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
