Monday, June 10, 2013

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