Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Mixing Systems
- _escription
The present invention relates to mixing
systems, and particularly to methods and apparatus for
mixing and circulating liquids having fibers suspended
therein.
The present invention is especially suitable
for use in mixing systems designed to treat waste water
by the circulation of the water in tanks with an axial
flow impeller in a draft tube. The invention is also
suitable for use in other mixing applications li~uid is
circulated with an axial flow impeller.
When waste water is mixed in tanks, as in the
course of waste water treatments such as aeration, the
power necessary to drive the impeller has been found to
increase. After a period of time, which may be from one
hour to four days, the required power to circulate the
waste water sufficiently to mix and aerate it may
increase up to 50~. The problem may be overcome by
2~ utilizing drive motors of higher power rating and bear-
ing the cost of the increased power.
The cause of this problem escaped appreciation
until the time of this invention. In accordance with
this invention, it was discovered that the drag on the
impeller increased with time as a consequence of the
accumulation of fibers on the leading edge of the
impeller. In waste water, which arrives from a sewage
system for treatment, the fibers are hair, rags (such as
diaper~ and feminine products) and plastic bags. Other
fiberous materials suspended in the liquid, which is
circulated and mixed, also accumulates on the leading
edge of the impeller with the same adverse effect.
MX-2~3
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The problem has been found, in accordance with
the invention, to be exacerbated when the liquid con-
tainin~ the fibers in suspension is circulated and mixed
with an axial flow impeller in a draft tube arrange-
ment. Then the velocity of the impeller blades throughthe liquid is several times the velocity of the liquid
in the direction axially of the impeller and draft
tube. There are components of drag on each of the ends
of the fiber which are disposed on opposite sides of the
leading edge of the blades of the impeller; thereby
leaving no net force tending to pull the fiber off the
impeller. The result of these drags is a force having a
component normal to the leading edges of the impeller
blades which holds the fibers on the leading edges, the
fibers therefore cling tenaciously to the leading
edges In time the fibers accumulate and present a
rough surface on the leading edge and around the leading
portion of the blades which progressively increases the
drag, decreases the lift and flow (pumping) and requires
more and more driving power to maintain the velocity of
the impeller necessary to obtain sufficient axial flow
and circulation of the liquid. The efficiency and per-
formance of the mixing system is therefore derrogated.
Accordingly, it is the principal object of the
present invention to provide an approved mixing system
whereby mixing can be carried out with low drag on the
impeller which circulates and mixes a liquid in spite of
the presence of fibers, in suspension, in the liquid.
It is a further object of the present invention
to provide improved methods of and apparatus for mixing
liquids, the performance and efficiency of which is not
reduced when fibers are suspended in the liquid bein~
mixed.
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It is a still further object of the present
invention to provide improved mixing apparatus wherein
- axial flow is produced within a draft tube with an
impeller having low drag characteristics even in the
presence of fibers in the liquid being mixed.
Briefly described, a system for mixing liquids
in a tank in accor2ance with the invention utilizes an
impeller which has opposite edges which respectively
lead and trail each other as the impeller rotates. This
system is operative to circulate the liquids through the
tank along a flow path which extends axially of the
impeller. The adhesion of fibers suspended in the
liquid along the leading edge of the impeller is
decreased, thereby preventing increased drag on the
impeller due to the accumulation of the fibers on the
leading edge as the impeller rotates. The adhesion is
decreased by inclining the leading edge with respect to
-- a radial line from the axis of the impeller sufficiently
to be greater than the angle of repose of the fibers on
2~ the impeller. The angle of repose is that angle where
the forces normal to the leading edge are sufficient to
hold the fibers against the leading edge, considering
the coefficient of friction of the surface of the
impeller at the leading edge with respect to the
fibers. Mechanically, the leading edge presents an
inclined plane with an inclination larger than the angle
of repose. Accordingly, the fibers are not held with
sufficient force to maintain them on the leading edge
and they continue to circulate with the liquid. This is
in spite of the absence of any significant radial com-
ponent of flow; the flow being essentially axial, par-
ticularly where the impeller is an axial flow impeller
in a draft tube. The adhesion of the fibers along the
3~
leading edge may also be decreased by providing the por-
tion of the impeller adjacent the leading edge with a
surface of low coefficient of friction material,.such a
low coefficient o~ friction may be provided by polish-
: 5 ing. Low coefficient of friction material is preferably
provided by utilizing a plastic material presenting a
low coefficient of friction to the fibers.
