Note: Descriptions are shown in the official language in which they were submitted.
~ ~67
Background of the Invention
Porous gas diffusion elements have been used since
the lg2~'s for bubbling air into sewage in the activated sludge
process.
Such elements are formed of a body of solid parti-
cles which has been shaped, pressed and rendered coherent by
bonding or sintering in a compacted form having pores. Such
compacts have been manufactured in a variety of forms of plate
or disc-like configuration and mounted in holders in or near
the bottom of sewage treatment-tanks.. ~ dër pressure, from
a plenum beneath the element is forced upwardly through pores
extending through the body of the element to its upper surface,
from which the air is released in the form of bubbles whose
fineness is controlled in part by the sizes of the pores at
the upper surface. The air encounters some resistance in pass-
ing from beneath the element into the water, and it is widely
known that this includes frictional losses resulting from
passing the air through the fine pores of the element.
With the exception of occasional defective elements
20 ~which are inevitably produced in most manufacturing processes,
the manufacturers of diffusion elements have apparently assumed
that the quantity of air released from their upper surfaces was
distributed with reasonable uniformity across the entire sur-
face of the element. Although highly detailed design and
performance specifications are regularly applied to most com-
ponents of sewage aeration systems, stringent distribution
uniformity specifications have not been developed for diffusion
elements. Also, a widely accepted test for uniformity of air
distribution in a sewage aeration air diffusion element has
been to merely make a visual examination of the bubble pattern
emitted by the element while it is operating submerged in water.
~, ~ .
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~`s~f~
~3~;7
Moreover, persons skilled in the art have accepted this type of
test for many years. They have done so despite the fact that
it is quite difficult to visually ascertain whether a submersed
diffusion element is distributing air uniformly, due to the
disturbance created by discharge of bubbles into the water.
Moreover, if accurate methods have existed heretofore by which
one could compare the air output of different portions of the
area of a diffusion element, such techniques have not been
generally known and applied in commercial practice by persons
active-in the manufacture of sewage aerat~on~ diffùsion elements
and associated aeration systems. From this it might appear
that there is little or no need for detailed or stringent speci-
fications for the air flow distribution uniformity of diffusion
elements for aeration.
A bubble release pressure test developed by the
present applicants has made it possible to compare the rela-
tive ease with which different portions of the gas discharge
surface of a diffusion element will discharge bubbles. Through
the use of this test it has been found that the gas distribu-
tion properties of diffusion elements are not nearly as uniformas previously supposed~ Although randomly disposed disuniformi-
ties of gas distribution have been observed, use of the bubble
release pressure test referred to abovehas shown a trend for
some diffusion elements to discharge a disproportionate amount
of their total flow through a central zone. A larger quantity
of flow through a given zone results in an increased rate, which
tends to produce larger bubbles. Due to their reduced surface
area per unit volume, larger bubbles tend toward reduced gas
transfer efficiency, e.g. OTE, oxygen transfer efficiency.
Thus, in a sewage aeration process, passing a disproportionate
share of the total flow through the central portion of the
.:
-- 3 --
3~3~
diffusion element, while the outward or surrounding zone of
the e:Lement is underused, produces excessively large bubbles
and therefore reduced oxygen transfer efficiency.
The tendency to release a disproportionate share
of the total flow through a central zone may arise fr~m a
variety of causes. For instance, a diffusion element whose
permeability is substantially uniform across its gas release
surface may release an excessive proportion of gas in its cen-
tral portion due to the design of associated components, such
as, for example, the holder or mount for the diffusion element.
The configuration of the holder or mount may concentrate flow
through the center of the element. Also, diffusion elements
are known which have been manufactured in such a manner as to
provide lesser permeability, greater density or lesser height
in a peripheral annular zone of relatively small proportions.
For example, U. S. Patent No. 4,046,845 to Richard K. Veeder
discloses the concept of subjecting a relatively narrow annular
zone of a diffusion element to sufficient extra pressing to
prevent discharge of bubbles from said zone. Application of
the above described bubble release pressure test to such ele-
ments has shown that the effects of the extra pressing extend
a considerable distance into the element from the annular zone,
thereby considerably affecting the air distribution through the
element and providing substantial encouragement for dispropor-
tionate flow through the central zone of the element. The
present invention is aimed at the correction of these diffi-
culties.
Summary of the Invention
The invention provides a rigid, monolithic, porous,
gas diffusion element having an enhanced apparent volumetric
compression ratio in a permeable central portion thereof. The
-- 4 --
~336~7
element is formed of a body of solid particles which has been
shaped, pressed and rendered coherent by bonding or sintering
in a compacted form having pores. As viewed in vertical cross-
section the element includes a generally horizontal portion
having a specific permeability in the range of about 6 to about
200 SCFM at 2 inches of water gauge. The maximum horizontal
dimension of the aforesaid portion is in a ratio of at least
about 4 to 1 relative to the thickness of said portion. The
said portion also includes an upper gas discharge surface,
lo which is génerally horizontal and which has a bubble release ~
pressure in water in the range of about 2 to 20 inches of water.
The central zone is beneath a portion of the upper gas discharge
surface. Within this central zone the solid particles have
been pressed to a greater apparent volumetric compression ratio
as compared to the material in an outward zone beneath the gas
discharge surface.
The invention can be embodied in a wide variety
of forms, some of which are also considered to be useful inven-
tions or discoveries. For example, the foregoing diffusion
20 element may be embodied in a form having a peripheral annular ,
zone in which the said æone has a lesser permeability, a
greater density or a lesser height than a portion of the afore-
mentioned outward zone or of the gas discharge surface above it.
Among the inventions disclosed herein axe diffusion
elements in which the apparent volumetric compression ratio
of the central zone has been enhanced by distributing the
particles in the aforementioned body, prior to or during press-
ing, for providing a larger mass of particles per unit horizon-
tal area in the central zone, as compared to the mass of
particles per unit area in the outward zone. The particles may
be distributed prior to or during pressing by providing a
-- 5 --
3~
larger mass of particles per unit volume in the central zone.
Moreover the larger mass per unit area of particles may be
provided by performing the pressing in a die having a cavity
and fllling a central portion and a surrounding portion of the
cavity with said particles in respectively greater and lesser
depths.
