steam battery and flows onwards over the pulp web to the next filter and steam battery. The loose fibres attach to the filter and, as time passes, the amount of fibres on the filters increases and the efficiency of the filter falls. This in turn leads to a decrease in the efficiency of the pulp drier.

In this way, the need arises to clean the filter from loose fibres. The cleaning of the filter is currently carried out by one or more people walking at different levels in the pulp drier and cleaning the filters from fibres with the aid of a portable vacuum cleaner of the type used in domestic application.

This vacuum cleaning is an important maintenance measure for the approximately 400 pulp dryers that exist in the world. The work is not popular. The environment is extremely noisy, warm and windy. Furthermore, the work with the vacuum cleaner is heavy and monotonous.

A device for cleaning an air filter in a feed channel for drying-air is shown in document DE, A, 3324372. The cleaning can take place during operation by closing one of several channels, and then guiding air to flow through the filter in the channel in the direction

opposite to the direction of the drying-air while the filter is simultaneously vibrated. Fibres are freed from the filter and captured, and the fibre-laden air is led out through an opening.

Pulp dryers are very large machines with a height of 5-20 metres. They are constructed from a number of identical vertical sections with steam batteries for warming of the drying-air.

Fans blow the warm air into jet boxes. The jet boxes distribute the air over the surface of the pulp web. After the air has passed the pulp web, the same air is re-warmed and blown in over the pulp web again. In order to optimise the heat efficiency, only a small portion of moist air is allowed to escape from the machine, to be replaced by a small portion of input air. In order to separate fibres out from the fibre-laden air, filters are arranged in the form of long nets in association with and along a complete pile of steam batteries.

Problems arise when the net lacks tension and is drawn in towards the steam battery. This results in, among other things, wear on the net. Problems also arise in that the amount of accumulated fibres varies over the net. It is difficult to know where and when the amount of fibres on the net passes the limit of what is acceptable. Cleaning will be uneven over the net when using a manual system, and it will be different from one occasion to another. It is particularly difficult to remove accumulated fibres from both sides of the net.

Thus the need for an automatic cleaning of the filters during the drying of pulp webs with warm air arises, as described above. It is required that the machine functions without manning and that it starts to operate when necessary. The machine must be constructed such that the net filter is not exposed to abnormal wear. The machine is to be able to clean both sides of the net filter. Furthermore, the machine must be able to take into consideration variations in the amount of accumulated fibres on the net.

The device according to the German document cannot satisfy these requirements.

DESCRIPTION OF THE INVENTION A pulp dryer using warm air as the drying medium comprises piles of steam batteries. Air is arranged to leave the steam batteries after warming, meet and dry the moist pulp web and circulate back to the steam batteries laden with loose fibres from the pulp web. The steam batteries are arranged with net filters, which filter out the loose fibres from the drying-air.

One pile of steam batteries has one common net filter.

The intention of the invention is to arrange automatic cleaning of the above-mentioned net filters in a pulp dryer. It is also the intention that it should be possible to install and use the invention in existing pulp dryers.

The solution according to the invention is characterised by the attributes specified in claim 1.

The pulp dryer is equipped with an automatic cleaning system for the removal of the loose fibres. The system comprises suction nozzles attached to a vacuum cleaning system. The net filter and the suction nozzles are arranged to move relative to each other. The net filters are placed in connection with that side of the steam batteries where the fibre-laden and moist air passes back into the steam battery in order to be warmed.

A filter net is arranged in a mobile manner between two motor-driven rollers. The filter net is rolled from the first roller to the second roller while at the same time it passes one or more fixed suction nozzles. The steam battery is equipped with spacer devices that prevent the net from being sucked into the battery. The net of the net filter is equipped with stretcher devices that tension the net in order to minimise wear of the net.

A method according to the invention in which the net is automatically cleaned by vacuum suction and in which the suction capacity can be regulated is also part of the solution. A pressure monitor measures the pressure difference across the net and gives a signal to the driving motor concerned to start the net when there is an accumulation of fibres on the net.

