Note: Descriptions are shown in the official language in which they were submitted.
1~21~8f~7
TWO LAYER WOUND-ON-CORE MEDIUM FILTER ELEMENT
BACKGROUND OF THE INVENTION
. .
The invention relates to a filter element for use in
a unit used to filter or purify liquid. In particular, the
invention is directed to a filter element having two types
of layers of windings over a central support core, and to a
method of purifying liquids with such an element.
In the prior art one type of apparatus for filtering
liquids typically includes a perforated cylindrical support
core and windings of strands of material around the support
core, such as disclosed in Goldman U.S. Patent No. 1,751,000.
These filters are adapted to remove undissolved particles
from liquid as the liquid is passed through the filters in
a direction from the outside, through the wound material,
and into the support co~e. The undissolved particulate
matter is trapped by the wound material.
Other filter ele;ments in the prior art disclose a
number of layers of material wound around a central support
core, wherein the layers vary in density. As disclosed in
Snow U.S. Patent No. 3,680,709, the density decreases in a
radial direction away from the support core because the windings
are tighter close to the support core and are gradually re-
duced toward the outside.
It has been found that such prior art filter elements
ha~ing a plurality of layers, wherein outermost layers are
less dense than the innermost layers, present difficulties
resulting from fouling tendencies, backwashing limitations,
and inadequate flow distribution. These disadvantages are
d~
~ --
.,, ~
~ 6 7
particularly noted when the wound filter elements of the
prior art are used to support a precoat of particles in the
si2e range of about 60 to 400 mesh. ~he precoat particles
typically are particles of cation and anion exchange resins.
These precoated filter elements are used to purify water
by reducing dissolved and undissolved impurity concentrations
from levels of approximately 50 parts per billion to less
than 10 parts per billion.
Filter elements of the prior art having a graduated
density of windings decreasing in a direction away from the
central support core are easily blinded by particulate matter
which infiltrates towards the center of the wound layer or
layers, as liquid is passed through the filter element in a
service cycle direction. Because of close spacing of windings
near the support core, it is difficult to achieve velocity
flows of liquid in a backwash direction through the filter
element sufficient to dislodge and remove particulate matteE
which is trapped within the filter element windings.
SUMMARY OF THE INVENTION
According to the present invention there is provided
a ilter element, having a plurality of layers of windings,
which overcomes the disadvantages of the prior art by improving
flow distribution patterns thereby facilitating backwashing
of the filter element, and by reducing fouling tendencies in
a service cycle direction. These advantages are obtained in
the present invention which comprises a perforated tubular
- 2 ~
;~
)867
support core, an innermost layer of filter material disposed
about the tubular core, and an outermost layer of filter
material disposed about the inner layer. The outermost
layer has the ability to trap smaller particles than the
innermost layer. This ability, expressed in terms of the
particle size, is called the nominal particle retention number.
In one broad aspect, the invention comprehends
a filter element, precoated with particles in the size
range of 60 to 400 mesh, consisting essentially of a
perforated tubular core. An inner layer of strand material
is wound about the core, the inner layer having a nomlnal
particle retention number between twenty-five and one hundred
microns, and an outer layer of strand material is wound
about the inner layer, the outer layer having a nominal
particle retention number between one and twenty-five microns.
A precoat of particles in the size range of 60 to 400 mesh
is supported on the outer layer.
The invention further comprehends a filter element,
precoated with particles in the size range of 60 to 400 mesh,
comprising a perforated tubular core with a plurality of
layers of strand material wound about the core. The nominal
particle retention number of the innermost layer is greater
than the nominal particle retention number of the outermost
layer and eaah of the layers, in addition to the innermost
layer, has a nominal particle retention number less than an
adjacent inner layer. The filter element is provided with
a plurality of layers having nominal particle retention
numbers which gradually decrease from the innermost layer
to the outermost layer, and a precoat of particles in
the size range of 60 to 400 mesh supported on the outer layer.
,, --.