In accordance with the invention, the enhanced
apparent volumetric compression ratio may also be provided
by effecting, during pressing, relatively larger and smaller
ratios of thickness reduction in the central and outward~z~nes
respectively; and this is true whether the height of particies
in that portion of the body of particles corresponding to the
central zone, prior to or during pressing, is substantially the
same or different, e.g. greater, than the height of the parti-
cles in that portion of the body corresponding to the outward
zone. These larger and smaller percentages of thickness reduc-
tion can be effected by performing the pressing in a press
having a ram and die cavity, with spaced, opposed compression
faces having respectivley smaller and larger clearances in
central and surrounding portions of the space between the
faces, whether the smaller clearances are provided by a pro-
tuberance on the compression face of the die cavity or by other
means.
The invention may be embodied in a wide variety of
forms including for example those having planar surfaces and
those having a depression above or below the central zone and
spaced inwardly from the periphery of the element. That is,
the element may be provided with a central depression or
depressions in its gas infusion surface, its gas discharge
surface or both. However, the aforesaid depression(s) may or
may not be coextensive with the central zone. The depth and
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3367
area of the depression(s) may be varied as desired for enhanc-
ing the uniformity of gas distribution laterally across the
horizontal gas discharge portion of the element, and the depth
may vary in portions of, or throughout, the depression(s).
There may be one or more areas within the outline of the
depression(s) which are not depressed.
According to a particularly preferred form of the
invention, there is provided a gas diffusion element having the
property of discharging gas in a substantially uniform manner
throughout an upper gas discharge surface of the element, said
surface having the property that its coefficient of variation
is not greater than about 0.25, said coefficient of variation
being based on the values of bubble release pressure measure-
ments at at least about 5 equally spaced points along each
of two mutually perpendicular reference lines extending across
said surface and through the center thereof.
Many other possible ~ariations of the invention
which are also considered to be inventions in their own right
are disclosed in conjunction with preferred and various other
embodiments discussed below and~or shown in the accompanying
drawings.
Brief DesCription of the Drawings
Figures 1 through 5 are fra~mentary diagrammatic
views in transverse, vertical cross-section, showing a prior
art technique'for shaping and pressing a body of particles
to form a gas diffusion element.
Figure 6 is similar to Figure 4, and illustrates
a modification of the process of Figures 1 through 5.
Figure 7 is a diagrammatic illustration, partly in
section, includlng a gas diffusion element as formed in Figure
6, an apparatus for determining the bubble releass pressure of
.,
~`
;7
-the element and a graphical representation of the bubble release
pressure and corresponding flow characteristics of the element.
Figure 7A is an enlarged portion of the probe of
Figure 7.
Figure 7B is an enlarged and forshortened portion
of the element and bubble release pressure curve of Figure 7.
Figures 8 through 16 are fragmentary, diagrammatic,
transverse sectional views disclosing diffusion elements in
accordance with the present invention and illustrative methods
for manufacturing same.
Figure 15A is a plan view from the top, of the
diffusion element of Figure 15.
Figure 17 is a transverse cross section of a
diffusion element in accordance with Figure 14 and an associated
1~ bubble release pressure plot exemplary of said element.
Figure 1~ is a transverse cross section of a
diffusion element in accordance with Figure 15 and an associated
bubble release pressure plot exemplary of said element.
iefinitions
Apparent Volumetric'Compression Ratio
For purposes of the present invention the "apparent
volumetric compression ratio" is used as a basis for comparison
of two or more zones of an element formed from a body of solid
particles which has been shaped, pressed and rendered coherent
by bonding or sintering in a compacted form havin~ pores. As
applied to a given zone, said ratio constitutes the quotient
obtained when the height of material prior to pressing is
divided by the height of material after pressing within said
- 7h -
~r~
zone, Although it is recognized that pressing may cause some
lateral migration of material from one zone to another, with
limited effects on the observed compression ratio, such migra-
tion can normally be ignored, hence the reference to the
compression ratio as "apparent". If the height of material
differs at different locations within a zone either prior to
or subsequent to pressing, an average height is used which
is weighted on the basis of area in plan view, The height
of all material subjected to compression is considered part
of the height prior to compression. Therefore, if an addi-
tional height of material is placed upon an original quantity
of material which has already been compacted, such as by
preliminary partial pressing or vibration compaction, the
height of the added material is included in the computation;
more specifically, in determining the quotient, the divisor
is the fully compacted height of all the material and the
dividend is the uncompacted height of both the original and
additional material.
Specific Permeability
The term "specific permeability" describes the
overall rate of passage of gas through a dry diffusion ele-
ment, and for purposes of the present invention is expressed
in standard cubic feet per minute per square foot of area
per inch of thickness at a driving pressure of 2 inches in
water gauge under standard conditions of temperature, pres-
sure and relative humidity (20C., 760 mm Hg. and 36~,
respectively). The specific permeability is calculated from
the equation G=Q(t/A), wherein G equals specific permeability,
Q equals flow in standard cubic feet per minute, t equals
thickness of the element in inches and A equals the mean
effective gas flow area through the element normal to the
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.
. ' `' "' .' ' ' ' ' ' '
33~7
direction of flow. If the gas discharge sur~ace of the
diffuser overlies portions of the element which are of varying
thickness, the average thickness is used, the thickness being
weighted on the basis of area.
Bubble Release Pressure
The "bubble release pressure" is used to characterize
resistance to discharge of air under water from a point or area
of the gas discharge surface of a diffusion element. As applied
to a given point of a given element, it constitutes the quasi-
static pressure which must be applied ~o réilease a ~ubble~--from
said point on the gas discharge surface. As applied to a given
area of the active gas discharge surface area of a diffusion
element, which given area may include all or a part of the
active area, the bubble release pressure is the mean of the
bubble release pressures observed at a statistically signifi-
cant number of points distributed over said area in a random
or uniform manner. For purposes of this disclosure the bubble
release pressure is expressed in terms of inches of water
gauge, after deduction of the hydrostatic head. The test may
be conducted using the apparatus disclosed in Figure 7 or
any other apparatus capable of producing data similar to or
convertible to an indication of bubble release pressure. The
values of bubble release pressure set forth herein were deter-
mined on a "quasi-static" basis in that the test apparatus
was adjusted to a sufficiently low rate of flow to release
bubbles slowly enough, so that the bubble release pressure
observed would be substantially that which would be obtained
under static conditions.
Coefficient of Variation
For purposes of the present invention, the "coefficient
of variation" is the quotient obtained from dividing the
, - . . . .
i7
"standard deviation" by the "mean"~ The "standard deviation"
represents the root mean square of the deviations from the
mean of a stated number of bubble release pressure measure-
ments. The "mean" is the arithmetic average of the aforesaid
bubble release pressure measurements.
