Welcome to our new vlog series on industrial mixing basics. These vlogs (a blog with primarily video content) focus on the fundamental principles of industrial mixing. They should be especially helpful to those new to industrial mixing, but “veterans” may also like them as a refresher.
The first topic in our industrial mixing vlog series focuses on using a baffled tank vs an unbaffled tank. Although this is a relatively basic topic for mixer design, we thought it would be illustrative to provide some videos to demonstrate flow patterns using a baffled tank and an unbaffled tank with various agitator-mounting configurations.
Each video uses a lab scale tank with two hydrofoil impellers (ProQuip HiFlow Impeller). The impeller diameter is 1/3 tank diameter. The impellers are operating at typical rotational speed for this size impeller in water. White plastic pellets with a specific gravity slightly less than water are added to aid in visualizing fluid flow. Colored pellets, with specific gravity slightly heavier than water, are added to represent suspension of solids.
Center-Mounted Agitator in an Unbaffled Tank
An unbaffled tank is a strong consideration for many applications. However, using a center-mounted mixer in an unbaffled tank with axial flow impellers provides very little top to bottom flow. In the video below, you can observe the primary flow pattern rotating horizontally and development of a center vortex (significance varies based on impeller diameter and rotational speed). Additionally, the heavier solids are not suspended and accumulate in the center of the tank.
Center-Mounted Agitator in a Baffled Tank
For comparison, video 2 shows the mixing pattern for the same impellers and rotational speed in a baffled tank. There is no longer a deep center vortex, and there is good top to bottom circulation and solid suspension.
Offset-Mounted Agitator in an Unbaffled Tank
One solution to providing better mixing in an unbaffled tank is to mount the mixer off-set from the tank center line. This is typically done by off-set mounting the agitator approximately 1/6 of the tank diameter as shown in Video 3. An off-set mounted mixer creates less of a vortex, and there is better top to bottom liquid flow pattern. The impeller power for this system is approximately the same as for a baffled tank. One disadvantage for this configuration is when there are a significant amount of solids in the tank. The solids may tend to accumulate on the opposite side of the off-set mount depending on their amount, size and specific gravity.
Angle-Offset Mounted Agitator in an Unbaffled Tank
This solution has the agitator mounted both offset, and the shaft angled into the tank. This configuration works well in unbaffled tanks for overall fluid circulation with top to bottom flow pattern. It also provides reasonable solid particle suspension. The angle-offset configuration is shown in video 4.