367
In another aspect, the lnvention comprises a method
for purifying liquids comprising precoating a filter having
layers of strand material with particles in the size range
of about 60 to 400 mesh. The filter has an outermost layer
with a nominal particle retention number less than the
nominal particle retention number of the innermost layer
of the ilter, with the nominal particle retention numbers
gradually decreasing from the innermost layer to the
outermost layer. The method delivers liquid to be filtered
through the precoated particles and the fllter periodlcally
backwashing the filter and removing the precoated partlcles,
precoating the filter with a fresh coatlng of particles in
the size range of about 60 to 400 mesh.
Such an arrangement of the windings, with the layer
having the largest nominal particle retention number closest
to the support core and the layer having the smallest nominal
particle retention number furthest away from the support
core, confines the area for filtering or purifying liquids
..~,.;~
passed through the filter elements to substantially the layer
at the outer surface of the element. Such confinement of
f:ilter area into a relatively thin annular volume is parti-
cularly desirable when the filter element is used in filter
UllitS in which the filter elements are precoated with finely
divided resin particles in the size range of 60 to 400 mesh.
In precoated filter elements according to the in~ention,
the area which has the tendency to foul is substantially re-
duced, and undissolved particles will not be lodged tightly
within the element below its outer surface so as ~o make them
difficult to remove by backwash methods.
Also, if resin or other particles are forced through
the outermost layer in the event of a flow surge of liquid
through the element, the particles will become distributed
throughout the layers of the wound element. In this manner,
some particles will be trapped by the windings as a result
of the circuitous path they take through the filter element,
but the open area remaining for flow of liquid will not be
diminished to a point where the filter's ability to pass
liquid is significantly reduced.
According to the present invention, backwashing is
also improved because the outermost layer presents a greater
pressure barrier to backwash liquid coming from the support
core than does the innermost layer of strand material.
Therefore, backwash liquid tends to distribute itself uni-
formly along the length of the filter element when confronted
with the increased pressure barrier produced by the outer
layer.
~ 7
Other advantages, objects, and features of the present
invention will become apparent upon reading the following
detailed description of the preferred embodiment in con-
junction with the accompanying drawings.
~RIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a partial cross-sectional view of a typical
filter and tank having replaceable, cylindrical filter elements
which embody the present invention;
FIGURE 2 is a perspective view of a filter element
according to the present invention, par~ially cut away to
show the wound layers of strand material and the central
support core; and
FIGURE 3 is a cross-sectional view of the filter
element of FIGURE 2, taken along line 2-2, and illustrating
the types of relationship of the wound layers of strand
material to the central support core.
DESCRIPTION OF THE PREFERRED E~ODIMENT
Referring now to the drawings, and more particularly
to FIGURE 1, a filtering~device which may be employed to
carry out the method of the present invention is generally
indicated by reference numeral 10. This device i9 of the
type which i8 shown and described in U.S. Patent No~ 3,279,608,
which is assigend to the assignee;of this application. The
filtering device 10 is adapted to receive an influent stream,
filter the influent stream, and discharge the filtrate or
effluent stream.
The filter tank 10 is a generally cylindrical vessel
made of steel or the like having an outwardly convex top 11
and an outwardly convex bottom 13. The tank 10 is divided
-- 6 --
~ 8 ~ ~
into an influent zone 15 and a filtrate zone 16 by a down-
wardly curved tube sheet plate 17 suitably secured to the
i.nterior of the tank 10 by welding or the like. The infl~ent
line 12 extends through the bottom 13 of the tank and communi-
cates with the influent zone 15 so that all the influentwater is passed directly to the influent zone 15. The in-
fluent pipe 12 is attached to the tube ~heet plate 17 by
welding or the like. In this manner, direct communication
between the influent zone 15 and the filtrate zone 16 is
precluded.
Mounted within the influent zone 15 are plurality of
filter elements 18 through which the influent stream must
pass before entering the filtrate zone 16 and being discharged
from the filter tank 10 through the outlet iine 14. The
filter elements 18 are annular-shapèd wound filter elements
having a plurality of layers with controlled particle re-
tention ability in accordance with the presènt invention.
