GB2264072A

GB2264072A - Hepa filter

Info

Publication number: GB2264072A
Application number: GB9303491A
Authority: GB
Inventors: Christopher William Pamplin
Original assignee: BTR PLC
Current assignee: BTR PLC
Priority date: 1992-02-11
Filing date: 1993-02-10
Publication date: 1993-08-18
Anticipated expiration: 2013-02-10
Also published as: GB2264072B; GB9202781D0; GB9303491D0

Abstract

In a HEPA type gas filter at least 95% by weight of the filter is substantially fully combustible such that when combusted the material of the filter reduces to an ash the weight of which is less than 1% by weight of the original material. The filter preferably includes a polymeric filter medium, such as polyester or polypropylene, which leaves no ash when combusted. The filter is used to remove radioactive waste from a gas stream and is then combusted, the resulting gases being filtered through a similar filter. As shown the filter medium is pleated and sealed into a combustible frame 1, with combustible spacers 3 and grilles 5. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO FILTERS This invention relates to gas filters and a method of use of filters of the so called high efficiency type, known commonly as HEPA filters.

Conventionally a HEPA filter utilizes glass as the filtration medium and whilst this performs effectively as a filter it presents a disposal problem particularly when the filter has been used in a radioactive environment. It can be difficult to separate the radioactive waste particles from the filtration medium. Thus the filtration material adds undesirably to the bulk of radioactive material which requires long term disposal provisions.

The present invention seeks to provide an improved filter and method of use of a filter in which the aforedescribed difficulties are mitigated or overcome.

In accordance with one of its aspects the present invention provides a filter of the HEPA type and which comprises substantially wholly substantially fully combustible material as herein defined, the filter comprising a filtration medium which is comprised by an organic polymeric material.

Preferably the organic material is a man-made type organic material. Examples of suitable organic polymeric materials include polyester and polypropylene.

By substantially wholly is meant that at least 95% and more preferably at least 99% by weight of the filter (prior to contamination) is substantially fully combustible.

By substantially fully combustible is meant that when combusted the material of the filter reduces to an ash the .weight of which is less than 1.0% and more preferably less than 0.5% by weight of the original material.

Preferably the filtration medium is ash-less by which is meant that it is wholly combustible so that when combusted it leaves no ash deposit.

The filter may comprise a filtration medium contained within a combustible case to which it is sealed by means of low ash content sealant. Examples of low ash content sealant include unfilled polyurethane. By low ash content is meant here a sealant which when combusted reduces to an ash the weight of which is less than 5.0% and more preferably less than 1.0% in weight of the original sealant.

Preferably the sealant is of a kind which does not generate toxic fumes during combustion.

It is further preferred that the filter medium be comprised of virgin as opposed to regenerated fibres.

In accordance with another of its aspects the present invention provides a method of use of a filter of the HEPA type comprising providing within a filter housing a filter at least the filtration medium of which comprises substantially wholly substantially fully combustible material as herein defined, using the filter to perform a filtration operation and then removing the filter from the housing and subjecting it to a combustion process to reduce the filter to ash.

Preferably, and especially when the filtration medium is contaminated by radioactive or toxic matter, the gases resulting from combustion are subjected to a filtration procedure which may comprise use of a filter in accordance with the present invention.

The filtration medium in sheet form preferably has a grammage i e weight in the range 50 to 200 grams per square metre, and more preferably in the range 120 to 180 grams per square metre.

An example of a HEPA filter in accordance with the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing which is a perspective view of a filter of the invention.

The filter 10 is substantially wholly combustible and comprises a combustible case 1 which defines a square section tubular passageway for containing an assembly of alternating layers or zones of corrugated combustible spacers 3 and layers 2 of an organic polymeric filtration material such as polypropylene or polyester. Although not illustrated, the spacers 3 are shaped, in known manner, to prevent direct flow of gases through the case, and to direct the gases into the layers 2 of filtration material.

Ends of the case are covered with protective grilles 5 of plastics mesh. The assembly of filtration layers and spacer layers are secured to inner surfaces of the case by a sealant such as unfilled polyurethane so as to prevent leakage between the filtration and spacer layers and the case.

The case 1 may be provided with a plastics handle 6 to facilitate transportation.

The filter of the present invention need not be of square or rectangular planar form. It may alternatively be of a cylindrical form comprising cylindrical layers of filtration material which may alternate with cylindrical spacer layers and which may be contained within a cylindrical housing case.

Claims

CLAIMS:

1. A gas filter of the HEPA type and which comprises substantially wholly substantially fully combustible material as herein defined.

2. A gas filter according to claim 1 wherein the filter comprises a filtration medium comprised by an organic polymeric material.

3. A gas filter according to claim 1 or claim 2 wherein the filtration medium is wholly combustible.

4. A gas filter according to any one of the preceding claims wherein the filtration medium is comprised by a man-made type organic material.

5. A gas filter according to claim 2, claim 3 or claim 4 and comprising a filtration medium in sheet form and having a grammage in the range 50 to 200 grams per square metre.

6. A gas filter according to claim 5 wherein the grammage is in the range 120 to 180 grams per square metre.

7. A gas filter according to any one of the preceding claims wherein at least 99% by weight of the filter prior to contamination is substantially fully combustible.

8. A gas filter according to any one of the preceding claims wherein combustion of the filter reduces it to an ash the weight of which is less than 0.5% by weight of the original material.

9. A gas filter according to claim 1 and constructed and arranged substantially as hereinbefore described.

10. Method of use of a filter of the HEPA type comprising providing within a filter housing a filter at least the filtration medium of which comprises substantially wholly substantially fully combustible material as herein defined, using the filter to perform a filtration operation and then removing the filter from the housing and subjecting it to a combustion process.

11. Method according to claim 10 wherein use is made Of a filter in accordance with any one of claims 1 to 9

12. Method of use of a filter of the HEPA type according to claim 10 or claim 11 and substantially as hereinbefore described.