GB2244011A

GB2244011A - Gas filter elements

Info

Publication number: GB2244011A
Application number: GB9011066A
Authority: GB
Inventors: Taliesin John Golesworthy
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1990-05-17
Filing date: 1990-05-17
Publication date: 1991-11-20
Also published as: GB9011066D0

Abstract

An inward-flow filter bag, for gases derived from combustion or gasification of coal, is internally supported by an extruded cage of solid, stellate cross-section made of high temperature polymer (exemplified).

Description

IMPRCVEMENTS IN CR RELATING TO FILTERS This invention concerns improvements in or relating to filters.

In particular, although not exclusively, the present invention has reference to filters employed for the filtration of flue gases derived from the combustion or gasification of coal.

One specific type of filter with which the invention is primarily concerned is an outside-in fabric filter, usually cleaned by pulse-jet, wherein generally tubular gas permeable filter bags are suspended vertically from a tube sheet, each bag being supported internally by a cage. The bags may be circular, elliptical or rectangular in cross section. Conventionally, the cage is manufactured from steel wire and comprises formers and stringers over which the fabric bag is disposed. In practice, the gas to be filtered passes through from the outside to the inside of the bag depositing particulate matter thereon, pulse jet cleaning being applied intermittently to the inside of the bag to displace and discharge the particles from the surface and/or the interstices of the fabric.

The profile of the cage may be of any convenient shape although cylindriform and cruciform designs have tended to be prevalent. It is also known to provide a cage of stellate cross-section with star-shaped formers having the stringers on the apices. Additionally, solid vaned stellate cages are known, but are usually of aluminium. The selection of the cage design is a balance between providing adequate support for the bag and allowing sufficient flexure during the pulse-jet cleaning step. Furthermore, due regard must be had to the environment in which the filter will be expected to operate; in particular the temperature parameter and the corrosive and erosive nature of the regime are important factors.In this connection, the operation of pulse-jet filters on industrial combustion equipment, as opposed to utility/power generation sized plant, is variable and can involve load changes and downtime involving acid and/or water dewpoint transitions.

Such a regime can cause severe acid dewpoint problems which render aluminium or mild steel inappropriate as corrosion of cage wires can cause bag damage. Only relatively expensive stainless type steels (304, 316) are acceptably resistant to typical conditions in a pulse-jet filter. All metallic wire cages suffer the problem that acid condensation on the wires in contact with the bag can cause bag failure due to acid attack of the fabric.

Investigations of cheaper painted (zinc based and phosphate based) mild steel cages have proved to be unsatisfactory. It is to be noted that the corrosion of the cage per se is not of primary concern, since the cage will retain its own mechanical integrity for a number of years. The greater concern is the effect of the corroded wire cage upon the fabric bag in contact with the metal. At best this leads, as the bag moves on the cage during cleandown, to abrasion of the bag by the corroded cage, and at worst to inclusion of fibres of the bag material in surface scale cracks and these fibres subsequently being torn out of the bag on clean down. The subsequent weakening of the bag and reduction in service life is of concern as bags are a consumable part of the filter but can represent up to 40% of the total capital cost of the filter.

Whilst stainless steel cages represent only about 10% of the total cost of the filter, their failure rate on mounting and demounting is high.

It is an object of the present invention to provide an improved cage support for pulse-jet fabric filters.

According to a first aspect of the invention there is provided a cage for an outside-in fabric filter, the cage being of solid stellate cross-section and of a high temperature polymer material.

According to a second aspect of the invention there is provided an outside-in fabric filter element including a cage of solid stellate cross-section and made of a high temperature polymer material, and a fabric bag mounted thereon.

The filter is conveniently of the pulse-jet type wherein cleaning of the filter fabric is effected by a pulse jet applied to the inside of the filter to dislodge and discharge particles lodged on the surface and in the interstices of the filter fabric.

Conveniently, the high polymer material has a service temperature in the range 200-250 C.

The material is preferably extrudable to facilitate cheap manufacture.

Materials of use in the present invention are by way of example only, polyphenylene sulphide (PPS), polyaryl sulphone, heat resistant epoxide, polyetherether ketone, and polyether ketone.

In principle, any polymer used to produce bag fabrics should have the requisite temperature/chemical resistance in its solid form, eg polyaramid.

The advantage of the cage of the present invention is its resistance to corrosion and increased durability compared with the conventional steel cages. The reduced corrosion which will essentially be non-existent, will result in greatly reduced wear on the filter bags. The enhanced durability will be of considerable advantage particularly in the mounting/demounting operation.

Moreover, the similarity of thermal properties as between the cage and the bag material means that there is less likelihood of acid condensation on the cage, thus resulting in a reduced chance of acid attack on the bag. Other metallic elements in the filter construction will preferentially experience acid condensation.

Furthermore, by virtue of the stellate design coupled with the material from which the cage is made, the cleaning of the filter element is likely to be more effective since the pulse energy will be evenly spread over the fabric surface thereby resulting in better fabric utilisation.

The present invention thus offers advantages over the conventional cages and filter elements both operationally and in cost since high temperature polymer materials are less expensive than stainless steel, particularly when cages manufactured from the latter demand frequent replacement, due to corrosion or mechanical damage or failure. The manufacture of the cage from high temperature polymer materials is also less costly being a simple extrusion process.

Claims

CLAIMS:

1. A cage for an outside-in fabric filter, the cage being of solid stellate cross-section and of a high temperature polymer material.

2. A cage according to claim 1 in which the high temperature polymer material is polyphenylene sulphide, polyaryl sulphone, heat resistant epoxide, polyetherether ketone, or polyether ketone, or polyaramid.

3. An outside-in fabric filter element including a cage of solid stellate cross-section and made of a high temperature polymer material, and a fabric bag mounted thereon.

4. An outside-in filter including a filter element according to claim 3.

5. A filter according to claim 4 in which the filter is a pulse-jet filter.

6. A cage for an outside-in fabric filter substantially as hereinbefore described.

7. An outside-in fabric filter element substantially as hereinbefore described.