LIQUID MIXING

For any application, choice of the proper impeller is critical for success. Impellers can be divided into two general classes, Axial flow, and Radial flow

Axial – flow impellers Axial – flow impellers include all impellers in which the blade makes an angle of less than 90 with the plane of rotation. Uses: Axial flow impellers are used at high speeds to promote rapid dispersion and are used at low speeds for keeping solids in suspension. It is also used for agitation in tanks smaller than 1000 gal. or <6 ft diameter, when <3 hp. is enough for the desired process results.

Fig: Pitched – blade turbine Fig: Marine – type mixing propeller

Different mounting positions of propellers: Propeller mixers may be clamped on the side of an open vessel in the angular, off – center position or bolted to a flange or plate on the top of a closed vessel with the shaft in the same angular, off – center position. This mounting results in a strong top to bottom circulation.

Fig: Side – entering propeller mixer

Off center top – entering propeller position Fig: Flow pattern for propeller in angular off – center position without baffles

Radial – flow impellers: Radial – flow impellers have blades which are parallel to the axis of the drive shaft. The smaller multi blade ones are known as “turbines”; larger, slower - speed impellers, with two or four blades, are often called “paddles” The diameter of a turbine is normally between 0.3 and 0.6 of the tank diameter. Curved blade turbines aid in starting an impeller in settled solids. A paddle agitator has a diameter usually greater then 0.6 of tank diameter and turns at a slow speed. Construction is similar to that of curved – blade turbine, but with two of four straight blades and a relatively smaller hub. Most large scale agitation of solid – liquid suspensions is done with top – entering turbines or paddles. The impeller speed is typically between 50 and 150 r.p.m; but depending on process conditions, it may go as high as 400 or as low as 15 r.p.m

Fig: Glassed – Steel impeller Fig: Anchor impeller Fig: Flat – blade turbine Fig: Curved – blade turbine Fig: Glassed – Steel impeller Fig: Anchor impeller

Radial flow impellers have blades that are parallel to the axis of the mixer shaft. The smaller, multi-blade types are called turbines. Larger slow-speed impellers, with two to four blades are known as paddles, while other larger two-sided types are known as gate or anchor impellers. For some pseudoplastic systems, stagnant fluid may be found next to the vessel walls in parts remote from propeller or turbine impellers. In such cases, an “anchor” impeller may be used. The fluid flow is principally circular in the direction of rotation of the anchor. Anchor agitators are used particularly to obtain improved heat transfer in high – consistency fluids. Uses: Turbines, which come in a variety of types, provide excellent circulation of fluid throughout the mixing vessel. The diameter of a turbine impeller is normally between 0.3 and 0.6 of the tank diameter. Paddles are used at slower speeds and normally have a diameter greater than 0.6 of the tank diameter. These two types are most often used for large scale mixing of solid/ liquid suspensions

Unbaffled tanks: (vortexing phenomenon) If a low – viscosity liquid is stirred in an unbaffled tank by an axially mounted agitator, there is a tendency for a swirling flow pattern to develop, regardless of the type of impeller. A vortex is produced owing to centrifugal force acting on the rotating liquid. Rotational speed is difficult to raise as severe air entrapment may occur once the vortex reaches the impeller. The swirling mass of liquid often generates an oscillating surge in the tank, which coupled with the deep vortex may create a large fluctuating force acting on the mixer shaft. Vertical velocities in a vortexing low viscosity liquid are low relative to circumferential rates may be obtained by mounting the impeller off – center.

Fig: Typical flow pattern for either axial – or radial – flow impellers in unbaffled tank

Fig: Typical flow pattern in baffled tank with propeller or axial flow turbine positioned on center

A common baffle width is one – tenth to one – twelfth of the tank diameter (radial dimension). For agitating slurries, the baffles often are located one – half their width from the vessel wall to minimize accumulation of solids on or behind them

Silverson emusifier

Silverson emulsifier: