Angular
Offset Mounting
The
intent of using an angular offset mounting arrangement is to eliminate the
need for using in-tank anti-swirl baffles. Plastic tanks, sanitary
application such as contact solutions, pharmaceuticals, and sticky process
materials that tend to hang-up on tank internals prefer an angular offset
mounting arrangement as anti-swirl baffles can generally be
eliminated. This arrangement is also used to save on the related
costs of having to add tank anti-swirl baffles to a stainless steel or
fiberglass tank.
Small
mixers generally defined by mixers with agitator shaft diameters of
1.5" or smaller tend to favor angular offset mounting
arrangements. The advantage is that the impeller to tank diameter
ratios {D/T ratio} lies within the range of 0.1 to 0.4. Although you
may see larger agitator shaft diameters using an angular offset mounting
arrangement, they generally do not fit well due in part to a larger D/T
ratio, but more importantly due to the additional overhung weight of the
shaft and impeller assembly as compared to that seen in a vertical
position.
HOW
IT WORKS:
The mixers impeller turns clockwise setting up a clockwise angular flow
pattern {swirl or vortex). The impeller discharges vertically
straight downward and due to its orientation sets up a counterclockwise
discharge. These motions, if oriented properly, counteract each
other which acts to disrupt the angular flow patter into the preferred
vertical and radial components of mixing. In effect, the mixing acts
much like a fully baffled tank.
Although
it is true that there are additional loads applied to the mixer due to the
overhung load of projecting the shaft and impeller at a 10 degree angle,
these effects are measurable and repeatable, allowing for a sound
design. As compared to an unbaffled tank, using a
vertical-off-tank-centerline mounting arrangement, the forces are
significantly lower. Even at very low horsepower per unit volumes, a
vertical-off-tank-centerline mounting arrangement will draw a vortex (air
incorporation) under water-like mixing conditions causing fluid forces
many times that seen under fully baffled full coverage conditions.
Also, when angular mixing is present, the radial resultant forces applied
by the fluid to the impeller are minimal at the tank centerline as
compared to that seen as you move away from the tank centerline to the
tank wall. In short, the vortex forces combine with the off-center
forces to cause either premature failure or limited mixer service
life.
APPLYING ANGULAR OFFSET MOUNTING:
-
Applications
requiring Impeller Diameter to Tank Diameter Ratio's {D/T ratio} >
0.4. Above this range, the tank walls constrain the location of
the impeller where an orientation lay-out will be required to maintain
the proper wall clearances.
-
Agitator
shaft diameter exceeding 2" in diameter. It can be done
however there will be a related cost penalty to handle the overhung
loads of the shaft and impeller on a 10 degree angle.
-
Heavy
solid suspension applications having free settling velocities greater
than 4 feet per minute.
-
Any
application requiring a significant vertical component of
mixing.
-
Cone
bottom tanks with internal wall angles < 119 degrees.
-
Solid
suspension mixing application in cone bottom tanks. The flow
patterns is base of a cone is virtually nonexistent, when the flow
pattern generated above acts to pack-in the solids below
it.
-
Tank
bottoms that have obstructions such as sump pumps, or other physical
constraints that might constrain or redirect flow
patterns.