Note: Descriptions are shown in the official language in which they were submitted.
~036S~7
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This invention relates to continuously operating sedimenta-
tion tanks wherein a rotating rake structure moves the sediment
or sludge over the tank bottom to a central outlet zone or sump,
while supernatant liquid overflows into a peripherally extending
launder.
Usually, a feed slurry or solids suspension is supplied
continuouYly to a central ~eed-well in the tank, while sludge
is pumped from the sump, and supernatant liquid overflows.
More in particular, the invention is concerned with
improvements pertaining to large sedimentation tanks for instance
in a size range of 300 ft. to 500 ft. diameter, and possibly
even larger, where the use of a traction-driven rake structure
is indiaated, in pre~erence to sel-contained pier-supported
rake structures requiring a central pier~supported drive head to
~upply the necessary driving torque. This latter type of rake `~
structure may become unwieldy, and uneconomical structurally as
well as with respect to torque requirements, when considering
those large size tanks.
With respect to this latter type, attempts have been made
to overcome suc~ limitations, for example as shown in the U. S. i
patent to Scott No. 2,122,385 and 2,122,383, both featuring
freely centilevering rake arms constructed and arranged to
automatically yield and override excessive sludge accumulations,
but neither one of which has been as satisfactory functionally
and structurally as the sludge raking mechanism-shown in U. S.
patent to Klopper No. 3,295,835 referred to below, although all
of these have practical tank size limitations.
By comparison, the traction type thickener as a class de~y
the aforementioned size limitations, larg~ly because o the
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US-1913 ~3~5~
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endwise support of the rake arm structure, and the provision
-- of the wheel-supported self propelled outer end of the arm.
This invention therefore is concerned with improvements
in the raking mechanism of the conventional traction type
sedimentation tanks, briefly termed "Traction Thickeners", as
exemplified in U. S. patent to Dorr No. 1,356,608 of 1920.
In this earlier type, the outer end of a rotating rake arm of
girder-like construction rests on a carria~e that is sélf-
propelled upon a peripheral track.
Supporting the inner end of the rake arm, and connected
rigidly therewith i~ a vertical cage structure surrounding the
pier, and provided with a vertical thrust bearing at the top.
In operation, this girder-~ike rake arm structure is
fully submerged, with the upper horizontal t.russ of the g~rder
extending adjacent the plane of the overflow level of the
tank, and the lower truss substantially conforming to the
bottom profile of the tank. In such large tanks it is
customary and economical as well as technically sound to have
the bottom profile comprise a shallow outer annular portion of
only slightly inclined inverted frusto-conical shape, surrounding
a central sludge accumulating area of a substantially deeper
conical shape, with an annular "break" representing the
intermediate line of demarkation.
Sludge raking blades are fixed directly to the underside
of the lower truss. A very rugged rigid rake arm structure of
this type was required to withstand excessive raking loads and
sludge accumulations occurring mainly in the central bottom
area of 30 large a tank. This problem was aggravated when
handling heavy metallurgical and viscous sludge, provided that
~lLppage between tho carriage and the track was evolded, and
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the raking load not so excessive as to cause overload responsive
devices to stop the operation.
Conventionally, the carriage is driven by a gear motor
receiving its power supply through a slide contact provided upon
the center pier, in~erconnecting an inleading cable extending
along a stationary radial walkway or access bridge, with an
outleading cable extending along the rotating rake arm structure.
A feed pulp supply channel or pipe is carried by the bridge.
This carriage requires drive means sufficiently powerful
to cope with the aforementioned loads or sludge accumulations
in so large a tank even though protected by overload responsive
device~ to stop the operation in case of excessive overloads.
Thereore, it is one ob~ect o~ this invention to provide
a traction-driven rake mechani~m Por large diameter tanks, ~ ;
capable of handling and re~olving the a~orementioned loads or
sludge accumulations, with minimum power requirements for
maintaining uninterrupted operation, and by the same token to
preserve and maintain the roller traction force of the self-
propelled carriage against slippage.
A further objcct is to provide lift capability to the rake
mechanism of a traction-dr1ven unit of a large sedimentation tank.
Another object is to provide such a traction-driven rake
mechanism to be of relatively light construction, and requiring
a minimum of submerged corrosion resistant expensive parts
usually of stainless steel, ~or operakion in corrosive liquids.