Central Zone
The "central zone" of a diffusion element in accordance
with the invention constitutes a central portion of the volume
of the diffusion element which lies beneath a central area
lQ constituting a st~alted pèrcentage--of the total!~ctive ^gas~ais~
charge area of the element, it being understood that the bounds
of said volume may or may not coincide with the position or
positions of the edges of certain depressions which may be
applied to the surface(s) of the element in accordance with
the present invention. The "central zone" applies to diffusion
elements of varying outline in plan view, whether circular,
oval, square, rectangular, polygonal, irregular or otherwise,
and the above~mentioned central area has a similar outline
to, and a common center with, the active gas discharge surface
2Q of the element. In general, the central area which establishes
the bounds of the central zone may constitute up to about 80%
of the total active gas discharge area, more preferably about
6Q% and still more preferably about 40%.
Outward Zone
The "outward zone" includes all or a substantial portion
of the body of the diffusion element beneath the total active
gas discharge surface other than the "central zone". As
compared to the central zone the outward zone lies further
outward from the center of the element than the central zone.
3Q Other than the central zone, the outward zone may or may not
be the sole additional zone in the element.
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1~336~7
Center
The term "center" refers to the position centroid
or geometric center of the active gas discharge surface or of
the center of the element itself in plan view, whether the
element is of regular or irregular shape,
Peripheral Zone
The "peripheral zone", if such is present, constitutes
a portion of the volume of the diffusion element at or along
the edge of the active gas discharge surface of the element
which normally constitutes'the~outermost ëdgè or periphery~~
of the element. The peripheral zone, whether annular or non-
circular, is one in which the element has been treated by
pressing, including a combination of pressing with other tech-
niques, to develop a zone having lesser permeability (including
no permeability), greater density or lesser height than all or
a portion of the outward zone. An element with a peripheral
zone may or may not include a boundary zone.
Boundary Zone
A "boundary zone" may be situated adjacent to and
inwardly of a peripheral zone, when such is present, and may
be situated between the peripheral zone and the outward zone
preferably adjacent the outer edge of the outward zone. It
is a zone in which there is a progressive increase, continu-
ous or stepwise, in the apparent volumetric compression ratio
of the element in the direction of the peripheral zone, or
towards a vertical surface which is near the periphery of
the element, which surface may for example be a portion
of the side of an element, which portion is inwardly of and
in or adjacent to the peripheral zone.
Vertical
~ '
The term "vertical" includes truly vertical and near
.
: - ' ~ ' :.: ' , '
( ~
~1~3~;7
vertlcal, e.g. within about 20 of vertical.
Various Preferred and Other Embodiments
In accordance with Figures 1-5, a known technique of
form:ing diffusion elements employs a die 1 having a cylindrical
cavity 2 defined in part by a compression face or bottom wall
3, and side walls 4 and 5. The die is filled with a loose mix
of solid organic or inorganic particles with or without binder
which can be pressed and rendered coherent. For instance, one
may employ beads or granules of synthetic resin, such as poly-
ethylene or polystyrene, glass beads, granulesiof inorganiematerials such as metal particles, alumina, mullite, silica
and others. Organic and inorganic binders may also be included
in the mix. The mixes may be designed for developing coherency
by pressing and sintering and/or bonding, such as for instance
organic adhesive bonding, glass bonding, or ceramic bonding.
According to one known technique the die 1 is filled
above its upper surface 6 with the body of loose particulate
solid material 10. Then, as shown in Figure 2, the excess
material 11 is struck off with a screed 12 which is shown as
having partially completed formation of a level surface 13
over the entire surfaee of the uncompacted body 10.
Next, as shown in Figure 3, ram 17 having compression
face 18 is moved toward surfaee 13 and eventually engages same.
Further travel of ram 17 as shown in Figure 4 presses the body
10 in die eavity 2, converting the body of partieulate material
to compacted form 19. Withdrawal of ram 17 as shown in Figure
S permits one to remove the eompaet 19 from die cavity 2.
Depending on the mix used, the compaet may or may not be baked
or fired to produce the completed element.
By a modifieation of die cavity 2 shown in Figure 6,
it is possible to produce a diffusion element having an annular
- 12 -
perlpheral zone having greater density, lesser permeability
and lesser height than the remainder of the element. In this
connection, a step is formed in die cavity 2 between bottom
wall 3 and side walls 4,5. This step includes a vertical
cylindrical surface 24 and a horizontal annular surface 25.
When one uses this die cavity with a ram 17 to perform the
same sequence of operations described above in connection
with Figures 1 through 5, one obtains a diffusion element
having an annular peripheral zone as above described. Figure
6 shows such element in a~stage of production~corréspon~ ~g~-to
Figure 4. The resultant element includes an integral annular
peripheral zone generally indicated by reference numeral 29,
having a vertical cylindrical edge 30, horizontal annular
surface 31 and vertical side surface 32, as well as gas infusion
surface 34 and discharge surface 33. It will be noted that
the surface 33 li~s against die cavity bottom 3 during the
pressing operation. While this surface has been denominated
the gas discharge surface for purposes of discussion, it
should be noted that surface 33 can also serve as the gas
infusion surface, in which case surface 34 would become the
gas discharge~surface.
The element produced in Figure 6 is shown in plan
view in Figure 7 wherein vertical cylindrical edge 30,
horizontal annular surface 31, vertical side surface 32 and
gas discharge surface 33 are visible. Figure 7 illustrates
the testing of this plate for bubble release pressure. One
possible apparatus for perorming such a test is shown in
the lower portion of Figure 7 and in Figure 7~.
The exemplary apparatus includes a tank 40 ~Figure 7)
having bottom wall 41 and side walls 42,43. Resting on bottom
wall 41 are supports 44,45 which support the diffusion element
,.
I - 13 -
t
~,.................................. .
with its gas discharge surface 33 facing upwardly beneath
the water level 46. Compressor C is connected via conduit
51, pressure regulating valve 52 and flowmeter 53 with a first
hose 54. The latter is in turn connected to the first horizon-
tal leg 56 of a tee 55 having a second horizontal leg 57 and
vertical leg 58. A sealing ring 59 is provided around the open
bottom end of vertical leg 58. Second horizontal leg 57 is
connected via second hose 63 with a manometer 64 having a
scale 65. By comparison of liquid levels. 66 and 67 with
scale.65;it i5 .possible to.determine.the~pressure.wi.th.~ he~
system, which is assembled carefully to provide gas tight
joints at all connections between components.
