Note: Descriptions are shown in the official language in which they were submitted.
FILTER ELEMENT WITH SLOTTED PRECOAT RETAINER SHEET
BACKGROUND OF THE INVENTION
The invention relates to a filter element for use
in a unit to filter or purify liquid. In particular, the
invention is directed to a filter element having a means
- or reducing gas entrapment in the ~ilter element during
S precoating and filtrat:ion.
Me~hods are known in ~he art for purifying liquitls
by passing them through a filter element which has been
precoated with a layer of ion exchange resîn par~icles,
such as a precoat medium in the size range of 60 to 400
mesh, as dlsclosed in U.S. Patent No. 3,250,703, issued
May 10, 1966, and a~signed to the assignee of this in~
vention.
In a typical system of this type R plurality of
filter elements are mounted within a filter tank. These
ilter elements of the prior art generally include a
stainle3s steel core havi.ng perforatlons or openings
therein, a layer of coarse wire screen positioned about
the core, and a layer of ine mesh wire screen surrounding
the coar~e screen for supporting the precoat ~llter
medium. A precoat layer of ~ilter medium is deposited
on the upstream sides of filter elements by passing a
water slurry of filter particles ~hrough the filter tank.
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The length of these filter elements i~ limited by the
expense of stainless elements and the size of the tanks that
can be effectively used for such filtration. Because of
this size limitation, it is desirable to precoat the entire
filter element with a filter medium.
However J during filling of the filter tank with
]iquidin order to deposit a slurry of precoat particles or
to commence purification ofliquid through the filter
elements, a certain amount of air or other gases typically
accumulates in the upper portion of each filter and is
trapped because of capillarity. As the pores of the filter
element are wetted by the liquid in the tank, an interface
is defined between the Liquid, air or other gases in the
filter tank, and the walls of each pore in the wire screen
supporting the filter medium. The surface tension of the
liquid across the pore creates a force that must be over-
come for gas to pass through the pore. The net gas pressure
in the pore which is in equilibrium with the surface tension
force caused by capillarity at the largest pore in a filter
element is defined as the bubble point pressure, according to
Aerospace Recommended Practice (ARP) 901 of the Society of
Automotive Engineers, Inc. The lower the bubble point
pressure for a given filter element, the lower the probability
that gas will be trapped in a filter element during pre-
coating or filtration.
It is known in the art that this entrapped gas hasat least two deleterious effects on the filtration system.
First, the entrapped gas prevents the flow of slurry
through and precoating of those areas o the filter elements
in which ~he gas is present~ thereby creating unprecoated
areas in which liquid with impurities can pass ~hrough the
filter element. Second, the presence of entrapped gas
during the filtering cycle of the system allows the gas
to be periodically released outwardly through the filter
3$ elements, which may disrupt or even remove portions of the
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precoated filter material.
The problem of entrapped gas within filter elements
is especially serious in filter elements wi-th absolute
particle retention abilities less than 50 microns and with
precoat filter media having low pressure drop characteristics r
such as powdered ion exchanye resins in the size range of
60 to 400 mesh, or smaller. The absolute particle retention
- ability is typically defined by the minimum dimension of the
largest pore in the element. Several attempts to overcome the
problem of accumulated gas have been disclosed and known
in the prior art, such as described in U.S~ Patent Nos.
3,680,700, issued August 1, 1972 and 3,779,386, issued
December 19, 1973, both of which are assigned to the
assignee of this invention. -
With the elements of the prior art a significant
volume of entrapped gas accumulates a-t the top of each
elemen-t. If the elements are mounted in a filter tank
having a bo-ttom tube sheet or plate between an upper
influent compartment and a lower filtrate compartment,
2~ the gas is trapped within the element core. If the filter
tank has a top tube sheet, the gas is trapped outside
each filter element. The outside surface area of each
element adjacent the entrapped gas volume is not pre~
coated. When the pressure in the filter tank increases
above the pressure during precoating causing the entrapped
gas volume to be compressed or the force of capillarity
to be exceeded or both~ unpurified li~uid flows through
the exposed unprecoated portion of the filter element.