Each filter element 18 is held in place in the influent zone
15 of the filter tank 1~ by a holding assembly indicated
generally by reference numeral 20.~ This holding assembly is
- ` adapted to releasably hold the filter element 18 in place
upon a filter seat means 30 which are attached to the tube
sheet plate 17. The filter elements 18 are placed into and
removed from the filter tank 10 through a small manhole
opening 22 in the filter tank lO. The manhole opening 22
has a cover means 24 which may be removed or opened, as
desired, to provide access to thq interior of the filter tank
10 .
867
The filter tank 10 is also provided with a vent 26
and a spare noz~le 28, which in this instance i~ capped.
The vent 26 may be of any suitable construction, the selection
of appropriate vent means being dependent generally upon the
use of the filter tank 10 and being within the ordinary skill
of one in the art.
The filter seat means 30 comprises a small pipe
made of steel or the like which extends through a hole in
the tube sheet plate 17 and i8 attached to the tube sheet
plate 17 by welding or other suitable means. The filter
seat means 30 i5 substantially parallel to the longitudinal
axis of the filter tank 10 and provides communication between
the lnfluent zone 15 with the filtrate zone 16. The filter
seat means 30 provides a base for the filter element 18,
which i8 held in position on the seat means 30 by the
holding assembly 20. The filter elements 18 are typically
fifty to eighty inches in length and one to three inches in
outside diameter, and may consist of a unitary element or
seYeral cartridges, usually ten inches in length, which are
combined to form a single element.
A filter element 18 according to the present invention
is illustrated in more detail in FIGURES 2 and 3. This element
18 includes a tubular support core 82 and a plurality of layers
84 and 86 of wound strand material. The tubular support
core 82 i8 preferably constructed of stainleos steel, and
provided with a plurality of symmetrically-spaced apertures
to produce approximately twenty percent open area on the
outer surface of the support core 82. The preferred range
8~7
of percent open area or perforation of the support core
82 is from five percent to sixty-five percent, and the inside
diameter of the support core ~2 preferably is between 3/4
inch and 1 3/8 inch.
An innermost layer 84 and an outermost layer 86 are
formed, as is known in the art, by winding a continuous strand
of yarn or other strand material, such as nylon, orlon, poly-
propylene, cotton, and the Like, onto the suppor~ core 82
in a helical fashion. The cross section of the yarn or ot~Ler
strand material may be round, oval, triangular, or the like,
as long as the particle retention ability according to the
present invention is obtained. In fact, the cross-section of
; the yarn may vary as the yarn is wound, depending upon the
proximity of adjacent strands and the winding tension.
~; 15 Spacing between adjacent strands of material is 1/16-
inch or less, producing less than a one percent open area,-
and allowing protruding fibers or other irregularities on
the yarn surface to trap particles and thereby to augment
the overall particle retention ability obtained by particle
contact with the relatively solid twist of each yarn strand.
The particle re~ention ability of each layer of yarn or other
material is dependent upon several factors in addition to
spacing. The particle retention ability may be varied, for
example, by varying the tension under which the yarn is wound,~
the thickness of each~layer of yarn, or by changing the
pattern which is formed as the strand is wound back and
: ::
~ forth. Ability to retain particles can also vary with yarn
l:ype, yarn ma~erial, and with the way the yarn is handled.
For example, nylon yarn can be napped to produce a roughened
surface, which affects the particle retention ability of
the ultimate filter cartridge.
The manner in which the proper particle retention
ability is produced is not important in the present invention,
and most o the above-mentioned techniques are well known to
those skilled in the art. However, it is essential that the
filter element 18 be wound to produce the varying ability to
retain particles as set forth herein and described by the
phrase "nominal particle retention number." As used herein
the nominal particle retention number of a filter element
layer is the longest dimension of the smallest particle
whose percent removal by the filter element layer is 90 or
greater when the particles are introduced at a flow rate of
3.5 gallons per minute per square foot in the form of Fine
Arizona Air Dust in aqueous suspension at about 70 F. For
example, if a filter element layer has a nominal particle
retention number of 25 microns, then if a quantity of Fine
Arizona Air Dust is passed through a filter element in aqueous
suspension as described above, 90 percent or more of the
particles having a maximum dimension of 25 microns or more
will be retained by the filter layer. The influent con-
centration of the Fine Arizona Air Dust is not critical,
and tests have been run with a concentration of 100+25
milligrams per liter. -
- 10 -
Fine Arizona Air Dust is a commercially available
material which i9 utilized for making measurements of the
type made herein~ The material i9 obtained from nat~ral
Arizona dust, and prepared by A. C. Sparkplug division of
5 General Motors Corporation. It has the following particle
distribution:
Micron Ran~e Percenta~ae
0-5 39+2
5-10 18+3
10 10-20 16+3
20-40 18+3
40-80 9+3
Any other material having the same particle distribution
could also be employed in accordance with the present in-
vèntion.
