U.S. patent number 3,885,734 [Application Number 05/432,258] was granted by the patent office on 1975-05-27 for centrifuge apparatus.
This patent grant is currently assigned to Pennwalt Corporation. Invention is credited to Chie-Ying Lee.
United States Patent |
3,885,734 |
Lee |
May 27, 1975 |
Centrifuge apparatus
Abstract
A decanter centrifuge having a screw conveyor within an
imperforate bowl is provided with an annular baffle carried by the
screw conveyor. Light phase material and heavy phase material is
separated from a mixture thereof fed to the centrifuge bowl. The
heavy phase discharge port, usually in a tapered portion of the
bowl, is preferably at a greater radial distance from the
rotational axis than the inner surface of the light phase material.
The periphery of the baffle is closely spaced from the bowl in
order to form a restricted passageway for the underflow of heavy
phase material from a separating zone within the cylindrical
portion of the bowl to a heavy phase discharge zone within the
tapered portion of the bowl. Pressure from the materials within the
separating zone, or from an independent source, is transmitted
through the restricted passageway and applied to the heavy phase
material undergoing discharge, thus facilitating its discharge with
minimum content of light phase material. With a conical baffle,
incoming feed is directed into the inwardly facing surface of the
baffle and accelerated in order to minimize turbulence in the
separating zone. Efficient separation is accomplished, even with
materials heretofore considered unsuited for separation by a
decanter centrifuge.
Inventors: |
Lee; Chie-Ying (Hatboro,
PA) |
Assignee: |
Pennwalt Corporation
(Philadelphia, PA)
|
Family
ID: |
26962570 |
Appl.
No.: |
05/432,258 |
Filed: |
January 10, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
284371 |
Sep 6, 1972 |
3795361 |
|
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Current U.S.
Class: |
494/25; 494/53;
494/56 |
Current CPC
Class: |
B04B
1/20 (20130101); B04B 2001/2033 (20130101); B04B
2001/2041 (20130101) |
Current International
Class: |
B04B
1/00 (20060101); B04B 1/20 (20060101); B04b
001/20 () |
Field of
Search: |
;233/1R,3,7,12,13,14R,19R,19A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Krizmanich; George H.
Attorney, Agent or Firm: Sager; Edward A.
Parent Case Text
This is a division of application Ser. No. 284,371, filed Sept. 6,
1972, now U.S. Pat. No. 3,795,361.
Claims
I claim:
1. In a decanter centrifuge for separating light and heavy phase
materials into respective inner and outer layers from a mixture
thereof and for separately discharging said materials, having an
elongated imperforate bowl with a tapered portion, said bowl being
adapted for rotation about its longitudinal axis, feed means for
delivering the mixture to be separated into said bowl, a screw
conveyor coaxially arranged with the bowl to revolve relative to
the bowl for advancing heavy phase material in the direction of the
tapered portion of the bowl, means for discharging light phase
material from the bowl, and means for discharging heavy phase
material from the bowl including a discharge port in the tapered
portion of said bowl, said discharge port having a weir surface,
that improvement comprising:
a baffle positioned between said axis and said bowl and dividing
the interior of the bowl into
a separating zone which communicates with said means for
discharging light phase material, and
a discharging zone for heavy phase material which is disposed
within the tapered portion of the bowl and communicates with said
means for discharging heavy phase material,
said baffle having its inner edge disposed closer to said axis than
the radial distance between said weir surface and said axis, and
extending through the entire layer of separated light phase
material in the separating zone and having its outer edge closely
spaced from the bowl to define therewith a restricted passageway
for the underflow of heavy phase material from the separating zone
to the discharging zone and to prevent the flow of separated light
phase material from said separating zone to said discharging
zone,
and means for pressurizing the heavy phase material in the
separating zone beyond the pressures applied thereto by the
centrifugal force of separated light phase material disposed
radially outwardly of the weir surface of said port, whereby the
additional pressure provided by said pressurizing means is
transmitted to the heavy phase material in said separating zone and
through said restricted passageway to the heavy phase material in
the discharging zone for promoting the advance of the heavy phase
material through the discharging zone to said port for discharge
thereform.
2. A centrifuge according to claim 1 wherein the pressurizing means
comprises means for delivering pressurized gas to the separating
zone.
