Note: Descriptions are shown in the official language in which they were submitted.
~L39~4
This application is a divisional of C~anadian Patent
Application Seridl No. 319,636 filed January 15, 1979.
This invention relates to apparatus for forming tubular
filter elements.
Canadian Patent No. 1,077,864 issued May 20, 1980 to
the present applicant and entitled "Filter Elements For
Gas or Liquid and Methods for Makin~ Such Elements" describes
a method of forming a filter element which comprises dispersing
a mass of fibres in a liquid to form a slurry, draining the
liquid throu~Jh a lilter surlace on whicll tllc Libres 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 ma-terial to a predetermined distance above
the supporting material, removing the collected fibres containincJ
the sheet oE supporting material Erom the Eilter surface, and
bonding the fibros to one another and to the supportincJ material
by means of a synthetic resin.
One aspect of -the present invention is based on further
~0 experiments that demonstrate unexpectedly satisfactory results
if, in the aforesaid method, the shee-t of supporting material
is omitted or, iE 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 is disclosed and which comprises dispersing
a mass of fibres in a liquid to form a slurry, applying
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the dispersioll under ~ressure to a f.i:Lter surface so that
the fibres collect as a layer covering the Eilter surEace
while the liquid passes throuc~h the filter surface, and
bonding the fibres in the collectecl mass of fibres, after
drying to one another by means oE a synthe-tic resin. A sheet
of material that is to provide a support :Eor 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. ~rhus~ when the
supL~ort sheet, wllich may L~e pl^ovi(le(l by vei-y fine nlesh material,
is removed, the fibres are found to have pene-trated through the
support sheet leaving their outer surface flush with the
outer surface oE the support sheet, which mcly be a .Laye:r of
expanded metal. :[n the past, in the case of cy:Lindric.~ .ilter
elements, these have :required the acldit:ion oE a separate
support sheet to (I:ive~ strength, but the p~esent mouldincJ nlethod
enables the filter cylinder and support sheet to be produced
as an integral part in one opera-tion wlth precision, saving
time and labour.
The present invention relates to apparatus for forming
a compacted mass of fibres comprising a cylindrical filter
screen, a central core centrally moun-ted in and spaced from
the filter screen, an annular filter screen ex-tending between
the core and the cylindrical f:ilter screen so as to provide
a base of a container providing a moulding space otherwise
defined by the core and the cylindrical filter screen, a
duct for delivering a dispersion of fibres in liquid peri-
pherally around the top of the moulding space for accumulating
a tubular mass of the fibres in the moulding space while the
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liquid is drained throucJh the scre~ens, means for receiving
-the drained liquicl ancl removing it from the vicinity of the
screens, means for applying pressure to the dispersion
throughout its introduction into the moul~ing space, and a
reciprocable imperEorate screen in sliding contact with the
cylindrical filter screen for progressively uncovering the
cylindrical filter screen as a mass of fibres builds upwarcls
along the moulding space from the annular filter screen, -the
imperforate screen thereby increasing the area of the filter
L0 screen through which the li~uid can drain as the mass of
fibres accumulates.
In general, it is desirable to make -the apertures and
open area of the support sheet as large as possibLe. ~lowever,
it is difficult to specify the~ largest aper-t-lre that can be
used. The smallest aperture at presen-t contemplated is 0.25 mm
diame-ter. tlowever, it must be~ remenlbered that certain fibres,
such as potassium polytitanate, e.g., potassium clititanate,
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 fil-ter surEace, expanded metal with
narrow flat strips between overlapping elonqated apertures,
an aperture size of 0.75 mm by 0.5 mm has been found to be
practical. This yives a good surface finish. 1 mm by 0.75 mm
will of course give a somewhat rougher Einish.
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
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of 1~ -to 2~ of the total ~low.
In a modification of the aEoresaid method, the support
sheet consists of a rigid ~oam or sintered material thereby
eliminating the necessity Eor -the use of the very fine mesh
material in the production of the filter element.
When, as in the aforesaid prior specification, the binder
is used not only to boncl the fibres bu-t also to the support
sheet, this may be, for example, silicone, polyurethane, expoxy
or phenolic resin. ~eat cured resins arc 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 majori-ty of the fibres
beiny disposed so that they are directed, in some measure,
approximately in parallel with Otle atlOther . '['tli5 gives
particularly aclvantacJeous results, whether or not a support
sheet is used. ~ccording to a further aspect of the invention,
therefore, a filter element comprises a mass of ibres compacted
together and bonded to one another wi-th a synthetic resin, a
majority of the Eibres being disposed so that -they are directed,
in some measure, approximately in parallel with one another.