The filter tank pressure increases typically in three
instances: first, when service flow commences; second,
if a flow surge occurs during service flow; and third,
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when undissolved impurities build up significantly on
the filter element. The flow of unpurified liquid
through the exposed portion o the element can result
in plugging of the element by particulates in the water
or premature termination of a filtering cycle by the
presence of impurities in -the effluent from the filter
system.
As disclosed in the above-men~ioned patent references,
attempts have been made in the prior art to overcome the
problem of entrapped gas. For instance, U.S. Patent Nos.
3,680,700 and 3,779,386 disclose a precoat filter element
having a dome-shaped cover or vent sleeve around the upper
portion of the element. The cover includes a vent hole
co~municating with the internal portion of the filter
elements. While this method is practical with filter
elements for general use with filter media in a size range
oE 60 to 400 mesh, it cannot be effecti.vely used with a
filter element having a low absolute particle retention
ability without substantial modiication of the apparatus.
When an absolute particle reten~ion ability of 50 microns
or less is required and a wire mesh cloth i~ used to achieve
this rating, such a filter element has been ~tneconomical be-
cause of the expense of material involved in providing the
necessary vent sleeve and the diminution in effective fil-
tering area of the filter element because of the length ofthe vent sleeve. This diminution is illustrated by the
critical length for sizing a vent sleeve, defined by the
height of the volume o entrapped gas which will be present
af~er the liquid fill step used in conjunction with the
filter tank if no vent sleeve were provided. This height
of the volume of entrapped gas is measured as the vertical
distance fro~ gas-liquid interface to the highest
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exposed portlon of the filter medium support layer at
the top of the filter element, and this height is pro-
portional to the bubble poin~ pressure of the layer of
the element having the highest bubble point pressure,
typically the outermost layer which supports the filter
medium.
Other attempts to overcome the problem of entrapped
gas in the prior art include addition of a surface-active
agent to reduce the surface tension of the liquid, and a
complete drying of the filter element before filling with
liquid, to eliminate the liquid providing the surface
tension force across an element pore. Addition of a
surface-active agent is not arl acceptable solution to the
problem because the surfactant promote~ an unacceptable
degree of ~oaming and the addition of such chemicals as
~urfactants to the liquid to be purified is often undesir-
able. Cornplete drying of the filter element be:Eore liqll:id
filling does elirninate entrapped gas, but utilizes a
significant amount of time, thereby lengthening ~he do~n
time of a filter tank system and reducingthe amount of ti~e
during which the system can be used for filtration.
SUMMARY OF THE INVENTION
. ~
Broadly, the in~ention contcmplates a filter element
adapted to be precoated with a filter medium, which comprises
a po:rous tubular core and means supporting the filter medium
precoat, the means including a discontinuously slotted shee-t
arranged concentrically about the -tubular core, each of the
slots having an absolute particle retention ability less than
about 50 microns, and an elongated configuration whereby gas
entrapment in the filter element during precoating and
filtration is reduced.
In a further aspect, the inven-tion comprehends a liquld
filtering apparatus including a filter tank and a plate
separating the tank into an influent compartment and a fil-
trate compartment. At least one filter element comprisesa vertically extending porous tubular core element and a
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discontinuously slotted sheet means positioned about the
tubula.r core for supporting a Eilter medi~ wherein the slots
are elongated, having a width of about 50 microns or less.
According to a preferred embodiment of the present
invention there is provided a filter element adapted to be
precoated with a filter medium including a porous tubular
core and a means for supporting -the filter medium precoat
including a discontinuously slotted sneet arranged con-
centrically about the tubular core. The bubble point
pressure of the filter element is generally decreased below
-that which would be expected from a filter element of the
prior art having a similar absolute particle retention
ability, thereb~ reducing gas en-trapment.
