Filter elements for gas or liquid and methods of making such filters

Tubular filter elements are formed by first feeding a slurry into a tubular moulding space between a vertical core and a cylindrical fine mesh screen. Air pressure is applied to the slurry so that the liquid drains through the screen and through a screen at one base of the space, while a mass of microfibres builds up to be removed from the space for bonding by a synthetic resin. A reciprocable sleeve increases the effective height of the screen as the mass builds up. Layers of microfibres having different qualities can be assembled by using different core diameters in succession. Filter elements that need not necessarily be cylindrical can be formed by this or analogous pressure methods in which the majority of the microfibres are directed approximately parallel to one another, and/or in which one or each face of the filter element has moulded into it a perforated sheet of supporting material.

This invention relates to filter elements and to methods and apparatus for 
filter elements. 
In U.S. Pat. No. 4,111,815 there is described a method of forming a filter 
element which comprises dispersing a mass of fibres in a liquid to form a 
slurry, draining the liquid through a filter surface on which the fibres 
collect while an apertured sheet of supporting material is located at a 
selected distance above the filter surface, so that the fibres build up 
from the filter surface through the apertures in the supporting material 
to a predetermined distance above the supporting material, removing the 
collected fibres containing the sheet of supporting material from the 
filter surface, and bonding the fibres to one another and to the 
supporting material by means of a synthetic resin. 
One aspect of the present invention is based on further experiments that 
demonstrate unexpectedly satisfactory results if, in the aforesaid method, 
the sheet of supporting material is omitted or, if provided, is located 
substantially in contact with the filter surface so that the sheet of 
supporting material becomes moulded into one surface of the filter 
element. The invention results in the production of a particularly 
efficient fibrous filter that is very economical to manufacture. 
According to the present invention, a method of forming a filter element 
comprises dispersing a mass of fibres in a liquid to form a slurry, 
applying the dispersion under pressure to a filter surface so that the 
fibres collect as a layer covering the filter surface while the liquid 
passes through the filter surface, and bonding the fibres in the collected 
mass of fibres, after drying, to one another by means of a synthetic 
resin. A sheet of material that is to provide a support for the filter 
element may be mounted in contact with at least a portion of the filter 
surface so that the support sheet becomes moulded into one surface of the 
collected mass of fibres. Thus, when the support sheet, which may be 
provided by very fine mesh material, is removed, the fibres are found to 
have penetrated through the support sheet leaving their outer surface 
flush with the outer surface of the support sheet, which may be a layer of 
expanded metal. In the past, in the case of cylindrical filter elements, 
these have required the addition of a separate support sheet to give 
strength, but the present moulding method enables the filter cylinder and 
support sheet to be produced as an integral part in one operation with 
precision, saving time and labor. 
In general, it is desirable to make the apertures and open area of the 
support sheet as large as possible. However, it is difficult to specify 
the largest aperture that can be used. The smallest aperture at present 
contemplated is 0.25 mm diameter. However, it must be remembered that 
certain fibres, such as potassium polytitanate, e.g., potassium 
dititanate, have a diameter of 0.5 microns and length of up to 0.15 mm and 
these can penetrate apertures of 0.25 mm and smaller. 
In the case of the filter surface, expanded metal with narrow flat strips 
between overlapping elongated apertures, an aperture size of 0.75 mm by 
0.5 mm has been found to be practical. This gives a good surface finish. 1 
mm by 0.75 mm will of course give a somewhat rougher finish. 
Other practical examples or rigid supports have had apertures of 2.8 mm by 
0.8 mm providing an open area of 26% of the area of the support sheet, and 
43 mm by 20 mm with an open area of 83%. In general it has been found that 
the support sheet results in a very small flow restriction, of the order 
of 1% to 2% of the total flow. 
In a modification of the aforesaid method, the support sheet consists of a 
rigid foam or sintered material thereby eliminating the necessity for the 
use of the very fine mesh material in the production of the filter 
element. 
