What to Know When Specifying Large Portable Mixers

The best large portable mixer selection is ultimately the one that can satisfy the specific needs of your operation as related to the following categories:

• Style: There are four basic styles: pail, which has the smallest capacity, drum, tote and light- or heavy-duty bulk tanks, which have the highest capacity. The most commonly used in industrial operations are the drum and tank styles. Most models are either gear-driven with speeds of 350 or 420 revolutions per minute (rpms), or direct-drive, with speeds of 1,750 rpm.
• Mounting: While some mixers can be permanently mounted to the drum, tote or bulk tank, in most instances a mounting clamp is used to attach the mixer to the vessel. There are three common portable mounting-clamp styles: C Clamp, which has an adjustable angle of entry that is controlled by a ball-and-socket adjustment design; Angled Riser, which provides a fixed 10-degree angle of entry into the mixing vessel; and Flange Mount, which attaches to a flange that is located on the lid of the drum, tote and bulk tank.
• Props: “Neptune recommends Square Pitch propeller, which is the most efficient prop and can displace eight inches of liquid in one revolution for tanks up to 5,000 Gallons” said Gurmeet Kharbanda, Director Product Management - Neptune.
• Shafts: Shaft lengths for most mixers are determined by critical speed. Operating at a rotational speed that is equal or close to critical speed will cause a multiplication factor on normal runout, but this will be mitigated by the speed damping that is produced by the liquid. There are, however, certain circumstances that will produce abnormal shaft vibration. These include operating the mixer through drawoff or at drawoff for a significant period of time, during the drawdown of solids, and at a natural frequency when mixing extremely light liquids that do not produce a damping effect.

Also contributing to the style of large mixer that should be specified are a number of operational characteristics, including mixing rate, or the time it takes to completely “turn” the product in the mixing vessel; mixer position, whether side-mounted, center-mounted, angled, offset or requiring baffles; and liquid type, with the liquid’s viscosity and particulate levels determining proper style and orientation.

Proper Mixer Mounting and Positioning

In most applications involving small mixing tanks of 1,000 gallons (3,785.4 liters) or less, the mixer is clamped to the side of the tank. Optimum mixing will be achieved when the mixer is angled from the tank wall, either off-center (figure 1) or on-center (figure 2). Improperly mounting a portable mixer will greatly lessen mixing efficiency, cause vibration, and could possibly damage the mixer.

Angling off-center is preferred when good material turnover is required, as in mixing slurry. Angling on-center is desired for more gentle mixing and when vortexing must be minimized.

Such baffling will avoid vortexing and allow mixing to be as thorough as possible. Neptune suggests that four baffles be used, located 90° apart and sized approximately 1/12 of the tank diameter. The baffle should not fully extend to the bottom of the tank and a gap should be provided between the baffle and the tank wall.