Background Art It is known to discharge bulk materials, such as cement powder, from vessels, using mobile discharging devices. Such discharging devices normally comprise pneu- matic equipment for transferring the bulk goods from the vessel to the device and from there to different types of storage container.

A first type of mobile discharging device is dis- closed in US-4,016,994. This device comprises two tanks which can be operated independently of each other. More specifically, one tank can be filled while the other is being emptied. For filling a tank, air is evacuated from the tank for application of a sub-atmospheric pressure in the same. This causes suction in a supply conduit con- nected to the tank, thus allowing supply of bulk goods to the tank. The air evacuated from the tank passes through a cyclone collector in the form of a venturi scrubber for collecting particles.

A great problem of the mobile discharging device of the first type described above is that it has a low de- gree of utilisation and a low discharging capacity. The reason for this is on the one hand that the particle material collected in the venturi scrubber must regularly be removed, which causes operational trouble. Moreover the requirements for cleaning of the air are very high since a vacuum pump is normally used for evacuation of

the air. The evacuated air which consequently passes through the vacuum pump must be essentially clean. For the discharging device to have a high discharging ca- pacity, large flows of air must be evacuated from the containers. To ensure the requisite cleaning of the evacuated air, the venturi scrubber must have such dimen- sions that it is not practically possible to arrange it on a mobile discharging device. This applies specifically to the case where the bulk goods are pulverulent since the evacuated air will then contain large amounts of particles.

A second type of mobile discharging device is dis- closed in US-A-4,659,262. This device solves the above problems and comprises at least two containers supported by a wheel-mounted frame and filter means made of a fibre material. The device is arranged in such manner that a container is being filled with bulk goods from the vessel as a second, previously filled container is being emptied for further transport of the bulk goods and vice versa.

This results in time-efficient discharging of the load of the vessel. The prior art device further comprises a movable arm which supports a suction conduit provided with a nozzle and connected to the containers.

For filling of a container, the suction conduit and its nozzle are arranged for a hold of the vessel, con- taining bulk goods. At the same time, a sub-atmospheric pressure is applied in the container, whereby the bulk goods are drawn into the container through the suction conduit. The sub-atmospheric pressure applied in the container is generated by means of a vacuum pump which draws air out of the container.

For emptying the tank, a pressure above atmospheric is applied in the container, whereby the bulk goods can be pressed out of the container.

During filling of a container, the sub-atmospheric pressure prevailing in the container forms an air flow in the suction conduit. This air flow entrains the bulk

goods from the hold to the container. When the air flow enters the container, its speed is significantly reduced, which means that the air flow can no longer carry the bulk goods which thus fall down in the container.

The filter means are arranged in the containers and the air, drawn out by the vacuum pump from the respective containers, entrains a small amount of particles and passes through the filter means made of a fibre material and arranged in the containers. The air drawn in by the vacuum pump from the respective containers will thus be essentially free from particles, which is necessary for the vacuum pump not to be damaged. In this context, it is the filter means made of fibre material which allow efficient cleaning of the air, also at high discharging capacities and when discharging pulverulent bulk goods, while the dimensions of the filter means can be reduced.

It will certainly be appreciated that the filter means must be cleaned regularly, but this is easily achieved by means of an opposed air blast, whereby the collected particles once more fall down in the containers.

However, there are some drawbacks of the discharging device described above.

A great drawback is related to the filter means of the device, which have been found to be extremely dis- posed to clogging and which have a fairly limited service life. This results in frequent operational trouble with the ensuing extensive maintenance work during which the device cannot be used for discharging bulk goods. The prior art discharging device thus has an unsatisfactory possible degree of utilisation. The filter means have also appeared to be fairly complicated and time consuming to change, which prolongs the shutdowns and thus further reduces the possible degree of utilisation of the dis- charging device.

One more drawback is related to the discharging ca- pacity of the device. The clogging of the filter means causes pressure drops in the entire system, which of

course reduces the discharging capacity of the device.