Whereas typical prior art element layers for support-
ing filter media have consisted of a fine screen or mesh
which is concentrically located over a coarse drain screen
ln turn positioned around a central tubular core, the
apparatus o-f the presen-t invention replaces the fine mesh
screen with a discontinuously slotted sheet. Each of the
slots of the present invention has an elongated configuration
with a width chosen to provide a desired absolute particle
retention ability, preferably less than 50 microns. The
length of the slot is chosen so that the bubble point
pressure is reduced significantly over a comparable fine
mesh, thereby diminishin~ the amount of entrapped gas with~
in the filter element.
According to another preEerred embodiment of the
present .~nven-tion, there is provided a filter element adapted
to be precoated with a filter medium, :i.ncluding a porous
tubular core and a means for supporting the fil.ter medium
precoat, positioned about the tubular core and terminating
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a distance from an end of the tubular core, so that
a relatively low bubble point pressure element section
is defined. The filter elemen-t also includes a
disconti.nuously slotted sheet means extending around the
circumEerence of the tubular core at the relatively low
bubble point pressure element section and sealed to the
precoat support means.
A further preferred embodiment includes a ven-t
sleeve positioned over and attached to the slotted sheet
means and having an aperture covered by the slotted sheet
means. Alternatively, the slotted sheet means is
positioned beneath the vent sleeve aperture, but does not
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extend around the circumference of the tubular core, and
the vent ~leeve is sealed to the precoat support means.
These embodiments achieve the advantages o~ reduction of
gas entrapment in the fllter element by providing a low
bubble point pressure means at a section of the element
having an area ~or gas release through a discontlnuously
slotted sheet~ This section is positioned in the filter
tank so that the maximum driving force of gas for ex-
pulsion acts on this section.
In the preferred embodiments, the slotted sheet means
is preferably diffusion bonded to an adjacent member of
the filter element, typically a coarse w:ire mesh layer.
Particularly for an embodiment having the slotted sheet
means as an outer layer extending the full length o the
filter element, this di~fusion bonding has several
advantages. Points of bonding contact between the slotted
sheet means and the coarse wire mesh are numerous, ~hereby
reducing the possibil.ity o~ folding or crimping of the
slotted sheet means during increase and reversals in the
liquid flow and force, and resultant stress and tearing
of the slotted sheet means. Also, dif~usion bonding at
numerous points of contaet provides substantial support
for ~he slotted sheet material, and reduces the possi-
bility of widening of the slots from physical mishandling
when compared with fine wire mesh screen of similar abs~lut~
particle retention ability.
According to some of the preferred embodiments, the
apparatus of the present invention having an outer slotted
sheet precoat support means along the entire length
of the filter element overcomes the disadvantages
of the prior art by significantly reducing the bubble point
pressure of the filter element, and in particular the
outer layer of the element, which generally has the greatest
bubble point pressure in the filter element and is there-
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fore the determinative layer in setting the hubble point
pressure. With the discontinuously slotted outer sheet
layer of the filter element o the present inven~ion,
particle retention of a si~e smaller than 50 microns can
be obtained, while simultaneously providing a filter
element that has a low resis-tance ~o the passage of gas
e~en when wetted. A filter element i9 thereby provided
with a par~icle retention equal to tha~ of an element using
fine woven wire cloth as a covering, bu~ with a superior
ability to release gas as compared to those filter elements
sing the size and mesh of woven wire cloth required for
the sa~e absolute particle retention ability.
Furthermore, the apparatus of the present invention
including the discontinuously slotted outer layer is less
prone to irreversible plugging than filter elements using
woven wire cloth with similar absolute particle retention
ratings.
The result o~ these advantages is that the preferred
embodiments of the present invention provide apparatus
which reduce the cost of a filter system having a given
effective filtering area by p~rmitting more of the filter
element area to be effectively precoated by a filter medium
and utilized during a filtration procéss.
Other advantages, objects, and features of the
present invention will become apparent upon reading the
following detailed description o~ the preferred embodiment
in conjunction with the accompanying drawings.