3. A centrifuge according to claim 2 wherein the means for
delivering pressurized gas includes a conduit extending through
said feed means.
4. A centrifuge according to claim 1 wherein the pressurizing means
comprises means for maintaining the inner surface of the light
phase material in said separating zone radially outward of the weir
surface of said port, whereby the centrifugal pressure head of that
portion of the light phase material disposed radially outward of
said weir surface provides said additional pressure.
5. A centrifuge according to claim 1 wherein said baffle is annular
and is carried by the screw conveyor.
6. A centrifuge according to claim 1 wherein the pressurizing means
comprises means for delivering pressurized fluid to the
pressurizing zone.
Description
BACKGROUND OF THE INVENTION
Decanter centrifuges usually include a rotating centrifuge bowl in
which a screw conveyor revolves at a slightly different speed.
Such centrifuges are capable of continuously receiving feed in the
bowl and of rapidly separating the feed into layers of light and
heavy phase materials which are discharged separately from the
bowl. It is the function of the screw conveyor to move the outer
layer of heavy phase material to a discharge port therefor, usually
located in a tapered or conical end portion of the bowl.
Effective and efficient centrifugal separation requires that the
light phase material, usually liquid, be discharged containing
little or no heavy phase material. In addition, the heavy phase
material should contain only a small amount of light phase
material. For example, if the light phase material is water and the
heavy phase material comprises soft solids, it is preferred that
fairly dry solids and clear water be separately discharged.
In addition to being capable of performing continuous separation,
decanter centrifuges have the advantage of being less susceptible
to pluggage by solids than other kinds of centrifuges. Furthermore,
decanter centrifuges may be shut down for long or short periods and
then restarted with minimum difficulty, whereas most other
centrifuges require cleaning to remove dried solids.
Despite these advantages, decanter centrifuges have seen limited
application for separating a mixture of materials having nearly the
same specific gravity, or wherein the heavier phase material is
slippery or very fine. When the materials of the light and heavy
phases have nearly the same specific gravity they separate slowly,
and they tend to re-mix when disturbed by turbulence. Turbulence
may be caused by axial flow within the bowl, by the splashing
introduction of feed, or by agitation of the revolving screw
conveyor. In conventional decanter centrifuges remixture is further
promoted by contact of the phases as the heavier phase material
passes through the inner layer of lighter phase material while
advancing along the tapered end of the bowl. It is therefore
difficult to keep such materials separated after they have been
separated. In addition, even if the separated materials could be
kept separated, conveying of slippery or very fine solid materials
by a screw conveyor is very difficult, just as it has been
virtually impossible to scroll separated liquids of low viscosity
by means of the slowly revolving screw conveyor.
The present invention is directed to overcoming the aforementioned
problems, thereby increasing the versatility and separating
efficiency of decanter centrifuges so that they will be operable
with commercially acceptable separating efficiency and
effectiveness on feed mixtures which heretofore have been difficult
or impossible for them to separate.
The disclosure of U.S. Pat. No. 3,172,851 teaches a method and
apparatus for centrifugal separation which improves the ability of
the conveyor to move light solids in the tapered part of the bowl
toward the solids discharge port. This is accomplished when the
solids discharge port is placed at a greater radial distance from
the bowl axis than is the liquid discharge port. However, with this
construction the solids in the tapered end of the bowl are totally
immersed in liquid until the moment before discharge, and therefore
the discharged solids will be quite wet. The present invention is a
substantial improvement over this prior disclosure because solids
may be discharged with an appreciably lower moisture content.
As further background, it is proposed in U.S. Pat. application Ser.
No. 15,238, assigned to the assignee of the present invention, that
a decanter centrifuge of the type normally used for liquid-solids
separation be used in triglyceride refining for the separation of
refined oil and viscous soapstock. The present invention may be
employed to facilitate liquid-liquid separation in triglyceride
refining, since the heavy phase soapstock will be discharged more
readily from the tapered end of the bowl together with any solids
present as impurities in the soapstock. Accordingly, the present
invention is further directed to the problem of enabling a decanter
centrifuge to separate two liquids, whether or not solids are
contained in the heavier liquid phase, yielding a clarified liquid
light phase and a liquid heavy phase with suspended solids.