What also affects the discharging capacity of the prior art device is the changes between the different phases for emptying and filling the respective containers. Each such change takes time and thus lowers the discharging capacity of the device. It will thus be desirable to have a discharging device which has a higher discharging ca- pacity. This second type of mobile discharging device thus eliminates the problems which exist in the first- mentioned type. At the same time, however, other problems arise, which result in also this second type of mobile discharging device having a poor degree of utilisation and insufficient discharging capacity.

Summary of the Invention The term"degree of utilisation"used above and also below relates to the extent at which operation of the device is allowed in consideration of the necessary shutdowns.

The term"discharging capacity"used above and also below relates to the capacity at which discharging is allowed during operation of the device.

In view of that stated above, the object of the present invention is to provide an improved discharging device of the second type described above and a corre- sponding improved method for discharging of bulk goods.

The discharging device as well as the method are advan- tageously to be especially suited for discharging of pulverulent bulk goods.

A special object is to provide a discharging device which allows an improved degree of utilisation and which preferably also allows an improved discharging capacity.

One more object of the invention is to provide a method for discharging of bulk goods by means of a dis- charging device of said second type, said method allowing an improved degree of utilisation of the device and pre-

ferably also an improved discharging capacity of the device.

Further objects will appear from the following description and the appended claims.

According to the present invention, the above objects are achieved by a discharging device having the features defined in claim 1 and a method having the features defined in claim 8. Preferred embodiments of the device are evident from claims 2-7.

More specifically, according to the invention a mobile discharging device for pneumatic handling of bulk goods is provided, comprising a wheel-mounted base frame on which at least two containers are arranged, an air unit and a filter means which comprises at least one filter element made of a fibre material, said air unit allowing application of a sub-atmospheric pressure in each of the containers for supplying bulk goods to the respective containers through a supply conduit, the fil- ter means being adapted to remove particles carried by air which is evacuated from the respective containers in connection with application of a sub-atmospheric pressure in the same, and said air unit allowing application of a pressure above atmospheric in each of the containers for discharging bulk goods from the respective containers to a discharge conduit, said discharging device being char- acterised in that the filter means is arranged outside the containers, and that the filter means communicates with the discharge conduit through a clearing conduit and comprises a means for releasing particles collected in said at least one filter element, the filter means being arranged to collect the released particles in a bottom region thereof and, when necessary, to discharge said particles to the discharge conduit through said clearing conduit.

As a result, a mobile discharging device is pro- vided, which allows time-efficient discharging of bulk goods from, for instance, a vessel.

The design of the inventive discharging device fur- ther allows extension of the service life of the filter means.

On the one hand, the thermal load applied to the filter means is reduced. In connection with application of a pressure above atmospheric in a container, com- pressed air is in fact introduced into the same. Such compressed air is so hot that it is difficult for normal fibre materials to withstand the thermal load for a pe- riod of considerable length. Thanks to the filter means being arranged outside the containers and separated, for example with a valve means, from the containers, it will be possible to prevent the filter means and its filter element made of a fibre material from being exposed to the hot air.

Moreover, also the load caused by pressure varia- tions and applied to the filter means is reduced. The reason for this is once more that the filter means is arranged outside the containers, which implies that the filter means need not be subjected to pressure above at- mospheric but is only subjected to a sub-atmospheric pressure in connection with evacuation of air from the respective containers.

When supplying the bulk goods to the container, a so-called spraying effect of particles normally arises, which occurs when bulk goods at high speed enter the con- tainer and hit the bulk goods already present in the con- tainer. This spraying effect can, if it hits the filter means, cause mechanical wear on the filter elements of the filter means. This mechanical wear is avoided due to the filter means being arranged outside the containers.

There is also a risk that the above-mentioned spray- ing effect, if it hits the filter elements of the filter means, clogs the same more than normal. As a result, the pressure difference over the filter means increases and thus also the risk of the filter elements collapsing.

Thanks to the filter means being arranged outside the

containers, the risk of clogging of the filter elements of the filter means, caused by said spraying effect, is eliminated.

Thus the composition of the inventive device means that the service life of the filter means increases sig- nificantly and that the risk of clogging of the filter elements of the filter means is reduced. This reduces the occurrence of operational trouble as well as the neces- sary maintenance work.