Alternatively, the net can be driven continuously, with constant or with adapted speed.

Assembly during installation of the invention onto existing pulp dryers is made easier in that the vacuum cleaning fan is common to all suction nozzles and that it can be placed separately at a distance from the battery piles. A scrubber for separation of the fibres can also be arranged at a distance from the battery piles. The amount of loose fibres on the pulp web varies considerably and depends on, among other things, earlier process steps, the type of pulp and its quality. The amount of loose fibres is greatest at the entry of the moist pulp web and at the turning rollers.

DESCRIPTION OF THE DRAWINGS The invention will now be explained in more detail by describing an embodiment with reference to the attached drawings, where

Figure 1 shows a pulp dryer with a pulp web that runs vertically and a cleaning machine according to the invention with net filters stretched between two motor-driven rollers.

Figure 2 shows sketches of the principle by which drying-air flows in a vertical pulp dryer.

Figure 3 shows net filters and suction nozzles seen in a direction outwards away from the steam battery.

Figure 4 shows the construction of the vacuum cleaning system.

Figure 5 shows a detailed view of a steam battery and a net filter in one embodiment of the invention.

Figure 6 shows a steam battery with spacer devices together with a net filter according to the invention, seen from the side.

Figure 7 shows an embodiment in which two piles of steam batteries have a commonly mobile net filter, in the form of an infinite band, and an embodiment with a mobile suction mouthpiece arranged next to a fixed net filter, in the form of a finite band.

Figure 8 shows an embodiment in which two piles of steam batteries have a commonly mobile net filter in the form of a finite band.

DESCRIPTION OF EMBODIMENTS A pulp dryer 1 (Fig. 1), in which the pulp web 2 runs vertically between turning rollers 3 and 4, comprises a number of identical vertical sections/piles 5 with steam batteries 6, for warming of drying-air. The drying-air is warmed in a first steam battery 6a. A first fan 7a (Figure 2) blows the warmed air into jet boxes 8a, which distribute the air over the surface of the pulp web 2. Warm air flows over the surface of the pulp web 2 and absorbs moisture from the pulp web 2 and at the same time carries with it all loose fibres from the surface of the pulp web 2. Once the air has passed the pulp web 2 it flows, cooled, towards a second steam battery 6b. The air is now laden with fibres, and a net filter 9b is stretched in front of the steam battery 6b in order to filter out the fibres. The air passes the net filter 9b, the fibres stick to the net, the air continues into the steam battery 6b in which it is warmed. The warm air leaves the steam battery 6b and a second fan 7b forces it onwards into the jet boxes 8b.

The drying-air flows out along the pulp web 2 and, laden with fibres, approaches a first steam battery 6a with a net filter 9a. The drying-air is filtered and warmed, after which it returns to the first fan 7a.

A small amount of moist wet air is released at the top of the machine, and a small amount of dry air is admitted at the bottom of the machine. Otherwise the system is a closed system in which air circulates.

An automatic cleaning system 1 a for removal of the loose fibres from the net filter 9 is arranged in the pulp dryer 1 and comprises suction nozzles 12 connected to a vacuum cleaning system 13.

The net filter 9 consists of a fine-mesh net 9 in the form of a band, which will be denoted in the following for simplicity's sake as"the net" (Figure 1). The net 9 is stretched between a first 10 and a second 11 rotatable roller. Each roller is driven by a motor arranged with a gear-box. When the relevant motor rotates the first/lower roller 10, the net 9 is rolled up onto the first roller 10 and rolled out from the second/upper roller 11. The second driving motor functions in this case as a brake which ensures that the net remains under tension during the transfer. The machine is constructed such that the net is transferred with a variable and controllable speed. The rollers 10 and 11 with their associated driving devices are placed in the vicinity of the relevant turning rollers 3 and 4, which means that they are placed in a relatively mild area from the point of view of climate (Figure 1).