Moreover, the improvements afforded by the present invention are
applicable to the separation of low viscosity liquids.
BRIEF SUMMARY OF THE INVENTION
The invention is applied to a decanter centrifuge of the type set
forth, and preferably to the kind having the weir surface of its
heavy phase discharge port at a greater radial distance from the
bowl axis than the inner surface of the light phase layer. The
centrifuge is provided with an annular baffle carried coaxially by
the screw conveyor, preferably for movement therewith. The baffle
may be flat and normal to the bowl axis, but preferably it is
frusto-conical in shape and formed about the axis. In either case
the baffle is positioned to divide the interior of the bowl into: a
first or separation zone, and also a second zone surrounded by the
tapered end portion of the bowl. The baffle has a generally
circular peripheral edge in closely spaced relation to the inner
surface of the bowl, thereby defining a restricted annular
passageway for the underflow of heavy phase material from the first
zone to the second zone. The flow area of this annular passageway
should be large enough to prevent an accumulation of heavy phase
material in the separation zone of the bowl, and not appreciably
larger than the flow area of the solids discharge port.
The baffle extends outwardly, beyond the annular interface between
the separated phases, a sufficient radial distance from the bowl
axis to prevent the flow of any portion of the light phase layer
from the first zone to the second zone. In this regard it is to be
distinguished from prior art baffles in decanter centrifuges. For
example, in U.S. Pat. No. 3,447,742 the baffles merely extend to
just beneath the surface of the light phase layer in order to block
the flow in axial direction of light solid particles floating on
the inner surface of the light phase layer.
The present invention is also distinguishable from many other prior
art constructions, e.g. U.S. Pat. No. 3,228,594, in that the flat
or conical baffle of the present invention is positioned between
the heavy phase discharge port and the entering path of feed to the
bowl. It is preferred that incoming feed be directed onto the
inwardly facing surface of a conical baffle so that, with the aid
of acceleration vanes on such surface, as the feed flows outwardly
therealong it will be given an angular velocity approaching that of
the separated phases in the separating zone; and the feed will
therefore enter the separating zone without creating excessive
turbulence.
The baffle is imperforate, at least for that portion thereof coming
into contact with the materials in the separating zone, in order to
prevent flow therethrough from one zone to the other zone. This is
to be distinguished from the perforated construction of some parts
shown in U.S. Pat. No. 2,593,278. Furthermore, with an imperforate
baffle extending almost to the inner surface of the bowl, it is
possible to establish favorable pressure relationships between the
heavy phase material in the second zone and the layers of light and
heavy phase materials in the separating zone.
According to the invention, the heavy phase material in the
separating zone is pressurized well beyond the usual pressure
applied in a conventional centrifuge, by the centrifugal force of
an inner layer of light phase material having conventional depth.
This is accomplished by setting the dam, discharge port, or other
discharge means for light phase material so that the inner surface
of the light phase material is maintained radially inward of the
heavy phase discharge port, instead of being conventionally
maintained radially outward of the heavy phase discharge port. The
deeper layer of light phase material, under centrifugal force,
subjects the underlying layer of heavy phase material to increased
pressure. The increased pressure on the heavy phase material in the
separating zone is transmitted therethrough via the restricted
passageway to the heavy phase material in the discharging zone,
thus helping to advance it to the discharge port.
Alternatively the heavy phase material in the separating zone may
be pressurized, without increasing the depth of the light phase
layer, by pressurizing the separating zone with gas introduced by a
suitable delivery means.