BRI~F DESCRIPTION 0~ T~IE DRAWINGS
FIGURE 1 is a partial cross-sectional view o~ a
typical filter tank having replaceable, cylindrical filter
eleme~ts which embody the present invention;
FIGURE 2 is a perspective view of a filter element
;
according to the present invention, partially cut away to
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show a central support core, a layer of coarse screen, a
discontinuously slotted sheet providing a support for the
filter medium, and a vent sleeve;
FIGURE 3 is a perspective view of an additional
preferred embodiment according to the present invention,
partially cut away to show -the relationship of the slotted
sheet means to other layers of the element and a vent
sleeve; and
FIGURE 4 is a perspective view of another preferred
embodiment according to the present invention, partially
cut away to show the relationship of the slotted sheet
means to other layers of the element and an aperture in a
vent sleeve.
DESCRIPTION OF THE PR~ERRED EM~ODIME,~TS
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Referring now to the drawings, and more particularly
to FIGURE 1, a filtering device utilizing the apparatus 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. Patent No. 3,27~,608, and is assigned to
the assignee of this invention. The filter device 10 is
adapted to receive an influent stream, filter the influent
stream, and discharge the filtrate of e~luent strea~n
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 down-
wardly curved tube sheet plate 17 suitably secured to the
interior of the tank 10 by welding or the like. The
in~luent line 12 extends through the bottom 13 o~ the tank
and communicates with the in~luent zone 15 ~o that all the
influent water is passed directly to ~he influent zone 15.
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The influent pipe 12 is at-tached to -the tube sheet plate 17
by welding or the like. In thi.s manner, direc-t communlca-tion
between the influent zone 15 and the filtrate zone 16
is precluded.
Mounted within the influent zone 15 are a 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 have a reduced bubble point
pressure in accordance with some preferred embodiments of
the present invention, provided by a discontinuously
slotted sheet 86 as shown in FIGURE 2. Alternativel.y the
filter elements 18 may embody the present invention as
illust.rated in FIGURES 3 and 4. Each filter element 18
is held in place ln -the influent zone ].5 of the filter
tank 10 by a holding assembly indica-ted generally by
reference numeral 20. Th:is holding assembly 20 includes a
centering pin 94 on a cover member 90 as shown i.n E'IGURES
2-4, and is adapted to releasably hold -the filter element
18 in place upon a filter seat means 30 which is attached
to the tube sheet plate 17. The fil-ter elements 18 are
placed into and removed from the filter tank 10 through a
small manhole opening 22 in the filter tank 10. The man-
hole openiny 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 f.ilter tank 10 is also prov,ided with a vent 26
and a spare nozzle 28, which in this instance is capped.
The vent 26 may be of any sultable construction, the
~election of appropriate vent means being dependent gener-
ally upon the use of the filter tank 10 and being within
the ordinary skill of one in the art.
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The filter seal 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 and the filtrate zone 16.
The filter seat means 30 provides a base for the filter
element 18, which is held in posltion on the seat means 30
by the holding assembly 20.
A filter element 18 accordlng to a preferred embodi-
ment of the present invention is illustrated in more
detail in FIGURE 2. This element 18 includes a tubular
support core 82, a layer of coarse screen 84 concentrically
disposed about the support core 82, and an outermost layer
of discontinuously slotted sheet metal 86 concentrically
disposed about and bonded to the coarse screen 84 for
supporting the filter medium of finely divided resin
particles, typically in the size range of 60 to 400 mesh
or smaller. The tubular support core 82 is porous, and
may be a perforated core 82 as shown in FIGURE 2, a core
of wound fibrous or packed granular material (not ShOWII),
or any suitable core member permeable to liquid. The
tubular support core 82 is preferably constructed of
stainless steelJ and provided with a plurality of symmetri-
cally spaced apPrtures to produce approximately twenty
percent open area on the outer surface of the support
core 82. The preerred range of percent open area or
perforation of the support core 82 i9 from five percent
to sixty-five percent, and the inside diameter of the
support core 82 preferably is between 3/4 inch and 2 1/2
inch.