The maintenance work is also simplified by the loca- tion of the filter means. The reason for this is that only one filter means for all the containers is required, which means that the time required for changing the fil- ter elements of the filter means can be reduced. More- over, it is not necessary to make any operation in the respective containers when changing the filter elements.

This is a considerable advantage since containers of this type are normally classified as pressure vessels. Thus, the containers must be inspected before being put into operation each time they have been opened. The filter means arranged outside the containers may in itself be classified as a pressure vessel, and thus also the filter means may need to be inspected each time it has been opened. Thanks to the extended service life of the filter means, however, the operations made in the filter means will be less frequent, which also applies to the neces- sary inspections.

The inventive discharging device thus has a filter means with a relatively long service life and a reduced tendency to clogging, also if the bulk goods are pulveru- lent. The maintenance work which is necessary for the filter means is reduced and will be easy to carry out. As a result, the occurrence of operational trouble is re- duced and the time required for the maintenance work is shortened. This minimises the shutdowns, which signifi- cantly improves the degree of utilisation of the device.

Furthermore, clogging of the filter elements of the filter means causes increased pressure drops in the en- tire system, which reduces the discharging capacity of the device. Thanks to the fact that the filter means is arranged outside the containers there is no such risk of clogging of the filter element caused by said spraying effect. Thus, pressure drops caused by clogging of filter elements are reduced, which of course has a positive effect on the discharging capacity of the discharging device.

Owing to the filter means being arranged outside the containers, it will also be possible to arrange the fil- ter means beside the containers. This means-without the permissible total height of the discharging device being exceeded-that the height of the containers can be in- creased, which results in an increase in the volume de- fined by each container. It goes without saying that this also has a positive effect on the discharging capacity of the device since the number of time-consuming changes between the different phases for filling and emptying is thus reduced. The reduced number of changes also means reduced wear on valves of the device.

According to a preferred embodiment of the device, the filter means is arranged in a vacuum conduit which is connectable to each container and through which the air unit merely acts to apply a sub-atmospheric pressure in the containers.

The inventive device further comprises preferably a pressure conduit which is connectable to each container and through which the air unit merely acts to apply a pressure above atmospheric in the containers.

The air unit advantageously comprises a vacuum pump for evacuation of air from the containers for applying a sub-atmospheric pressure in the same. Further the air unit advantageously comprises a compressor for introduc- tion of air into the containers for applying a pressure above atmospheric in the same.

The air unit is advantageously adapted to apply a sub-atmospheric pressure in one of the containers while simultaneously applying a pressure above atmospheric in the other one of the containers.

According to one more preferred embodiment, the in- ventive device comprises an operable arm which is mounted on the base frame on which the supply conduit is sup- ported, a nozzle mounted on an outer end of the supply conduit being movable by operation of the arm.

According to the present invention, also a method is provided for pneumatic discharging of bulk goods by means of a mobile discharging device, which comprises a wheel- mounted base frame on which at least two containers and an air unit are arranged, comprising the steps of apply- ing by means of the air unit a sub-atmospheric pressure in one of the containers in order to thus establish a suction effect in a supply conduit connected to the con- tainer, the air evacuated for application of a sub-atmos- pheric pressure in the container being cleaned with the aid of a filter means with at least one filter element made of a fibre material, and arranging a nozzle, mounted on an outer end of the supply conduit, for the bulk goods that are to be discharged, whereby said bulk goods are drawn into the supply conduit and transferred to the con- tainer, said method being characterised by the steps of conducting the air evacuated from the container through a vacuum conduit to the filter means, which is arranged outside the container, releasing, with the aid of a means of the filter means, particles collected in said at least one filter element, in such manner that said particles are collected in a bottom region of the filter means, and discharging, when necessary, the collected particles from said bottom region through a clearing conduit to a dis- charge conduit connected to said container.

The invention will now be described by way of ex- ample with reference to the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a perspective view of a mobile discharging device according to the present invention.

Fig. 2 is a partly sectional perspective view of a part of the device shown in Fig. 1.