With the aforesaid pressurizing means, the flow of heavy phase
material is promoted from the outer layer of the separating zone
through the restricted passageway and then axially and inwardly
along the inner surface of the tapered portion of the bowl. Such
promotion of heavy phase flow augments the action of the screw
conveyor as it conveys heavy phase material to the discharge port
in the tapered portion of the bowl. Even if the heavy phase is
mostly liquid, it will readily flow together with sedimented solids
to the heavy phase discharge port according to this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view, partly in section, of a centrifuge
embodying one form of the invention;
FIG. 2 is an elevational view, partly in section, of a centrifuge
embodying another form of the invention;
FIG. 3 is an enlarged elevational view, partly in section, of a
portion of the centrifuge shown in FIG. 1;
FIG. 4 is a fragmentary bottom view of the centrifuge portion of
FIG. 3, with screw flights omitted for clarity; and
FIG. 5 is a schematic illustration of a centrifuge embodying the
invention in further modified form.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Shown in FIG. 1 is a centrifuge 10 comprising a frame 12 having
main bearings 14 in which are journaled the ends of a hollow,
elongated centrifuge bowl 16 of circular cross section. The bowl 16
is adapted for rotation about its longitudinal axis within a
housing 18. A plurality of discharge ports or openings 20 are
formed in one end wall 22 of the bowl 16 and annularly disposed
about the rotational axis for the discharge of liquid or light
phase material. A plurality of similarly disposed solids or heavy
phase discharge ports or openings 24 are provided adjacent the
other end wall 26.
In other respects the peripheral wall of the bowl 16 is of
imperforate tubular construction, a major portion 28 thereof being
cylindrical.
The end portion 30 of the bowl 16 adjacent the end wall 26 is
tapered or convergent, its inner surface gradually decreasing in
diameter towards and beyond the solid discharge openings 24. The
liquid discharge openings 20 and the heavy phase discharge ports or
openings 24 are at selectively adjustable radial distances from the
rotational axis, preferably so that during proper operation the
inner surface of the light phase material will be disposed radially
inward of the weir surfaces of the heavy phase discharge ports
24.
Mounted coaxially of the bowl 16 in suitable bearings 31, adjacent
the ends of the bowl 16, is a screw conveyor 32. The bowl 16 is
rotated by connection through a pulley 34 to suitable drive means,
such as a motor (not shown). In order to rotate the bowl 16 and the
conveyor 32 at slightly different speeds the rotation of the bowl
16 is transmitted to a gear box 36 having torque control means 38
and thence through a spline shaft 40 within the bowl shaft to the
conveyor 32.
The process feed stream, or mixture to be separated, is delivered
to the interior of the centrifuge through a stationary feed tube
42. The latter projects in axial direction and terminates
concentrically of a feed chamber 44 partly defined by the interior
of a hub 46 having an internal lining 48.
The hub 46, which is part of the conveyor 32, carries outwardly
projecting, cylindrically coiled screw flights 50, and also
outwardly projecting, conically coiled screw flights 52. The
flights 50 and 52 are mounted with small clearance from the bowl 16
for rotation with the hub 46 relative to the bowl 16, preferably at
a speed suitably different from the speed of the bowl to move
settled solids toward the discharge openings 24 for discharge
therethrough. The hub 46 is further provided with one or more feed
passages 54 which also extend through the lining 48 in order to
discharge the feed outwardly from the feed chamber 44 for
separation within the bowl 16.
The feed chamber 44 within the hub 46 extends in axial direction
from a partition 56 to an accelerator 58. The latter comprises a
cup-shaped plate secured in sealing relationship with the inner
surface of the hub 56 and having a vane assembly 60 secured thereto
for imparting radial and tangential velocity to the feed mixture
delivered thereto by the feed pipe 42. As shown, the feed pipe 42
lies concentrically within the feed chamber 44.
An annular seal (not shown) may be secured to the partition 56 to
close the space between the outer surface of the feed pipe 42 and
the portion 56.
A broken line a designates the maximum and desired level of
materials within the cylindrical portion 28 of the bowl 16 which is
maintained by the discharge ports 20. The outermost portion of the
surface defining each port 20 acts as a weir over which light phase
material flows when discharged from the bowl 16.
As shown in FIG. 1, the coiled flights 52 are welded on the outer
surface of a baffle 62 of frusto-conical shape. The baffle 62
tapers in the same direction as the tapered end portion 30 of the
bowl 16. The smaller end 64 of the baffle 62 is securely attached
to the hub 46; and the coiled flights 52 are structurally connected
between the outer surface of the larger end of the baffle 62 and
the hub 46. Reference is made to FIG. 3 for an illustration of the
latter.
The conical baffle 62 of FIG. 1 is preferred to the modified
construction of FIG. 2 in which a flat annular baffle 66 is
employed.
The baffle 62 is positioned within the bowl 16 to divide the
elongated chamber between the outer surface of the hub 46 and the
inner surface of the bowl 16 into two axially adjacent zones: a
first or separating zone 68, and a second or discharging zone 70.