In the preferred embodiment of the invention, there
are provided at least two layers of strand material, an
outermost layer 86 and an innermost layer 84. As used
herein, the term "layer" refers to windings of strand
material sufficient to produce a desired nominal particle
retention number uniformly along a filter element. Thus,
a "layer" may include many overlapped strands of material,
depending upon the partlcle winding pattern employed.
Conversely, one layer overlying another may not have any
visible discrete separation from the underlying layer, having
the same nominal particle retention number.
1~.2~867
The number of passes of the winding unit along the
support core and the incremental lead sittings for the
winding unit are determined by the desired particle retention
ability. The nominal particle retention number of the outer-
most layer 86 is less than the nominal particle retentionnumber of the innermost layer 84.
If layers àre included in the filter element 18 in
addition to the innermost layer 84 and the outermost layer 86,
it is preferred that each of the layers in addition to the
innermost layer has a nominal particle retention number less
than an adjacent inner layer. In this manner, the filter
element 18 is provided with a plurality of layers having
nominal particle retention numbers which decrease from the
innermost layer 84 to the outermost layer 86.
- For use with filter units adapted to reduce impurities
in water from about fifty parts per billion to about ten parts
per billion, and precoated with a mixture of cation and anion
exchange resins in the size range of 60 to 4Q0 mesh, the
preferred embodiment of the invention includes an outermost
layer 86 having a nominal particle retention number between
one and twenty-five microns, and an innermost layer 84 having
a nominal particle retention number between twenty-five and
one hundred microns. According to this preferred arrange-
ment of the invention, the filter element 18 is a surface
filter, i.e., a filter with a m~nimal depth of working filter
area, effectively equivalent to the depth af the outermost
element layer 86.
~ .
- 12 -
~lZ~8~i7
According to the present invention, the innermost
layer 84 of wound material provides improved back~ash flow
distribution by adding to the prPssure drop across the path
which backwash liquid must take before reaching the outer-
most layer 86. Furthermore, the innermost layer 84 augments
the area of the surface filter provided by the outermost
layer 86, in a manner which is less expensive than increasing
the diameter, and therefore the amount of material involved,
in the core 82. This increased area markedly increases the
efficient length of the run available before exhaustion of
the filter element 18 in a comparison with a filter element
having a primary fllter area near a relatively small diameter
support core 82. The utilization of a filter element 18
according to the invention including an outermost layer 84
: 15 with a nominal particle retention number less than the
~ innermost layer 86 also provides the advantages of an inner
: volume of wound layers which trap particles forced through
~- the outer st layer 86, particularly during flow surges
of influent liquid. Such l'bleed-through" particles are
::~ 20 therefore usualLy trapped but do not significantly impair
~: fLow~af liquid through the filter in a service cycle direction~
In the operation of the apparatus shown in FIGURE 1,
a water slurry of the precoat medium, in this instance finely
divided ion exc~ange resin particles in the size range of
~: 25 about 60 to 400 mesh, is stored in a precoat tank 32. A
slurry line 34, controlled by a slurry valve 36, connects
the precoat tank with a slurry pump 38. A transfer line
:.
- 13 -
367
40 connects the pump 38 with the inlet line 12 of thé filter
tank 10. A transfer valve 42 adjacent the pump 38 and in
the transfer line 40 controls the passage of slurry or
liquid from the pump 38.
The water to be treated enters the filter system
through a feed line 44 having an intake control valve 46.
The feed line 44 is connected to the transfer line 40 between
the control transfer valve 42 and the inlet line 12.
The outlet line 14 from the filter tank 10 is con-
nected to a service line 48 and a precoat return line 50 at
a T-juncture indicated by reference numeral 52. The service
line 48 is connected to service units not shown, such as a
steam generator and the like, and has a service valve 54.