When, as in the aforesaid prior U.S. Pat. No. 4,111,815, the binder is used 
not only to bond the fibres together but also to the support sheet, this 
may be, for example, silicone, polyurethane, expoxy or phenolic resin. 
Heat cured resins are preferred though air drying resins can be used. The 
weight of the resin binder depends on the strength required. Generally the 
weight of the binder is no more than 100% of the weight of the fibres. 
It has been found that the use of pressure in the method according to the 
invention results in a majority of the fibres being disposed so that they 
are directed, in some measure, approximately in parallel with one another. 
This gives particularly advantageous results, whether or not a support 
sheet is used. According to a further aspect of the invention, therefore, 
a filter element comprises a mass of fibres compacted together and bonded 
to one another with a synthetic resin, a majority of the fibres being 
disposed so that they are directed, in some measure, approximately in 
parallel with one another.

The portion of the filter element shown in FIGS. 1 and 2 may be part of the 
wall of a cylindrical filter element although it can equally well be 
regarded as part of a disc, sheet or conical or frusto conical cylindrical 
shape (for example closed at one end as shown in FIG. 7). A similar method 
may also be used for the production of concave or convex discs. The bulk 1 
of the filter element comprises fibre material; for example, glass, 
ceramic, synthetic fibres, asbestos, mineral wool, organic or silicate 
fibres. Raw borosilicate microfibre is a preferred material. For cartridge 
type filters to be used in liquid filtration, cellulose, wool, synthetic 
polymer (e.g. polypropylene and acrylic) fibres, and combinations of 
these, also such combinations containing a portion of borosilicate 
microfibre, can very advantageously be used. These combinations can also 
be used for gas filtration. Both faces of the fibre mass 1 have an 
apertured support sheet 2 moulded thereto so that the fibrous mass 
penetrates through the apertures in the sheets to present surfaces that 
are flush with the outer surfaces of the sheets (FIG. 2). Each support 
sheet consists of an apertured or open pore rigid material such as a 
perforated, expanded or woven material which, in turn, may be of metal, 
plastics, glass or ceramic. Expanded metal is a preferred material. The 
total area of the openings in the support sheets depends on the use to 
which the filter element is to be used. 
The filter element of FIG. 3 is similar to that of FIG. 2, but only one 
support sheet 2 is used. Where one support sheet is used, this is 
generally located on the downstream side of the fibres. This not only 
gives strength where it is required but does not reduce the inlet surface 
area of the filter, thereby increasing the dirt holding capacity. For low 
pressure use as for example in vacuum systems, the support sheet can be of 
comparatively light construction but, when used in a high pressure system, 
either with gas or liquid, the support sheet can be of heavier 
construction. 
In a further example, no support sheet is used. This example consists of a 
tube made from raw borosilicate microfibre moulded by pressure forming 
into the cylindrical shape by a method as described below with reference 
to FIGS. 4 to 6. The moulded tube is then dipped into a solution of resin 
in a solvent so as to impregnate the fibrous material and is then heat 
cured. By using a method as described below a filter element without any 
support sheet can be constructed with very advantageous properties. For 
example such filter elements 54 mm long, 44 mm outside diameter and 34 mm 
inside diameter have been constructed and tested to give the following 
characteristics: 
______________________________________ 
D.O.P. BURST FLOW .DELTA.p 
p O.D.T. 
% bar NM.sup.3 /H 
bar bar % w/w 
______________________________________ 
&gt;&gt;99.999 &gt;7.0 45 .069 7.0 15.0 
99.97 &gt;7.0 45 .069 3.0 27.0 
99.90 &gt;7.0 48 .035 3.0 25.0 
99.80 &gt;7.0 52 .035 3.0 21.0 
99.80 &gt;7.0 50 .035 4.0 35.0 
______________________________________ 
In the above table p is the operating test pressure, .DELTA.p is the 
pressure loss across the filter below, and O.D.T. is the ratio of the oven 
dried total weight of resin to the fibre content of the filter element. 