In order that the invention may be clearly understood and
readily carried into efEect, exan)ples of the invention will now
be described with reference to the accompanying drawings, in
which:
Figure 1 is a sectional elevation of part of a filter
element;
Figure 2 is an enlargement of a portion of Figure l;
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Fic~ure 3 is a sectiorla:L el.evatioll of part of another
filter element;
Figure 4 is a diagralll showinq appa.ratus for manufacturing
a filter element;
Figure 5 is a sectional elevation o~ a detail of the
apparatus of Figure 4;
Figure 6 is similar to Fic~ure 5 bu-t relates ta a different
phase in the operation O e the apparatus;
Figure 7 is a sectional elevation of a further filter
element; and
Figures 8 to 18 show portions of various sealing arrangements
~ for the ends of filter elements.
; The portion o:E the fil-ter element shown in Fi~ures 1 and
2 may be part of the wall oE a cylindrical :ilter element
although i-t can equally well be regarded as part o~ a disc,
sheet or conical or frus-to conical cylindrical shape (for
example closecl at one end as shown in F`igure 7). A similar
me-thod may also be used for the production of concave or convex
discs. The bulk 1 of the filter element comprises fibre
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material; for example, glass, ceramic, synthetic fibres,
asbestos, mineral wool, organic or silicate fibres. Raw
borosilicate microEibre 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 suppor-t sheet
2 moulded thereto so that the fibrous mass penetrates through
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the aper-tures in the sheets to present surfaces tha-t are flush
with the outer surfaces of the sheets (Figure 2). Each support
sheet consists of an apertured or open pore ri~id material
such as a perforated, expanded or woven material which, in turn,
may be of metal, phastics, ~lass or ceramic. Expanded metal
is a preferred material.
The filter element of Figure 3 is simi:Lar to that of
Figure 2, but only one support sheet 2 is used. Where one
support sheet is used, this is ~enerally located on the downstream
10 side of the fibres. This not only cJives 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 SUppOl^t sheet
can be of comparatively light cons-truction but, when usec:l in a
high pressure syst.em, either with cJas or liquicl, the support
sheet can be of lleavier construction.
In a further example consisting of a cylindrical ~ilter
element, no support sheet is used. This example COllSiS ts of a
tube made from raw borosilica-te microfibre moulded by pressure
2~0 : formin~ into the cylindrical shape by a method as described
below with reference to Figures 4 to 6. The moulded tube is
then dipped into a solution oE resin in a solvent so as t:o
: impregnate the Eibrous 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:
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V.O.P. UlJI~S~ LO~ ~p p O.D.T.
~ bclr NM~I bar bar % w/w
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9'3.999 7.0 45 .069 7.0 15.0
99.97 7.0 45 .06~ 3.0 27.0
99.90~7.0 48 .035 3.0 25.0
99.80~7.0 52 .035 3.0 21.0
99.80~7.0 50 .035 ~.0 35.0
In the above table p is the operatinc~ test pressure, L\p iS -the
pressure loss across the filter below ancl O.D.T. is the ratio
L0 of the oven clried total wei-ltlt of resin to tl~e fibre conten-t of
the filter element. The binder used in all the filter elements
represen-ted in the above table was a silicone resin, which is
preferred, but many other binclers carl be llsecl-to give comparative
test results. 'rhe highest res~ content wll:ictl is in the :la~t
tabulated examEjle, is 35'~ but this can be raisecl as h:iclll as 100
while still prov:kling ~aL:isl.lctory chalacteristics. Ilowever,
25~ has been ~oulld adlnirably satis~actory t`or most aE)E)lications.
The effect Oll performance of varying the wall thickness
of an unsupported tubular filter element is shown i.n 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 nlnl inside ~ meter Lor ~xa~ le (b):-
SAMPI.E D.O.P. FLOW ~p p O.D.T
_% MN~/II bar bar % w/w
(a)99-99 306 .017 4.2 16.0
(b)99.999 107 .017 4.2 16.0
In the above table the pressure p is a gauge pressure
above atmospheric pressure while L~p, of course, is a pressure
differential.
