Corrugated filter element with external spiral tape support

A corrugated cylindrical filter element is provided having an external spiral tape support, comprising a corrugated filter sheet material in the form of a cylinder having an end cap at each end of the cylinder sealingly bonded to the filter sheet material at that end, the corrugations of the filter sheet material having a tendency to become distorted and displaced with respect to each other under the conditions to which the material is subjected in use, and a relatively narrow strip to continuous high-modulus fiber-reinforced synthetic plastic tape spirally wound around the cylinder, spanning the corrugations and extending between each end and bonded to the end cap and to the tips of the corrugations in a manner to prevent distortion and displacement thereof.

Corrugated filter elements which are designed for use in high pressure 
fluid systems must, of necessity, possess sufficient strength to withstand 
the fluid pressure to which they are subjected during use. As the 
contaminants accumulate upon the element, the differential pressure 
resulting from such accumulation increases, causing the corrugations to 
become distorted or displaced, or even to collapse, reducing the 
contaminant capacity of the filter element by 25% or more, and requiring 
that the element be replaced prematurely. This problem is particularly 
acute when the filter element is made of a very fine wire mesh, thin 
plastic membrane paper sheet, or other structurally weak material. 
Further, the elements are attached to or inserted into a variety of 
housings, and must withstand without damage the stresses imposed on them 
during assembly and removal. Examples of such stresses are the axial load 
imposed when an O-ring seal at the element open end is forced into its 
socket, and the torque imposed when a threaded end fitting is made up. 
A corrugated mesh filter element of fine wires can be strengthened by 
interposing a coarser mesh material of heavy wires between or upon the 
mesh of finer wires. Wires of larger diameter can also be interwoven with 
the finer wires. Both these expedients reduce the open pore area, and thus 
to a certain extent are self-defeating because reduction in pore area 
increases pressure drop across the filter, and a lower contaminant 
capacity results as well. The problem is to provide a sufficient 
resistance to distortion without reducing porosity, or increasing the 
pressure drop across the filter, or reducing contaminant capacity. 
Pall, Verrando and Silverwater U.S. Pat. No. 3,165,473, patented Jan. 12, 
1965, provide a corrugated metallic filter element having superior 
resistance to distortion, a high contaminant capacity, and a substantially 
undiminished open pore area, comprising a corrugated foraminous metallic 
material which has a tendency to become distorted under the conditions to 
which it is subjected to use, and a relatively narrow metallic cross-strip 
bonded to a surface thereof and following the corrugated contour of the 
sheet, in and out of the corrugations thereof in a manner to prevent 
distortion thereof. 
Humbert U.S. Pat. No. 3,241,680 shows a corrugated cylindrical filter 
element having spiral wrap of string or twine, which is glued to the tips 
of the corrugations. The spiral wrap does not extend into the end cap 
bonding area, so far as the drawings show. The string or twine has 
strength under tension, but readily collapses when stressed axially in the 
opposite direction, that is, under compression. For that reason, the use 
of string or twine has been proven to be unsatisfactory when applied to 
elements having corrugations which are made using less rigid filter media. 
With such filter media, string or twine-reinforced elements tend to 
collapse, due to compression by the hands of the operator, while being 
assembled or disassembled from their housings, with consequent damage to 
the filter element. 
Murphy et al U.S. Pat. No. 3,246,765 shows an inside-out filter cartridge 
in which the corrugated filter element is enclosed within two concentric 
foraminous tubes. The corrugated element is held in position by a 
plurality of adhesive strips, disposed between the upstream support tube 
and the filter element. The filter element has the undesirable feature 
that a substantial amount of filter area is blocked by the foraminous 
upstream support, and further, the adhesive strips both block area on the 
filter and close off a substantial number of apertures in the foraminous 
upstream support. Moverover, the adhesive strips shown by Murphy must be 
applied by hand, and therefore involve a time-consuming and costly 
assembly. 
Fricke et al U.S. Pat. No. 2,749,265 shows a corrugated filter sheet bonded 
to a foraminous core by cement that is applied to the core as a layer of 
sheet cement. Fricke et al disclose no perforations or holes in this sheet 
cement, but apparently Fricke et al intended the sheet to disintegrate 
when heated, as can be seen by reference to FIG. 7 of the Fricke et al 
patent. 
U.S. Pat. No. 3,570,675, patented Mar. 16, 1971 to David B. Pall and Tadas 
K. Jasaitis, provides a corrugated cylindrical filter element in which a 
cylindrical filter sheet is supported on a central core. A perforated 
plastic bonding sheet is disposed between the core and the filter. The 
bonding sheet is bonded in situ to the inner tips of the corrugations 
only, so that this provides resistance to back pressure but not to 
distortion or displacement of the corrugation folds. 