The binder used in all the filter elements represented in the above table 
was a silicone resin, which is preferred, but many other binders can be 
used to give comparative test results. The highest resin content which is 
in the last tabulated example, is 35% but this can be raised to as high as 
100% while still providing satisfactory characteristics. However, 25% has 
been found admirably satisfactory for most applications. 
The effect on performance of varying the wall thickness of an unsupported 
tubular filter element is shown in the following table relating to a 
larger element 200 mm long, 66 mm outside diameter and 54 mm inside 
diameter for sample (a) but 46 mm inside diameter for sample (b): 
______________________________________ 
D.O.P. FLOW .DELTA.p 
p O.D.T. 
SAMPLE % NM.sup.3 /H 
bar bar % w/w 
______________________________________ 
(a) 99.99 306 .017 4.2 16.0 
(b) 99.999 170 .017 4.2 16.0 
______________________________________ 
In the above table the pressure p is a gauge pressure above atmospheric 
pressure while .DELTA.p, of course, is a pressure differential. 
The above table shows that it is effectively only the flow capacity and 
efficiency that is affected by the increase in wall thickness. In 
practice, it is thought that about 3 mm will prove to be a lower limit for 
the wall thickness. 
The good results, exemplified by the above tables, are believed to arise 
from the packing pattern of the fibres that arise as a result of a method 
of manufacture such as described below with reference to FIGS. 4 to 6. 
This packing pattern results from the fibres lying in some measure more 
uniformly in a circumferential direction around the filter element, than 
is possible with known vacuum methods which display a totally random 
packing pattern. The more regular packing in the filter elements of the 
invention does not detract from their efficiency. 
Although the filter elements described immediately above have no rigid 
support sheet, they can be provided with an inner, outer, or both inner 
and outer layer of woven or non-woven flexible material to improve the 
handling characteristics. Such a layer can be incorporated during the 
manufacture of the filter element by a method as described below. The 
fibres would generally penetrate through an aperture or pore structure of 
the flexible material. Moreover in the case of a filter element with a 
single rigid suport sheet as shown in FIG. 3, the opposite face of the 
fibrous structure can be provided with a layer of flexible material. 
Simple, unsupported tubular filter elements as described above may be 
formed with a variety of surface patterns, for example circumferential or 
longitudinal grooves, to increase the surface area. 
FIG. 4 shows diagrammatically apparatus for forming a tubular filter 
element. When this apparatus is in operation, water and borosilicate 
microfibres are fed into a blending tank 31. Hydrochloric or sulphuric 
acid is added until the pH value reaches 2.8 to 3.5. Borosilicate 
microfibres are found to disperse more readily at this value. It has also 
been found that the fibres disperse more readily if the solution 
temperature is increased to about 35.degree. C. The quality of the fibres 
that are used depends on the grade of the filter element that is to be 
used. The fibre to water ratio (by weight) is generally 0.05% but can vary 
between 0.01% and 0.5%. A binder such as colloidal silica may be 
introduced into the slurry at this stage. It has been found advantageous 
to use this type of binder to impart additional strength prior to resin 
impregnation. The final dispersion is effected by a mechanical agitator 32 
and takes about 15 minutes. 
With valves 33 and 34 closed and valve 35 open, a pump 36 transfers the 
dispersion to a pressure vessel 37. The precise quantity transferred 
depends on the fibre/water ratio and the size of the filter element to be 
produced. 
Next the valve 35 is closed and the valve 33 is opened to admit compressed 
air to the pressure vessel 37. Generally the pressure used is 3.5 bar. 
This top pressure is the forming pressure and can be varied according to 
the efficiency required. The efficiency can be varied within a range, 
e.g., 99.9% to 99.999%, using the same fibre blend. The forming pressure 
may be as low as 0.3 bar, but a pressure of 3.5 bar has been found highly 
satisfactory with the fibre blend adjusted to suit the required 
efficiency. 