The second zone 70 lies radially outwardly of the baffle 62 and
inwardly of the bowl 16, being surrounded by the tapered portion 30
of the bowl 16. The second zone 70 extends in axial direction from
a peripheral edge or outer edge 72 of the baffle 62 to the end wall
26, although for all practical purposes the second zone 70
terminates with the discharge ports 24 for heavy phase material.
The ports 24 communicate with the second zone 70. The first or
separating zone 68 lies outwardly of the hub 46 and extends
outwardly thereof to the inner surface of the baffle 62 on one side
of the peripheral edge 72 and to the inner surface of the bowl 16
on the other side of the peripheral edge 72. The axial extent of
the first or separating zone 68 is from the small end 64 of the
baffle 62 to the end wall 22, terminating in the discharge ports 20
for light phase material. The cylindrical portion 28 of the bowl 16
surrounds the first zone 68. The ports 20 communicate with the
first zone 68.
Preparatory to further description of the baffle 62, it is to be
understood that feed entering the separating zone 68 within the
rapidly rotating bowl 16 is subjected to high centrifugal forces
which are usually 2000 to 4000 times gravitational force. This
separates the mixture of light and heavy phase materials in zone 68
into an inner annular layer of light phase material and an outer
annular layer of heavy phase material. The annular interface
between the two layers in zone 68 is shown by a broken line
designated e. The layer of heavy phase material lies outwardly of
the e line; and the layer of light phase material lies inwardly of
the e line. The inner surface of the light phase layer is
approximately in axial alignment with the outermost or weir surface
portion of the structure surrounding each port 20, with some
allowance for cresting of the liquid discharging from the ports
20.
The e line is adjustable by adjusting the level of the ports 20.
This is commonly done by providing an end wall 22 having the ports
20 in the desired location. This adjustment is usually suited to
the specific gravities of the materials comprising the feed
mixture, the percentage of each in the feed, the inflow rate of the
feed, and various other factors. In any event, the e line may be
established by known procedures.
It is important that the peripheral edge 72 of the baffle 62 be
carefully positioned relative to the inner surface of the bowl 16
and the e line.
Firstly, the baffle 62 must extend outwardly beyond the inner layer
of light phase material, i.e., the e line, in order to prevent the
flow of light phase material from the first zone 68 to the second
zone 70. It is better practice to have the peripheral edge 72
disposed outwardly of the e line a substantial distance in order to
ensure that some light phase material will not be entrained by
heavy phase material flowing from the first zone 68 to the second
zone 70. It can also be seen that the baffle 62 must be imperforate
at least for the radial distance it contacts the light phase layer,
again to prevent flow of light phase material into the second zone
70.
Secondly, the peripheral edge 72 is positioned inwardly of the bowl
16 to define a restricted annular passageway 74 between them for
the underflow of heavy phase material therethrough from the first
zone 68 to the second zone 70. The spacing between the peripheral
edge 72 and the bowl 16 determines the flow area of the passageway
74; and it should be large enough to prevent an excessive
accumulation of heavy phase material in the separating zone 68,
that is, at least large enough to permit passage of the heavy phase
in the feed at the rate the heavy phase material is separated in
the separating zone. The peripheral edge 72 must always extend in
radial direction, relative to the axis, at least to the weir
surface of the discharge port 24 for heavy phase material.
The centrifugal force applied to the light and heavy phase
materials in the separating zone 68 produces a centrifugal pressure
head which is transmitted to the heavy phase material in the
discharging zone 70. This pressure head, when combined with the
pressure applied by the screw conveyor 32, overcomes the oppositely
directed centrifugal head of the heavy phase material in zone 70.
The level of the heavy phase material in zone 70 is shown by a
broken line identified by the letter x. The level designated x is
slightly inwardly of the weir surfaces of the discharge ports 24,
whereby heavy phase material is discharged from ports 24.
The light phase material has a lower specific gravity than the
heavy phase material; and therefore a layer of light phase material
which is thicker than a layer of heavy phase material is required
to provide an equivalent centrifugal pressure head. Consequently,
level x is more distant from the rotational axis, in radial
direction, than is level a. An advantage of a so-called deep pond
of all materials in the separating zone 68, which the baffle 62
permits, is that larger volumes of feed are accommodated therein,
and therefore greater throughput capacities are obtainable.