As shown in greater detail in Figure 7A, the vertical
leg 58 of tee 55 and the sealing ring 59 constitute a test
probe.. It may for example be fabricated from a standard labora-
tory glass tee having a sufficient internal diameter to readily
deliver the desired gas flow and from a standard rubber stopper : .
assembled as shown, the bottom of the stopper constituting the
end of the probe and having outside and inside diameters 59A
and 59B of 3/8 inch and 3/16 inch, respectively. The probe can
be manually pressed against the gas discharge surface 33 of the
diffusion element with its inside diameter surrounding a test
site and with the lower surface of sealing ring or stopper 5~
.~ forming a gas-tight seal, sealing off the surrounding surface
from th.e test site and the interior of vertical leg 58 and the
remainder of the attached pressure and flow producing and
measuring components. Air flow under pressure from the probe
enters surface 33, travels outwardly from the test site through
the body of the element beneath the sealing ring and emerges
as bubbles 60 through a pore or pores nearby. The pressure
required to release the bubbles can be read from the manometer.
- 14 -
.
~ ~ ,
Since the stopper 59 is non-adherent relative to surface 33,
the probe may be readily moved from one test site to another
to take a series of pressure readings from which bubble
release pressure can be calculated.
Returning to Figure 7, mutually perpendicular reference
lines 71 and 72 are drawn on gas discharge surface 33 of the
diffusion element. Equally spaced reference marks 73 situated
along reference lines 71 and 72 identify test sites over which
the open end of the above-described probe of Figure 7A is
sequentially positioned in such a way as to produce an effec-
tive seal as described above. Regulating valve 52 is adjusted
against a relatively high pressure from compressor C to a
relatively low rate of flow, i.e. 2 x 10 3 C.F.M. When a
bubble or bubbles 60 are produced through a portion of the gas
discharge surface adjacent the probe, the pressure in the
system is read from manometer scale 65. The bubble release
pressure at the test point is obtained by subtracting the
hydrostatic head H, between gas discharge surface 33 and
water level 46, from the pressure read from the manometer.
Taking bubble release pressure measurements at a statistically
significant number of randomly or uniformly established points
on gas discharge surface 33 enables one to determine the bubble
release pressure of said surface. However, in actual practice
it has been found to be reasonably accurate and convenient to
establish all of the pressure testing points along two mutually
perpendlcular reference lines as shown. In a diffusion element
;~ manufactured with reasonable care, conducting the tests along
two such reference lines provides a reasonably accurate
approximation of the uniformity of distribution of air flow
capabilities across the gas discharge surface.
- 15 -
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', : ' ' ' `~ ' ' ;'
The central portion of Figure 7 includes a graph
having a horizontal coordinate 74 with divisions 74A and
corresponding in scale and position to reference mark 73 on
reference line 71. Vertical coordinate 75 of this graph
includes an appropriate scale of pressure values 75 whereby
the pressure readings taken at reference marks 73 on reference
line 71 may be plotted between the coordinates to develop a
bubble release pressure profile or curve 76. In a tank 40 with
sufficient space between side walls 42,43 and the sides of the
diffusion element, it is possible also..to take~ b.ble,~re.~
pressure readings at the vertical cylindrical edge 30, horizon-
tal annular surface 31 and vertical side surface 32 as well
as at a point on gas discharge surface 33 which is quite close
to the vertical side surface 32. The positions of the test
sites Oll the diffusion element and the corresponding positions
of the pressures as plotted in the graph are shown by dashed
reference lines 80A,80B (vertical cylindrical edge 30), 81A,
81B, thorizontal annular surface 31), 82A, 82B (vertical side
surface 32) and 83A, 83B (edge of gas discharge surface 33).
The aforementioned testing and plotting positions are shown in
greater detail in Figure 7B, an enlarged and foreshortened
portion of Figure 7.
The bubble release pressure is an indication of the
pressure required for bubbles of air to overcome surface ten-
sion upon discharge from the pores of the plate. It has been
found that this pressure requirement can considerably exceed
the pressure losses due to friction in pushing the gas from
the air infusion surface to the gas discharge surface of the
plate. This is particularly true where the plate is fabricated
of hydrophilic materials which are readily wetted by the water
as compared to hydrophobic materials,
- 16 -
r
' : :
.
9~7
The graph in Figure 7 shows that the plate produced
in accordance with Figure 6 exhibits minimum bubble release
pressure (s.R.P.) in a central region of the element. In
surrounding regions of the element, the bubble release
pressure grows gradually higher, climbing towards a maximum
85A, 85B, indicated by reference lines 83A, 83B, based on
tests made on gas discharge surface 33 adjacent vertical side
surface 32. Bubble release pressure tests on side surface 32
indicated by reference lines 82A, 82B indicate that the bubble
release pressure reaches a second minimum 86A,86B~in~th~ ea.
A further measurement of bubble release pressure on horizontal
annular surface 31, indicated by dashed lines 81A,81B shows
that the pressure can reach a second maximum 87A,87B in this
area. A final observation taken on vertical cylindrical
edge 30 as indicated by reference lines 80A,80B indicates there
can be some reduction of bubble release pressure in this area
as compared to the second maximum 87A,87B. Using data accumu- -
lated along reference line 72, it is also possible to develop
a bubble release pressure curve (not shown) for the test
sites along reference line 72.
The existence of a low bubble release pressure
region in vertical side surface 32 as indicated by the
second minimum 86A,86B of bubble release pressure curve 76
~; is unexpected. Perhaps this phenomenon may be explained in
retrospect by a theoretical consideration of the flow of
particles within the die cavity (see Figure 6). Inasmuch as
the solid particles between ram compression face 18 and
horizontal annular surface 25 are subjected to greater com-
pression than the adjoining particles between ram surface 18
and die bottom wall 3, downward and inward force vectors
may possibly develop in the material above surface 25 as
- 17 -
,.3~7
some of the particles, under compression, migrate downwardly
and inwardly in the compact. Vertical side surface 32 may be
shielded somewhat from such vectors by the inner edge of hori-
zontal annular surface 25, whereby the material along surface
32 is less compacted than that along surface 31, imparting
respectively greater and lesser permeability to said surfaces.
Inasmuch as the rate of flow of gas through a given
region of the diffuser elemen-t will be an inverse function of
the bubble release pressure in said region, it is possible to
devélop a flow ;curvei 77 whicfi !is consider`ed ~ep~ésenta~i~e~
o~ the flow profile of the plate across reference line 71.
