WO2005000472A1

WO2005000472A1 - Device for minimising dust emission from shredder

Info

Publication number: WO2005000472A1
Application number: PCT/GB2004/002689
Authority: WO
Inventors: Roger Morgan
Original assignee: Manara Business Machines Limited
Priority date: 2003-06-26
Filing date: 2004-06-23
Publication date: 2005-01-06
Also published as: GB0314872D0

Abstract

A method and apparatus for minimising dust emission from a shredder in which the apparatus includes an air extraction device to extract air from a shredder in an extracted airstream and a filter to filter dust from the extracted airstream wherein the filter is configured to filter the dust from the extracted airstream by accumulating the dust into a solid powder.

Description

DEVICE FOR MINIMISING DUST EMMI SSION FROM SHREDDER

Introduction

This invention relates to a device for minimising dust emission, in particular to a device for minimising dust emission from a paper shredder.

It is known to use shredders to shred paper documents to prevent confidential and/or secure information becoming available to undesired persons. However, the process of shredding paper documents generally results in considerable amounts of paper dust being generated. Such paper dust is a potential health hazard to humans. In particular, in the case where paper dust passes into the human respiratory system, significant medical complaints may result.

There is therefore a need for a device for minimising dust emission from a shredder.

Statements of Invention

According to the invention there is provided a device for minimising dust emission fro a shredder, the device comprising: - an air extraction device to extract air from a shredder in an extracted airstream; and a filter to filter dust from the extracted airstream; the filter being configured to filter the dust from the extracted airstream by accumulating the dust into a solid powder. By accumulating the dust into a solid powder the device for minimising dust emission according to the invention enables dust, in particular paper dust, to be filtered from the airstream. Thus, the potential health hazard represented by paper dust is significantly reduced.

Because the device according to the invention operates by accumulating dust into a solid powder, the device does not suffer from blockage problems during a filtration operation. Thus there is no increase in pressure drop or reduction in airflow between cleanings, h particular it has been found that the device enables effective filtration to be achieved with a low pressure drop, and also sufficient airflow may be maintained without a powerful fan being required.

The filtration process using the device according to the invention is effective even with very fine particles, such as micro-dust which is a particular problem with paper shredding machinery. The electrostatic filter is particularly effective even with a low pressure drop at filtering very fine dust, such as the micro-dust generated during paper shredding.

By accumulating the dust into a solid powder, the device according to the invention enables the accumulated dust to be more easily handled. For example, the accumulated dust may be subsequently washed off as a slurry or shaken off as a powder. Furthermore the device according to the invention may be reusable.

In one embodiment of the invention the filter is configured to apply an electrostatic field to the extracted airstream to accumulate the dust. Preferably the filter comprises a repelling element and a collecting element, the collecting element being oppositely polarised to the repelling element to apply the electrostatic field. Ideally the element is polarised by applying an electrical charge to the element. Most preferably the element comprises an electrode. The collecting element may be configured for the accumulation of the dust as a solid powder on a surface of the collecting element. Preferably the device comprises means to remove the solid powder from the collecting element. The removal means may comprise a shaker to shake the collecting element to remove the solid powder. The removal means may comprise a hammer to strike the collecting element to remove the solid powder.

In one case the collecting element comprises one or more plates. The collecting element may comprise a plurality of plates arranged in substantially parallel planes. The collecting element may comprise a plurality of plates arranged substantially in a mesh arrangement.

In another embodiment the repelling element comprises one or more tubular elements. Preferably the repelling element comprises a plurality of tubular elements arranged substantially parallel to one another.

In a preferred embodiment of the invention the device comprises means to determine a fault in operation of the filter. Preferably the device comprises means to suspend operation of a shredder in response to a fault in operation of the filter. Ideally the means to suspend shredder operation comprises a contactor to disconnect a shredder from an electrical supply.

The filter may be located upstream of the air extraction device. Preferably the air extraction device is mounted to the filter.

The filter may be located downstream of the air extraction device. Preferably the filter is mounted to the air extraction device.

In one case the air extraction device comprises a fan. In another case the air extraction device comprises a blower. In another preferred embodiment the invention provides a device for shredding material, the device comprising: - a shredder; and a device for minimising dust emission of the invention.