In accordance with the present invention, a corrugated cylindrical filter 
element is provided comprising a corrugated filter sheet material in the 
form of a cylinder having an end cap at each end of the cylinder sealingly 
bonded to the filter sheet material at that end, the corrugations of the 
filter sheet material having a tendency to become distorted and displaced 
with respect to each other under the conditions to which the material is 
subjected in use, and a relatively narrow strip of continuous high modulus 
fiber-reinforced tape of synthetic plastic material spirally wound around 
the cylinder, spanning the corrugations and extending between each end, 
and bonded to the end cap and to the tips of the corrugations in a manner 
to prevent distortion and displacement thereof. 
The continuous high modulus fiber-reinforced tape of synthetic plastic 
material serves as an external spiral support for the corrugations and 
imparts an impressive rigidity to the element. It accordingly makes 
unnecessary other external supports such as foraminous cylindrical sheaths 
or tubes, although such supplemental supports can also be used, if 
desired. 
Continuous high modulus glass fiber-filled synthetic plastic tape is 
commercially available, with or without a pressure sensitive facing, and 
has a continuous warp of glass filaments or fibers extending lengthwise, 
embedded in a matrix of the synthetic plastic tape material. The tape is 
relatively thin, in the range from 0.1 to 0.4 mm, with glass fibers or 
filaments of a diameter less than this, so as to be embedded therein. The 
tape is also relatively narrow, in the range from 3 to 8 mm, so as to 
block off as little as possible of the filter surface area at the 
corrugation tips to which it is bonded. 
The tape can be made with a continuous high modulus warp of materials other 
than glass fiber, for example carbon, Kevlar or high strength steel fibers 
could be used. Fibers with modulus in excess of about 5.times.10.sup.6 psi 
are preferred, and it is further preferred that all of the fibers be 
oriented lengthwise, parallel to the tape. 
Any suitable plastic can be used, provided that it has good aging 
properties when exposed to air and a variety of liquids, and providing it 
adheres well, or can be made to adhere well, to the reinforcing fiber. 
The adhesive can be applied at the time of laydown, and may be of the hot 
melt, solvent, emulsion, or pressure-sensitive type. An aggressive, very 
tacky glass fiber pressure-sensitive tape, of the type commercially 
available from companies such as Minnesota Mining and Manufacturing 
Company and widely used for package sealing, is preferred. An example is 
No. 890 filament tape, a packaging tape, which is a "Scotchpar" film 
backed tape reinforced with continuous glass yarn filaments. 
Synthetic thermoplastic materials that can be used are film-forming, and 
can be shaped into flexible strong tape form. Suitable thermoplastic 
materials include polypropylene, polyethylene, polyisobutylene, 
polyesters, polyamides, cellulose acetate, copolymers of vinyl chloride 
and vinyl acetate, polyvinyl chloride, polyvinylidene chloride, polyvinyl 
butyral, polystyrene, polytrifluorochloroethylene, synthetic rubbers such 
as butadiene-styrene and ABS polymers. 
The corrugated filter sheet of the instant element can comprise any of the 
filter sheets known to those skilled in the art, and can be made from any 
filter medium. Fibrous filter sheets made of materials such as paper, 
asbestos, paper-asbestor combinations, textile fibers, regenerated 
cellulose, microcrystalline cellulose, casein fibers, zein fibers, 
cellulose acetate, viscose rayon, hemp, jute, linen, cotton, silk, wool, 
mohair, glass, polyvinyl chloride, polystyrene, polyethylene, 
polypropylene, and polyacrylonitrile are generally preferred. A fibrous 
filter sheet can be woven, or it can be nonwoven, such as a felt, mat or 
bat. The filter can also comprise a plurality of woven and/or nonwoven 
layers, and can be resin-impregnated. 
The filter sheet can also comprise a fibrous or nonfibrous base upon which 
a fibrous and/or particulate layer is laid down. Preferable examples are 
disclosed in U.S. Pat. Nos. 3,246,767 to Pall et al, and 3,238,056 to 
Pall. Other filter media, such as wire meshes of stainless steel, sintered 
stainless steel, brass, Monel, iron, copper, aluminum and plastic can be 
used. These can have a fibrous or particulate material bonded thereto. 
Examples of a suitable metallic filter element are disclosed in U.S. Pat. 
Nos. 2,925,650 to Pall and 3,241,681 to Pall. Thin plastic filter 
membranes of cellulose acetate, polyamides, polypropylene, polyethylene, 
polysulfone, polyimides, and polyesters can be used. 
