GB2240485A - Moving band filter - Google Patents

Abstract

A filter comprises two axially spaced, rotatable discs 6 with an endless filter band 12 entrained round the lower parts of their peripheries to define a chamber for liquid 30 to be filtered. Each disc carries two peripheral sealing rings 9a, 9b, and a second liquid 34 is fed into the space 10 between the rings and maintained at a level higher than liquid 30 to prevent leakage of liquid 30 from the chamber between discs 6 and band 12. Liquid 34 may be water or filtrate. Band 12 supports a non-endless length of filter paper (not shown) and moves intermittently in response to rising level of liquid 30. <IMAGE>

Description

FILTER ASSEMBLY The present invention relates to an improved deep bed filter assembly.

Conventionally such a filter assembly may comprise a filtering band wrapped part way around the circumferential walls of a pair of equi-sized discs rotatable about a common axis, so that the filtering band forms the floor of a reservoir to hold the unfiltered liquid. The filtering band may be a continuous belt driven by a series of rollers, the discs rotating as the band is advanced.

One of the problems with such an assembly is that the seal between each disc and the filtering band is prone to leak unfiltered liquid into the filtered liquid in the lower volume of the assembly, the risk of leakage increasing as greater filtration rates are achieved by increasing the diameter of the discs, and hence the depth of the liquid in the reservoir.

The present invention provides a filter assembly comprising a reservoir for an unfiltered first liquid, the two end-walls of the reservoir comprising a pair of rotable discs having the same dimensions and rotatable about a common axis, the curved bottom wall of the reservoir being formed by a filtering band passing around the circumferential walls of the discs, an inlet for the first liquid and an outlet for the filtrate, at least two seals surrounding the circumferential wall of each disc, spaced so as to provide an annular space therebetween, and means for introducing a second liquid into the space.

The outer seal preferably has an annular lip directed into the space between the outer seal and the next seal in towards the unfiltered liquid.

The second liquid is preferably filtered first liquid. The means for introducing the second liquid preferably includes control means so that the liquid level may be maintained at a level at least as high as that of the unfiltered liquid in the reservoir.

Other liquids, for example water, may be used as the second liquid.

The introduction of the second liquid into the annular space between the seals reduces the difference in pressure across the inner rubber seal. The advantage of lowering this pressure difference is that the seal is less likely to leak unfiltered liquid.

A further advantage of the present invention is that the risk of leakage of unfiltered liquid into the lower volume of the filter assembly beneath the filtering band, and hence into the filtered liquid, is considerably reduced. By keeping the level of the second liquid higher than that of the unfiltered liquid, if the inner seal breaks, the second liquid will flow into the unfiltered liquid. If the outer seal breaks the second, clean liquid will escape from the annular space between the seals into the lower volume of the filter assembly. The third area where leakage can occur is through the filtering band in the region of the annular space between the seals, but again this will be of clean liquid.Even if the level of the liquid inbetween the seals falls beneath that of the liquid in the reservoir, and the inner seal fails, the leakage will be of unfiltered liquid into the space between the seals, not into the lower volume of the assembly. Thus the risk of recontaminating the filtered liquid with unfiltered liquid that has by-passed the filtering band is lowered and the filtration efficiency of the present invention is greater than that of conventional filter assemblies.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings; in which: Fig. 1 is a perspective view of a filter assembly according to the present invention; Fig. 2 is a front view of the assembly shown in Fig.1 with parts removed; Fig. 3 is a sectional view through line I-I of Fig. 2; Fig. 4 is a schematic representation of the seal and disc arrangement; Fig. 5 is an enlargement of a part of Fig 4 marked by the dashed circle II.

Referring to Figure 1, a filter assembly comprises a reservoir 1 into which is introduced unfiltered liquid through a slot 2 in a feed pipe 4. The end walls of the reservoir comprise two equi-sized discs 6 which are rotatable about a common axis.

Surrounding the circumferential wall of each disc is a pair of rubber seals 9, spaced so as to provide an annular space 10 therebetween.

The discs have a circular cross-sectional end wall and a circumferential wall and are fixedly mounted on a spindle 8.

