GB2177321A - Dust filter - Google Patents

Abstract

A filter for removing dust from a gas flow includes a number of fabric bags (30) on wire cages (36) and supported at their open ends (34) by an insulating plate (14). End portions (54) of the bags (30) remote from the plate (14) are restrained by engaging with complementarily- shaped end-cups (68) which prevent the bags (30) from wobbling when exposed to a gas flow. Adjacent bags (30) are raised to high voltages of opposite polarity so that dust deposition takes place in an electric field, and hence the dust cake is relatively porous. The end-cups (68) prevent the bags from shaking, which would cause sparking. End- cups (68) are spring-loaded and adjustable (Fig. 3) on support bars (48). <IMAGE>

Description

SPECIFICATION Dust filter This invention relates to dust filters utilizing fabric filters for removing dust or other particles from a gas stream.

Conventional dust filters comprise a layer of cloth or other fabric through which the gas stream is passed. They are very effective at dust collection,-but their effectiveness is due to a large extent to a cake or layer of dust which builds up on the surface of the fabric, and which itself acts as a filter. However this dust layer tends to become substantially impervious, which results in excessive pressure drop across the filter and consequently a large energy consumption in bringing about the gas flow.

It is also known that the application of an electric field in the vicinity of such a filter can lead to the production of a more permeable dust cake and hence a reduction in pressure drop. For example US Patent No 3 966 435 (Penney) describes a dust filter including a number of fabric filter members supported by metal grids, in which an electric field is applied between adjacent filter members to make the dust cake more porous. The filter members and grids are rectangular in shape, unlike a conventional dust filter in which cylindrical tubes or bags are more usual; and no indication is given of how to cope with problems such as sparking caused by movement of the filter elements brought about by the gas flow.

According to the present invention there is provided a dust filter comprising a duct into which an unfiltered gas flow is passed, a partition in the duct to separate the unfiltered gas from the filtered gas, and a plurality of bags suspended from the partition, means being provided to supply high electrical voltages to the bags so as to generate electric fields between each bag and its neighbours, each bag to which a high voltage is supplied being supported by electrically insulating means, and bag restraint means being provided remote from the partition to restrain the bags from moving relative to each other.

The fields may be generated by applying a high voltage of for example + 20 kV to alternate bags, the other bags being earthed; or preferably (as smaller absolute values of voltage are involved) by applying high voltages of for example + 15 kV to every bag, the polarity of the voltage applied to adjacent bags being of opposite sign. In a preferred embodiment of the invention, electrical insulation of the bags is provided by forming the partition of an electrical insulator.

The bag restraint means desirably is arranged so as not to trap dust, not to significantly alter the gas flow through the dust filter, and not to require any holes through the bag through which dust might pass. For example a restraint member might be of complementary shape to an end portion of the bag so as to mate therewith. In a desired embodiment the end portion of the bag is of tapered convex shape, and the bag restraint means includes an end cup of substantially complementary shape to receive the end portion of the bag. If the bags are cylindrical, the end portions and the end cups may be of conical shape. The bag restraint means desirably incorporates axially resilient support means for the end cups to accommodate bags of slightly different lengths.

The invention enables the advantages of electrostatic augmentation of dust filtration to be obtained in industrial dust filters while making only a small number of changes to known designs of dust filter, so that the invention can be retrofitted to existing plant, and so that in the event of a breakdown of the electrical supply means the dust filter will continue to operate although producing a less permeable dust cake.

The present invention also provides a bag restraint means for use in such a dust filter.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a sectional view in a vertical plane through a dust filter; Fig. 2 shows a sectional view on the line 1111 of Figure 1; and Fig. 3 shows a sectional view to an enlarged scale on the line Ill-Ill of Figure 1.

Referring to Figure 1, a dust filter 10 comprises a rectangular chamber 12 divided by a horizontal plate 14 of insulating material into a lower portion 16 and an upper portion 18. An inlet duct 20 is arranged to supply an unfiltered gas stream to the lower portion 16, and an outlet duct 22 is provided for filtered gas from the upper portion 18. The lower part of the lower portion 16 tapers, forming a hopper 24 in which dust collects in operation of the filter 10, and at its base is a removable door 26 by which the dust can be extracted.

The plate 14 supports sixteen cylindrical fabric bags 30 equally spaced in a square array (only four are shown in Figure 1). Each bag 30 passes through a respective circular hole in the plate 14 and includes two spacedapart snap rings 32 near its mouth 34 which seal against the top and bottom surfaces of the plate 14, so holding the bag 30 in position. In Figure 1, one of the bags 30 is shown partially broken away; rigidity of the bag 30 is provided by an open-mesh cylindrical wire cage 36 which extends the length of the bag 30 and at its lower end includes a circular end plate 38 (shown in Figure 3), and a metal venturi tube 40 fits in the mouth 34 of the bag 30. Each bag 30 is the same, each including a cage 36 and a venturi tube 40. An inlet baffle 42 is supported adjacent the inlet duct 20 to ensure a more uniform distribution of the gas stream throughout the chamber 12.

