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Angular Offset Mounting Arrangement

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:

  1. 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.

  2. 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.

  3. Heavy solid suspension applications having free settling velocities greater than 4 feet per minute. 

  4. Any application requiring a significant vertical component of mixing. 

  5. Cone bottom tanks with internal wall angles < 119 degrees.

  6. 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.  

  7. Tank bottoms that have obstructions such as sump pumps, or other physical constraints that might constrain or redirect flow patterns.   

 

05.06.11

 

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