An arrangement with a tube-shaped filter in combination with a cyclone as a coarse filter for a first filtration and the removal of the largest particles and then the use of a microfilter to remove the remaining particles is already known. From time to time, the collecting device needs to be stopped for emptying, changing the bag and cleaning the filter. While this is being carried out, the work using the floor sander must also be interrupted.

The object of the invention is therefore to achieve an improved method and a device of the kind mentioned in the introduction, that can be in continuous operation for a longer period of time.

This is possible with a method and a device as described in the following patent claims.

An example of an embodiment of the invention is described in the following with reference to the attached schematic drawings.

Figure 1 shows a vertical view in cross-section of a filter arrangement in a device for collecting particles according to the invention and Figure 2 shows a block diagram of the valve control in a further development of the device, according to which two identical filter arrangements, such as the one shown in Figure 1 , are operated simultaneously using a control unit. Identical components have the same reference numerals in the figures, in this case with the addition of the prime sign to distinguish components in the first filter - arrangement from corresponding components in the second filter arrangement.

Figure 1 shows a filter arrangement 2 with a casing 3, an inlet 4 and an outlet 5. The inlet is connected, either directly or, in the case of simultaneous operation, via a branch pipe 6 shown in Figure 2, to a particle-filled air flow that is generated by an attached machine. The outlet 5 is connected to the suction side of a fan 7 of conventional type that is only shown in Figure 2. The inlet 4 leads, via a first valve 8, to a conventional cyclone device 10 with a coarse filter 12 and a bottom part 14 for separating off most of the particles from the air flow in a known way. The bottom part has a hinged trap door 16 that forms a seal against the lower edge of the bottom part 14 and thereby can be held closed by atmospheric pressure when a vacuum is created in the cyclone 10 by the fan 7. The coarse filter unit 12 is similarly of conventional type and contains a suitable number of known filter cartridges or cloth filters 18.

The coarse filter unit 12 has an outlet side 20 which is connected via a second valve 22 to an inlet side of a microfilter unit 24 which, where applicable, can contain a plurality of parallel-connected microfilters 26 of conventional type and which leads into the outlet 5 at its outlet side.

In the casing 3 of the filter arrangement 2, there is a third valve 28 for admitting atmospheric air, mounted in such a way that it is directed towards the outlet side of the coarse filter unit 12.

The first 8, second 22 and third 28 valves are preferably of an identical construction, for example sliding valves of conventional type, and can be operated electrically, pneumatically or electro-pneumatically by means of a control unit CU and an operating unit MU. The fan 7 can also be operated via the control unit CU. The control unit is preferably a microprocessor (that is part of a so-called PLC system). The function of the filter arrangement 2 is described in greater detail in the following. Emptying and cleaning the filter arrangement is carried out by operating the valves by means of the control unit CU as follows:

The first valve 8 is closed to create a vacuum in the filter arrangement 2, whereupon the second valve 22 is closed to isolate the microfilter unit 24 from the coarse filter unit 12. The third valve 28, mounted in the casing 3 of the filter arrangement and pointing towards the outlet side of the coarse filter unit 12, is opened quickly in direct association with the closing of the second valve 22, whereupon atmospheric air rushes in towards the outlet side (the clean side) of the coarse filter unit 12 and dislodges particles that have collected on the opposite side of the coarse filter unit, which fall towards the bottom part 14 of the cyclone arrangement 10. As soon as the atmospheric pressure has created an equalization of the pressure in the cyclone arrangement 10, the trap door 16 is opened by gravity and releases the collected particles, which are collected in a plastic bag, a collection vessel or a tubular bag system in the conventional way. The third valve 28 is then closed again and the second valve 22 is opened, whereupon the procedure is repeated a predetermined number of times, preferably twice, after which the first valve 8 and the second valve 22 are opened for continued operation of the filter arrangement 2.

The time sequence and duration of each step in the procedure described above can be controlled by means of the control unit CU and emptying and cleaning of the filter arrangement 2 can be carried out in response to signals sent from the operating unit MU to the control unit CU.

In an advantageous way, two filter arrangements according to Figure 1 , a first filter arrangement 2 and a second identical filter arrangement 2' can be operated simultaneously by means of the control unit CU, in the way that is shown in Figure 2. This is possible as particle-filled air from the machine in - question is connected by means of the branch pipe 6 to each inlet on the respective filter arrangement 2, 2' and the outlet side of the microfilter unit 24, 24' in the respective filter arrangement is connected to the suction side of the fan 7. The control unit CU is then arranged to carry out emptying and cleaning of the first filter arrangement 2 while the second filter arrangement 2' is in operation and vice versa, in response to the signals from the operating unit MU.

The operating unit MU can be programmed with data relating to the duration of the times ti, t.2, t.3 ... , whereby it is possible to control the times for the alternating operation and emptying/cleaning of the first filter arrangement 2 and second filter arrangement 2'.