GB2120569A

GB2120569A - Tube pressure filters

Info

Publication number: GB2120569A
Application number: GB08215295A
Authority: GB
Inventors: John Lawrence Martin
Original assignee: Alfa-Laval Co Ltd; Alfa Laval Ltd Great Britain
Current assignee: Alfa-Laval Co Ltd; Alfa Laval Ltd Great Britain
Priority date: 1982-05-26
Filing date: 1982-05-26
Publication date: 1983-12-07

Abstract

The filter cloth of a tube pressure filter is secured to the inner tubular member of the filter by forming a band of non-metallic strapping material into a loop, tensioning the loop, and welding the overlapping ends of the loop together to maintain the tension.

Description

SPECIFICATION Tube pressure filters This invention relates to tube pressure filters, of the type known, for example, from British Patent Specification 907485, comprising a pair of coaxial, generally tubular assemblies arranged one within the other, and an elastically extensible tubular membrane arranged between the assemblies to define an inner slurry compartment bounded by the inner assembly and the membrane, and an outer, pressure compartment bounded by the membrane and the outer assembly. In use, slurries are fed to the inner compartment and fluid pressure is applied to the outer compartment to cause the membrane to express liquid from the inner compartment through the inner assembly, the main part of which is constructed as an annular filter.

The membrane in such a press is secured to the outer assembly, and the inner assembly is axially displaceable relative to the outer assembly to facilitate removal of the filter cake which forms on it and for occasional maintenance of the inner assembly. The inner assembly comprises a rigid tubular, cylindrical member having a perforated wall and carrying at each end a fairing which tapers in diameter towards the adjacent end of the cylindrical member. The cylindrical member is normally covered by a layer of wire mesh, over which is laid an optional felt backing layer, and over which in turn is laid a filter cloth, the felt backing (if provided) and filter cloth being secured at opposite ends of the inner assembly.

Such a tube pressure filter is hereinafter referred to for convenience as being "of the type specified". The present invention is concerned with the problem of satisfactorily securing the filter cloth at its ends to the assembly.

Manufacturers of tube pressure filters of the type specified have made considerable efforts in recent years to improve on the current practice, which is to secure the ends of the filter cloth with twine or string, usually tying the cloth into a circumferential groove in each fairing. This procedure requires considerable skill and care to avoid two common problems, one being rucking of the cloth in the groove and adjacent the groove, and the other being a tendency to pull out the end which has first been secured when stretching the cloth axially and securing it at the opposite end.

Rucking of the material is significant, since the membrane is pressed against the filter cloth with a pressure in the order of 2000 p.s.i.

The present invention aims at the provision of an alternative technique of securing the filter cloth which obviates or reduces these problems.

In the particular embodiments of the invention described below, this is achieved by use of nonmetallic strapping material and securing tools, both commercially available and used in the packaging art, so as to secure each end of the filter cloth by a band of the strapping material which is tensioned to a degree predetermined by the tool with overlapping end portions which are welded together to maintain the said tension.

These embodiments will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of the inner assembly with its wire mesh already in position; Figures 2, 3 and 4 are scrap views on a larger scale, and partially in section, showing successive stages in the process of securing a felt and a filter cloth at one end of the assembly; and Figures 5 and 6 illustrate an alternative embodiment of the invention.

Figure 1 illustrates a rigid, generally tubular inner member, the major part of the length of which comprises a cylindrical section 1 having a perforated wall, this section being covered by a layer or sleeve of wire mesh 2 tightly fitted over the section 1, secured in place by spaced straps 3 of any suitable and convenient material. At each end the member carries a fairing 4, which tapers in diameter towards the adjacent end of the section 1, and which is formed with a circumferential groove 5.

As seen in Figure 2, a sleeve of felt backing 6 is laid over the wire mesh, and is of sufficient length to extend beyond the grooves 5. The felt may be wrapped around the inner member or be preformed into a tube by stitching along an axial seam. The felt 6 is drawn into and secured in the bottom of the groove 5 by a strapping 7 of nonmetallic material as described in more detail below.

Next, as shown in Figure 3, a sleeve of filter cloth 8 is drawn over the assembly, being stretched over the fairing and drawn down into the groove by another band of strapping 9, doubled back over the strapping 9 and a further band of strapping 10 is applied. Finally, the excess felt and filter cloth is folded back into the groove, over the strapping 10, and, as shown in Figure 4, a final band of strapping 11 is applied to secure and tidy up the attachment.

The felt is secured at both ends before the filter cloth is attached, and when the filter cloth has been secured at one end, as just described, it is stretched axially of the assembly to make it smooth and tight and then secured in the groove 5 in the other fairing in the manner just described.

The bands of strapping 7, 9, 10 and 1 1 are commercially available banding materials used in the packaging art being of synthetic plastics material, such as polyamides or polyesters having a degree of inherent elasticity. They are applied by commercially available tools which are pneumatically operated and which form the banding material into a loop of any desired size, tension the loop to a preselected degree and then weld the overlapping ends together by friction welding.

In use of this equipment as described above, the base portion of the tool is too wide to enter the groove and each band is tensioned, secured and then allowed to snap tightly into the groove, with residual tension in the band acting to secure the underlying layer of material.

The use of strapping bands of this form assist in reducing to a minimum rucking of the secured material and also the ensuring of the requisite degree of securing force applied. In both respects, the technique has shown itself superior to conventional methods of tying with twine or string which is difficult to perform without rucking of the materials, and the danger of the first end pulling away when the second end is secured.

The plastics banding material is relatively gentle on the filter cloth, being of a flexible nature, so that there is little risk of tearing the cloth locally when securing it.

These advantages are of considerable practical significance, bearing in mind that the replacement of filter cloths, and backing felts has to be effected fairly frequently to maintain a high quality of filtration.

In the modified embodiment, illustrated in Figures 5 and 6, both the felt backing 6 and the filter cloth 8 are secured at the ends of the cylindrical section 1 by bands of strapping 12, 13 in the manner described above.

This and many other variations will, of course, be possible within the scope of the present invention.

Claims

1. A method of securing each end of the filter cloth of a tube pressure filter of the type specified, wherein a band of non-metallic strapping material is formed into a loop about the filter cloth, elastically tensioned, and whilst held under tension, has its adjacent overlapping portions welded together to maintain tension in the band.

2. A method in accordance with claim 1, wherein the fairing at each end of the rigid tubular member is formed with an external annular groove within which the filter cloth is secured by the said band of strapping material.

3. A method in accordance with claim 1 or 2, wherein at each end of the cloth, an excess end portion of the cloth is doubled back over the said band, and a further said band is applied over the doubled-back portion.

4. A method in accordance with claim 1, substantially as herein described with reference to the accompanying drawings.

5. A tube pressure filter of the type specified, wherein the ends of the filter cloth have been secured by a method in accordance with any preceding claim.