Preferably the shredder has an air outlet through which the extracted airstream passes, and one or more one air inlets, the air extraction device being configured for inward air flow into the shredder through all air inlets.

Because the air flow through all air inlets is inwardly into the shredder, this ensures that the only air exiting the shredder passes through the filter, and therefore all air exiting the shredder is filtered of paper dust.

The air extraction device may be located adjacent the air outlet. Ideally the air extraction device is located externally of the shredder. The air extraction device may be mounted to the shredder.

In one case wherein the filter is located adjacent the air outlet. Preferably the filter is located externally of the shredder. Ideally the filter is mounted to the shredder.

hi one embodiment the device comprises means to detect combustion of material within the shredder. Preferably the combustion detection means comprises means to monitor the temperature of material within the shredder. Ideally the device comprises means to deliver a fire-extinguishing substance into the shredder. Most preferably the delivery means comprises a spraying means to spray a fire- extinguishing substance into the shredder.

The shredder may be a cam-type shredder. Preferably the invention provides a device for shredding paper. hi another aspect the invention provides a device for minimising dust emission from a shredder, the device comprising:- an air extraction device to extract air from a shredder in an extracted airstream; a filter to filter dust from the extracted airstream; and means to suspend operation of the shredder in response to a fault in operation of the filter.

In one embodiment of the invention the means to suspend shredder operation comprises a contactor to disconnect a shredder from an electrical supply. Preferably the device comprises means to determine a fault in operation of the filter.

According to a further aspect the invention provides a device for shredding material, the device comprising:- means to shred material fed into the device: and means to detect combustion of material within the shredder.

In one case the combustion detection means comprises means to monitor the temperature of material within the device. Preferably the device comprises means to deliver a fire-extinguishing substance into the device. Ideally the delivery means comprises a spraying means to spray a fire-exting ώhing substance into the device.

hi a further aspect of the invention there is provided a method for mirώnising dust emission from a shredder, the method comprising the steps of:- extracting air from a shredder in an extracted airstream; and filtering dust from the extracted airstream by accumulating the dust into a solid powder.

In one embodiment of the invention an electrostatic field is applied to the extracted airstream to accumulate the dust.

A repelling element and/or a collecting element may be polarised to apply the electrostatic field.

Ideally the dust is accumulated as a solid powder on a surface of a filter. The method may comprise the step of removing the solid powder from the surface on which the dust has accumulated. Preferably the solid powder is removed by shaking the surface. The solid powder may be removed by striking the surface.

In another case the method comprises the step of determining a fault in the filtering operation. Preferably the method comprises the step of suspending a shredding operation in response to a fault in the filtering operation.

In a preferred embodiment the invention provides a method of shredding material, the method comprising the steps of:- feeding material to be shred into a shredder; shredding the material in the shredder; and minimising the dust emission from the shredder. Preferably the method comprises the step of detecting combustion of material within the shredder. Ideally the method comprises the step of delivering a fire-extinguishing substance into the shredder. Most preferably the method comprises the step of spraying a fire-extinguishing substance into the shredder.

The invention provides in one case a method of shredding paper.

According to a further aspect of the invention there is provided a method for minimising dust emission from a shredder, the method comprising the steps of:- extracting air from a shredder in an extracted airstream; filtering dust from the extracted airstream; and suspending a shredding operation in response to a fault in the filtering operation.

hi one case the method comprises the step of determining a fault in the filtering operation.

In a further aspect of the invention there is provided a method of shredding material, the method comprising the steps of:- feeding material to be shred into a shredder; shredding the material in the shredder; and detecting combustion of material within the shredder. hi one embodiment of the invention the method comprises the step of delivering a fire-extinguishing substance into the shredder. Ideally the method comprises the step of spraying a fire-extinguishing substance into the shredder. hi another embodiment of the invention, a fan is mounted on one of the existing ventilation apertures of the shredder cabinet, or a new aperture can be cut into the cabinet if necessary. The fan draws air through the shredder cabinet in such a way as to ensure that all air flow through all remaining ventilation apertures are inward, and that any air flow through other apertures, gaps, etc are also inward. This ensures that no dust can escape outwards from the shredder except through the aperture on which the fan is mounted. The control of air flow in a shredder is an important feature of this invention.

