Selecting Mixers -- Points to Consider

Baffles are used to control vortexing. Most portable mixers should operate successfully without baffles. Portables are designed to be lifted from one tank to another and are easily positioned for varying mixing patterns. Effective mixing patterns can be developed by angling the shaft off the center of the tank. The mixer may be moved as much as 15° – 20° off the tank centerline, and angled into the vessel 10° – 15° from the vertical.

The definition of a good mixer flow pattern is the degree of agitation required, plus a good top-to-bottom turnover of the contents of the tank.

When a mixer is mounted on the center of the cylindrical tank without baffles, a deep vortex is created. The contents of the swirl are moved around the tank without much top-to-bottom turnover. The vortex becomes less severe as viscosity increases. The vortex will almost disappear and be replaced by good turnover for viscosity's between 2000 and 10,000 centipoise.

Low viscosity applications with the mixer mounted on the center of the cylindrical tank will require baffles. Four baffles should be equally spaced around the tank. The width of the baffle should be 1/12 of the tank diameter. Baffles used when mixing solids should be mounted with approximately ½ " gap between the baffle and the tank wall to avoid the build-up of solids or slow moving material at the point where the baffle would join the tank wall.

The shape of square or rectangular vessels offers partial baffling. No additional baffling is generally required for square or rectangular tanks when the mixer is clamped to the side wall of the tank. Low viscosity applications on square or rectangular tanks, using center-mounted mixers, should be baffled. Proper baffling of a rectangular tank would be to locate a baffle at the mid-point of each of the four tank walls.

Baffles should extend from near the bottom of the tank to above the liquid level for low viscosity applications. Baffles should be truncated below the liquid level for higher viscosity applications.

Baffles are used to control vortexing, however, the elimination of vortexing is not always required.

Vortexing

Creation of a vortex is desirable for pulling in solids or powders added to the top of the batch, or for drawing liquids rapidly down into the batch.

It may be desirable to begin a batch in a vortex condition to rapidly incorporate added material and then to continue mixing without vortexing but with a mixing pattern to produce good top-to-bottom turnover.

Vortexing in unbaffled cylindrical vessels can be easily controlled by changing the mixer position. Begin the batch with the mixer to an angled on center to incorporate materials, and then move the mixer to an off center position to eliminate the vortex and to provide good top-to-bottom turnover.

If the mixer cannot be adjusted completely, angled on center positioning will produce the vortex for incorporating solids and will still produce acceptable mixing. It may be difficult to create a vortex in a baffled tank. Use of dual propellers with one propeller located near the surface, can produce a vortex needed to draw in powder or liquid materials, even in a baffled tank.

Also, the baffles may be cut off to a point of the vertical distance below the liquid level, to encourage vortexing at the top of the tank when powders or solid are to be added.

Dual Propellers

Dual propellers may be used for unusual batch configurations or to improve the drawing-in of solids or liquids being added to a solution.

Most mixing applications will require a propeller to be located 1 to 2 diameters off the bottom of the tank. A good flow pattern will be produced if the impeller has at least 2 diameters of liquid above it.

A propeller submerged too deeply in a tank will result in a short-circulating of the flow; i.e., the bottom of the tank will be well mixed but very slow moving material will appear at the top of the vessel. A second propeller placed near the top of the vessel will correct this problem.

Generally, dual propellers will be required on any vessel where the height of the vessel is greater than 1.5 times the diameter. The propellers should be located at least 2 propeller diameters apart to ensure good mixing. The upper propeller should remain submerged at least 2 propeller diameters below the liquid surface.

An exception to this discussed earlier, is when a vortex is desired to incorporate solids. The upper propeller should then be located approximately one propeller diameter below the liquid surface.

Neptune Chemical Pump Co., Inc., P.O. Box 247, Lansdale, PA 19446. Tel: 215-699-8700; Fax: 215-699-0370.