. The foregoing and other objects, features and
: advantages of the invention, as well as presently pre-
ferred embodiments thereof and the best modes presently
~ known for practicing the invention, will become more
: apparent from a reading the following description in
connection with the accompanying drawings in which:
FIG~ 1 is a sectional view, in elevation~ of a
mixing system embodying the inven~ion;
FIG. 2 is a sectional view taken along the line
2-2 in FIG. 1, illustrating the low drag impeller pro-
vided in accordance with an embodiment of the invention
in greater detail;
FIG. 3 is an enlarged view, in elevation,
: illustrating one of the blades of the impeller shown in
FIGS. 1 and 2;
FIGo 4 is a sectional view of one of the blades
of the impeller illustrating its airfoil shape and .
designating the chord, camber and thickness of the
blade, the section being taken along the line 4-4 in
FIG. 2.; and
FIG. 5 is a view similar to FIG. 2 illustrating
a low drag impeller in accordance with another embodi-
ment of the invention.
Referring more particularly to FIG~ 1~ there isshown a tank 10 which may be used for the mixing of
liquids, such as waste water, having fibers in suspen-
sion. A draft tube 12, having a conical upper section
~l 2 4 . 3 ~
14 and a cylindrical lower section 16, is suspended from
a beam 18 by posts 20. The lower section may also be
diverging. The beam 18 also carries a drive assembly 22
including an electric motor 2~ and gearing 26. Suitable
bearings in the gearing section 26 rotatably support a
drive shaft 28. An impeller 30 having a plurality of
blades 32 is connected to a hub 34 which is connected to
and driven by the drive shaft 28. The draft tube sec-
tions 14 and 16 define an impeller way 36. When the
system is used for mixing and aerating, a sparge ring
may be mounted below the impeller 30 in the lower sec-
tion 16. Alternatively, air may be inducted through
holes in the blades for sparging purposes as described
in U.S. Patent No. 4,231,974. The operation and advan-
tages of the impeller way 36 are described in U.SOPatent No. 3,477,382. Further information respecting
the design of draft tube mixing systems may also be
obtained from U.S. Patent No. 4,385,206.
The impeller 34 is an axial flow impeller.
Each of the blades have an airfoil shape and cause the
li~uid in the tank to be circulated through the draft
tube, preferably downwardly when the system is used for
mixing and aerating, although upward circulation may
also be used. Vanes (not shown), for example as
described in U.S. Patent No. 4,231,974, may be used to
further direct the flow axially of the impeller. The
impeller has a plurality of blades, three blades 32
being used as shown in FIG. 2. Each of the blades is
identical to the others. They are mounted 120 apart on
the hub 34 and may be welded at their bases 35 to the
hub 34.
As shown in FIGS. 3 and 4, each b~ade 32 has an
airfoil profile. The chord of the blade ~CL) is
2 ~3~30~
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measured between its leading edge 38 and its trailing
edge 40. The blade has camber as measured between its
midline 42 and chord. The blade also has twist,, as
shown in FIG. 3, in that the angle between the chord and
the lower surface of the blade is greater at the base 3
of the blade than at the tip 44 thereof, and may be
10-18 greater. In order to provide for axial flow, the
' camber may have a maximum length or value of from about
' 4 to 8 percent of the chord length. The location of the
maximum chamber length may be from about 20% to about
60% of the chord length away from the leading edge 38
towards the trailing edge 40. The blade also has thick-
ness between its upper and lower surfaces preferably
from about 6 to 14 percent of the chord length. The
width of the blade at the hub may be 22-28% of the chord
length. The width of the blade at the tip may be 14-20%
- of the chord lengthO The tip chord angle (between the
chord at the tip and the horizontal may be 5-25. The
blade may be constructed from a pair of plates 46 and
48, as shown in FIG. 4 which may be welded together near
the leading and trailing edges. An insert 50 is pro-
vided in a portion extending approximately 10 percent of
the length of each of the upper and lower surfaces from
the leading to the trailing edges 38 and 40 of each
blade 32. This insert is preferably a body of plastic
materials which presents a surface having a low coeffi-
-~ cient of friction. ~ suitable material is ultra high
' molecular weight polyethlene (nUHMWn). The molecular
weight of this material is approximately 5 million.
- 30 Suitable material is available froM Poly-Hi/Menasha
Corp. under the trade name TIVARR. Other material
presenting a low coefficient of friction is suitable,
- for example a flouropolymer (such as kn~wn by the trade
t~`?8~{3
- ~ 7 --
name Teflon). Ultra high molecular weight polyethelene
is preferred.
It will also be observed that the tips 44 of
the blades 32 have the shape of a sector of a circle and
conform to the cylindrical inner periphery of the draft
~ tube; extending to the inner periphery of the lower se~-
- tion 16, and separated therefrom only the necessary
clearance to maintain manufacturing tolerances. The
leading edge 38 is also longer than the trailing edge
40. The leading edge has an arcuate section 52 near the
base end 3S and extends to a straight section 54. The
trailing edge 40 is entirely straight. The arcuate sec-
tion 52 i~ provided in order that the inclination of the
leading edge can be sufficient so that the angle of
- 15 repose of fibers which tend to accumulate on the leading
edge is not reached. This angle of repose may be
measured between the leading edge and radial lines
extending from the axis 56 of the impeller (the center
line of the shaft 28 and hub 34). In a preferred
embodiment, the angle indicated as, 0, in FIG. 2 between
- a radial line 60 in a plane perpendicular to the axis 56
which intersects the leading edge 38 at a point 62,
where the radial line has a length, RL, equal to 70 per-
cent of the radius of the blade from the tip 44 to the
25 axis 56, is 40. This angle of inclination, 0, may be
in a range from 20 to 60 depending upon the nature of
the fibers which are suspended in the liquid being mixed
and the coefficient of friction of the surface provided
by the insert 50.
In some cases, in lieu of such an insert with
low coefficient of friction, a sufficient inclination
angle ~ will suffice. Also, instead of providing an
insert to present the portion of the surface at the
leading edge with a low coefficient of friction, the
portion may be polished. In lieu of an insert 50 the
low coefficient of friction material may be coated or
otherwise bonded onto the surface. For further informa-
tion on coating techni~ues reference may be had to
D. P. Willis, Jr., Increasing Lifetime with Flouro~
polymer Coatings, Appliance Engr. Vol. 7, NoO 1 (1973
and D. P. Willis, Jr., Machine Design, April 10, 1980,
pp. 123-12?~
In the event that increased angles of inclina-
tion are desired, for example 50 or more tne impeller
30 may be provided with blades 64 of a design shown in
FIG. 5. These blades may be of airfoil profile their
leading edges 66 are sectors of a circle. Thé angle of
inclination, 9, as in FIG. 2, is measured at the inter-
section 62 of the radial line 60 sf length 70% of the
radius to the top 70 of the blade with the leading edge
- 66. The angle included between the radial line 60 and a
line 72 tangent to the leading edge 66 at the point 68.
The center of the sector is shown at 68 along a line
perpendicular to tangent line 72~
~ he trailing edge 74 of the blade is made up of
two arcs, one of which 76 is a sector of a circle having
its center at 68 and the other of which 78 is also a
sector of a circle having its center 80 within the blade
64.
- The portion 81 of the blade adjacent the lead-
ing edge 66 may be provided by an insert of low coeffi-
cient of friction material, similar to the insert 50.
The other techniques mentioned above, for providing low
coefficient of friction in the surfaces of the portion
81, may alternatively be used.
From the foregoing description it will be
apparent that there has been provided an improved mixing
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system in which axial flow for circulation and mixing of
liquids is provided without the derrogation of effi-
ciency and performance which results from fibers sus-
pended in the liquid being circulated and mixed. Varia-
tions and modifications in the herein described system,within the scope of the invention, will undoubtedly
suggest themselves to those skilled in the art. Accord-
ingly, the foregoing description should be taken as
illustrative and not in a limiting sense.