On, or adjacent to the fan, an electrostatic filter or other type of dust filter may be mounted, in such a way as to ensure that all air flow through the fan also flows through the filter. Normally the filter will be sited so as to intercept the air before it flows through the fan, so as to remove the dust before the air flow enters the fan, in order to keep the fan clean, but the filter might alternatively be sited so it intercepts the air after it has flowed through the fan, if that is more convenient. The function of the filter is to remove all or almost all dust from the air flow, and thus to prevent the emission of dust from the shredder. The inclusion of a filter into a shredder to remove dust is an advantageous feature of this invention.

The electrostatic filter may be equipped with a switch which switches off the high voltage supply to the filter if the casing around the filter is removed. The filter may be equipped with an alarm which detects discharge in the filter due to dust overload, and to switch off the electrical power to the filter. When it is activated, the alarm function is arranged in this invention to switch off the electrical power to the shredder, thus preventing the shredder from being used with a malfunctioning filter. The inclusion of a safety feature to prevent operation of a shredder with a malfunctioning filter is another important feature of this invention. Paper dust is potentially flammable. An automatic fire extinguisher may be included, which detects a rise in temperature above a set level, and activates a spray of fire- extinguishing material into the shredder. It is recommended that a 'dry powder' type of extinguisher is used, because such material is effective in the presence of ventilation and is both non-toxic and non-asphyxiant. The inclusion of a fire extinguisher into a shredder is a further advantageous feature of this invention.

Brief Description of the Drawings

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which: -

Fig. 1 is a schematic illustration of a shredder and a device for minimising dust emission from the shredder according to the invention;

Fig. 2 is a front, perspective view of the shredder and device of Fig. 1; Fig. 3 is an enlarged, front, perspective view of the device of Fig. 1; Fig. 4 is another front, perspective view of the shredder and device of Fig. 1; Fig. 5 is a rear, perspective view of the shredder and device of Fig. 1;

Fig. 6 is an exploded, perspective view of a fan of the device of Fig. 1; Fig. 7 is aperspective view of a filter of the device of Fig. 1; Fig. 8 is a schematic illustration of the filter of Fig. 8; Fig. 9(a) is a schematic illustration of apart of the filter of Fig. 8;

Fig. 9(b) is a schematic illustration of a part of a filter of another device for minimising dust emission from a shredder according to the invention; and

Fig. 10 is a schematic illustration of a shredder and a further device for minimising dust emission from the shredder according to the invention.

Detailed Description

Referring to Figs. 1 to 9(a), there is illustrated a shredder, in this case a paper shredder 1, and a device 2 for minimising dust emission from the shredder 1 according to the invention. The shredder 1 has a plurality of air inlets 3 through which air may enter the shredder 1 in entry airstreams B and an air outlet 4 through which air may exit the shredder 1 in an extracted airstream A. Together the air inlets 3 and the air outlet 4 provide a means of ventilating the shredder 1.

The dust minimisation device 2 comprises an air extraction device, provided in this case by a fan 5, to extract air from the shredder 1 in the extracted airstream A, and a filter 6 to filter dust from the extracted airstream A. As illustrated in Fig. 1, both the fan 5 and the filter 6 are located externally of the shredder 1 adjacent to the shredder air outlet 4. The filter 6 is located upstream of the fan 5 so that dust is filtered from the extracted airstream A after the extracted airstream A exits the shredder 1 and before the extracted airstream A enters the fan 5. In this case, the filter 6 is mounted to the shredder 1, and the fan 5 is mounted to the filter 6, as illustrated in Figs. 2 to 5.

The paper shredder 1 is a cam-type shredder with shredding shafts of high-quality, hardened steel. Thus, the shredder 1 is capable of shredding large quantities of paper, even if paper clips are present in the paper. Means may be provided to automatically lubricate the shredding shafts using oil.

The shredder 1 senses when paper to be shredded is being fed into the shredder 1 using photo cells. In this manner the shredding operation is automatically started when a batch of paper is fed into the shredder 1, and automatically stopped when all of the batch of paper has been shredded.

The shredder 1 has means of sensing if there is a paper jam, or if a shred bag is full, or if a door of the shredder is open. If any of these occurrences are sensed, the shredder 1 automatically suspends the shredding operation.

The shredder 1 may also have means to pack the shredded paper into bails.

