The tank mixer design, other than the mixer itself, is essential in achieving a successful outcome in any procedure.
It is vital to create a setting that supports both perfect impeller alignment and enough impeller liquid coverage in order to enhance the efficiency of a mixer’s impeller. Misalignment can negatively affect the effectiveness of a mixer, the integrity of a product, and perhaps even the mixer drive’s operating life.
Tank Mixer Design
Vertical cylindrical, square, or rectangular mixer tank designs are the most frequent. There are specific guidelines to follow while choosing the best tank design for a given application.
- Level-to-Diameter Ratio
Most mixing tasks work best with a ratio of 0.8 between the liquid level and the tank’s diameter. In real life, though, a ratio of about 1:1 will work. A too-small ratio makes it impossible for a tank to do axial mixing well. We advise staying away from anything with a ratio of less than 0.6.
When the ratio exceeds 1.4, you should use two impellers. When the liquid-to-tank diameter ratio exceeds 2.0, reconsider the tank selection because this narrow tank mixer design is not the most cost-effective method to mix.
The price of the mixer increases as the shaft length grows. For illustration, consider a standard 5,000-gallon tank in both the optimal and narrow tank situations.
To get the equivalent diameter of a rectangular tank, just square root (Length x Width) 1.13. The ratio of liquid height to tank diameter is around 0.8.
Rectangular tanks may be used efficiently for mixing since they are self-baffling. On the other hand, rectangular tanks are not recommended for solid suspensions owing to the creation of solid clumps in the corners. In a tank with corners, there will be “dead spots.” As a result, more mixing is necessary to get the same effect as in a cylindrical tank of the same size.
Vertical Cylindrical Tanks
The most popular kind of tank is a vertical cylindrical tank. Cylindrical tanks must be either baffled or positioned at an angle o prevent whirling from occurring.
Smaller tanks often do not need baffles (capacity 5,000 gallons or height 10 feet). However, installing baffles in larger tanks is more cost-effective than investing in an expensive, offset-mounted, heavy-duty mixer.
Round and Cone Bottom Tanks
Some tank mixer design have spherical (dish-shaped) bases, while others have conical ones. The following is a high-level plan for achieving satisfactory tank mixing.
- Cone bottom: The ideal angle for a cone is 15 degrees or less. However, angles between 15 and 30 degrees are fine. But because of this, it becomes increasingly more difficult to produce efficient mixing within a deep cone.
- Round Bottom: If you want to follow the guidelines for a cone, the same as in a circular base. A tank with a circular bottom is preferable for suspending solids, as corners cause no dead patches.
- Baffles: If a tank with a cone or circular bottom is exceptionally deep, you may add baffles inside this portion to increase axial mixing and reduce spinning.
A baffle is a long, flat plate attached to a tank’s side to prevent swirling and promote fluid movement from top to bottom.
Because rectangular tanks are self-baffling, using baffles is essential when only using cylindrical tanks. When the agitator is in the middle of a cylindrical tank without a baffle, it makes a swirling motion that is very inefficient. For example, think of two particles moving around in a circle. It will always be a race between them, and they will never be able to mix.
Consider these two options:
The best solution is to place baffles in the tank.
Avoid spinning by installing the mixer at an angle of about 1/6 the diameter of the tank. This method has the downside that the unbalanced forces will exert a more significant strain on the mixer shaft, requiring a mixer with a more robust, heavy-duty design. This becomes highly costly for larger applications needing shaft lengths over 10 feet.
Baffles are usually placed at a distance of 1/72 of the tank’s diameter from the wall and have a width of 1/12 of the tank’s diameter. The ideal number of baffles to place within a tank is four. However, three baffles will do for most applications.
The baffles should spread across the whole tank, going a gap at the base to prevent sediments from settling. So, if you have a tank mixer design that’s 12 feet in diameter, you may want to install baffles that are 1 foot wide and spaced out 2 inches from the tank wall.