The layer of coarse drain mesh or screen 84 is used
to support the outer slotted sheet layer 86 and to dis-
tribute the flow of liquld between the slotted sheet 86
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and the support core 82. The screen 84 is sized so as no~
to have any measurable impact upon the bubble point
pressure of the filter element in general. In the pre-
ferred embodiment the coarse screen 84 is abou~ 100 mesh
in siæe. The limi.ting member in setting the bubble point
pressure for the filter element 18 is the outer slotted
sheet layer 86. This sheet is formed of a suitable metal,
preferably .002 inch thick nickel, which can accept
rectangular slots 88 having a width of about 30 microns
and a length of about 430 mi.crons. The slotted sheet 86
is arranged for dimensional stability so that the length
of each slot 88 extends along the circumference of the
filter element 18, as shown in FIGU~ES 2, 3, and 4.
Although the slots 88 are shown as rectangular in
shape, they may be oval or any elongated shape, having a
width suitable to uniformly define an absolute particle
retentlon ability or the slotted sheet 86. The length oE a
slot 88 is at least about one and one-half times the smaller
dimension, thereby producing a large slot cross-sectional
area and promoting a low bubble pressure, while the desired
low micron rating for the absolute particle retention
ability of the sheet is achieved. It can be shown that
in a case where the slot length is much greater than the
slot width, the bubble point pressure of such a slot 88
approaches one-half the bubble point pressure of a
circular pore, the diameter of which is equal to the width
o~ the slot 88. For a slot with a width of 30 microns,
the length o the slot is preferably at least 45 microns.
However, the length of each slot cannot ~e so great
that the width dimension loses its required tolerance,
as may occur when each slot is effectively a continuous
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opening around the circumference of an element 18. Since
the degree of gas entrapment in a cylindrical element 18
increases as the bubble point pressure of the fil-tration
layer 86 incre~ses, the degree of gas entrapment associated
with some preferred embodiment of the element 18 covered
with the slotted sheet 86 is less than that associated
with a similar element covered by a filtration layer
containing circular pores or capillaries of the same
absolute particle retention ability.
A~ ~hown in FIGURE 2, the filter element 18 also
includes a dome-shaped cover member or vent sleeve 9û
which closes off the upper portion of the element 18. At
least one vent hole 92, and preferably three or four, passes
through theupper portion of the vent sleeve 90 so as to
permit fluid con~unication between the inside o~ the filter
element 18 and the area outside the filter element 18.
Although the holes 92 are sho~l in FIGURES 2, 3, and 4 as
being on a cylindrical side wall of the vent sleeve 90,
one or more apertures rnay also be placed on the uppermost
surface of the vent sleeve 90 and accomplish the desired
- results, if a slotted sheet 86, 96 or 98 is used according
to the present invention. In the preferred embodiroent of
FIGURE 2, the support core 82, coars~ screen 84, and slotted
sheet 86 all extend upward behind the vent holes 92. The
purpose of the vent sleeve 90 is to prevent entrapped gas
from contacting the precoatable surface of the element 18,
as defined by the slotted sheet 86 in the preferred embodi-
ment of FIGUP~ 2, and a fine mesh s creen 102 in FIGURES 3
and 4.
When the vent sleeve 90 is properly sized, a gas-
liquid interface occurs at the bottom of the vent sleeve
90 after the tank 10 is filled with liquid, and the
presence of this interface underneath the vent sleeve 90
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prevents gas from contacting the precoatable surace of
the elements 18. As the pressure of the filter -tank is
increased, which typically occurs during precoating of
~ilter elements 18, entrapped gas is compressed, moving
the gas-liquid interface upward and further insuring -that
gas is prevented from contacting -the surface of the filter
element 18 on which the precoat layer is to be placed.
The critical length A needed for sizing the vent
sleeve 90 is equal to the height of the volume of entrapped
gas which would be present after a water fill step if no
vent sleeve 90 is used. The critical length A is measured
as the vertical distance from the gas-li~uid inter:Eace to
the highest exposed slots at the top of the element, and
the distance is proportional to the bubble point pressure
of the slotted sheet ~6. The gas-liquid interface occurs
within the tubular support core for elements which are
mounted in a filter tank lO having a bo~tom tube sheet 17
as shown in FIGURE 1, and the interface occurs on the ves~sel
side of the filter element when a top tube sheet design
~not shown) is utilized, as is known in the art.