As illustrated in Fig. 6, the fan 5 comprises an inner grille AA attached to a fan assembly C by means of screws N. A back draught shutter BB is enclosed between the inner grille AA and the fan assembly C. The fan assembly C includes a rating plate H, a selector switch J and a customer connector K.

The fan assembly C is secured within a fan mounting box D by means of screws P, and the fan mounting box D is secured within a wall tube G by means of retainers E and ladder strips F.

The filter 6 comprises a first filtration layer 10 which includes an aluminium filter, a second filtration layer 11 which includes an electrostatic filter, and a third filtration layer 12 which includes a charcoal filter, as illustrated in Fig. 8. The first filtration layer 10 is relatively coarse to filter relatively large particles from the extracted airstream A, and the third filtration layer 12 is suitable for removing odours from the extracted airstream A. The second filtration layer 11 applies an electrostatic field to the extracted airstream A to accumulate the dust in the extracted airstream A into a solid powder. By accumulating the dust into a solid powder, the second filtration layer 11 filters the dust from the extracted airstream A. In this manner, the filter 6 is suitable for filtering micro-particles from the extracted airstream A.

The filter 6 comprises a repelling electrode and a collecting electrode to apply the electrostatic field across the extracted airstream A. In this case, the collecting electrode is provided in the form of a plurality of plates 7 arranged in substantially parallel planes, and the repelling electrode is provided in the form of a corresponding plurality of tubular elements 8 arranged substantially parallel to one another, as illustrated in Fig. 9(a). A rod or a wire may be used for the tubular element 8 in this parallel plate arrangement. The plates 7 are oppositely polarised to the tubular elements 8 by applying electrical charges to the plates 7 and the tubular elements 8. In this maimer the electrostatic field is applied across the extracted airstream A, and the dust filtered from the extracted airstream A accumulates as a solid powder on the surface of the plates 7.

Because the second filtration layer 11 employs an electrostatic field, no pressure drop will occur across the second filtration layer 11. In this manner, the filter 6 is particularly suitable for filtering dust from the extracted airstream A.

A typical volumetric flow rate through the filter 6 is approximately 1000 m³/hour.

In use, the fan 5 is mounted to the filter 6, and the filter 6 is mounted to the shredder 1. With the fan 5 and the filter 6 in operation, paper to be shred is fed into the shredder 1. As the paper is shredded within the shredder 1, dust is generated as a result of the shredding process. The fan 5 extracts air in the extracted airstream A from the shredder 1 through the filter 6. The fan 5 also causes air to flow inwardly in the entry airstream B into the shredder 1 through all air inlets 3, as illustrated in Fig. 1. Electrical charges are applied to the plates 7 and the tubular elements 8 to oppositely polarise the plates 7 and the tubular elements 8, and thereby apply an electrostatic field across the extracted airstream A. The electric field is at a maximum close to each tubular element 8. This is because lines of electrical force are concentrated by the small diameter of each tubular element 8. In this way the particles of dust are ionised close to each tubular element 8. The ionised dust is then attracted to the plates 7 of opposite polarity, hi this manner the electrostatic field causes the dust in the extracted airstream A to accumulate as a solid powder on the surface of the plates 7. Thus the paper dust generated during the shredding process is effectively filtered from the extracted airstream A.

The solid powder may be subsequently removed from the surface of the plates 7, for example by shaking the plates 7 at 50Hz using an AC solenoid, or by striking the plates 7 with an electrically powered hammer or other suitable implement. Alternatively, the plates 7 may be thoroughly washed and dried, in which case the dust comes away as a slurry.

Because air flows inwardly in the entry airstreams B into the shredder 1 through all air inlets 3, this ensures that the only air exiting the shredder 1 is the extracted airstream A which passes through the filter 6. Thus all air exiting the shredder 1 is filtered to filter paper dust from the air.

It will be appreciated that, as an alternative, the collecting electrode may be provided in the form of a plurality of plates 9 arranged substantially in a mesh arrangement, as illustrated in Fig. 9(b). This mesh plate arrangement may be suitable for filtering larger quantities of dust than the simpler parallel plate arrangement described previously with reference to Fig. 9(a). Referring to Fig. 10, there is illustrated another device 22 for rm^'nimising dust emission from the shredder 1 according to the invention, which is similar to the device 2 described previously with reference to Figs 1 to 9(a), and similar elements in Fig. 10 are assigned the same reference numerals.

