Dos and Don’ts of Mixer Selection

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Mixer Agitation Rate

When sizing a portable mixer it is important to know the degree of agitation required for the application. Mild agitation is normally 1/2 to 1 tank turnover per minute, medium agitation is 1-1/2 to 2 tank turns per minute, vigorous agitation is 2-1/2 to 3 turns per minute and violent mixing has tank-turnover rates of more than 3 turns per minute. The prop’s pumping rate is available from mixer manufacturers for each mixer model type and size at the various rpms at which they will operate, and is normally measured in the gallon-per-minute (gpm) pumping rate of water. Sizing and selecting a mixer can be easily done with this information. For example, a 100-gallon tank of a water-like chemical requires vigorous agitation, or 2-1/2 turnovers. Knowing that a 4-inch square pitch prop at 1,750 rpm delivers a pumping rate of 250 gpm, a portable mixer could be selected. Keep in mind, however, that as liquid viscosity increases the pumping rate decreases.

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Creating a vortex

Another consideration in the realm of vortexing may be the need to use two props on the shaft. Since it may be difficult to create a vortex in a baffled tank, a solution can be to use dual props. In this case, one prop is placed at the top of the shaft, near the surface of the liquid, while the second is located in its traditional position at the bottom of the shaft. This configuration can produce the vortex that is required to draw in powder or liquid materials, even in a baffled tank. In general, dual props will also be required in any container whose height is greater than 1.5 times its diameter. In this case, the props should be located at least two prop diameters apart and the upper prop should be submerged at least two prop diameters below the fluid surface to ensure good mixing.

Keep in mind, though, that when determining mixing rate that dual props do not produce twice the flow of single props. This is because a mixer with dual props of the same diameter has a higher head capacity and will produce an increase of flow of around 20 % (dependent on propeller spacing), but will require an 80 % increase in power to create that increased flow.

Mixer Positioning

The goal of any portable mixer is to either blend, dissolve or disperse, and to do this in the most efficient manner. With that in mind, the position of the shaft and prop in the mixing container is a critical concern.

In most mixing applications involving small cylindrical tanks of 1,000 gallons or less, the mixer is clamped to the side of the container. Effective mixing patterns will be achieved if the mixer is angled 10 to 15 degrees away from the vertical, either off-center or on-center. Angling the mixer 15 to 20 degrees off of the tank’s centerline is preferred when good material turnover is required, as when mixing a slurry. On-center angling is better for gentler mixing and creates a vortex. Vortexing occurs when the contents of the tank swirl around the walls of the tank without much top-to-bottom turnover; this creates a less-efficient mixing operation and the possibility of uneven mixing or blending.

Cylindrical tanks with capacities of more than 1,000 gallons may require that the mixer be mounted directly in the center of the tank with the shaft in a vertical orientation. In this configuration, it is recommended that the operator put baffles on the walls of the tank in order to prevent the contents of the tank from turning in the direction of the mix, or creating an inefficient vortex. When this occurs, the mixing action in the tank will be poor.

It is suggested that four baffles be used in this situation with them placed 90 degrees apart on the walls of the container and sized approximately 1/12th of the tank’s diameter. The baffle should not fully extend to the bottom of the tank and a noticeable gap, usually 1/2 to 1-inch in size, should be left between the baffles and the tank walls if solids are present or for viscosities over 500 cPs to prevent a buildup where the baffle connects to the tank wall. If baffles are required in a square or rectangular tank, they should be placed at the midpoint of each of the four walls, with the same sizing and positioning parameters for baffles that are placed in circular tanks acknowledged.

Be advised, though, that the presence of vortexing during the mixing or blending process is not always detrimental. For example, creation of a vortex is desirable when solids or powders are added to the top of the batch, or liquids need to be drawn rapidly into the batch. The level of vortexing also becomes less severe with fluids that have higher viscosities.

Conclusion: Portable mixers can play a significant role in optimizing the effectiveness of the entire production operation. Knowing the the right way to select and operate mixers, as well as the handling characteristics of the fluid to be mixed or blended, will play a major role in selecting the technology that is perfectly suited for the specific operation.

* The Author is Director of Business Development for Neptune Chemical Pump Company and PSG, Grand Terrace/USA.

(ID:45446026)

A Guide to Configuring Portable Mixers

Dos and Don’ts of Mixer Selection

Mixer Agitation Rate

Mixer Positioning