The bubble point pressure referred to in this appli-
cation is defined by the bubble point test method used in
Aerospace Recommended Practice (ARP) 901, issued March
1, 1968, by the Society of Automotive Engineers, Incor-
porated. In short, to obtain a bubble point pressure fora particular filter, the filter i5 immersed in the test
liquid to wet and saturate the pore structure of the
filter. Gas pressure is applied to the inside of the
filter element so that the liquid that has wetted the
filter element is displaced by the gas. The gas pressure
is slowly increased until the first steady stream of gas
bubbles is observed as emitting from a point on the filter
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element. The bubble point pressure is defined as the
measured gas pressure required to for~l the first bubble
released from the filter element and is essentially equal
to the pressure force which is in equllibrium with the
surface tension force at ~he largest opening in the outer
filter element layer.
According to the preferred embodiment of the present
invention as shown in FIGURE 3, the filter element includes
a discontinuously slotted shee~ band 96 positioned under
the vent sleeve 90. The slotted sheet band 96 has the
same preferred slot dimensions, material constxuction,
and other limitations as the slotted sheet ~36. However,
the filter precoat is supported along the exposed length
of the filter element 18 by a fine mesh screen 102,
positioned around the coarse screen 84 and the support
core 82. As illustrate~ ln FIGURE 3, the :Eine mesh screen
102 terminates a distance from the end of the tubular core
82 so that an element section is de:Eined having a bubble
point pressure determined by the coarse screen 84, and
therefore relatively low by comparison with the fine mesh
screen 102. The slotted sheet band 96, having a lower
bubble point pressure than the fine mesh screen 102 but
greater than the coarse screen 84, is positioned over
the relatively low bubble pressure element section
and sealed to the fine mesh screen 102 at point B by
welding or other suitable methods, to prevent passage
of liquid through the filter element 18 without puri-
fication. The vent sleeve 90 is positioned over the
slotted sheet band 96, and is attached to the band 96
by any suitable means also to prevent passage of liquid
through the filter element without purification. As
illustrated in FIGURE 3, vent holes 92 of the vent sleeve
90 are covered by a portion of the slotted sheet band 96,
and entrapped gas is released from the filter element 18
throush the relatively low bubble pressure slotted sheet
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band 96 and the vent holes 92. l~e dimensi.ons of the slots
in the band 96 are chosen to have an absolute particle
retentiOn ability equal to tha-t of the fine mesh screen
102, in order that impurities which reach the slo~ted
sheet band 96 will not pass through.
With the preferred embodiment of the apparatus of
the present invention shown in FIGURES 2 and 3, it may be
found, depending on the parameters of the filter element 18
and filter unit 10, that no air is entrapped in the filter
elemen-t 18, as discussed in Example II below. In such
circumstances, the critical length A for sizing the vent
sleeve 90 may be substantially diminished and the structure
of the vent sleeve 90 is changed to operate as a cover
member, as is known in the art, with release of gas
accomplished through the slotted sheet 86 or the slotted
sheet band 96.
According to an additional preferred embodiment of
the present invention, illustrated in FIGURE 4, the fine
mesh screen 102, coarse screen 84, and support tube 82
are arranged as in the embodiment of FIGURE 3. However,
a discontinuously slotted sheet disc 98 is secured to the
vent sleeve 90 immediately below the aperture 92 by
tack welding, diffusion bonding, or any suitable means,
thereby providing a relatively low bubble pressure member
for release o:f entrapped gas while also permitting
impurities which inadvertently reach the vent sleeve
aperture 92 to be trapped by the slotted 3heet disc 98.
Therefore, the slot dimensions of the disc 98 are chosen
so that the disc 98 has an absolute par~icle retention
ability equal to that of the fine mesh screen 102.