The dust minimisation device 22 comprises means to determine a fault in operation of the filter 6, and a contactor 13 to suspend operation of the shredder 1 in response to a fault in operation of the filter 6. A typical filter fault may be a blockage which leads to electrical arcing. i this case, the contactor 13 is located between the shredder 1 and an electrical supply 14 to facilitate disconnecting of the shredder 1 from the electrical supply 14 in response to a fault in operation of the filter 6. A feedback line 15 runs from the filter 6 to the contactor 13 to inform the contactor 13 of a fault in operation of the filter 6. The feedback line 15 may be provided in the form of a single phase mains supply from an "on" lamp of the filter 6 to the contactor 13.

In this case, the contactor 13 is an electrically controlled three-phase contactor, introduced into the electrical supply to the shredder 1, so that when an ordinary mains single-phase 230 volt AC electrical supply is connected to the contactor coil, the contactor 13 pulls in, making contact between the mains supply and the shredder 1. Removal of the electrical supply to the contactor 13, for any reason, will cause the contactor 13 to fall out and disconnect the shredder 1 from the mains.

A feed is taken at ordinary single-phase mains voltage from the filter "on" lamp to the contactor coil. As a result, when the filter "on" lamp is lit, the contactor 13 pulls in and allows the shredder 1 to operate. Any failure of the filter 6 will cause the supply to the filter "on" lamp to be cut off, causing the contactor 13 to fall out and disconnect the shredder 1 from the mains. By suspending operation of the shredder 1 in the event of a fault in operation of the filter 6, the device 22 according to the invention prevents emission of paper dust from the shredder 1. In particular the device 22 prevents the dust emission without modifications to the shredder being required. hi another device for minimising dust emission from the shredder 1 according to the invention, means may be provided to detect combustion of paper within the shredder 1, for example by monitoring the temperature of the paper within the shredder 1. In the event that combustion of the paper within the shredder 1 is detected, a fire- extinguishing substance, such as a dry-powder, may be delivered into the shredder 1 to extinguish the fire, for example by spraying the fire-extinguishing substance, into the shredder 1.

It will be appreciated that the filter 6 may alternatively be located downstream of the fan 5. The filter 4 may be mounted to the fan 5.

Further it will be appreciated that the air extraction device may be provided by any suitable device, such as a blower. The fan 5 described previously with reference to Fig. 6 is presented as a suitable air extraction device by way of example only.

An advantage of the dust-free shredder described herein is that the air flow through the cabinet of the shredder is controlled so that all egress of air goes through a filter, and all other air flows are ingress.

The volume of air-flow into the shredder needed to cool energy-dissipating items such as the main drive motor, or the cutters may be estimated from the measured or tabulated electric power consumption of the shredder, the density of air, specific heat of air at constant pressure, and the allowable temperature rise, according to: v = - P TpC_F where: V = volume flow at point of measurement (cubic metres) P = average power consumption (watts) T = temperature rise (degrees Celsius) p = density of air at point of measurement (kilograms per cubic metre) Cp = specific heat of air at constant pressure over the expected temperature range (joules per kg per degree C).

This equation assumes that there are no losses of heat from the shredder to the environment other than those in the air flow. This is a 'safe' assumption in the sense that there will indeed be losses additional to those calculated, and they will lower the temperature rise at a given air flow, or alternatively allow a lower flow rate than calculated at a given temperature rise, thus making the system able to take flow restrictions without danger.

The point of measurement for the calculation of required volume flow will normally be the output of the extract fan, and the temperature used in the calculation will thus be the elevated temperature, that is, the room air temperature plus the temperature rise, rather than the intake air temperature from the room.

Example

A three-phase shredder consumes 4kW (total, over all three phases) when working under load. It consumes IkW when running light. If the machine has a duty cycle of 50% under normal conditions, the volume of air flow needed to ensure a temperature rise in the air flow which cannot exceed 10 degrees C is calculated as follows: P = Average power consumption = (4 x 0.5) + (1 x 0.5) = 2.5 kW taking T = temperature rise = 10 degrees C and given p = density of air at point of measurement = 1.25 kg per cubic metre Cp = specific heat of air at constant pressure = 1.005 kilojoules per kg per degree C

P V = - TpC_P

= 2.5 / (10 x 1.25 1.005) = 0.2 cubic metres per second = 0.2 x 3600 = 720 cubic metres per hour

In the worst-case situation where 100% duty cycle is reached, the maximum possible temperature rise at the same air flow is calculated as follows:

At 100% duty cycle, power consumption rises from 2.5 kW to 4 kW so temperature rise increases to

10 x 4 / 2.5 = 16 degrees C

The invention is not limited to the embodiments hereinbefore described, with reference to the accompanying drawings, which may be varied in construction and detail.

