High solidity counterflow impeller system

A mixing apparatus has a tank for holding a material to be mixed, a drive shaft rotatable in the tank, a radially inner impeller on the drive shaft with blades pitched to produce axial flow of the material in a first direction, and a radially outer impeller with blades pitched to produce axial flow in an opposite direction. The radially inner impeller can be a high solidity impeller disposed in a preferably-stationary flow shield occupying a portion of a circumference between the inner and outer impellers, and providing a barrier between the material flowing axially in opposite directions while leaving spaces for recirculation of material by radial flow at the ends of the opposite axial flows. The outer impeller can be coupled to the drive shaft by connecting members protruding radially through axial spaces provided in or around the flow shield. Baffles are fixed in the tank and support the flow shield. The baffles have inclined inner and outer sections that extend axially and are pitched to intercept circumferential flow produced by the inner and outer impellers, respectively, redirecting the flow axially in the appropriate direction. A number of axially spaced impeller stages are provided, each having an inner impeller in a section of the flow shield and an outer impeller on connecting members that extend through axial gaps between sections of flow tube supported on the baffles.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of rotational mixing apparatus, and in 
particular concerns an impeller system employing high solidity radially 
inner impeller blades for pumping in one axial direction, coupled to 
radially outer impeller blades for pumping in an opposite axial direction, 
and with fixed baffles and flow shields that provide distinct inner and 
outer axial flow paths. 
2. Prior Art 
A high solidity impeller structure is disclosed in U.S. Pat. No. 
5,326,226--Wyczalkowski et al., which is hereby incorporated in its 
entirety. The impeller has a plurality of blades mounted on a rotational 
drive shaft to provide an axial flow with rotation of the impeller. Such 
blades are generally known as hydrofoil impeller blades, and are useful 
for mixing and aerating operations, in particular producing a circulating 
axially downward flow along the center line of a tank, with an axially 
upward flow around the periphery. Gas may be sparged into the tank, e.g., 
below the impeller, where the gas bubbles rise against the axially 
downward flow. 
An object of impeller blade design is to obtain the greatest efficiency of 
fluid movement, namely to maximize the volume of fluid moved per unit of 
power expended to rotate the impeller. Another object of impeller design 
is to reduce the cost of manufacture without adversely affecting the 
efficiency of the impeller or the attributes of the impeller for use in 
its particular mixing application. In the Wyczalkowski et al. patent, 
these objects are addressed by providing blades formed of plate stock, 
rolled along the axis of a cylinder such that the roll axis lags the 
radius at which the blades are attached to the drive shaft, for example by 
45.degree.. The blades thus approximate the shape of a hydrofoil, although 
they are made of rolled plate stock rather than being cast. 
The Wyczalkowski blades are dimensioned to form a "high solidity" impeller, 
namely an impeller in which the plurality of blades when viewed along the 
rotation axis, occupy a high proportion of the area of axial projection of 
the impeller, preferably about 90% of the area. High solidity impellers 
are particularly useful in sparging applications wherein a rising column 
of gas bubbles is opposed by an axial downward flow of liquid, because the 
impellers reduce the tendency of the rising gas to produce an upward flow 
leading to flooding, foaming or splashing. 
The required configuration of an impeller blade is complicated by the fact 
that the radially outer portion has a greater linear speed than the 
radially inner portion. The inner portion must be pitched more steeply 
than the outer portion to equalize the axial flow rates at different 
radii. The pitch of the impeller produces a resultant force component 
causing liquid to rotate with the impeller. In a high solidity blade 
configuration, the blades are relatively wide and paddle-like, such that 
the rotational displacement of the liquid can be substantial. Resulting 
centripetal acceleration causes a radially outward liquid flow component. 
Finally, eddy currents and turbulence occur adjacent to the edges of the 
impeller blades. 
High viscosity mixing applications can benefit particularly if axial flow 
is improved. As viscosity increases there is a tendency for the liquid to 
rotate locally with the impeller. In order to achieve overall fluid motion 
in high viscosity mixing applications (e.g., over about 50,000 
centipoise), it is sometimes necessary to provide a large diameter anchor 
agitator or a helical ribbon agitator that moves the fluid in the manner 
of an auger. Such "large" diameter agitating structures extend, for 
example, to 90% of the vessel diameter, and are relatively expensive. 