Also, the vent sleeve 90 is attached to ~he fine mesh
screen 102 by welding or other suitable means at point C.
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The slots of the disc 98 are climensioned as discussed
with reerence to the slotted sheet 86, and the prefer~ed
material construction and other limitations of the slotted
sheet 86 are applicable to the disc g8.
Particularly with respect to the preferred embodimen~s
o~ FIGURES 2 and 3, it is preferred that the slotted sheet
86 and theslotted sheet band 96 be se!cured to the coarse
wire screen 84 by a process of diffusion bonding at
numerous points on the sheet 86 and band 96. However,
because the band 96 is positioned beneath the vent sleeve
90, other suitable bonding and sealing processes may also
be used for that embodiment. For use with a slotted sheet
86 or band 96 of nickel and a coarse screen 84 of stain-
less steel, the preferred diffusion bonding method, some-
times referred to as sintering, includes pressing the
sheet 86 or band 96 against the coarse screen 84 in a
hydrogen atmosphere or a vacuum, and bringing the temper-
ature of the materia]. to about 2000~ F :Eor a period o:E
time, thereby causing a diffusion or particles between
the sheet 86 or band 96 and the coarse screen 84
effecting bonding.
In the operation of the apparatus shown in FIGURE 1,
a liquid slurry of the precoat medi~, in this instance
finely divided ion exchange resin particles in the size
range of about 60 to 400.mesh or smaller, 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 co~nects 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 from the pump 38.
The liquid 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 be-
tween the control transfer valve 4~ and the inlet line 12.
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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 slurry 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 is deposited upon the
upstream sides of the filter elements 18, i.e., the sides
where the liquid 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 finely divided ion
exchange resin particles.
~ uring 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 pre-
coating step is i~itiated 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
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the inlet line 12 into the filter tank 10. The pressure
of the incoming slurry forces the demineralized water in
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 l:ine 54 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 oE the
precoat medium are separated from the slurry and deposited
as the precoat layer upon the upstream suraces of the
filter elements 18. The slurry i~ circulated through the
filter system in this manner until a sufficient depth of
lS the resin precoat layer ls deposited upon the upstream .
surface of the filter elements 18. The precoating step is
terminated by closlng the slurry valve 36 and the return
valve 56. Then the filter system is ready to be used t~
treat the feed water or liquid. The thickness o the
pr~coat layer on the slotted sheet 86 is not critical,
but it is preferred that the layer have a thickness in
the range o 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
liquid enters the fllter system through the eed 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 liquid forces :it through the resin preeoat layer,
the fllters 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.
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As the untreated liquid passes through the precoat
layer, an ion exchange reaction takes place to remove
dissolved impurities from the liquid. In addition, undis-
solved impurities are removed from the untreated liquid by
virtue of the liquid passing through the precoated filter
elements 18. Filter cake, consisting of the undissolved
impurities, builds up within and on the precoat layer as
the process continues. The purified liquid is directed to
a supply tank or suitable equipment by the service line 48.
Eventually the resins will become exhausted and must
~e 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 gasl 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 bottom. The air is introduced into the interiot
of the filter element 18 by opening a valve 66 in the air
line 68 communicating with -the out`:Let 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 fil~er
ele~ent 18. Preferably, the :Elow rate of the air is in the
range of about 1 to 2 standard cubi.c feet per minu~e per
square foot of filter surface area, while the water flow
range 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 mi.nute. The air and water entering the filter
tank lO therefore tend to pass first through the upper
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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 el~ment 18.
After the tank 10 fills ~o a level about six inches above
the tops of the filter element 18, the ~alves 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, respectlvely. A somewhat
higher liquid flow rate, e.g., 1-2 gallons per minute per
square foot of filter, is employed during this step. Aîr
is also delivered at about 1.5 standard cublc feet per
minute per square foot. After the tank 10 has filled to a
level above the tops of the filter elements 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 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 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 or a third time.
Ater 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 appli-
cation of a new precoat, as previously described.