Actual flow data may be obtained for the central portion of the
element by operating the element for timed intervals with an
inverted graduated cylinder over the test sites. Estimated
flows based on bubble release pressure may be derived for
the edges of the element. Analysis of the resulting flow
curve provides a practical indication of the uniformity of gas
flow distribution across the element, As shown by gas flow
curve 7?, peak flow occurs in the central region of the diffu-
sion element, tapering off to a first minimum 90A,9OB(corresponding to bubble release pressure maximum 85A,85Bl near
the outer edge of gas discharge surface 33. There are also
flow peaks 91A,9lB and second minima 92A,92B corresponding
inversely and respectively to bubble release pressure minima
86A,86B and second maxima 87A,87B. Thus the testing technique
illustrated in Figure 7 has provided a much clearer and more
quantatively accurate indication of the flow profile of the
diffusion element than has heretofore generally been available
in the industry. Moreover it has made it possible to see that
such diffusion elements may exhibit their maximum flow in cen-
tral regions, while outward regions of the elements are
- 18 -
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.
367
underused. Excessive gas flow through the central region
of the plate tends to produce excessively large bubbles,
thereby impairing the oxygen transfer efficiency of the plate.
Figures 8 through 19 disclose various forms of diffu-
sion elements configured in such a manner as to avoid the
above described difficulty. The diffusion elements may be
manufactured according to any convenient method. However
Figures 8 through 16 disclose illustrative methods which may
be used if desired. These methods are modifications of the
technique disclosed in Figures 1 through 5. ~s~
A first example shown in Figures 8 through 10, hegins
with filling and levéling off the contents of a die cavity
as shown in Figures 1 and 2. After the level surface 13
(Figure 2) has been prepared, a shaping ring 100 is placed on
the die upper surface 6. Shaping ring 100 has planar upper
and lower surfaces 101 and 102 and perpendicular peripheral
surface 103 representing its outer edge. The ring also has an
internal conical surface 104 defining a central frustro-conical
chamber open at the top and bottom. The aforesaid chamber is
;~ 20 initially empty because the body of particulate material has
previously been screeded off flush to the die upper surface 6.
However the chamber within internal conical surface 104 is
now filled with exces= particulate material 105 which is
then struck off with screed 12 to a level surface 106 flush
~ with upper surface 101 of shaping ring 100. Upon careful
;~ removal of shaping ring 100, there is left in die cavity 2
~ a body 107 of particulate material having an elevated central
::
portion 108 with a flat top 109 and conical sides 110; however
portion 108 can have various shapes.
Body 107 and the above desaribed technique of preparing
same constitute one example of providing a larger ~ass of
19 -
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33~
particles per unit of horizontal area in a central zone or
region, as compared to the mass of particles per unit area in
an outward surrounding zone or region. In this case, the larger
mass of particles per unit of horizontal area is provided by
filling a portion of the die to a greater depth than the sur-
rounding portions.
However, a larger mass of particles per unit area
can also be provided by filling a central portion of the die
with particles at a greater density. For instance, one may
proceed in accordance with Figures I and 2 and then densi~-y~
the particles in a central region of the die by localized
vibration or pressing, which will cause the material in such
region to sink lower in the die than the surrounding material.
The resulting depression can be filled with additional particu-
late material prior to pressing or final pressing as the case
may be. This is an illustration of distributing the particles
prior to or during pressing for providing a larger mass of
particles per unit volume in a central region or zone as
comparea to the mass of particles per unit volume in an
outward or surrounding region or zone.
Irrespective of whether the central region or zone
of the die is filled to a greater height and/or greater density
the body of material is then pressed as shown in Figure 10.
This produces an element having an enhanced apparent volumetric
compression ratio in its central zone 113 (bounded by reference
~ lines 113A,113B) as compared to the volumetric compression
;~ ratio in an outward zone 114, which in this case is an annular
zone bounded at its inner edges by reference lines 113A,113B
and by the peripheral edges of the element indicated by 114A,
114B.
- 20 -
, . .
~3~6t~
Figures 11 through 16 illustrate the preferred
technique of providing enhanced apparent volumetric compression
ratio. More specifically these figures disclose the technique
of effecting, during pressing, relatively larger and smaller
ratios of thickness reduction in the aforementioned central
and outward zones, respectively.
According to one of the preferred techniques disclosed
in Figures 11 and 12, the relatively larger and smaller ratios
of thickness reduction are obtained with the assistance of
an annular insert 121 constituting a protuberan~c~e-~on~he
compression face 3 of die cavity 2. Insert 121 includes a
lower cylindrical projection 122 which mates with a correspond-
ing socket 120 in die cavity lower wall 3. The upper portion
of insert 121 is a shaping member which includes a flat top
124 surrounded by a conical surface 125. One may carry out the
procedure of Figures 1 through 5 except that the annular insert
121 is present as shown in Figure 11 during formation of the
compact. The resultant compact 126 shown in Figure 12 after
removal from die cavity 2 includes a planar air infusion
surface 130 and a generally horizontal air discharge surface
131 including a central depression 132 having a flat area 133
and a beveled edge 134. This is an example of enhancing the
apparent volumetric compression ratio of the central zone of
a diffusion element by effecting, during pressing, relatively
larger and smaller ratios of thickness reduction in the cen-
tral and outward zones, respectively. In this case, the
respective larger and smaller percentages of thickness reduc-
tion have been effected by performing the pressing in a press
having a ram and die cavity with spaced, opposed compression
faces with respectively larger and smaller clearances in cen-
tral and surrounding portions of the space between the faces.
- 21 -
. . :
: ~ .
In this case, the smaller clearance is provided by a protuber-
ance, i.e, insert 121, on the compression face of the die
cavity; but it is also possible to carry out this technique
using a die cavity with a flat bottom wall and a protuberance
on the compression face of the ram.
One can make a wide variety of modifications to
the article and manufacturing technique shown in Figures 11 and
12 without departing from the invention. The shape, depth
and area of the depression 132 may be freely varied to obtain
the desired level of uniformity of air distribution~at~the
gas discharge surface 131 of the element. The shape of the
depression may include any desired outline which contributes
to uniform distribution of air flow; but preferably the outline
of the depression is similar io the outline of the element.
Within the outline of the depression, there may be a wide
variety of shapes as viewed in transverse cross section. The
floor of the depression may be composed wholly of stra~ght or
curved surfaces or a combination of straight or curved sur-
faces. The center of the depression may be flat as shown,
; 20 or gently curved throughout or may be in the form of an
extremely flat cone or may have any other convenient or desire-
able shape which accomplishes the purposes of the invention.
The showings of depressions including flat areas and beveled
edges shown in the drawings herein are simple and preferred but
are by no means intended to limit the invention.