In conjunction with the above stated objactives, the
invention is furthermore aoncerned with the problem to overcome
limitations inherent in the above mentioned rotary rake structure
of Klopper Patent No. 3,295,835, ~eaturing a rake arm structure
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- ~ US-1~13 ~0365~7
of great simplicity and low torque requirement, and capable of
yielding to overloads to resolve excessive sludge accumulations,
but for practical reasons limited to tank sizes much smaller
than those contemplated for the traction thickener of this
invention.
In the Klopper patent, a linear or straight tubular bladed
rake arm has a compound hinge connection between its inner end
and the lower end of a vertical drive shaft. This hinge
connection allows the arm to swing vertically as well as
horizontally, while preventing rotation of the arm about its
own longitudinal axis. Sloping guy wires connect the rake arm
with the upper end portion of the shaft so as to maintain the
raking blades o~ the arm clear ~æ the tank bottom under normal
sludge raking conditions.
A horizontal drive arm extends rigidly from the upper end
portion of the vertical drive shaft. A downwardly and rear-
wardly sloping draft element connects the drive arm with the
rake arm, causing the rake arm to follow the drive arm in its
rotation, while the compound hinge connection allows the rake
arm to yield and override any undue sludge accumulations, thus
gradually allowing the same to become resolved.
~owever, for practical reason~, the Klopper type of linear
rake arm structure is limited to only a length at which its `
weight can still be supported effectively by the sloping guy
wires.
Moreover, if the length of suoh a rake arm were to be
extended well beyond such a practiaal limi~, its overall raking
capacity would be impaired, with the outer end of the arm
rising much higher than desired, when yielding to excessive
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~ludge accumulations. Yet further, it would be di~ficult or
at least awkward to attempt to adapt this tubular rake arm to
the aforementioned "break" in the bottom contour of those large
size tanks, compounding the gu~ wire supporting problem.
Accord~ng to one aspect of the present invention, there
ls provided in a continuously operating large diameter settling
tank havlng means ~or supplying feed slurry to the tahk, sediment
discharge means at the foot Or a center pier, and overflow means
for supernatant liquid, the combination which comprises a center -
pier, a vertical cage structure surrounding said pier, and
supported upon said pier by a thrust bear~ng at the top thereo~
for rotation about a vertical axis, a main drive arm of girder
type construction having an upper and a lower truss and it~
lnner end rlgldly connected to sald cage structure to rotate
therewith, and having its outer end supported by a track for
travel along the perlphery of the tank, means for propelllng the
outer end Or said drive arm along sald track, ror rotatlon to-
gether with said cage structure, a bladed rake arm extending
from the lower end of said cage structure, and by its length ~
deflning the radlus and extent Or an lnner bottom zone of sedi- -
mentatlon sludge accumulatlon around sald pler, surrounded by
an outer annular bottom sedimentation zone, wlth the outer end
portion Or said drive arm extending a slgniflcant dlstance beyond
sald lnner zone and radlally across sald outer zone, hlnge means `~`
connectlng the inner end of said bladed rake arm to the lower end I
of sald cage structure, constructed and arran~ed so as to allow ;
sald rake arm to swing from a predetermlned }owermost normal
raklng posltlon rearwardly upwardly along a predetermlned path,
while preventing said rake arm ~rom rotatlng about lts own longl~
tudinal axls, a ~loping guy wlre connecting sald rake arm wikh
the upper end portion of said cage structure, and normally ~ ;
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extending in a substantially vertical plane, said guy wire
being effective to support the rake arm in said lowermost posi-
tion relative to the tank bottom under normal sludge raking load
conditions, downwardly and rearwardly sloping draft means having
its lower end connected to said rake arm, and its upper end
connected to said drive arm by a drive connection whereby said
drive arm and said rake arm are rotated together, said rake
arm during suoh rotation being adapted to move the sludge in
said inner zone towards said sludge discharge means, and to
yield rearwardly upwardly along said predetermined path when
; overriding excessive sediment accumulation in said lnner zone ~;
while resolvln~ a sludge overload condition therein during
continued rotation of the drive arm and rake structure, and outer
complementary raking means connected to said outer end portlon
of the drive arm, and con~tructed~and arranged for movlng
relatlvely light sedimentatlon sludge load rrOm said outer
annular zone to said inner zone of sedimentatlon sludge
accumulation.