The precoat return line 50 is connected to the precoat tank
32 and has a return valve 56 to control the flow of liquid
~back to the precoat tank 32.
A bridge line 58 with a bridge valve 60 interconnects
the precoat return line 50 and the slurry line 34. A drain
line 62 with a valve 64 communicates with the inlet line 12.
20 ~ During the precoating step a precoat layer of finely
divided ion exchange resin particles in the size range of
about 60 to 400 meah i9 deposited upon the upstream sides of
the filter elements 18, i.e., the sides where thie water is
: ~,
introduced into the filter element 18. Similarly, during
the filtering step a filter cake builds up within and on
the upstream side of the precoat layer.
:, .
- 14 -
l~.Z~8~7
In preparing the fil~er system for operation the
:initial s~ep is to precoat the filter elements 18. To these
ends, the filter tank 10 is filled with low impurity water,
such as demineralized water. A slurry of precoat medium and
demineralized water is prepared in the precoat tank 32, the
precoat medium being ion exchange resin particles in the
size range of about 60 to 400 mesh.
During the precoating step all the valves are closed,
except the slurry valve 36, the transfer valve 42, the
return valve 56, and the bridge valve 60. The precoating
step is Lnitiated by starting the pump 38, thereby drawing
the resin precoat slurry from the precoat tank 32 and through
the slurry line 34 to the pump 38. The slurry is forced by
the pump 38 through the transfer line 40 and the inlet line
12 into the filter tank 10. The pressure of the incoming
slurry forces the demineralized water in the filter tank lO
through the filters 18 and out of the filter tank 10 via the
filtrate zone 16 and the outlet line 14. A portion of
~ demineralized water enters the precoat tank 32 through the
:~ 20 return line 50~ and a second portion is delivered to the
s~lurry line 54 through the bridge line 58.
As cycling continues the precoat slurry i9 brought
into contact with the upstream surfaces of the filter elements
~: 18. The finely divided resin particles of the precoat medium
~ 25 are separated from the slurry and deposited as the precoat
;~: layer upon the upstream surfaces of the filter elements 18.
The slurry is circulated through the filter system in this
J~ J. ?~ i 7
manner until a sufficient depth of the resin precoat layer
is deposited upon the upstream surface of the filter elements
18. The precoating step is terminated by closing the slurry
valve 36 and the return valve 56. Then the filter system
is ready to be used to treat the feed water. The thickness
of the precoat layer is not critical, but it is preferred
that the layer have a thickness in the range of about l/16
to 2 inches, more preferably about 1/8 to 1 inch, ~nd most
preferably 1/8 to 5/8 inch.
The service run is begun by opening the service valve
54 and the intake valve 46. In this manner, untreated water
enters the filter system through the feed line 44 and passes
through the transfer line 40 and the inlet line 12 into the
filter tank 10. The pressure of the incoming untreated
water forces it through the resin precoat layer, the filters
18 and the filtrate zone 16 into the outlet line 14
Following the establishment of the service flow, the transfer
valve 42 and bridge valve 60 are closed and the pump 38 is
stopped.
As the untreated water passes through the precoat
layer, an ion exchange reaction takes place to remove dissolved
impurities from the water. In addition, undissolved im-
purities are removed from the untreated water by virtue of
the water passing through the precoated filter elements 18.
Filter cake? consisting of the undissolved impurities, builds
up within and on the precoat Iayèr as the process continues.
The purified or treated water flows through the filtrate
zone 16 and the outlet line 14 to the service line 48.
- 16 -
~.~867
The purified water is directed to a supply tank or suitable
~equipment by the service line 48.
Eventually the resins will become exhausted and must
be regenerated or discarded. At this time the filtering or
S service cycle is stopped by closing the intake valve 46 and
the service valve 54. The filter tank 10 is then cleaned.