As indicated above, the area of the central depression
is not necessarily coextensive with the area which establishes
the central zone of the element. The depression may terminate
within or extend beyond the area which defines the central
zone. However, it is convenient for purposes of design to
establish the area of the depression in such a way that it
- 22 -
;,7
is coincident with the area which defines the central zone
of the element.
One may select any combination of area and average
depth for the depression which are sufficient for significantly
enhancing the uniformity of gas distribution laterally across
the gas discharge portion of the element. For example, the
area of the depression may comprise about 10 to about 80
percent, preferably about 25 to about 75 percent and more
preferably about 45 to about 65 percent of the total area of
the diffusion`element gas discharge surface-~o~o~he~t~
area of the element, while the average depth of the depression
may be about 2 to about 20 percent, preferably about 4 to
about 15 percent and more preferably about 5 to about 10 per-
cent of the average thickness of the horizontal portion of the
element.
The depth of the element may vary within its outline,
in either a stepwise or gradual fashion, the latter being
preferred. Most preferably the variation occurs gradually
along gradually sloping portions of the gas discharge surface,
It is also contemplated that there may be certain areas
within the outline of the depression in which there is no
depression. Such is illustrated by Figure 13.
Figure 13 discloses an element which can be formed
for example, by a modification of the technique shown in
Figure 12. The insert 121 is replaced by an annular insert
140 having an annular rib 141 in its underside which engages
a correspondingly shaped annular channel 142 in the bottom wall
3 of die cavity 2. Annular insert 140 includes a flat top 143
with beveled inner and outer edges 144, 145. When the thus
modified die cavity 2 is employed to produce a part following
the technique of Figures 1 through 5, the resultant piece,
- 23 -
-
... ,. :
~3~367
shown in the upper portion of Figure 13, includes a central
depression 149 of annular shape, having tapered edges 150, 151
and including a non-depressed center portion 152. Within the
body of the circular element thus formed is a circular central
zone whose boundaries are indicated by 113A, 113B, surrounded
by an outward zone e~tending from 113A, 113B to the cylindri-
cal peripheral surface of the element indicated by 114A, 114B.
As illustrated in Figure 14 the outward zone of the
element may not necessarily extend to its extreme peripheral
edge. Figure 14 illustrates an element, and an illustrative
method of production thereof, combining the features of
previously described Figures 6, 11 and 12. The cylindrical
die cavity 2 includes a circular insert 121 in its bottom
wall 3. A step is formed about the periphery of bottom wall
3 where it joins side walls 4 and 5, said step including
vertical cylindrical surface 24 and horizontal annular surface
25 as in Figure 6. Production of an element with such a die
following the sequence of operations disclosed in Figures 1
through 5 produces an element as illustrated in the upper por-
tion of Figure 14. This element includes a gas discharge
surface 131 having depression 132 with tapered edges 134 with-
in the bounds 113A, 113B of the central zone of the element.
The vertical cylindrical surface 24 and horizontal annular
surface 25 of the die cause the extreme peripheral portion of
the element to include a vertical cylindrical edge 30, hori-
zontal surface 31 and vertical side surface 32 defining a
step in the edge of the element. That portion of the volume
: of the element bounded by planar air infusion surface 130,
vertical cylindrical edge 30, horizontal annular surface 31
and reference lines 160A,160B define an annular peripheral
zone of reduced permeability, greater density and lesser
- 24 -
., ............................. . - , . ~ .
'.: .~ ' ' ' ' ' "'
.
~1~336~7
average height as compared to the permeability, density average
height of the relatively inward and adjoining portions of the
element. In this element the outward zone is bounded inwardly by
reference lines 113A,113B and at its outer edge by reference
lines 160A,160B.
Figure 14 represents a way of improving the uniformity
of air distribution of a porous gas diffusion element such as
for instance that shown in Figure 6. Formation of an annular
zone of lesser height or greater density or lesser permeability
affects not only the flow characteristics of the peripheral
zone itself, but also the characteristics of the relatively
inward portions of the element, tending to concentrate flow
in a central zone. By providing an enhanced apparent volumetric
compression ratio in the central zone, the foregoing tendency
can be countered, equalized or eliminated.
Figure 15 illustrates how the present diffusion
element may include a boundary zone adjacent to a peripheral
zone. From the description of the element without a depression
in Figure 6 and the testing thereof as illustrated in Figure 7,
it will be recalled that the vertical side surface 32 of said
element tended to exhibit a second minimum 86A,86B of bubble
release pressure and a corresponding peak 91A,9lB on the flow
rate curve 77, indicated by dashed lines 82A,82B. The tendency
for the flow to peak in this area of the element may be con-
sidered undesireable depending on a number of factors such
as for instance the type of holder and sealing arrangement
adopted in mounting the element in a diffusion system. Where
the element is mounted in such a manner that it is free to
discharge bubbles through vertical side surface 32 into the
medium to be aerated, there is a tendency for the surface to
produce undesireably large bubbles and high flux rate. More-
over, if the surface 32 discharges into a crevice which can
- 25 -
be swept clear of water (and thereby freed of surface tension)
by the bubbles emanating from the pores in surface 32, a dis-
proportionate share of the total air flow will be shifted to
said surface. This tendency may be countered, equalized or
overcome by a variety or combination of techniques including,
for example, covering the surface 32 with an impermeable layer
which is held or adhered in place, by utilization of seals and
diffusion element holders of proper design, by the modification
shown in Figure 15, by a combination of these measures or by
any other desired means. 2 . ~
Figure lS illustrates a diffusion element which
includes a boundary zone adjacent to and inwardly of the peri-
pheral zone, the solid particles having been pressed to a
greater apparent volumetric compression ratio in the boundary
zone as compared to the aforementioned outward zone. This is
accomplished for instance by modifying the die cavity 2 to
include a fillet 161 at the base of, and extending inwardly of,
the step formed by vertical cylindrical surface 24 and hori-
zontal annular surface 25. This fillPt may for instance be
at an angle relative to the horizontal, or relative to the
surface of the aforementioned outward zone (especially if the
latter is not perfectly horizontal), in the range of
about 10 to 70, In other respects the die of Figure 15 is
identical to that shown in Figure 14. When an element is
pressed in the die of Figure 15 following the techniques of
Figures 1 through 5, the resultant element shown in the upper
portion of Figure 15 includes a beveled edge 162 which is at
an angle alpha relative to the horizontal. For the style
of plate shown in Figure 15, 25 is considered the optimum
value of alpha.