The foregoing concept of a novel combination embodied
;~ 20 in the composite sludge raking mechanism, incorporates the
structural advantages and functional effectiveness of the
load-yielding linear rake member, with~ut exceeding its pr~c- ;
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tical size limitations, to operate in the critical central
sludge accumulation area of the tank, and to resolve potential ~`
sludge overloads.
The surrounding bottom area is served by the auxiliary
outer rake structure for moving the outer lighter portlon of
the sludge load lnto the central zone of sludge accumulation.
The traction-driven girder which may be elevated to
a position above the overflow level o~ the tank, thus provides
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the common motivating member or drive arm for ~he c~mplementary
outer end rake structure and for the draft-connected yieldable
sludge conveying arm.
As a result, the composite raking mechanism as a whole is
relatively lightened, while the extent of submergence is
minimizea, combined with lessened drive tor~ue requirements
and assured continuity of operation. Moreover, tbe composite -
~ .
rake mechanism in this combination is readily adaptable to the ; -
profile of the tank bottom involving the aforementioned
intermediate "break", while only a minimum of submerged structure ;~
is located in a position where it must react against being
dragged through more viscous sludge material. `
Specific features reside in the provision o~ supplemental
load-responsive means integrated into the composite raking
mechanism of the invention to provide additional sludge raking
capacity in the central sludge accumulation zone.
Other specific features are concerned with the provision
of means for efficient feed introduction into the tank, ;~
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~ integrated into the composite rake mechanism of this invention.
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As this invention may be embodied in several forms without -
departing from the spirit or essential characteristics thereof,
the present embodiment is illustrative and not restrictive.
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The scope of the invention is defined by the appended claims
rather than by the description preceding them, and all embodi- ~
' 25 ments which fall within the meaning and range of equivalency ~;
of the claims are therefore intended to be embraced by those
' claims.
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~ ` US-1913 1~365~7
Fig. 1 is a schematic vertical sectional view of the
settling tank r showing one embodiment of the invention in a
center pier supported traction-driven sludge raking mechanism.
~S~eet ~)
~A-~ Fig. 2~is fragmentary plan view of the raking mechanism,
taken on line 2-2 in Fig. 1.
Fig. 3 is a side view of the raking mechanism, taken on
, line 3-3 of Fig. 2.
Fig. 4 is a side view similar to Fig. 3 of the raking
mechanism according to another embodiment of the invention.
Fig. 5 is a detail plan view of t.he rake arm taken on
Line 5-5 of Fig. ~, showing the attachment of drag elements for
the rake member.
, Fig. 6 is an enlarged vertical sectional view of the center
;~ pier supported cage portion o~ the raking mechanism, and of a
; composite feedwell construction rotating therewith.
Fig. 7 is an enlarged cross-sectional view of a double
hinge conn0ction of the raking mechanism, taken on line 7-7
of Fig. 6.
Fig. 8 is a sid~ view taken on line 8-8 of Fig. 7.
~ Fig. 9 is an enlarged vertical sectional view of the center
"~
support bearing means of the raking mechanism, taken from Fig. 6.
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Fig. 10 is a cross-section~l detail view of the raking
mechanism taken on line 10-10 in Fig. 3.
Fig. 11 is another cross-sectional detail view of the raking
mechanism taken on line 11-11 in Fig. 3.
::1
J Fig. 12 is a cross-sectional view taken on line 12-12 in
Fig. 6, showing the auxiliary ~ee~well from khe top.
Fig. 13 is a cross-sectional view taken on line 13-13 in
~l Fig. 9, showing the top end portion of the cage structure.
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.~ .,--~ Fig. 14 i~ a cross-sectional view taken on line 14-14 -;
in Fig. 6, showing the main feedwell from the top.
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US-1913 ~Q36S~7
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As exemplified in Figs. 1, 2 and 3, the invention is
embodied in a settling tank 10 of relatively large diameter for
example in the order of 250 ft. or larger, having a traction
driven rotating sludge raking mechanism of the general type
exempliied in U. S. Patent to Dorr No. 1,356,608.
In this mechanism, the inner end of a rotating girder type
rake arm structure 11 i9 supported upon a center column or
pier 12, for instance by means of a known self-adjusting
spherical thrust bearing later on to be more fully disclosed
by reference to Figs. 6 and 9. The outer end of the rake arm
structure operates upon a peripheral track 13 provided atop the
wall of the settling tank, and is propelled thereon by an
otherwise known motorized traction drlven carxiage or mechanism
indicated at 14 in Fig. 2.
