Anti-Swirl Baffling, Vertical Tank Configuration:

ANTI-SWIRL TANK BAFFLING
Applying the 80/20 rule, most fluids fall under the classification of water-like viscosity where either proper Anti-Swirl Tank Baffling is required or an angular offset mounting arrangement should be considered instead of baffles.  These configurations are not only important to ensure satisfactory mixing results, they are also required to protect the mixer from premature failure.    Anti-Swirl Tank Baffles ensure good mixing by redirecting the angular component of mixing, also known as swirl or vortexing, into the preferred vertical and radial components of mixing.  On the other hand, an over baffled configuration, normally associated with higher viscosity applications, will localize the mixing action and will reduce the overall mass flow such that the mixing will be unsatisfactory.  As such, it is always best to secure an Anti-Swirl Baffle recommendation from your mixer supplier.  To protect your equipment and to ensure its longevity, monitoring and modifications to this initial recommendation may be required ⁴.     

ANTI-SWIRL BAFFLE CONFIGURATION: STANDARD CONDITIONS:
The term “standard (width) baffles”, actually refers to a standard tank & fluid conditions.  Vertical cylindrical tanks with water like viscosities where the mixer is mounted on-(the-tanks) centerline will require standard baffles.  Standard baffles are defined as follows:

Baffle Width = 1/12^th  Tank Diameter
Baffle Off Wall Distance = 1/6^th  to 1/12^th  Baffle Width
Baffle Length = Full Straight Side^1,2,3
Off Bottom = 1/8^th  to 1/16^th  baffle length

NOTE:
1) Three bladed impellers such as hydrofoils and propellers typically require three (3) anti-swirl tank baffles, oriented 120 degree apart.  All other impellers that typically have either a higher radial discharge component or that transform a great degree of the available horsepower into fluid shear require four (4) anti-swirl tank baffles, oriented 90 degrees apart.  On ASME or 2:1 semi-elliptical Dish (heads) top & bottoms, baffles usually begin & end where the dish & straight wall meet.  Physically taller dishes may require special baffling.  2) To promote additional surface motion for the additions at the surface, anti-swirl baffles of 80 to 85% of the straight side can be used.
3) On flat bottom tanks, locating the baffle approximately 1/8^th  to 1/16^th  the anti-swirl baffle length off the tank bottom is acceptable to prevent potential hang-up points on the bottom where the baffle meet the tank bottom.-
4) An under baffled system while mixing a low viscosity fluid is not only inefficient from a mixing standpoint, it may also result in damage to the mixer and tank internals (unless the mixer is specifically designed to handle these excessive loads).  If you are not sure, check your operation and maintenance manual where it should be noted on the Outline and Installation Drawing of your mixer.  Under baffling is normally indicated by the angular component of mixing also known as swirl and/or vortexing.  Power variations or under loading, aeration, surface wave phenomena, and excessive shaft run out are other characteristics of an under baffled condition.  Even systems with "VERY LOW POWER" levels in cylindrical tanks will exhibit these characteristics and will require anti-swirl tank baffling.   

06.27.08