The filter sheet is corrugated and is formed in a closed configuration as a 
corrugated tube about a temporary or permanent support or core. The side 
seam of the filter sheet can then be sealed, and the composite is ready 
for bonding of the tips of the corrugated filter sheet to the tape. 
The tape in the completed assembly bonds the tips at the corrugations of 
the filter sheet in a fixed relation to each other. By forming such a 
bond, the filter element is given lateral support. This is due to the fact 
that substantially each corrugation in this assembly is bonded to the tape 
and held in a fixed relation thereto. Thus each corrugation can no longer 
flex away from the next, and the external periphery of the filter element 
therefore has a fixed circumference. 
Further, due to the high modulus and parallel disposition of the 
reinforcing fibers, each short section of tape joining the tips of two 
corrugations has high column strength, and cannot be readily compressed. 
Probably for this reason, a filter comprising a relatively non-rigid 
corrugation combined with the tape winding of this invention feels and 
behaves similar to a filter made with a more rigid filter medium, and can 
be manipulated and stressed without the damage which would otherwise tend 
to occur. The importance of the high column strength can be seen when a 
pressure sensitive tape backed by woven cloth, or made without 
reinforcement is used on the same basic corrugated element; the product is 
limp and collapses readily when compressed or otherwise manipulated and 
may be damaged as a result. 
This difference in behavior manifests itself particularly when the filter 
medium is thin, and itself relatively non-rigid and is deeply pleated. An 
example of such a filter medium is a lightly resin-bonded glass fiber laid 
down on a thin supporting medium such as a cellulosic paper, or on Reemay 
(a non-woven polyester made by DuPont) of low basis weight. The use of 
thin media as contrasted with thick, rigid medium results in more pleats 
for a given element size, hence more effective filter area. The use of low 
resin content, as contrasted with high resin content to get rigidity, is 
also preferred, as the resulting filter medium has higher open area, hence 
lower clean pressure drop and higher service life. Deep pleats also add 
surface area, hence result in increased service life for an element of a 
given size. 
It should be understood that the use of continuous high modulus warp filled 
tape will benefit any element by making it more rigid, but that the 
additional rigidity is most useful when applied to deeply pleated elements 
made with relatively thin non-rigid media. 
The tape is first spirally wound tightly about the filter cylinder, from 
end to end. Before so doing, a bonding agent or adhesive can be applied to 
the inner surface of the tape, in contact with the cylinder. A 
pressure-sensitive adhesive-backed tape can also be used. After doing so, 
the end caps are bonded to the filter cylinder. The tape may extend into 
the end bonding area, in which case the tape is bonded too, in the seal, 
using either a bonding agent, or embedding or potting the cylinder end in 
the end cap material. In this embodiment, the end cap has a peripheral 
flange, and the circumferential wrap is within the flange. 
If the tape is made of a thermoplastic material, the assembly of the 
temporary support, the tape and the filter element can be subjected to an 
elevated temperature appropriate to soften the plastic. This causes the 
tape to soften and the corrugation tips become embedded therein. When the 
tape cools, the corrugations of the filter sheet are bonded to the sheet 
such that each corrugation is fixed relative to the next, in a continuous 
manner, across the corrugations and along the length of the sheet. 
The temperature to which a thermoplastic or thermosetting tape is exposed 
is determined by the particular plastic used; the softening points and 
curing temperatures are well known to those skilled in the art. The 
material should not be melted except at the surface, since there otherwise 
is danger of disintegration. 
If a plastic tape softened by a solvent is used, the entire assembly of the 
temporary support, the tape and the filter sheet can be immersed in a 
solvent bath. The plastic sheet is softened thereby, and the corrugation 
tips become imbedded therein. When the solvent evaporates, the bonding 
sheet hardens, and it bonds the corrugations of the filter element 
together in the manner described above. 
The side seam of a corrugated filter sheet can also be sealed by applying a 
strip or layer of a bonding agent, such as a thermosetting resin, a resin 
cured by a curing agent, liquid or semisolid thermoplastic material and 
the like, between side edges of the filter sheet. 
By using the same or a similar bonding agent for both the tape, the end 
caps and the side seam, valuable time can be saved in the manufacture of 
the filter element. For example, if the corrugations are fixed by a 
thermoplastic tape, a thermosetting resin can be disposed along the side 
seam of the filter sheet. When heat and pressure are applied to the 
package, curing of the resin takes place. The filter sheet thus is sealed 
along its side seam and when the thermoplastic sheet cools, its 
corrugations are bonded together. 