Claims

1. A device for minirnising dust emission from a shredder, the device comprising:- an air extraction device to extract air from a shredder in an extracted airstream; and a filter to filter dust from the extracted airstream; the filter being configured to filter the dust from the extracted airstream by accumulating the dust into a solid powder.

2. A device as claimed in claim 1 wherein the filter is configured to apply an electrostatic field to the extracted airstream to accumulate the dust.

3. A device as claimed in claim 2 wherein the filter comprises a repelling element and a collecting element, the collecting element being oppositely polarised to the repelling element to apply the electrostatic field.

4. A device as claimed in claim 3 wherein the element is polarised by applying an electrical charge to the element.

5. A device as claimed in claim 4 wherein the element comprises an electrode.

6. A device as claimed in any of claims 3 to 5 wherein the collecting element is configured for the accumulation of the dust as a solid powder on a surface of the collecting element.

7. A device as claimed in claim 6 wherein the device comprises means to remove the solid powder from the collecting element.

8. A device as claimed in claim 7 wherein the removal means comprises a shaker to shake the collecting element to remove the solid powder.

9. A device as claimed in claim 7 or 8 wherein the removal means comprises a hammer to strike the collecting element to remove the solid powder.

10. A device as claimed in any of claims 3 to 9 wherein the collecting element comprises one or more plates.

11. A device as claimed in claim 10 wherein the collecting element comprises a plurality of plates arranged in substantially parallel planes.

12. A device as claimed in claim 10 or 11 wherein the collecting element comprises a plurality of plates arranged substantially in a mesh arrangement.

13. A device as claimed in any of claims 3 to 12 wherein the repelling element comprises one or more tubular elements.

14. A device as claimed in claim 13 wherein the repelling element comprises a plurality of tubular elements arranged substantially parallel to one another.

15. A device as claimed in any of claims 1 to 14 wherein the device comprises means to determine a fault in operation of the filter.

16. A device as claimed in any of claims 1 to 15 wherein the device comprises means to suspend operation of a shredder in response to a fault in operation of the filter.

17. A device as claimed in claim 16 wherein the means to suspend shredder operation comprises a contactor to disconnect a shredder from an electrical supply.

18. A device as claimed in any of claims 1 to 17 wherein the filter is located upstream of the air extraction device.

19. A device as claimed in claim 18 wherein the air extraction device is mounted to the filter.

20. A device as claimed in any of claims 1 to 17 wherein the filter is located downstream of the air extraction device.

21. A device as claimed in claim 20 wherein the filter is mounted to the air extraction device.

22. A device as claimed in any of claims 1 to 21 wherein the air extraction device comprises a fan.

23. A device as claimed in any of claims 1 to 21 wherein the air extraction device comprises a blower.

24. A device for minimising dust emission from -a shredder substantially as hereinbefore described with reference to the accompanying drawings.

25. A device for shredding material, the device comprising:- a shredder; and a device for minimising dust emission as claimed in any of claims 1 to 23.

26. A device as claimed in claim 25 wherein the shredder has an air outlet through which the extracted airstream passes, and one or more one air inlets, the air extraction device being configured for inward air flow into the shredder through all air inlets.

27. A device as claimed in claim 25 or 26 wherein the air extraction device is located adjacent the air outlet.

28. A device as claimed in any of claims 25 to 27 wherein the air extraction device is located externally of the shredder.

29. A device as claimed any of claims 25 to 28 wherein the air extraction device is mounted to the shredder.

,

30. A device as claimed in any of claims 25 to 29 wherein the filter is located adjacent the air outlet.

31. A device as claimed in any of claims 25 to 30 wherein the filter is located externally of the shredder.

32. A device as claimed in any of claims 25 to 31 wherein the filter is mounted to the shredder.

33. A device as claimed in any of claims 25 to 32 wherein the device comprises means to detect combustion of material within the shredder.