When a dusty gas stream is passed into the inlet duct 20 some sedimentation of the larger dust particles occurs on entering the lower portion 16, while the remainder of the dust is collected as a cake on the outside of the bags 30 and filtered gas passes into the upper portion 18 and so out of the outlet duct 22.

In the upper portion 18 are four pulse tubes 44 (only one is shown) connected to a source of high pressure gas (not shown) each with four nozzles 46. Each nozzle 46 points down the mouth 34 of one of the bags 30. If it is desired to remove the dust cake from the bags 30, a pulse of gas is emitted by each nozzle 46; as a result of the venturi tubes 40 a large gas flow takes place into each bag 30, accelerating and then decelerating the bag wall abruptly, and causing the dust cake to fall off and into the hopper 24.

Below the lower ends of the bags 30 four bars 48 (only one is shown) extend across the chamber 12, each bar 48 carrying four bag restraint units 50 (to be described in more detail later) arranged so that the lower end of each bag 30 is restrained by a respective restraint unit 50.

Referring also to Figure 2, each bag 30 is raised to a high negative or positive voltage by electrical connections 51, 52 to the venturi tubes 40 and consequently to the cages 36 with which they are in contact. An earth connection 53 is made to the wall of the chamber 12, while alternate bags 30 are raised to plus or minus 15 kV, so that nearest-neighbours are of opposite polarity.

Referring now to Figure 3, a lower end portion 54 of each bag 30 is of conical shape, and the space 56 between the end plate 38 of the cage 36 and the conical fabric end portion 54 is filled with polystyrene balls, so that the end portion 54 is substantially resistant to any deformation. Each bag restraint unit 50 includes an externally-threaded support tube 58 closed at its lower end 59 and clamped in position through a hole 60 in the bar 48 by nuts 62. The tube 58 contains a helical spring 64 which abuts its lower end 59, and the tube 58 acts as a guide for a cylindrical stem 66 of a conical end-cup 68 in which the lower end portion 54 locates. The stem 66 and the end-cup 68 are of an electrically insulating plastics material.The spring 64 ensures the end-cup 68 is resiliently urged into engagement with the end portion 54 of the tube 30, and allows slight variations in length between different tubes 30 to be accommodated.

In operation of the dust filter 10 it has been found that the bag restraint units 50 prevent the bags 30 wobbling, and so prevent sparking between adjacent bags 30. The bag restraint means 50 do not trap dust, and do not significantly affect the gas flow through the region between the bags 30 causes the dust to form a relatively permeable cake, so reducing the pressure drop across the dust filter 10.

It will be appreciated that if the bags 30 are all of identical length then the springs 64 can be dispensed with, the end-cups 68 being rigidly connected to the support tubes 58. Indeed, with such a modification of the bag restraint units 50, bags 30 of slightly different lengths can be accommodated by adjustment of the nuts 62, if access to the nuts 62 is available. It will be understood that, in the embodiment described, the conical end portions 54 may be filled by a packing material other than polystyrene balls. It will also be appreciated that the lower end portions of the bags 30 might be of a different shape to that described above as long as the end cups are of substantially complementary shape. In yet a further modification the lower end portion of each bag 30 might define a concave recess, and the restraint unit include a projection to engage the recess.

Claims (8)

1. A dust filter comprising a duct into which an unfiltered gas flow is passed, a partition in the duct to separate the unfiltered gas from the filtered gas, and a plurality of bags suspended from the partition, means being provided to supply high electrical voltages to the bags so as to generate electric fields between each bag and its neighbours, each bag to which a high voltage is supplied being supported by electrically insulating means, and bag restraint means being provided remote from the partition to restrain the bags from moving relative to each other.

2. A dust filter as claimed in Claim 1 wherein the partition is an electrical insulator.

3. A dust filter as claimed in Claim 1 or Claim 2 wherein the bag restraint means comprises a restraint member of complementary shape to an end portion of a bag so as to mate therewith.

4. A dust filter as claimed in Claim 3 wherein the end portion of the bag is of tapered convex shape, and the restraint member comprises an end cup of substantially complementary shape to receive the end portion of the bag.

5. A dust filter as claimed in Claim 3 or Claim 4 wherein the bag restraint means incorporates axially resilient support means for the restraint member, to urge the restraint member resiliently into contact with the end portion of the bag.

6. A bag restraint means intended for use in a dust filter as claimed in any one of the preceding claims.

7. A dust filter substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

8. A bag restraint means substantially as hereinbefore described with reference to, and as shown in, Figures 1 and 3 of the accompanying drawings.