GB2236961A - Apparatus for filtering and coalescing liquids - Google Patents

Abstract

A tubular, inward flow filter element 3 comprises fibres extending between lower support 5 and upper support 6 which is axially and rotatably movable to compress the fibres in the filtering position and to separate them in the cleaning position. So that pore size decreases in the flow direction, both the fibre spacing and the unconstrained length of the fibres decreases inwards. The fibres form the central parts of an array of concentric tubes (Fig. 2) whose ends are of woven construction. <IMAGE>

Description

APPARATUS FOR FILTERING AND COALESCING LIQUIDS The present invention relates to apparatus for filtering solids from liquids or for coalescing droplets of one liquid dispersed in another liquid.

EP 0 004 724 discloses an apparatus for removing contaminants from a liquid which comprises bundles of fibres aligned parallel to one another between two end plates. The end plates may be moved axially to stretch or compress the fibres. Liquid flows through the apparatus parallel to the fibre bundles.

EP 0 207 797 discloses another apparatus for removing contaminants from a liquid. This apparatus also comprises fibres connected at each end to end supports, which end supports can be moved away from or towards each other to place the fibres under tension or to relax any applied tension. The end supports can also be rotated relative to each other. The fibres are attached to the end supports so as to form a tube of fibres surrounding a central tubular core provided with perforations. The liquid to be treated passes across the fibres between the central core and an outer compartment surrounding the fibres. In use the end supports are moved towards one another and rotated relative to one another to twist the fibres and to compress them against the perforated tube.

Our copending unpublished British patent application 8822249.2 corresponding to our Case 7075 discloses an apparatus in which the inner perforated tube which restrains the fibres from radial movement is replaced by a helical spring.

We believe it is desirable to provide a filter or coalescor in which the filtering or coalescing medium becomes progressively finer from the feed side to the filtrate side. In the filtration device described in EP 207 797 the device is put into a condition to carry out filtration or coalescence by rotating the end supports relative to each other and moving them longitudinally towards each other so as to compress the fibres against the radial constraining means (e.g. the inner perforated tube). When using fibres of uniform length and uniform spacing on the end support the maximum fibre stress will be at the outside of the tubular filtration bed. The bed density will then be highest at the outside of the tubular filtration bed.

In the arrangement described above it is generally preferred to feed the liquid to be treated (whether by filtration or coalescence) to the outside surface of the tubular filtration bed as this maintains the fibrous filter in compression for stability and avoids channelling.

It is desirable to devise a filtering or coalescing apparatus which has improved filtration characteristics for liquid fed from the outside of the tubular filtration bed.

According to the present invention an apparatus for removing contaminants from a liquid, which apparatus comprises a casing, divided into two compartments by a tubular fibrous filter, said fibrous filter being formed of fibres attached at each end to an end support, said end supports being moveable axially relative to one another, and rotable relative to one another about the longitudinal axis of the tubular fibrous filter, and with the fibres disposed around a radial constraining means to resist radial inward movement of the fibres when the end supports are rotated relative to one another and are moved longitudinally towards each other, characterised in that the spacing between the fibres and the unconstrained length of the fibres decreases from the outside of the tubular filter to the inside.

The radial constraining means may correspond to the perforated tube of EP 0 207 797 or to the helical spring of copending unpublished application 8822249.2 (Case 7075).

For reasons of manufacturing convenience the spacing between the fibres and the unconstrained length of the fibres will not normally vary continuously across the tubular filter. The filter can conveniently be divided into zones, with each zone having the same fibre spacing and unconstrained length, but with the spacing and length decreasing from the outermost to the innermost zone.

The main body of the tubular filter is formed of fibres which are not bound to one another so that they are free to slide over one another so that the degree of packing of the fibres can be changed by axial and rotational movement of the end supports. However it may be convenient to form the tubular filter from individual tubes in which the fibres are bound together at each end by woven portions. Such materials are known as semi-woven fabrics.

It is desirable to arrange the fibres so that the differences in strain on the fibres in the packed state, when the end supports have been moved so as to minimise the spacing between the fibres, is minimised when moving from the outside to the inside of the tubular filter. Preferably the degree of strain in the packed state is substantially constant across the tubular filter.