Support vanes 11 radiate from the spindle 8 to the circumferential wall of each disc. The spindle is rotatably mounted on bearings positioned on the side walls of the filter assembly.

A filtering band 12 is fed around a series of guide rollers 14a,b,c and d and a driven roller 15, all of which rollers extend the full width of the filtering band. The filtering band is tensionned against the circumferential walls of the discs to form the curved bottom wall of the reservoir. The filtering band 12 comprises a continuous belt supporting a sheet of filter medium. The arrangement of the rollers 14a,b,c,d and 15 is shown more clearly in Figs. 2 & 3, which show their arrangement with the filtering band removed. The support belt passes around the driven roller 15 and then around the top front guide roller 14a and part way around the circumferential walls of the discs before passing around the rear top guide roller 14b to form the curved bottom wall of the reservoir. The further guide rollers 14c and 14d guide the band on its return to the driven roller 15.

The driven roller 15 is rotated by a drive mechanism 16, acutated by a float switch 18 triggered by the position of a float 19 when the level of the liquid in the reservoir reaches a certain pre-determined level, indicating that the maximum practical filtering capacity of the band has been reached.

Rotation of the driven roller 15 causes the advancement of the filtering band 12 in the direction towards the driven roller, from which the spent filtering medium can be conveniently removed for disposal.

The filter medium carried by the support band is preferably a sheet of filter paper of appropriate density. Supports 20 carry a supply of paper from which fresh paper is taken as the support belt advances on actuation of the drive mechanism of the driven roller 15, the discs 6 rotating with the spindle 8 as the band advances to draw the spent filter paper away from the top front guide roller 14a towards the driven roller.

The liquid to be filtered is introduced into the reservoir through the feed pipe 4. A distribution plate 24 is arranged so that the unfiltered liquid is introduced to a particular area of the filtering band beneath the slot 2 at a velocity sufficient to wash away some of the residue that will build up in this area. This prevents localised build-up of residue in the area directly beneath the slot 2 and enables a greater area of the filtering band to be used for filtration before the maximum filtration capacity is reached, that is when the level of the liquid in the reservoir reaches the height at which the float switch 18 actuates the drive mechanism 16 of the driven roller 15 to advance the filtering band.

Filtered liquid is removed from the lower volume of the assembly beneath the filtering band through a sump outlet 26.

Fig. 4 shows schematically the sealing arrangement between the discs 6 and the filtering band 12. The two rubber seals 9 surround the circumferential wall of each disc, spaced to give an annular space 10 therebetween. The first liquid 30 is held before filtration in the reservoir 1 formed by the discs 6 and the filtering band 12. Feed means 32 introduce a second liquid 34 into the annular space 10 between the two seals 9.

The feed means 32 includes control means to ensure that the level of the second liquid 34 in the annular space 10 is kept at a level at least as high as that of the unfiltered liquid 30 in the reservoir 1. This level is preferably higher than that of the unfiltered liquid in the reservoir.

Referring to Figure 5, the outer seal 9a has an annular lip 36 directed towards the inner seal 9b. The pressure of the second liquid 34 acts on the lip to give a seal notably improved over that afforded by a lipless rubber seal.

Claims (6)

1. A filter assembly comprising a reservoir for a unfiltered first liquid, the two end-walls of the reservoir comprising a pair of rotatable discs, the discs having the same dimensions and rotatable about a common axis, the curved bottom wall of the reservoir being formed by a filtering band passing around the circumferential walls of the discs, an inlet for the first unfiltered liquid and an outlet for the filtrate, at least two seals surrounding the circumferential wall of each disc, the seals spaced so as to provide an annular space therebetween, and means for introducing a second liquid into the annular space.

2. A filter assembly according to claim 1, wherein the means for introducing a second liquid into the annular space include control means to maintain the level of the second liquid at a level at least as high as the level of the unfiltered liquid in the reservoir.

3. A filter assembly according to claim 1 or 2, wherein the second liquid is filtered first liquid.

4. A filter assembly according to any of the above claims, wherein the outer seal has an annular lip directed into the annular space between the outer and the next seal in.

5. A filter assembly according to any of the above claims, wherein the filter band comprises a support band carrying a filter medium.

6. A filter assembly according to claim 5 wherein the filter medium is paper.