Note: Descriptions are shown in the official language in which they were submitted.
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When mixing liquids with propeller mixers two methods
have been used up to now. According to one, the propeller is
arranged on a vertical or somewhat inclined shaft, while,
according to the other, the driving shaft is horizontal with
the driving unit arranged outside the tank. In both cases the
object is to avoid circulation around the symmetry axle since
this decreases the mixing effect.
In order to save energy during mixing, translation
movements are preferred as turbulent energy has a worse distri-
bution ability. The turbulent movements shall only be bigenough to secure the essential mixing. It is preferable to
obtain a rotating movement where the central part is involved,
without unnecessarily extensive micro turbulence.
The possibility of obtaining such a flow is decreased
when particles are sedimented in the liquid, as these particles
are drawn towards the center due to the so-called tea-cup
ef`fect.
By the methods used up to now, these particles cannot
leave the center part because of the bad media exchange. Mixers
having horizontal shafts have therefore been directed across the
tank or in an angle less than 10 from the diame-ter. The
central dead zone and the co-rotation are then eliminated. It
turns out, however, that another two rotation centers are built
up on each side of the flow centerline.
When using mixers having vertical sha-Ets a central
dead zone may be avoided if the co-rotation is checked by
baffles arrangecl at the tanksides. A toroidal spiral movement,
; controlled to a certain extent by an upwards or downwards going
flow and an opposite periphery flow is then obtained, which is
superposed a checked rotation movement.
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This method has however several disadvantages. A
long propeller part must be used, which means strong
loads on motor and ge!ar box. Further a slow rotating
mixer must be used as well as expensive baffles~ which
often consume more energy than what is necessary for
the mixing process. Another way to solve the problem
to make the central part take part in the mixing
process is to inject air in this part, thus obtaining,
secondary flows which ensure media exchange and
diminish sedimentation. This method means however
often problems with vibrations in the mixer depending
on the air influencing the propeller and the hydraulic
connection between the geometry of the tank and the
propeller. This has been solved by providing the
propeller with a flow ring which stops radial flows
and prevents air from streaming from the pressure side
to the suction side oi the impeller.
The technique to inject air is of course only
profitable if aeration is demanded during the mixing.
According to the invention it is however possible
to use a free propeller without any ring, which is
arranged on a horizontal shaft and having a big
diameter when compared with the depth of the tank in
such a way, that a co-operation between co-rotation,
the main flow rotation of the propeller and the free
tip turbulences brings about a media exchange at the
central part, the tip turbulence then sweeping in a
spiral form through the bottoln layer of the tank thus
preventing sedimentation. This is obtained by help of
a special propeller design in combination with the
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tank design.
The invention provides a method for mixing a fluid
contained in a tank having either one of a circular wall or an
oval wall, comprisinc; the steps of: rotating the Eluid about
the center axis of the tank; simultaneously creating a tu.rbulent
flow in said fluid in the shape of a toroid having a plane sub-
stantially parallel t.o the bottom of the tank, said toroidal
turbulence being formed by the step of mounting a propeller
horizontally, the propeller having a diameter which is at least
40% of the depth of t.he fluid in the tank and having blades that
rotate in a plane substantially perpendicular to a tangential
poin-t on said wall, said toroidal turbulence having an upward
flow of fluid near the center of said tank, outwardly at the
surface of the fluid, angularly downwardly at the periphery of
the tank wall and inwardly at the bottom of the tank when the
blades are in a predetermined direction of rotation and having
a downward flow of fluid near the center of the tank when the
blades are rotated in a direction opposite to said predetermined
rotation direction to cause sediment on the bottom of the tank
to move.
The invention also provides a method for mixing a
fluid medium contained in a tank having either one of a circular
or an oval side wall centered on a vertical axis, comprising the
steps of: mounting a propeller having at least two blades
horizontally in the tank, the propeller having a diameter of at
least 40~ of the depth of the fluid medium; creating a laminar
flow in the fluid medi.um by rotating the medium around the
vertical axis of the t:ank; displacing the lifting force centers
in the fluid medium to a location withirl 65-75% of each blade
diameter taken from th.e center of the pr.opeller; and generatinc~
a turbulent flow in the fluid medium in the form of a toroid
having a plane substantially parallel tc t~e bottom of the tank
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and having a diameter of approximately 10~ of the propeller
diameter to stir up sediment in the tank, the turbulent flow
surrounding the periphery of the laminar flow and orbiting
spirally thereabout.
The invention which gives the possibility of energy
saving and extensive mixing, without risking propeller vibra-
tions, is described more closely below with reference to enclosed
drawings.