Space between the fibres may be varied by increasing the distance between the fibres measured from centre to centre or by keeping the distance measured centre to centre constant but increasing the thickness of the fibre or by a combination of methods.

A specific embodiment of the invention will now be illustrated by reference to the accompanying drawing, in which Figure 1 is a cross-section through an apparatus according to the invention, and Figure 2 is a diagramatic representation of the arrangement of semi-woven tubes forming the fibrous filter.

The apparatus comprises a casing (1), having a vent (2) at its upper end. The vent (2) will normally be closed off by a pressure relief valve or a pressure gauge in use . The casing is divided into two compartments by a tubular fibrous filter, indicated generally at (3). The fibrous filter cloth tubes are secured at each end to end supports (5) and (6) and shown in Figure 2. End support (5) is fixed to the casing (1). End support (6) is attached to to an actuating rod (7) by which can it can be moved axially towards end support (5) and also rotated relative to end support (5). A liquid inlet (8) is provided in casing (1), opening into the outer compartment surrounding the tubular filter. A liquid outlet (8) is provided in the base of the casing,opening into the compartment within the tubular filter.

A drain outlet (10) is provided through which the casing can be drained if required. A helical spring (11) is disposed within the space within the tubular filter (3). One end (12) of the spring (11) is anchored in an opening in fixed end support (5). The other end (13) is anchored in moveable end support (6).

In use the moveable end support (6) is moved from the position shown in dotted lines in Figure 1 to the position shown in full lines by retraction and rotation of the actuating rod. As a consequence the fibres move from a position in which they are parallel to the axis of the filter to one in which they are inclined to the axis. As a result the individual fibres are forced closer together so increasing their efficiency at trapping particles in the fluid or coalescing droplets of immisicible liquid.

As indicated in Figure 2 the fibrous filter is formed from semi-woven cloth tubes attached at their ends to the end supports (5) and (6). The tubes have fibres running parallel to the axis of the filter in the unconstrained state, but with woven top and bottom selvedges. The cloth tubes are grouped into three zones (14), (15), and (16). The fibres in the outer most zone (14) have a low thread density of 10 threads per inch. The cloth tubes in (15) have a density of 15 threads per inch while those in zone (16) have a density of 20 threads per inch. The cloth tubes are all cylindrical in their unconstrained state. However as the bed length of the tubes increases going from the inside to the outside of the tubular filter the amount of compression to which the fibres are subjected by rotation of the end support (6) and movement of end support (6) towards end support (5) is less than for the tubes in the inner zone. As a result the outer zone will filter out coarser particles than the suceeding zones when liquid to be filtered is fed through inlet (8).

The movement of the actuating rod compresses the spring so that it continues to provide support over the whole length of the fibres without the complex mechanical arrangement shown in EP 0 207 797.

When the tubular fibrous filter is rotated and compressed the spring provides radial support for the stressed fibres and allows the end plates to rotate and compress the tubular fibrous bed by accepting torsion and compression within the spring due to the low stiffness of the spring in the torsional and axial axes but high rigidity in the axial direction.

When it is desired to clean the filter the actuating rod (7) is extended and rotated to return the moveable end support to the position shown in dotted lines. This increases the spacing between the fibres so facilitating backwashing.

Claims (3)

Claims:

1. An apparatus for removing contaminants from a liquid, which apparatus comprises a casing, divided into two compartments by a tubular fibrous filter, said fibrous filter being formed of fibres attached at each end to an end support, said end supports being moveable axially relative to one another, and rotable relative to one another about the longitudinal axis of the tubular fibrous filter, and with the fibres disposed around a radial constraining means to resist radial inward movement of the fibres when the end supports are rotated relative to one another and are moved longitudinally towards each other, characterised in that the spacing between the fibres and the unconstrained length of the fibres decreases from the outside of the tubular filter to the inside.

2. An apparatus according to claim 1 wherein the tubular filter is divided into zones, with each zone having the same fibre spacing and unconstrained length, but with the spacing and length decreasing from the outermost to the innermost zone.

3. An apparatus according to either one of claims 1 or 2 wherein the strain on the fibres when the end supports have been moved so as to minimise the distance between the fibres is substantially constant from the outside of the tubular filter to the inside.