Furthermore, with a deep pond in the separating zone 68 greater
centrifugal forces are imposed upon sedimented solids therein,
resulting in better solids compaction. The more compact the solids
are in the separating zone 68, the clearer will be the separated
light phase material. Compact solids also lend themselves to more
effective conveying by the screw conveyor 32.
The baffle 62 is disposed between the discharge ports 24 for heavy
phase material and the path traveled by feed entering the
separating zone 68. This keeps the feed out of the discharging zone
70. Preferably, with a conical baffle 62 the feed passages 54 are
disposed radially inward of the baffle 62, for directing feed onto
the inner surface of the baffle 62 intermediate the ends thereof.
Feed travels outwardly and axially along the innersurface of the
baffle 62 and joins the separated materials in the separating zone
68 where it also undergoes separation. This arrangement avoids any
splashing introduction of feed which might create turbulence and
tend to re-mix separated materials of similar specific gravity.
It is preferred that the inner surface of the baffle 62 be provided
with annularly spaced accelerator vanes 76 which extend in
generally axial direction. It is the function of the vanes 76 to
accelerate incoming feed and thereby bring it up toward the angular
velocity of the separated layers already in the zone 68. This also
minimizes turbulence in the separating zone and improves
clarification.
The screw conveyor 32 and conical baffle 62 of FIG. 3 shows that a
portion 78 of the baffle 62 adjacent the peripheral edge 72 is
cylindrical to provide an additional structural support for the
large end of the baffle 62. With this arrangement, such peripheral
portion 78 is welded to the screw flights 50, thereby improving the
structural integrity of the conveyor 32 and baffle 62
combination.
As shown in FIG. 4, the peripheral portion 78 has an axially facing
edge 80 which generally follows the trailing edge of the associated
helical screw flight 50, except that upon completion of one full
turn of such screw flight a short length of the edge 80 extends in
axial direction. The significance of this provision is that feed
leaving the inner surface of the baffle 62 will join the materials
in the separating zone 68 on the trailing side of the closest screw
flight 50, the side where the least amount of heavy phase material
is present. Since the screw flights 50 and 52 push heavy phase
material with the leading side thereof toward the tapered end 30 of
the bowl 16 there will be a build-up of heavy phase material on the
leading side and very lilttle heavy phase material on the trailing
side. Introduction of feed to the separating zone 68 where there is
little heavy phase material present minimizes the chance of
disturbing settled heavy phase material and thereby improves
clarification of light phase material.
MODIFICATION
The modification of the invention shown in FIG. 2 employs an
annular baffle 66 which is a flat annular plate carried coaxially
by the hub 46.
Where parts in FIG. 2 are similar to parts in FIG. 1, like
reference numerals are employed, and descriptions thereof will not
be repeated, for the sake of brevity.
The baffle 66 operates similar to the baffle 62 for creating a
restricted passageway 74 for the underflow of heavy phase material.
Therefore, the same considerations apply when selecting the radial
distance between the peripheral edge 72 and the bowl 16, and when
establishing pressure equilibrium between the phases in separating
zone 68 and the heavy phase material in discharging zone 70.
It is to be noted that the feed passages 54 in the hub 46 are
disposed immediately adjacent the side of the baffle 66 facing the
separating zone 68. This is also in keeping with the broad concept
of arranging the baffle between the discharging zone 70 and the
entering path of the feed entering the separating zone 68.
OPERATION
In the decanter centrifuge of FIG. 1 feed is introduced via the
feed tube 42, the feed chamber 44, and the feed passages 54. Feed
travels radially outwardly into contact with the inner surface of
the conical baffle 62, and it is accelerated by the vanes 76 while
flowing outwardly and axially along such inner surface. The feed
now has an angular velocity approaching that of the contents of the
separating zone 68 as it comingles with the phases therein.
Separation of the feed into an inner layer of light phase material
and an outer layer of heavy phase material takes place by
centrifugal action. The e line between the phases is at a level
with a baffle surface, with the result that no light phase material
can flow to the discharging zone 70. Light phase material exits the
bowl 16 from the discharge ports 20; and a level along line a is
maintained in the separating zone 68.