A stationary girder type access bridge struature 15
extending above the rake arm, has one end supported upon the
center pier, while the other end had a support column 16 located
outwardly adjacent to the circular wall of the tank. This
bridge carries the feed supply conduit 17 delivering the pulp
or feed suspensions to the feedwell construction 18 surrounding
the center pier, and furthermore to be described by reference
to Figs. 6 and 9. Also carried by the bridge is an inleading
cable 19 which via slide contact rings (see Fig. 9) connects
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~i to an outward leading cable 20 carried by the rake arm structure
to energize the motor of the traction driven mechanism.
As shown in this example ~See Fig. 3), the shapa o the
tank bottom compri~es an outer annular settling area A-l o
shallow slope, surrounding an inner annular area A-2 o an
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us-lgl3 ~036~7
adequately steeper conical lncline, which in turn surrounds an
annular sump "S" around the center pier 12. The shallow slope
of the outer area is for receiving the lighter portion of the
sludge load to be moved inwardly by the rotating rake structure,
while the area of ~teeper conical incline is adequate to handle
the accumulating heavy portion o~ ~he loadO
A usual sludge withdrawal pipe "P" delivers collected
sludge from the sump.
There now follows a more detailed description of the rake
structure and its associated part~, embodying one form of this
invention.
A pre~erred rotating girder 11 ~truature ~or the a~ore-
mentioned large size tank may be o rotating simple and relatively
light con~truction and otherwise constructed in the manner
indicated in Fig. 11. ~hi~ girder 11 rigidly extends from a
vertical cage structure 21 surrounding the center pier (see Figs. 3
and 6) supported upon the pier by the aforementioned sel~-
adju~ting bearing to be furthermore described by reference to
ig. 9. The cage structure 21 square shaped in cross-section,
is generally defined by vertical corner members 21a(see Fig. 14)
interconnected by suitable diagonal bracing members.
A novel combinatLon of sll~dge raklng means according to
this invention, enables the-traction-driven raking mechanism to
handle heavy sludge loads or aacu~ulations in large slze tanks
, .
with relative ease, that is to say, without requiring undue
traction ~orce such as may re~ult in track slippage of the
traction drive, yet with a lighter rotary girder arm construction
. ~ .
: as compared with prior art rake mechanisms for comparable tank
~, sizes.
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US-i913 1~365~7
This is accompli~hed by a combination o~ a composite raking
mechanism wherein a txaction driven girder arm structure is
combined ~ith two different types of sludge engaging raking
~.
means each to operate in their respsctive annular bottom zones
A-l and A-2, each with optimum effectiveness.
Accordingly, for handling ~he lighter sludge load in the
i outer settling zone A-l, a set of plowing raking blades is
rigidly connected to the ou~er end of the girder. In the
embodiment of Fig~. 3, 10 and 11, such raking blades 22 are
connected or welded to a pair o horizontally spaced parallel
stringers 23 and 24 which in turn are further rigidly intex-
connècted by mean~ of horizont-~ cros~ members 25. ~his rake
structure designated "S" comprising part~ 22, 23, 24 and 25,
i~ rigidly connected to the glrder arm above, for in~tance by
vertical frame structures 26 such as indicated in Figs. 3 and 11.
Th~ much heavier sludge accumulation in the inner settling
i' zone A-2 i9 handled effectively and without overload risk by
~` a yieldable rake arm 27 of linear or tubular configuration, to
which are fixed or welded a series of plowing raking blades 28.
.
The inner end of the auxiliary arm is connected to the lower end
of the vertical cage structure 21 by means of a double hinge
device 29 indicated in Fig. 6, and more clearly in the enlarged
details of Fig~. 7 and 8.
The double hinge device in this embodiment comprises a
l horizontal bracket plate 30 fixea to a corner member 21a of
;, the vertical cage ~tructure. A vertical pin or pivot member 31
li in this bracket plate i9 secured in place as by nut~ 32 at the
~ top.
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US-1913 1 V3 6 S ~i7
At the bottom, this pin has an inverted head portion or
vertical plate 33 to which the auxiliary rake~arm 27 is hinged
by means of an horizontal pin or pivot member 34. ~-
This double hinge connection allows the linear rake member
27 to swing up and down as well as laterally, but is prevented
from rotating about its longitudinal axis. For this rake arm
to become operative, there is provided a set of divergent guy
wires 35 (see Fig. 3) connecting an intermediate point "P-1"
of the cage structure with points A, B, C of the rake arm.
