GB2181365A - Cleaning an air filter - Google Patents

Abstract

A hydrophobic HEPA air filter partially blocked by small ferric oxide and iron particles, such as arise from the plasma arc cutting of iron or steel, is unblocked by subjecting the filter to a flow of a vapour (which term includes droplets), atomised liquid or solution, aerosol or the like, of such concentration and duration as not to block the filter but to dissolve the oxide, which is redistributed to leave gas-conducting voids when the liquid subsequently dries out. The vapour may be a cold water fog produced by solid carbon dioxide and hot water, or an acid or organic solvent may be used. The unblocked filter may be coated with silane-coated glass spheres to facilitate subsequent filtration. A particular application involves plasma cutting of equipment contaminated with plutonium.

Description

SPECIFICATION Improvements in or relating to the unblocking of filters This invention relates to the unblocking of filters.

Iron and steel equipment used in the processing of radioactive materials such as plutonium becomes contaminated with such materials, and the eventual disposal of such equipment presents difficulties.

Disposal is made easier by cutting the equipment into small pieces, and plasma arc cutting is a suitable technique. However the arising fumes, principally ferric oxide and metallic iron particles, are themselves contaminated, so that the cutting must be done within an enclosure and the fumes collected on filters. Unfortunately it is found that HEPA (High Efficiency Particle Air) filters used for this purpose block rapidly, owing to the deposition of the fumes on the fibre-glass filter surface as a coherent, uniform, brittle crust. This crust causes blockage by bridging the air-conducting voids between the fibre-glass threads. Further, the crust only forms on the surface of the filter, preventing uniform loading of the entire volume of the filter.

This causes an unacceptable degree of blockage at low mass loadings with consequent high wastage of filters, and has led to the abandonment of this method of size-reduction by some organisations.

The present invention provides a method of unblocking such filters in situ which allows their repeated use.

According to the present invention a method of unblocking a filter partially blocked by deposition of small particles comprises subjecting the filter, in situ or otherwise, to a flow of vapour (which term includes droplets), atomised liquid or solution, aerosol or the like, of such concentration and duration that no substantial blockage is effected by the flow thereof itself, and which produces gasconducting voids in the particle deposit due to shrinkage of said deposit.

The particles may be substantially ferric oxide and metallic iron particles such as those present in fumes arising from the plasma arc cutting or iron or steel, and the filter material be hydrophobic.

The filter may be an HEPA filter and may be made of fibre-glass treated to be hydrophobic. The vapour, atomised liquid or solution, aerosol or the like may be simply water or may be a concentrated acid, an organic solvent, a wetting agent or a detergent.

Water may be used as an aerosol which may be formed by spraying. For treating HEPA filters the droplet diameter, of water or otherwise, is preferably less than 10 microns and the aerosol is preferably formed as a cool fog. A suitable water fog can be produced by immersing solid carbon dioxide in hot water.

The acid may be nitric or hydrochloric and may be vapourised by bubbling a gas such as air therethrough. The organic solvent may be trichloro, trifluoroethane (Freon) which is also a wetting agent and is particularly suitable for plutoniumcontaminated fumes because it poses few criticality problems. It may be used as an atomised dilute solution.

Using any of the aforesaid substances, the pressure drop across a piece of Vokes Type 55 hydrophobic fibre-glass filter paper partially blocked by fumes as aforesaid, at constant flow-rate, decreased to near that of unused paper, allowing its repeated use. Atomised water droplets of about 135 microns (using a garden-type spray) and 40 microns (using an air paint spray) diameter were equally effective.

For unblocking in situ hydrophobic HEPA filters, which comprise a long sheet of filter paper concertina'd into layers separated by sheets of corrugated paper or aluminium foil, the above droplets are found too large to penetrate far into the filter along the corrugations. Smaller droplets, of 10 microns or less, are found to penetrate the full depth and cannot be formed as a cool fog by immersing solid carbon dioxide in hot water. Commercially available theatrical fog-producing equipment can be used for this purpose. In one experiment the pressure drop across a hydrophobic HEPAfilter partially blocked by the aforesaid fumes, was reduced from 7.2 swg to 2.0 swg using such a fog, the whole paper surface being unblocked.

The fog can be introduced into the duct leading to an in situ filter upstream of the filter and drawn thereto by the existing extraction means, eg, a fan.

Being cool, the droplets do not evaporate en route to deposition on the filter; nor is there substantial deposition of water on the duct walls. In another experiment the fog was introduced into a duct about 2 metres upstream of a 66 cm square by 33 cm deep HEPA filter at a rate of about 200 g of water per minute. Unblocking was effected in 1 minute or less.