Above mentioned and other features and objects of this
invention will become more apparent by reference to the follow-
ing description taken in conjunction with the accompanying
drawings in which:
Figure 1 is an illustration of a horizontally arranged
propeller mixer;
Figure 2 is an illustration of a vertically arranged
propeller mixer with baffles;
Figure 3 is an illustration of wing tip turbulence;
Fiyure 4 is an illustration of a propeller with flow
ring;
Figure 5 is an elevational view of a propeller without
a flow ring;
Figure 6 is an illustration of a propeller according
to the present invention in an oval tank; and
Figure 7 is an illustration of the propeller of the
present invention directed towards the periphery of the tank.
; When regarding a wing tip (Figure 3) in a laminar
liquid flow, a t:ip turbulence occurs caused by the over pressure
on the underside and the under pressure on the upper side. This
turbulence follows the liquid backwards. Such a flow can be
counteracted by help of a tip plate on the wing, but such a
plate is in its turn the origin of corresponding turbulence
phenomena that remove the gain by eliminating -the turbulence
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first mentioned.
When regaraing a propeller having a flow ring (Figure
~), it can be seen that the liquid leaves the propeller along a
spiral formed line, the rotation of which originates from the
torque of the propeller shaft. If the flow ring is removed, a
tip turbulence occurs. The latter is directed in the opposite
direction of the main flow rotation and, if it becomes too
extensive, an important loss and a diffuse flow from the
impeller occurs (Figure 5). The problem is stressed if the
propeller diameter is big, compared with the water depth, as the
circumstances differ very much between the moments when the
propeller blades are vertical and horizontal respectively.
According to the invention the propeller is so
designed, that tip turbulence is limited and moving like a
spiral-formed
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whirl along the cylindrical surface of the main flow. It is
also possible to so dimension the tip turbulence, that it
mainly corresponds with the layer at the tank bottom and
that it sweeps in spiral form over the layer, thus preventing
sedimentation and resuspending particles already sedimented.
~; The size of the tip turbulence by a free flcwing
propeller depends on the pressure dlfference between the two
~ sides of the blade at the tips and o-f the position of the
t~ thrust center. As the thrust is proportional to the dynamic
~~ 10 pressure, a lifting force for a certain area is obtained
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which is proportional to the square radius to the center. By
conventionally designed propellers, the tips then give a
very strong lifting force. By forming the tips to vary
elipsoidally and displace the blade area towards the center,
a limitation of the tip turbulence is obtained. If the lifting
force center is located within 70% of the blade counted from
-the center, tip turbulences are obtained which have a diameter
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of about 10~ of the propeller diameter. This dimension
corresponds quite well with the layer thickness in a tank
having a depth of 1,1 - 1,2 times the propeller diameter. By
such dimensioning the spiral-formed circulation flow depending
on the torque, will be big enoucJh ~o exchange the liquid in
the tank center if the ratio tank diameter-liquid depth is
~- within the area 1-5. By a greater ratio, the number of mixers
can be increased to obtain the same spiral movement and to
keep the tip turbulence effect in the bottom layer.
- In the description above, circular tanks have been
referred to. The method can, however, also be used for oval
tanks (Fig.6). In such a tank, where there is a rotating flow
by the ends and a laminar flow thexebetween, a special
advantage is obtained by a free flow propeller because of the
overflow of tip turbulence which occurs at the partition
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wall (Fig. 6). In~ediately behind the linking by the
partition wall, a death zone normally occurs where particles
sediment. If the propeller then originates distinct tip
turbulences, these will sweep regu:Larly with the rotation of
the main flow into this zone ancl resuspend the material.
Like in circular tanks, it is important that the tip turbulence
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is small when compared with the total flow diameter, since
otherwise the tip turbulence will diffund into the main
flow and stop acting at the tank bottom.
The spiral-formed toroid movement as an energy saving
mixing method may be improved in various ways. The machine
may be arranged to oscillate in the horizontal plane in
order that the spiral movement switches its rotation center
and the tip turbulences sweep more congruently over the tank
~¦ bottom. More energy may then be saved as the effect necessary
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Another way to decrease the energy demand and the varia-
tions in the propeller load further is to gradually set the
angle of incidence for the blade depending on whether the
blade is directed towards the center or the periphery of
the tank as shown in Fig. 7. There is always a risk that
- the propeller blade will be overloaded at its center where
the flow speed is lower than at the periphery. In combina-
tion with an oscillating movement, this setting gives
-- possibilities to exactly adjust the flow for the mixing
intensity demanded. There is, of course, an advantage but
not a necessity that the whole unit is submersible.
In the description above a mixer having a propeller with
two blades has been referred to. The theory is, however,
~`~ valid for any suitable number of blades, but the effect is
the effect is not as great for propeller with a greater
number of blades.
While I have described abovle the principles of my
- invention in connection with specific apparatus, it is to be
clearly un~erstcod that this description is made only by way
~r 30 of example and not as a limitation to the scope of my inven-
tion as s~t forth in the accompanying claims.
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