To these ends, the vent 26 and the drain valve 64 are opened,
and water plus a cleansing gas, usually air, are passed into
the interior of the filter element 1~ at its lower end to
clean the filter element 18 progressively from top to the
bottom. The air is introduced into the interior of the
filter element 18 by opening a valve 66 in an air line 68
communicating with the outlet line 14. At the same time,
water is introduced into the filter element 18 by opening
a valve 74 in the backwash line 76. Air under pressurè and
backwash water thereby enter the filtrate zone 16 and pass
upwardly into the interior of the filter element 18. Pre-
ferably, the flow rate of the air is in the range of about 1
to 2 standard cubic feet per minute per square foot of filter
surface area, while the water flow rate is about 0.5 gallons
per minute per square foot of filter. The drain valve 62 is
controlled 90 that the water level falls slowly, preferably
at a rate of about 10-15 inches per minute. The air and
water entering the filter tank 10 therefore tend to pass first
through the upper portion of the filter element 18 and remove
the precoat layer therefrom.
3867
After the filter tank 10 has been drained, the drain
valve 64 is closed, and the tank begins to refill with liquid,
which passes in reverse flow through the filter element 18.
After t~e tank 10 fills to a level about six inches above
S the tops of the filter element 18, the valves 66, 74 in the
air line 68 and backwash line 76 are closed, and the backwash
water is removed from the tank 10 by opening the valve 64.
The drain valve 64 is closed, and the filter elements
18 are again backwashed by opening the valves 66, 74 on the
air line 68 and backwash line 76, respectively. A somewhat
higher liquid flow rate, e.g., 1-2 gallons per minute per
square foot of filter, is employed during this step. Air
is also delivered at about 1.5 standard cubic feet per
minute per square foot. After the tank lO has filled to a
level above the tops of the filter element 18, the drain
valve 64 is again opened to permit the liquid level to fa~l
at a rate of about ten to fifteen inches per minute, whil~
the flow of air and backwash liquid is continued. The back-
wash valve 74 is closed, and draining with the introduction
of air only is continued for a short time to assure complete
draining. After the tank lO empties, the drain valve 6
and the air valve 66 are closed. The backwash valve 74 i5
opened, and the tank is permitted to fill for a third time.
After the tank 10 has filled, vent 26 and valve 74 on the
backwash line are closed. The tank 10 is filled wit'n water,
and the filter elements 18 are now ready for the application
of a new precoat, as previously described.
- 18 -
~ 6 7
Though air has been discussed as the cleansing gas,
other gases may be used a~ the cleansing gas, such as nitrogen,
oxygen and the like. Air, however, is generally ~peaking,
the most economical as it i~ readily available in most plants.
Similarly, liquids other than water may be used during the
backwashing cycle. Exemplary of such liquid~ are alcohols,
carbon tetrachloride and detergent and soap solutions. It
is preferred that the liquids have a temperature in the range
of about 100 to 200 F.
Typical solid cation exchange resin particles which
may be employed in the specific filtering method discussed
herein are the divinylbenzene-styrene copolymer type, the
acrylic type, the sulfonated coal type and the phenolic type.
These may be used in the sodium, hydrogen, or ammonium form,
for example. Typical solid anion exchange resin particles
that may be employed are the phenolformaldehyde type, the
divinylbenzene-styrene copolymer type, the acrylic type and
the epoxy type. The anion resin particles may be used in the
hydroxide or chloride form, for example. Suitable resins
are sold co~mercially in the large bead form under the trade
marks Amberline IR-120 and Amberline LRA-400, sold by Rohm
& Haas Company; and Dowex HCR-S and Dowex SBR-P, sold by Dow
Chemical Company. The finely divided resins are prepared by
reducing the particle size of these well known large bead
resins to the desired size range. These resin particles are
regenerated and washed prior to use.
- 19 -
)867
Though the filtering method has been discussed in
relation to precoat layer of finely divided ion exchange
precoat particles, the method is likewise applicable where the
precoat layer is treated or untreated diatomaceous earth,
cellulose fibers, polyacrylonitrile fibers, or any other
precoat material, as will be understood by one with ordinary
skill in the art. Moreover, though the embodiments herein-
before described are preferred, many modifications and refine-
ments which do not depart from the true spirit and scope of
the present invention may be conceived by those skilled in
the art. It is intended that all such modifications be
covered by the following claims.
:
:: ~
~ ~ .
- 20 -
.