The benefits of providing the beveled edge 162 were
- 26 ~
1J~43367
not foreseen when the concept of central volumetric compression
ratio enhancement was developed. However experience gained
from working with the central depression concept and the above
described measuring technique has led to some hypothetical
explanations for the possible effects of beveled edge 162.
It is theorized that the fillet 161 in the die exerts a crowd-
ing effect on the material above it, creating next to the
outward zone a ring-like boundary zone bounded at its outer
edges by references lines 160A, 160B adjacent annular peri-
pheral zone 29 and bounded at its inner edge by referenceslines 163A,163B. Thus Figure 15 is one example in which
larger and smaller percentages of thickness reduction have been
effected by performing the pressing in a press having a ram
and die cavity with spaced, opposed, compression faces and
wherein there are relatively smaller and larger clearances in
those portions of the space between the faces which correspond
to the positions of the boundary zone and the aforementioned
outward zone in the diffusion element. In this particular
instance, the smaller clearance is provided by a protuberance
on the compression face of the die cavi~y, i.e. fillet 161.
Figure 16 discloses an alternative techni~ue for
providing the larger and smaller percentages of thickness reduc-
tion referred to above. More specifically, in Figure 16 the
smaller clearance is provided by a protuberance on the compres-
sion face of the ram. For example, as shown in Figure 16 the
compression face 18 of ram 17 includes an annular rib which
may for instance extend full circle around compression face
18 a short distance inwardly of its peripheral edge. This rib
may be of any desired cross section but is preferably arcuate.
It may have`any suitable depth consistent with the structural
integrity of the peripheral edge of the element and which is
- 27 -
. ,~
. ~, . . .
, ~ , ,
suitable for producing the desired enhancement of apparent
volumetric compression ratio. Representative depths would be
those stated above for the central depression 132.
In this embodiment, the die cavity 2 may include the
fillet 161 of Figure 15 but preferably is like the die cavity
of Figure 14 having insert 121 and a step defined by surfaces
24 and 25. When this die cavity and the ram 17 of Figure 16
are employed to produce an element following the general tech-
nique of Figures 1 through 5, the resultant element is as shown
in the central portion of Figure 16. The annular rib lh~i*pro-
duces a corresponding annular groove 167 in the air infusion
surface 130 of the element. The groove 167 may be positioned
so that its shape intersects with or is slightly inward of
the edge of the peripheral zone or the projected surface of
vertical side surface 32. Thus, while annular groove 167 should
be situated at least in part within the boundary zone of the
element, it may project to some extent into the peripheral
annular zone 29. An element having an annular groove 167
positioned as shown in Figure 16 will include central, outward,
boundary and annular peripheral zones delineated by reference
lines 113A, 113B; 163A, 163B; 160A, 160B; and vertical cylin-
drical edges 30 in the same general manner as the element of
Figure 15.
Elements with boundary zones are illustrated by
Figures 15 and 16 and have the advantage that their side
surfaces 32 have an increased bubble release pressure.
Thus they may suffer less or not at all from the disadvantages
described above in respect to the Figure 6 and 14 embodiments.
Thus, diffusion elements with central enhancement of apparent
volumetric compression ratio, and with peripheral zones that
have been rendered semi-permeable or substantially non-permeable
- 28 -
rr. ~ , , ~ .
11~3367
can be improved, if desired, by enhancement of the apparent
volumetric compression ration in a boundary zone adjacent to
and inwardly of the peripheral zone.
The benefits of providing a boundary zone are
illustrated graphically in Figures 17 and 18, which show
respectively the diffusion elements of Figures 14 and 15, without
and with boundary zones, respectively. Using the bubble release
pressure testing procedure of Figure 7 and a form of graphical
representation similar to that Figure, one may develop bubble
release pressure ~B.R.P.~ curves A and B for the respective
elements. Comparison of these curves shows that without the boundary
zone there is a minimum M in the bubble release pressure curve
at the element vertical side surface 32. When the boundary
zone is provided, the bubble release pressure is increased in
the area of surface 32 as shown by curve B in Figure 18. In
view of the inverse functional relationship of flow to bubble
release pressure, the presence of the boundary zone enables
one to control the f,low from surface 32, making it less than
the flow through the center of the element.
Although the benefits of the boundary zone have
been illustrated above by its effect upon the vertical surface,
i.e. surface 32, the vertical surface is not required. The
upper surface of the boundary zone may for example be a non-
vertlcal surface, such as an outwardly and downwardly inclined
surface extending all the way from the upper surface of the
element to the upper surface of the peripheral zone.
- 29 -
A
~143367
In Figures 10 through 16 there have been shown
diffusion elements in which references lines such as 113A,
113B; 160A, 160B; 163A, 163B and so forth have been used to
generally indicate the lateral bounds of various zones such
as the central zone, outward zone, boundary zone and peripheral
zone. These references lines have not been drawn to scale nor
should they be taken to mean that there should be a clearly
identifiable vertical line of demarcation between the respective
zones in actual products according to the invention. In
diffusion elements within the scope of the invention, it may
not be possible to draw any line of division between zones where
material of significantly greater and lesser density or
compaction will be found immediately to either side of the
division between two zones. Rather these bounds have been given
to illustrate the lateral extent of a volume which, when compared
as a whole with an adjacent volume, exhibits the desired
difference in apparent volumetric compression ratio.
Now that the principle of the invention has been
taught, it should bé apparent that it is capable of general
application without necessarily being limited to products with
limited ranges of properties. However, to assist those skilled
in the art in practicing some of the more preferred alternative
forms of the invention, some representative and preferred
parameters and properties of the diffusion elements are described
below.
It is believed that many applications of the invention
will invol~e elements wherein the apparent volumetric compression
ratio of the central zone has been enhanced, relative to
_ 30 -
. ~
:1143367
the outward zone, by at least about 2%, more particularly
about 2 to about 20% and preferably about 3 to about 15%. The
foregoing percentages are obtained by expressing the difference
in apparent volumetric compression ratios of the two zones as
a percentage of the volumetric compression ratio of the outward
zone. Similarly, where a boundary zone is provided, most appli-
cations of the invention will involve enhancement of the
apparent volumetric compression ratio of the boundary zone,
relative to the outward zone, by at least about 10% more
particularly about lO to about 35% and preferably about 35 to
about lO0~.
In principle, the invention is not limited to diffusion
elements of a specific pore size, but many applications of the
invention will involue diffusion elements in which the pore
size is in the range of about 60 to about 600 microns, more par-
ticularly about 90 to about 400 microns and preferably
about 120 to about 300 microns as computed in applying the
bubble release pressure to the equation shown in ASTM E-128
D=30~/p, wherein D= maximum pore diameter, ~= surface tension
of the test liquid in dynes/c~, and p--pressure in mm of Mercury.