These guy wires support the weight of the arm, while normally
maintaining it clear of the tank bottom, when in its lowermost
position. A set of rearwardly inclined divergent drag-or draft
elements 36 (see Figs. 2 and 3) connects a point "P-2" o~ the
tractlon driven girder with points D, E, F, G o~ the rake arm
27.
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The rake arm 27 thus connected to the cage portion and to
.. . .
il the girder respectively, under normal load conditions will
,1 //
travel or trail behind the traction driven girder ~ as
ind1cated by angle "T" in Fig. 2. But when excessive sludge
accumulation is encountered, this arrangement allows the rake
arm to yield and override obstructions while nevertheless
continuing in its sludge conveying function.
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Howev~r, for pxactical reasons previously set forth, the
length of the yieldable rake arm and its raking capacity are
~ limited. Therefore, in the large settling tank equipped with
i the composite rake mechanism o the invention, and with the
outer rakes in zone A-l adding to the slud~e accumulation in
the inner zone A-2, additional raking capacity may be provided
to cope with that accumulation.
For that purpose (see Figs. 2 and 3), the rake mechanism
is shown to comprise a set o~ auxiliary short rake arms 37, 38
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US-1913
and 39 constructed an ~ ~r3r ~ ed to operate in the manner of
` the yieldable rake arm 27. Accordingly, the rake arm 37 has
draft elements 40, 41 and 42 connecting it with point "P-3"
i~
of a foreshortened auxiliary rake arm 42a which extending from
the cage structure appears as a rearward structural girder-like
; extension of the traction driven main girder arm. While
, .. .
yieldable by way of a double hinge connection similar to the
one described above, the weight supporting guy wires for this
rake arm are indicated at 43, 44 and 45 respectively in Fig. 3.
A pair of similar foreshortened auxiliary arms 46 and 47
extend in opposite direction to each other and at right angles
:, to arms 11 and 42a. A set of draft elements 48, 49 and 50
~,~ connect the rake arm 38 with a point "P-4" of auxiliary drive
,4 arm ~6. Similarly a set o draft elements 51, 52 ana 53
connect the rake arm 39 with point "P-5" of auxiliary drive
: arm 47. It will be understood khat the auxiliary yieldable
rake arms 38 and 39 are provided each with kheir own weight
, supporting guy wires extending in a vertical plane when the rake
arms are in their lowermost or normal raking position, similar
'i to guy wires 43, 44 and 45 (see Fig. 3) shown to support the: i
rake arm 37.
As shown in Figs. 6 and 9, an example of the self-adjusting
,.,;
spherical bearing supporting the inner end of the traction
`i driven girder arm and rake structure, comprises a hollow bearing
base member 54 bolted to the top end of the center pier or
column 12. This bearing base member is in khe Eorm of an
inverted flange cup having an upward cylindrical extension
i 55 providing in effect an upward extension of the center column,
; for supporting the access bridge 15.
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~Q365~7
An annular spherical bearing face 56 of this base member
cooperates with, and is engaged by the inner spherical face 57
of the surrounding bell shaped bearing member 58 which has a
top opening 59 accommodating said extension 55. This outer :.
bearing member is self-adjusting spherically, but has rotation
preventing lugs 60 and 61 cooperating with lugs 62 and 63
provided on the bearing base member 54. ';
The outer bearing member 57 has a peripheral bottom portion
forming an annular thrust bearing 64 supporting a horizontal
bearing plate 64a bolted to a surrounding square shaped frame .
member 65 which in turn constitutes the top end of the cage
structure 21 (as also seen from Fig. 13).
A slide contact device 66 ~nnects the inleading cable 19
with the outleading cable 20, schematically shown to compri~e
a contact ring 67 mounted on the outer spherical bearing member
58, and contact brushes 68 mounted on the annular bearing plate
64a rotating with the cage structure 21.
The slurry feed pipe 17 supported on the access bridge 15
delivers to the feedwell construction 18 mounted on the rake
structure and rotating therewith.
