Apparatus and method

A filter element having a perforated tubular core and a plurality of layers of strand material wound about the tubular core. The outermost layer has the ability to retain particles smaller than those retained by the innermost layer. The size range of particles retained by the outermost layer is from one to twenty-five microns and the size range of particles retained by the innermost layer is from twenty-five to one hundred microns. According to the method of invention, the filter element is used to purify liquids through the steps of precoating, delivering liquid through the filter, periodically backwashing the filter, and precoating the filter again.

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. Pat. 
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 core. 
The undissolved particulate matter is trapped by the wound material. 
Other filter elements 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. Pat. 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 reduced 
toward the outside. 
It has been found that such prior art filter elements having 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 
particularly noted when the wound filter elements of the prior art are 
used to support a precoat of particles in the size range of about 60 to 
400 mesh. The 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 concenrations 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 matter which is trapped within the filter element 
windings. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a filter 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 
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. 
According to a preferred embodiment of the invention the filter element 
comprises a plurality of layers of strand material, and is adapted to be 
precoated with particles in the size range of 60 to 400 mesh. The filter 
elements are usually arranged in a filter tank unit through which liquid 
to be purified is passed, according to methods which are well known in the 
art. 
According to the method of the invention, liquids are purified by 
precoating a wound filter element constructed in accordance with the 
present invention with particles in the size range of about 60 to 400 
mesh. Liquid to be filtered is delivered through the precoated filter 
elements. Periodically the filter is backwashed to remove the precoat 
particles and entrapped impurities, and the filter is precoated with a 
fresh layer of particles. 
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 filter area into a 
relatively thin annular volume is particularly desirable when the filter 
element is used in filter units 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 invention, the area which 
has the tendency to foul is substantially reduced, and undissolved 
particles will not be lodged tightly within the element below its outer 
surface so as to 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 uniformly 
along the length of the filter element when confronted with the increased 
pressure barrier produced by the outer layer. 
Other advantages, objects, and features of the present invention will 
become apparent upon reading the following detailed description of the 
preferred embodiment in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, and more particularly to FIG. 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 is 
of the type which is shown and described in U.S. Pat. No. 3,279,608, which 
is assigned 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 into an influent zone 15 and a filtrate zone 16 by 
a downwardly curved tube sheet plate 17 suitably secured to the interior 
of the tank 10 by welding or the like. The influent line 12 extends 
through the bottom 13 of the tank and communicates with the influent zone 
15 so that all the influent water is passed directly to the influent zone 
15. The influent pipe 12 is attached to the tube sheet 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 
line 14. The filter elements 18 are annular-shaped wound filter elements 
having a plurality of layers with controlled particle retention ability in 
accordance with the present invention. Each filter element 18 is held in 
place in the influent zone 15 of the filter tank 10 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 10. The manhole opening 22 has 
a cover means 24 which may be removed or opened, as desired, to provide 
access to the interior of the filter tank 10. 
The filter tank 10 is also provided with a vent 26 and a spare nozzle 28, 
which in this instance is 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 is attached to 
the tube sheet plate 17 by welding or other suitable means. The filter 
seat means 30 is substantially parallel to the longitudinal axis of the 
filter tank 10 and provides communication between the influent zone 15 
with the filtrate zone 16. The filter seat means 30 provides a base for 
the filter element 18, which is 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 several 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 FIGS. 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 is preferably constructed of stainless 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 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 82 preferably is between 3/4 inch and 13/8 
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, polypropylene, cotton, and the like, onto the 
support core 82 in a helical fashion. The cross section of the yarn or 
other 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. 
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 retention 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 type, yarn 
material, 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 of 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 regularly shaped 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.degree. 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 
concentration of the Fine Arizona Air Dust is not critical, and tests have 
been run with a concentration of 100.+-.25 milligrams per liter. 