The next step is to open the valve 34 to enable the dispersion to flow into 
a moulding rig 38 shown in detail in FIGS. 5 and 6. The moulding rig 
includes inner and outer vertical cylinders 39, 40 defining a space 41 
through which the dispersion can flow into a cylindrical moulding space 42 
defined between a fine mesh screen 44, supported by a machined perforated 
cylinder 45, and a core 43 when in the position of FIG. 5. FIGS. 5 and 6 
show the filter element being moulded as a unit with an outer rigid 
cylindrical support sheet 2, but it will be appreciated that for a simple 
borosilicate microfibre filter tube, this can be omitted. Alternatively, 
of course, an inner support sheet can be moulded into the inside surface 
of the tube, either instead of or as an addition to the outer sheet 2. The 
bottom of the moulding space is covered by a fine mesh screen 46. A 
reciprocable sleeve 47 is mounted to slide outside the cylinder 40 and 
perforated cylinder 45. 
With the core 43 and sleeve 47 in the positions shown in FIG. 5, the water 
drains away through the screen 46 and lower end of the screen 44 into a 
tank 48 (FIG. 4) while the mass of fibres begin to build up in the 
moulding space 42. After all the fibres have accumulated in the moulding 
space, the air pressure is maintained so as to remove residual water from 
the fibres and so dry the formed filter. The valve 34 is then closed. 
During the moulding process, a pump 49 continuously pumps the water from 
the tank 48 to a holding tank 50 from which the water is recycled. 
Finally the core 43 is removed to enable the formed filter element to be 
removed from the rig 38. The process can then be started once more. As an 
example, it has been found that the time taken to mould a tubular filter 
element 250 mm long, 65 mm outside diameter with a wall thickness of 10 mm 
takes approximately one minute. The formed filter element is removed to a 
hot air dryer for final drying and is then resin impregnated and oven 
cured to harden the resin. 
Particularly in the case of long filter elements, e.g., over 50 mm, it has 
been found desirable progressively to raise the sleeve 47, substantially 
at the same rate that the height of the fibre mass increases, in order to 
maintain an uninterrupted flow of the dispersion to the point where the 
mass of fibres is building up. The movement of the sleeve 47 then 
terminates as shown in FIG. 6. 
The core 43 is formed with an upper portion 51 of reduced diameter. This is 
to enable an additional internal layer of fibrous filter material to be 
added to the filter material formed in the moulding space 42, by feeding a 
further dispersion through the cylinder 39 into a moulding space 52 (FIG. 
6) between the moulding space 42 and the core portion 51 when the core 43 
is lowered. The water from the new layer escapes through the fibres in the 
space 42. The new layer may be of higher or lower efficiency than the 
tubular element formed in the space 42. This arrangement enables a filter 
element of graded density to be produced as part of an integral process. 
Investigations have shown that the fibres in a finished filter element 
produced by the method described above with reference to FIGS. 4 to 6 are 
predominantly layered in planes perpendicular to the direction in which 
the dispersion flows into the moulding space. It has further been found 
that the same packing pattern arises throughout the range of forming 
pressures that can be used effectively in practice. Advantages of this 
packing pattern appear from the results tabulated above. 
For some applications of the invention, where cellulose fibres or 
combinations of celluose fibres with borosilicate fibres are used, a 
melamine or phenolic resin binder may advantageously be used for the 
bonding material. Cellulose when bonded with melamine resin is approved as 
being suitable for potable water and sanitary conditions. Phenolic resin 
is preferred for higher temperature work. The combination of cellulose 
fibres with other fibres provides economies both in regard to cost and 
production time, good flow characteristics and chemical resistance, and 
controlled selection of pore size by blending different fibre materials 
with cellulose. It has been found that by blending 20% borosilicate 
microfibre with 80% cellulose by weight the production time for the filter 
can be reduced by 30%. In this case when the fluid is water the pressure 
drop (.DELTA.p) across the filter was 0.15 bar with a flow rate of 16 
liters per minute. With a weight to weight ratio of 50%, .DELTA.p was 
found to be 0.15 bar with a flow rate of 22 liters per minute. The glass 
fibre size (diameter) was 3.8 to 5.1 microns and the cellulose a bleached 
softwood kraft. The bonding material, e.g. melamine resin, phenolic resin 
or other synthetic resin, can be applied in one of three different ways. 