Description of Embodiments Figs 1 and 2, to which reference is now made, illus- trate a preferred embodiment of the inventive mobile dis- charging device.

The discharging device is designed as a trailer 1 of a traction vehicle (not shown) and thus comprises a base frame 3 mounted on wheels 2. On the base frame 3 there are arranged an air unit 4 with a compressor (not shown) and a vacuum pump (not shown), a filter means 6, two con- tainers 7a, 7b and a movable arm 8 with a supply conduit 9 in the form of a suction conduit 10.

One end 11 of the suction conduit 10 supports a nozzle 12 and the other end 13 is, with the aid of valve means 14a, 14b for supplying bulk goods, connected to the respective containers 7a, 7b at a top region 15 thereof.

A valve means 16a, 16b for discharge of air is also arranged at the top region 15 of the respective con- tainers 7a, 7b, through which valve means 16a, 16b the containers 7a, 7b are connected to a vacuum conduit 17 which extends to the filter means 6. The filter means 6 is in turn connected to the vacuum pump of the air unit 4 via an evacuation conduit 18.

A valve means (not shown) for supplying air is arranged in the respective containers 7a, 7b, through which valve means the containers 7a, 7b are connected to a common pressure conduit 21 which in itself is connected to the compressor of the air unit 4.

Each container 7a, 7b communicates in its bottom region 20, via a valve means 22a, 22b for discharging bulk goods, with a discharge conduit 23. The discharge conduit 23 comprises a branch pipe 24a, 24b connected to

each container 7a, 7b, said branch pipes being in turn connected to a common conveying conduit (not shown).

Pneumatic conveying means 26a, 26b operated by the com- pressor are connected to the branch pipes 24a, 24b.

Finally a clearing conduit 27 extends between a bottom region 28 of the filter means 6 and one of the branch pipes 24a. A valve means 29 is arranged in this clearing conduit 27.

The mobile discharging device is mainly intended for discharging bulk goods, such as cement powder, from vessels. The discharging device is moved by means of a traction vehicle along a quay and arranged in a position in connection with the vessel involved. The nozzle 12 of the suction conduit 10 supported by the arm 8 is then arranged, by operating the movable arm 8, in one of the holds of the vessel for accessing the bulk goods located therein.

The discharge of the bulk goods occurs by alter- natingly filling and emptying the containers 7a, 7b, i. e. one container 7a; 7b is being filled as the other con- tainer 7b; 7a is being emptied and vice versa.

For filling e. g. the container 7a, the associated valve means 14a and 16a for supplying bulk goods and dis- charging air are opened, while the valve means 22a for discharging bulk goods and the valve means for supplying air are closed. Subsequently the vacuum pump for evacuat- ing air from the container 7a via the vacuum conduit 17 is started. The air is drawn to the filter means 6 and from there to the vacuum pump. As a result, a sub-atmos- pheric pressure is generated in the container 7a, which in turn generates a suction effect in the suction conduit 10. Bulk goods will thus be drawn in through the nozzle 12 of the suction conduit 10 and carried by the air flow generated, to be conveyed to the container 7a. When the air flow enters the container 7a, its speed, as well as its carrying capacity, is considerably reduced, whereby the bulk goods fall down in the container 7a. The air

evacuated from the container 7a will entrain a small amount of particles, which are separated from the air in filter elements 30 of the filter means 6. From there the thus cleaned air is conducted to the vacuum pump through the evacuating conduit 18. The filter elements 30 are more precisely made of a fibre material, and when the evacuated air passes through the filter elements 30, the particles carried by the air will be deposited in said fibre material. The advantage of filter means 6 of this type is that a large air flow can be cleaned while the dimensions of the filter means will be relatively small.

When emptying, for example, the container 7a, the previously open valve means 14a and 16a are closed while the previously closed valve means are opened. Air is in- troduced by means of the compressor into the container 7a via the pressure conduit 21 and the pressure above atmos- pheric generated in the container 7a forces the bulk goods to the branch pipe 24a. The air flow generated by the conveying means 26a arranged adjacent to the branch pipe 24a entrains the bulk goods and conducts it to the conveying conduit (not shown) for further transport, for instance to a silo.