Heavy phase material in the separating zone 68 is moved by the
screw conveyer toward the tapered end 30 of the bowl, there being a
small accumulation at the entrance to the restricted passageway 74.
Under pressure from the rapidly rotating materials in the
separating zone 68, the heavy phase material flows through the
passageway 74 and establishes its own level x in the discharging
zone 70. Level x is such that heavy phase material flows out of the
discharge ports 24. Heavy phase particles such as coarse solids
which offer resistance to flow, frictional or otherwise, are
readily conveyed by the screw conveyor 32 to the discharge port
24.
The invention is ideally suited for the separation of secondary
sewage, sludge, and other materials wherein the light and heavy
phase materials are of similar specific gravity, and wherein the
solids may be fine and very slippery when wet. These materials have
heretofore defied separation by a decanter centrifuge without
polyelectrolytes or additives.
The invention also finds meaningful application to the separation
of two liquids where solid impurities in the heavy phase liquid
would plug other kinds of centrifuges which continuously separate
and discharge two liquids. Accordingly, the terms "heavy phase" and
"light phase" have been employed to describe the materials which
are separable by the centrifuge of the present invention, since the
light phase material will usually be a liquid and the heavy phase
material will usually be a mixture of solids or a mixture of solids
and liquid.
A feature of the present invention is the provision of a
discharging zone 70 within the bowl 16, in which the heavy phase
material is segregated from the light phase material during the
discharging operation. A drier heavy phase discharge results.
Furthermore, whereas previous decanter centrifuges required the
screw conveyor to move the heavy phase material through and out of
the layer of light phase material, tending to cause remixture of
the phases, the present invention moves the heavy phase material
alone through the discharging zone. This yields a well clarified
light phase material, and a separated heavy phase material which
contains a commercially acceptable amount of light phase
material.
A further modification of the invention is shown schematically in
FIG. 5. There, the centrifuge bowl 16 also has a heavy phase
discharge port 24 in the tapered portion 30. At the opposite end of
the bowl 16, light phase discharge ports 20 are provided to
maintain level a within the bowl. In addition, a circular baffle 86
is carried by the hub 46 of the screw conveyor 32 to seal the
separating chamber 68 from the atmosphere via ports 20. A still
further addition is an auxiliary baffle 88 of annular shape,
carried by the bowl 16. The auxiliary baffle 88 extends radially
inward a sufficient distance beyond the e line to prevent the flow
of heavy phase material toward ports 20.
As in the embodiment of FIG. 2, a flat annular baffle 66 is
provided the FIG. 5 embodiment. Baffle 66 is carried by the hub 46
and it extends radially outwardly toward closely spaced
relationship with the bowl 16, in order to provide a restricted
passageway 74 therebetween. Baffle 66 serves the same function
described with reference to the embodiment of FIG. 2 and it also
seals chamber 68 from the ports 24 leading to the atmosphere.
A major distinction between the embodiments of FIG. 2 and FIG. 5 is
that the embodiment of FIG. 5 effects pressurization of the heavy
phase material in the discharging zone 70 by means of an external
pressure source. Such pressure source is preferably a conduit 90,
extending from outside the centrifuge bowl 16 through the feed tube
42 to the feed chamber 44. Pressurized gas is delivered through the
conduit 90, the chamber 44 and the feed passages 54 to the
separating zone or chamber 68, where pressure is applied to the
inner surface of the light phase layer and transmitted through the
heavy phase material in chamber 68, thence through passageway 74,
to the heavy phase material in zone 70.
The pressurized gas is delivered to the separating zone 68 at
pressures above the pressure which would be centrifugally developed
by any portion of a light phase layer disposed radially outward of
the heavy phase port 24. With this arrangement, the inner surface
of the light phase material in the separating zone disposed between
the baffles 66 and 86 is maintained at a level b which is radially
outward of the levels a and x. The pressure from the gas, together
with the compressive force applied to the heavy phase material by
the screw conveyor 32, plus the centrifugal pressure head of the
light phase layer, all combine to overcome the centrifugal pressure
head of the heavy phase material having level x in the discharging
zone 70.
* * * * *