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The above table shows that it is effectively only the
flow capacity and efficiency that is affectecl 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, exempliEied by the above tables, are
believed to arise from the packing pattern of the fihres that
arise as a result of a methocl of manufacture such as described
below with reference to Figures 4 to 6. This packing pattern
results from the fibres lying in some measure more uniformly
L0 in a circumferential direction round the Lilter elelne~nt, 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 eEEiciency.
A}though the fil-ter elelllent describe(~ immediateLy above have
no rigid support shee-t, they can be provi~led with an inner, outer,
or both inner ancl ou~t~r lay~r oE woven or non-woven Elexible
material to improve -the hanclling characteristics. Such a layer
can be inc~rporated during the manuEac-ture of the filter element
; by a method as described below. The fibres would generally
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penetrate through an aperture or pore structure of the flexible
material. Moreover ln the case of a filter element with a
slngle rigid suuport sheet as shown in Figure 3, the opposi-te
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.
Figure 4 shows diagrammatically apparatus for forming
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a tubular filter ~lelllent. When this apparatus is in operation,
water and borosilicate miel~o~i~res are fecl into a blending
tank 31. ~Iydrochloric or sulphuric acid is added until the
pH value reaches 2.~ 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 solu-tion
temperature is increasecl to about 35C. The quality of the
fibres that are used depends on the grade of the filter elemen-t
that is to be used. rrhe 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 adclitlonal streng-th prior to resin impregnation.
The final dispersion is eE~Eectecl by a mecllanical ac~i-tator 32 and
takes about 15 minutes.
With valves 33 allcl 34 cLosed ancl 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
adrnit compressed air to the pressure vessel 37. Generally the
pressure used is 3.5 bar. 'I'his top prcssure is the forming
pressure and can be varied according to the efficiency required.
The efficiency can be varied within a ranye, e.g., 99.9~ to
99.999~, usiny the same fibre blend. The forming pressure may
be as low as 0.3 bar, bu-t 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
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dispersion to LLow into a molllclLncJ rig 38 shown in detail in
Figures 5 and 6. The mouldincJ ric) includes inner and outer
vertical cylinders 39 40 defininy a space 41 through whieh
the dispersion can flow into a cylindrica~ mouldinc~ spaee 42
defined between a fine mesh screen 44 supported by a maehined
perforated cylinder 45 and a core 43 when in the position of
Figure 5. Yigures 4 and 5 show the filter element being moulded
as a unit with an outer rigid cylinclrical support sheet 2 but
it will be appreciated tha-t for a simple borosilica-te mircofibre
filter tube this can be omitted. Alternatively of course
an inner support sheet can be moulded into the inside surface
of the tube eithe~r insteacl o~ or as an adclition -to the outer
sheet 2. The bottom of the moulclirlcJ space is eovere(l by a
fine mesh sereen 46. A reciproca~:le slee~ve 47 is moullted to
slide outside the eylinder 40 and perEoratecl c~ylindcI^ 45.
With the core ~3 and sleeve 47 in the positions shown in
F'igure 5 the water drains away throuc~h the screen 46 and lower
end oE the screen 44 into a tank 48 (FicJure 4) while the
mass of fibres becJin to build up in the moulding space 42.
After all the fibres have accumulated in the moulding spaee the
air pressure is maintained so as to remove residual water from
the fibres and so dry the ~ormed Lilter. The valve 34 is then
elosed. Durinc3 the moulding process a pump 49 continuously
pumps the water from the tank 48 to a holding tank 50 ~rom 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 i-t has been found
that the time taken to moulcl a tubular filter element 250 mm
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lon~, 65 null outside di~ t~l with a wall thickness oC 10 mm
takes ayproximcltely one~ minute. 'I'he formed fil-ter 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 Eilter 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 poin-t where the mass of fibres
is building up. The movemen-t oE the sleevc 47 then terminates
as shown in Fiyure 6.
The core 43 is formed wi-th an upper por-tion 51 of reduced
diameter. This is to enable an additional internal layer of
fibrous filter material to be added to the ~ilter mate~idl
formed in the mouldlng space 42, by Eeecting a further dispersion
through the cylinder 39 into a mouldinq space S2 (FicJure 6)
between the moulding space 42 and the core portlon Sl 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
c~raded de~nsity to be produccd ~s part or un integr.ll process.
Investigations have shown that the fibres in a finished
filter element produced by the method described above with
reference to Figures 4 to 6 are predominantly layered in planes
perpendicular to the direction in which the dispersion flows
into the moulding space. Lt has further been found that the
the same packing pattern arises -throughout -the range of forming
pressures that can be used effectively in practice. Advantages
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of this packin-J pattern appear from -the results -tabulated
above.