This feedwell construction generally speaking is similar .:
to one shown in U. S. Patent to Stansmore No. 3,542,207,
comprising a main feedwell 69 and an auxiliary feedwell 70
which has a pair of downwardly directed feed pipes or downcomers :
71 and 72 delivering respectively half portions of the feed
slurry into the main feedwell under a head as represented by
the length or vertiaal height o~ these pipes.
The main feedwell 69 (see Figs. 6 and 14~ is connected
to the cage structure 21 as indicated by horizontal support
members 73 and 74. Normally, this feedwell is submerged with
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US-1913 1 Q3 6 S ~ ~
only a top edge portion "d" shown to extend above the tank
overflow level "L".
Internally, this feedwell is formad with a pair of annular
channels 75 and 76 providing concentric annular horizontal
raceways one above the other, for the two counter-currently
directed streams of feed slurry from pipes 71 and 72. These
raceways are de~ined by three horizontal annular baffles 77,
78 and 79.
Accordingly, from these pipes two separate streams of
the feed slurry having velocity energy are fed into the
respective annular channals or raceways, each in a direction
opposite to the other~ Thus, the two influent streams are
directed int~ annular countercurrent paths located one above the
other, and in such a manner that the in~luent energy i0
dissipated, and the feed pulp i~ distributed radially evenly
in all directions, while entering the surrounding body of
li~uid undergoing sedimentation.
The auxiliary feedwell 70 (see Figs. 6, 9 and 12) comprises
a bottom plate 80 mounted atop the cage structure 21, and
surrounding the spherically self-adjusting bearing described
above. The bottom plate therefore has a central opening 81a
and a pair of syn~etrically disposed eccentric openings 81b
and 81c connected to the respective aforementioned downcomers
71 and 72. An inner cylindrical wall 82 is connected to the
central opening, concentric with the center column 12. An outer
peripheral wall 83 extends along the edge or peripheral contour
of the bottom plate, shown as of the same higher "h" as the inner
cylindrical wall 82. A pair of arcuate lower walls 84 and 85
provide overflow weirs 84a and 85a.
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These arcuate walls or weirs merge at points designated
"B-l", "B-2", "B-3" and "B-4" with respective adjacent arcuate
portions "B-5" and "B-~" of the peripheral wall, and define
therewith and with the inner cylindrical wall 82, an annular
feedwell area A-3 receiving feed slurry from feed pipe 17.
That is to say, feed slurry overflow the two arcuate weirs in
oppoi~ite directions into respective receiving areas A-5 and A-6,
then plunging through the two downcomer pi~es into the main
feedwell 69 below.
In this way, there is achieved an effective introduction
o~ feed slurry from the stationary ~eed pipe 17 located on the
bridge, into the body of liquid undergoing sedlmentation, by
., i .
way o~ the composite rotating ~eedwell construation 18 described
above.
In Fig. 4, the invention is embodied in a rake mechanism
which di~fers from the embodiment of Figs. 1 to 3, in that the
, I , .
rotary rake struature comprises a traction driven rake arm 86
connected to an intermediate portion of a vertical cage
structure 87, and otherwise arranged to be submerged in the body
of liquid undergoing sedimantation. ~his embodiment allows for
correspondingly decreasing the height of the center column, and
the elevation of the access bridge, as well as the height to
which the feed slurry must be pumped.
This submerged low level girder arm is of simple and
relatively }ight construction, having parallel upper and lower
j chords 88 and 89, in this respect resembling the emerged or
high level girder arm o~ ~igs~ 1 to 3. Yet, its low level
l submerged arrangement allows for raking blades 90 operating in
I the outer bottom ~one A 7, to be ~ixed directly to the lower
chord 89.
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36507
A yieldable rake arm 91 is supported and operates in the
;
inner bottom area A-8, and in the manner similar to the one
in Figs. l to 3. Accordingly, there are shown draft elements
92, 93, 94 and 95 connecting the rake arm 91 with point "P-5"
on trailing side of the upper chord 88.
By way o comparison of the two embodiments, it should
be understood that the high level girder arm arrangement in
Figs. 1 to 3 i8. of practical importance where the liquid under-
going sedimentation is of a corrosive nature requiring the use
of expensive corrosive resistant steel ~or the submerged parts
of the rake structure. The high level emerged position of the
girder arm of Figs. 1 to 3, although requiring the vertical
connecting frame structure 26 for the fixed raking blades,
minlmlzeo thc expcn~ic for the corroslvo rc~lstant ~tecl.
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