The filter means 6 is continuously cleaned with the aid of a means for releasing particles collected in the filter elements 30 of the filter means 6. Said means may comprise, for example, a compressor (not shown) which regularly generates an air blast directed towards the filter element 30 and causing the particles collected in the filter element 30 to be released and fall down in the bottom region 28 of the filter means 6. The filter means 6 may comprise a level indicator which indicates when it is time to empty the filter means 6. Its valve means 29 opens, whereby the released particles are discharged through the clearing conduit 27 to the branch pipe 24a connected thereto, for further transport to the conveying conduit.

The filter means 6 is arranged outside and at a dis- tance from the containers 7. This prevents clogging of and mechanical wear on the filter elements 30 of the fil- ter means 6 as a consequence of the spraying effect which arises when the bulk goods are supplied to the containers 7a, 7b and hits the bulk goods already positioned in the containers 7a, 7b.

The compressed air pumped into each container 7a; 7b for applying a pressure above atmospheric in the same is normally fairly hot. More specifically, the air is so hot that when contacting the filter element 30 it may have a detrimental effect on the fibre material thereof. How- ever, the filter means 6 is arranged outside and in com- munication with the containers 7a, 7b through a vacuum conduit 17 which is closeable with the aid of valve means 16a, 16b, which makes it possible to prevent such contact between the filter element 30 of the filter means 6 and the hot air. Thus the thermal load on the filter means 6 is reduced.

The fact that the filter means 6 is arranged outside the containers 7a, 7b implies that it will not be sub- jected to said pressure above atmospheric either, which for obvious reasons reduces the load on the same, caused by pressure variations over the filter means 6.

Summing up, a mobile discharging device is provided, which has both a high degree of utilisation and a high discharging capacity. The filter means arranged outside the containers allows cleaning of large air flows, which in turn enables a high discharging capacity. Besides the filter means can be efficiently cleaned, and the par- ticles released from the filter elements can be recircu- lated to the discharged bulk goods. Moreover the mechan- ical and thermal load, as well as the load caused by pressure variations, on the filter means 6 and its filter element 30 have been reduced compared with conventional mobile discharging devices of the second type described by way of introduction. As a result, the service life of

the filter means 6 is extended, which in turn implies that the occurrence of operational trouble is reduced and that the maintenance work required for maintenance of the filter means 6 is minimised. Less frequent cleaning and change of the filter element 30 is required. Moreover, the filter means 6 is arranged in such manner that the risk of clogging of the filter element 30 of the filter means 6 owing to the same collapsing, has been signifi- cantly reduced. Finally, in case of cleaning or change of filter element, an operation is required in one position only, which of course facilitates the operation. This is, of course, due to the fact that the filter means 6 acts to clean air which is evacuated from both containers 7a, 7b. All in all, this allows a relatively seen high degree of utilisation of the inventive discharging device since stillstands caused by operational trouble or maintenance work are considerably reduced.

The filter means 6 is, as mentioned above, arranged in such manner that the risk of clogging of its filter element 30 owing to said spraying effect has been elimi- nated. As a result, the pressure drops caused by clogging are reduced, which, of course, implies that the capacity of the device increases further.

The total height of the discharging device is nor- mally restricted by various rules and regulations. Thanks to the arrangement of the filter means 6 outside and be- side the containers 7a, 7b, the containers can, with their total height maintained, be given a relatively seen larger volume compared with the known case where a filter means is arranged in the top region of each container.

The relatively larger volume of the containers 7a, 7b re- sults in a reduction of the number of changes, required during the discharging work, between filling and emptying of the containers 7a, 7b. Each change takes time, and ow- ing to the reduction of the number of changes the dis- charging capacity of the discharging device increases. A reduced number of necessary changes also implies that the

valve means of the device are subjected to less wear, which of course results in an extended service life of said valve means.

It will be appreciated that the inventive discharg- ing device is not limited to the shown embodiment and that several modifications and variations are feasible.

The invention is thus defined exclusively by the appended claims.