Firstly, by forming a mass of fibres in a moulding rig such as shown in 
FIGS. 5 and 6, then impregnating the mass after drying by dipping in a 
resin solution and curing the resin in an oven. Secondly, by preparing the 
cellulose fibre and separately mixing the borosilicate fibre with a resin 
solution, bringing the two mixtures together, forming the mass under 
pressure in the moulding rig and curing the mass. Thirdly, all the fibres 
and resin solution can be mixed in a single tank, passed to the moulding 
rig, the mass being subsequently cured. 
A cylindrical filter element for liquid filtration having a combination of 
fibres as described above may have an outside diameter of 64 mm, a wall 
thickness of 18 mm and various lengths, such as 250 mm. No support sheet 
is necessary for many uses but can be added when necessary. The filter is 
preferably arranged for flow from outside to inside the cylinder to give 
greater surface area for collection of dirt. This area can be increased by 
forming longitudinal or circumferential grooves in the outside surface of 
the cylinder. 
Instead of using a compressed gas to apply pressure to the slurry in the 
moulding rig, a hydraulic pump may be used, this pump being arranged to 
withdraw the slurry from the blending tank and force it into the moulding 
rig. 
Tubular or cylindrical filter elements made in accordance with the 
invention may be mounted in a variety of filters, in particular those 
shown in FIGS. 5, 6, 7 and 13 in the aforesaid prior U.S. Pat. No. 
4,111,815. As in that prior patent, also the ends of the cylindrical 
filter elements may be fitted into end caps in a variety of ways. Such 
ways are shown in FIGS. 8 to 17 of the present specification. 
FIGS. 8 to 13 show cases where the end of a cylindrical, unsupported filter 
element 10 is fitted into an end cap 11 using a gasket seal 12 (FIG. 8), a 
double taper seal 13 (FIG. 9), an outside taper seal 14 (FIG. 10) an 
inside taper seal 15 (FIGS. 11 and 12) and a double taper flange seal 16 
(FIG. 13). For a cylindrical filter element with an inside support sheet 
17 an outside taper seal 14 (FIG. 14) may be used. For an outside support 
sheet 18 (FIG. 15) an inside taper seal 15, or a single taper flange seal 
19 (FIG. 16) may be used. In the case of a filter element having inside 
and outside support sheets 20, 21 (FIG. 17) a gasket seal 12 (FIG. 17) can 
be used. In all forms of the filter element constructed according to the 
invention, an open pore filter layer or sleeve, as shown in FIGS. 12 and 
17, can be used if required to act as a pre-filter or as an after-filter 
to drain coalesced liquids. This layer or sleeve can be an open pore 
plastic or metal foam or a layer or layers of non-woven material such as 
felt. As a further alternative the filter element can be dip sealed into 
end caps as shown in FIG. 6 of the aforesaid prior patent. FIG. 18 shows 
an arrangement similar to FIG. 15 with an internal supporting spring 24 in 
place of any inside support sheet. 
Filters made in accordance with the invention can be used for either gas or 
liquid filtration. The efficiency can be as high as 99.99998% when tested 
to BS 4400 or can be produced with a micron rating in various stages 
between 1 and 50 microns. A further method of increasing the efficiency of 
the moulded filter material is by compressing the material while being 
resin impregnated and cured. 
A further material that can be used for the support sheet is a rigid metal 
foam. The fibres can be moulded directly onto such foam so that they 
penetrate only so far into the thickness of the foam sheet, but the fine 
mesh screen 44 can be eliminated in this process because the foam sheet 
itself provides the filter surface through which the water is drained. The 
same method can be used in the case of the aforesaid sintered support 
sheet. The same method can also be used with foam consisting of plastics 
material, which may be flexible or semi-rigid. However, very 
advantageously a rigid polyvinyl chloride coated plastic foam can be used. 
Among the many possible uses of the filter according to the invention are 
the removal of oil from compressed air, pre-filtration, aeration, vacuum 
filtration, liquid filtration, air sterilization and for pneumatic 
silencing.