A suction nozzle 12 is arranged close to each roller such that the net 9 passes the nozzle 12, is cleaned of fibres and is subsequently rolled up onto the roller. The length of the suction nozzle 12 is in this case equal to the width of the net. The suction nozzle 12 is connected to a vacuum cleaning system 13 (Figure 4).

The complete vacuum cleaning system 13 comprises a number of steam battery piles and filter nets. The complete system is controlled by a PLC system. The system 13 includes a vacuum fan 14, a water pump 15 for a scrubber 16 and a magnetic valve 17 for water. At the start of a vacuum cleaning sequence, the vacuum fan 14 and water pump 15 are started, and the magnetic valve 17 is opened for water. A damper S1 is opened and a motor Ml is started.

The relevant net filter 9 is rolled up onto the equivalent lower roller 10 while at the same time the relevant suction nozzle 12 cleans the net 9. The equivalent upper motor M2 functions in this case as a brake and ensures that the net filter 9 is held under tension. When a length of the net filter 9 equal in length to one steam battery pile 5 has been rolled up, the motor M1 is stopped by a limit position. Damper S3 opens while damper S 1 closes and motor M3 starts.

The sequence continues with all of the lower motors, and all nets have been cleaned when all of these have been activated.

Depending on the amount of fibre dust that arises and the size of the machine, time- dependent stops can be established for the vacuum fan, the water pump and the magnetic valve for the water. After the programmed time, the sequence recommences, but in this case dampers and motors with even numbers, 2,4,6, etc., are activated.

In order to avoid collision of the net 9 with the steam battery 6, all steam batteries 6 are equipped with spacer devices in the form of an extra flange pipe 18, which will be denoted by frameshaped flange pipe in the following (Figure 5). Each frameshaped flange pipe 18 is equipped with vertical tubes 19 that prevent the net 9 from being sucked into the steam battery 6.

In order to avoid abnormal wear of the net 9, the net is equipped with stretching devices 19 in the form of horizontal supporting beading 20. The supporting beading 20 stretches the net 9 out across its width and is arranged perpendicular to the direction of motion of the net 9.

ALTERNATIVE EMBODIMENTS The invention is to be installed in pulp dryers with pulp webs that move horizontally. The pulp dryer is equipped with piles of steam batteries at one edge of the pulp web 2. After passage over the pulp web in one direction, the warm air reverses direction and is recirculated, now laden with fibres, between the jet boxes back to the steam battery.

The net filter 9 consists of a infinite band 9d. The pulp dryer is equipped with at least one driving unit and a number of supporting rollers 21 in order to circulate the infinite band past one or more steam battery piles. One or more suction nozzles 12 are placed at a freely chosen position or positions along the pathway of the net band (Figure 7). In this way the net is fully exploited and cleaning can be carried out in an optimal manner. The driving unit circulates the infinite band either continuously or intermittently.

Pulp dryers that have steam battery piles 5 arranged in pairs have a common net filter 9 in the form of a finite band 9c for two battery piles 5 (Figure 8). In this way, the required numbers of rollers, driving units and suction nozzles are minimised.

Pulp dryer in which the net filter 9 is firmly fixed and in which at least one mobile suction nozzle 12a is displaced along the net filter 9, as is exemplified at the left of Figure 7.

Fibre dust on the reverse side, that is, the side of the net that is not cleaned, is sucked away with a further suction nozzle 12.

A blower nozzle 22 on one side of the net 9 blows the loose fibres in the direction of a suction nozzle 12 arranged on the opposite side of the net (Figure 6).

The design of the suction nozzle is adapted to the geometry of the net.

The suction nozzles are placed at freely chosen locations on one or on both sides of the net.

Independently of whether it is net filters, suction nozzles, blower nozzles or a combination of these that are arranged to be mobile, they are driven such that they move either continuously or intermittently.