While the diffu~ion. elements may include a wide variety
of particulate (including fibrous) materials of both organic
and inorganic character, they are clearly distinguishable from
pressed, open, fibrous filters and the like in terms of modu-
lus of compression, or specific permeability, or bubble release
pressure,.or a combination thereof. Thus, many applications
of the invention will involve diffusion elements having a modu-
lus in compression of at least about 0.2 X 105 psi, more par-
ticularly about 0.2 X lO to about 4 X lO psi in applications . -
involving softer particulate materials, and ~referablv about4 X lO to about 6 X 10 psi when working with.the harder in-
- 31 -
.~. ' ' .
367
organic material. Generally, unused diffusion elements in
accordance with the invention will have a specific permeability
in the range of about 6 to about 200 SCFM, more particularly
about 12 to about 70 SCFM and most preferably about 15 to about
35 SCFM in the case of alumina and silica sewage aeration
diffusion elements. While most diffusion elements in accor-
dance with the invention will have a bubble release pressure 3
in the range of about 2 to about 20, and more particularly about
4 to about 15, the most preferred bubble release pressure for
10 the preferred sewage aeration diffusion elements disclosed here-
in about 5 to about 10.
For the most part the generally horizontal portions
of the diffusion elements according to the invention will exhi-
bit a ratio of maximum horizontal dimension relative to thick-
ness of a least about 4 to 1, it being understood that non-
circular shapes such as ovals and rectangles will have both
maximum and minimum horizontal dimensions when viewed in plan
view. Preferred and more preferred values for the aforesaid
ratio are about 6 to 1 and about 8 to 1.
The invention is useful whenever it produces any func-
tionally significant improvement in the uniformity ~f flow dis-
tribution across a diffusion element, and it would be virtually
impossible to predict the minimum degree of improvement which
might ever be considered functionally significant, given the
potential for changes in the quality of instrumentation and
technological need. However, certain classes of diffusion
elements are illustrative of the type of benefits which can be
~ ~ .
produced by the invention. These include diffusion elements
wherein the coefficient of variation of the gas discharge sur-
face is not greater than about 0.25, based on the values of
bubble release pressure measurements of at least five equally
f - 32 -
:
3367
spaced polnts along each of two perpendicular reference lines
extending across the surface of the element and through the
center thereof. More preferred examples include diffusion
elements, as just described, in which the coefficient of vari-
ation is in the range of about 0.05 to about 0.25 or more pre-
ferably less than about 0.05.
As previously disclosed the solid particles in the
boundary zone of a diffusion element according to the invention
can be pressed to a greater apparent volumetric compression
ratio as compared to the particles in the outward zone by
forming the upper gas discharge surface above the boundary
æone with a downward and outward slope at an angle of depression
in a range of about 10 to about 80 degrees relative to the
horizontal, with many applications of the invention falling in
the range of about 20 to about 70 degrees and more preferably
about 25 to about 65 degrees, with about 25 degrees being con-
sidered the optimum for the preferred sewage aeration elements
being produced in accordance with the example which follows.
EXAMPLE
A diffusion element is fabricated in accordance with
Figures 15 and 15A, which together illustrate the most pre-
ferred embodiment of the present invention. The respective
outer diameters of flat area 133, beveled edge 134, flat sur-
face 164, beveled surface 162 and horizontal annular surface
31 are 4.5, 6.5, 7.6, 8.7, and 9.25 inches, respectively.
Surface 133 lies 0.070 inches below surface 164. Beveled sur-
face 162 is at an angle of inclination ~ or 25 relative to
the horizontal and its common edge with vertical side surface
32 has a 1/16th inch radius as viewed in transverse cross sec-
tion. The respective overall heights of horizontal annular
surface 31, the top edge of vertical side surface 32 and hori-
- 33 -
11~3367
zontal flat surface 164 are 0.5, 0.741 and 1.000 inch respec-
tively.
The plate is formed from a mix containing particles of
alumina with mean transverse and longitudinal dimensions of
0.020 and 0.032 inch respectively, and 20 parts by weight of
ceramic bonding agent, per hundred parts by weight of alumina
particles. The mix is compacted in a press having a ram with
a planar surface and a cylindrical die cavity with a shape
corresponding to the illustration in Figure 15, the height of
the die from its bottom wall to its upper edge being 1.5 inches.
The mix is struck off level with the top of the die as shown
in Figure 2 and is then compressed to the dimensions previously
given under a pressure of approximately 900 psi. The resultant
compact, after removal from the press, is fired in a kiln at a
temperature sufficient to fuse the bonding agent and is then
gradually cooled. The resultant product is a coherent porous
ceramic diffusion element having a specific permea~ility of 25
SCFM - 3 SCFM, and a pore size of 165p .
The diffusion element produced according to the above
illustrative example has certain additional characteristics and
properties which are optional but preferred features of the
invention. The product of the example has a gas discharge sur-
face which is free of bubble emitting macro openings such as
those shown for instance in U.S. Patent 3,970,731 to Oksmann.
The element will emit bubbles from random locations throughout
the gas discharge surface. The gas infusion surface of the
element is free of air transmitting holes longer than 0.3T,
wherein T is the average thickness of the element weighted on
an area basis, or is free of such holes. A macro hole is an
intentionally or unintentionally produced hole larger than that
normally produced by compaction of the particulate material.
- 34 -
,~ .
.....
Substantially all gas paths through the body of the element
to its gas discharge surface, as installed in the plenum or
other holder, are about the same length and substantially
parallel. Moreover, it has a bubble release pressure of about
7 inches. The value of bubble pressure given is for bubble re-
lease pressure in water of an element as manufactured, i.e.
prior to use. The element is fabricated of hydrophilic
materials,i.e, materials which are hydrophilic in the element
as manufactured and prior to use. Also, the element has a
side surface, particularly vertical edge 32 in Figure 15,-whLch
is at least semi-permeable and free of adherent material pre-
venting bubble emission. The gas discharge surface will be
free of through-holes other than pores. One may prepare
diffusion elements having any one or all of the above preferred
characteristics.
Based on the foregoing description, it should be
apparent that the present invention may be embodied in a wide
variety of forms, and that the invention is not limited to the
precise embodiments disclosed in the foregoing description and
drawings. Thus the appended claims should be construed to
cover the subject matter described in said claims and all
equivalents thereof.
- 35 -