When the sheet undergoes solidification by cooling, evaporation of a 
solvent, polymerization, cross-linking or the like, the temporary support 
is replaced by the permanent support core. This can readily be 
accomplished since the temporary support is not bonded to the corrugated 
filter sheet. 
The selection of the core and material used therefor are not dependent upon 
the bonding agent used to bond the tape to the filter element, since the 
corrugated filter sheet is not bonded to the core. The core need not be 
placed within the filter element until the bonding operation is completed. 
The core generally can be of any material, including metals such as 
aluminum, magnesium, stainless steel, steel, brass, iron, and alloys 
including these, and the like. Ceramic cores and polycarbonate cores can 
also be used. Plastic cores made of other materials such as polyvinyl 
chloride, polypropylene, polyethylene, phenol-formaldehyde, 
urea-formaldehyde, polystyrene, nylon, polytetrafluoroethylene, and the 
like are also suitable for use in the instant filter assembly. The 
material used for the core and its thickness will be selected by reference 
to the strength needed to withstand the pressures encountered in a 
particular fluid system. 
The permanent support core is preferably generally tubular in shape. 
However, it can be formed in any shape desired. All that is necessary is 
that the core fit within or around the filter element and have an open 
interior passage of fluid therethrough. 
The core can be formed with a smooth surface or it can be formed with a 
plurality of circumferential alternately raised and depressed portions 
along its length. These portions can be defined by a series of 
circumferential grooves or circumferential troughlike indentations in the 
surface of the core. The distance between one raised portion and the next 
preferably should not exceed two inches. This configuration facilitates 
the passage of fluid through the filter element since it reduces the 
amount of filter area blocked by the core and provides flow paths for the 
fluid. 
The core has a plurality of holes therethrough, well distributed along its 
length, in sufficient number to permit the passage of fluid from the 
filter element through the core. The number and size of the holes are 
determined by the acceptable pressure drop through the filter element. 
These, in the preferred embodiment, are disposed in the depressed portions 
of the core. This construction facilitates the passage of fluid through 
the core. The higher the acceptable pressure drop, the fewer the number of 
and the smaller the holes that need be provided. 
To remove the temporary support and replace it with the permanent core, the 
permanent core can be placed with its butt end against the temporary 
support and forced axially against it. This forces the temporary support 
outwardly from the tubular corrugated sheet, and replaces it with the 
permanent support. This can be accomplished manually, pneumatically, or by 
any other means known to those skilled in the art. It has been found 
advantageous to use a guide, such as a centering piece or boss to 
facilitate the removal of the temporary support member and the insertion 
of the permanent core into position. 
Naturally, if a permanent support that is deflatable or contractable is 
used, the temporary support need not be forced from the assembly but can 
be merely removed and replaced by the permanent support core. 
When the temporary support is removed, and the permanent support core has 
replaced it, the filter element is completed. 
If heat bonding is used, the permanent core can be introduced into the 
assembly before the filter assembly becomes completely cooled. In such a 
case, after the permanent core is introduced and the assembly cools 
further, the bonding sheet will contract and tightly grip the permanent 
core. 
The corrugations of the completed filter element are bonded to the tape and 
held thereby in a fixed relation to each other, such that they no longer 
are free to flex relative to each other. Thus, they will not be able to 
move away or be displaced away from the core, and the complete element 
will resist back pressures. However, since the corrugations of the filter 
sheet are not bonded to the permanent core and since it is not exposed to 
the method of bonding or the bonding agent used, the permanent core can be 
of any desired material.

To the corrugation tips 1 of the corrugated filter sheet 2, best shown in 
FIG. 1, is bonded, such as by a pressure-sensitive adhesive backing layer 
3, a tape 4 of glass fiber-reinforced "Scotchpar" film. The glass fibers 
are actually a warp 5 of glass yarn filaments extending lengthwise of the 
tape, and the tape is 6 mm wide and 0.012 mm thick. The tape does not 
reduce appreciably the flow area of the sheet, because of the narrowness 
of the tape. The tape spirals are parallel to each other, and terminate in 
one circumferential wrap about each end of the filter cylinder in the end 
cap bonding area. 
Next, the end caps are applied to each end of the filter cylinder, as seen 
in FIG. 2, melt-bonding the end caps to the ends over the ends of the 
tape, using the process of U.S. Pat. No. 3,354,012, patented Nov. 21, 
1967, to Foreman and Pall, producing the structure shown in FIG. 3. 
It will, of course, be apparent that the corrugated structure of FIG. 3 
need not be in cylindrical form, but could be employed in any closed 
tubular shape, as required by the particular filter unit in connection 
with which it is to be used. It will also be apparent that the tape can be 
applied after the element has been side sealed and end capped.