Insofar as the chosen structure of the rotating impeller is axially 
continuous, the impeller structure may preclude the possibility of placing 
fixed baffles between axially spaced impeller blades or sections, to 
better guide the flow in an axial direction as opposed to rotating the 
fluid. The absence of baffles also can make the mixing apparatus less than 
suitable for lower viscosity mixing applications (e.g., below about 20,000 
centipoise). 
It would be advantageous to optimize a mixing system for high solidity 
impellers and thereby to improve on the efficiency of fluid flow volume 
per unit of expended power. It would further be advantageous if this could 
be accomplished in a mixing apparatus that is efficient over a wide range 
of viscosities. It is an aspect of the invention that certain rotating 
counterflow impeller structures are employed with a high solidity impeller 
for mixing applications having radially inner and outer flow, together 
with fixed inclined baffles and flow shields, which work together with a 
high solidity impeller as in Wyczalkowski, for maximizing axial flow in 
both opposite directions with rotation of the impeller and over a wide 
range of viscosities. 
SUMMARY OF THE INVENTION 
It is an object of the invention to optimize the operation of a high 
solidity impeller for mixing applications over a range of viscosities, 
involving radially inner and outer axial flow in opposite directions. 
It is another object to couple sets of impeller blades structured for 
forcing a liquid in opposite directions, to a common drive shaft. 
It is a further object to intercept inefficient circumferential and radial 
flows produced by an impeller blade and to direct such flows axially. 
It is also an object to provide a structure to isolate radially inner and 
outer flow paths in a mixing apparatus as described, with connecting 
structures for impeller blades in the radially outer flowpath extending 
through the isolating structure, and such that the isolating structure 
does not impede recirculation of fluid to flow from one opposite axial 
path into the other, e.g., at the surface of whatever level of fluid is in 
the tank. 
It is another object to mount a partial flow shield separating radially 
inner and outer zones via baffles pitched to redirect circumferential flow 
axially. 
It is another object to optimize the impeller blades of a mixing apparatus 
such that the outer blades, which move linearly faster than the inner 
blades, can provide a substantial driving force while the inner blades 
efficiently return liquid in a circulating path. 
These and other objects are accomplished by a mixing apparatus including a 
tank for holding a material to be mixed, a drive shaft rotatably supported 
in the tank, a radially inner impeller on the drive shaft with blades 
pitched to produce axial flow of the material in a first direction 
(especially downwardly), and a radially outer impeller on the drive shaft 
with blades pitched to produce axial flow in an opposite direction 
(upwardly). The radially inner impeller can be a high solidity impeller 
disposed in a flow shield between the inner and outer impellers, providing 
a barrier between the material flowing axially in said first and second 
directions. The outer impeller is coupled to the drive shaft by connecting 
members protruding radially through axial spaces provided in or around the 
flow shield, which extends only partially around a full circumference to 
leave spaces permitting fluid to recirculate from one axial direction to 
the other at the end of the axial path, regardless of the surface level of 
the fluid as compared to the position of the flow shield. Baffles are 
fixed in the tank and preferably the flow shield is fixed to the tank by 
the baffles. The baffles have inclined inner and outer sections that 
extend axially and are pitched to intercept circumferential flow produced 
by the inner and outer impellers, respectively, and to redirect the flow 
axially in the appropriate direction of flow. The apparatus can include a 
number of axially spaced impeller stages, each having an inner impeller 
encompassed by a section of flow shield and an outer impeller on 
connecting members that extend through axial gaps between the flow shield 
sections or stages.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, a mixing apparatus 22 is provided with flow guiding and 
confining structures that cooperate with oppositely pitched inner and 
outer impellers 32, 34 in order to maximize mixing efficiency. In 
particular, liquid moved by the rotating impellers 32, 34 is caused to 
move substantially axially in opposite directions at radially inner and 
outer areas of a tank 40 holding a material to be mixed. 