Since the total filter area is 20 square metres, this represents the deposition of only 10 g of water per square metre. This illustrates a further advantage in using smaller water droplets with plutoniumcontaminated fumes from a criticality viewpoint, since smaller droplets allow a given filter area to be covernd,frnm geometrical considerations, by a smaller mass of water. The above rates and times are exemplary, not critical; they may be determined by experiment for particular applications.

The number of cycles of use, unblocking and reuse obtainable before permanent blocking makes the filter unusable will depend on circumstances.

Without wishing to be bound by any particular theory of operation, it is believed, as a result of microscopic examination that, with the aforesaid fumes, the present method functions, at least in part, as follows. The blocking is caused by the presence in the arising fumes of sensibly spherical particles of iron and amorphous particles of ferric oxide. The iron spheres are larger (1 to 40 microns) than the oxide particles ( < 0.3 micron), and although the iron spheres are stopped at the filter surface, being unable to penetrate between the parallel glass fibres, it is the smaller oxide particles which gradually form the non-porous surface crust.

When the fume flow is replaced by a flow of a chemical in accordance with the present method, liquid droplets thereof are believed to form on the surface and to dissolve the ferric oxide particles, thereby converting each droplet to a sol or solution of the oxide. As these droplets dry out, they apparently contract towards the iron spheres and the glass fibres, taking the ferric oxide crust with them. The droplets then form "windows" lodged between adjacent iron spheres, and cylindrical menisci surrounding the glass fibres. When these windows and menisci finally dry out they leave ferric salts or ferric oxide (depending on the chemical used) as entities of much lower volume then the crust and which do not clog the airconducting voids between the fibres. The filter thus becomes substantially unblocked. The iron spheres are not affected to the same degree.The gradual accumulation of dried ferric droplets during repeated cycles will determine the total life of the filter.

It is found that fibre-glass filters blocked with iron/ ferric oxide as aforesaid are not unblocked by the present method, using water, unless the glass has been treated in a known manner to be hydrophobic.

With untreated (hydrophilic) filters the ferric oxide appears to adhere preferentially to the glass fibres and does not contract to leave voids upon contacting with the water droplets.

The present water droplets remain on the paper surface and their water content eventually evaporates without saturating it; no increase in the pressure differential across the filter occurs during subjection to the aforesaid fog. Thus the paper does not saturate and block with water, which would lead to physical damage to the filter paper (due to the high differential pressure) and to criticality problems, and the volume of water present, from the criticality aspect, is small.

After unblocking a filter, its subsequent life, before reblocking, may be prolongable by applying a layer of, eg, glass, microspheres over its surface, these serving to distribute the fume flow over the whole surface instead of being directed straight into the voids produced by the unblocking. With iron/ ferric oxide fumes at least, the spheres are desirably pre-treated, eg, with a silane, to make them hydrophobic, so that the present unblocking method may be re-applicable when the filter reblocks.

Claims (10)

1. A method of unblocking a filter partially blocked by deposition of small particles comprises subjecting the filter, in situ or otherwise, to a flow of vapour (which term includes droplets), atomised liquid or solution, aerosol or the like, of such concentration and duration that no substantial blockage is effected by the flow thereof itself, and which produces gas-conducting voids in the particle deposit due to shrinkage of the deposit.

2. A method as claimed in claim 1 wherein the particles are substantially ferric oxide and metal iron particles such as those present in fumes arising from the plasma arc cutting of iron or steel, and the filter material is hydrophobic.

3. A method as claimed in claim 1 or claim 2 wherein the filter is a High Efficiency Particle Air (HEPA) filter.

4. A method as claimed in any of claims 1 to 3 wherein the filter is made of fibre-glass treated to be hydrophobic.

5. A method as claimed in any preceding claim wherein the filter is subjected to a flow of liquid droplets of less than 10 microns diameter.

6. A method as claimed in claim 5 wherein the droplets form a cool fog.

7. A method as claimed in claim 6 wherein the droplets are water droplets.

8. A method as claimed in claim 7 wherein the fog is formed by immersing solid carbon dioxide in hot water.

9. A method as claimed in claim 7 or claim 8 wherein the fog is introduced into a duct leading to an in situ HEPA filter upstream of the filter and drawn thereto.

10. A method of unblocking a filter as claimed in claim 1 and substantially as hereinbefore described.