For some ap~lications of the invention, where cellulose
fibres or combinations of cellulose fibres with borosilica-te
fibres are used, A melamine or phenolic resin binder may
advantageously be used Eor 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 wi-tll 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 difEererlt fibre materials with cellulose. It has
been found that by blending 20'~ borosilicate microibre wi.th 80
cellulose by weight the production tinle Eor the E:ilter can be
reduced by 30~. Ln this case whe~n the fluicl is water the
pressure drop (~p) across the Eilter was 0.15 bar with a flow
rate of 16 litres per minute. With a weiqht to weight ratio
of 50%, Ap was found to be 0.15 bar with a Elow rate oE 22
litres per minute. The glclss fibre size (diameter) was 3.8 to
5.1 microns and the cellulose a bleached sof-twood kraft. The
bonding material, e.g. melamine resin, phenolic resin or other
synthetic resin, can be a~plie~ in one of three difEerent ways.
Firstly, by forming a mass of fibres in a moulding rig such
as shown in Figures 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
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pressure in the mouldincJ ri~J ancl curincl the mass~ Thirclly, all
the fibres and resin solution can be mixed in a single tank,
passed to the mouldiny rig, the mass beiny subsequently eured.
~ eylindrieal filter element for liquid filtration haviny
a eombination of fibres as described above may have an outside
diameter of 64 mm, a wall thickness of 18 mm and various lengths,
sueh as 250 mm. No support sheet is necessary for many uses but
ean be added when necessary. lrhe Eilter is preferably arranyed
for flow from outside to inside the cylinder to give grea-ter
surfaee area ~or eollection ol dirt. 'l'~-~is ared ean be inereased
by forming lonyitudinal or eircumferential yrooves in -the
outside surfaee of the eylinder.
Instead oE usincJ a coMpressecl clas to apply ~ressure to
the slurry in the moulclin~ ricl, a hydr.l-llie pump lll,ly L)e used,
this pump being arran~Jecl to withclraw the slurry From tl~e ~lenclillq
tank and force it into tlle moulclirlcl riLcl.
Tubular or cylindrical ~iL-ter e:lements made in aeeordanee
with the inven-tion may be mounted in a variety of Eil-ters, in
partieular those shown in l;`icJures 5, 6, 7 and 13 in the
aforesaid speeifieation. ~s in tha-t speeiEieation, also the
ends of the eylindrieal filter elemen-ts may be fitted into end
eaps in a variety oE ways. ~ueh ways are shown in FicJures 8 to
17 of the present speeifieation.
Figures 8 to 13 show eases where the end of a eylindrieal,
unsupported filter elemen-t 10 is fitted into an end eap 11 using
a gasket seal 21 (Figure 8), a double taper seal 13 (Figure 9),
an outside taper seal 14 (Fiyure 10~ an inside taper seal 15
(Figures 11 and 12) and a double taper flange seal 16 (Figure 13).
For a eylindrieal filter element with an inside support sheet 17
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an outside taper seal 14 (l;`i~lure l4) may be used. F`or an
outside suppor-t sheet 18 (~igure :L5) an inside taper seal 15,
or a single taper flange seal 19 (Figure 16) may be used. In
the case of a filter element having insid~ and outside support
sheets 20, 21 (l~ligure 17) a (Jasket seal 12 (Figure 17) can be
used. In all forms of the Filter element constructed according
to the invention, an open pore ~ilter layer of sleeve, as shown
in Figures 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 C.lll be all open ~ore plastic or metal foanl 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 FicJure 6 ol- the aforesa:id specification.
~'igure 18 shows an arran{Jealllent similar to Figllre 15 with an
internal supportincJ spring 2~ in place of any inside support
sheet.
Fil-ters made in accordance with the invention can be usecl
Eor either yas or liquid ~Eiltra-tion. The efEiciency can be
as hiyh as 99.99998% when testecl as ss 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 compressincJ the material whi:Le being resin
impre~Jnated 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 Eoam so that they pene-trate 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 i-tself provides the
filter surface through which the water is drained. The same
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method can be used in the case of the aforesaid sintered
SuppQrt sheet. rt'he same me-thod 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 EiLter according to
the invention are the removal oE oil from compressed air,
pre-filtration, aeration, vacuum filtration, liquid filtration,
air sterilisation and for pneumatic silencing.
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