34. A device as claimed in claim 33 wherein the combustion detection means comprises means to monitor the temperature of material within the shredder.

35. A device as claimed in claim 33 or 34 wherein the device comprises means to deliver a fire-extinguishing substance into the shredder.

36. A device as claimed in claim 35 wherein the delivery means comprises a spraying means to spray a fire-extinguishing substance into the shredder.

37. A device as claimed in any of claims 25 to 36 wherein the shredder is a cam- type shredder.

38. A device for shredding paper as claimed in any of claims 25 to 37.

39. A device for shredding material substantially as hereinbefore described with reference to the accompanying drawings.

40. A device for minimising dust emission from a shredder, the device comprising: - an air extraction device to extract air from a shredder in an extracted airstream; a filter to filter dust from the extracted airstream; and means to suspend operation of the shredder in response to a fault in operation of the filter.

41. A device as claimed in claim 40 wherein the means to suspend shredder operation comprises a contactor to disconnect a shredder from an electrical supply.

42. A device as claimed in claim 40 or 41 wherein the device comprises means to determine a fault in operation of the filter.

43. A device for shredding material, the device comprising:- means to shred material fed into the device: and means to detect combustion of material within the shredder.

44. A device as claimed in claim 43 wherein the combustion detection means comprises means to monitor the temperature of material within the device.

45. A device as claimed in claim 43 or 44 wherein the device comprises means to deliver a fire-extinguishing substance into the device.

46. A device as claimed in claim 45 wherein the delivery means comprises a spraying means to spray a fire-extinguishing substance into the device.

47. A method for minimising dust emission from a shredder, the method comprising the steps of:- extracting air from a shredder in an extracted airstream; and filtering dust from the extracted airstream by accumulating the dust into a solid powder.

48. A method as claimed in claim 47 wherein an electrostatic field is applied to the extracted airstream to accumulate the dust.

49. A method as claimed in claim 48 wherein a repelling element and/or a collecting element is polarised to apply the electrostatic field.

50. A method as claimed in any of claims 47 to 49 wherein the dust is accumulated as a solid powder on a surface of a filter.

51. A method as claimed in claim 50 wherein the method comprises the step of removing the solid powder from the surface on which the dust has accumulated.

52. A method as claimed in claim 51 wherein the solid powder is removed by shaking the surface.

53. A method as claimed in claim 51 or 52 wherein the solid powder is removed by striking the surface.

54. A method as claimed in any of claims 47 to 53 wherein the method comprises the step of determining a fault in the filtering operation.

55. A method as claimed in any of claims 47 to 54 wherein the method comprises the step of suspending a shredding operation in response to a fault in the filtering operation.

56. A method for minimising dust emission from a shredder substantially as hereinbefore described with reference to the accompanying drawings.

57. A method of shredding material, the method comprising the steps of:- feeding material to be shred into a shredder; shredding the material in the shredder; and minimising the dust emission from the shredder as claimed in any of claims 47 to 56.

58. A method as claimed in claim 57 wherein the method comprises the step of detecting combustion of material within the shredder.

59. A method as claimed in claim 58 wherein the method comprises the step of delivering a fire-extinguishing substance into the shredder.

60. A method as claimed in claim 59 wherein the method comprises the step of spraying a fire-extinguishing substance into the shredder.

61. A method of shredding paper as claimed in any of claims 57 to 60.

62. A method of shredding material substantially as hereinbefore described with reference to the accompanying drawings.

63. A method for minimising dust emission from a shredder, the method comprising the steps of:- extracting air from a shredder in an extracted airstream; filtering dust from the extracted airstream; and suspending a shredding operation in response to a fault in the filtering operation.

64. A method as claimed in claim 63 wherein the method comprises the step of determining a fault in the filtering operation.

65. A method of shredding material, the method comprising the steps of:- feeding material to be shred into a shredder; shredding the material in the shredder; and detecting combustion of material within the shredder.

66. A method as claimed in claim 65 wherein the method comprises the step of delivering a fire-extinguishing substance into the shredder.

67. A method as claimed in claim 66 wherein the method comprises the step of spraying a fire-extinguishing substance into the shredder.