A drive shaft 42 is supported for rotation in tank 40 on a rotation axis 
44, and is coupleable to a gear motor (not shown) or similar powered 
device for rotating the drive shaft. A radially inner impeller structure 
32 is fixed to drive shaft 42, and has at least two inner blades 52 
pitched to produce axial flow of the material in a first direction with 
rotation of drive shaft 42 in the direction shown. The inner impeller 32, 
which preferably comprises a number of axially spaced stages 53, drives 
the material downwardly in the embodiment shown in FIG. 1, as indicated by 
arrows. 
A radially outer impeller structure 34 is also fixed to drive shaft 42, and 
has at least two outer blades 54 pitched to produce axial flow of the 
material in a second direction with rotation of drive shaft 42, namely 
upwardly in FIG. 1. Thus the material circulates in tank 40 with rotation 
of drive shaft 42 and the impeller blades 52, 54 thereon. 
The radially inner impeller structure 32 preferably comprises a high 
solidity type impeller blade, for example as disclosed in U.S. Pat. No. 
5,326,226--Wyczalkowski et al., which is hereby incorporated. In the 
preferred arrangement as shown in FIGS. 2 and 3, two inner blades 52 and 
two outer blades 54 are provided at 90.degree. intervals. The outer blades 
can comprise flat plates as in FIGS. 2 and 3, pitched for example at about 
20.degree., or can be curved (concave up) or pitched at a different angle. 
Whereas the outer blades are at a greater radius than the inner blades, 
they move linearly faster and provide good pumping efficiency driving the 
liquid upwardly in an annular space at the walls of the tank. The inner 
blades drive the liquid downwardly, returning the liquid in a circulating 
path. 
The inner impeller blades 52 are formed of plate stock, rolled along the 
axis of a cylinder such that the roll axis lags the radius at which the 
blades are attached to drive shaft 42, for example by 45.degree.. The 
blades 52 thus approximate the shape of a hydrofoil. The radially outer 
impeller structure 34 comprises blades 54 with flat plates, curved as 
shown in plan in FIG. 2 to fit in the available annular space, and 
inclined relative to rotation axis 44. The outer blades are carried on 
connecting members 56 extending to the central hub 62 to which inner 
impeller blades 52 are also attached. 
The tendency of the radially inner and outer opposite axial flows of liquid 
to interfere turbulently with one another is minimized by a flow shield 64 
in tank 40, disposed substantially between the inner and outer impeller 
structures 32, 34, and providing a barrier that tends to isolate the flows 
of material in the first and second axial directions. Flow shield 64 is 
substantially tubular and extends for an axial length encompassing the 
inner impeller structure 32 while providing gaps or spaces for clearance 
for the connecting members 56 carrying outer blades 34 (i.e., axial gaps). 
Flow shield 64 is radially closely adjacent to inner impeller 32 and 
confines radially outward flow from the inner impeller which would 
otherwise occur due to centripetal acceleration as impeller 32 is rotated 
by drive shaft 42. The connecting members 56 for outer impeller blades 54 
protrude radially through or around flow shield 64. Flow shield 64 
preferably is rigidly fixed relative to tank 40. 
The flow shield preferably extends less than 360.degree. around the axis, 
thus leaving gaps 65 of a certain circumferential or angular width between 
segments of the flow shield (i.e., longitudinal gaps). The flow shield is 
thereby structured to permit liquid to flow in a radial direction through 
the longitudinal gaps, particularly at one or both ends of the opposite 
axial paths where the liquid changes direction in the recirculating path 
shown. Assuming that the depth of liquid in the tank may vary, providing 
the longitudinal gaps permits the liquid to reverse direction without 
necessarily passing around an axial end of a section of the flow shield, 
which otherwise could impede recirculation when the tank is not full. 
Preferably, flow shield 64 extends circumferentially about 180.degree., 
namely in two 90.degree. sections attached to a baffle structure at 
opposite sides. However, flow shield 64 can also extend around a larger or 
smaller proportion of the circumference. 