Fine Arizona Air Dust is a commercially available material which is 
utilized for making measurements of the type made herein. The material is 
obtained from natural Arizona dust, and prepared by A. C. Sparkplug 
division of General Motors Corporation. It has the following particle 
distribution: 
______________________________________ 
Micron Range Percentage 
______________________________________ 
0-5 39 .+-. 2 
5-10 18 .+-. 3 
10-20 16 .+-. 3 
20-40 18 .+-. 3 
40-30 9 .+-. 3 
______________________________________ 
Any other material having the same particle distribution could also be 
employed in accordance with the present invention. 
In the preferred embodiments 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 particle 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. 
The number of passes of the winding unit along the support core and the 
incremental lead settings for the winding unit are determined by the 
desired particle retention ability. The nominal particle retention number 
of the outermost layer 86 is less than the nominal particle retention 
number of the innermost layer 84. 
If layers are 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 
400 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 arrangement of the invention, the filter element 18 is a 
surface filter, i.e., a filter with a minimal depth of working filter 
area, effectively equivalent to the depth of the outermost element layer 
86. 
According to the present invention, the innermost layer 84 of wound 
material provides improved backwash flow distribution by adding to the 
pressure drop across the path which backwash liquid must take before 
reaching the outermost 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 filter area near a relatively small diameter support core 
82. The utilization of a filter element 18 according to the invention 
including an outermost layer 86 with a nominal particle retention number 
less than the innermost layer 84 also provides the advantages of an inner 
volume of wound layers which trap particles forced through the outermost 
layer 86, particularly during flow surges of influent liquid. Such 
"bleed-through" particles are therefore usually trapped but do not 
significantly impair flow of liquid through the filter in a service cycle 
direction. 
In the operation of the apparatus shown in FIG. 1, a water slurry of the 
precoat medium, in this instance finely divided ion exchange resin 
particles in the size range of 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 40 
connects the pump 38 with the inlet line 12 of the 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 connected 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. 
During the precoating step a precoat layer of finely divided ion exchange 
resin particles in the size range of about 60 to 400 mesh is deposited 
upon the upstream sides of the filter elements 18, i.e., the sides where 
the 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. 
In preparing the filter system for operation the initial step 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 initiated 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 10 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 return line 50, and a second 
portion is delivered to the slurry line 34 through the bridge line 58. 
As cycling continues the precoat slurry is brought into contact with the 
upstream surfaces of the filter elements 18. The finely divided resin 
particles of the precoat medium 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 
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 1/16 to 2 inches, more 
preferably about 1/8 to 1 inch, and 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 impurities are removed from the untreated water by 
virtue of the water passing through the precoat filter elements 18. Filter 
cake, consisting of the undissolved impurities, builds up within and on 
the precoat layer 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. 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 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 18 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 pressure and backwash water thereby enter the 
filtrate zone 16 and pass upwardly into the interior of the filter element 
18. Preferably, 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 so 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. 
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 the tank 10 fills to a level about 
six inches above 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 10 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 fall at a rate of about ten to fifteen inches per minute, 
while the flow of air and backwash liquid is continued. The backwash 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 10 empties, 
the drain valve 64 and the air valve 66 are closed. The backwash valve 74 
is 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 with water, and the filter elements 18 are now ready 
for the application of a new precoat, as previously described. 
Though air has been discussed as the cleansing gas, other gases may be used 
as the cleansing gas, such as nitrogen, oxygen and the like. Air, however, 
is generally speaking, the most economical as it is readily available in 
most plants. Similarly, liquids other than water may be used during the 
backwashing cycle. Exemplary of such liquids are alcohols, carbon 
tetrachloride and detergent and soap solutions. It is preferred that the 
liquids have a temperature in the range of about 100.degree. to 
200.degree. 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 commercially in the large bead form under the 
trade names Amberlite IR-120 and Amberlite 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. 
Though the filtering method has been discussed in relation to precoat layer 
of finely divided ion exchange resin 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 hereinbefore described are preferred, 
many modifications and refinements 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.