Though air has been discuss~d as the cleansing gas,other gases ~ay be used as the cleansing gas, such as
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nitrogen, oxygen and the like. Air, however, is generally
speaking, the most economical and it is readily available
in most plan~s. Similarly, liquids other than water may be
used during the backwashing cycle. Exemplary of such liquids
are alcohols, cabron tetrachloride and detergent and soap
solutions. It is preferred that the liquids have a tempera-
- ture 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 phelonic
type. These may be used in sodium, hydrogen, or ammonium
form, for example. Typical solid anion exchange resin
particles that may be employed are the phenol~ormaldehyde
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 ~orm, for example.
Suitable resins are sold commerclally in the large bead
form under the trade names Amberlite IR-120 and ~mberlite
IRA-400, sold by Rohm & Haas Company; and Dowex HCR-S
and Dowex SBR-P, sold by Dow Chemical Company. The finely
di~ided resins are prepared by re~ucing the particle size
range. These resin particles are regenerated and washed
prior to use.
I~e examples below are intended-to set Eorth appli-
cations o~ the apparatus of t~e present invention, and not
to limit the scope of -the present invention.
_xample I
~ Bubble point pressure measurements were made on
filter elements having an outer layer comprising 165 x 800
stainless steel wire mesh cloth and on a ~ilter eLemene
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having a slotted sheet material according to the pres~nt
invention as an outer layer. The slotted sheet material
had a slot dimension of 30 x 430 microns. When used as
filtration layers, both the 165 x 800 steel wire mesh cloth
and 30 x 430 microns slotted sheet material have an absolute
particle retention ability of about 30 microns.
The bubble point tests were run according to the
procedure recommended in the Aerospace Recommended Practice
document (ARP) 901 published by the Society of Automotive
Engineers, Inc., issued March 1, 1968. Raw data were
corrected, also according to ARP-901, to obtain the standard
bubble point pressure for each element based in isopropanol
at 77 F. The results were that the standard bubble point
pressure in isopropanol at 77 F for 165 x 800 wire mesh
clo-th is 6.9 inches of water, and the standard bubble pOillt
pressure for the slotted sheet ~ilt~r element of the precoat
invention with 30 x 430 micron slo-ts is 4.2 inches of water.
Therefore, the filter element having an outer slotted
sheet layer according to the present invention had a lower
bubble point pressure than a prior art element having the
same absolute particle retention ability. This reduction
in bubble point pressure allows more air to escape through
the filter element of the present in~ention during pre-
coating and filtering steps than through prior art ele~ents.
~xample II
A filter element constructed with 165 x 800 wire
mesh cloth was tested in a pilot laboratory ~or use as a
precoat support filter. After fill and precoat steps on
the filter system about 9 inches of the top of the filter
element were left unprecoated because of gas entrapment.
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In light of the bubble point pressure data which were
accumulated in Example I, the degree of gas entrapment
expected with an element employing a 30 x 430 micron
slotted sheet as an outer Layer would be less than that
for the wire mesh cloth element, however it would be
expected that some gas would still be entrapped, and a
portion of the element wowld be unprecoated~ calcula~ed
by the ratio of bubble point pressures of the slotted
sheet and wire mesh elements, as about 5.5 inches.
However, when an element covered with the 30 x
430 micron slotted sheet layer of the present invention
was built and tested, it was found that the element could
be precoated over its entire length, indicating that no
gas was entrapped. Thus, gas is eliminated by the present
invention without using a vent sleeve or other cover
member.
Though the filtering apparatus described above has
been di~cussed in relation to a precoat l.ayer o finely
divided lon exchange precoat particles, the apparatus is
likewise applicable where the precoat layer is diatomaceous
earth, cellulose fibers, polyacrylonitri`le fibers, or
any other precoat material, as will be understood by one
with ordinary skill in the art. Moreover, though the
embodiments and refinements which do not depart from the
~5 true spirit and scope o the present invention may be con-
ceived by those skilled in the art. It i9 intended that
all such modifications be covered by the following
claims.
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