Mixing School - Tank Baffles

chapter 1: vertical cylindrical tanks, waterlike products                                                

To guarantee satisfactory mixing performance, careful consideration must be given to tank baffles and their design. Mixing low viscosity products in an unbaffled, or underbaffled tank will result in the product swirling round and round the tank with relatively little top to bottom motion, resulting in vortexing and ineffective mixing. The vortexing also creates greater unbalanced fluid forces that act upon the mixer. A larger mixer may be required to overcome these increased forces. If solids are present, they will tend to remain on the tank bottom unsuspended. Overbaffling or otherwise improperly designed baffles on a tank will also reduce mixing efficiency and may create dead zones behind the baffles. 

 

UNBAFFLED TANK, OFF-CENTER MOUNTING, WATERLIKE PRODUCTS

Many customers do not want baffles on their tank due to cleaning problems or other considerations.  Although not as efficient as baffles, there are options to provide adequate mixing for many applications in an unbaffled tank by mounting the mixer off center to minimize swirling.

1.  Off-Center Mounting of a vertical mixer, usually 1/6 the tank diameter, creates an eccentric flow pattern in the tank that reduces some swirling.  This design works satisfactorily for gentle mixing, but as the agitation intensity grows the swirling will increase.  The swirling action in the tank can also create a significant side load on the impeller and deflection of the mixer shaft.  This must be taken into account when designing the mixer, and may result in a larger mixer drive and shaft to accommodate these loads.  The mechanical load can be as much as 250% as high as in a fully baffled tank.  This system will often end up being more expensive and less effective than a properly baffled or angle mounted design.

2.  Angled Off-Center Mounting of the mixer positions the mixer at a 10 – 15 degree angle from vertical.  The mixer drive is mounted off-center, pointing the shaft into the opposite quadrant of the tank.  This creates an asymmetrical flow pattern and the discharge from the impeller pumps against the natural rotation of the fluid, reducing swirling. The fluid will swirl in the same rotation as the mixer, so it is important to match the position of the mixer with the rotational direction of the mixer. This concept is most often seen in the mounting of typical clamp-mounted portable mixers.  The weight and subsequent bending moment of the impeller and shaft mounted at an angle must be included in the design of the mixer.

 

BAFFLED ON-CENTER MOUNTING, WATERLIKE PRODUCTS

On vertical cylindrical tanks, baffles should be used for mixing waterlike products when the mixer is mounted on the tank centerline, unless the power level is extremely low (i.e., less than 0.1 hp/1000 gal.).  Baffles create flow patterns that assure the entire batch will pass through the impeller where there is the highest level of mixing intensity. Other benefits of a baffled tank include:

                       

§    Prevention of excessive swirling, vortexing and air induction into the batch.

§    Stabilization of mixer horsepower draw.

§    Reduction of unbalanced fluid forces acting upon the mixer.

§   Improvement of mass transfer for heating or cooling the product with tank jackets or cooling coils within the tank.

 

Extensive testing has proven that for mixing waterlike products in vertical cylindrical tanks, the optimum baffle design uses (4) baffles, mounted radially at 90 degree spacing.  Baffle widths are 1/12 of the tank diameter, mounted off the tank wall by 1/6 of the baffle width.

 

            …next: rectangular tanks and horizontal cylindrical tanks

Submitted by Jay Dinnison

 

Shear vs. Viscosity

What is shear vs. viscosity?

A Pascal second is 1,000 cp (most viscosity is reported as mPs; milli Pascal seconds these days) So 0.5 Pascal seconds is 500 cp

Inverse second is the shear rate. So this result would imply that the viscosity is shear dependent (non Newtonian)

By taking a minimum of three readings of viscosity at different shear rates we calculate the shear rate dependence using the least squares method of curve fitting. As the relationship is exponential it can be plotted on log/log and calculated for any shear rate. Note that we use different spindle speeds for this method, we know (or can calculate) the so called shear rate constant for the known geometry of the specific spindle. Also note that only cylindrical spindles can be used for this method because the geometry is well defined. Disc spindles are not appropriate.

For mixer design the “apparent” viscosity is dependent on the impeller speed and the impeller’s own shear rate constant. Although Sharpe has not determined the shear rate constant for our impellers we do use published values, which are fine as the margin of error is no big deal when working with power law fluids.

For the HF218 the constant is 10

For Axial it is 11

For anchors and double helix it is 20

So the “apparent” viscosity of a HF218 at 350 rpm would be determined from the viscosity vs. shear rate at a shear rate of 350rpm/60 X 10 = 58/sec

Baffle recommendations need to be determined at low shear rates (as at the tank wall the viscosity is by definition infinite; the shear rate would be zero and dividing the shear stress by zero is infinite)

Typically a shear rate of 2/sec is used for baffle design. In most cases baffles need to be small or not used. For example a lime slurry may have a viscosity close to the tank wall of over 10,000 cp. Standard baffles for this would simply create dead zones and effectively reduce the tank diameter for mixing by the baffle width x 2. So for stuff like fruit slurries (ketchup, pop tart fillings, etc.) with very high “shear thinning index” on center, no baffles, slow speed to minimize cavity formation is the correct design.

By Jeremy Higginson, P.E.
Direct of Engineering