According to an inventive aspect, mixing apparatus 22 further comprises at 
least one and preferably a plurality of baffles 66, 68, fixed in tank 40. 
Each of the baffles 66, 68 is axially adjacent to an impeller 32, 34 
disposed upstream in the direction of flow. The baffles 66, 68 extend 
radially through an area of at least one of the inner and outer impeller 
structures 32, 34. The baffles 66, 68 are inclined relative to a 
circumferential path of the liquid, preferably by about 45.degree.. 
Whereas the liquid is in part moved circumferentially by rotation of the 
associated impeller 32, 34, the inclined baffles 66, 68 convert the 
direction of flow from circumferential to substantially axial. Thus, 
considering the direction of rotation of impeller blades 52, 54, the 
baffles 66, 68 each have a leading edge directed toward the impeller blade 
52, 54, which leading edge is ahead of the position of the trailing edge 
in the rotation direction. In other words, baffles 66, 68 and their 
associated impeller blades 52, 54 are inclined or pitched in opposite 
directions from one another. The baffles 66, 68 are rigidly mounted in 
tank 40, for example by welding. Baffles 66, 68 are also attached to flow 
shield 64 and thereby rigidly support the flow shield sections in tank 40. 
In the embodiment of FIG. 1, three impeller stages 53 are provided; however 
any number is possible. Each stage 53 has inner and outer impeller 
structures 32, 34 fixed to drive shaft 42. The impeller stages 53 are 
spaced axially along drive shaft 42, and the baffles 66, 68 are disposed 
between impeller stages 53. The inner baffles 66 can have journal 
couplings 74 that rotatably support drive shaft 42 between impeller stages 
53, permitting a long length of drive shaft 42 with many impeller stages 
53 but without the tendency to wobble the drive shaft. 
Flow shield 64 likewise has axially spaced stages or sections 76. Tank 40 
is preferably tubular and the sections of flow shield 64 are 
correspondingly tubular but preferably have longitudinal gaps 65, as 
discussed. The flow shield stages 76 form barriers that isolate the 
radially inner and outer opposite axial flows, each stage 76 extending for 
an axial length encompassing a respective stage 53 of the impeller 
structures 32, 34. Axial gaps 78 are provided between the sections of flow 
shield stages 76, through which the connecting members 56 for outer 
impeller blades 54 protrude radially. The outer impeller blades 54 can be 
welded to the connecting members 56, and the connecting members can be 
welded to the hubs 62. 
In the embodiment shown in FIG. 1, two inner impeller blades 52 and two 
outer impeller blades 54 are shown with four baffles 66, 68 for each bank 
(or eight, counting the inner and outer baffles separately). It is 
possible to use any number of blades 52 54, baffles 66, 68 and/or flow 
shield sections for the inner and outer impellers. The depicted embodiment 
has the respective banks of impeller blades, baffles and flow shield 
sections mounted angularly in registry. These banks can be angularly 
offset as well. 
The size of the inner and outer impeller blades is chosen to achieve 
substantially equal fluid movement capacity for maximum efficiency. The 
linear speed of outer impeller blades 54, at 90 to 95% of the tank 
diameter, is substantially greater than that of inner blades 52, which 
preferably encompass about 60% of the tank diameter. The faster moving 
outer blades provide good pumping efficiency due to the large diameter. To 
equalize the pumping rate of the inner and outer blades, the outer blades 
54 can be smaller in area than inner blades 52, less numerous and/or less 
steeply pitched than inner blades 52. The particular size of the blades 
52, 54 and the speed at which they are rotated, can be varied as known in 
the art to reflect the characteristics of the fluid being mixed. However, 
the disclosed embodiment has been found to be efficient over a range of 
mixing conditions and power levels. In addition, the mixing structure is 
efficient over a wide range of liquid viscosities. 
The invention having been disclosed in connection with the foregoing 
variations and examples, additional variations will now be apparent to 
persons skilled in the art. The invention is not intended to be limited to 
the variations specifically mentioned, and accordingly reference should be 
made to the appended claims rather than the foregoing discussion of 
preferred examples, to assess the scope of the invention in which 
exclusive rights are claimed.